New Concepts and Applications in the Macromolecular Chemistry of Fullerenes

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New Concepts and Applications in the Macromolecular Chemistry of Fullerenes

By Francesco Giacalone * and Nazario Martín *

A new classifi cation on the different types of fullerene-containing polymers is presented according to their different properties and applications they exhibit in a variety of fi elds. Because of their interest and novelty, water-soluble and biodegradable C 60 -polymers are discussed fi rst, followed by polyfullerene-based membranes where unprecedented supramolecular structures are presented. Next are compounds that involve hybrid materials formed from fullerenes and other components such as silica, DNA, and carbon nanotubes (CNTs) where the most recent advances have been achieved. A most relevant topic is still that of C 60 -based donor-acceptor (D–A) polymers. Since their application in photovoltaics D–A polymers are among the most realistic appli-cations of fullerenes in the so-called molecular electronics. The most relevant aspects in these covalently connected fullerene/polymer hybrids as well as new concepts to improve energy conversion effi ciencies are presented. The last topics disccused relate to supramolecular aspects that are in involved in C 60 -polymer systems and in the self-assembly of C 60 -macromolecular structures, which open a new scenario for organizing, by means of non-covalent interactions, new supramolecular structures at the nano- and micrometric scale, in which the combination of the hydrofobicity of fullerenes with the versatility of the noncovalent chemistry afford new and spectacular superstructures.

1. Introduction

Fullerene-containing polymers is a very active fi eld that inte-grates two expanding scientifi c areas, i.e., those of fullerenes and polymers. However, whereas polymers have shown their applicability for social needs in a huge number of examples, fullerenes despite their outstanding mechanical, [ 1 ] chemical, [ 2 ]

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[∗] Dr. F. Giacalone Department of Organic Chemistry “E. Paternò”Università di PalermoViale delle Scienze s/n, Ed. 17, 90128 Palermo (Italy)Fax: + 39-091-566825 E-mail: [email protected] Prof. N. Martín Departamento de Química Orgánica IFacultad de Ciencias QuímicasUniversidad Complutense de MadridCiudad Universitaria s/n, 28040 Madrid, Spain IMDEA- NanocienciaCampus de la Universidad Autónoma de Madrid28049 Madrid (Spain) E-mail: [email protected]

DOI: 10.1002/adma.201000083

electrochemical [ 3 ] and photophysical [ 4 ] properties still are not widely employed in real applications. In this regard, the combination of the unique fullerenes with the highly versatile and easily processable polymers merges into a new fi eld in which novel materials exhibiting the properties of fullerenes can be handled with the advan-tages of processability of polymers.

Although a variety of previous review papers partially covered the many exam-ples of polymers that make use of fuller-enes, only recently we have carried out a comprehensive revision of this fi eld and organized the rich existing bibliography from a chemical viewpoint, with special emphasis on the synthesis and properties exhibited by the different materials. [ 5 , 6 ]

The aim of the present paper is to update the previous work by bringing together the most signifi cant advances that have occurred in the fi eld of fullerene polymers during the last few years involving different macromolecular structures combined with fullerenes. Furthermore, we have added some recent hybrid systems involving fuller-

enes and carbon nanotubes (CNTs). In fact, CNTs can be used as polydisperse materials with variable length and diameters, and are able to allocate C 60 molecules inside their cavity forming nano peapods which can undergo a further and amazing poly-merization reaction to generate double-walled CNTs.

On the other hand, in addition to new issues such as water-soluble C 60 -polymers, biodegradable C 60 -polymers or polyfullerene-based membranes, new fullerene-containing hybrids such as fullerene-silica hybrid materials or DNA-C 60 hybrids are also discussed. Donor-acceptor (D-A) fullerene poly-mers maintain the interest of the scientifi c community due to their implications for photovoltaic applications and, thus, the most recent advances have also been discussed in the text.

The last part is dedicated to the increasing fi eld of supramo-lecular polymers involving fullerenes, which opens a new avenue for facilitating the integration of fullerenes into poly-mers and other supramolecular organizations such as the inter-esting self-assembling polyfullerene-based systems.

Finally, we would like to remark that this review is mainly focused on the singular features and applications of the different classes of fullerene-containing polymers and, consequently, they have been organized in a systematic and unprecedented way according to the different properties they exhibit.

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Francesco Giacalone is an associate researcher at the University of Palermo, Italy. He received his Ph.D. from the University Complutense of Madrid (UCM) in 2004, under the guidance of Prof. N. Martín and Dr. J. L. Segura. Since June 2005 he worked at the UCM as postdoctoral scientist as a ESF Fellow in the fi eld of molecular elec-

tronics. He is co-editor, for the book “Fullerene-Polymers: Synthesis, Properties and Applications” published by Wiley-VCH

Nazario Martín is Full Professor of Organic Chemistry at the UCM. He worked with Prof. M. Hanack in Tübingen and Prof. F. Wudl in Santa Barbara (California). His research interest is focused on the covalent and supramolecular chemistry of carbon nanostructures such as fullerenes and carbon nanotubes, p-conjugated

systems, and electroactive molecules, in the context of electron transfer processes, photovoltaic applications and nanoscience.

2. Water Soluble C 60 -Polymers

In recent years, several groups have developed different syn-thetic strategies with the aim to obtain water-soluble C 60 poly-mers able to exploit the unique features of fullerenes in a hostile environment such as the aqueous one, in which fullerenes are practically insoluble. The interest of these water soluble C 60 -containing polymers lies in their use as effi cient DNA-cleaving agents in the photodynamic cancer therapy (PCT), [ 7 ] as mimic of the superoxide dismutase, [ 8 ] as carriers for drug/gene delivery applications, [ 9 ] and for biological studies in general [ 10 ] just to name a few potential applications. Among the different strate-gies used to solubilize fullerenes in water, the covalent linkage of a polysaccharide such as pullulans [ 11 ] or curdlan sulfates [ 12 ] to the C 60 cage has been very successful.

In 2006, Jérôme and coworkers covalently grafted four poly(vinyl acetate) chains end-capped by a Co(acac) 2 complex to C 60 via cobalt-mediated radical polymerization at low tem-perature (30 ° C). [ 13 ] Once hydrolyzed the acetate groups, the resulting star (PVOH) 4 -C 60 became water-soluble. This fi nding prompted the authors to study whether such hybrid material may behave as a photosensitizer to produce singlet oxygen. For this purpose, they successfully tested (PVOH) 4 -C 60 in the photooxidation of 9,10-anthracenedipropionic acid (ADPA) to the corresponding endoperoxide in D 2 O ( Scheme 1 ). More-over, (PVOH) 4 -C 60 proved to be low cytotoxic toward THP-1 cells. All the above results make such star-polymers promising candidates for PCT. On the contrary, a random C 60 -acrylamide copolymer exhibited cytotoxicity toward bone tumoral cells after irradiation. [14] In fact, the growth of tumoral cells may be inhibited by more than 90% in a polymer concentration of 100 μ g/mL.

Recently, a series of C 60 -anchored multi-armed polyacrylic acids (PAA) has been synthesized by hydrolysis of the parent poly( tert -butyl acrylates). [ 15 ] These polymers are able to interact with cationic porphyrins by mean of electrostatic interactions. [ 15 ] In this way, such donor-acceptor supramolecular complexes show strong interactions between fullerenes and porphyrin moieties both in the ground and in the excited states. The authors foresaw that this system may have some exploitation as photophysical probe in the study of polycationic proteins with a centered heme group.

In 2007, Geckeler exploited the solubility of β -cyclodextrins (CD) in polar solvents in order to solubilize the poly(azomethine) formed by the condensation of poly(oxypropylene)diamine and terephthaldehyde, by forming the corresponding poly-pseudor-otaxane 3 ( Scheme 2 ). [ 16 ]

Hence, reaction of terminal amine moieties of 3 with C 60 led to the water-soluble polyrotaxane 4 in which the fullerenes act as stoppers, thus avoiding the de-threading of cyclodextrins.

© 2010 WILEY-VCH Verlag GmAdv. Mater. 2010, 22, 4220–4248

Scheme 1 . Photooxidation of 9,10-anthracenedipropionic acid ADPA.

CH2CH2COOH

CH2CH2COOH

1O2

CH2CH2COOH

CH2CH2COOH

OO

It is interesting to note, however, that CD units make the poly(oxypropylene)diamine fragment linear, able to react readily with terephthaldehyde affording high molar mass ensembles.

