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Coordination Chemistry Reviews 282–283 (2015) 87–99

Contents lists available at ScienceDirect

Coordination Chemistry Reviews

j ourna l h om epage: www.elsev ier .com/ locate /ccr

eview

n the role of ligand-field states for the photophysical properties ofuthenium(II) polypyridyl complexes

inchao Sun, Sandra Mosquera-Vazquez, Yan Suffren, Jihane Hankache, Nahid Amstutz,atévi Max Lawson Daku, Eric Vauthey, Andreas Hauser ∗

épartement de chimie physique, Université de Genève, 30 Quai Ernest-Ansermet, 1211 Genève 4, Switzerland

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 872. Setting the stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 883. The complexes, experimental details and computational methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 914. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

4.1. Ultrafast transient absorption in solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 914.2. [Ru(m-bpy)3]2+ and [Ru(terpy)2]2+ doped into the corresponding Zn matrices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

4.2.1. Temperature and pressure dependence of the luminescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 944.3. DFT calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

5. Discussion and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

r t i c l e i n f o

rticle history:eceived 8 May 2014eceived in revised form 7 July 2014ccepted 7 July 2014vailable online 31 July 2014

eywords:uthenium(II) polypyridyl complexesigand-field statesuminescence quenchinghotophysical propertiesigh-pressure

a b s t r a c t

The role of ligand-field states for the photophysical properties of d6 systems has been discussed in a largenumber of publications over the past decades. Since the seminal paper by Houten and Watts, for instance,the quenching of the 3MLCT luminescence in ruthenium(II) polypyridyl complexes is attributed to thepresence of the first excited ligand-field state, namely a component of the 3T1(t2g

5eg1) state, at similar

energies. If this state lies above the 3MLCT state, the luminescence is quenched via thermal population atelevated temperatures only. If it lies well below, then the luminescence is quenched down to cryogenictemperatures. In this contribution we present transient absorption spectra on non-luminescent ruthe-nium polypyridyl complexes such as [Ru(m-bpy)3]2+, m-bpy = 6-methyl-2,2′-bipyridine, in acetonitrileat room temperature, which reveal an ultra-rapid depopulation of the 3MLCT state but a much slowerground state recovery. We propose that in this and related complexes the methyl groups force longermetal-ligand bond lengths, thus resulting in a lowering of the ligand-field strength such that the 3dd

3 3

state drops to below the MLCT state, and that furthermore the population of this state from the MLCTstate occurs faster than its decay to the ground state. In addition we demonstrate that in this complexthe luminescence can be switched on by external pressure, which we attribute to a destabilisation of theligand-field state by the pressure due to its larger molecular volume compared to the ground state aswell as the 3MLCT state.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

The photophysical and photochemical properties of ruthe-ium(II) diimine complexes have been investigated by virtually

∗ Corresponding author.E-mail address: andreas.hauser@unige.ch (A. Hauser).

ttp://dx.doi.org/10.1016/j.ccr.2014.07.004010-8545/© 2014 Elsevier B.V. All rights reserved.

thousands of researchers over the past 50 years, the reason beingthat they are not only interesting for fundamental studies of,for instance, light-induced electron [1–4] and energy transferprocesses [5–9], but they are also becoming increasingly impor-

tant in technological and biomedical applications, such as solarenergy conversion [10,11], photo-catalysis [12,13], cancer pho-totherapy [14–18], and sensors [19,20]. This all goes back tothe pioneering work of Demas and Crosby [21], who reported

8 istry Reviews 282–283 (2015) 87–99

tfatbadtpohctccVscHsmcfcibpomopain(2rsestdftep

2

ptsirlfaddrltsa

Fig. 1. Potential energy diagram along a reaction coordinate involving essentiallythe Ru N bond lengths of the 1A ground state, the 1MLCT and the 3MLCT states, and

8 Q. Sun et al. / Coordination Chem

he intense orange luminescence for the model complex of theamily, namely [Ru(bpy)3]2+ (bpy = 2,2′-bipyridine) in 1971, andttributed it to luminescence due to the spin-forbidden transi-ion from a triplet metal-ligand charge transfer (3MLCT) stateack to the 1A1(t2g

6) ground state. Shortly thereafter, Harrigannd Crosby [22], postulated that the rather strong temperatureependence of both the lifetime and the quantum efficiency ofhe luminescence below 120 K is due to three close lying com-onents of the 3MLCT state in thermal equilibrium with eachther and each with its own lifetime and quantum efficiency. Atigher temperatures, both the observed lifetime and the lumines-ence intensity decrease rapidly. This, in turn, has been attributedo quenching by the thermal population of a non-luminescent,lose lying metal-centred 3dd state, namely the lowest energyomponent of the 3T1(t2g

