Photochemical Conversion of Solar Energy

DOI: 10.1002/cssc.200700087

Photochemical Conversion of Solar EnergyVincenzo Balzani,* Alberto Credi, and Margherita Venturi[a]

In memory of Giacomo Ciamician (1857–1922) in the 150th anniversary of his birth

26 � 2008 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim ChemSusChem 2008, 1, 26 – 58

1. Introduction

Energy is the most important issue of the 21st century.[1] Fossilfuels have offered astounding opportunities during the 20thcentury in the rich countries of the western world, but now re-serves of fossil fuels are progressively decreasing[2,3] and theircontinued use produces harmful effects such as pollution thatthreatens human health and greenhouse gases associated withglobal warming.[4] Currently, the world’s growing thirst for oilamounts to almost 1000 barrels a second, which correspondsto about 2 liters per day per person living on Earth.[5] Theglobal energy consumption is equivalent to 15 trillion watts(15 TW) of power demand, which is expected to increase by50% by 2030.[6,7]

The goal of ecological sustainability is even more imperativeif we consider the problem of disparity. As an example, a UScitizen consumes as much energy as two Europeans, 10 Chi-nese, 20 Indians, or 30 African people.[8] Disparity is, indeed,the most prominent characteristic among Earth’s inhabitantsand also the most difficult problem to solve. We are wellaware that the stability of human society decreases with in-creasing disparities.How long can we keep running this road? Here is the funda-

mental challenge we face; here are many vital and entangledquestions that we are called to answer.[1] As informed citizens,we have the duty to speak up with decision-makers and politi-cians on the key issues of the irresponsible depletion of resour-ces, the reckless increase of pollution, and the intolerable andever-increasing disparity between the rich and the poor. Aschemists, we can help by improving energy technologies and,hopefully, finding a scientific breakthrough capable of solvingthe energy problem at its root.We are lucky that spaceship Earth, which is otherwise a

closed system, receives an inexhaustible power flow from thesun: 120000 TW of electromagnetic radiation. It is a quantityof energy far exceeding human needs. Covering 0.16% of theland of the Earth with 10% efficient solar-conversion systemswould provide 20 TW of power,[6] nearly twice the world’s con-sumption rate of fossil energy and the equivalent of 20000 nu-clear fission plants of 1 GWe each. Sunlight is our ultimateenergy source, and we need to learn not only how sunlight isused by nature to power life[9] but also how we can convert

sunlight into forms of energy useful for the development ofour civilization.[4,10–14]

Sunlight in the geological eras has also provided us withfossil fuels, the nonrenewable energy source that we are so ea-gerly consuming. In contrast, we are not yet able to take fulladvantage of the extraordinary amount of energy that the sunsupplies us with every day. This paradox was first pointed outby the Italian scientist Giacomo Ciamician in a famous lectureentitled “The Photochemistry of the Future” delivered in NewYork at the VIII International Congress of Pure and AppliedChemistry (1912):[15] “So far human civilization has made usealmost exclusively of fossil solar energy. Would it not be advanta-geous to make a better use of radiant energy?” Ciamician alsorealized that a civilization based on solar energy could re-equi-librate the economic gap, already existing at that time, be-tween northern and southern regions of the world: “Solarenergy is not evenly distributed over the surface of the earth.There are privileged regions, and others that are less favored bythe climate. The former ones would be the prosperous ones if weshould become able to utilize the energy of the sun. The tropicalcountries would be conquered by civilization which would in thismanner return to its birth-place”. The final sentence of thatpaper presents a concept quite meaningful even today (weshould only add oil, gas, and nuclear energy to coal): “If ourblack and nervous civilization, based on coal, shall be followedby a quieter civilization based on the utilization of solar energy,that will not be harmful to the progress and to human happi-ness.”Solar energy has an enormous potential as a clean, abun-

dant, and economical energy source, but cannot be employedas such; it must be captured and converted into useful formsof energy. Because solar power is diffuse (ca. 170 Wm�2) andintermittent, conversion should involve concentration and stor-age. The ultimate challenge remains the production of a fuel

Energy is the most important issue of the 21st century. About85% of our energy comes from fossil fuels, a finite resource un-evenly distributed beneath the Earth’s surface. Reserves of fossilfuels are progressively decreasing, and their continued use produ-ces harmful effects such as pollution that threatens humanhealth and greenhouse gases associated with global warming.Prompt global action to solve the energy crisis is thereforeneeded. To pursue such an action, we are urged to save energyand to use energy in more efficient ways, but we are also forcedto find alternative energy sources, the most convenient of whichis solar energy for several reasons. The sun continuously providesthe Earth with a huge amount of energy, fairly distributed all

over the world. Its enormous potential as a clean, abundant, andeconomical energy source, however, cannot be exploited unless itis converted into useful forms of energy. This Review starts with abrief description of the mechanism at the basis of the naturalphotosynthesis and, then, reports the results obtained so far inthe field of photochemical conversion of solar energy. The “grandchallenge” for chemists is to find a convenient means for artificialconversion of solar energy into fuels. If chemists succeed tocreate an artificial photosynthetic process, “… life and civilizationwill continue as long as the sun shines!”, as the Italian scientistGiacomo Ciamician forecast almost one hundred years ago.

[a] Prof. V. Balzani, Prof. A. Credi, Prof. M. VenturiDipartimento di Chimica “G. Ciamician”Universit> di BolognaVia Selmi 2, 40126 Bologna (Italy)Fax: (+39)051-209-9456E-mail : [email protected]

ChemSusChem 2008, 1, 26 – 58 � 2008 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim www.chemsuschem.org 27

Photochemical Conversion of Solar Energy

www.chemsuschem.org

capable of being stored and transported, such as hydrogen, aprocess that would solve both the energy crisis and the envi-ronmental emergency.Light excitation can induce a variety of chemical reactions.

For energy-conversion purposes, photoinduced electron trans-fer is by far the preferred reaction in nature. This process gen-erates a charge-separated state, which is then used to preparethe various high-energy molecules that fuel an organism. Afterthe energy crisis of the 1970s, several types of endoergonicphotochemical reactions (e.g. photodissociation, valence pho-

toisomerization)[16] were proposed for the artificial conversionand storage of solar energy, but the results have been disap-pointing. Once the mechanism of natural photosynthesis wasat least in part elucidated, mimicry of this natural process forartificial solar energy conversion began to be pursued by sev-eral research groups, as forecast by Ciamician:[15] “The photo-chemical processes, that hitherto have been the guarded secretof the plants, will have been mastered by human industry, whichwill know how to make them bear even more abundant fruitthan nature, for nature is not in a hurry but mankind is.” Itshould be pointed out, however, that only hopes, not fruit, areabundant so far. Hopefully, a more detailed knowledge of nat-ural photosynthesis coupled with a much stronger researcheffort in the field of chemistry will succeed to create an artifi-cial photosynthetic process.

2. Natural Photosynthesis

2.1. Introduction

Photosynthetic organisms are ubiquitous in nature; they areresponsible for the development and sustenance of all life onEarth. They may be quite different, but all of them use thesame basic strategy, in which light is initially absorbed by an-tenna proteins containing many chromophores, followed byenergy transfer to a specialized reaction center protein, inwhich the captured energy is converted into chemical energyby means of electron-transfer reactions.[17]

2.2. Natural Antenna Systems

The better-known natural antennae are the light-harvestingcomplexes of photosynthetic purple bacteria.[18] A major break-through in the field was the high-resolution X-ray crystal struc-ture of the light-harvesting antenna complex LH2 of the pho-tosynthetic unit of Rhodopseudomonas acidophila (Figure 1).[19]

The complex is composed of two rings of bacteriochlorophyll(BChl) molecules, namely 1) a set of 18 molecules close to themembrane surface in almost a face-to-face arrangement like aturbine wheel, and 2) another set of nine molecules all lying ina plane that is perpendicular to the earlier ring of BChl mole-cules, in the middle of the bilayer. These structures are con-

Vincenzo Balzani was born in Forlim-

popoli (Italy) in 1936. He received his

“Laurea” in chemistry at the University

of Bologna in 1960. Following an assis-

tant professorship at the University of

Ferrara, he joined the faculty at the

University of Bologna in 1969 and has

remained there to this day. His scientif-

ic activity is documented by three

monographs and about 500 papers in

the fields of photochemistry, supra-

molecular chemistry, molecular-level

devices and machines, and solar energy conversion. He is one of

the 50 most-cited scientists in the field of chemistry (ISI).

Alberto Credi was born in Bologna

(Italy) in 1970. He received his “Laurea”

(1994) from the University of Bologna,

where, after a research period in the

U.S. , he later earned his PhD (1999).

He is currently Associate Professor of

Chemistry there. He has received sev-

eral scientific awards, including the

IUPAC Prize for Young Chemists (2000)

and the Grammaticakis–Neumann

Prize for Photochemistry (2006), and

co-authored about 140 scientific

papers and several books in the fields of molecular and supra-

molecular photochemistry and electrochemistry.

Margherita Venturi is Professor of

Chemistry at the University of Bologna.

From 1972 to 1991, she worked at the

National Research Council of Bologna,

where she mainly studied, by means

of pulsed and continuous radiolytic

techniques, electron-transfer processes

involved in model systems for the con-

version of solar energy. She joined the

group of Prof. Balzani in 1992. Her

present research interests are in the

field of supramolecular photochemis-

try and electrochemistry. She is the co-author of about 150 publi-

cations in international journals as well as the monograph “Molecu-

lar Devices and Machines” (with V. Balzani and A. Credi).

Figure 1. Structure of the LH2 light-harvesting antenna system of R. acido-phila which contains rings of 18 (a) and nine (b) bacteriochlorophyll mole-cules. See text for details. Reprinted with permission from Ref. [19] .

28 www.chemsuschem.org � 2008 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim ChemSusChem 2008, 1, 26 – 58

V. Balzani et al.

www.chemsuschem.org

tained within the walls of two protein cylinders with radii of1.8 and 3.4 nm. Because of the different chemical environ-ments, the two sets of BChl molecules have different absorp-tion and photophysical properties. The 18 BChl molecules be-longing to the larger wheel have the lowest-energy absorptionmaximum at 850 nm (and are therefore named B850), and thenine BChl molecules in the middle of the bilayer have thelowest-energy absorption maximum at 800 nm (B800). Thereare other significant differences between the two sets of pig-ments:[20] the B800 species are largely monomeric, whereas theB850 molecules are strongly exciton-coupled, with the excitonstate delocalized over several (presumably four) BChl mole-cules. All the BChl molecules are maintained in a fixed spatialrelationship by the surrounding polypeptides. Carotenoids arealso associated within the LH2 structure with the dual functionof contributing to light harvesting and protecting the systemagainst photooxidation by quenching the singlet oxygen mole-cules produced by photosensitization. The light absorbed bythe B800 array is transferred to the B850 wheel within 1 ps.Energy migration among the various exciton states of B850then occurs on the 300-fs timescale.The energy collected by the LH2 antennae is then trans-

ferred to another antenna complex, LH1, which surrounds thereaction center (RC). The reaction center is the final destinationof the collected energy, and it is the site where charge separa-tion takes place. A schematic view of the overall light-harvest-ing process is shown in Figure 2. The structure of LH1 is not

known at the same level of definition as that of LH2, but ananalysis by electron crystallography of two-dimensional crystalsof the LH1 complex of Rhodospirillum rubrum[21] has revealedthat, although LH1 is much larger, there is a clear similitudebetween LH1 and LH2: its 32 BChl molecules are indeed ar-ranged as the B850 molecules of LH2. LH1 absorbs at 880 nm(B880) and, because LH1 and LH2 are in close contact (estimat-ed to be shorter than 30 N), LH2!LH1 energy transfer is quitefast (3 ps). The rate of the successive energy-transfer step fromLH1 to the embedded RC is more than 10 times slower (35 ps).As the molecules of the LH1 wheel are exciton-coupled likethose of B850, such an energy-transfer process should occur

from approximately eight sites of LH1, each one comprisingfour delocalized BChl subunits, to RC (assuming one RC per32 BChl LH1 molecules).[20]

In conclusion, in natural light-harvesting antennae ultrafastenergy migration within almost isoenergetic subunits of asingle complex is followed by fast energy transfer to a lower-energy complex with minimal losses. All processes are believedto occur by a Fçrster mechanism.Recent studies have provided evidence for wavelike energy

transfer through quantum coherence.[22] Superposition statesallow excitation to reversibly sample relaxation rates from allcomponent exciton states, thereby efficiently directing theenergy transfer to find the most effective sink for excitationenergy. When viewed in this way, the system is essentially per-forming a single quantum computation, sensing many statessimultaneously and selecting the correct answer, as indicatedby the efficiency of energy transfer.The light-harvesting complexes of green plants are not well

known and, likely, they are more complicated than those ofbacterial photosynthesis.[23,24] There are good reasons to be-lieve, however, that the governing principles of operation aresimilar to those discussed above.

2.3. Natural Reaction Centers

The simplest and best understood reaction center is thatfound in purple bacteria, which can be taken as a model of allthe photosynthetic reaction centers.[25–27] The most importantsolar-energy-conversion process, however, is that occurring ingreen plants,[28–30] where the reaction center of Photosystem IIhas an electron-acceptor site quite similar to that of the bacte-rial reaction center and a very peculiar donor site, which canuse water as an electron source and produce dioxygen as a“waste” product. Because this peculiar feature is particularlyrelevant to the design of artificial systems capable of perform-ing photoinduced water splitting (Section 3.7), the donor siteof the reaction center of Photosystem II will also be illustrated.

2.3.1. Bacterial Photosynthesis

The structures of several bacterial reaction centers are knownprecisely as a result of X-ray crystallographic investigations.[31,32]

The photosynthetic reaction centers of bacteria and other or-ganisms consist mainly of a protein, which is embedded in andspans a lipid bilayer membrane. The basic photochemistry isperformed by some cofactors buried within it.[33] A simplifiedview of the structure of the reaction center of Rhodopseudomo-nas viridis is sketched in Figure 3. Detailed photophysical stud-ies of this reaction center have led to a precise picture of thesequence of events participating in photoinduced charge sepa-ration.[33,34] The key molecular components are a bacteriochlor-ophyll “special pair” (P), a bacteriochlorophyll monomer (BC), abacteriopheophytin (BP), a quinone (QA), and a four-heme c-type cytochrome (Cyt). These molecules are held in a fixed ge-ometry by surrounding proteins, so that the twofold axis ofP[35] is perpendicular to the membrane, the periplasmic facelies approximately between P and Cyt, and the cytoplasmic

Figure 2. Schematic representation of the overall light-harvesting process byLH2 and LH1 antenna complexes in bacterial photosynthesis. RC denotesthe reaction center.



www.chemsuschem.org

face lies at the level of QA. In the reaction center, excitation ofP by absorption of light or, more commonly, by singlet–singletenergy transfer from various antenna systems, is followed byvery fast (~3 ps) electron transfer to the BP “primary” acceptor(whether the interposed BC plays the role of mediator in a su-perexchange mechanism or directly intervenes as an inter-mediate electron acceptor has been the object of debate).[36]

The next step involves fast (~200 ps) electron transfer from BPto QA, followed by slower (~270 ns) reduction of the oxidizedP by the nearest heme group of Cyt.[37] At that stage, trans-membrane charge separation has been achieved with an effi-ciency approaching unity and an extremely long lifetime withrespect to charge recombination. The rate constants of the var-ious electron-transfer steps involved in the charge-separationprocess are summarized in the approximate energy-level dia-gram of Figure 4, together with those of the non-occurringBP�!P+ and QA

�!P+ charge-recombination steps (as deter-

mined from experiments with modified reaction centers lack-ing the possibility of competing forward processes).[38]

Figure 3 and Figure 4 point out the importance of the supra-molecular structure of the reaction center. The achievement ofefficient photoinduced charge separation over a large distanceis made possible by optimization of several aspects of thisphotochemical device: 1) the organization of the molecularcomponents in space, 2) the thermodynamic driving force ofthe various electron-transfer steps, and 3) the kinetic competi-tion between forward (useful) over back (dissipative) electron-transfer processes. How this occurs can be reasonably well un-derstood in terms of electron-transfer theory.[25,39] In particular,it can be noted that the high efficiency of the charge-separa-tion process is due to the fact that the charge-recombinationsteps are slow because they lie in the Marcus invertedregion.[40] Furthermore, in order to have a high efficiency ofcharge separation, the photoinduced electron-transfer processmust proceed only along one of the two branches of the appa-rently symmetric reaction center (Figure 3). It is likely that mu-tations have broken the symmetry, imposing unfavorableFranck–Condon factors on the disfavored side. It has also beenobserved[25] that if the distance between BP and QA was just afew angstroms longer, or the driving force of this reaction wasseveral tenths of an eV larger or smaller, then the quantum ef-ficiency of the reaction center would suffer as charge recombi-nation became more common. On the other hand, if the driv-ing force for the BP�-to-P+ ground-state reaction was de-creased, then this inverted region reaction would accelerateand also lower the efficiency of the productive charge separa-tion. Experiments with reaction centers oriented in an externalelectric field have provided some evidence that this is true.[41]

In the process described above, the ultimate electron ac-ceptor is a quinone QA. Then, the process continues with manyother steps. The electron migrates to a second quinone QB,and, after reduction of the oxidized special pair P+ by a c-typecytochrome (see below), the energy of a second photon isused to transfer a second electron to QB. Reduction of QB to itshydroquinone form involves the uptake of two protons fromwater on the internal cytoplasmic side of the membrane. Thehydroquinone then diffuses to the next component of the ap-paratus, a proton pump, denoted the cytochrome bc1 complex(Figure 5). This complex oxidizes the hydroquinone back to a

Figure 3. A simplified view of the structure of the reaction center of R. viridis.See text for details.

Figure 4. Energy-level diagram and rate constants of the electron-transfersteps involved in the charge-separation process in the reaction center ofR. viridis.

Figure 5. Schematic representation of the bacterial photosynthetic mem-brane and of the different protein components.


V. Balzani et al.

www.chemsuschem.org

quinone and uses the energy re-leased to translocate protonsacross the membrane and es-tablish a proton concentrationand charge imbalance (proton-motive force). The oxidationprocess is ultimately driven,through various cytochromeredox relays, by the oxidizedspecial pair P+ , which becomesreduced to its initial state. Final-ly, a rotary motor, the enzymeadenosine triphosphate (ATP)synthase,[42,43] allows protons toflow back across the membrane,down the thermodynamic gradi-ent, driving the release of ATP formed from adenosine diphos-phate (ADP) and inorganic phosphate (Pi). The ATP fills the ma-jority of the energy needs of the bacterium.

2.3.2. Photosystem II

Photosystem II (PSII) carries out all the processes needed forphotosynthesis in green plants: light absorption in antennacomponents, energy transfer to a reaction center, charge sepa-ration, and charge stabilization.[30] Furthermore, it is capable ofusing water as the reductant of the quinone which is at theend of the acceptor side. In order to do that, PSII must1) reach potentials high enough to oxidize water (>+0.9 V rel-ative to the normal hydrogen electrode, NHE), 2) handle such ahigh oxidation potential in fragile biological structures, and3) couple the one-photon/one-electron charge-separation pro-cess to the four-electron water oxidation process. The wateroxidation moiety of PSII (Figure 6a)[28,44,45] consists of a triadcomposed of a multimer of chlorophylls (named P680), a redox-active tyrosine aminoacid (TyrZ, Y161 of the D1 polypeptide),and the so-called oxygen-evolving complex (OEC), a clustercontaining four Mn atoms and a Ca atom (Mn4Ca) connectedby mono-m-oxo, di-m-oxo, and/or hydroxo bridges. The specificprotein environment and one chloride ion are also essential forthe water-splitting activity.PSII spans the thylakoid mem-

brane in the chloroplasts, andthe water-oxidizing triad is lo-cated closely to one side of themembrane. On direct absorptionof a photon or energy transferfrom the antenna units, P680 isexcited and becomes a strongreductant. An electron is thentransferred from excited P680 tothe acceptor system (pheophy-tin and two quinones, QA andQB) on the other side of themembrane (Figure 6a). The oxi-dized primary donor, P680

+ , isone of the most oxidizing spe-

cies found in nature and reaches a potential of +1.2 V relativeto NHE. P680

+ is rapidly reduced by TyrZ (t=20–200 ns, de-pending on the state of the Mn4Ca cluster; see below) which,in its reduced state, is hydrogen-bonded to a nearby histidineresidue.[46] Such a hydrogen bond facilitates oxidation of TyrZwhich occurs with concomitant deprotonation (proton-coupledelectron transfer, PCET). PCET plays indeed a fundamental rolein biological processes, as electron transfer in many proteinsand enzymes is supported along pathways exhibiting hydro-gen-bond contact between amino acid residues and polypep-tide chains.[47,48] Electron and proton transfer influence eachother thermodynamically and kinetically.[49] Extensive investiga-tions have been perfomed in the last few years in an attemptto throw light on these complex processes.[50]

As illustrated in Figure 7,[51] PCET offers an energy advantageof 9.5 kcalmol�1 compared to simple electron transfer becauseit avoids oxidation of TyrZ to the high-energy radical cation in-termediate TyrzOH

+ . The oxidized TyrZ radical so obtained is re-duced by electrons, which ultimately are derived from water.How this happens is still largely unknown. When PSII works atfull speed, approximately 200 water molecules can be oxidizedper second. This suggests that the kinetic barriers must bevery low.

Figure 6. a) Schematic representation of the charge-separation process in PSII. b) The five redox states (S0–S4) ofthe Mn4Ca cluster.

Figure 7. Comparative energetics of oxidation of TyrZ by electron transfer and by histidine-assisted PCET.[51]



www.chemsuschem.org

Oxidation of water to dioxygen is a four-electron process, sothe results of four charge-separation events must be accumu-lated. This role is played by the Mn4Ca cluster, which is closeto TyrZ and is oxidized stepwise by the TyrZ radical to a seriesof states Si (i=0–4), as shown in Figure 6b. Dioxygen evolutionoccurs when the most oxidized cluster state, S4, returns, in afour-electron reduction process, to the most reduced state, S0.This process involves the oxidation of two water molecules,which have probably been coordinatively bound to the man-ganese cluster.The structure of the Mn4Ca cluster has been the object of

extensive investigation with a variety of techniques, includingX-ray diffraction (XRD) studies of single crystals of PSII at 3.5[52]

and 3.0 N resolution,[53] and Mn X-ray absorption near-edgestructure (XANES)[54,55] and Mn X-ray absorption fine structure(EXAFS) studies.[55] In the S1 state, the Mn–Mn distances areabout 2.7 N, except for two Mn atoms which are separated by3.3 N, and the Ca atom is 3.4 N from two Mn atoms. The avail-able data constrain the Mn4Ca cluster geometry to a set ofthree similar high-resolution structures.[55] The nature of the in-teraction of the manganese cluster with the TyrZ radical is notfully understood. As mentioned above, a concerted electron-proton transfer of the manganese-bound water molecules bythe tyrosine radical is most likely on thermodynamic groundsas it would avoid the formation of high-energy intermedi-ates.[56]

Some theoretical clues to the reaction mechanism of dioxy-gen formation at the oxygen-evolving complex, OEC, have alsobeen discussed.[57] A reliable mechanism for the very complexwater oxidation process, however, will only be obtained whenthe structures of the various Si states are available.