Recently, a pioneering work has been carried out by Steinmetz and co-workers, [ 17 ] who succeeded in the covalent linking of fullerene with viral nanoparticles (VNPs), natu-rally occurring self-assembling protein structures useful in biomedicine. They exploited the presence of several units of lysine residues exposed to solvents in the capsid of the VPNs, namely Cowpea mosaic virus (CPMV) and bacteriophage Q β , as multivalent linking sites for functionalization. In this manner, the authors coupled PCBA both to CPMV or Q β , or conjugated a PCBA-PEG derivative 7 with Q β -PEG via a “click” coupling protocol, as depicted in Scheme 3 . The obtained hybrid complexes remained water-soluble and stable for several months, with VNP-PEG-C60 9 resulting the more fullerene-loaded than 6a-b (45-50 C 60 molecules per Q β par-ticle). Studies on the cellular uptake of hybrids in the HeLa human cancer cell line revealed that internalization was not inhibited by the presence of attached fullerene moieties. In this way, VNP may be used as vehicles for C 60 delivery into the cells, with potential applications in photoactivated tumor therapy.

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Scheme 2 . Synthesis of C 60 doubly end-capped polyrotaxane 4.

Scheme 3 . (CNPV and Q β reproduced from VIPERdb).

3. Biodegradable C 60 -Polymers

In the search of bio-applications for poly-fullerenes, the main task is to conjugate the well-known properties of C 60 with non-toxic and biocompatible macromolecular backbones since the application of fullerene C 60 and its derivatives for biomedical pur-poses has been strongly limited by their potential toxicity. [ 18 ] In this regard, poly( ε -caprolactones) (PCLs) as well as aliphatic polyesters have attracted a lot of interest in recent years because they are biodegradable, biocompatible, and non-toxic to living organ-isms. [ 19 ] These features make PCLs suitable candidates for applications in the biomedical fi eld as controlling drug delivery systems, [ 20 ] and recently several groups have tried to con-jugate the properties of both C 60 and PCL. For example, Rashkov, Jérôme, and co-workers have reported in 2006 two different examples in which they synthesized two star-shaped PCL-fullerene hybrids: one by reacting an average of six azide-terminated PCL chains with C 60 ; [ 21 ] and the second one by success-fully anchoring six amino-terminated PCLs onto the fullerene cage. [ 22 ] Once prepared, these star-PCL-C 60 hybrids were tested in the generation of singlet oxygen by irradiation with visible light, using the photooxidation of ADPA as reaction probe (Scheme 1 ). Interest-ingly, both polymers showed remarkable gen-eration of 1 O 2 , being the amino-PCL the most active. In addition to the PCL-C 60 polymers, an amino-PEG has also been prepared and reacted with [60]fullerene. [ 22 ] The so obtained star-shaped PEG-C 60 also showed good singlet oxygen generation and, because of its good

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Figure 1 . SEM images of electrospun mats of PCL/C 60 [N-PCL] 6 (2:1, w/w) × 200. Reproduced with permission [ 22 ] . Copyright 2006 ACS.

water solubility, its cytotoxicity has been tested revealing low tox-icity against human promonocytic THP-1 cells. This fi nding could fi nd application in the treatment of multidrug resistant patho-gens. Afterwards, star-azido and star-PEG polymers were mixed with PCL and electrospinned, see Figure 1 , giving rise to the for-mation of monofi bers with a diameter distribution spread in the 4000–8000 nm range, which are very promising materials for sev-eral applications such as tissue engineering and drug delivery.

© 2010 WILEY-VCH Verlag GAdv. Mater. 2010, 22, 4220–4248

Figure 2 . Gelation of C 60 -PCL fi lm: (a) original fi lm, (b) fi lm in chloroformof (c). Reproduced with permission [ 23 ] .

Inoue and co-workers have also been engaged in the syn-thesis and studies on C 60 -PCLs systems. After preparing and characterizing a PCL copolymer endowed with 2.6% units with azido groups in the side-chain, they grafted approximatively 80% of fullerene. [ 23 ] It is worth noting that in the formed mate-rial the crystallization temperature and the crystallinity resulted to be higher and lower, respectively, than the original PCL poly mer. This was probably due to nucleating and confi nement effect of C 60 aggregates on the poly(caprolactone) chains, that fi nally lead to the formation of a pseudo-network, as noticed by the reversible gelation in chloroform ( Figure 2 ).

Analogously, the same behavior has been displayed by an end-capped C 60 -PCL polymer. [ 24 ] On the other hand, a novel star-shaped fullerene-PCL hybrid with 2-3 arms per molecule has been prepared by reacting fullerenol with caprolactone via ring open polymerization. [ 25 ] Finally, a PCL with a terminal polihedral oligomeric silsesquioxane (POSS) has been capped at the other end with [60]fullerene, affording a polymer with two different points of aggregation of different force: weaker in the POSS side and stronger in the C 60 side. [ 26 ]

4. Polyfullerene-Based Membranes

The fi rst example of gas separation membranes based on poly-fullerene derivatives has been reported by Stamatialis et al. in 2004. [ 27 ] They attached fullerene moieties on the side chain of a poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) polymer. The

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for 5 min, (c) fi lm in chloroform for 1 h, and (d) fi lm after drying the fi lm

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subsequent membranes showed an increased permeability of up to 80% with respect to pure PPO, probably because the incorporation of C 60 moieties gives rise to an increase of the polymer free volume, leading to a considerable stiffening of linked PPO-C 60 .

Vinogradova et al. have recently prepared and tested two different star-shaped fullerene-polystyrene (PS) hybrids as a membrane for gas separation. [ 28 ] The fi rst polymer, PS 6 -C 60 , was obtained by grafting six polystryrillithium arms onto the C 60 sphere. The second one has been synthesized by reacting the PS 6 -C 60 in the hexaanionic form with chlorodimethylsilane (CDMS), in such a way to afford two fullerene cages connected each other having twelve arms in total, PS 6 -C 60 -DMS-C 60 -PS 6 . Membranes were prepared by casting the polymers from chlo-roform solutions and, after evaporation of the solvent, they were dried under vacuum at 40 ° C. Once formed, the mem-branes proved to be effective for gas separation, especially showing a good selectivity factor for the O 2 /N 2 (5.9 for the hexa-arm, 4.0 for the twelve arms) and CO/N 2 pairs, and which are higher than the corresponding values obtained with linear PS membranes.

Slightly lower selectivity for the O 2 /N 2 pair (3.6) and good selectivity for He/CH 4 (7.4) have been displayed by aged mem-branes made of fullerene fi lms grown and photopolymerized onto a porous polycarbonatesiloxane substrate. [ 29 ] The authors found a strong infl uence for the age of the membrane on the gas permeability properties.

Finally, Tasaki, Wudl, and co-workers have prepared com-posite membranes for proton exchange fuel cells. [ 30 ] They mixed Nafi on with a fullerene superacid, HC 60 (CN) 3 , by employing a fullerene star-polymer as compatibilizer. Such membranes showed enhanced proton conductivity under low relative humidity regimes in comparison with recasted Nafi on membranes.

5. Fullerene-Silica Hybrid Materials

Since the availability of fullerene C 60 in multigram quanti-ties, [ 31 ] several research groups have tried to exploit its exotic properties. Soon after, a number of attempts to incorporate C 60 into inorganic matrices appeared with the hope that three-dimensional inorganic networks prepared via the sol-gel route may lead to the development of highly stable, resistant, and long-lasting fullerene-based materials. [ 32 ] Unfortunately, unsat-isfactory results were obtained mainly because of the low solu-bility of fullerene and its tendency to aggregate, which resulted in inhomogeneous mixtures or nontransparent glasses. Hence, C 60 has been functionalized with the aim to covalently link it on inorganic oxides and several sol-gel processes have subse-quently been developed.

In some cases, fullerene derivatives were anchored into silicate glasses, showing interesting optical limiting proper-ties, [ 33 ] which so far is likely the most promising application for this kind of material, as well as excellent thermal and optical stability. [ 34 ]

Another extensive application for these hybrid deriva-tives stem from their use in HPLC stationary phases after their grafting onto silica, leading to materials with excellent

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separation properties. [ 35 ] Such modifi ed silicas have also been employed in gas chromatographic separation. [ 36 ] In addition, fullerenol C 60 (OH) x , has been used in sol-gel technology as a novel coating for solid-phase microextraction fi bers. [ 37 ] The so prepared inexpensive and durable fi ber showed higher sen-sitivity and faster mass transfer velocities for aromatic com-pounds than commercial stationary phases.

In 2007, Ozin and co-workers reported on the synthesis of “periodic mesoporous buckysilica” prepared by self-assembly of a multi-substituted fullero-tri(ethoxy)silane derivative hydrolized in the presence of tetraethyl orthosilicate (TEOS) under basic and acid conditions. [ 38 ] The latter resulted in the best homo-geneous mesoporous C 60 -silica hybrids obtained so far. The structure of these materials integrating fullerene-moieties into the silica channel walls, has been studied by means of STEM, PXRD, nitrogen absorption and energy dispersive X-ray fl uores-cence (EDX), in which after fullerene was fi rst incorporated and then labelled with OsO 4 .