5eg1) state of octahedral parentage by

an Houten and Watts [23]. In the following years, a number oftudies aimed to pin down this state in the parent [Ru(bpy)3]2+

omplex as well as in related ruthenium(II) diimine complexes.owever, despite intensive investigations using classical spectro-

copic methods [24–28] and lately also with modern ultrafastethods [29–33], the evidence for its existence and role remained

ircumstantial. For instance, it is generally held responsible not onlyor the quenching of the luminescence but also for the photode-omposition of ruthenium(II) complexes [23,23,34–39], as indeedn systems with ligands having a weaker ligand-field strength thanpy, the 3MLCT luminescence is quenched down to lowest tem-eratures and photodecomposition is often quite efficient [40–45],r yet again, encapsulating [Ru(bpy)3]2+ in a confining environ-ent such as the cavity of a zeolite [46] or a three-dimensional

xalate network [47] seems to destabilise the 3dd state via chemicalressure, so that the thermal quenching occurs at higher temper-tures. In this contexts we recently reported the observation of anntermediate state in the deactivation of the 3MLCT state in theon-luminescent [Ru(m-bpy)3]2+ and [Ru(tm-bpy)3]2+ complexesm-bpy = 6-methyl-2,2′-bipyridine, tm-bpy = 4,4′,6,6′-tetramethyl-,2′-bipyridine) with lifetimes of 450 and 7.5 ps, respectively, atoom temperature in solution. We attributed this intermediatetate to the elusive 3dd state [48]. In the present paper, we willxpand on this observation to other complexes using ultrafast tran-ient absorption spectroscopy and we will try to rationalise aso why the above complex is indeed the perfect complex for airect observation of the 3dd state, that is, as to why its populationrom the initially excited MLCT state is ultrafast, thus quenchinghe 3MLCT luminescence completely, and why its lifetime is nev-rtheless long enough for it to result in an observable transientopulation.

. Setting the stage

Extensive reviews of the photophysical and photochemicalroperties of ruthenium tris-diimine complexes and their applica-ions have been published at regular intervals over the past decades,ome containing several hundred references [49–53]. We do notntend to add another one to the list; rather we restrict the bibliog-aphy to the key publications with respect to the discussion of theigand-field states. As mentioned above, the seminal paper camerom Van Houten and Watts [23], who postulated that the thermallyctivated quenching of the 3MLCT luminescence of [Ru(bpy)3]2+ isue to the thermal population of the lowest energy 3dd state asepicted in Fig. 1. The key idea is that as in the (t2g

5eg1) configu-

ation of this state, the metal-ligand bond length is substantially

onger than both in the ground state as well as in the MLCT states,he non-radiative deactivation from this state as well as ligand dis-ociation are favoured. For [Ru(bpy)3]2+ in aqueous solution, anctivation energy of around 3200 cm−1 was determined [23,25].

1

the 3dd state. Radiative processes are shown as full lines, non-radiative processesas broken lines.

Already at this early stage, the discussion regarding the interpreta-tion of the observed activation energy was lively. The question was,does it correspond to the classical activation energy Ea across thebarrier or does it correspond to the zero-point energy difference�E0 between the two states in the form of a rapid pre-equilibriumbefore deactivation via the 3dd state [49–53]. Indeed, at that stage itwas not even clear as to whether for [Ru(bpy)3]2+ �E0 is positive ornegative. Of course, related systems were studied in order to arriveat a comprehensive and quantitative understanding of the quench-ing process, but even so, discussions and conclusions remainedcontroversial. At least for [Ru(bpy)3]2+ it is now generally acknowl-edged that �E0 is positive and indeed corresponds to the observedactivation energy [52], in particular based on the direct observa-tion of the decay of the 3dd state to the 3MLCT state and thermalequilibration with a time constant of 29 ns at room temperaturein aqueous solution via two-photon absorption by Thompson et al.[28]. Of course, in addition to the relaxation processes depictedin Fig. 1, ligand dissociation also plays an important role in thephotophysics and photochemistry of ruthenium(II) complexes, inparticular for very strained or monodentate ligands [14–17]. For thebi- and tridentate ligands of this study the quantum efficiencies ofthe photodissociation are <10−3 [25,48], and photodissociation assuch is not subject of the present work.