3. Artificial Photosynthesis

3.1. Introduction

The conversion of solar energy into fuel by artificial photosyn-thetic systems is certainly one of the most challenging goals inchemistry.[1, 58–68] For the production of solar fuel to be econom-ically and environmentally attractive, the fuels must be formedfrom abundant, inexpensive raw materials such as water andcarbon dioxide. Water should be split into molecular hydrogenand molecular oxygen, and carbon dioxide in aqueous solutionshould be reduced to ethanol with the concomitant genera-tion of dioxygen.[69]

From many points of view, the most attractive fuel-generat-ing reaction is the cleavage of water into hydrogen andoxygen [Eq. (1)]:

2 H2Oþ 4 hn! 2H2 þ O2 ð1Þ

Such a process, of course, has to be sensitized as watercannot be electronically excited by sunlight.[58] Combustion ofmolecular hydrogen, H2, with oxygen produces heat and water,and combination of molecular hydrogen and oxygen in a fuelcell generates electricity, heat, and water. Once obtained, hy-drogen could also be used to obtain methanol, a liquid fuel.[3]

Clearly, if hydrogen could promptly replace oil, both theenergy and the environmental problems of our planet wouldbe solved.

3.2. Hydrogen Economy

The media, press, and even policy-makers often talk about the“hydrogen economy” and sometimes describe hydrogen as afuel available or obtainable for free from water. This (wrong)message suggests that the energy problem will soon besolved. Most scientists, however, believe that the shift to a hy-drogen economy will not occur soon and might even notoccur at all unless a large research effort is set up to overcomeseveral scientific and technological obstacles.[1, 70,71]

Since there is no molecular hydrogen on the Earth, molecu-lar hydrogen cannot be mined but instead has to be “manufac-tured”, starting from hydrogen-rich compounds, by usingenergy. Therefore, hydrogen is not an alternative fuel, but asecondary form of energy. This is the central (but not unique)problem of a hydrogen economy. Like electricity, hydrogenmust be produced by using fossil, nuclear, or renewableenergy, and then it can be used as an energy vector with theadvantage, with respect to electricity, that it can be stored.Although a proper use of hydrogen is not expected to cause

big environmental problems, one cannot say that hydrogen isa “clean” form of energy. In fact, hydrogen is “clean” or “dirty”depending of the primary energy form used to produce it. Hy-drogen obtained by spending fossil fuels or nuclear energy in-corporates all the problems of using those primary energysources. Burning fossil fuels in remote regions to produce hy-drogen as a clean fuel for metropolitan areas would be an inef-fective solution owing to the trans-boundary nature of atmos-pheric pollution.[72]

Clearly, clean hydrogen can only be obtained by exploitingrenewable energies, and this can be done, in principle, by pho-tochemical water splitting or through the intermediate produc-tion of electricity (e.g. by wind or photovoltaic cells) followedby water electrolysis.

3.3. Components of an Artificial Photosynthetic System

The best way to construct artificial photosynthetic systems forpractical solar fuels production is that of mimicking the molec-ular and supramolecular organization of the natural photosyn-thetic process: light harvesting should lead to charge separa-tion, that must be followed by charge transport to deliver theoxidizing and reducing equivalents to catalytic sites, whereevolution of oxygen and hydrogen (or CO2 reduction) shouldseparately occur. Therefore, a plausible artificial photosyntheticsystem should include the following basic features(Figure 8):[65] 1) an antenna for light harvesting, 2) a reactioncenter for charge separation, 3) catalysts as one-to-multielec-tron interfaces between the charge-separated state and thesubstrate, and 4) a membrane to provide physical separationof the products.The complexity of the natural photosynthetic systems is

clearly out of the reach of the synthetic chemist. This complex-


V. Balzani et al.

www.chemsuschem.org

ity, however, is largely related to their living nature. Today, weknow that single photosynthetic functions, such as photoin-duced energy and electron transfer, can be duplicated bysimple artificial systems. The important lesson from nature isthat the achievement of efficient conversion of light intochemical energy requires the involvement of supramolecularstructures with very precise organization in the dimensions ofspace (relative location of the components), energy (excited-states energies and redox potentials), and time (rates of com-peting processes). Such an organization, which in natural sys-tems comes as a result of evolution and is dictated by intricateintermolecular interactions, can be imposed in artificial systemsby molecular engineering exploiting covalent or noncovalentbonding.[73] Today, while some progress has been made oneach aspect of artificial photosynthesis, integration of the vari-ous components in a working system has not yet been ach-ieved.

3.4. Artificial Antenna Systems

3.4.1. Introduction

The antenna effect can only be obtained in supramoleculararrays suitably organized in the dimensions of time, energy,and space. Each molecular component has to absorb the inci-dent light, and the excited state so obtained (donor) has totransfer electronic energy to a nearby component (acceptor)before undergoing radiative or nonradiative deactivation (or-ganization in the time dimension). For energy transfer tooccur, the energy of the acceptor excited state has to be loweror, at most, equal to the energy of the excited state of thedonor (organization in the energy dimension). Finally, the suc-cessive donor-to-acceptor energy-transfer steps must result inan overall energy-transfer process that leads the excitationenergy to a selected component of the array (organization inthe space dimension). In recent years, the development ofsupramolecular chemistry (particularly, of dendrimer chemistry)and the high level of experimental and theoretical efficacy

reached by photochemistryhave enabled scientists todesign and construct many arti-ficial antenna systems.Dendrimers[74] (Figure 9) con-

stitute a class of well-definedmacromolecules exhibiting atreelike, nanometer-size archi-tecture, reminiscent of the ar-chitecture of natural light-har-vesting complexes. Therefore,dendrimer structures are veryattractive for the constructionof artificial antennae,[75–78] alsobecause their convergent and/or divergent synthesis[74] allowsthe assembly, in a few syntheticsteps, of a large number ofchromophores in a restricted

space and with high topological control. Photoactive units canbe directly incorporated or appended with covalent or coordi-nation bonds in different regions of a dendritic structure andcan also be noncovalently hosted in the cavities of a dendri-mer. Because of their proximity, the various functional groupsof a dendrimer may easily interact with one another.Collecting light by an antenna systems may also be useful

for purposes different from artificial photosynthesis, for exam-ple, for signal amplification in luminescence sensors,[79] photo-dynamic cancer therapy,[80] and up-conversion processes.[81] Alarge system, where an array of chromophoric units absorblight and transfer energy to a luminescent center, can also beconsidered a spatial and spectral energy concentrator (“molec-ular lens”).[82]

We will illustrate a few selected examples of artificial anten-na systems.

3.4.2. Dendrimers Based on Metal Complexes

Oligopyridine ligands have extensively been used to build uppolynuclear complexes with dendritic structures.[75,83–86] In such

Figure 8. Schematic representation of photochemical water splitting (artificial photosynthesis).[65] Five fundamen-tal components can be recognized—an antenna for light harvesting, a charge-separation triad D-P-A, a catalystfor hydrogen evolution, a catalyst for oxygen evolution, and a membrane separating the reductive and the oxida-tive processes.

Figure 9. Schematic representation of a dendrimer.



www.chemsuschem.org

dendrimers (see, for example, Figure 10), the metal units arelinked together by bridging ligands. The choice of suitablebridging ligands is crucial in determining the properties ofdendrimers, because 1) their coordinating sites (together with

those of the “terminal” ligands) influence the spectroscopicand redox properties of the active metal-based units, 2) theirstructure and the orientation of their coordinating sites deter-mine the architecture of the dendrimer, and 3) their chemicalnature controls the electronic communication between themetal-based units.The more carefully investigated dendrimers of this kind are

those containing RuII and OsII as metal ions, 2,3- and 2,5-bis(2-pyridyl)pyrazine (2,3- and 2,5-dpp, respectively) as bridging li-gands, and 2,2’-bipyridine (bpy) and 2,2’-biquinoline (biq) asterminal ligands (Figure 10).[75] The typical strategy used to pre-pare these dendrimers is the so-called “complexes as metalsand complexes as ligands” approach,[87] which has enabled theconstruction of species containing four, six, 10, 13, and 22metal-based units. A docosanuclear dendrimer of that family,such as that schematically shown in Figure 10, is a cationicspecies carrying a charge of 44+ and comprising 1090 atoms,with an estimated size of 5 nm. Besides 22 metal atoms, it con-tains 24 terminal ligands and 21 bridging ligands.

From a photophysical viewpoint, such dendrimers, whichcan be viewed as ordered ensembles of [M(L)n(BL)3-n]

2+ com-plexes (M=RuII or OsII ; L=bpy or biq; BL=2,3- or 2,5-dpp), areknown to have 1) intense ligand-centered (LC) absorptionbands in the UV region and moderately intense metal-to-ligand charge-transfer (MLCT) bands in the visible region, and2) a relatively long-lived luminescence in the red spectralregion, originating from the lowest 3MLCT level. In the den-drimers of this family, there is only a small electronic interac-tion between nearby mononuclear units and, therefore, the ab-sorption spectrum is practically the “sum” of the spectra of theconstituent units. In the dendrimers of higher nuclearity, as aconsequence, the molar absorption coefficient is hugethroughout the entire UV/Vis spectral region (e=

202000 Lmol�1 cm�1 at 542 nm for a docosanuclear dendrimerin which all the metal ions are RuII), so that most of the photo-chemically active part of sunlight can be absorbed. The smallbut not negligible electronic interaction between nearby unitsis sufficient to cause in these dendrimers a very fast energytransfer that leads to the quenching of the potentially lumines-cent units having higher-energy 3MLCT levels and the sensitiza-tion of the luminescence of the units having lower-energy3MLCT levels. Energy transfer between nearby units, however,occurs mainly in the femtosecond timescale from non-thermal-ized singlet excited states, in competition with intersystemcrossing.[88]

The energy of the excited states of each unit depends onmetal and ligands in a predictable way, and the modular syn-thetic strategy[75] enables high synthetic control in terms of thenature and position of metal centers, bridging ligands, and ter-minal ligands. Such synthetic control translates into a highdegree of control on the direction of energy flow within thedendritic array. On increasing nuclearity, however, a unidirec-tional gradient (center-to-periphery or vice versa) for energytransfer cannot be obtained with only two types of metals (RuII

and OsII) and ligands (bpy and 2,3-dpp). An extension of thiskind of antennae is represented by heterometallic dendrimerswith appended organic chromophores.[89]

For the heptanuclear complex made up of a [Cl2Ru(m-2,3-dpp)2] core, two [Ru(m-2,3-dpp)3]

2+ branch units, and four[(bpy)2Ru(m-2,3-dpp)]

2+ in the periphery, it has been shownthat the peripheral units transfer energy to the core throughthe intermediate higher-energy units taking advantage of a se-quential two-step electron-transfer process.[90]

3.4.3. Dendrimers Based on Porphyrins

Porphyrins, the main chromophores of natural photosynthesis,are obvious candidates for the design of artificial antenna sys-tems. In this section, we illustrate a few typical examples;more extensive coverage can be found in several reviews.[91]

By means of a modular approach and the use of an ethynelinkage between aryl groups on adjacent tetraarylporphyrinmacrocycles, a variety of di-, tri-, tetra-, and pentameric por-phyrin arrays have been obtained.[91c] Photophysical investiga-tions have shown[92] that 1) singlet excited-state energy trans-fer from Zn porphyrin to free-base porphyrin is extremely effi-

Figure 10. Schematic representation of a dendrimer containing Ru and/orOs complexes in each branching site.[75] The formulae of the 2,3- and 2,5-bis(2-pyridyl)pyrazine (2,3- and 2,5-dpp) bridging ligands and of the 2,2’-bi-pyridine (bpy) and 2,2’-biquinoline (biq) terminal ligands are also shown.


V. Balzani et al.

www.chemsuschem.org

cient (95–99%), 2) competitiveelectron-transfer reactions arenot observed, 3) the mechanismof energy transfer predominant-ly involves through-bond com-munication via the ethynelinker, and 4) energy transfer be-tween two isoenergetic Zn por-phyrins is very fast. These stud-ies demonstrate that extendedmultiporphyrin arrays can be de-signed in a rational manner withpredictable photophysical fea-tures and efficient light-harvest-ing properties.Efficient excitation energy

transfer has been shown tooccur in giant wheels (about 7-nm diameter) composed of 24porphyrin units with a rate of35 ps�1 for energy hopping be-tween neighboring tetrapor-phyrins moieties.[93] Attempts tobuild up artificial antennae byself-assembling of porphyrincomponents have also been re-ported.[94,95]

Morphology-dependent an-tenna properties have been re-vealed[96] for a series of dendrim-ers that have the general formu-la (L)nP, where P is a free-baseporphyrin core bearing differentnumbers (n=1–4) of poly(ben-zyl ether) dendrons (L) at itsmeso positions (Figure 11a). Indichloromethane solutions, exci-tation of the chromophoricgroups of the dendrons causessinglet–singlet energy-transferprocesses that lead to the exci-tation of the porphyrin core.The (L)4P dendrimer, which has aspherical morphology, exhibits amuch higher energy-transferquantum yield (0.8) than thepartially substituted (L)1P, (L)2P, and (L)3P species (quantumyields less than 0.32). Fluorescence polarization studies on (L)4Pshowed that the excitation energy migrates very efficientlyover the dendrons within the excited-state lifetime, so that thefour dendrons can be viewed as a single, large chromophoresurrounding the energy trap. Temperature-dependent effectssuggested that increased flexibility and conformational free-dom are responsible for the decreased energy-transfer efficien-cy on decreasing the number of dendrons. Only the highlycrowded (L)4P dendrimer retains a constant level of energytransfer, even at high temperatures. It was also postulated that

cooperativity between dendrons, which decreases with increas-ing conformational mobility, is necessary for efficient energytransfer.[96] Such a behavior would mimic that of natural photo-synthetic systems, where energy migration within “wheels” ofchromophoric groups results in an efficient energy transfer tothe reaction center. The morphology effect has also been in-vestigated by appending to the free-base porphyrin core P ofdendrimers (L)nP up to four, much larger dendrons each con-taining seven Zn-porphyrin units (e.g. L is compound 1 shownin Figure 11b).[97] The presence of poly(benzyl ether) dendriticwedges at the periphery makes such dendrimers soluble in

Figure 11. Light-harvesting dendrimers with the general formula (L)nP, where P is a free-base porphyrin core bear-ing different numbers of dendrons, L: a) L=poly(benzyl ether) dendrons;[96] b) L=compound 1 containing sevenZn-porphyrin units.[97]



www.chemsuschem.org

common organic solvents. In the star-shaped (1)4P dendrimer,energy transfer from the excited singlet states of dendrons 1to the focal P core takes place with a rate constant of 1.0T109 s�1 and 71% efficiency, whereas in the conically shaped(1)1P dendrimer the energy-transfer rate constant is 10 timessmaller and the efficiency is 19%. This result shows that mor-phology has indeed a noticeable effect on the energy-transferrate. Excitation of (1)4P at 544 nm with polarized light results ina highly depolarized fluorescence from the Zn-porphyrin units(fluorescence anisotropy factor 0.03, to be compared with 0.19of a monomeric reference compound), indicating an efficientenergy migration among the Zn-porphyrin units before theenergy is transferred to the free-base core. In the case of theconically shaped (1)1P compound, the fluorescence anisotropyfactor is much higher (0.10). These results suggest a coopera-tion of the four dendrons of (1)4P in facilitating the energy mi-gration among the Zn-porphyrin units. Clearly, the (1)4Psystem, which incorporates 28 light-absorbing Zn-porphyrinunits into a dendritic scaffold that has an energy-acceptingcore, mimics several aspects ofthe natural light-harvesting LH1complex.A study on a nonameric por-

phyrin assembly made up of acentral free-base and eight pe-ripheral Zn porphyrins connect-ed by flexible nucleoside linkershas provided evidence for thepresence of several nonequili-brated conformations.[98] Bothsinglet–singlet and triplet–trip-let energy transfer from the pe-ripheral Zn porphyrins to thefree-base porphyrin and triplet–triplet annihilation have beendetected. Association of biden-tate bases increases the rigidityof the structure and improvesthe energy collection ability.

3.4.4. Dendrimers Based on Or-ganic Molecules

Many antenna systems basedon organic molecules havebeen constructed.[99] Energytransfer in a series of shape-per-sistent polyphenylene dendrim-ers substituted with perylenei-mide and terryleneimide chro-mophoric units has been inves-tigated in toluene solution.[100]

Energy hopping among the per-yleneimide chromophores, re-vealed by anisotropy decaytimes,[101] occurs with a rateconstant of 4.6T109 s�1. When

three peryleneimide and one terryleneimide chromophores areattached to the dendrimer rim, energy transfer from theformer to the latter units takes place with an efficiency of over95%. All the observed energy-transfer processes can be inter-preted on the basis of the Fçrster mechanism. Polyphenylenedendrimers with a perylene diimide as a luminescent corehave also been investigated.[102] In films, the dendrons suppressthe interaction of the emissive cores that causes loss and redshifting of the emission.Several studies on single dendrimer molecules have been re-

ported.[103–106] In polyphenylene dendrimer 2 (Figure 12), whichconsists of a terrylenediimide (TDI) core, four perylenemono-ACHTUNGTRENNUNGimides (PMIs) attached to the scaffold, and eight naphthalene-monoimides (NMIs) at the rim, the antenna effect has beenstudied at the ensemble and single-molecule level.[106] Efficientenergy transfer from the PMIs to the core and from the NMIsdirectly or via PMIs to the core has been observed. In single-molecule experiments, the NMI chromophores are the first tobleach.

Figure 12. Dendrimer 2 consisting of a terrylenediimide (TDI) core, four perylenemonoimides (PMIs) attached tothe scaffold, and eight naphthalenemonoimides (NMIs) at the rim.[106]


V. Balzani et al.

www.chemsuschem.org

Oligo(p-phenylene vinylene) (OPV) units are extensively stud-ied as ideal model compounds for the corresponding poly(p-phenylene vinylene) (PPV) polymers that can be used for light-emitting diodes (LEDs),[107] field-effect transistors (FETs),[108] andsolar cells.[109] OPV units are also increasingly used to obtainphotoactive dendrimers.[110] Energy transfer in single OPV vesi-cles,[111] chiral co-assemblies of hydrogen-bonded OPV and por-phyrin,[112] and self-assembled OPV functionalized with pery-lene-bisimide units[113] have been recently investigated.

3.4.5. Dendrimers Based on Host–Guest Systems

An important property of dendrimers is the presence of inter-nal cavities where ions or neutral molecules can be hosted.[114]

Such a property can potentially be exploited for a variety ofpurposes, which include catalysis[115] and drug delivery.[116]

Energy transfer from the numerous chromophoric units of asuitable dendrimer to an appropriate guest may result in alight-harvesting antenna system.[117, 118] An advantage shown bysuch host–guest light-harvesting systems is that the collectedenergy can be delivered by the same dendrimer to suitablytuned guests.An interesting example is given by dendrimer 3 (Figure 13),

which consists of a hexaamine core surrounded by eightdansyl-, 24 dimethoxybenzene-, and 32 naphthalene-typeunits.[118] In dichloromethane solution, compound 3 exhibits

the characteristic absorption bands of the component unitsand a strong dansyl-type fluorescence. Energy transfer fromthe peripheral dimethoxybenzene and naphthalene units tothe fluorescent dansyl units occurs with over 90% efficiency.When the dendrimer hosts a molecule of eosin (3�eosin), thedansyl fluorescence, in its turn, is quenched and sensitizationof the fluorescence of the eosin guest can be observed. Quan-titative measurements showed that the encapsulated eosinmolecule collects electronic energy from all 64 chomophoricunits of the dendrimer with an efficiency of over 80% (partialoverlapping between dansyl and eosin emissions precludes abetter precision). Both intramolecular (i.e. within dendrimer)and intermolecular (i.e. dendrimer host!eosin guest) energy-transfer processes occur very efficiently by a Fçrster-typemechanism because of the strong overlap between the emis-sion and absorption spectra of the relevant donor and accept-or units.Dye molecules can also be hosted into poly(propylene

amine) dendrimers surface-modified with OPV units.[119] Inthese systems, energy transfer from the OPV fluorescent units(lmax=492 nm) to the enclosed dye molecules is not efficientin solution (40% efficiency at maximal loading), but is very effi-cient in spin-coated films of dendrimer/dye assemblies. Energytransfer has been found to occur from 1,3,4-oxodiazole den-drons to hydrogen-bonded OPV derivatives[120] and has beeninvestigated at the single-molecule level in host–guest systems

consisting of a second-genera-tion polyphenylene dendrimerand a cyanine dye.[121]

Investigations on dendrimerscapable of hosting metal ionshave been recently reviewed.[122]

3.4.6. Other Systems

Energy transfer in derivatizedpolymers with attached chro-mophores has been extensivelyinvestigated.[123,124] RuII and OsII

polypyridine complexes havebeen attached by amide linkag-es to a 1:1 styrene-p-aminome-thylstyrene copolymer with apolydispersivity of 1.5 and anaverage of 16 repeatingunits.[123] A mixed polymer wasprepared by sequential cou-pling, first with a limitedamount of the less reactive OsII

complex, and then with themore reactive RuII complex tofill all the remaining free sites.In a mixed polymer containingthe lower-energy OsII complexand the higher-energy RuII com-plex in a 3:13 ratio[123a] (subse-quently corrected to 5:11[123d]),

Figure 13. Schematic representation of the energy-transfer processes occurring in dendrimer 3, which containsthree different types of light-harvesting chromophoric units.[118] All the excitation energy can be channelled to ahosted eosin molecule.



www.chemsuschem.org

triplet–triplet energy transfer from the excited RuII complex tothe OsII complex was observed with an efficiency higher than90% in acetonitrile solution. Poly(amino acids) have also beenused as backbones to construct antenna systems.[123e]

Zeolite L, a crystalline aluminosilicate in which the SiO4 andAlO4 tetrahedra give rise to one-dimensional channels ar-ranged in a hexagonal structure, has been used as a host forthe organization of dyes to furnish antenna properties.[125] Azone of the zeolite nanocrystal (e.g. the central zone in eachchannel) can be filled with molecules of a specific dye (dye1;e.g. 1,2-bis-(5-methyl-benzoxazol-2-yl)ethene); then, under ap-propriate experimental conditions, a second (dye2; e.g. pyro-nine) and a third (dye3; e.g. oxonine) dye are successively in-serted into the channels. If the three dyes are suitably chosen,light excitation of dye1 located in the middle part leads toenergy-migration processes on both sides of the channels. Clo-sure (stopcock) molecules can be used both to prevent thedyes from leaving the channels and to interface the dye mole-cules contained in the channels with the external environment.The linear [Ru ACHTUNGTRENNUNG(bpy)2 ACHTUNGTRENNUNG{bpy-(Ph)4-Si ACHTUNGTRENNUNG(CH3)3}]

2+ complex can be usedas a functional stopcock that transfers excitation energy to theacceptor dye oxazine contained inside the zeolite.[126] Energytransfer along a specified direction has been obtained[127] anddye–zeolite crystals have been organized as oriented monolay-ers.[128]

Light harvesting has also been investigated in a variety ofother systems such as multichromophoric cyclodextrins,[129]

phthalocyanines,[130] metallosupramolecular squares,[131] rotax-anes,[132] and polyelectrolytes.[133]

A combination of self-organizing biological structures andsynthetic building blocks has been used as a flexible methodfor the construction of antenna systems. Recently,[134] buildingblocks were prepared by attaching fluorescent chromophoresto cysteine residues introduced on tobacco mosaic virus coatprotein monomers. When placed under the appropriate bufferconditions, these conjugates could be assembled into stacks ofdisks or into rods that reached hundreds of nanometers inlength. Efficient energy transfer from a large number of donorsto a single acceptor was observed in such systems.