On the other hand, Song, Chung, and co-workers have described the synthesis of highly photoluminescent fullerene-silica nanoparticles (FSNP). [ 39 ] This material has been obtained by a reverse microemulsion method in which fullerene was reacted with TEOS in the presence of TX-100 in n-hexanol as surfactant and cosurfactant, respectively, employing NH 4 OH as catalyst. In such a way, monodisperse, spherical nanoparticles with an average diameter of 61 ± 6 nm, have been obtained. Interestingly, this material showed an excellent PL inten-sity and it was successfully employed as bioimaging material ( Figure 3 ). In fact, C 60 -silica nanoparticles penetrated the cell without entering the nuclei, thus avoiding genetic disruption. Moreover, the nanospheres displayed higher photostability than Alexa Fluor 488, the best alternative to fl uorescein for its brightness and stability, but they also resulted to have a very low cytotoxicity. For this novel photoluminescent material for biosensing, the authors suggested that the increased photo-luminescence, compared to both C 60 and silica nanoparticles, aroose from the C-O-Si linkages that seems to be formed in the sol-gel process.

In 2007 Bakry et al. reported the modifi cation of a series of aminopropyl-functionalized silica of different porosity, with fullerene epoxide or fullereneacetic acid ( Figure 4 ). [ 40 ] The formed materials were successfully used as reversed-phase com-ponents for solid-phase extraction (SPE) of phosphopeptides, usually lost with the normal sorbents. They were employed in desalting and preconcentration of protein and peptides as well. Furthermore, recoveries of up to 99% were achieved in the SPE of fl avonoids.

Very recently, fullerene has been covalently linked to mag-netic silica nanoparticles. [ 41 ] Such fullerene–silica hybrids have been applied with high effi ciency in the enrichment of low-concentration peptides in complex biological samples.

Finally, Carofi glio, Maggini, and co-workers used a fullerene-derivative covalently supported on silica as a highly effi cient catalytic bed in fl ow microreactors for the photooxidation of methionine sulfoxide. [ 42 ] They also used Tentagel functionalized with fullerene as singlet oxygen sensitizer for the conversion of α -terpinene to ascaridole. For these experiments the authors used a self-built microreactor, which in the former reaction worked perfectly under white LED illumination.

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Figure 3 . Photostability and cytotoxicity of the FSNP. a) PL image of FSNPs incorporated in macrophages (RAW 264.7) under (492 ± 18) nm excita-tion and > 617 nm detection during the initial stage of irradiation. b) PL image of the same cells after continuous irradiation with (492 ± 18) nm light for 600 s. c) Fluorescence image of macrophage cells stained by Alexa 488-SA at the initial stage of irradiation ( λ ex = 492 ± 18 nm, λ det > 525 nm). d) Fluorescence image of the same cells after continuous irradiation by 492 ± 18 nm light for 600 s. Reprinted with permission [ 39 ] .

6. DNA-C 60 Hybrids

In 1994 Hélène and Nakamura fi rst reported a C 60 -linked deoxynu-cleotide (16-mer) that effectively formed double and triple helices with complementary single-stranded DNA, duplex DNA, and DNA duplex with hairpin structure ( 12 , Figure 5 ). [ 43 ] Moreover, they found that exclusive cleavage at guanidine bases occurs after irra-diation with light. The same fi nding has been later confi rmed by Rubin, who employed C 60 -oligonucleotide conjugate 13 (38-mer) to shed light on the cleavage mechanism that, in contrast to that previously supposed, did not involve 1 O 2 as the active species but a single electron-transfer process between 3 C 60 and guanosine. [ 44 ]

Since then, a new strategy has been widely employed by researchers, namely the supramolecular complexation between fullerene-derivatives and DNA. In fact, DNA may be regarded as


Figure 4 . Structures of C 60 modifi ed aminopropyl-functionalized silica.

SiOSiOH

OHH2N

SiOSiOH

OHNH

O

10

an anionic polyelectrolyte able to form polyelectrolyte-surfactant complexes (PSCs), in which the surfactant molecules are positively charged. [ 45 ]

The fi rst researcher to exploit the PSC formation between C 60 - N , N -dimethylfulleropyrrolidinium and DNA was Tour in 1998. [ 46 ] Unfortunately, once fullerenes were placed at the outer side of DNA double helix, the hybrids became hydrophobic and strongly aggregated in water.

Undoubtly, Nakamura and his group were pioneers in this fi eld. [ 47 ] In 2000 they designed and prepared a tetracati-onic C 60 derivative with four anchoring points, able to strongly interact with DNA due to the perfect complementarity of the diammonium–fullerene side-chains with two parallel phos-phate backbones along the major groove of a DNA duplex (both separated from each other by 1.2 nm) ( 14 , Figure 6 ). [ 48 ]


SiOSiOH

OHH2N

SiOSiOH

OHNH

OH

11

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Figure 5 . Structures of C 60 -oligonucleotide conjugates 12 – 13 .

H O

O

O

OS

NH

PO

OO

pNu1

pNu1 = TCTTTCCTCTTCTT

12

O

O

S PO

OO

O

pNu2

N

NH

O

O

13

pNu2 = 3'-TCTACGATGTTAATCCGAACATGTATAACAGCAATC

This compound was found to be an effective transfection reagent for gene transfer, capable to bind 4-40 kbp DNA vector and to deliver it to mammalian cells after incubation. They also observed that this approach results in the folding of a super-coiled DNA molecule by forming disk-like condensates which are composed of one to few DNA double strands ( Figure 7 ). [ 49 ]

Interestingly, such aggregates were reversible since extrac-tion with chloroform dissolves 14 leaving the free DNA mol-ecules. Further studies in collaboration with Chu’s group gave more insights about the complexation mechanism. This seems to proceed in two steps: fi rstly, as soon as mixed, 14 and DNA chains aggregate and then a restructuring process takes place ( Figure 8 ). [ 50 ] In-depth biological and chemical investigations allowed the authors to observe that cell uptake of 14 /DNA PSC occurs trough endocytosis mechanism in which the DNA is protected by the fullerene derivative against enzymatic diges-tion. [ 51 ] The release of the plasmid, achieved both by loss of the ammonium groups of 14 or their transformation into neutral derivatives, leads in turn to expression of the gene either tran-siently or stably. Finally, Nakamura and co-workers successfully prepared and tested other gene delivering aminofullerenes, such as 15 , and they were able to give general rules for their future chemical design. [ 52 ]

More recently, other multicationic fullerene-derivatives have been tested as vectors for gene transfer ( Figure 9 ). Prato et al. studied DNA complexation and plasmid delivery ability of multi- N , N -dimethylfullerenopyrrolidinium derivatives 16 and 17 capable to strongly electrostatically bind plasmid DNA. [ 53 ] On the other hand, Engler and coworkers studied several cationic, neutral and anionic Bingel-type C 60 -multiadducts ( 18 - 23 ) as nonviral gene delivery vectors. [ 54 ] Among them, only octa- and dodeca- ammonium derivatives 20 and 21 displayed effi cient

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Figure 6 . DNA-interacting tetracationic fullerene derivatives 14–15 .

H

OO

N

NMe2HO

O

N

HMe2N

H

14

N

N+H2N

+H2N

O

15

in vitro transfection. Moreover, all the derivatives presented dose-dependent increasing toxicity.

Very recently, a pH-driven enthalpic molecular nanomachine in which two fullerene moieties have been covalently incorpo-rated at both 3’ and 5’ends of i-motif DNA, has been reported ( Figure 10 ). [ 55 ] This DNA-fullerene hybrid was able to change its conformation from “close” to “open” by varying the pH value from 5 to 8. Interestingly, such molecular machine showed cycling effi -ciency of ∼ 100% for 10 consecutive cycles with a remarkable low response time of 20 sec, the same of non-functionalized DNA. From SAXS data, the authors estimated a contraction strain of up to 86%, being this value higher than that for free DNA. Moreover, van ‘t Hoff analysis of CD melting curves of both free and C 60 -functionalized DNA revealed a more stable folded struc-ture in the latter (–26.3 kJ/mol vs. – 11.1 kJ/mol). All these data clearly reveal that fullerene drives a faster response and increases the stability, probably due to strong hydrophobic C 60 –C 60 interac-tions in aqueous media.