The complexity of the problem lies in the multitude ofpotentially competing processes having different temperaturedependencies and possibly also in the non-transferability of someof the key parameters as for instance structural parameters even forclosely related systems. In order to illustrate this let us just considerthe most simple model shown in Fig. 1, with an intrinsic decay rateconstant for the 3MLCT state, k , which comprises both radia-

MLCTtive and non-radiative processes from the 3MLCT state directly tothe ground state, forward and backward rate constants, k1 and k−1,for the internal conversion between the 3MLCT and the 3dd states,

Q. Sun et al. / Coordination Chemistry Reviews 282–283 (2015) 87–99 89

F k1, anc oximas

acw

lt(pdtmizsb[

dcttBrngrctpewbce

k

k

�i

ig. 2. Schematic representation of the dependence of the rate constants kMLCT, kdd,onstant �E0

MLCT (b). The numerical values for the different rate constants and apprhould not be over interpreted.

nd a rate constant, kdd, for the non-radiative decay via intersystemrossing of the 3dd state. In the following, these are to be discussedith reference to the schematics in Fig. 2.

kMLCT is quite strongly temperature dependent, in particular atow temperatures because of the above mentioned different elec-ronic sublevels, and can reach values of as low as 104 s−1 [22]intrinsic lifetime of the order of 100 �s) at liquid helium tem-erature. Towards room temperature it becomes less temperatureependent and is of the order of 106 s−1 [25] (intrinsic lifetime ofhe order of �s) for systems where it could be accurately deter-

ined, with a tendency to increase only slightly with a furtherncrease in temperature and with decreasing emission energy orero-point energy difference between the 3MLCT and the groundtate, �E0

MLCT, because of an increase in the non-radiative contri-ution according to the energy gap law in the weak coupling limit6,42,54,55].

The internal conversion from the 3MLCT to the 3dd state is bestescribed as a non-adiabatic multi-phonon process in the strongoupling limit, that is, with a large horizontal and a compara-ively small vertical displacement of the potential wells relativeo each other. In principle, the rate constants can be calculated asoltzmann weighted sums of vibrational overlap integrals whileespecting energy conservation. However this involves a certainumber of structural and energetic parameters such as the reor-anisation energy along the reaction coordinate, the nature of theeaction coordinate itself, the corresponding vibrational frequen-ies, the zero-point energy difference between the two states andhe nature of the electronic coupling between them [56,57]. In theresent case, rather than use the corresponding somewhat complexxpressions, we will use the simple phenomenological formulation,hich captures the essential properties of k1 and k−1 characterised

y low-temperature tunnelling and thermal activation as the rateonstants increase exponentially with the thermal population ofxcited vibrational levels

1 = kT→01 + A1e−Ea

1/kBT (1a)

= kT→0 + A e−Ea−1

/kBT (1b)

−1 −1 −1

At low temperature, that is, in the tunnelling region, and forE0 < 0 the tunnelling rate constant for the forward reaction kT→0

1ncreases exponentially with |�E0|, whereas for the backward

d k−1 on temperature for �E0 ≈ 0 (a), and on �E0 at around room temperature andte ranges are estimated from the wealth of literature data on this subject, but they

reaction kT→0−1 → 0, and vice versa for �E0 > 0 [57]. In the thermally

activated region, the corresponding activation energies are some-what smaller than the classical barrier height, and the zero-pointenergy difference �E0 relates k1 and k−1 via the equation ofdetailed balance

k1

k−1= ge−�E0/kBT = e−�G0/kBT (2)

where �E0 = Ea−1 − Ea

1, and g = gdd/gMLCT = A1/A−1 is the ratio of theeffective densities of state, gdd and gMLCT, in the respective elec-tronic state. g is related to the corresponding entropy change �S via

�S = kb ln g (3)

In the literature [27], �S is usually set to 0. In view of the higherelectronic as well as the higher vibrational density of states for the3dd state, a value of �S of around ∼2 cm−1 K−1 (∼20 J K−1 mol−1)corresponding to g ≈ 10, in analogy for instance to the values foundfor the one-electron spin transition in cobalt(II) complexes [47,58],seems more appropriate.

Since for �E0 > 0, k1 < k−1, and for �E0 < 0, k1 > k−1, only the lat-ter case can result in an appreciable transient population of the3dd state, and this only for the case kdd < k1. This brings us tothe third difficulty: the intersystem crossing process from the 3ddstate back to the ground state, like the internal conversion and thedirect non-radiative decay of the 3MLCT state, is a non-adiabaticmulti-phonon process, but this time in the weak to intermediatecoupling region. Therefore kdd depends both on temperature and onthe zero-point energy difference between the 3dd and the groundstate, �E0

dd, again according to the energy gap law. In particular,in the intermediate and strong coupling regimes, subtle variationsin the actual geometry changes along the reaction coordinate asdictated for instance by steric constraints in multi-dentate ligandsor Jahn–Teller distortions can have dramatic and opposite effectson k1 and kdd. Thus trying to find a general and unified descriptionof the quenching process constitutes a multi-parameter problemwith a large number of temperature, pressure and energy depend-ent parameters. We can of course follow the literature and discuss

some limiting cases based on the set of differential Eq. (4), wherewe have implicitly assumed that the population of the 1MLCT stateis negligible even in pulsed experiments a few 100 fs after the pulse,due to the fact that ultrafast transient absorption spectroscopy on a