3.5. Artificial Reaction Centers

3.5.1. Introduction

As we have seen in Section 2.3, photoinduced charge separa-tion taking place in a reaction center (Figure 4) is the key pro-cess that converts light energy into chemical energy in nature.In recent years, many attempts have been performed to con-struct artificial systems capable of mimicking the function ofthe natural reaction center.The minimum model for a charge-separation system is a

dyad, which consists of an electron-donor (or -acceptor) chro-mophore, an additional electron-acceptor (or -donor) moiety,and an organizational principle that controls their distance andelectronic interactions (and therefore the rates and yields ofelectron transfer). Dyads of this type have been constructedand investigated.[34,91a,c,d,135–140] The energy-level diagram for a

dyad is shown in Figure 14. All the dyad-type systems suffer toa greater or lesser extent from rapid charge recombination(process 4).

As we have also seen in Section 2.3, the problem of rapidcharge recombination has been overcome in nature with aseries of short-range, fast, and efficient electron-transfer pro-cesses that lead to a charge separation over a long distance.This model has inspired the construction of systems consistingof three or more components.Charge-separation in three-component systems (triads) is il-

lustrated in Figure 15a. The functioning principles are shownin the orbital-type energy diagrams in the right part of thefigure. Excitation of a chromophoric component (step 1) is fol-lowed by a primary photoinduced electron transfer to a pri-mary acceptor (step 2). This process is followed by a secondarythermal electron-transfer process (step 3), namely, electrontransfer from a donor component to the oxidized chromophor-ic component. The primary process competes with excited-state deactivation (step 4), whereas the secondary processcompetes with primary charge recombination (step 5). Finally,charge recombination between remote molecular components(step 6) leads the triad back to its initial state. The sequence ofprocesses indicated above (1-2-3) is not unique. Actually, thealternate sequence 1-3-2, would also lead to the same charge-separated state. The performance of a triad for energy conver-sion purposes is related to the quantum yield (F) of formationof the charge-separated state (depending on the competitionbetween forward and back processes, F = [k2/ ACHTUNGTRENNUNG(k2+k4)]-ACHTUNGTRENNUNG[k3/ ACHTUNGTRENNUNG(k3+k5)]), the lifetime (t) of charge separation (dependingon the rate of the final charge-recombination process, t=1/k6 ), and the efficiency of energy conversion (hen.conv.= FTF,where F is the fraction of the excited-state energy conservedin the final charge-separated state). To put things in a real per-spective, it should be recalled that the “triad portion” of the re-action center of bacterial photosynthesis discussed in Sec-tion 2.3.1 converts light energy with t~10 ms, F =1, andhen.conv.~0.6.Several mechanistic investigations on photoinduced charge

separation in artificial systems have been performed on cova-lently linked organic compounds. Recent reviews can be con-sulted for a detailed discussion of the various aspects of this

Figure 14. Schematic energy-level diagram for a dyad.


V. Balzani et al.

www.chemsuschem.org

topic (through-bond compared with through-space transfer, in-terference effects in through-bond coupling, orientation ef-fects, modulation of electron-transfer dynamics by electricfields, etc.).[141]

3.5.2. Triads

Three examples of triads are shown in Figure 15b. The 4-nm-long triad 43+ consists of an IrIII bis-terpyridine complex con-nected to a triphenylamine electron donor (D) and a naphtha-lene-bisimide electron acceptor (A).[142] Upon excitation of theelectron donor D (or even the Ir-based moiety), a charge-sepa-rated state D+-Ir�-A is formed with 100% yield in less than20 ps that successively leads to D+-Ir-A� with 10% efficiency in400 ps. Remarkably, the fully charge-separated state D+-Ir-A�

has a lifetime of 120 ms at room temperature in deaerated ace-tonitrile solution.Triad 5 (Figure 15b) consists of a porphyrin as light-absorb-

ing chromophore, naphthoquinone as acceptor, dimethylani-

line as donor, and triptycenebridges as connectors.[143]

Charge separation is very effi-cient (F=0.71) and long-lived(t=2.5 ms) at room temperaturein butyronitrile, and the charge-separated state has an energyof 1.39 eV. The charge-separa-tion function is completely sup-pressed by cooling the systemto low-temperature rigid solu-tions, behavior that is contraryto what happens for naturalphotosynthetic systems. Thereason is that blocking the reor-ientation of solvent dipolesstrongly destabilizes the charge-separated states relative to fluidsolution.[144] In particular, theforward secondary electron-transfer step 3 (which in fluidsolution is exergonic by 0.14 eV)becomes endergonic in rigidmatrix. As proof, in a triad con-taining a higher-energy chro-mophore and better electron-acceptor and -donor compo-nents (the driving force of sec-ondary electron-transfer step influid solution is 0.45 eV), photo-induced charge separation pro-ceeds even at 5 K in 2-methylte-trahydrofuran.[145] Interestingly,under these conditions thecharge-separated state can bedetected as a spin-polarizedtransient EPR signal, whichgives information about donor–

acceptor electronic coupling.In triad 6 (Figure 15b), that will later be compared to a

pentad of the same family, the quantum yield of charge sepa-ration is 4%, the efficiency of energy conversion is 2%, andthe lifetime of the charge-separated excited state is 300 ns indichloromethane solution.[135]

Triad 7 (Figure 16) consists of a porphyrin (P) bearing a full-erene C60 and a carotenoid (C) secondary electron donor. Exci-tation of the porphyrin unit of the triad causes in 2-methylte-trahydrofuran the events shown schematically in the energy-level diagram shown in Figure 16.[146,147] The C-1P-C60 excitedstate decays almost exclusively by electron transfer (step 2)with formation of C-P+-C60

� with k2=3.3T1011 s�1 and a quan-tum yield of unity. A small fraction of C-P-1C60 excited states isalso obtained (step 3), but these also decay to C-P+-C60

� byelectron transfer (step 4). Experiments performed on dyad P-C60 show that C-P+-C60

� can return to the ground state bycharge recombination with k7=2.1T109 s�1. Electron transferfrom the carotenoid to the porphyrin is, however, much faster

Figure 15. a) Schematic representation of charge separation in a triad. Other arrangements of the componentsare possible. b) Triads 43+ ,[142] 5,[143] and 6.[135]



www.chemsuschem.org

(k8=1.5T1010 s�1). The C+-P-C60� state is therefore produced

with an overall quantum yield of 0.88. It decays slowly bycharge recombination to yield the carotenoid triplet (k9=2.9T106 s�1). In conclusion, the two-step electron-transfer sequencein triad 7 has increased the lifetime of charge separation by afactor of nearly 103 relative to that of dyad P-C60. Triad 7 hasalso been found to have other properties that are present inthe reaction centers but not common in artificial biomimeticsystems.[148, 149] For example, the formation of C+-P-C60

� occurseven in a glass at 8 K. Charge recombination of C+-P-C60

�

yields 3C-P-C60 with a unique EPR-detectable spin-polarizationpattern and occurs by a radical-pair mechanism,[148] as ob-served in natural reaction centers. Transfer of triplet excitationenergy by a relay mechanism related to that found in some re-action centers is, furthermore, also observed for this triad. Incontrast with what happens in 2-methyltetrahydrofuran, in tol-uene solution the C-P-1C60 excited state of 7 does not undergoelectron transfer, decaying instead by intersystem crossing toyield C-P-3C60. This triplet excited state decays with a rate con-stant of 9T106 s�1 to give the carotenoid triplet, 3C-P-C60, viathe slightly endergonic formation of the C-3P-C60 excitedstate.[150] It can be recalled that in nature the quenching of thechlorophyll triplets by carotenoids is an important protectivemechanism against photodegradation caused by formation ofsinglet oxygen.

3.5.3. More Complex Systems

The introduction of further molecular components (tetrads,pentads, hexads) leads to the occurrence of further electron-

transfer steps, which, in suitablydesigned systems, producecharge separation over largerand larger distances. As thenumber of molecular compo-nents increases, also the mecha-nistic complexity increases andcharge separation may involveenergy-transfer steps.A tetrad comprising a ferro-

cene group, a Zn porphyrin, afree-base porphyrin, and a C60unit, Fc-ZnP-P-C60, has been re-ported.[151] In this system, excita-tion of the Zn or free-base por-phyrin causes electron transfer,leading to Fc-ZnP-P+-C60

� ; suc-cessive charge-shift processesproduce the final charge-sepa-rated state, Fc+-ZnP-P-C60

� ,with F =0.24 and hen.conv.=0.13(benzonitrile solution). Lifetimemeasurements reveal that, insolution, charge recombinationoccurs mainly through bimolec-ular processes, that is, the intra-molecular charge recombination

is too slow to compete with diffusion-limited intermolecularelectron transfer. The lifetime of charge separation has beenmeasured in frozen matrix by time-resolved EPR experiments;it was found to be remarkably long (380 ms in benzonitrile at193 K). In the related tetrad Fc-(ZnP)2-C60,

[152] the final charge-separated state, Fc+-(ZnP)2-C60

� , is obtained with F=0.80(hen.conv.~0.4) by excitation of the zinc-porphyrin dimer in ben-zonitrile solution at 295 K. In this case, charge recombination(t=19 ms) apparently occurs through a reversed stepwise pro-cess, that is, a rate-limiting electron transfer from (ZnP)2 to Fc

+

followed by a fast electron transfer from C60� to (ZnP)2

+ , whichregenerates the ground state.Pentad 8 (Figure 17) is formally obtained[135,153] by introduc-

ing in triad 6 (Figure 15b) a secondary acceptor and a secon-dary donor/chromophore. The various charge-separation path-ways of 8 are indicated in the state-energy diagram shown,[154]

which features also an energy-transfer step. The improvementin performance with increasing complexity can be seen bycomparing data for the triad and the pentad: for 6, t=300 ns,F=0.04, and hen.conv.=0.02 (dichloromethane); for 8, t=55 ms,F=0.83, and hen.conv.=0.5 (chloroform).Several multicomponent systems do exhibit both energy-

and electron-transfer photoinduced processes. However, theydo not properly couple antenna and reaction-center compo-nents as needed to mimic the natural photosynthetic appara-tus. Examples of systems in which light harvesting and chargeseparation can be clearly identified are illustrated in the nextsection.

Figure 16. Structure of triad 7, and the corresponding energy-level diagram for the charge-separation process-es.[146]


V. Balzani et al.

www.chemsuschem.org

3.6. Coupling Artificial Antennas and Reaction-CenterBuilding Blocks

3.6.1. Introduction

As we have seen in Section 2.3, in natural systems the solarenergy collected by the antenna devices at the end of theenergy-transfer chain is used to induce a charge-separation re-action, that is, to obtain redox energy. Coupling energy- andelectron-transfer processes is a very demanding task. In thissection, we examine attempts to couple these two functions inartificial systems. Several types of chromophores have beenused as antennas, whereas a porphyrin–fullerene moiety isoften used as a charge-separation device as it leads, with highefficiency, to long-lived charge-separated states.[147]

3.6.2. Systems Based on Metal Complexes

Because there are significant limitations in the preparation ofmolecular assemblies by sequential covalent-bond formation,an alternative strategy involves the derivatization of preformedpolymers, particularly with metal complexes.[123e]

A derivatized polystyrene assembly has been constructedwith appended [Ru ACHTUNGTRENNUNG(bpy)3]

2+-type units (Figure 18).[155] In someof the metal complexes (three out of 20), a bpy ligand bearsan electron donor (phenothiazine, PTZ) and an electron accept-

or (a derivative of 1,1’-dimethyl-4,4’-bipyridinium, methylviolo-gen, MV2+) to yield a structure that mimics the reaction center.Following excitation of the metal complexes by visible laserflash photolysis, the PTZ+-MV+ state was observed, with a sig-nificant (ca. 30%) contribution from excitation at antenna Rusites not adjacent to the functionalized Ru units. In the overallreaction, the 2.13 eV excited energy of the antenna excitedstates is transferred to the reaction-center model, where it isconverted into 1.15 eV of stored redox energy. The efficiencyof formation of the redox-separated state varies from 12 to18% depending on the laser intensity. At high intensities, mul-tiphoton excitation and excited-state annihilation competewith sensitized electron transfer. The lifetime of the charge-separated state is 160 ns. It should be noted, however, that asmall amount (0.5%) of a long-lived transient (about 20 ms)was also observed and attributed to polymers in which PTZ+

and MV+ were formed on spatially separated sites. This obser-vation is potentially important because it suggests that photo-chemically generated redox equivalents can be created andstored on the polymers for extended periods.[123e]

3.6.3. Systems Based on Organic Compounds and Porphyrins

Light-harvesting and charge-separation coupling has been ob-tained in modified windmill porphyrin arrays.[156] In the com-

Figure 17. Structure of pentad 8, and the corresponding energy-level diagram for the charge-separation processes.[154]



www.chemsuschem.org

pounds shown in Figure 19 which bear a naphthalenetetracar-boxylic diimide or a meso-nitrated free-base porphyrin electronacceptor, A, attached to the two core ZnPc units, energy trans-fer from the peripheral porphyrins, ZnPp, to the two ZnPc por-phyrins is followed by electron transfer to the A unit, and then

by hole transfer from the oxidizedZnPc

+ units to a peripheral Zn porphy-rin, as indicated in the following reac-tions [Eqs. (2)–(5)] (for the sake of clari-ty, only one ZnPp, ZnPc, and A unitsare indicated):

1ZnPp-ZnPc-A! ZnPp-1ZnPc-A ð2Þ

ZnPp-1ZnPc-A! ZnPp-ZnPc

þ-A� ð3Þ

ZnPp-ZnPcþ-A� ! ZnPp

þ-ZnPc-A� ð4Þ

ZnPpþ-ZnPc-A

� ! ZnPp-ZnPc-A ð5Þ

The charge-separation efficiency is,however, low because the hole-trans-fer reaction [Eq. (4)] is slower than thecharge-recombination reaction in theZnPp-ZnPc

+-A� species.An interesting attempt to couple light-harvesting antennas

and a charge-separation module is represented by compound9 (Figure 20), which consists of four covalently linked zinc tet-raarylporphyrins, (ZnPp)3-ZnPc (p stands for peripheral, c standsfor central), covalently joined to a free-base-porphyrin–fuller-ene dyad, P-C60, to form the (ZnPp)3-ZnPc-P-C60 hexad.

[157] Re-sults obtained from time-resolved emission and absorption in-vestigations in 2-methyltetrahydrofuran solution comparedwith those obtained for some model compounds have led tothe following picture: 1) excitation of any peripheral zinc por-phyrin is followed by singlet–singlet energy transfer to thecentral zinc porphyrin [Eq. (6)] with a rate constant k6=2.0T1010 s�1; 2) singlet–singlet energy transfer from the central zincporphyrin to the free-base porphyrin [Eq. (7)] occurs with arate constant k7=4.1T109 s�1; 3) electron transfer from the ex-cited free-base unit to the fullerene unit [Eq. (8)] is very rapid,k8=3T1011 s�1; and 4) the lifetime of the charge-separatedstate is 1.3 ns [Eq. (9), k9=7.5T108 s�1] .

1ðZnPpÞ3-ZnPc-P-C60 ! ðZnPpÞ3-1ZnPc-P-C60 ð6Þ

ðZnPpÞ3-1ZnPc-P-C60 ! ðZnPpÞ3-ZnPc-1P-C60 ð7Þ

ðZnPpÞ3-ZnPc-1P-C60 ! ðZnPpÞ3-ZnPc-Pþ-C60� ð8Þ

ðZnPpÞ3-ZnPc-Pþ-C60� ! ðZnPpÞ3-ZnPc-P-C60 ð9Þ

The quantum yield of the charge-separated state is unity onexcitation of the free-base porphyrin because of the very largerate constant for photoinduced electron transfer [Eq. (8)] ; onexcitation of the Zn-porphyrin units, however, the quantumyield drops to 0.70 because of the competition between intrin-sic decay and energy transfer to the central free-base porphy-rin. The non-unity quantum yield of charge separation and theshort lifetime of the charge-separated state left room for im-provement of the performance of the hexad by clever molecu-lar engineering of the free-base porphyrin unit. Replacementof the free-base diaryloctaalkylporphyrin in 9 with a meso-tet-raarylporphyrin gives compound 10 (Figure 20).[158] In 2-meth-

Figure 18. Sequence of energy- and electron-transfer processes in a polystyrene derivatized with [Ru-ACHTUNGTRENNUNG(bpy)3]

2+-type units.[155]

Figure 19. Antenna–reaction-center complexes based on a windmill porphy-rin array.[156]


V. Balzani et al.

www.chemsuschem.org

yltetrahydrofuran solution, this hexad leads to faster energytransfer from the central Zn porphyrin to the free-base porphy-rin as compared with 9, thereby increasing the overall yield ofcharge separation. Because the tetraarylporphyrin employed in10 has a higher oxidation potential than its octaalkylporphyrinanalogue in 9, migration of the positive charge from the free-base porphyrin to the Zn-porphyrin system [Eq. (10)] occurs,moreover, with a rate constant k10=2.6T109 s�1. The lifetimeof the final charge-separated state is increased to 240 ns[Eq. (11), k11=4.2T106 s�1] .

ðZnPpÞ3-ZnPc-Pþ-C60� ! ½ðZnPpÞ3-ZnPc�þ-P-C60� ð10Þ

½ðZnPpÞ3-ZnPc�þ-P-C60� ! ðZnPpÞ3-ZnPc-P-C60 ð11Þ

A further improvement has been obtained with heptads 11and 12 (Figure 21),[159, 160] in which the hexaphenylbenzenescaffold provides a rigid and versatile core for organizing an-tenna chromophores and coupling them efficiently with acharge-separation moiety. Such compounds contain five bis-ACHTUNGTRENNUNG(phenylethynyl)antracene (BPEA) antennas and a porphyrin–

fullerene electron-donor–accept-or module. The BPEA antennachromophore was chosen be-cause it absorbs strongly in the430–475 nm region, as do caro-tenoid polyenes in photosyn-thetic organisms. Energy transferfrom the five antennas to theporphyrin occurs on the picosec-ond timescale with a quantumyield of 1.0, comparable to thoseseen in some photosynthetic an-tenna systems. The Fçrstermechanism plays the major rolein energy transfer, but athrough-bond, electron-ex-change mechanism also contrib-utes. After light harvesting, thefirst singlet excited state of theporphyrin donates an electronto the attached fullerene to yielda P+-C60

� charge-separatedstate, which has a lifetime ofseveral nanoseconds. The quan-tum yield of charge separationbased on the light absorbed bythe antenna chromophores is80% for the free-base com-pound 11 and 96% for the zincanalogue 12. The rate constantsof the energy- and electron-transfer processes are indicatedin Figure 21.Biomimetic reaction centers

have also been constructed byself-assembly of a porphyrin

dimer with functionalized fullerenes.[161]

3.7. Coupling Single-Photon Charge Separation withMultielectron Redox Processes

3.7.1. Introduction

The main problem of artificial photosynthesis is perhaps thecoupling of photoinduced charge separation, which is a one-photon, one-electron process, with oxygen evolution, which isa four-electron process. As we have seen in Section 2.3.2, na-ture’s answer is the Mn4Ca cluster, a catalyst for multielectrontransfer that is capable of 1) releasing electrons in a stepwisemanner at constant potential, and 2) oxidizing water moleculesin a concerted way, so as to avoid the formation of high-energy intermediates. The design of specific multielectronredox catalysts is a fascinating and challenging problem ofmodern chemistry.[51,162–164]

From the standard redox potentials of the two correspond-ing half-reactions, the free energy demand for water splitting[Eq. (1), Section 3.1] is 1.23 eV. For many charge-separated

Figure 20. Schematic representation of the energy- and electron-transfer processes that occur in hexads 9[157] and10.[158]



www.chemsuschem.org

states discussed in previous sections, the difference in redoxpotentials of the oxidized and reduced molecular componentsis larger than this number. Thus, by the use of such systems,light-energy conversion by means of photoinduced chargeseparation followed by the reactions shown in Equations (12)and (13) is thermodynamically feasible.

Dþ-P-A� þ H2O! Dþ-P-Aþ 1=2H2 þ OH� ð12Þ

Dþ-P-Aþ 1=2H2O! D-P-A þ 1=4O2 þ Hþ ð13Þ

None of these systems, however, would evolve hydrogenand oxygen upon simple irradiation in aqueous solution. Thereis, in fact, a fundamental kinetic problem. The photoinducedcharge separation is a one-electron process (i.e. D+ and A� areone-electron oxidants and reductants). On the other hand, thereactions depicted in Equations (12) and (13), although writtenin one-electron terms for stoichiometric purposes, are inher-ently multielectron processes: two electrons for [Eq. (12)] andfour electrons for [Eq. (13)] . Thus, although relatively long-livedcharge separation can be achieved with supramolecular sys-tems, the reactions depicted in Equations (12) and (13) arehopelessly slow to compete with charge recombination. Thisproblem is common to any conceivable fuel-generating pro-cesses.