7. C 60 -Carbon Nanotubes Hybrids

In the past two decades an extraordinary amount of work has been dedicated to disclosing the physical and chemical proper-ties of carbon nanotubes (CNTs), especially in the search for novel commercial and industrial applications. [ 56 ] From a chem-ist’s viewpoint, besides covalent and supramolecular modifi ca-tion of the external wall of CNTs, fi lling the hollow cavity of carbon nanotubes has attracted much interest. [ 57 ]

In 1998 Luzzi accidentally discovered that several C 60 units, byproduct in the synthesis of single-walled NTs (SWNTs), were trapped inside of open-ended nanotubes, forming quasi-1D

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N

NH2+

NH2+

arrays, [ 58 ] and afterwards he proposed their formation mechanism. [ 59 ] The high geo-metrical match between C 60 and SWNTs is responsible for the very strong van der Waals forces present in such peapods (as they have commonly been coined) with a strong inter-action of 3 eV between the two carbon allo-tropes, making encapsulation a spontaneous and irreversible process. [ 60 ]

Several groups have been engaged in the study of these spectacular structures and a number of techniques have been employed in order to achieve more insight of their

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Figure 7 . AFM images of pBR322 DNA in the absence (a) and in the presence (b–d) of 14 at various reagent/base pair ratio (R) values. a) Intact DNA, R = 0; b) partially folded single DNA molecule, R = 0.65; c) partially condensed DNA with unfolded double strands, R = 0.65; d) fully condensed fullerene-DNA disk, R = 0.65. All scale bars 200 nm. Reproduced with permission [ 49 ] .

properties such as Raman [ 61 ] and in situ Raman [ 62 ] spectro-scopies, EPR, [ 63 ] electron energy loss [ 64 ] and sub Kelvin trans-port [ 65 ] spectroscopies, among others. Beside pristine [60]fullerene, a number of different endohedral metallofullerenes have been successfully used to fi ll up SWNts in order to pre-pare what was usually abbreviated (M@C n )@NT, namely Sc@C 82 , [ 66 ] Sc 2 @C 84 , [ 67 ] Dy@C 82 , Dy 3 N@C 90 , [ 68 ] La@C 80 , [ 66 , 69 ] Gd @C 82 , [ 70 ] Eu@C 82 , [ 71 ] Sm@C 82 [ 72 ] and Er@C 82 . [ 73 ]

In 2003 Terrones achieved the fullerene coalescence inside nanopeapods both thermally or by electron irradiation that ini-tially resulted in a 1D polyfullerene, which for further annealing became a kind of nanotube fi nally forming a double-walled NT (DWNT) ( Figure 11 ). [ 74 ] In the polymerization of C 60 fi rstly short nanotubes with diameters of ∼ 0.7 nm are formed. Then, the short nanotubes merge together growing in length. [ 75 ] The polymerization can be avoided or limited up to 1100 ° C by pre-heating peapods in the presence of H . [ 76 ]


2

Some years later, Shinohara and coworkers unequivocally showed stable phases of encapsulation of C 60 and C 70 in DWNT and in triple-walled NT (TWNT) ( Figure 12 ). [ 77 ] They observed that fullerene moieties adopt a zig-zag phase when introduced inside DWNT or TWNT with inner diameter over 2.34 nm. Once again, thermal treatment of these superpeapods results in the coalescence of fullerenes with formation of a new smaller nanotube.

The last example we report in which SWNT and C 60 interact not covalently regards a supramolecular assembly formed between a pyrene-fullerene conjugated and the outer wall of HiPCO SWNTs via π − π interactions. [ 78 ] The effective direct interaction of pyrene and nanotube was defi nitively confi rmed by electrochemical studies.

On the other hand, only recently chemists have been involved in the covalent modifi cation of NTs linking them with fullerene moieties. In this regard, the synthesis of a hybrid material


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Figure 8 . Molecular model shows a possible structure of the complex between DNA and two-handed tetraaminofullerene 14 . Reproduced with permission [ 50 ] . Copyright 2005 ACS.

consisting of fullerene units fused to the outer wall of the nano-tubes has been reported. [ 79 ] HR-TEM images showed that the majority of these fullerenes are C 42 and C 60 .

In 2007 Langa and coworkers, chemically modifi ed for the fi rst time, the ending rims of SWNTs by means of amidation reactions between the carboxylic groups of NTs and an amine-functionalized C 60 derivative. [ 80 ] The formation of SWNT-C 60 hybrid 24 ( Figure 13 ) has been directly confi rmed by HR-TEM, FTIR and Raman spectroscopies and, undirectly, by acid hydrol-ysis. In fact, mixing 24 with a CHCl 3 /HCl (1:1 v/v) mixture


Figure 9 . Fullerene derivatives 16–23 tested as gene delivery vectors.

N

N

n = 4-7

O

OO

O

H3N NH3n

18: n = 219: n = 320: n = 421: n = 6

HO

HO

16

allowed to recover both SWNT-COOH and the fullerene derivative.

Two years later, Bonifazi, Prato, and co-workers employed the same approach in order to covalently functionalize SWNTs. This time they anchored D-A dyads in which the acceptor was C 60 and the donors were ferrocene, porphyrin and N , N -dimeth-ylaniline moieties ( 25a-c , Figure 13 ). [ 81 ] Once again, confi r-mation of linking was furnished by TGA analysis, solid-state UV-vis-NIR, Raman and XPS spectroscopies. HR-TEM images showed that, in addition to the ending functionalization, some

mbH & Co. KGaA, Weinheim Adv. Mater. 2010, 22, 4220–4248

N

N

n = 2-5

O

O NH3

17

O

NH

NH

O O

OO

O

3

OH

OH

22 23


RO

GRES

S R

EPO

RT

Figure 10 . a) Schematic diagram of i-motif DNA and sequence of base with hybridized on two end-sides (50 and 30) of the i-motif DNA with a car-boxyl functionalized C 60 . The C + :C hemiprotonated base pairs are the ‘‘building blocks’’ for quadruplex formation. The contraction is induced by the folding of the 21-base oligonucleotides that contain six C + :C hemiprotonated base pairs at pH 5. The DNA has three TAA loops (yellow line) which are following Hoogsteen base pairs. b) Representative models of a C 60 -DNA nanomachine and the working switching cycle by protons in the absence of fuel DNA. Reproduced with permission [ 55] .

wall functionalization also occurs ( Figure 14 ). Probably, –COOH groups present in wall-defects were also amidifi ed.

On the other hand, Fang and coworkers reported three-step chemical functionalization of multi-walled NTs (MWNTs) with C 60 . [ 82 ] They fi rst hydroxylated MWNT by treating with KOH, then reacted with aminopropyltriethoxysilane (APTES). Finally, fullerene was anchored by nucleophilic addition of the amino groups ( Figure 15 ). The so-obtained C 60 -decorated MWNT hybrid has proven to be better fl ame retardant that pristine nanotubes. Finally, a covalent hybrid between two


Figure 11 . Sequence of TEM images showing irradiation induced coales-cence of C 60 within a SWNT. (a) Starting confi guration. (b-h) Consecu-tive images recorded at 60-90 s intervals. Reprinted with permission [ 74 ]. Copyright 2003 ACS.

different allotropes of carbon, namely C 60 and diamond, has been prepared by deposition of evaporated fullerene onto the bare-diamond surface. [ 83 ] The so formed interphase showed to be robust and stable to electrochemical investigation. Once C 60 has been linked, the interphase resulted to be passivated against oxidation and hydrogenation. Heating over 500 ° C produced desorption of C 60 molecules. Hydroxylated diamonds have also been used for the covalent linking of C 60 F 48 molecules, although they decompose by annealing over 200 ° C. [ 84 ]

8. Donor-Acceptor C 60 Containing Polymers

8.1. Miscellaneous Polymeric π -Donor Backbones

The design of intrinsically ambipolar electron donor–electron acceptor (D-A) polymers in order to simultaneously control the electronic properties and the degree of D–A phase separa-tion within the photoactive layer has attracted much of atten-tion because of its practical aspects. The covalent binding of acceptor moieties such as fullerenes onto π -conjugated polymeric backbones appeared as the fi rst choice toward the preparation of such ambipolar organic semiconductors. [ 85 ] To date one of the most promising practical application of D-A fullerene polymers is their use as active layer in organic photo-voltaic devices. [ 86 ]

In 2006, Li and coworkers reported the synthesis of a perylene-porphyrin-fullerene triad copolymer synthesized trough Sonogashira and Suzuki coupling protocols ( 26 , Figure 16 ). [ 87 ] Good photoconductivities have been measured and the photoconductivity could be enhanced by increasing the fullerene content. The authors observed that rapid, steady, and reproducible cathodic photocurrent was produced when a fi lm of polymer 26 was irradiated with a white light. In addition, they argued that π − π interactions are responsible for the formation


4230


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Figure 12 . TEM images for the single chain phase of C 60 molecules in (a,b) a SWNT, (c) a DWNT, and (d) a TWNT. Close views magnifi ed twice for the highlighted parts indicated by the arrows in parts a and d are shown in parts e-g. The scale bar is 5 nm. Reprinted with permission [ 77] . Copyright 2003, ACS.

of spherical superstructures with diameters in the 25–100 nm range, as evidenced by SEM and TEM studies.