9 istry

nca

ci

M

D

a

I

wk

i

a

M

D

w

A

a

a

w

p

wt

a

we

a

wtcabt

k

0 Q. Sun et al. / Coordination Chem

umber of systems revealed that the 1MLCT to 3MLCT intersystemrossing generally takes only of the order of 100 fs and occurs with

quantum efficiency close to unity [31,59,60].

dM

dt= kexG − (kMLCT + k1)M + k−1D (4a)

dD

dt= k1M − (kdd + k−1)D (4b)

dG

dt= −kexG − kMLCTM + kddD (4c)

For continuous irradiation, the excitation rate constant, kex, isonstant, and the three equations can be solved for the steady staten the limit of M, D � G.

= kexN0

(kdd + k−1

kMLCTkdd + kMLCTk−1 + kddk1

)(5a)

= kexN0

(k1

kMLCTkdd + kMLCTk−1 + kddk1

)(5b)

nd the 3MLCT luminescence intensity is therefore given by

= krMLCTM∼�r

MLCT

(kMLCT(kdd + k−1)

kMLCTkdd + kMLCTk−1 + kddk1

)(6)

here N0 is the total concentration of chromophores and �rMLCT =

rMLCT/

(kr

MLCT + knrMLCT

)is the intrinsic luminescence quantum yield

n the absence of the quenching by the 3dd state.For pulsed irradiation, M(t0) = M0 and kex = 0 during the relax-

tion and the solution of the differential equations gives

= M0{A1e−a1t + A2e−a2t} (7a)

= M0A3{e−a1t − e−a2t} (7b)

ith

1 = k−1 + kdd − a1

a2 − a1, A1 = k−1 + kdd − a2

a1 − a2, A3 = k1

a2 − a1(8a)

nd

1,2 = p ±√

p2 − 4q

2(8b)

ith

= kMLCT + kdd + k1 + k−1 and q = kMLCTk−1

+ kMLCTkdd + k1kdd (8c)

(i) If �E0 is positive and comparatively large, then quenchingill set in at elevated temperatures only. But at elevated tempera-

ures, in general k1 + k−1 � kdd > kMLCT. In this limit

1 = k1 + k−1 (9a)

hich is the rate constant establishing the thermal pre-quilibrium, and

2 = kMLCT + kddge−�E0/kBT

1 + ge−�E0/kBT(9b)

hich corresponds to the observed decay rate constant kobs ofhe 3MLCT state. At elevated temperature, where in the presentase quenching becomes relevant, kMLCT is only weakly temper-ture dependent, but not necessarily so kdd, which can likewise

e expressed by the sum of a low-temperature tunnelling and ahermally activated process according to

dd = kT→0dd + Adde−Ea

dd/kBT (10)

Reviews 282–283 (2015) 87–99

At elevated temperature the tunnelling term in Eq. (10) can beneglected, and Eq. (9b) becomes

kobs = kMLCT + Addge−(

�E0/Eadd

)/kBL = kMLCT + Ae−Ea/kBL (11)

The last equality is the most often used equation in the litera-ture in conjunction with the 3MLCT luminescence quenching. In theliterature A is usually identified with kdd and Ea with �E0. This isclearly an approximation, which neglects the entropy contributionin the 3MLCT to 3dd internal conversion, and which assumes thatthe intersystem crossing from the 3dd state to the ground state isactivation less. As mentioned above, in the intermediate couplingregion kdd and therefore Add and Ea

dd are quite strong functions ofthe difference in equilibrium geometry between the 3dd state andthe ground state and the corresponding zero-point energy differ-ence �E0

dd, and therefore some caution in the interpretation of theobserved activation energy and the pre-exponential factor is calledfor. Additionally, the �S = kB ln g term is somewhat temperaturedependent, easily accounting for an increase in the pre-exponentialof an order of magnitude from systems with low quenching tem-peratures to systems with high quenching temperatures.

(ii) If �E0 is negative and has a comparatively large abso-lute value, quenching of the 3MLCT luminescence sets in alreadyat low temperatures, and the sequence of rate constants isk−1 � kMLCT � k1, kdd such that the back reaction can be neglectednot only at low temperatures but also up to fairly high tempera-tures. In this case

a1 = kMLCT + k1 (12a)

a2 = kdd (12b)

where a1 corresponds to the observed decay of the 3MLCT state,because A1 ≈ 1 and A2 ≈ 0. Whether or not a sizeable populationof the 3dd state is built up, depends upon the value of k1 rela-tive to the ones of kMLCT and kdd. The coefficient A3 = k1/(kMLCT + k1)gives the quantum efficiency of the internal conversion, which forthe present case approaches unity irrespective of the fact that atlow temperatures kMLCT is quite strongly temperature dependent,because the low-temperature tunnelling rate constant for k1 isalways larger than kMLCT. The maximum transient population ofthe 3dd state is therefore given by the ratio of kdd to k1. This is amore delicate matter. k1 and kdd have the same tendencies withrespect to their dependence on �E0, that is the more negative thevalue of �E0, the larger k1 in the strong coupling limit, but at thesame time, the larger the negative value of �E0, the smaller thecorresponding value of �E0

dd, and the larger also kdd according tothe energy gap law in the weak to intermediate coupling regime.However, as schematically shown in Fig. 2b the dependence of k1on �E0 is much stronger than the one of kdd.