The answer to this general problem lies in thepossibility of accelerating multielectron redox reac-tions by the use of artificial catalysts.[51,164] A catalystfor multielectron redox processes is essentially a“charge pool”, that is, a species capable of 1) acquir-ing electrons (or holes) from a one-electron-reducing(or -oxidizing) species in a stepwise manner at con-stant potential, and 2) delivering these electrons (orholes) to the substrate in a “concerted” manner toavoid the formation of high-energy intermediates.From the field of heterogeneous catalysis, metals

and metal oxides are known to be good candidatesfor this type of processes. Photochemical “watersplitting” cycles based on bimolecular reactions be-tween molecular photosensitizer, acceptor, anddonor species were actively studied some timeago.[59–62] The main result of such studies (which didnot lead to any practical success with regard towater splitting) was the optimization of several het-erogeneous catalysts.[165] For example, colloidal plati-num was found to be a superior catalyst for photo-chemical hydrogen evolution and colloidal RuO2 wasidentified as a moderately efficient catalyst for pho-tochemical oxygen formation.Apart from solid-state materials, discrete supra-

molecular species can also be conceived as catalystsfor multielectron redox reactions. This is what hap-pens, as we have seen in Section 2.3.2, in green-plant photosynthesis whereby an enzyme (still notwell characterized, but containing four manganesecenters) catalyzes the oxidation of water. A supra-

molecular catalyst for multielectron redox processes must con-tain several equivalent redox centers (at least as many as theelectrons to be exchanged), with the appropriate redox prop-erties to mediate between the charge-separated state and thesubstrate. The electronic coupling between such centersshould be not too strong, otherwise the “charging” process(stepwise one-electron transfer to the catalyst) could not occurat a reasonably constant potential. The centers should, on theother hand, be sufficiently close to be able to cooperate inbinding and reducing (or oxidizing) the substrate. Apparently,electron transfer alone cannot satisfy these requirements.The lesson to be learnt from nature (Section 2.3.2) is that

multiple electron-transfer processes can be profitably accom-plished when accompanied by proton transfer. This can occurby a sequence of two distinct reactions or by a concerted pro-cess (proton-coupled electron transfer, PCET).[49] PCET process-es are advantageous from the thermodynamic viewpoint, butthey are inevitably more complex than either electron orproton transfer because both electrons and protons must betransferred simultaneously. For example, electron–proton trans-fer between the relatively simple cis-[RuIVACHTUNGTRENNUNG(bpy)2(py)(O)]

2+ andcis-[RuIIACHTUNGTRENNUNG(bpy)2(py) ACHTUNGTRENNUNG(H2O)]

2+ complexes involves electron transferfrom a dp ACHTUNGTRENNUNG(RuII) orbital to a dp ACHTUNGTRENNUNG(RuIV) orbital and proton transferfrom sO�H to a lone pair on the oxo group (Figure 22).[51,166]

Therefore, stepwise and concerted electron-transfer processesmay compete, depending on the particular conditions

Figure 21. Energy- and electron-transfer processes occurring in heptads 11 and 12[160]

(subscripts: e=energy transfer, o=ortho, m=meta, p=para, cs=charge separation,cr=charge recombination).


V. Balzani et al.

www.chemsuschem.org

(pH, driving force, reorganizational barrier, dielectric con-stant).[167]

To obtain photoinduced evolution of hydrogen and oxygen,catalysts must be coupled with a charge-separation unit. Muchhas to be learnt in this field, but some interesting studies havestarted to appear.

3.7.2. Two-Electron Mixed-Valence Systems

An interesting approach is that pursued by expanding the re-activity of metal complexes in electronically excited statesbeyond the conventional one-electron transfer. The types oftwo-electron mixed valency that have been investigated areshown in Figure 23:[164] a) bimetallic complexes that rely on

ligand sets favoring a ground state Mn-Mn+2 species, which isstabilized relative to its Mn+1-Mn+1 congener; b) porphyrino-gens that store two-electron equivalency in the framework ofa macrocyclic ligand; and c) MIII-O-MIII macrocycles tethered toa rigid spacer that upon excitation produce a two-electronmixed-valence intermediate, which is a reactive oxidant.Rh complexes offer interesting examples of binuclear mixed-

valence systems (Figure 23a). Hydrogen has been producedfrom hydrohalic acid (HX) solutions upon irradiation of a dirho-dium complex LRh0-Rh0L with generation of a two-electronmixed-valence LRh0-RhIIX2 species. In the presence of a halogentrap, LRh0-RhIIX2 can be converted back by light irradiation intothe coordinatively unsaturated species LRh0-Rh0L, thereby gen-erating a photocycle for H2 production without any heteroge-neous electron mediator.[168] The detailed kinetic aspects andthe intermediate compounds involved in the photocycle havebeen investigated.[169]

Iron porphyrinogens (Figure 23b) have been prepared thatdisplay one-electron metal-based and four-electron ligand-based chemistry.[170] A series of cofacial “Pacman” bisporphyrins(Figure 23c) bridged by xanthene and diphenyl-furan havealso been prepared. For the case of di-iron ACHTUNGTRENNUNG(III) m-oxo porphyr-ins, light excitation breaks the FeIII-O-FeIII bond to produce a(PFeII) ACHTUNGTRENNUNG(PFeIV=O) cofacial intermediate, which oxidizes simple

electron-rich substrates.[171] Re-action of two ferrous porphyrinsubunits with O2 reforms the di-iron ACHTUNGTRENNUNG(III) m-oxo complex.[172]

3.7.3. Systems Based on RuACHTUNGTRENNUNGOligopyridine Complexes

RuII oligopyridine complexes, in particular [Ru ACHTUNGTRENNUNG(bpy)3]2+ , were

again chosen[173–179] because of their well-known photochemi-cal, photophysical, and electrochemical properties.[180] A varietyof mono- and multinuclear MnII complexes have been cova-lently linked to a Ru complex. Furthermore, inspired by thepresence of tyrosine as a mediator in the photooxidation ofthe Mn cluster in the natural process (Section 2.3.2), tyrosineitself or another type of phenolate moiety (hereafter indicatedby ArOH) were introduced in the model systems designed.Some of the investigated systems are shown in Figure 24

and Figure 25. It should be noted that the oxidizing species isnever the excited state of [Ru ACHTUNGTRENNUNG(bpy)3]

2+ , a relatively weak oxi-dant (*[Ru ACHTUNGTRENNUNG(bpy)3]

2+/[Ru ACHTUNGTRENNUNG(bpy)3]+ =+0.84 V relative to NHE), but

is the [Ru ACHTUNGTRENNUNG(bpy)3]3+ unit ([Ru ACHTUNGTRENNUNG(bpy)3]

3+/[Ru ACHTUNGTRENNUNG(bpy)3]2+ =+1.26 V rel-

ative to NHE) generated by bimolecular reaction of the excited[Ru ACHTUNGTRENNUNG(bpy)3]

2+ moiety with a sacrificial electron acceptor, methyl-viologen (MV2+) or [CoACHTUNGTRENNUNG(NH3)5Cl]

2+ . In the example illustrated inFigure 24a,[173,175] the intermolecular photoinduced electron-transfer process (step 2) is followed by electron transfer fromthe tyrosine moiety to the [Ru ACHTUNGTRENNUNG(bpy)3]

3+ unit with generation ofa tyrosyl radical (as revealed by EPR experiments), which isable to oxidize the dinuclear Mn complex. In fact, competingenergy- and electron-transfer processes also occur. In thesystem shown in Figure 24b,[175] a dinuclear Mn complex islinked to the RuII complex. Flash photolysis experiments re-vealed that the excited state of the RuII complex is quenchedby intermolecular electron transfer to MV2+ or [CoACHTUNGTRENNUNG(NH3)5Cl]

2+

(step 2) and the RuIII complex obtained is reduced by rapid(k>1T107 s�1) intramolecular electron transfer from the Mncomplex that is oxidized to the MnII-MnIII state. Figure 25ashows a complex consisting of three [Ru ACHTUNGTRENNUNG(bpy)3]

2+-type units at-tached to a MnIV complex in which Mn is coordinated to elec-tron-rich phenols.[178] Photoexcitation in the presence of MV2+

or [Co ACHTUNGTRENNUNG(NH3)5Cl]2+ leads to the formation of a [Ru ACHTUNGTRENNUNG(bpy)3]

3+-typeunit (step 2). Intramolecular electron transfer (k5T107 s�1)from the phenolate ligands to the oxidized Ru complex thenoccurs, with formation of a (complexed) phenoxyl radical. Inthe system shown in Figure 25b, however, in which [Ru-ACHTUNGTRENNUNG(bpy)3]

2+-type units are linked to a trinuclear MnII complex, thesource of the electron that reduces the photochemically gener-ated [Ru ACHTUNGTRENNUNG(bpy)3]

3+-type species is not a phenolate but a MnII

ion.[178]

3.7.4. Other Systems

Other studies have been performed in an attempt to achievephotoinduced water splitting. Some binuclear Ru-Ir,[181] Ru-Rh,[182] and Ru-Pt[183] species undergo a photochemical two-electron reduction process, and a binuclear complex based on

Figure 22. Concerted electron–proton transfer between cis-[RuIVACHTUNGTRENNUNG(bpy)2(py)(O)]2+ and cis-[RuII ACHTUNGTRENNUNG(bpy)2(py) ACHTUNGTRENNUNG(H2O)]

2+

(py=pyridine).[51]

Figure 23. Types of two-electron photocatalysts (see text for details).[164]



www.chemsuschem.org

the [Ru ACHTUNGTRENNUNG(phen)2]2+ (phen=1,10-phenanthroline) moiety[184] can

be reversibly photoreduced on the bridging ligand by fourelectrons. Several dendrimers are capable of storing severalredox equivalents[185] and, in some cases, the reduction processcan be carried out by photoexcitation.[186]

Artificial photosynthetic systems could be realized by con-necting catalysts for water reduction and oxidation to a photo-voltaic cell (Section 5.2).[13,187] In such constructs, the spatiallyseparated electron–hole pairs provided by the photovoltaicjunction are captured by the catalysts, and the energy isstored in the bond rearrangement of water to H2 and O2. Sys-tems in which light is converted into chemical energy throughelectricity as a discrete intermediate are, however, affected byconstraints associated with electrical contacts. To overcomethese problems, a more intimate integration of the charge-sep-aration and chemical bond-forming functions is required.Systems based on the semiconductor–liquid interface have

been studied[68,188–190] starting from pioneering work on single-crystal TiO2 electrodes.

[191] The most difficult problems are sta-bility of the semiconductors and their band gap, which shouldbe small enough to absorb visible light. Alternatively, the pro-cess can be sensitized by dyes.[192] Recently, aqueous suspen-sions of oxides or (oxy)nitrides semiconductors capable of ab-sorbing visible light,[193] sometimes impregnated with metaloxide nanoparticles,[194] have been used.

3.8. Assembly Strategies

3.8.1. Introduction

Any efficient artificial photosynthetic system must satisfy an-other requirement—the oxidized and reduced products should

be produced in physically sepa-rated compartments of thesystem to avoid uncontrolledenergy-wasting back reactionsand to facilitate collection andstorage of the fuel. As occurs innatural systems, therefore, somekind of membrane is needed toseparate the oxidative and re-ductive parts of the process(Figure 8). This requirement, inturn, needs that every charge-separating molecular device isspecifically organized and ori-ented with respect to such amembrane. This problemshould be addressed by re-search on self-assembling pro-cesses and organized media. Aswe will see in Section 4, artificialtriads have already been suc-cessfully inserted into bilipidmembranes.

3.8.2. Self-Assembly

Once organization at the supramolecular level has been ach-ieved by covalent synthesis of appropriate building blocks, thesupramolecular entities should self-assemble (or be assembled)into structures that can bridge length scales from nanometersto macroscopic dimensions. When the building blocks used toobtain light harvesting and/or charge separation have particu-lar shapes, sizes, and capacity to give hydrogen bonds or p-pinteractions, they can self-assemble.[141b,195,196] For example,compound 13 (Figure 26), which is made up of four perylene-diimide peripheral units (PDIp) attached to a perylenediimidecore (PDIc), self assembles into stacked dimers (13)2 in solu-ACHTUNGTRENNUNGtion.[195a] Femtosecond transient absorption spectroscopyshowed that energy transfer from (PDIp)2 to (PDIc)2 occurs witht=21 ps, followed by excited-state symmetry breaking of 1*-ACHTUNGTRENNUNG(PDIc)2 to produce PDIc

+-PDIc� quantitatively with charge-sepa-

ration time of 7 ps. The ion-pair recombines with t=420 ps.Electron transfer occurs only in the dimeric system and doesnot occur in the disassembled monomer, thus mimicking bothantenna and special pair functions in natural photosynthesis.A photoactive layer consisting of electron-donating zinc-por-

phyrin and electron-accepting fullerene arrays was constructedby using dendrimers appended with multiple porphyrin units(DPm, m=6, 12, 24) capable of hosting bis-pyridine compoundscarrying multiple fullerene units (BFn, n=1–3).[197] Compoundsof the type DPm�BFn were obtained in which photoexcitationof the zinc-porphyrin units results in electron transfer to thefullerene. The charge-separation rate constant (109-1010 s�1) in-creases with increasing m and n values, whereas the charge-re-combination process is much slower (about 5T106 s�1) in allcases. Langmuir–Blodgett films containing zinc-porphyrin–full-erene dyads have also been constructed.[198]

Figure 24. Model systems for the PSII center: a) intermolecular and b) intramolecular photochemical oxidation ofbinuclear MnII complexes.[173, 175, 176]


V. Balzani et al.

www.chemsuschem.org

Mixed self-assembled siloxane monolayers containing cou-marin-2 and coumarin-343 have been constructed on a siliconwafer.[199] Single coumarin-2 molecules and dendron-type struc-tures containing two and four coumarin-2 donor units wereused. The energy-transfer efficiency from excited coumarin-2to coumarin-343 was found to depend, as expected, on thecomposition of the mixture and on the branched nature of theenergy donor.Mixed self-assembled monolayers (SAMs) have been pre-

pared on gold surfaces to study light harvesting and photo-currrent generation.[200] Pyrene or boron-dipyrrin were used aslight-harvesting units and porphyrins as acceptors. Energy-transfer efficiency was 100% for a donor/acceptor ratio of 7:3.In SAMs containing the boron-dipyrrin energy donor and a fer-rocene-porphyrin-fullerene (Fc-P-C60) triad, energy transfer

from boron-dipyrrin to porphy-rin is followed by electron trans-fer from porphyrin to fullereneand then by electron transferfrom ferrocene to the oxidizedporphyrin, with formation ofthe Fc+-P-C60

� charge-separatedstate of the triad. In the pres-ence of an electron carrier suchas methylviologen, MV2+ , thereduced fullerene is reoxidizedin a bimolecular process andthe ferrocenium ion receives anelectron from the gold elec-trode, resulting in current gen-eration with an incidentphoton-to-current efficiency of1–2%.[201]

Rigid p-octiphenyl rods wereused to create tetramericp stacks of blue–red-fluorescentnaphthaline-diimides that canspan lipid bilayer mem-branes.[202] In lipid vescicles con-taining quinone as electron ac-ceptor that is surrounded byethylenediaminetetracetic acidas hole acceptor, the p-stackedarrays accept an electron fromthe outer sacrificial electrondonor and transport the elec-tron to the quinone containedinside the vesicle which is thusreduced to hydroquinone. Byadding an electron-rich dialkox-ybenzene derivative to thesystem, the naphthalene-dii-mide array transforms in ahollow photoinactive coassem-bly. Under such conditions, theelectron-transport function be-comes disabled and the scaffold

is transformed into an ion channel.

3.8.3. Bilayer Membranes

A bilayer membrane made of two amphiphiles has been con-structed.[203] One amphiphile contains an N-ethylcarbazolyllight-absorbing group, A, and the other has an energy-accept-or anthryl group, B, appended to an electron-accepting violo-gen group, C. Light excitation of the absorbing species A is fol-lowed by energy migration among the A groups until energyis irreversibly transferred to the B group with 70% efficiency.The excited B group then transfers an electron to the append-ed viologen unit with 95% efficiency.

Figure 25. Model systems for the PSII center. Intramolecular photochemical oxidation of a) a phenolate ligand andb) a trinuclear MnII complex.[178]



www.chemsuschem.org

4. Hybrid Systems

4.1. Semibiological Photosynthetic Reaction ACHTUNGTRENNUNGCenters

The artificial photosynthetic reaction center 146+ (Figure 27),in which a crown ether containing a [Ru ACHTUNGTRENNUNG(bpy)3]

2+-type unit ismechanically linked in a catenane fashion to a cyclobis(para-quat-p-phenylene) (CBPQT4+) moiety, has been constructed ona protein surface by cofactor reconstitution.[204,205] Recostitutionof apo-myoglobin (Mb) with 146+ afforded the Mb-based artifi-cial triad Mb ACHTUNGTRENNUNG(FeIIIOH2)-[Ru ACHTUNGTRENNUNG(bpy)3]

2+-CBPQT4+ , in which excita-tion of the [Ru ACHTUNGTRENNUNG(bpy)3]

2+ moiety in aqueous solution causesphotoinduced electron transfer from the Ru complex to theCBPQT4+ electron-acceptor unit. This process is followed by aproton-coupled electron transfer that leads, with a quantumyield of 0.005, to the final charge-separated state containing aporphyrin cation radical and a reduced viologen radical, Mb-ACHTUNGTRENNUNG(FeIV=O)-[Ru ACHTUNGTRENNUNG(bpy)3]

2+-CBPQT3+ . This species lies 1 eV above theground state and has a lifetime (>2 ms) comparable to that ofthe charge-separated state in the natural photosynthetic pro-cess. In the case of the analogous Mb(Zn)-[Ru ACHTUNGTRENNUNG(bpy)3]

2+-CBPQT4+ species, the charge-separated state Mb ACHTUNGTRENNUNG(Zn+)-[Ru-ACHTUNGTRENNUNG(bpy)3]

2+-CBPQT3+ is obtained with the same excess of energy,a much higher quantum yield (0.08), and moreover a much

faster (102-103-fold) recombination rate. The longerlifetime of the 146+ derivative is attributed to the in-volvement of protons in the electron-transfer pro-cess.

4.2. Conversion of Light to Proton-Motive Force

Although the conversion of light into chemicalenergy by means of artificial systems seems to be asomewhat distant goal, a hybrid natural–artificialsystem capable of using light to obtain proton-motive force and then ATP synthesis has been con-structed.[91a,206,207] These results have been achievedby coupling the photoinduced electron-transfer ca-pability of artificial triads with the movement of pro-tons across a membrane.As illustrated in Figure 28,[206] the molecular triad 6

(Figure 15b), consisting of a light-absorbing tetraar-ylporphyrin (P) covalently linked to a quinone ac-

ceptor (Q) and a carotenoid donor (C), was incorporated intothe bilayer of a liposome. When excited in various solvents,this triad undergoes photoinduced electron transfer from thesinglet excited state of the porphyrin moiety to yield the inter-

mediate charge-separated stateC-P+-Q� with quantum yield ofapproximately one. Subsequentelectron transfer from the carote-noid to the porphyrin cationcompetes with charge recombi-nation to give the C+-P-Q�

charge-separated state with aquantum yield of 0.15. Lipo-somes were prepared from aliquid mixture that contained thelipid-soluble 2,5-diphenylbenzo-

Figure 26. Structural formula of compound 13. See text for details.[195a]

Figure 27. Structural formula of compound 146+ .[204]

Figure 28. Schematic representation of the liposome-based proton pumppowered by a photoinduced charge-separation process (PS=pyraninetrisul-fonate).[206]


V. Balzani et al.

www.chemsuschem.org

quinone, Qs, and pyraninetrisulfonate (PS), a water-soluble dyewhose fluorescence indicates the pH of the solution. Vectorialelectron and proton transport requires asymmetric insertion ofthe triad into the liposomal bilayer membrane. This result wasachieved because the negatively charged carboxylate groupclose to the quinone resides near the outside of the liposomesurface, whereas the lipophilic carotenoid extends into the oilyportion of the bilayer. It was found that excitation of the por-phyrin moiety of 6 in liposomes with 5-ns light pulses resultsin the formation of C+-P-Q� , as detected by the characteristictransient absorbance of the carotenoid cation at 930 nm. Theyield of this species is around 0.1; its lifetime, which is about110 ns in the absence of Qs, is reduced to approximately 60 nswhen Qs is present in the liposomes. Under the latter condi-tions, the PS dye indicates that light excitation drives protonsinto the interior aqueous phase of the liposomes, as expectedon the basis of the shuttling mechanism involving Qs shownschematically in Figure 28. Step 1 includes excitation and two-step charge separation. In step 2, Qs, near the external aqueousphase, accepts an electron from the C+-P-Q� species, as ex-pected on thermodynamic grounds. In step 3, Qs

� accepts aproton from the aqueous phase, as required by the pKa valueof Qs. The semiquinone so formed diffuses across the bilayer(step 4) and it is oxidized by the carotenoid cation (step 5). Theprotonated quinone then releases a proton (step 6) with thedriving force related to its pKa, and Qs diffuses back to the ex-terior region of the bilayer (step 7). A pH gradient is thus creat-ed between the inside and outside of the liposome. The quan-tum yield of the proton transport in the first minute of irradia-tion was found to be around 0.004. The efficiency of thesystem can be, however, increased if an ionophore, such as va-linomycin, is added to relax the membrane potential.In this system, photon energy is transduced into vectorial in-

tramembrane redox potential and then into proton-motiveforce, that is, the biological analogue of electromotive force,by a chemically cyclic mechanism. It does not require sacrificialelectron acceptors or donors and, as happens in natural sys-tems, the redox potential remains confined to the membrane.

4.3. Light-Driven Production of ATP

In principle, proton-motive force generated by the light-drivenprocess described above can be used to perform work. Thisresult has been achieved[207] by the system illustrated inFigure 29. F0F1-ATP synthase has been incorporated, with theATP-synthesizing portion extending out into the external aque-ous solution, into liposomes containing the components of theproton-pumping photocycle. F0F1-ATP synthase is a molecular-scale rotary motor moved by a proton gradient and capable ofsynthesizing ATP from adenosine diphosphate (ADP) and inor-ganic phosphate (Pi).