The synthesis and photovoltaic characteristics of new donor–acceptor materials based on random polynorbornenes bearing pendant phthalocyanine (Pc) and fullerene units ( 27a-b ) pre-pared by using ruthenium catalyzed ring-opening metathesis polymerization (ROMP) technique has recently been reported. [ 88 ] Long-lived charge-separated states (average lifetime of 0.4 ms) formed by photoinduced electron transfer from the donor Pc units to the C 60 acceptor units of the copolymers have been detected. Preliminary photovoltaic devices based on copoly mer 27a have been prepared displaying very modest conversion and photocurrent ( ∼ 0.07% under AM1.5, 100 mW·cm − 2 ) although the copolymer absorbs photons with the wavelength of up to 800 nm.

Also block copolymers 28a-f have been prepared by means of living ROMP polymerization (Figure 16 ). [ 89 ] The authors fi rstly polymerized the donor block and, subsequently, the C 60 -con-taining monomer in the presence of Grubbs’ fi rst generation catalyst were added and polymerized. The so-obtained polymers formed 1D nanostructures when spin-coated or drop-casted from their chloroform solutions ( Figure 17 ). TEM images showed that such nanowire-like aggregates are composed by alternating D-A domains disposed perpendicularly to their long axes in which the diameter seems to be determined by the length of the copolymer chain while the internal domain size is determined by the width of the polymer building block. Inter-estingly, 28a showed a high charge mobility of 0.26 cm 2 V − 1 s − 1 ,


in the range of those displayed by highly ordered columnar assemblies of discotic materials. Furthermore, fi lms of the same copolymer assembled onto gold gap electrodes give rise to per-sistent photocurrent switching after several light/dark cycles.

Very recently, Manners, Sargent, and co-workers have reported the synthesis of a series of polyferrocenylsilane random copolymers containing covalently bound pendant [60]fullerene cages. [ 90 ]

The syntheses were performed as depicted in Scheme 4 . After preparing polymers 29a-d by Pt-catalyzed ring opening polymerization, the resulting copolymers were functionalized with 11-azido-1-undecanol in the presence of triethylamine and fi nally reacted with C 60 to obtain polymers 31a-d . The C 60 -containing polyferrocenylsilanes resulted air-stable and soluble in several solvents and were explored as active layer in an all-solid-state photodiode. The device prepared by spin-coating 10 wt% of a solution of 31b,c in chlorobenzene onto PEDOT:PSS/ITO, with Mg as contact metal protected by a Ag layer, showed photoconductive and photovoltaic responses under white light illumination, although small photocurrents on the order of nanoamperes were measured.

On the other hand, Natori reported the synthesis of a C 60 -end-cappped poly( N -vinylcarbazole) (PVK) by simply grafting poly(N-vinylcarbazolyl)lithium chains onto the fullerene cage. [ 91 ] Kang and co-workers exploited the D-A behavior of a soluble and processable PVK-C 60 polymer to fabricate a nonvolatile fl ash memory device. [ 92 ] In fact, the device with architecture ITO/PVK-C 60 /Al exhibited a high ON/OFF



RO

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S R

EPO

RT

no C 60 in its structure.

O

O

O

O

N

RNH

N

R NH

N

RHN

O

N

R

NH

N

RHN

N

RHN

N

RHN

O

O

N

Fe

25a-c

a

R =

N

N

N

NZn

b c

NN

F3C

CF3

HN

O

O

NN

CF3

F3C

NH

NN

F3C

CF3

HN

NN

CF3

F3C

NH

O

O

CO2C5H11C5H11O2C

C5H11O2CCO2C5H11

24

Figure 13 . Structures of SWNT-C60 hybrids 24–25 .

current ratio and behaved as a rewritable memory with acces-sible electronic states that could be written, read and erased, working for more than 100 million read cycles under ambient conditions.


Finally, a poly- p -phenylenevinylene-polystyrene block copolymer has been prepared in which the pendant fullerenes deeply affect the self-assemblying behavior of the parent poly mer with

[93]


4232


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Figure 14 . HR-TEM images for nanohybrid 25b showing dark spherical features (attributed to the [60]fullerene cage) almost in front of each other separated by two dark lines representing the SWCNT’s walls. Inset: zoomed region. Reprinted with permission from [81] . Copyright 2003 Elsevier.

8.2. Polythiophene-Based Donor-Acceptor Fullerene-Polymers

D-A double-cable polymers (DC) represent the most exciting examples, under a chemist viewpoint, in which the researchers have developed all their imagination in order to overcome the major problems dealing with this kind of mate-rials: the solubility and hence the processability of such poly-mers as well as the high C 60 content. These are fundamental issues to be addressed for better performing solar cells. [ 94 ] Several groups have focused their researches on polythi-ophene (PT) as electron-donor materials because of its high hole mobility, relatively low band gap and processability. [ 95 ] In this regard, major progress has been reported in 2007, when a C 60 moiety was introduced in every two units of thiophene of a polythiophene-based DC-polymers ( 32 , Figure 18 ). [ 96 ] The authors observed that acceptors and donors preserved their original electronic properties and no ground state inter-actions were found. On the contrary, strong interactions between the C 60 units and the donor backbone emerged in the excited state. A remarkable power conversion effi ciency of


0.52% was reached when polymer 32 was sandwiched onto a photovoltaic device and irradiated under AM1.5, 100 mW/cm 2 conditions, being this effi ciency the best ever reported for a DC polymer.

One year later, Dagron-Lartigau reported two different DC-polymers: the fi rst one was a random copolymer, 33 , whilst the second one was a block-copolymer in which the fi rst block was random as the previous polymer, but the second block was constituted by 3-hexylthiophene units, 34 , (Figure 18 ). [ 97 ] Once again, no interactions in the ground state were observed between donor and acceptor units, and the block copolymer 34 showed improved quenching of the photoluminescence in com-parison with 33 as well as a better control on the nanophase separation with a more homogeneous morphology with small aggregates.

Very recently, a doubly C 60 -end-capped regioregular poly-3-hexylthiophene (P3HT) has been prepared by means of Grig-nard metathesis (GRIM) and subsequent post-polymerization functionalization (Figure 18 , polymer 35 ). [ 98 ] This interesting macromolecule presents two different semicrystalline regimes,



RO

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EPO

RT

Figure 15 . Schematic synthesis route for C 60 - d -CNTs. Note: the self-condensation of APTES actually occurred, as evidenced by IR spectrometry, but this is not described in the above scheme. Reprinted with permission from [ 82]. Copyright 2009 RSC.

as shown by powder X-ray scattering. The authors argued that PT-rich and fullerene-rich domains are formed after micro-phase separation of end groups from the main chain.

Hiorns and co-workers reported a series of thiophene-C 60 polymers in which two or even three small PT chains were linked onto the fullerene surface both via pyrrolidine ring formation or via ATRA. [ 99 ] The photovoltaic behavior of some of these copolymers was explored, but only low effi ciencies ( η ≈ 0.05%) were obtained when the copolymer was used alone as the active layer. Improved effi ciencies were found when PCBM in 1:1 ratio was added ( η = 0.2%).

Besides the chemical formation of PT-based DC-polymers, another strategy involves the electropolymerization of suitably functionalized thiophene, bisthiophene and terthiophene mon-omers. In this light two different papers appeared in 2007 in which a terthiophene and a thiophene derivatives were used as DC precursors ( Figure 19 ).

In the fi rst one, Wallace electropolymerized monomer 36 on a stainless steel mesh substrate galvanostatically, and this material was in turn used as the cathode in lithium cells. [ 100 ] Surprisingly, after 50 testing cycles the discharge capacity of poly- 36 still was 2.5 times higher than that of the parent poly-terthiophene with no fullerenes. in the second example, ter-thiophene S , S -dioxide was reacted with C 60 by Diels-Alder cycloaddition to form monomer 37 after extrusion of SO 2 . [ 101 ] Afterwards, such a monomer was easily electropolymerized to produce double donor-acceptor molecular wires. Analogously, very recently, Chandezon and Sadki reported the synthesis, electropolymerization and properties of bisthiophenylcarbazole derivative 38 . [ 102 ] In solution, intramolecular charge transfer has been noticed and, little ground state interactions between the conjugate backbone and the fullerene moieties has been revealed by means of differential pulse voltammetry.


Much attention has been paid to the development of a new strategy involving the synthesis of PT-based block copolymers, especially for photovoltaic purposes. [ 103 ] A very interesting example has been reported by Holdcroft and coworkers who prepared a PT endowed of a polystyrene fragment departing from each repeating thiophene unit ( 40 , Scheme 5 ). [ 104 ] The polystyrene chains were grown through nitroxide-mediated radical polymerization from the suitable monomer 39 , and sub-sequently covalently modifi ed with fullerene to afford the title poly mer 41 . It is worth noting that in polymers with a C 60 con-tent as high as 47 wt%, the electronic excitation of the PT-back-bone was completely quenched by fullerenes, whilst in blends of C 60 and P3HT the quenching was only partial. This fi nding proves the unique donor–acceptor properties of polymer 41 , making it a good candidate for organic solar cells.