(iii) The most complex behaviour is to be expected when�E0 ≈ 0. k1 and k−1 are both strongly temperature dependent andmay not be competitive with kMLCT in the low-temperature tun-nelling region. They become competitive at higher temperature,with apparent activation energies approaching the classical valuesonly at high temperature. Since all involved rate constants, includ-ing kdd, are temperature dependent, the luminescence quenchingbehaviour is complex. Luminescence decay curves can be biphasic,and the observed time constants and the corresponding ampli-tudes are of course also strongly temperature dependent. In order toextract a meaningful quenching rate constant kq = A exp(−Ea/kBT),one has to know the temperature dependence of kMLCT, which can

only be inferred indirectly by analogy with, say, a non-quenchedsystem. And even then kq = kobs − kMLCT plotted as ln kq versus 1/Tdoes not necessarily result in a straight line, because the variousrate constants change from the low-temperature tunnelling region

istry

ts

3

atpaiiisbiselste

3m

tLb(betwdaiAlm

oatrsfittaamRppDcci

dbTswo

Q. Sun et al. / Coordination Chem

o the thermally activated region across the experimentally acces-ible temperature interval.

In view of the difficulty to unambiguously determine thedd − 3MLCT energy difference �E0 experimentally, computationalpproaches have increasingly been called upon [61–65]. Essentiallyhese calculations show that for ruthenium(II)–polypyridyl com-lexes, �E0 is around zero. However, computational approachesre usually performed in the gas phase, and it is well known thatn solution the energy of MLCT states depends on solvent polar-ty [24,66], and that in the solid state the one of the 3dd state isnfluenced by a confining environment akin to a chemical pres-ure effect [46,47]. Furthermore, the most often used approachesased on DFT have an inherent difficulty with regard to the accuracy

n the determination of relative energies of states having differentpin multiplicities [67] or big differences in electronic densities andquilibrium geometries [68]. They are nevertheless useful in estab-ishing trends in a series of related complexes. Thus the detailedtudy of Alary et al. [65] estimates that for [Ru(bpy)3]2+ in acetoni-rile the 3MLCT state is slightly lower in energy than the 3dd statessentially due to the solvent stabilisation of the former.

. The complexes, experimental details and computationalethods

All complexes investigated spectroscopically, with one excep-ion, were tris-diimine complexes of the form [Ru(L)3]2+, with

= m-bpy (6-methyl-2,2′-bipyridine), dm-bpy (6,6′-dimethyl-2,2′-ipyridine), tm-bpy (4,4′,6,6′-tetramethyl-2,2′-bipyrdine), m-phen2-methyl-1,10-phenantroline), and for comparison bpy (2,2′-ipyridine), in acetonitrile solution at room temperature. Thexception was the [Ru(terpy)2]2+ complex with the tridentateerpy = 2,2′:6′,2′′-terpyridine. [Ru(m-bpy)3]2+ and [Ru(terpy)2]2+

ere also investigated as a function of temperature and pressureoped into the corresponding zinc host lattices [Zn(m-bpy)3](PF6)2nd [Zn(terpy)2](PF6)2. All compounds were prepared as describedn Refs. [69–72]. Extra dry acetonitrile (Acros) was used as received.ll of the complexes, with the exception of [Ru(bpy)3]2+, are non-

uminescent at room temperature in de-oxygenated solutions or asicrocrystalline powders.UV–vis absorption and standard emission spectra were recorded

n a double beam absorption spectrometer (Varian, Cary 5000)nd a commercial fluorimeter (Horiba, Fluorolog 3). When men-ioned, emission spectra were corrected for the instrumentalesponse. For temperature dependent emission spectra, powderamples of the doped compounds were mounted on the coldnger of a closed cycle cryostat capable of temperatures downo 3 K (Janis–Sumitomo). Luminescence lifetimes on nanosecondimescales were recorded with pulsed irradiation at 458 nm from

MOPO (Opotek Magic Prism) pumped by the third harmonic of Nd:YAG laser (Quantel Brillant) and detection at 620 nm via aonochromator (Spex 270 M) and photomultiplier (Hamamatsu

928) coupled to a digital oscilloscope (Tektronix TDS-540B) with areamplifier (LeCroy, 500 MHz). Luminescence spectra under highressure were recorded using a diamond anvil cell (MiniDAC of’Anvils Ltd.), which could also be inserted into the closed cycleryostat. Pressure was calibrated by using the 5D0 → 7F0 lumines-ence of Sm2+ doped BaFCl with a shiftrate of −21 cm−1 GPa−1 asnternal pressure sensor [73].