[42,43] Irradiation of the membrane(Figure 29) with visible light leads to the charge-separationprocess that causes the above-described proton translocation,with generation of a proton-motive force. On accumulation ofsufficient proton-motive force, protons flow through the F0F1-ATP synthase, with the formation of ATP from ADP and Pi. Thefunctioning of the system was monitored by means of the luci-

ferin–luciferase fluorescence assay. The results show that thesynthesis of ATP occurs against an ATP chemical potential ofapproximately 12 kcalmol�1 and with a quantum yield of morethan 7%. One molecule of ATP is synthesized per 14 absorbedphotons of 633 nm light, an observation which means that upto 4% of the initial energy incident on the sample is stored bythe system. The photocyclic system operates efficiently over atimescale of hours with a turnover number of seven ATP mole-cules per F0F1 per second. This number is similar to that ob-served in bacteriorhodopsin/ATP synthase constructs.[208]

This is the first complete biomimetic system which effective-ly couples electrical potential, derived from photoinduced elec-tron transfer, to the chemical potential associated with theADP-ATP conversion, thereby mimicking the entire process ofbacterial photosynthesis. It constitutes a synthetic biologicalmotor that, in principle, can be used to power anything whichrequires a proton gradient or ATP to work, for example, topump calcium ions across a lipid bilayer membrane[209] or evennanomachines.

5. Conversion of Light into Electricity

5.1. Introduction

Solar power can be converted directly into electrical power byphotovoltaic (PV) cells and photoelectrochemicalcells.[13,14,68, 187,189,190] Solar electricity can be profitably exploitedin developing as well as developed countries.[1] By 2005, morethan 2 million households in developing countries receivedelectricity from solar home systems. One gets an idea of hownecessary development in this field is and how huge thismarket is from the estimation that 350 million householdsworldwide do not have access to central power networks. Thespreading of decentralized electricity-generation systems couldeliminate the need to build up an extensive and costly trans-mission grid, in the same way as mobile telecommunicationshas allowed the leapfrogging of cabled telephone lines insome developing regions of the world. In wise developedcountries, grid-connected PV systems grow so much that pro-duction does not satisfy demand. The potential of solar energy

Figure 29. Schematic representation of the process leading to light-drivenproduction of ATP.[207]



www.chemsuschem.org

generation in the European Union member states has been re-cently evaluated in detail.[210]

Here, we will not discuss the solid-state PV cells based on in-organic semiconductors, present on the market,[13,14, 187,211] andthose based on organic semiconductors[212,213] or organic–inor-ganic hybrids,[68,214] which are the object of extensive investiga-tions. We will only mention a few issues related to the im-provement of PV cells and describe briefly the photoelectro-chemical cells which exploit scientific principles quite similar tothose used in the above-described photosynthetic processes.

5.1. Photovoltaic Cells

Photovoltaic cells capture photons by exciting electrons acrossthe band gap of a semiconductor. This process creates elec-tron–hole pairs that are subsequently separated, typically byp–n junctions introduced by doping.[211,215] In the n-type re-gions of the device, conduction-band electrons can flow easilyto and from cell contacts, whereas valence holes cannot; thep-type regions have the opposite properties. Such an asymme-try causes a flow of photogenerated electrons and holes in op-posite directions, which generates a potential difference at theexternal electrodes.First-generation PV cells, which make up 85% of the current

commercial market,[14] are based on expensive (poly)crystallinesilicon wafers. Shipped PV modules have efficiencies of 15 to20% and a lifetime on the order of 30 years.[187] However, forsolar electricity to be cost-competitive with fossil-based elec-tricity at utility scale, manufacturing costs must be substantial-ly reduced. This goal has been partly reached with second-generation cells, which are based on thin films of less expen-sive materials such as amorphous or nanocrystalline Si, CdTe,or CuInSe2. However, research is needed to improve the effi-ciency of these cells in order to render them economicallycompetitive.A key issue in the manufacturing of PV cells is the trade-off

between material purity and device performance.[187] A mini-mum thickness of the cell is set by the thickness of the materi-al required to absorb most of the incident sunlight. However,the thickness of the material imposes a constraint on the re-quired purity, because the photoexcited charge carriers mustlive sufficiently long within the absorbing material to arrive atthe electrical junction, where they can be separated to pro-duce an electrical current. Impure absorber materials are char-acterized by short charge-carrier lifetimes and are thereforeunable to convert effectively the absorbed sunlight energyinto electricity. On the other hand, materials with the necessarypurity are expensive to produce and manufacture. This cost–ef-ficiency compromise could be circumvented in systems inwhich the collection of the charge carriers takes place along adirection orthogonal to the direction of light absorption. High-aspect-ratio nanorods, for example, can provide a long dimen-sion for light absorption, while charge carriers move radiallyalong the short axle of the nanorod to be separated by thejunction and collected as electricity (Figure 30).[187,216]

5.3. Photoelectrochemical Cells

Photoelectrochemical cells, often called GrWtzel cells after thescientist who has developed them,[217] are based on the sensiti-zation of wide-gap semiconductors by dyes capable of exploit-ing sunlight (i.e. visible light). Although the basic principles ofdye sensitization of semiconductors have long been estab-lished,[215] the application of such techniques to light-energyconversion became appealing only when new nanocrystallinesemiconductor electrodes of very high surface area were devel-oped.[218–221]

The working principle of a dye-sensitized solar cell is shownin Figure 31a.[222] The system comprises a photosensitizer (P)linked in some way (usually, by -COOH, -PO3H2, or -B(OH)2 func-tional groups) to the semiconductor surface, a solution con-taining a redox mediator (R), and a metallic counter electrode.The sensitizer is first excited by light absorption. The excitedsensitizer then injects, on the femto- to picosecond time-scale,[223] an electron into the conduction band of the semicon-ductor (step 1 in Figure 31a). The oxidized sensitizer is reducedby a relay molecule (step 2), which then diffuses to dischargeat the counter electrode (step 3) which is a conductive glass.As a result, a photopotential is generated between the twoelectrodes under open-circuit conditions, and a correspondingphotocurrent can be obtained on closing the external circuitby use of an appropriate load. A great number of photosensi-tizers of the Ru-oligopyridine family, which display metal-to-ligand charge-transfer excited states, have been employed.The most efficient ones are those bearing two NCS� and twosubstituted bpy ligands (see, for example, compound 15 inFigure 31[224]) which show intense absorption bands in the visi-ble region. A variety of solvents of different viscosity and ofredox mediators have been used, the most common being theI3�/I� couple in acetonitrile solution. A global efficiency up to11% has been reported.[190] Zn porphyrins have also proved tobe promising sensitizers.[225] Recently, corroles[226] and cyclome-talated IrIII dyes displaying ligand-to-ligand charge-transfer ex-cited states have been used.[227]

Figure 30. Scheme of a PV cell based on nanorod arrays, illustrating an ap-proach to orthogonalization of the directions of light absorption (along thelength of the rods) and charge-carrier collection (radially outward to the sur-face of the rods).[187, 216]


V. Balzani et al.

www.chemsuschem.org

Photoelectrochemical cells might look at first sight quite dif-ferent from the photosynthetic systems discussed in Section 2.On closer inspection, however, analogies are apparent. Thesystem is clearly based on photoinduced charge separation.From this viewpoint, it can be regarded as a heterogeneous“pseudo-triad”, in which the semiconductor surface acts as theprimary acceptor and the relay as the secondary donor. As inany triad, the efficiency of charge separation and energy con-version depends critically on the kinetic competition betweenthe various forward processes and charge-recombinationsteps. The main difference from photosynthetic systems issimply that the redox potential energy of the charge-separatedstate is not stored in products of subsequent reactions, butrather it is directly used to produce a photocurrent.[228] Hybridphotoelectrochemical biofuel cells have also been construct-ed.[229]

Taking this comparative analysis a step further, one mightconsider applying some of the strategies of photosynthesis toincrease the efficiency of photoelectrochemical cells.[230–232] Inthis regard, note that several possible processes can provide“short-circuit” paths within the photoelectrochemical cell. Themost important of such dissipative processes is the charge re-combination between the hole in the oxidized sensitizer andthe injected electron (step 4, Figure 31a). This process isalways thermodynamically allowed and can be avoided only ifit is disfavored by kinetic reasons compared with the other“useful” processes. To prevent the detrimental charge-recombi-nation step, a simple heterotriad system, as shown schemati-cally in Figure 31b, could be used to produce the hole at spa-tially remote sites. For this idea to be implemented, of course,several nontrivial problems must be solved.[233] Experimentshave been performed on TiO2 electrodes with dyads consistingof a [Ru ACHTUNGTRENNUNG(bpy)3]

2+-type complex linked to phenothiazine,[234] RuII

and OsII oligopyridine compounds,[235] a RuII-RhIII dyad,[236] andRu complexes with one (compound 16 in Figure 31)[237] ortwo[238] triphenylamine moieties appended.The antenna effect could also find useful application in

these systems. With a conventional semiconductor electrodeand a simple molecular sensitizer, light absorption is oftenquite inefficient at monolayer coverage. Multilayer adsorption,on the other hand, does not help because the inner layerstend to act as insulators relative to the outer layers.[239] Al-though this type of limitation is now much less severe becauseof the introduction of nanostructured electrodes of exception-ally high surface area,[219] the search for sensitizers with high in-trinsic light-harvesting efficiency is still of considerable interestin the field. One possibility in this direction is to replace thesensitizer molecule at the semiconductor–solution interfacewith an antenna sensitizer molecular device (Figure 31c).[240] Afurther advantage offered by antenna devices is that appropri-ate selection of the spectral properties of the various chromo-phoric groups can lead to better matching between the ab-sorption spectrum and the solar emission spectrum.The trinuclear complex 172+ (Figure 31) has been devel-

oped[240] in a first attempt to demonstrate the applicability ofthe antenna effect in semiconductor sensitization. The pres-ence of the carboxylate groups, besides being relevant to theenergetics of the system, is essential for grafting the complexthrough its central component to a TiO2 surface. Experimentsperformed using TiO2-coated electrodes (aqueous solution,pH 3.5, NaI as electron donor) showed that the photocurrentspectrum reproduces closely the absorption spectrum of thecomplex. This indicates that, as a consequence of efficientenergy transfer to the central unit bound to the semiconduc-tor, all the light energy absorbed by the trinuclear complex, in-cluding that absorbed by the peripheral units, is used for elec-

Figure 31. a) Working principle of a photosensitized (n-type) semiconductor cell ; P denotes a sensitizer linked to the semiconductor electrode, and R repre-sents an electron relay molecule.[222] b) Photosensitization of a semiconductor by a dyad.[233] c) Photosensitization of a semiconductor by an antennasystem.[240] Compounds 15,[224] 16,[237] and 172+ [240] are examples of a photosensitizer, a dyad, and an antenna system, respectively.



www.chemsuschem.org

tron injection. High conversion efficiency (~7%) of incidentlight to electricity has been obtained by use of this trinuclearcomplex on high-surface-area nanocrystalline TiO2 films.[217]

Substantial efficiencies have also been achieved with relatedcompounds.[241] One-dimensional systems that could stack per-pendicularly to the surface (with the different antenna unitsworking in series rather than in parallel) would be an evenmore interesting means of increasing the ratio of chromophor-ic components to the occupied surface area. Supramoleculararrays of porphyrin with fullerene have also been used.[242]

6. Conversion of Light into Mechanical Workby Molecular Machines

6.1. Introduction

In the past decade, there has been an extraordinary develop-ment of studies aimed at using light to cause mechanical mo-tions at the molecular level.[243–248] Although these processeswill hardly be exploited to convert sunlight into mechanicalwork, it is worthwhile to illustrate a few of the most recentachievements.[249]

6.2. Molecular Rotary Motors Based on -C=C- Photoisomeri-zation

In suitably designed alkene-type compounds containing ste-reogenic centers, the relative direction of the movement lead-ing to geometrical photoisomerization can be controlled.[250,251]

Each of the two helical subunits of compound 18 (Figure 32)can adopt a right-handed (P) or a left-handed (M) helicity. As aresult, a total of four stereoisomers are possible for this com-pound. The cis–trans isomerizations are reversible and occuron irradiation at appropriate wavelengths. In contrast, the in-versions of helicities, while maintaining a cis or a trans configu-ration, occur irreversibly under the influence of thermal energybecause of the strain associated with the equatorial methylsusbstituents. Thus, a sequence of light- and temperature-in-duced isomerizations can be exploited to move this molecularrotor in one direction only. Like natural motor proteins,[252] asystem of this type can operate autonomously; that is, in aconstant environment and without the intervention of an op-erator, as long as the energy source (continuous light irradia-tion at a suitable temperature) is available. As the photoisome-rization process in such systems is extremely fast (picosecondtimescale), the rate-limiting step is the slowest of the thermallyactivated isomerization (relaxation) reactions. The effect of mo-lecular structure on this rate has been investigated in a seriesof derivatives.[253] The rotary motor was then redesigned sothat it had distinct upper and lower parts, with the lower halfthat could be connected to other molecules or surfaces. Ratesof up to 44 rotations per second were achieved, and a deriva-tive bearing donor and acceptor substituents was prepared ca-pable of operating with visible light.[254, 255] Photoinduced uni-directional rotation has also been observed for molecularmotors anchored on the surface of Au nanoparticles[256] and ofa quartz plate.[257]

A molecular rotary motor of the above-described kind wasemployed to construct a prototype of a light-powered “nano-car” designed to move on an atomically flat surface (19,Figure 33).[258] The molecule comprises the motor unit, an oli-go(phenylene ethynylene) chassis, and an axle system withfour carborane wheels. It was shown that the light-poweredmotor of 19 does work in solution, but light-driven movementacross a surface poses several problems and has not yet beendemonstrated.It was observed[259] that on doping a liquid-crystal film with

a chiral light-driven molecular motor related to 18, the helicalorganization induced by the dopant results in a fingerprint-likestructure to the surface of the film. Irradiation of the film withlight changes the distribution of the isomers, and, as they havedifferent helical twisting power, the organization of the liquidcrystal is changed. This process results in a rotational reorgani-

Figure 32. Compound 18 undergoes unidirectional rotation in four steps;each light-driven, energetically uphill process is followed by a thermal, ener-getically downhill process.[250]


V. Balzani et al.

www.chemsuschem.org

zation of the surface structure, which can be followed by aglass rod sitting on the film.[259] When a photostationary stateis reached, the rod’s rotary motion ceases. Removing the lightexcitation enables the population of the unstable isomer todecay, returning the system to its starting state, accompaniedby rotation of the rod in the opposite direction. Very recently,a molecular rotary motor was attached to the terminus of a dy-namically racemic helical polymer, containing equal amountsof left- and right-handed helices.[260] Effective chiral inductionby the molecular motor allows fully reversible control of thepreferred helical sense of the polymer backbone by photo-and thermal isomerization of the alkene chromophore. Allthese effects, however, are related to the state of the system(i.e. the distribution of its various forms) modified by the pho-toinduced rotary motion, rather than to the unidirectional tra-jectory of the motor. In other words, the operation of thesystem is related to its switching properties.[247]

Strategies to obtain unidirectional light-induced ring rota-tion in catenanes have also been explored.[261]

6.3. A Molecular Shuttle Powered by Sunlight

Molecular shuttles, that is, rotaxanes in which the ring compo-nent can be controllably displaced between “stations” locatedalong the axle, constitute the most common examples of artifi-cial molecular machines.[243–248,262] Photoinduced ring shuttlingin a rotaxane containing two different recognition sites in theaxle component has been achieved with the compound 206+

(Figure 34).[263] This compound consists of six molecular com-ponents suitably chosen and assembled to achieve the devisedfunction. It comprises a bis-p-phenylene-34-crown-10 electron-donor macrocycle R (hereafter called the ring), and a dumb-

bell-shaped component which contains two electron-acceptorrecognition sites for the ring, namely a 4,4’-bipyridinium (A1

2+)and a 3,3’-dimethyl-4,4’-bipyridinium (A2

2+) unit, that can playthe role of “stations” for the ring R. Furthermore, the dumb-bell-shaped component incorporates a [Ru ACHTUNGTRENNUNG(bpy)3]

2+-type elec-tron-transfer photosensitizer P2+ , which is able to operate withvisible light and also plays the role of a stopper, a p-terphenyl-type rigid spacer S, which has the task of keeping the photo-sensitizer far from the electron-acceptor units, and finally a tet-raarylmethane group T as the second stopper. The stable trans-lational isomer is that in which the R component encircles theA1

2+ unit because this station is a better electron acceptorthan the other.In acetonitrile solution, the absorption of a visible photon by

the Ru-based unit of 206+ causes the forward and backwardshuttling of the ring R between the two stations. The opera-tion of this system is based on a “four-stroke” synchronized se-quence of electronic and nuclear processes.[263] The quantumyield of ring shuttling is 0.02, and it can be estimated thatabout 10% of the photon’s energy is used for the mechanicalmotion. The somewhat disappointing ring-shuttling efficiencyis compensated by the fact that the investigated system is aunique example of an artificial molecular machine because itgathers together the following features: 1) it is powered byvisible light (in other words, sunlight) ; 2) it exhibits autono-mous behavior ; 3) it does not generate waste products ; 4) itsoperation can rely only on intramolecular processes, allowingin principle operation at the single-molecule level; 5) it can bedriven at a frequency of about 1 kHz; 6) it works under mildenvironmental conditions (i.e. fluid solution at ambient tem-perature); and 7) it is stable for at least 103 cycles.Although the system in its present form cannot develop a

net mechanical work in a full cycle of operation (as for any re-versible molecular shuttle, the work done in the forward strokewould be cancelled by that performed in the backwardstroke[264]), it shows that the structural and functional integra-tion of different molecular subunits in a multicomponent as-sembly is a powerful strategy to construct light-powered nano-scale machines.[265, 266]

7. Concluding Remarks

About 85% of our energy comes from fossil fuels (“fossil solarenergy”),[15] a finite resource unevenly distributed beneath theEarth’s surface. Reserves of fossil fuels are progressively de-creasing,[2,3] and their continued use produces harmful effectssuch as pollution that threatens human health and greenhouse

Figure 33. Prototype of the light-powered nanocar 19 based on a photo-chemical unidirectional motor, an oligo(phenylene ethynylene) chassis, andan axle system with four carborane wheels.[258]

Figure 34. Structural formula of the multicomponent rotaxane 206+ designed to work as an autonomous molecular shuttle powered by sunlight.[263]



www.chemsuschem.org

gases associated with global warming.[4] Prompt global actionto solve the energy crisis is needed. Such an action should beincorporated in a more general strategy based on the con-sciousness that the Earth’s resources are limited.[1] We areurged to save energy and to use energy in more efficientways, but we are also forced to find alternative energy sourcesas soon as possible. The ultimate choice is between nuclearenergy and renewable energies (essentially, solar energy).[267]

Nuclear energy obtained with the currently available technolo-gies is neither clean nor inexhaustible. It must be producedunder severe technical, political, and military control becauseof its high capital cost, possible catastrophic accidents, hugedifficulties to dispose waste, possible misuse of nuclear materi-al, and proliferation of nuclear armaments.[268, 269] Poor countrieswill not be able to develop an independent energy policybased on nuclear energy.The sun continuously provides the Earth with a huge

amount of energy, fairly distributed all over the world. Theamount of energy mankind uses annually, about 4.6T1020 J, isdelivered to Earth by the sun in one hour. The enormous po-tential of sunlight as a clean, abundant, and economicalenergy source, however, cannot be exploited unless it is con-verted into useful forms of energy. As solar energy is diffuseand intermittent, conversion should involve concentration andstorage. These two requirements are hardly met by the cur-rently available artificial conversion technologies, namely con-version into thermal and electrical energy. Chemists can play akey role in improving thermal and electrical conversion tech-nologies by finding new materials and new processes. There isalso plenty of room for improvement in increasing solar bio-mass production.[270–273] But the “grand challenge” of chemistryis to find a convenient means for artificial conversion of solarenergy into fuels.[10,13, 67]

Acknowledgements

Financial support from the University of Bologna is gratefully ac-knowledged. We thank Dr. Fausto Puntoriero for his help withthe preparation of the frontispiece image.

Keywords: artificial photosynthesis · electron transfer ·energy transfer · photochemistry · photosynthesis

[1] N. Armaroli, V. Balzani, Angew. Chem. 2007, 119, 52; Angew. Chem. Int.Ed. 2007, 46, 52.

[2] M. R. Simmons, Twilight in the Desert, John Wiley & Sons, Hoboken, NJ,2005.

[3] G. O. Olah, A. Goepert, G. K. S. Prakash, Beyond Oil and Gas: The Metha-nol Economy, Wiley-VCH, Weinheim, 2006.

[4] Special section on “Sustainability and Energy”: Science 2007, 315, 781–813.

[5] P. Tertzakian, A Thousand Barrels a Second, McGraw-Hill, New York,2006.

[6] Key World Energy Statistics 2007, International Energy Agency, Paris,2007; http://www.iea.org/textbase/nppdf/free/2007/key_stats_2007.pdf.

[7] International Energy Outlook 2007, Energy Information Administration,U.S. Department of Energy, Washington, DC, 2007; http://www.eia.-doe.gov/oiaf/ieo/index.html.

[8] V. Smil, Energy, Oneworld, Oxford, 2006.[9] This fundamental issue of scientific research has been beautifully ex-

pressed in poetical sentences by Wilhelm Ostwald: “Life is a watermill : the effect produced by the falling water is achieved by the rays ofthe sun. Without the sun the wheel of life cannot be kept going. But wehave to investigate more closely which circumstances and laws ofNature bring about this remarkable transformation of the sunrays intofood and warmth” [translated from German “Die Rolle des fallendenWasser aber wird bei der Maschine des Lebens von den SonnenstralenZbernommen; ohne die Sonnenstralen kann das rad des Lebens nichtim Gang erhalten werden und wir werden noch genauer erforschenmZssen, auf welchen VerhWltnissen und Naturgesezen diese merkwZr-dige Umwandlung der Sonnenstralen in Nahrungsmittel und WWrmeberuht”, W. Ostwald, Die Muehle des Lebens, Thomas, Leipzig, 1911)into English by Horst Hennig (Leipzig): H. Hennig, R. Billing, H. Knoll, inPhotosensitization and Photocatalysis (Eds. : K. Kalyanasundaram, M.GrWtzel), Kluwer, Dordrecht, 1993] .

[10] R. F. Service, Science 2005, 309, 548.[11] Basic Energy Sciences Report on Basic Research Needs for Solar Energy

Utilization, Office of Science U.S. Department of Energy, Washington,DC, 2005 ; http://www.sc.doe.gov/bes/reports/files/SEU_rpt.pdf.

[12] Special issue on “Forum on Solar and Renewable Energy”: Inorg. Chem.2005, 44(20).

[13] N. S. Lewis, D. G. Nocera, Proc. Natl. Acad. Sci. USA 2006, 103, 15729.[14] G. W. Crabtree, N. S. Lewis, Phys. Today 2007, 60(3), 37.[15] G. Ciamician, Science 1912, 36, 385. This paper has been published in

four languages (English, German, French, and Italian). Giacomo Ciami-cian, one of the most important pioneers of photochemistry, was Pro-fessor of Chemistry at the University of Bologna, where the chemistrydepartment is now named in his honor. For more information aboutCiamician, see, for example: N. D. Heindel, M. Pfau, J. Chem. Educ.1965, 42, 383.