In a more recent example, Brochon and Hadziioannou pre-pared and characterized a number of block copolymers in which to the GRIM synthesized regioregular P3HT block, a radical initiator has been attached in order to grow the second block (poly(butylacrylate- stat -chloromethylstyrene) ( 42 , Figure 20 ). [ 105 ] In this way, they obtained several interesting rod-coil block copolymers to be further investigated as devices’ active layers.

A third PT-based block copolymer was reported in 2009. Once again, the regioregular P3HT fragment has been synthesized via GRIM polymerization and, in turn, methylmethacrylate and 2-hydroxyethylmethacrylate were copolymerized via ATRA to constitute the second block. [ 106 ] Fullerene was then introduced by esterifying with a PCBA analogue ( 43 , Figure 20 ). TEM images of thin fi lms of polymer 43 showed that, after annealing, the copolymer presents nanometric phase separation, making 43 suitable candidate for photovoltaic devices due to its good morphology for enabling the easy movement of excitons from donor to acceptor domains.


4234


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RT

N

N

O O

OO

N

N

N

N

Zn

OO

O O

O O

n m p

26

mn

O

N

N

N

N N

N

N

N

Zn tButBu

tBu27a n = 2, m = 327b n = 1, m = 4

OO

N

Ph

N NOO OO

n m

X

N

Y

N

N

N

N

RZnR

R

(OCH2CH2)3OCH3R =

D1: X = -(OCH2CH2)3)O-D2: X = -O(CH2)6O-A1: Y = -O(CH2)8O-A2: Y = -O(CH2)12O-A3: Y = -(OCH2CH2)3)O-

28a poly(D1)20-block-poly(A1)2028b poly(D1)20-block-poly(A1)728c poly(D1)20-block-poly(A2)2028d poly(D1)20-block-poly(A2)728e poly(D1)20-co-poly(A2)2028f poly(D2)20-block-poly(A3)20

Figure 16 . Chemical structure of D-A-polymers 26–28.

Finally, Wudl, Heeger, and co-workers described a few months later the synthesis, characterization, and application in photovoltaic devices of the novel block copolymer 44 . [ 107 ] After GRIM polymerization, the P3HT fragment has been in turn copolymerized in the presence of styrene and a suitably func-tionalized acrylate monomer able to subsequently incorporate fullerene. The so obtained rod–coil block copolymer 44 , showed an interesting nanofi brillar structure and it has been used at var-ious concentrations as surfactant/compatibilizer for the active layer of bulk-heterojunction solar cells in blends with PCBM. This approach resulted in 35% increase of the photocurrent effi ciency, arising this from 2.6 to 3.5% when the copoly mer was used in 5 wt%. The outstanding enhancement has been


accounted for by the authors to the improvement in the biconti-nous interpenetrating network due to the compatibilizing action of the copolymer, as also evidenced by AFM studies.

9. Supramolecular C 60 -Polymer Systems

In the last recent years a growing interest has been devoted to the synthesis of supramolecular polymers of C 60 gaining momentum as an effective method for synthesizing functional, novel carbon-based materials. [ 5 , 108 ]

In 2006 Liu reported the formation of a supramolecular polymer by the inclusion of C 60 in the wider rim of the



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RT

Figure 17 . A,B) SEM and TEM images respectively of 28a nanowires dropcast from CHCl 3 solution. C) SEM image of 28b nanowires. D,E) 3D-AFM and TEM images respectively of 3c nanowires. F) TEM image of 28c nanowires with mutually aligned internal domains. Reprinted with permission from [89] . Copyright 2009 ACS.

Tb 3 + CD dimer complex 47 and its exhaustive characterization ( Scheme 6 ). [ 109 ]

The formation of the superstructure has been unambigously demonstrated by means of FTIR, UV-vis, and XRD techniques. TG-DTA studies suggested a 1:3 molar ratio between 47 and C 60 , hence three fullerene cages for each six CD units. STM

© 2010 WILEY-VCH Verlag GAdv. Mater. 2010, 22, 4220–4248

Scheme 4 . Synthesis of polyferrocenylsilanes 5a–d

Fe Fe

Si

Me Ph

Et3Si SiMe Cl

H

29a: m:n = 93:7; 29b: m:n 87:1329c: m:n 80:20; 29d: m:n 76:24

HO(CH2)11N3

Et3N / Toluene Et3

C60

Toluene Fe Fe

Si

Me Ph

Et3Si SiMe

O(CH2)11

H

31a-d

m n

m n

images of dilute solutions of 3 (1 × 10 − 5 M) showed a double-lined array (width 5.5 nm, height 2.2 nm ca.) that defi nitely assembly into a trifoil bundle-shaped nano-supramolecular structure ( Figure 21 ). Interestingly, the Tb 3 + complexes still retain their luminescence properties although a quenching due to energy transfer to C 60 occurs (in comparison with pure 47 ).

4235mbH & Co. KGaA, Weinheim wileyonlinelibrary.com

Fe Fe

Si

Me Ph

Si SiMe

O(CH2)11N3

H

30a-d

m n

N

4236


PRO

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RT

C7H15

SS

OO

n

32

S SSBr

C6H13

OO

8

N

x y y'

7

S

C6H13

Hz

34 x = 25%; y = 10%; y' 15%; z = 50%

S

C6H13

n

N N

S SSBr

C6H13

OO

8

N

x y y'

7

33 x = 75%; y = 10%; y' 15%

35

Figure 18 . Structures of C 60 -containing polythiophenes 32–35 .

Recently, the same group has obtained linear supramo-lecular architectures in water by complexing a fullerene-bridged bis(permethyl- β -cyclodextrin) together with tetrakis(4-sulfonat-ophenyl)porphyrin, both in the free base or as the zinc complex form. [ 110 ] The complexation and formation of linear superstruc-tures has been confi rmed by NOESY experiments as well as TEM, SEM, and AFM. It is noteworthy that the photoinduced electron transfer (PET) process takes place in both complexes as noticed in fl uorescence decay and nanosecond transient absorption experiments.

Analogously, Jayawickramarajah and co-workers have also prepared water-soluble nanorods that comprise double-sided zinc porphyrin endowed with four permethyl- β -cyclodextrins on each side and pristine fulllerene. [ 111 ]

In 2007, Tani described the crystal structure of a tubular supramolecular complex formed by cyclic cobalt-porphyrin dimers (Ni 2 CPD Py ), inside of which a fullerene was hosted. [ 112 ] In solution, the rectangular Ni 2 CPD Py (11.6 × 13.6 Å) complex


Figure 19 . Electropolymerizable C 60 -containing thiophene monomers 36 – 38 .

N S

S

S

S

36

C 60 with an association constant of 2 × 10 5 M, and such com-plexes could be easily detected by ESI-MS. The reddish-black crystals of the supramolecular tubular complex were obtained by adding an equivalent of fullerene in toluene during the Ni 2 CPD Py crystallization in chloroform. As can be noticed in Figure 22 , a tubular assembly is formed – through C-H···N hydrogen bonds and π − π interactions of the pyridyl groups – in which the fullerenes are linearly disposed. Successively, photo-dynamic studies have been carried out on this system. [113] Inter-estingly, anisotropic high electron mobility along the fullerene array has been determined by means of fl ash-photolysis time-resolved microwave conductivity measurements.

On the other hand, a fullerene array has been obtained by simply ordering C 60 molecules inside helical syndiotactic polymethylmethacrylates ( st -PMMA). [ 114 ] Yashima and co-workers found that upon heating st -PMMA and C 60 in toluene at 110 ° C, followed by cooling to room temperature, a gel was formed ( Figure 23 ). They were able to trap up to 23.5 wt%


S

37

N

S S

5

38

N


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EPO

RT

Scheme 5 . Synthesis of polythiophene-based polymer 41 .

S n

O N

39

C6H13

125 ºC

Cl

+

S

C6H13

Cl

x

y

n40

C60BiPy

CuBr, CuS

C6H13

x

y

n41

z

x ~ 16.4; y ~ 7.5; z ~ 0.53

of C 60 roughly fi lling 86% of PMMA helical hollow, when 1,2-dichlorobenzene was used as co-solvent. Also C 70 and C 84 were encapsulated onto st -PMMA forming new gels and fi lms, respectively, in which the polymer adapts its helical cavity in order to better fi t these wider guests. Furthermore, in the case of C 60 , the helical sense may be easily modulated by mixing (R)- or (S)-1-phenylethanol, thus leading to chiral materials. [ 115 ]

Once solubilized in polar solvents such as acetonitrile or ace-tone (in which fullerene is not soluble), st -PMMA was able to incorporate fullerene by forming inclusion complexes. [ 116 ] After evaporation of acetonitrile, homogeneous fi lms with high heat resistance and UV-fi ltering properties were formed.