The femtosecond transient absorption (TA) setup has beenescribed elsewhere [74,75]. Excitation was performed at 400 nmy the frequency-doubled output of a standard 1 kHz amplified

i:Sapphire system (Spectra-Physics). The pump intensity on theample was approximately 1.5 mJ cm−2 (3 �J focused on a spotith ∅ = 300 �m). The probe pulse was a white light continuum

btained from a 3 mm CaF2 window from which the fundamental

Reviews 282–283 (2015) 87–99 91

was removed using a HR800HT400–720 nm filter. The polarisationof the probe pulses was at magic angle (54.70) relative to that ofthe pump pulses. The instrumental response function (IRF) wasapproximately 150 fs FMHM. Further details regarding chirp cor-rection may be found in Ref. [76]. The TA data was analysed by amulti-exponential global fit analysis. The first step is to reproducethe data with a sufficient number of exponential decays and of vari-able amplitudes. Such an analysis yields a series of rate constantsand the corresponding decay associated difference spectra (DADS)[77].

X-ray structural parameters were taken from the literaturewhere available or from Ref. [48]. DFT calculations were per-formed using the BLYP functional [78a,b] augmented with thesemi-empirical dispersion correction of Grimme (BLYP-D3) [78c]and the hybrid Gaussian and plane wave (GPW) method [78d], asimplemented in the CP2K/Quickstep program [79]. True minima ofthe different states were checked via vibrational analysis and ver-ified by the absence of imaginary frequencies. Further details aregiven in the supporting information to Ref. [48].

4. Results

4.1. Ultrafast transient absorption in solution

The absorption spectra of all compounds studied show the typi-cal intense 1MLCT band centred at around 455 nm (∼22,000 cm−1).The intensities are the highest for [Ru(bpy)3]2+ and [Ru(terpy)2]2+,they are substantially lower for those complexes having methylgroups in alpha position to nitrogen (see Fig. 3). This is due to thesomewhat longer metal-ligand bond lengths as a result of the sterichindrance of the methyl groups in the latter.

At room temperature in de-oxygenated acetonitrile solutiononly [Ru(bpy)3]2+ is luminescent with values of 9% and 850 ns forthe luminescence quantum efficiency �MLCT and the decay time�MLCT, respectively [80]. Excited state dynamics must therefore befollowed by ultrafast transient absorption techniques. The corre-sponding transient absorption (TA) spectra for all six complexeslikewise in de-oxygenated acetonitrile solutions upon irradiationwith an 80 fs pulse at 400 nm are shown in Fig. 3. As has beendescribed in the literature by different groups [28,29,31], for thereference complex [Ru(bpy)3]2+ the interpretation is compara-tively straight forward. Following the initial excitation to the1MLCT, intersystem crossing takes the complex to the correspond-ing 3MLCT state within less than 100 fs. With an IRF of 150 fs, thisstep is not resolved in the spectra of Fig. 2a. The TA spectrum imme-diately after the pulse shows strong ground state bleaching centredat 454 nm (22,025 cm−1) and characteristic excited state absorp-tions (ESA) at 370 nm (27,000 cm−1) and above 500 nm. The formerhas been attributed to a ligand-centred transition on the formallyreduced bpy−, the latter to an overlap of a bpy− centred transitionand ligand to metal charge transfer (LMCT) transition from one ofthe neutral ligands to the formally oxidised Ru3+ of the 3MLCT state[28,29,33]. This spectrum decays mono-exponentially at all wave-lengths with the same time constant of 850 ns as the luminescencedecay, indicating that 3MLCT decay and ground state recovery gohand in hand. Thus in accordance with published work [28,29,31]for [Ru(bpy)3]2+, there is no evidence for the build up of any sizeablepopulation of an intermediate state in the deactivation cascade ofthe 3MLCT state.