[16] See, for example: Solar Energy: Chemical Conversion and Storage (Eds. :R. R. Hautala, R. B. King, C. Kutal), Humana Press, Clifton, 1979.

[17] R. E. Blankenship, Molecular Mechanisms of Photosynthesis, BlackwellScience, Oxford, 2002.

[18] X. Hu, A. Damjanovic, T. Ritz, K. Schulten, Proc. Natl. Acad. Sci. USA1998, 95, 5935.

[19] G. McDermott, S. M. Prince, A. A. Freer, A. M. Hawthornthwaite-Lawless,M. Z. Papiz, R. J. Cogdell, N. W. Isaacs, Nature 1995, 374, 517.

[20] T. Pullerits, V. Sundstrçm, Acc. Chem. Res. 1996, 29, 381, and referencestherein.

[21] S. Karrash, P. A. Bullough, R. Ghosh, EMBO J. 1995, 14, 631.[22] G. S. Engel, T. R. Calhoun, E. L. Read, T.-K. Ahn, T. Mancal, Y.-C. Cheng,

R. E. Blankeship, G. R. Fleming, Nature 2007, 446, 782.[23] W. KZhlbrandt, D. N. Wang, Y. Fujiyoshi, Nature 1994, 367, 614.[24] a) E. Formaggio, G. Cinque, R. Bassi, J. Mol. Biol. 2001, 314, 1157; b) H.

Rogl, R. Schodel, H. Lokstein, W. KZhlbrandt, A. Schubert, Biochemistry2002, 41, 2281; c) A. N. Melkozernov, V. H. R. Schmid, S. Lin, H. Paulsen,R. E. Blankenship, J. Phys. Chem. B 2002, 106, 4313.

[25] C. C. Moser, C. C. Page, P. L. Dutton, in Electron Transfer in Chemistry,Vol. 3 (Ed. : V. Balzani), Wiley-VCH, Weinheim, 2001, p. 25.

[26] T. Ritz, A. Damjanovic, K. Schulten, ChemPhysChem 2002, 3, 243.[27] W. Zinth, J. Wachtveitl, ChemPhysChem 2005, 6, 871.[28] Oxygenic Photosynthesis : The Light Reactions (Eds. : D. R. Ort, C. F.

Yocum), Kluwer, Dordrecht, 1996.[29] K.-H. Rhee, E. P. Morris, J. Barber, W. KZhlbrandt, Nature 1998, 396, 283.[30] N. Krauss, Curr. Opin. Chem. Biol. 2003, 7, 540.[31] a) J. Deisenhofer, O. Epp, K. Miki, R. Huber, H. Michel, Nature 1985, 318,

618; b) J. Deisenhofer, H. Michel, Angew. Chem. 1989, 101, 872; Angew.Chem. Int. Ed. Engl. 1989, 28, 829; c) R. Huber, Angew. Chem. 1989, 101,849; Angew. Chem. Int. Ed. Engl. 1989, 28, 848.

[32] J. P. Allen, G. Feher, T. O. Yeates, H. Komiya, D. C. Rees, Proc. Natl. Acad.Sci. USA 1987, 84, 5730.

[33] J. R. Norris, M. Schiffer, Chem. Eng. News 1990, 68(31), 22.[34] V. Balzani, F. Scandola, Supramolecular Photochemistry, Horwood, Chi-

chester, 1991, Ch. 5, and references therein.[35] Of the two structurally equivalent branches, usually called A and B,

only one (branch A) is used for electron transfer ; see, for example: E.Katilius, Z. Katiliene, S. Lin, A. K. W. Taguchi, N. W. Woodbury, J. Phys.Chem. B 2002, 106, 1471.


V. Balzani et al.

http://dx.doi.org/10.1002/ange.200602373

http://dx.doi.org/10.1002/anie.200602373


http://dx.doi.org/10.1126/science.309.5734.548

http://dx.doi.org/10.1073/pnas.0603395103


http://dx.doi.org/10.1073/pnas.95.11.5935


http://dx.doi.org/10.1038/374517a0

http://dx.doi.org/10.1021/ar950110o

http://dx.doi.org/10.1038/nature05678

http://dx.doi.org/10.1038/367614a0

http://dx.doi.org/10.1006/jmbi.2000.5179

http://dx.doi.org/10.1021/bi015875k

http://dx.doi.org/10.1021/bi015875k

http://dx.doi.org/10.1021/jp014271n

http://dx.doi.org/10.1002/1439-7641(20020315)3:3%3C243::AID-CPHC243%3E3.0.CO;2-Y

http://dx.doi.org/10.1002/cphc.200400458

http://dx.doi.org/10.1016/j.cbpa.2003.08.011

http://dx.doi.org/10.1038/318618a0

http://dx.doi.org/10.1038/318618a0









http://dx.doi.org/10.1021/jp013265o

http://dx.doi.org/10.1021/jp013265o

www.chemsuschem.org

[36] a) R. A. Marcus, Chem. Phys. Lett. 1987, 133, 471; b) M. Bixon, J. Jortner,M. E. Michel-Beyerle, A. Ogrodnik, W. Lersch, Chem. Phys. Lett. 1987,140, 626; c) G. R. Fleming, J. L. Martin, J. Breton, Nature 1988, 333, 190;d) W. Holzapfel, U. Finkele, W. Kaiser, D. Oesterhelt, H. Scheer, H. U.Stilz, W. Zinth, Chem. Phys. Lett. 1989, 160, 1.

[37] For a detailed discussion, see Ref. [25] .[38] P. L. Dutton, J. S. Leigh, R. C. Prince, D. M. Tiede, in Tunneling in Biologi-

cal Systems (Eds. : B. Chance, D. C. DeVault, H. Frauenfelder, R. A.Marcus, J. R. Schrieffer, N. Sutin), Academic Press, New York, 1979,p. 319.

[39] R. A. Marcus, N. Sutin, Biochim. Biophys. Acta Rev. Bioenerg. 1985, 811,265.

[40] R. A. Marcus, Annu. Rev. Phys. Chem. 1964, 15, 155.[41] C. C. Moser, R. J. Sension, A. Z. Szarka, S. T. Repinec, R. M. Hochstrasser,

P. L. Dutton, Chem. Phys. 1995, 197, 343.[42] a) P. D. Boyer, Biochim. Biophys. Acta Bioenerg. 1993, 1140, 215; b) P. D.

Boyer, Angew. Chem. 1998, 110, 2424; Angew. Chem. Int. Ed. 1998, 37,2296.

[43] a) J. E. Walker, Angew. Chem. 1998, 110, 2438; Angew. Chem. Int. Ed.1998, 37, 2308; b) D. Stock, A. G. W. Leslie, J. E. Walker, Science 1999,286, 1700.

[44] B. A. Dinner, G. T. Babcock, in Oxygenic Photosynthesis : The Light Reac-tions (Eds. : D. R. Ort, C. F. Yocum), Kluwer, Dordrecht, 1996, p. 213.

[45] R. Danielsson, M. Suorsa, V. Paakkarinen, P.-N. Albertsson, S. Styring, E.-M. Aro, F. Mamedov, J. Biol. Chem. 2006, 281, 14241.

[46] F. Mamedov, R. T. Sayre, S. Styring, Biochemistry 1998, 37, 14245.[47] D. N. Beratan, J. N. Onuchic, J. R. Winkler, H. B. Gray, Science 1992, 258,

1740.[48] For a perspective article on long-range electron transfer, see: H. B.

Gray, J. R. Winkler, Proc. Natl. Acad. Sci. USA 2005, 102, 3534.[49] a) C. Turr[, C. K. Chang, G. E. Leroi, R. I. Cukier, D. G. Nocera, J. Am.

Chem. Soc. 1992, 114, 4013; b) C. J. Chang, M. C. Y. Chang, N. H. Damra-uer, D. G. Nocera, Biochim. Biophys. Acta Bioenergetics 2004, 1655, 13.

[50] a) J. Stubbe, D. G. Nocera, C. S. Yee, M. C. Y. Chang, Chem. Rev. 2003,103, 2167; b) J. M. Hodgkiss, N. H. Damrauer, S. Press\, J. Rosenthal,D. G. Nocera, J. Phys. Chem. B 2006, 110, 18853; c) J. Rosenthal, J. M.Hodgkiss, E. R. Young, D. G. Nocera, J. Am. Chem. Soc. 2006, 128,10474; d) S. Y. Reece, M. R. Seyedsayamdost, J. Stubbe, D. G. Nocera, J.Am. Chem. Soc. 2007, 129, 8500; e) J. D. Soper, S. V. Kryatov, E. V.Rybak-Akimova, D. G. Nocera, J. Am. Chem. Soc. 2007, 129, 5069.

[51] J. H. Alstrum-Acevedo, M.K: Brennaman, T. J. Meyer, Inorg. Chem. 2005,44, 6802.

[52] K. N. Ferreira, T. M. Iverson, K. Maghlaoui, J. Barber, S. Iwata, Science2004, 303, 1831.

[53] B. Loll, J. Kern, W. Saenger, A. Zouni, J. Biesiadka, Nature 2005, 438,1040.

[54] J. Yano, J. Kern, K.-D. Irrgang, M. J. Latimer, U. Bergmann, P. Glatzel, Y.Pushkar, J. Biesiadka, B. Loll, K. Sauer, J. Messinger, A. Zouni, V. K. Ya-chandra, Proc. Natl. Acad. Sci. USA 2005, 102, 12047.

[55] J. Yano, J. Kern, K. Sauer, M. J. Latimer, Y. Pushkar, J. Biesiadka, B. Loll,W. Saenger, J. Messinger, A. Zouni, V. K. Yachandra, Science 2006, 314,821.

[56] C. Tommos, G. T. Babcock, Acc. Chem. Res. 1998, 31, 18.[57] G. Aull[n, E. Ruiz, S. Alvarez, Chem. Eur. J. 2002, 8, 2508.[58] V. Balzani, L. Moggi, M. F. Manfrin, F. Bolletta, M. Gleria, Science 1975,

189, 852.[59] Photochemical Conversion and Storage of Solar Energy (Ed. : J. S. Con-

nolly) Academic Press, New York, 1981.[60] Energy Resources through Photochemistry and Catalysis (Ed. : M. GrWtzel),

Academic Press, New York, 1983.[61] T. J. Meyer, Acc. Chem. Res. 1989, 22, 163.[62] Photocatalysis : Fundamentals and Applications (Eds. : N. Serpone, E. Pel-

izzetti), Wiley, New York, 1989.[63] I. Willner, B. Willner, Top. Curr. Chem. 1991, 159, 153.[64] A. J. Bard, M. A. Fox, Acc. Chem. Res. 1995, 28, 141.[65] V. Balzani, A. Credi, M. Venturi, Curr. Opin. Chem. Biol. 1997, 1, 506.[66] Artificial Photosynthesis (Eds. : A. Collings, C. Critchley), Wiley-VCH,

Weinheim, 2005.[67] R. Eisenberg, D. G. Nocera, Inorg. Chem. 2005, 44, 6799.[68] P. V. Kamat, J. Phys. Chem. C 2007, 111, 2834.

[69] For a recent study concerning the catalytic reduction of CO2, see, forexample: a) S. Ogo, R. Kabe, H. Hayashi, R. Harada, S. Fukuzumi, DaltonTrans. 2006, 4657. Comprehensive reviews: b) H. Arakawa, M. Aresta,J. N. Armor, M. A. Barteau, E. J. Beckman, A. T. Bell, J. E. Bercaw, C.Creutz, E. Dinjus, D. A. Dixon, K. Domen, D. L. DuBois, J. Eckert, E.Fujita, D. H. Gibson, W. A. Goddard, D. W. Goodman, J. Keller, G. J.Kubas, H. H. Kung, J. E. Lyons, L. E. Manzer, T. J. Marks, K. Morokuma,K. M. Nicholas, R. Periana, L. Que, J. Rostrup-Nielson, W. M. H. Sachtler,L. D. Schmidt, A. Sen, G. A. Somorjai, P. C. Stair, B. R. Stults, W. Tumas,Chem. Rev. 2001, 101, 953; c) G. Centi, S. Perathoner, Stud. Surf. Sci.Catal. 2004, 153, 1; d) M. Aresta, A. Dibenedetto, Dalton Trans. 2007,2975; e) T. Sakakura, J.-C. Choi, H. Yasuda, Chem. Rev. 2007, 107, 2365.

[70] R. Coontz, B. Hanson, Science 2004, 305, 957.[71] S. G. Chalk, J. Romm, M. Jacoby, Chem. Eng. News 2005, 83(34), 30.[72] H. Akimoto, Science 2003, 302, 1716.[73] V. Balzani, F. Scandola, in Comprehensive Supramolecular Chemistry,

Vol. 10 (Eds. : J. L. Atwood, J. E. D. Davies, D. D. MacNicol, F. Vçgtle),Pergamon, Oxford, 1996, p. 687.

[74] a) D. A. Tomalia, H. D. Durst, Top. Curr. Chem. 1993, 165, 193; b) J. M. J.Fr\chet, Science 1994, 263, 1710; c) M. Venturi, S. Serroni, A. Juris, S.Campagna, V. Balzani, Top. Curr. Chem. 1998, 197, 193; d) A. W.Bosman, H. M. Janssen, E. W. Meijer, Chem. Rev. 1999, 99, 1665; e) D.Astruc, F. Chardac, Chem. Rev. 2001, 101, 2991; f) G. R. Newkome, C. N.Moorefield, F. Vçgtle, Dendrimers and Dendrons, Wiley-VCH, Weinheim,2001; g) M. Ballauff, C. Likos, Angew. Chem. 2004, 116, 3060; Angew.Chem. Int. Ed. 2004, 43, 2998; h) D. A. Tomalia, Mater. Today 2005, 8,p. 34; i) F. C. De Schryver, T. Vosch. M. Cotlet, M. Van der Auweraer, K.MZllen, J. Hofkens, Acc. Chem. Res. 2005, 38, 514; j) F. Vçgtle, G. Ri-chardt, N. Werner, Dendritische MolekIle, Teubner, Stuttgart, 2007.

[75] V. Balzani, S. Campagna, G. Denti, A. Juris, S. Serroni, M. Venturi, Acc.Chem. Res. 1998, 31, 26.

[76] A. Adronov, J. M. J. Fr\chet, Chem. Commun. 2000, 1701.[77] V. Balzani, P. Ceroni, M. Maestri, V. Vicinelli, Curr. Opin. Chem. Biol.

2003, 7, 657.[78] P. Ceroni, G. Bergamini, F. Marchioni, V. Balzani, Prog. Polym. Sci. 2005,

30, 453.[79] V. Balzani, P. Ceroni, S. Gestermann, C. Kauffmann, M. Gorka, F. Vçgtle,

Chem. Commun. 2000, 853.[80] M. A. Oar, W. R. Dichtel, J. M. Serin, J. M. J. Fr\chet, J. E. Rogers, J. E.

Slagle, P. A. Fleitz, L.-S. Tan, T. Y. Ohulchanskyy, P. N. Prasad, J. Am.Chem. Soc. 2006, 128, 3682.

[81] X. Yan, T. Goodson III, T. Imaoka, K. Yamamoto, J. Phys. Chem. B 2005,109, 9321.

[82] S. Hecht, J. M. J. Fr\chet, Angew. Chem. 2001, 113, 76; Angew. Chem.Int. Ed. 2001, 40, 74.

[83] a) V. Balzani, P. Ceroni, A. Juris, M. Venturi, S. Campagna, F. Puntotiero,S. Serroni, Coord. Chem. Rev. 2001, 219, 545; b) F. Puntoriero, F. Nastasi,M. Cavazzini, S. Quici, S. Campagna, Coord. Chem. Rev. 2007, 251, 536.

[84] M. Pleovets, F. Vçgtle, L. De Cola, V. Balzani, New J. Chem. 1999, 23, 63.[85] X. Zhou, D. S. Tyson, F. N. Castellano, Angew. Chem. 2000, 112, 4471;

Angew. Chem. Int. Ed. 2000, 39, 4301.[86] M. Kimura, T. Shiba, T. Muto, K. Hanabusa, H. Shirai, Tetrahedron Lett.

2000, 41, 6809.[87] S. Campagna, G. Denti, S. Serroni, A. Juris, M. Venturi, V. Ricevuto, V.

Balzani, Chem. Eur. J. 1995, 1, 211.[88] a) H. B. Baudin, J. Davidsson, S. Serroni, A. Juris, V. Balzani, S. Campa-

gna, L. Hammarstrom, J. Phys. Chem. A 2002, 106, 4312; b) J. Ander-sson, F. Puntoriero, S. Serroni, A. Yartsev, T. Pascher, T. Pol]vka, S. Cam-pagna, V. Sundstrçm, Faraday Discuss. 2004, 127, 295.

[89] a) N. D. McClenaghan, F. Loiseau, F. Puntoriero, S. Serroni, S. Campa-gna, Chem. Commun. 2001, 2634; b) J. Larsen, F. Puntoriero, T. Pascher,N. McClenaghan, S. Campagna, V. Sundstrçm, E. Nkesson, ChemPhys-Chem. , DOI: 10.1002/cphc.200700539.

[90] F. Puntoriero, S. Serroni, M. Galletta, A. Juris, A. Licciardello, C. Chiorbo-li, S. Campagna, F. Scandola, ChemPhysChem 2005, 6, 129.

[91] a) D. Gust, T. A. Moore, A. L. Moore, Acc. Chem. Res. 2001, 34, 40;b) A. K. Burrell, D. L. Officer, P. G. Plieger, D. C. W. Reid, Chem. Rev. 2001,101, 2751; c) D. Holten, D. F. Bocian, J. S. Lindsey, Acc. Chem. Res. 2002,35, 57; d) D. M. Guldi, Chem. Soc. Rev. 2002, 31, 22; e) M.-S. Choi, T. Ya-mazaki, T. Aida, Angew. Chem. 2004, 116, 152; Angew. Chem. Int. Ed.2004, 43, 150; f) D.-L. Jiang, T. Aida, Prog. Polym. Sci. 2005, 30, 403;



http://dx.doi.org/10.1016/0009-2614(87)80061-1

http://dx.doi.org/10.1016/0009-2614(87)80500-6

http://dx.doi.org/10.1016/0009-2614(87)80500-6

http://dx.doi.org/10.1038/333190a0

http://dx.doi.org/10.1016/0009-2614(89)87543-8

http://dx.doi.org/10.1146/annurev.pc.15.100164.001103

http://dx.doi.org/10.1016/0301-0104(95)00164-J

http://dx.doi.org/10.1016/0005-2728(93)90063-L

http://dx.doi.org/10.1002/(SICI)1521-3757(19980904)110:17%3C2424::AID-ANGE2424%3E3.0.CO;2-T

http://dx.doi.org/10.1002/(SICI)1521-3773(19980918)37:17%3C2296::AID-ANIE2296%3E3.0.CO;2-W


http://dx.doi.org/10.1002/(SICI)1521-3757(19980904)110:17%3C2438::AID-ANGE2438%3E3.0.CO;2-3





http://dx.doi.org/10.1074/jbc.M600634200

http://dx.doi.org/10.1021/bi980194j

http://dx.doi.org/10.1126/science.1334572



http://dx.doi.org/10.1016/j.bbabio.2003.08.010

http://dx.doi.org/10.1021/cr020421u


http://dx.doi.org/10.1021/jp056703q

http://dx.doi.org/10.1021/ja062430g

http://dx.doi.org/10.1021/ja062430g

http://dx.doi.org/10.1021/ja0704434



http://dx.doi.org/10.1021/ic050904r

http://dx.doi.org/10.1021/ic050904r








http://dx.doi.org/10.1021/ar9600188



http://dx.doi.org/10.1021/ar00161a001


http://dx.doi.org/10.1016/S1367-5931(97)80045-2

http://dx.doi.org/10.1021/ic058006i

http://dx.doi.org/10.1021/jp066952u

http://dx.doi.org/10.1039/b607993h


http://dx.doi.org/10.1039/b700658f





http://dx.doi.org/10.1021/cr970069y

http://dx.doi.org/10.1021/cr010323t




http://dx.doi.org/10.1021/ar950202d

http://dx.doi.org/10.1021/ar950202d

http://dx.doi.org/10.1039/b005993p



http://dx.doi.org/10.1016/j.progpolymsci.2005.01.003


http://dx.doi.org/10.1039/b002116o

http://dx.doi.org/10.1002/1521-3757(20010105)113:1%3C76::AID-ANGE76%3E3.0.CO;2-F

http://dx.doi.org/10.1002/1521-3773(20010105)40:1%3C74::AID-ANIE74%3E3.0.CO;2-C

http://dx.doi.org/10.1002/1521-3773(20010105)40:1%3C74::AID-ANIE74%3E3.0.CO;2-C

http://dx.doi.org/10.1016/S0010-8545(01)00351-4

http://dx.doi.org/10.1016/j.ccr.2006.04.005

http://dx.doi.org/10.1002/1521-3757(20001201)112:23%3C4471::AID-ANGE4471%3E3.0.CO;2-5

http://dx.doi.org/10.1002/1521-3773(20001201)39:23%3C4301::AID-ANIE4301%3E3.0.CO;2-9

http://dx.doi.org/10.1016/S0040-4039(00)01154-0

http://dx.doi.org/10.1016/S0040-4039(00)01154-0

http://dx.doi.org/10.1002/chem.19950010404

http://dx.doi.org/10.1021/jp012770i

http://dx.doi.org/10.1039/b316160a

http://dx.doi.org/10.1039/b109068b



http://dx.doi.org/10.1021/cr0000426

http://dx.doi.org/10.1021/cr0000426

http://dx.doi.org/10.1021/ar970264z

http://dx.doi.org/10.1021/ar970264z






www.chemsuschem.org

g) C. C. You, R. Dobrava, C. R. Saha-Mçller, F. WZrthner, Top. Curr. Chem.2005, 258, 39.

[92] J.-S. Hsiao, B. P. Krueger, R. W. Wagner, T. E. Johnson, J. K. Delaney, D. C.Mauzerall, G. R. Fleming, J. S. Lindsey, D. F. Bocian, R. J. Donohoe, J.Am. Chem. Soc. 1996, 118, 11181.

[93] T. Hori, N. Aratani, A. Takagi, T. Matsumoto, T. Kawai, M.-C. Yoon, Z. S.Yoon, S. Cho, D. Kim, A. Osuka, Chem. Eur. J. 2006, 12, 1319.