Analogously, it is very likely that even when fullerene was mixed with polystyrene–poly(dimethylsiloxane) PS–PDMS diblock copolymer, random helical complexes were formed. [ 117 ]


Figure 20 . Structures of PT-C 60 block copolymers 42–44 .

SSBr

n

C6H13

ONH2

x y z

OO

N

S

C6H13

S

n

O

OO

x

O

C6H13

S S

S7

42

44

Carboxylated PDMS has been supramolecularly crosslinked with [1-(4-methyl)piperazinyl] 9 fullerene in order to produce a composite with enhanced thermal mechanical stability, storage and loss moduli. [ 118 ]

In the examples above examined, nanotubular or helical structures accommodate fullerene as an array in their inner cavities. Very recently, Granja et al. reported a nice example of supramolecular arrangement having the fullerene moieties allo-cated in parallel outer of the nanotubular surface formed from the self-recognizing octacyclopeptide 49 ( Figure 24 ). [ 119 ] This 1D arrangement may be directed by salt-bridge interaction, Figure 24 a, or induced by surface properties, namely anionic mica may direct all the Arg residue toward the same side, ori-enting the fullerenes into two parallel wires (Figure 24 b). This outstanding new approach pave the way to the preparation of novel supramolecular organization in which electron donor and


n

O

O

Br

O OO

x y

OO

y

O

43

4238


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RT

Scheme 6 . Reproduced with permission from [109] . Copyright 2006 ACS.

acceptor moieties should lie on the different side of a nanotube, constituting the elemental unit of an electron/hole highway. [ 120 ]

Another type of monodimensional assembly has been very recently reported by Ogawa and co-workers. [121] They prepared photo-responsive electronic devices by bridging the nanoscale gap between gold electrodes (10 nm), with a porphyrin polymer endowed with thiol groups at both ends ( ∼ 17 nm) complexed with a fullerene derivative bearing a pyridinyl moiety ( Figure 25 , 50 ). Interestingly, the so-obtained devices showed photocurrent during light irradiation only when both fullerene derivative and poly-porphyrin were present. These authors were also able to determine that multiple conduction mechanisms take place: tunneling at low temperatures and thermoionic emission at high temperatures.

Very recently, Martín and co-workers have described the ability of an extended tetrathiafulvalene (exTTF) isophthalic


Figure 21 . a,b) STM images of assembly 48 on a HOPG surface (tunneling curof 48 . Reproduced with permission [ 109 ] . Copyright 2006 ACS.

diester molecular tweezer to recognize and complex C 60 through complementary π − π concave-convex orbital overlapping interactions [ 122 ] enhanced by mean of donor-acceptor interac-tions. [ 123 ] These fi ndings have been exploited by the authors to synthesize and study the behavior of self-complementary mon-omers in which a fullerene moiety is covalently linked to one, 51 , [ 124 ] or two pincer-like exTTF, 52 , [ 125 ] dimers ( Scheme 7 ). In such a way, they have obtained self-organized supramo-lecular oligomers and polymers or polydisperse dendrimers, as evidenced by means of NMR studies (variable concentration experiments and pulsed-fi eld gradient), MALDI-TOF, DLS, and AFM. It is noteworthy that in both cases cyclic voltammetry and UV-vis spectra showed electronic communication between the electroactive parts in the ground state, which is further enhanced by the complexation. Further photophysical studies by means of fl uorescence and transient absorption spectroscopy revealed


rent 1.0 nA). c) Line profi le of image shown in (b). d) Schematic structure


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Figure 22 . Crystal structure of tubular assemblies of C 60 @Ni 2 -CPDPy. Hydrogen atoms are omitted for clarity. a) front view; b) side view; c) top view. In (a, b), Ni green, N blue. Adapted with permission [112] .

that, upon light irradiation, a photoinduced charge separated state is formed in the exTTF molecular tweezer/[60]fullerene complex, with lifetimes in the range of a few picoseconds, due to the close proximity between both electroactive units. [ 126 ]

Interestingly, unexpected self-association between exTTF and C 60 moieties in exTTF-oligo- m -phenyleneethynylene-C 60 triads has been very recently detected by means of MALDI-TOF spectrometry. [ 127 ] Subsequently, this fi nding has also been confi rmed by NMR studies and AFM spectroscopy. Moreover, fl uorescence spectroscopy allowed to calculate the association constant (K ass ∼ 10 3 M − 1 ).

10. Self-Assembling C 60 -Macromolecular Systems

Self assembly refers to the natural tendency of molecules and macromolecular systems to associate spontaneously to form, by means of non-covalent bonds, ordered aggregates with well-defi ned shapes and/or functions, in a process which is widely common throughout nature. [ 128 ] In the last two decades self-assembly has emerged as a novel and useful strategy for syn-thetic chemists, due to its potential for generating non-biological


structures in the nano- and micro-metric scale. [ 129 ] In this con-text, in the last recent years several groups investigated the self-assembling processes involving fullerene-containing polymers, especially exploiting the hydrophobic C 60 core that strongly infl uences the equilibrium conditions of well-known behaving polymers, leading to novel spectacular superstructures.

In 2006, Ikkala and co-workers showed that charge transfer complexation between polystyrene- block -poly(4-vinylpyridine) (PS-b-P4VP) occurs at the pyridine level of the copolymer. [ 130 ] Interestingly, the morphology of C 60 /PS-b-P4VP mixtures changes from cylindrical to spherical when xylene solutions were aged and casted. The authors argued this change in mor-phology was due to the fact that a single fullerene molecule can bind several pyridine moieties.

At the same time, Yashima and co-workers fi rstly prepared stereoregular C 60 end-capped it - and st -PMMA via stereospecifi c anionic living polymerization of methylmethacrylate (MMA). [ 131 ] Such polymers independently aggregate in H 2 O/MeCN (1:9 v/v) mixtures to form self-aggregated core-shell supramolecular nanospheres with the fullerene moieties allocated at the core due to their solvophobic behavior. Interestingly, they found that, when mixed two fresh solutions of it -PMMA-C 60 and


4240


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Figure 23 . a) Schematic illustration of the encapsulation of C 60 in the st -PMMA helical cavity upon gelation. Right- (blue) and left-handed (green) helical complexes are equally produced. b) Photographs of a toluene solution of C 60 (1 mgmL − 1 , 1 mL; left), st -PMMA/C 60 gel after the addition of st-PMMA (10 mg) with subsequent heating to 110 ° C and then cooling to room temperature (middle), and st-PMMA/C 60 complex gel after centrifugation (right). Reproduced with permission from [114] .

st -PMMA-C 60 aggregates forming spherical nanoparticles as the stereocomplex ( Figure 26 ), as evidenced by DSC, SEM and TEM analyses. Surprisingly, when solutions containing aggregates of it - and st -PMMA-C 60 were mixed, large tridimensional nanon-etworks were formed in which it - and st -PMMA-C 60 clusters self-assembled with each other through iterative stereocomplex formation. The authors found these monoaggregates thermally stabilized and controlled the morphology directly acting on PMMA’s molecular weights.

© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weiwileyonlinelibrary.com

Figure 24 . Model for peptide nanotube formation from α , γ -CP-1 49 in which the self-assembly p(red and blue balls) or (B) induced by the surface properties. Adapted with permission [119]. Co

Liu et al. prepared a PEO-(C 60 )- b -PS block copolymer ( 53 ) by combining end group transformation and ATRA strategies ( Figure 27 a ). [ 132 ] Once prepared, the so syn-thesized amphiphilic block copolymers were dissolved in dioxane and, after adding water slowly under vigorous stirring, self-assembled forming unilamellar vesicular nanostructures ( Figure 27 b–c ) in a hollow sphere fashion.

On the contrary, lamellar, hexagonal packing of cylinders and gyroid microstruc-tures have been obtained by self-assembling of star fullerene-polyisoprene- b -polystyrene C 60 (PI- b -PS) block copolymers simply by var-ying the monomers ratio. [ 133 ] A PEG-type of block copolymer with hexadecaaniline (PEG-C 60 -A 16 ) has been successfully self-assembled into packed Langmuir monolayer at air-water interface. [ 134 ] This donor-acceptor layer has subsequently been transferred onto a glass via Langmuir-Blodgett technique, forming a multilayer showing strong intermolecular electronic interactions between fullerenes and A 16 fragments. Finally, the supramo-lecular self-assembly has been photopoly-merized at the oligoaniline level in order to obtain an irreversible cross-linked fi lm.

Recently, the search for durable and thermally stable self-assembled mate-rials prompted Nakanishi to cross-link the

fullerene derivative endowed with three diacetylene groups 54 once it was organized into fl akelike nano- and microparticles in order to fi nally freeze it ( Figure 28 ). [ 135 ] The fi nal material showed remarkable resistivity both to heat and organic solvents, displaying high water repellency.