For the other members of the [Ru(L)3]2+ family the TA spec-tra immediately after the pulse are very similar to the one of

[Ru(bpy)3]2+ with the ESA in the UV and in the red, and strongground state bleaching at around 450 nm. In accordance withthe fact that these complexes are non-luminescent, all TA signalsdecay much faster than for [Ru(bpy)3]2+. Qualitatively, the most

92 Q. Sun et al. / Coordination Chemistry Reviews 282–283 (2015) 87–99

Fig. 3. Transient absorption spectra at selected time delays of (a) [Ru(bpy)3]2+, (b) [Ru(m-bpy)3]2+, (c) [Ru(tm-bpy)3]2+, (d) [Ru(dm-bpy)3]2+, (e) [Ru(m-phen)3]2+, (f)[ the cob

ioatdc3

a

ejFpa0artGD�Ei[Tg

fadc�ga

Ru(tpy)2]2+, c = 2 × 10−5 M, �ex = 400 nm, in CH3CN at 295 K. For direct comparison, and c are adapted from Ref. [48].

mportant finding is not just the overall faster decay, but also thebservation of a biphasic process, in which the ESA at 370 nm andbove 500 nm characteristic of the 3MLCT state decays much fasterhan the ground state recovery. This is a clear indication that theepopulation of the 3MLCT state occurs with high quantum effi-iency to an intermediate state having a longer lifetime than theMLCT state and thus builds up a sizeable population in the relax-tion cascade.

The data of Fig. 3 can be analysed more thoroughly using multi-xponential global analysis of the TA spectra when possible, or byust looking at the decay curves at some characteristic wavelengths.ig. 4 shows the TA profiles at 370, 454 and 630 nm for all com-lexes of the series and additionally at 528 nm for [Ru(m-bpy)3]2+

nd [Ru(m-phen)3]2+. For [Ru(bpy)3]2+ in the time interval from to 10 ps within which the 3MLCT state does not decay notice-bly, the fit function f(t) = A1 exp(−t/�1) + Ainf, using an iterativeeconvolution procedure with an IRF of 150 fs, gives a very good fito the experimental curves at these wavelengths with �1 = 1.8 ps.lobal analysis of the full spectra results in the correspondingADS shown in Fig. 5a. The fast component with the time constant1 = 1.8 ps having derivative type wiggles in the region of strongSA, corresponds to a combination of the ultrafast 1MLCT → 3MLCTntersystem crossing, charge localisation on one of the ligands81–83], solvent reorganisation and vibrational relaxation [31,59].he Ainf component corresponds to the ESA of the 3MLCT state andround state bleaching as discussed above.

For the other four complexes of the [Ru(L)3]2+ series the fitunction is f(t) = A1 exp(−t/�1) + A2 exp(−t/�2). For [Ru(m-bpy)3]2+

nd [Ru(m-phen)3]2+ the time profiles at the specific wavelengthsepicted in Fig. 4b and e are very similar to each other, having a fast

omponent with �1 = 1.6 and 4.4 ps and a slower component with2 = 450 and 452 ps, respectively. In Fig. 5b and d the DADS from thelobal fit analysis are shown. As already pointed out qualitativelybove, the characteristic ESA of the 3MLCT state at 370 and above

rresponding stationary absorption spectra are shown in grey (right axis). Panels a,

500 nm as given by the A1 component completely disappears withina few ps. The slow component is characterised by the strong groundstate bleaching and weak ESA at around 600 nm. Together this con-stitutes a definite proof of the existence of an intermediate state,which is rapidly populated from the 3MLCT state, thus quench-ing the 3MLCT luminescence totally, and which has a sufficientlylong lifetime to result in the build up of a substantial transientpopulation. The natural candidate for this intermediate state is, ofcourse, the low-lying 3dd state, 3T1(t2g

5eg1), despite the fact that

with 450 ps it seems longer lived than hitherto assumed. This pointwill be discussed below, in particular also based on experimentalresults obtained upon application of external pressure.

The TA spectra of the complexes with methyl groups in bothalpha position of nitrogen in bipyridine, [Ru(dm-bpy)3]2+ and[Ru(tm-bpy)3]2+, are also very similar to the ones of the othercomplexes, and the time profiles shown in Fig. 4c and d canbe reproduced with a bi-exponential function. However, with�1 < 150 fs and �2 = 6.6 and 7.5 ps, respectively, both time constantsare substantially smaller than for the complexes with only onemethyl group in alpha position to nitrogen for each bipyridine.

For the at room temperature only very weakly luminescent[Ru(terpy)2]2+ complex [40], the situation is different again. TheTA spectrum just after the pulse is still composed of the groundstate bleach at 450 nm and ESA at 370 and above 500 nm. Againthe decay is much faster than for [Ru(bpy)3]2+, but this time, like in[Ru(bpy)3]2+, the overall spectrum decays with the same time con-stant of � = 132 ps, as depicted in Fig. 4f showing the time profilesat different wavelengths and in Fig. 5c showing the correspondingDADS. This signifies that the quenching state must be either muchshorter lived than the feeding process, or that a fast pre-equilibrium

between the quenching state and the 3MLCT state is established.The latter is supported by the experiments of Hewitt et al. [30],who found a small but fast transient reduction of the ESA intensityat 370 nm immediately after the laser pulse with a time constant

Q. Sun et al. / Coordination Chemistry Reviews 282–283 (2015) 87–99 93

F +, (c) [c ll line

occ

fetaid

Fa

ig. 4. Time profiles at different wavelengths for (a) [Ru(bpy)3]2+, (b) [Ru(m-bpy)3]2

orresponding ground state recovery at around 450 nm; symbols: experimental; fu

f ∼3 ps. Since kMLCT is much smaller than the observed decay rateonstant, the latter corresponds essentially to kdd. The above rateonstants are summarised in Table 1.