[94] a) A. Prodi, C. Chiorboli, F. Scandola, E. Iengo, E. Alessio, R. Dobrawa, F.WZrthner, J. Am. Chem. Soc. 2005, 127, 1454; b) F. Scandola, C. Chior-boli, A. Prodi, E. Iengo, E. Alessio, Coord. Chem. Rev. 2006, 250, 1471;c) A. Prodi, C. Chiorboli, F. Scandola, E. Iengo, E. Alessio, ChemPhys-Chem 2006, 7, 1514.

[95] a) T. S. Balaban, A. Eichhçfer, J.-M. Lehn, Eur. J. Org. Chem. 2000, 4047;b) T. S. Balaban, H. Tamiaki, A. R. Holzwarth, Top. Curr. Chem. 2005, 258,1; c) C. Rçger, M. G. MZller, M. Lysetska, Y. Miloslavina, A. R. Holzwarth,F. WZrthner, J. Am. Chem. Soc. 2006, 128, 6542.

[96] D.-L. Jiang, T. Aida, J. Am. Chem. Soc. 1998, 120, 10895.[97] a) M.-S. Choi, T. Aida, T. Yamazaki, I. Yamazaki, Angew. Chem. 2001, 113,

3294; Angew. Chem. Int. Ed. 2001, 40, 3194; b) M.-S. Choi, T. Aida, T. Ya-mazaki, I. Yamazaki, Chem. Eur. J. 2002, 8, 2668.

[98] L. Flamigni, A. M. Talarico, B. Ventura, C. Sooambar, N. Solladi\, Eur. J.Inorg. Chem. 2006, 2155.

[99] See, for example: a) S. L. Gilat, A. Adronov, J. M. J. Fr\chet, Angew.Chem. 1999, 111, 1519; Angew. Chem. Int. Ed. 1999, 38, 1422; b) A.Adronov, S. L. Gilat, J. M. J. Fr\chet, K. Ohta, F. V. R. Neuwahl, G. R.Fleming, J. Am. Chem. Soc. 2000, 122, 1175; c) K. R. J. Thomas, A. L.Thompson, A. V. Sivakumar, C. J. Bardeen, S. Thayumanavan, J. Am.Chem. Soc. 2005, 127, 373; d) T. S. Ahn, A. L. Thompson, P. Bharathi, A.MZller, C. J. Bardeen. , J. Phys. Chem. B 2006, 110, 19810.

[100] M. Maus, R. De, M. Lor, T. Weil, S. Mitra, U.-M. Wiesler, A. Hermann, J.Hofkens, T. Vosch, K. MZllen, F. C. De Schryver, J. Am. Chem. Soc. 2001,123, 7668. See also: T. Weil, E. Reuther, K. MZllen, Angew. Chem. 2002,114, 1980; Angew. Chem. Int. Ed. 2002, 41, 1900.

[101] M. Maus, S. Mitra, M. Lor, J. Hofkens, T. Weil, A. Hermann, K. MZllen,F. C. De Schryver, J. Phys. Chem. A 2001, 105, 3961.

[102] J. Qu, J. Zhang, A. C. Grimsdale, K. MZllen, F. Jaiser, X. Yang, D. Neher,Macromolecules 2004, 37, 8297.

[103] a) J. Hofkens, L. Latterini, G. De Belder, T. Gensch, M. Maus, T. Vosch, Y.Karni, G. Schweitzer, F. C. De Schryver, A. Hermann, K. Mullen Chem.Phys. Lett. 1999, 304, 1; b) Y. Karni, S. Jordens, G. De Belder, J. Hofkens,G. Schweitzer, F. C. De Schryver, A. Hermann, K. MZllen, J. Phys. Chem.B 1999, 103, 9378; c) J. Hofkens, M. Maus, T. Gensch, T. Vosch, M.Cotlet, F. Kçhn, A. Hermann, K. Mullen, F. C. De Schryver, J. Am. Chem.Soc. 2000, 122, 9278.

[104] R. Gronheid, J. Hofkens, F. Kçhn, T. Weil, E. Reuther, K. MZllen, F. C. DeSchryver, J. Am. Chem. Soc. 2002, 124, 2418.

[105] R. M\tivier, F. Kluulzer, T. Weil, K. MZllen, T. Basch\, J. Am. Chem. Soc.2004, 126, 14364.

[106] M. Cotlet, T. Vosch, S. Habuchi, T. Weil, K. MZllen, J. Hofkens, F. C. DeSchryver, J. Am. Chem. Soc. 2005, 127, 9760.

[107] S.-C. Lo, T. D. Anthopoulos, E. B. Namdas, P. L. Burn, I. D. W. Samuel,Adv. Mater. 2005, 17, 1945.

[108] M. Muccini, Nat. Mater. 2006, 5, 605.[109] S. R. Forrest, Nature 2004, 428, 911.[110] N. Armaroli, G. Accorsi, J. N. Clifford, J.-F. Eckert, J.-F. Nierengarten,

Chem. Asian J. 2006, 1, 564.[111] F. J. M. Hoeben, I. O. Shklyarevskiy, M. J. Pouderoijen, H. Engelkamp,

A. P. H. J. Schenning, P. C. M. Christianen, J. C. Maan, E. W. Meijer,Angew. Chem. 2006, 118, 1254; Angew. Chem. Int. Ed. 2006, 45, 1232.

[112] F. J. M. Hoeben, M. J. Pouderoijen, A. P. H. J. Schenning, E. W. Meijer,Org. Biomol. Chem. 2006, 4, 4460.

[113] J. Zhang, F. J. M. Hoeben, M. J. Pouderoijen, A. P. H. J. Schenning, E. W.Meijer, F. C. De Schryver, S. De Feyter, Chem. Eur. J. 2006, 12, 9046.

[114] a) F. Vçgtle, S. Gestermann, C. Kauffmann, P. Ceroni, V. Vicinelli, V. Bal-zani, J. Am. Chem. Soc. 2000, 122, 10398; b) M. W. P. L. Baars, E. W.Meijer, Top. Curr. Chem. 2000, 210, 131; c) L. S. Kaanumalle, R. Ramesh,V. S. N. Murthy Maddipatla, J. Nithyanandhan, N. Jayaraman, V. Rama-murthy J. Org. Chem. 2005, 70, 5062.

[115] S. Nlate, L. Plault, D. Astruc, Chem. Eur. J. 2006, 12, 903.[116] T. Darbre, J.-L. Reymond, Acc. Chem. Res. 2006, 39, 925.

[117] a) V. Balzani, P. Ceroni, S. Gestermann, M. Gorka, C. Kauffmann, M.Maestri, F. Vçgtle, ChemPhysChem 2000, 1, 224; b) V. Balzani, P. Ceroni,S. Gestermann, M. Gorka, C. Kauffmann, F. Vçgtle, Tetrahedron 2002,58, 629.

[118] U. Hahn, M. Gorka, F. Vçgtle, V. Vicinelli, P. Ceroni, M. Maestri, V. Balzani,Angew. Chem. 2002, 114, 3747; Angew. Chem. Int. Ed. 2002, 41, 3595.

[119] A. P. H. J. Schenning, E. Peeters, E. W. Mejier, J. Am. Chem. Soc. 2000,122, 4489.

[120] C.-W. Wu, H.-C. Lin, Macromolecules 2006, 39, 7985.[121] F. Kçhn, J. Hofkens, U.-M. Wiesler, M. Cotlet, M. van der Auweraer, K.

MZllen, F. C. De Schryver, Chem. Eur. J. 2001, 7, 4126.[122] V. Balzani, G. Bergamini, P. Ceroni, F. Vçgtle, Coord. Chem. Rev. 2007,

251, 525.[123] a) L. M. Dupray, M. Devenney, D. R. Striplin, T. J. Meyer, J. Am. Chem.

Soc. 1997, 119, 10243; b) D. A. Friesen, T. Kajita, E. Danielson, T. J.

Meyer, Inorg. Chem. 1998, 37, 2756; c) C. A. Slate, D. R. Striplin, J. A.Moss, P. Chen, B. W. Erickson, T. J. Meyer, J. Am. Chem. Soc. 1998, 120,4885; d) C. N. Fleming, L. M. Dupray, J. M. Papanikolas, T. J. Meyer, J.Phys. Chem. A 2002, 106, 2328; e) M. H. V. Huynh, D. M. Dattelbaum,T. J. Meyer, Coord. Chem. Rev. 2005, 249, 457.

[124] a) S. Hecht, N. Vladimirov, J. M. J. Fr\chet, J. Am. Chem. Soc. 2001, 123,18; b) X. Schultze, J. Serin, A. Adronov, J. M. J. Fr\chet, Chem. Commun.2001, 1160.

[125] G. Calzaferri, S. Huber, H. Maas, C. Minkowski, Angew. Chem. 2003, 115,3860; Angew. Chem. Int. Ed. 2003, 42, 3732.

[126] O. Bossart, L. De Cola, S. Welter, G. Calzaferri, Chem. Eur. J. 2004, 10,5771.

[127] C. Minkowski, G. Calzaferri, Angew. Chem. 2005, 117, 5459; Angew.Chem. Int. Ed. 2005, 44, 5325.

[128] A. Zabala Ruiz, H. Li, G. Calzaferri, Angew. Chem. 2006, 118, 5408;Angew. Chem. Int. Ed. 2006, 45, 5282.

[129] L. Jullien, J. Canceill, B. Valeur, E. Bardez, J.-P. Lef^vre, J.-M. Lehn, V.Marchi-Artzner, R. Pansu, J. Am. Chem. Soc. 1996, 118, 5432.

[130] X. Li, L. E. Sinks, B. Rybtchinski, M. R. Wasielewski, J. Am. Chem. Soc.2004, 126, 10810.

[131] C.-C. You, C. Hippius, M. GrZne, F. WZrthner, Chem. Eur. J. 2006, 12,7510.

[132] M. Tamura, D. Gao, A. Ueno, Chem. Eur. J. 2001, 7, 1390.[133] M. Nowakowska, J. Storsberg, S. Zapotoczny, J. E. Guillet, New J. Chem.

1999, 23, 617.[134] R. A. Miller, A. D. Presley, M. B. Francis, J. Am. Chem. Soc. 2007, 129,

3104.[135] D. Gust, T. A. Moore, A. L. Moore, Acc. Chem. Res. 1993, 26, 198.[136] H. Kurreck, M. Huber, Angew. Chem. 1995, 107, 929; Angew. Chem. Int.

Ed. Engl. 1995, 34, 849.[137] D. Gust, T. A. Moore, A. Moore, in The Porphyrin Handbook, Vol. 8 (Eds. :

K. M. Kadish, K. M. Smith, R. Guillard), Academic Press, San Diego,

2000, p. 153.[138] M. N. Paddon-Row, in Electron Transfer in Chemistry, Vol. 3 (Ed. : V. Balza-

ni), Wiley-VCH, Weinheim, 2001, p. 179.[139] D. Gust, T. A. Moore, A. L. Moore, in Electron Transfer in Chemistry, Vol. 3

(Ed. : V. Balzani), Wiley-VCH, Weinheim, 2001, p. 272.[140] F. Scandola, C. Chiorboli, M. T. Indelli, M. A. Rampi, in Electron Transfer

in Chemistry, Vol. 3 (Ed. : V. Balzani), Wiley-VCH, Weinheim, 2001, p. 337.[141] a) E. A. Weiss, M. R. Wasielewski, M. A. Ratner, Top. Curr. Chem. 2005,

257, 103; b) M. R. Wasielewski, J. Org. Chem. 2006, 71, 5051.[142] L. Flamigni, E. Baranoff, J.-P. Collin, J.-P. Sauvage, Chem. Eur. J. 2006, 12,

6592.

[143] M. R. Wasielewski, M. P. Niemczyk, W. A. Svec, E. B. Pewitt, J. Am. Chem.Soc. 1985, 107, 5562.

[144] G. L. Gaines III, M. P. O’Neil, W. A. Svec, M. P. Niemczik, M. R. Wasielew-ski, J. Am. Chem. Soc. 1991, 113, 719.

[145] M. R. Wasielewski, G. L. Gaines III, M. P. O’Neil, W. A. Svec, M. P. Niemc-zyk, J. Am. Chem. Soc. 1990, 112, 4559.

[146] P. A. Liddell, D. Kuciauskas, J. P. Sumida, B. Nash, D. Nguyen, A. L.Moore, T. A. Moore, D. Gust, J. Am. Chem. Soc. 1997, 119, 1400.

[147] For the electron-transfer chemistry of fullerene, see: S. Fukuzumi,D. M. Guldi, in Electron Transfer in Chemistry, Vol. 2 (Ed. : V. Balzani),Wiley-VCH, Weinheim, 2001, p. 270.


V. Balzani et al.

http://dx.doi.org/10.1021/ja961612f



http://dx.doi.org/10.1021/ja045379u




http://dx.doi.org/10.1002/1099-0690(200012)2000:24%3C4047::AID-EJOC4047%3E3.0.CO;2-8





http://dx.doi.org/10.1002/ejic.200501042

http://dx.doi.org/10.1002/ejic.200501042



http://dx.doi.org/10.1002/(SICI)1521-3773(19990517)38:10%3C1422::AID-ANIE1422%3E3.0.CO;2-V

http://dx.doi.org/10.1021/ja993272e

http://dx.doi.org/10.1021/jp056831e



http://dx.doi.org/10.1002/1521-3757(20020603)114:11%3C1980::AID-ANGE1980%3E3.0.CO;2-Z

http://dx.doi.org/10.1002/1521-3757(20020603)114:11%3C1980::AID-ANGE1980%3E3.0.CO;2-Z


http://dx.doi.org/10.1021/jp003643&TR_opa;+&TR_ope;

http://dx.doi.org/10.1021/ma0494131

http://dx.doi.org/10.1021/jp9923670




http://dx.doi.org/10.1021/ja017442a

http://dx.doi.org/10.1021/ja042656o

http://dx.doi.org/10.1002/adma.200500020

http://dx.doi.org/10.1038/nmat1699


http://dx.doi.org/10.1002/asia.200600082






http://dx.doi.org/10.1021/ar050203y

http://dx.doi.org/10.1002/1439-7641(20001215)1:4%3C224::AID-CPHC224%3E3.0.CO;2-2

http://dx.doi.org/10.1016/S0040-4020(01)01094-8

http://dx.doi.org/10.1016/S0040-4020(01)01094-8

http://dx.doi.org/10.1002/1521-3757(20021004)114:19%3C3747::AID-ANGE3747%3E3.0.CO;2-F

http://dx.doi.org/10.1002/1521-3773(20021004)41:19%3C3595::AID-ANIE3595%3E3.0.CO;2-B

http://dx.doi.org/10.1021/ja000099&TR_opa;+&TR_ope;

http://dx.doi.org/10.1021/ja000099&TR_opa;+&TR_ope;

http://dx.doi.org/10.1021/ma0610358

http://dx.doi.org/10.1002/1521-3765(20011001)7:19%3C4126::AID-CHEM4126%3E3.0.CO;2-M



http://dx.doi.org/10.1021/ja970660c


http://dx.doi.org/10.1021/ic9710420



http://dx.doi.org/10.1021/jp012990w

http://dx.doi.org/10.1021/jp012990w




http://dx.doi.org/10.1039/b103792g

http://dx.doi.org/10.1039/b103792g











http://dx.doi.org/10.1021/ja954332t

http://dx.doi.org/10.1021/ja047176b

http://dx.doi.org/10.1021/ja047176b



http://dx.doi.org/10.1002/1521-3765(20010401)7:7%3C1390::AID-CHEM1390%3E3.0.CO;2-L

http://dx.doi.org/10.1039/a901026b

http://dx.doi.org/10.1039/a901026b







http://dx.doi.org/10.1021/jo060225d



http://dx.doi.org/10.1021/ja00305a059





www.chemsuschem.org

[148] D. Carbonera, M. Di Valentin, C. Corvaja, G. Agostini, G. Giacometti,P. A. Liddell, D. Kuciauskas, A. L. Moore, T. A. Moore, D. Gust, J. Am.Chem. Soc. 1998, 120, 4398.

[149] D. Kuciauskas, P. A. Liddell, A. L. Moore, T. A. Moore, D. Gust, J. Am.Chem. Soc. 1998, 120, 10880, and references therein.

[150] D. Gust, T. A. Moore, A. L. Moore, D. Kuciauskas, P. A. Liddell, B. D. Hal-bert, J. Photochem. Photobiol. B 1998, 43, 209.

[151] H. Imahori, D. M. Guldi, K. Tamaki, Y. Yoshida, C. Luo, Y. Sakata, S. Fuku-zumi, J. Am. Chem. Soc. 2001, 123, 6617.

[152] H. Imahori, K. Tamaki, Y. Araki, Y. Sekiguchi, O. Ito, Y. Sakata, S. Fukuzu-mi, J. Am. Chem. Soc. 2002, 124, 5165.

[153] D. Gust, T. A. Moore, Science 1989, 244, 35.[154] D. Gust, T. A. Moore, A. L. Moore, A. N. Macpherson, A. Lopez, J. M. De-

Graziano, I. Gouni, E. Bittersmann, G. R. Seely, F. Gao, R. A. Nieman,X. C. Ma, L. J. Demanche, S.-C. Hung, D. K. Luttrull, S.-J. Lee, P. K. Kerri-gan, J. Am. Chem. Soc. 1993, 115, 11141.

[155] M. Sykora, K. A. Maxwell, J. M. De Simone, T. J. Meyer, Proc. Natl. Acad.Sci. USA 2000, 97, 7687.

[156] A. Nakano, A. Osuka, T. Yamazaki, I. Nishimura, S. Akimoto, I. Yamazaki,A. Itaya, M. Murakami, H. Miyasaka, Chem. Eur. J. 2001, 7, 3134.

[157] D. Kuciauskas, P. A. Liddell, S. Lin, T. E. Johnson, S. J. Weghorn, J. S.Lindsey, A. L. Moore, T. A. Moore, D. Gust, J. Am. Chem. Soc. 1999, 121,8604.

[158] G. Kodis, P. A. Liddell, L. de La Garza, P. C. Clausen, J. S. Lindsey, A. L.Moore, T. A. Moore, D. Gust, J. Phys. Chem. A 2002, 106, 2036.

[159] Y. Terazono, P. A. Liddell, V. Garg, G. Kodis, A. Brune, M. Hambourger,A. L. Moore, T. A. Moore, D. Gust, J. Porphyrins Phthalocyanines 2005, 9,706.

[160] G. Kodis, Y. Terazono, P. A. Liddell, J. Andr^asson, V. Garg, M. Ham-bourger, T. A. Moore, A. L. Moore, D. Gust, J. Am. Chem. Soc. 2006, 128,1818.

[161] F. D’Souza, R. Chitta, S. Gadde, L. M. Rogers, P. A. Karr, M. E. Zandler,A. S. D. Sandanayaka, Y. Araki, O. Ito, Chem. Eur. J. 2007, 13, 916.

[162] W. RZttinger, G. C. Dismukes, Chem. Rev. 1997, 97, 1.[163] a) J. Limburg, J. S. Vrettos, J. M. Liable-Sands, A. L. Rheingold, R. H.

Crabtree, G. W. Brudvig, Science 1999, 283, 1524; b) D. Astruc, PureAppl. Chem. 2003, 75, 461; c) H. Chen, R. Tagore, S. Das, C. Incarvito,J. W. Faller, R. H. Crabtree, G. W. Brudvig, Inorg. Chem. 2005, 44, 7661;d) H. Chen, M.-N. Collomb, G. Blondin, E. Rivi^re, J. W. Faller, R. H. Crab-tree, G. W. Brudvig, Inorg. Chem. 2005, 44, 9567; e) S. Das, C. Incarvito,R. H. Crabtree, G. W. Brudvig, Science 2006, 312, 1941.

[164] J. Rosenthal, J. Bachman, J. L. Dempsey, A. J. Esswein, T. G. Gray, J. M.Hodgkiss, D. R. Manke, T. D. Luckett, B. J. Pistorio, A. S. Veige, D. G.Nocera, Coord. Chem. Rev. 2005, 249, 1316.

[165] See, for example: H. Kotani, K. Ohkubo, Y. Takai, S. Fukuzumi, J. Phys.Chem. B 2006, 110, 24047, and references therein.

[166] E. L. Lebeau, R. A. Binstead, T. J. Meyer, J. Am. Chem. Soc. 2001, 123,10535.

[167] M. Sjçdin, S. Styring, H. Wolpher, Y. Hu, L. Sun, L. Hammarstrçm, J. Am.Chem. Soc. 2005, 127, 3855.

[168] A. F. Heyduk, D. G. Nocera, Science 2001, 293, 1639.[169] A. J. Esswein, A. S. Veige, D. G. Nocera, J. Am. Chem. Soc. 2005, 127,

16641.[170] J. Bachmann, D. G. Nocera, J. Am. Chem. Soc. 2005, 127, 4730.[171] J. M. Hodgkiss, C. J. Chang, B. J. Pistorio, D. G. Nocera, Inorg. Chem.

2003, 42, 8270.[172] For related studies, see: a) J. Rosenthal, B. J. Pistorio, L. L. Chng, D. G.

Nocera, J. Org. Chem. 2005, 70, 1885; b) J. Rosenthal, T. D. Luckett,J. M. Hodgkiss, D. G. Nocera, J. Am. Chem. Soc. 2006, 128, 6546.

[173] L. Sun, L. Hammarstrçm, B. Nkermark, S. Styring, Chem. Soc. Rev. 2001,30, 36.

[174] L. Sun, H. Berglund, R. Davidov, T. Norrby, L. Hammarstrçm, P. Korall, A.Bçrje, C. Philouze, K. Berg, A. Tran, M. Anderson, G. Stenhagen, J. M_r-tensson, M. Almgren, S. Styring, B. Nkermark, J. Am. Chem. Soc. 1997,119, 6996.

[175] L. Sun, M. K. Raymond, A. Magnuson, D. LeGourri\rec, M. Tamm, M.Abrahamsson, J. M_rtensson, G. Stenhagen, L. Hammarstrçm, S. Styr-ing, B. Nkermark, J. Inorg. Biochem. 1999, 78, 15.

[176] A. Magnuson, Y. Frapart, M. Abrahamsson, O. Horner, B. Nkermark, L.Sun, J.-J. Girerd, L. Hammarstrçm, S. Styring, J. Am. Chem. Soc. 1999,121, 89.

[177] L. Sun, M. Burkitt, M. Tamm, M. K Raymond, M. Abrahamsson, D. Le-Gourri\rec, Y. Frapart, A. Magnuson, P. H. Ken\z, P. Brandt, A. Tran, L.Hammarstrçm, B. Nkermark, S. Styring, J. Am. Chem. Soc. 1999, 121,6834.

[178] D. Burdinski, K. Wieghardt, S. Steenken, J. Am. Chem. Soc. 1999, 121,10781.