In the recent past, several stimuli-responsive C 60 -containing polymers—smart macromolecules displaying drastic changes in properties after small variations of temperature, pH, or ionic species—have been synthesized and their aggregation

nheim Adv. Mater. 2010, 22, 4220–4248

rocess can be directed by (A) salt-bridge interactions pyright 2009 ACS.



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Figure 25 . Structure of supramolecular donor-acceptor polymer 50 .

Scheme 7 . Formation of linear ( 51 ) and dendrimeric ( 52 ) supramoleculardonor exTTF units and fullerenes.

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behavior has been investigated ( Figure 29 ). In 2007, Gan reported the synthesis of a water-soluble [C 60 -DMAEMA-C(CH 3 ) 2 COOCH 2 ] 2 , 55 , fullerene-end-capped at both ends. [ 136 ] Flower-like micelles were observed when 55 was dissolved in unbuffered water with an aggregation number larger than in pH = 3 solution, although in the latter the micellar size was bigger due to the strong charge repulsion in the polymeric backbone.

Among the stimuli-responsive polymers, poly(N-isopropy-lacrylamide) (PNIPAM) is probably the most studied because of its temperature-tunable solubility. It readily dissolves in cold aqueous solution but PNIPAM became reversibly insoluble because of rapid and reversible hydration-dehydration cycles by heating above 32 ° C, its lower critical solution temperature


structures based on π − π concave-convex interactions between electron

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Figure 26 . Schematic representation of the formations of core − shell nanospheres and nanonetworks based on it - and st -PMMA-C 60 ’s through their stereocomplex formation combined with self-assembly of the terminal C 60 . Reproduced with permission from [131] . Copyright 2006 ACS.

(LCST). [ 137 ] Geckeler and co-workers prepared a hybrid PNIPAM-C 60 end-capped polymer ( 56 ) by reversible addition-fragmentation chain-transfer (RAFT) polymerization. [ 138 ] Such polymers were found to be soluble in common organic solvents


Figure 27 . a) Chemical structure and schematic illustration of the fabricativia the self-assembly of PEO-(C 60 )- b -PS 53 in water; b), c) HRTEM images oself-assembled hybrid vesicles in aqueous solution using 1,4-dioxane as thewith permission from [132] .

as well as in water, in which it showed the same LCST than the parent PNIPAM. Moreover, in water 56 undergoes self-association by forming micelles of ∼ 1 μ m of diameter and it was found to possess interesting radical scavenging activity, as

bH & Co. KGaA, Weinhei

on of hybrid vesicles f PEO-(C 60 )- b -PS 53 cosolvent. Adapted

evidenced by cell viability and metabolic activity tests carried out with fi broblasts and NOR-3 radicals. The same authors, one year later, described the synthesis and characterization of two C 60 -end capped PNIPAM polymers terminated at the other end with an electroactive tris(bipyridine)ruthenium complex ( 57a-b ), in which the PNIPAM was composed by 78 and 146 units, respectively. [ 139 ] These polymers formed core-shell micellar microstructures in water, with the fullerenes allocated at the core ( Figure 30 ). Interestingly, the self-assembled vesicles showed micrometric diameters (Figure 30 a) which turned into vesicles nanometric in size after 3-fold dilution (Figure 30 c,d). Further dilution (10-fold with respect to the original concen-tration) resulted in changes in the shape of the aggregates, namely from spherical to rod-like (Figure 30 e,f).

More recently, a pyrene-PNIPAM-C 60 ( 58 ) and pyrene-PNIPAM-(C 60 ) 2 ( 59 ) with two fullerenes allocated along the poly-meric backbone, have been reported. [ 140 ] These two polymers were tested on tem-perature-dependent quenching of the pyrene fl uorescence. Both polymers suf-fered drastic on-off quenching in sharp temperature intervals. This sensor-type behavior is due to the drastic changes in the conformation of the main chain, from coil to globule, which strongly reduces the distance between C 60 and pyrene units, thus increasing the quenching effi ciency.

m Adv. Mater. 2010, 22, 4220–4248


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Figure 28 . Schematic representation of the photo-cross-linking process in the bilayer structural subunit of the fullerene endowed with diacetylene groups 54 and fl akelike particles formed upon cross-link. Adapted with permission from [135] .

As already seen for the covalent linking of fullerene with the viral nanoparticles bacteriophage Q β in order to obtain hybrid 9 (Scheme 3 ), the Huysgen’s “click” protocol has been recently used with success for the chemical functionalization of fullerenes. [ 141 ] In fact, following such a chemical strategy both hexakis-adducts [ 142 ] and C 60 -polymer conjugates [ 143 ] have been prepared. In this respect, three novel PNIPAM-C 60 hybrids through “click” coupling protocols were prepared: mono-C 60 end-capped PNIPAM ( 60 ); (C 60 ) 2 -PNIPAM ( 61 ) and PEG-C 60 -PNIPAM copolymer ( 62 ) ( Figure 31 ). [ 144 ] Interestingly, all


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Figure 29 . Structure of thermoresponsive C 60 -containing polymers 55–59 .

the hybrid polymers retained the thermoresponsiveness afforded by PNIPAM fragments in aqueous solutions, in which supramolecular self-assembly exhibited thermo-induced collapse-aggregation cycles due to the LSCT phase transitions.

11. Outlook and Perspectives

In recent years we have witnessed the development of a variety of new materials constituted by fullerenes and different


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© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2010, 22, 4220–4248

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Figure 30 . TEM images of Ru(bpy) 3 2 + -PNIPAM 78 -C 60 57a (a, c, e) and Ru(bpy) 3 2 + -PNIPAM 146 -C 60 57b (b, d, f), in water at different concentrations (for images a–b, the sample concentration was 5.0 mg/ml; for images c and d, the sample concentration was 1.67 mg/ml, for images e and f, the sample concentration was 0.5 mg/ml,). Reproduced with permission from [139] Copyright 2009 RSC.

Figure 31 . C 60 end-capped PNIPAM polymers 60-62 prepared via “click ” chemistry.


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polymers combining the singular properties of fullerenes with the many advantages related to processing polymers. From the earlier times of this new interdisciplinary fi eld in polymer chemistry where efforts were focused on synthetic aspects, nowadays the interest is mainly devoted to the understanding of the unusual properties of these new materials as well as on their potential applications in a variety of areas ranging from mechanical to opto-electronic technological usefulness.

The aim of this review is to show to the reader the different goals achieved so far, organized in a systematic way in which compounds have been gathered according to the features and properties they exhibit. This will allow to gain a better under-standing of the different chemical structures prepared in the search for a specifi c property as well as the different approaches to solve a specifi c problem. Thus, it is surprising to see how almost antagonistic aspects such as water-soluble or biodegrad-able fullerene-containing polymers have become a reality of great technological interest. In this regard, those new hybrid materials formed by combining fullerenes with other well-known technological compounds such as silica, DNA or carbon nanotubes are expected to open the way to new applications in a variety of fi elds, including biomedical applications.

Because of the interest in fullerene-containing semicon-ducting polymers in the photovoltaic fi eld, the most recent achievements have also been presented in this review. Interest-ingly, despite the conceptual beauty of the double-cable approach to organic photovoltaic solar cells, effi ciencies achieved by means of covalently linked fullerene-polymer systems are rela-tively low when compared with those obtained by simply mixing both photo- and electro-active components. In this regard, the last part of this work is centered on the great advances achieved along the last recent years on the supramolecular chemistry of this interdisciplinary fi eld. New structures have been achieved forming materials that exhibit unprecedented properties where order plays a fundamental role. No doubt, supramo-lecular organization and, more particularly, self-assembly have emerged as useful strategies for generating artifi cial micro- and nanostructures where the hydrofobicity of fullerenes has been successfully exploited. A variety of formations such as nano-spheres, vesicles, fl akelike and rodlike particles, etc., in which further photo-crosslinked processes are possible, represent a new scenario where designing properties at will in the search for specifi c technological application is becoming a reality.

Polymers are certainly among the most important contri-butions of chemistry to address societal needs. A new genera-tion of polymers enriched with fullerenes is opening up new avenues where the imagination of the chemist will be the only limit in the search of new properties and applications.

Acknowledgements Financial support from the MICINN of Spain (projects CTQ2008-00795, Consolider-Ingenio 2010 CSD2007-0010, Nanociencia Molecular) and the CAM (project S2009/PPQ-1533 Madrisolar-2) and from University of Palermo and Italian MIUR (project 2008KRBX3B) are greatly appreciated.

Received: January 9, 2010Published online: August 26, 2010

© 2010 WILEY-VCH Verlag GmbAdv. Mater. 2010, 22, 4220–4248

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New Concepts and Applications in the Macromolecular Chemistry of Fullerenes