[179] D. Burdinski, E. Bothe, K. Wieghardt, Inorg. Chem. 2000, 39, 105.[180] a) A. Juris, V. Balzani, F. Barigelletti, S. Campagna, P. Belser, A. von Ze-

lewsky, Coord. Chem. Rev. 1988, 84, 85; b) V. Balzani, G. Bergamini, F.Marchioni, P. Ceroni, Coord. Chem. Rev. 2006, 250, 1254.

[181] S. M. Molnar, G. Nallas, J. S. Bridgewater, K. J. Brewer, J. Am. Chem. Soc.1994, 116, 5206.

[182] M. Elvington, K. J. Brewer, Inorg. Chem. 2006, 45, 5242.[183] H. Ozawa, M. A. Haga, K. Sakai, J. Am. Chem. Soc. 2006, 128, 4926.[184] a) R. Konduri, H. Ye, F. M. MacDonnell, S. Serroni, S. Campagna, K. Ra-

jeshwar, Angew. Chem. 2002, 114, 3317; Angew. Chem. Int. Ed. 2002, 41,3185; b) N. R. de Tacconi, R. O. Lezna, R. Konduri, F. Ongeri, K. Rajesh-war, F. M. MacDonnell, Chem. Eur. J. 2005, 11, 4327.

[185] Supramolecular species of this type are often referred to as “molecularbatteries”; see Ref. [75] and: a) I. Cuadrado, M. Mor`n, C. M. Casado, B.Alonso, J. Losada, Coord. Chem. Rev. 1999, 193–195, 395; b) D. Astruc,Acc. Chem. Res. 2000, 33, 287; c) D. Astruc, in Electron Transfer inChemistry, Vol. 2 (Ed. : V. Balzani), Wiley-VCH, Weinheim, 2001, p. 174;d) D. Astruc, M.-C. Daniel, S. Nlate, J. Ruiz, in Trends in Molecular Elec-trochemistry (Eds. : A. J. L. Pombiero, C. Amatore), FontisMedia S.A. andMarcel Dekker Inc. , Lausanne/New York, 2004, p. 283.

[186] F. Marchioni, M. Venturi, P. Ceroni, V. Balzani, M. Belohradsky, A. M. Eliz-arov, H.-R. Tseng, J. F. Stoddart, Chem. Eur. J. 2004, 10, 6361.

[187] N. S. Lewis, Science 2007, 315, 798.[188] a) A. Heller, Science 1984, 223, 1141; b) O. Khaselev, J. A. Turner, Science

1998, 280, 425.[189] N. S. Lewis, Inorg. Chem. 2005, 44, 6900.[190] C. A. Bignozzi, R. Argazzi, S. Caramori, in Inorganic and Bioinorganic

Chemistry (Ed. : I. Bertini) in Encyclopedia of Life Supporting Systems(EOLSS), Developed under the Auspices of the UNESCO, Eolss Publish-ers, Oxford, UK, 2007; http://www.eolss.net.

[191] A. Fujishima, K. Honda, Nature 1972, 238, 37.[192] R. Abe, K. Sayama, H. Arakawa, J. Photochem. Photobiol. A 2004, 166,

115.[193] See, for example: K. Maeda, T. Takata, M. Hara, N. Saito, Y. Inoue, H. Ko-

bayashi, K. Domen, J. Am. Chem. Soc. 2005, 127, 8286.[194] See, for example: K. Maeda, K. Teramura, D. Lu, T. Takata, N. Saito, Y.

Inoue, K. Domen, Nature 2006, 440, 295.[195] a) B. Rybtchinski, L. E. Sinks, M. Wasielewski, J. Am. Chem. Soc. 2004,

126, 12268; b) R. F. Kelley, W. S. Shin, B. Rybtchinski, L. E. Sinks, M. Wa-sielewski, J. Am. Chem. Soc. 2007, 129, 3173; c) R. F. Kelley, R. H. Gold-smith, M. R. Wasielewski, J. Am. Chem. Soc. 2007, 129, 6384.

[196] a) M. Segura, L. Sanchez, J. de Mendoza, N. Martin, D. M. Guldi, J. Am.Chem. Soc. 2003, 125, 15093; b) L. S`nchez, M. Sierra, N. Martin, A. J.Myles, T. J. Dale, J. Rebek, W. Seitz, D. M. Guldi, Angew. Chem. 2006,118, 4753; Angew. Chem. Int. Ed. 2006, 45, 4637; c) M. S. Rodriguez-Morgade, T. Torres, C. Atienza-Castellanos, D. M. Guldi, J. Am. Chem.Soc. 2006, 128, 15145.

[197] W-S. Li, K. S. Kim, D.-L. Jiang, H. Tanaka, T. Kawai, J. H. Kwon, D. Kim, T.Aida, J. Am. Chem. Soc. 2006, 128, 10527.

[198] R. Marczak, V. Sgobba, W. Kutner, S. Gadde, F. D’Souza, D. M. Guldi,Langmuir 2007, 23, 1917.

[199] L. A. J. Chrisstoffels, A. Adronov, J. M. J. Fr\chet, Angew. Chem. 2000,112, 2247; Angew. Chem. Int. Ed. 2000, 39, 2163.

[200] H. Imahori, H. Norieda, H. Yamada, Y. Nishimura, I. Yamazaki, Y. Sakata,S. Fukuzumi, J. Am. Chem. Soc. 2001, 123, 100.

[201] For more recent studies, see, for example: V. Chukharev, T. Vourinen,A. Efimov, N. V. Tkachenco, M. Kimura, S. Fukuzumi, H. Imahori, H. Lem-metyinen, Langmuir 2005, 21, 6385.

[202] S. Bosahle, A. L. Sisson, P. Talukdar, A. FZstenberg, N. Banerji, E. Vau-they, G. Bollot, J. Mareda, C. Rçger, F. WZrthner, N. Sakai, S. Matile, Sci-ence 2006, 313, 84.

[203] T. Morita, S. Kimura, Y. Imanishi, J. Am. Chem. Soc. 1999, 121, 581.[204] Y.-Z. Hu, S. Tsukiji, S. Shinkai, S. Oishi, I. Hamachi, J. Am. Chem. Soc.

2000, 122, 241.







http://dx.doi.org/10.1016/S1011-1344(98)00110-9

http://dx.doi.org/10.1021/ja004123v

http://dx.doi.org/10.1021/ja016655x





http://dx.doi.org/10.1002/1521-3765(20010716)7:14%3C3134::AID-CHEM3134%3E3.0.CO;2-3

http://dx.doi.org/10.1021/ja991255j

http://dx.doi.org/10.1021/ja991255j

http://dx.doi.org/10.1021/jp012133s




http://dx.doi.org/10.1351/pac200375040461

http://dx.doi.org/10.1351/pac200375040461


http://dx.doi.org/10.1021/ic051462m



http://dx.doi.org/10.1021/jp065215v

http://dx.doi.org/10.1021/jp065215v






http://dx.doi.org/10.1021/ja043132r

http://dx.doi.org/10.1021/ic034751o

http://dx.doi.org/10.1021/ic034751o

http://dx.doi.org/10.1021/jo048570v

http://dx.doi.org/10.1021/ja058731s

http://dx.doi.org/10.1039/a801490f

http://dx.doi.org/10.1039/a801490f





http://dx.doi.org/10.1021/ja991402d

http://dx.doi.org/10.1021/ja991402d

http://dx.doi.org/10.1021/ic990755a

http://dx.doi.org/10.1016/0010-8545(88)80032-8





http://dx.doi.org/10.1021/ja058087h

http://dx.doi.org/10.1002/1521-3757(20020902)114:17%3C3317::AID-ANGE3317%3E3.0.CO;2-R

http://dx.doi.org/10.1002/1521-3773(20020902)41:17%3C3185::AID-ANIE3185%3E3.0.CO;2-Z

http://dx.doi.org/10.1002/1521-3773(20020902)41:17%3C3185::AID-ANIE3185%3E3.0.CO;2-Z


http://dx.doi.org/10.1016/S0010-8545(99)00036-3







http://dx.doi.org/10.1021/ic051118p

http://dx.doi.org/10.1038/238037a0

http://dx.doi.org/10.1016/j.jphotochem.2004.04.031

http://dx.doi.org/10.1016/j.jphotochem.2004.04.031


http://dx.doi.org/10.1038/440295a





http://dx.doi.org/10.1021/ja036358n

http://dx.doi.org/10.1021/ja036358n




http://dx.doi.org/10.1021/la0628266



http://dx.doi.org/10.1002/1521-3773(20000616)39:12%3C2163::AID-ANIE2163%3E3.0.CO;2-X

http://dx.doi.org/10.1021/ja002154k

http://dx.doi.org/10.1021/la0500833


http://dx.doi.org/10.1021/ja991406i

http://dx.doi.org/10.1021/ja991406i

www.chemsuschem.org

[205] Y.-Z. Hu, H. Takashima, S. Tsukiji, S. Shinkai, T. Nagamune, S. Oishi, I. Ha-machi, Chem. Eur. J. 2000, 6, 1907.

[206] G. Steinberg-Yfrach, P. A. Liddell, S.-C. Hung, A. L. Moore, D. Gust, T. A.Moore, Nature 1997, 385, 239.

[207] G. Steinberg-Yfrach, J.-L. Rigaud, E. N. Durantini, A. L. Moore, D. Gust,T. A. Moore, Nature 1998, 392, 479.

[208] B. Pitard, P. Richard, M. Dunach, J.-L. Rigaud, Eur. J. Biochem. 1996, 235,779.

[209] I. M. Bennet, H. M. Vanegas Farfano, F. Bogani, A. Primak, P. A. Liddell,L. Otero, L. Sereno, J. J. Silber, A. L. Moore, T. A. Moore, D. Gust, Nature2002, 420, 398.

[210] M. Sfflri, T. A. Huld, E. D. Dunlop, H. A. Ossenbrink, Solar Energy 2007,81, 1295.

[211] M. A. Green, Third Generation Photovoltaics : Advanced Solar EnergyConversion, Springer-Verlag, Berlin, 2004.

[212] S. Gunes, H. Neugebauer, N. S. Sariciftci, Chem. Rev. 2007, 107, 1324.[213] R. Koeppe, O. Bossart, G. Calzaferri, N. S. Sariciftci, Solar En. Mater. Solar

Cells 2007, 91, 986.[214] S. E. Gledhill, B. Scott, B. A. Gregg, J. Mater. Res. 2005, 20, 3167.[215] H. Gerischer, F. Willig, Top. Curr. Chem. 1976, 61, 31.[216] B. M. Kayer, H. A. Atwater III, N. S. Lewis, J. Appl. Phys. 2005, 97,

114302.[217] a) B. O’Regan, M. GrWtzel, Nature 1991, 353, 737; b) K. Kalyanasundar-

am, M. GrWtzel, Coord. Chem. Rev. 1998, 77, 347.[218] N. Vlachopoulos, P. Liska, J. Augustynski, M. GrWtzel, J. Am. Chem. Soc.

1988, 110, 1216.[219] A. Hagfeldt, M. GrWtzel, Acc. Chem. Res. 2000, 33, 269.[220] P. Atienzar, S. Valencia, A. Corma, H. Garcia, ChemPhysChem 2007, 8, 1115.[221] Direct or dye-sensitized excitation of semiconductors can also be used

for several photocatalytic processes. See, for example: H. Kisch, W.Macyk, ChemPhysChem 2002, 3, 399.

[222] M. GrWtzel, Inorg. Chem. 2005, 44, 6841.[223] a) G. Benkç, J. Kallioinen, J. E. I. Korppi-Tommola, A. P. Yartsev, V. Sund-

strçm, J. Am. Chem. Soc. 2002, 124, 489; b) B. Wenger, M. GrWtzel, J. E.Moser, J. Am. Chem. Soc. 2005, 127, 12150.

[224] P. Wang, C. Klein, R. Humphry-Baker, S. H. Zakeeruddin, M. GrWtzel, J.Am. Chem. Soc. 2005, 127, 808.

[225] L. Schmidt-Mende, W. M. Campbell, Q. Wang, K. W. Jolley, D. L. Officer,Md. K. Nazeeruddin, M. GrWtzel, ChemPhysChem 2005, 6, 1253.

[226] D. Walker, S. Chappel, A. Mahammed, B. S. Brunschwig, J. R. Winkler,H. B. Gray, A. Zaban, Z. Gross, J. Porphyrins Phthalocyanines 2006, 10,1259.

[227] E. I. Mayo, K. Kilsa, T. Tirrell, P. I. Djurovich, A. Tamayo, M. E. Thompson,N. S. Lewis, H. B. Gray, Photochem. Photobiol. Sci. 2006, 5, 871.

[228] In open systems, water splitting by visible light with very low quan-tum yields can be observed by using metal-doped semiconductors.See, for example, Refs. [193, 194] and: K. Sayama, K. Mukasa, R. Abe, H.Arakawa, Chem. Commun. 2001, 2416.

[229] A. Brune, G. Jeong, P. A. Liddell, T. Sotomura, T. A. Moore, A. L. Moore,D. Gust, Langmuir 2004, 20, 8366.

[230] C. A. Bignozzi, J. R. Schoonover, F. Scandola, Prog. Inorg. Chem. 1997,44, 1.

[231] C. A. Bignozzi, R. Argazzi, C. J. Kleverlaan, Chem. Soc. Rev. 2000, 29, 87.[232] P. Qu, G. J. Meyer, in Electron Transfer in Chemistry, Vol. 4 (Ed. : V. Balza-

ni), Wiley-VCH, Weinheim, 2001, p. 353.[233] C. A. Bignozzi, R. Argazzi, M. T. Indelli, F. Scandola, Solar En. Mater.

Solar Cells 1994, 32, 229.[234] R. Argazzi, C. A. Bignozzi, T. A. Heimer, F. N. Castellano, G. J. Meyer, J.

Phys. Chem. B 1997, 101, 2591.[235] A. C. Lees, C. J. Kleverlaan, C. A. Bignozzi, J. G. Vos, Inorg. Chem. 2001,

40, 5343.[236] C. J. Kleverlaan, M. T. Indelli, C. A. Bignozzi, L. Pavanin, F. Scandola,

G. M. Hasselmann, G. J. Meyer, J. Am. Chem. Soc. 2000, 122, 2840.[237] N. Hirata, J. J. Lagreff, E. J. Palomares, J. R. Durrant, M. K. Nazeeruddin,

M. GrWtzel, D. di Censo, Chem. Eur. J. 2004, 10, 595.[238] S. A. Haque, S. Handa, K. Peter, E. Palomares, M. Thelakkat, J. R. Dur-

rant, Angew. Chem. 2005, 117, 5886; Angew. Chem. Int. Ed. 2005, 44,5740.

[239] See, however: A. Nomoto, Y. Kobuke, Chem. Commun. 2002, 1104.[240] R. Amadelli, R. Argazzi, C. A. Bignozzi, F. Scandola, J. Am. Chem. Soc.

1990, 112, 7099.

[241] R. Argazzi, C. A. Bignozzi, T. A. Heimer, G. J. Meyer, Inorg. Chem. 1997,36, 2.

[242] a) T. Hasobe, Y. Kashiwagi, M. A. Absalon, J. Sly, K. Hosomizu, M. J.Crossley, H. Imahori, P. V. Kamat, S. Fukuzumi, Adv. Mater. 2004, 16,975; b) T. Hasobe, S. Hattori, P. V. Kamat, Y. Urano, N. Umezawa, T.Nagano, S. Fukuzumi, Chem. Phys. 2005, 319, 243; c) T. Hasobe, S. Hat-tori, P. V. Kamat, S. Fukuzumi, Tetrahedron 2006, 62, 1937.

[243] a) V. Balzani, A. Credi, M. Venturi, Molecular Devices and Machines — AJourney into the Nanoworld, Wiley-VCH, Weinheim, 2003 ; b) An updat-ed and enlarged edition of this monograph is in press.

[244] W. R. Browne, B. L. Feringa, Nat. Nanotechnol. 2006, 1, 25.[245] A. Credi, Aust. J. Chem. 2006, 59, 157.[246] S. Saha, J. F. Stoddart, Chem. Soc. Rev. 2007, 36, 77.[247] E. R. Kay, D. A. Leigh, F. Zerbetto, Angew. Chem. 2007, 119, 72; Angew.

Chem. Int. Ed. 2007, 46, 72.[248] R. Ballardini, A. Credi, M. T. Gandolfi, F. Marchioni, S. Silvi, M. Venturi,

Photochem. Photobiol. Sci. 2007, 6, 345.[249] Reversible size and shape changes have been recently observed on

photoirradiation of micrometer-scale molecular crystals of a diaryle-thene photochrome. See: S. Kobatake, S. Takami, H. Muto, T. Ishikawa,M. Irie, Nature 2007, 446, 778.

[250] N. Koumura, R. W. J. Zijlstra, R. A. van Delden, N. Harada, B. L. Feringa,Nature 1999, 401, 152.

[251] B. L. Feringa, Nature 2000, 408, 151.[252] Molecular Motors (Ed. : M. Schliwa), Wiley-VCH, Weinheim, 2003.[253] M. K. J. ter Wiel, R. A. van Delden, A. Meetsma, B. L. Feringa, J. Am.

Chem. Soc. 2005, 127, 14208.[254] R. A. van Delden, N. Koumura, A Schoevaars, A. Meetsma, B. L. Feringa,

Org. Biomol. Chem. 2003, 1, 33.[255] M. M. Pollard, M. Klok, D. Pijper, B. L. Feringa, Adv. Funct. Mater. 2007,

17, 718.[256] R. A. van Delden, M. K. J. ter Wiel, M. M. Pollard, J. Vicario, N. Koumura,

B. L. Feringa, Nature 2005, 437, 1337.[257] M. M. Pollard, M. Lubonska, P. Rudolf, B. L. Feringa, Angew. Chem.

2007, 119, 1300; Angew. Chem. Int. Ed. 2007, 46, 1278.[258] J.-F. Morin, Y. Shirai, J. M. Tour, Org. Lett. 2006, 8, 1713.[259] R. Eelkema, M. M. Pollard, J. Vicario, N. Katsonis, B. S. Ramon, C. W. M.

Bastiaansen, D. J. Broer, B. L. Feringa, Nature 2006, 440, 163.[260] D. Pijper, B. L. Feringa, Angew. Chem. 2007, 119, 3767; Angew. Chem.

Int. Ed. 2007, 46, 3693.[261] D. A. Leigh, J. K. Y. Wong, F. Dehez, F. Zerbetto, Nature 2003, 424, 174.[262] UV light has been employed to drive the ring distribution in a two-sta-

tion molecular shuttle away from equilibrium by an information ratch-et mechanism. See: V. Serreli, C.-F. Lee, E. R. Kay, D. A. Leigh, Nature2007, 445, 523.

[263] V. Balzani, M. Clemente-Le[n, A. Credi, B. Ferrer, M. Venturi, A. H.Flood, J. F. Stoddart, Proc. Natl. Acad. Sci. USA 2006, 103, 1178.

[264] E. R. Kay, D. A. Leigh, Nature 2006, 440, 286.[265] a) S. Saha, L. E. Johansson, A. H. Flood, H.-R. Tseng, J. I. Zink, J. F. Stod-

dart, Small 2005, 1, 87; b) S. Saha, L. E. Johansson, A. H. Flood, H.-R.Tseng, J. I. Zink, J. F. Stoddart, Chem. Eur. J. 2005, 11, 6846.

[266] S. Saha, A. H. Flood, J. F. Stoddart, S. Impellizzeri, S. Silvi, M. Venturi, A.Credi, J. Am. Chem. Soc. 2007, 129, 12159.

[267] K. W. J. Barnham, M. Mazzer, B. Clive, Nat. Mater. 2006, 5, 161.[268] a) M. Inman, Science 2005, 309, 1170; b) J. Giles, Nature 2006, 440, 984;

c) J. Bohannon, Science 2007, 315, 791.[269] a) J. Johnson, Chem. Eng. News 2005, 83(23), 32; b) G. Hess, Chem. Eng.

News 2006, 84(13), 34; c) J. Johnson, Chem. Eng. News 2007, 85(25),48.

[270] Catalysis for Renewables (Eds. : G. Centi, R. A. van Santen), Wiley-VCH,Weinheim, 2007.

[271] J. Goldemberg, Science 2007, 315, 808.[272] a) A. H. Tullo, Chem. Eng. News 2007, 85(7), 53; b) R. Van Noorden,

Chem. Biodiversity Chem. World UK 2007, 4(7), 8.[273] How Much Bioenergy can Europe Produce without Harming the Environ-

ment? The European Environment Agency, Copenhagen, 2006 ; http://reports.eea.europa.eu/eea_report_2006_7/en/eea_report_7_2006.pdf.

Received: September 17, 2007Published online on December 4, 2007


V. Balzani et al.

http://dx.doi.org/10.1002/1521-3765(20000602)6:11%3C1907::AID-CHEM1907%3E3.0.CO;2-9

http://dx.doi.org/10.1038/385239a0

http://dx.doi.org/10.1111/j.1432-1033.1996.t01-1-00779.x

http://dx.doi.org/10.1111/j.1432-1033.1996.t01-1-00779.x

http://dx.doi.org/10.1021/cr050149z

http://dx.doi.org/10.1016/j.solmat.2007.01.008

http://dx.doi.org/10.1016/j.solmat.2007.01.008

http://dx.doi.org/10.1557/jmr.2005.0407



http://dx.doi.org/10.1021/ar980112j


http://dx.doi.org/10.1002/1439-7641(20020517)3:5%3C399::AID-CPHC399%3E3.0.CO;2-H





http://dx.doi.org/10.1039/b608430c


http://dx.doi.org/10.1021/la048974i

http://dx.doi.org/10.1039/a803991g

http://dx.doi.org/10.1016/0927-0248(94)90261-5

http://dx.doi.org/10.1016/0927-0248(94)90261-5










http://dx.doi.org/10.1039/b201251k



http://dx.doi.org/10.1021/ic960717g

http://dx.doi.org/10.1021/ic960717g



http://dx.doi.org/10.1016/j.chemphys.2005.06.035

http://dx.doi.org/10.1016/j.tet.2005.05.113

http://dx.doi.org/10.1038/nnano.2006.45

http://dx.doi.org/10.1071/CH06025





http://dx.doi.org/10.1039/b613411d


http://dx.doi.org/10.1038/35041665


http://dx.doi.org/10.1002/adfm.200601025

http://dx.doi.org/10.1002/adfm.200601025





http://dx.doi.org/10.1021/ol060445d

http://dx.doi.org/10.1038/440163a








http://dx.doi.org/10.1038/440286b

http://dx.doi.org/10.1002/smll.200400055



http://dx.doi.org/10.1038/nmat1604


http://dx.doi.org/10.1038/440984a



www.chemsuschem.org

Documents

Photochemical Conversion of Solar Energy