Qualitatively, the interpretation of the above data is straight-orward. For [Ru(bpy)3]2+, the quencher state is at sufficiently highnergy for it to be only active via thermal activation at around roomemperature. Indeed the single-exponential luminescence decays

nd the perfectly straight lines in Arrhenius plots of the quench-ng rate constant of the luminescence for [Ru(bpy)3]2+ doped intoifferent host lattices [45] with activation energies between 2000

ig. 5. Decay-associated difference spectra from global analysis of the TA spectra for (a)nd b are adapted from Ref. [48].

Ru(tm-bpy)3]2+, (d) [Ru(dm-bpy)3]2+, (e) [Ru(m-phen)3]2+, (f) [Ru(terpy)2]2+, insets:s: least squares fits. Panels a, b and c are adapted from Ref. [48].

and 4000 cm−1 depending upon the size of the cavity provided bythe host lattice constitute a textbook example of case (i) behaviourwith a large and positive value of �E0 corresponding to the experi-mental activation energy. The methyl groups in the alpha-positionof the nitrogen atoms of bpy and phen obviously force a longerRu N bond length, which reduces the ligand-field strength, andthis in turn results in values of �E0, which are close to zero or even

negative, such that the luminescence is totally quenched at roomtemperature. The effect is larger for dm-bpy and tm-bpy, and there-fore the driving force for the internal conversion from the 3MLCT

[Ru(bpy)3]2+, (b) [Ru(m-bpy)3]2+, (c) [Ru(terpy)2]2+, (d) [Ru(m-phen)3]2+. Panels a

94 Q. Sun et al. / Coordination Chemistry

Table 1Time constants of the major amplitude of the decay of the transient absorption at370 nm (�1) characteristic for the 3MLCT state and at 450 nm (�2) for the groundstate recovery.

�1 at ∼370 nm �2 at ∼450 nm

[Ru(bpy)3]2+ 850 ns 850 ns[Ru(m-bpy)3]2+ 1.6 ps 450 ps[Ru(m-phen)3]2+ 4.4 ps 452 ps[Ru(dm-bpy)3]2+ <0.2 ps 6.6 ps[Ru(tm-bpy)3]2+ <0.2 ps 7.6 ps[Ru(terpy)2]2+ 3 psa 132 ps

a From Ref. [30] for the 3dd − 3MLCT equilibration.

2.5

2.0

1.5

1.0

0.5

0.0

OD

700600500400wavelength [nm]

Intensity [a.u.]

[Zn1-xRux(terpy)2](PF 6)2, x = 0.2%Absorption at RT

E parallel molecular C2 axis E perpendicular molecular C2 axis

luminescence at 77 K

Fig. 6. Polarised single crystal absorption spectra of [Zn Ru (terpy) ](PF ) ,xe

stlsriae

4c

mlucswt[atl

iFrt

Obviously, pressure moves the quencher state to higher energywith respect to the 3MLCT state, so that above a certain pressure,it is no longer active. The change in energy difference between thetwo states can be due to two effects: either the 3MLCT is stabilised

1−x x 2 6 2

= 0.2%, at room temperature, and powder luminescence spectrum at 77 K withxcitation at 480 nm corrected for spectral response of the spectrometer.

tate to the 3dd state is larger for these two systems compared tohe monosubstituted m-bpy and m-phen. In the strong couplingimit, this results in �1 < 150 fs for dm-bpy and tm-bpy being sub-tantially smaller than the 1.6 and 4.4 ps for m-bpy and m-phen,espectively. With regard to the lifetimes of the intermediate state,t is longer for m-bpy and m-phen with the smaller value of �E0

nd thus the larger value of �E0dd, because this process obeys the

nergy gap law.

.2. [Ru(m-bpy)3]2+ and [Ru(terpy)2]2+ doped into theorresponding Zn matrices

Doping chromophores in low concentrations into inert hostatrices allows studying their intrinsic properties over a much

arger temperature and pressure range than accessible for liq-id solutions. Of course, solvent reorganisation, important forharge transfer states, is non-existent or largely inhibited inuch solid solutions. Photochemical ligand dissociation is like-ise inhibited. In the following, we present results on the

emperature and pressure dependence of the luminescence ofRu(m-bpy)3]2+ and [Ru(terpy)2]2+ doped into [Zn(m-bpy)3](PF6)2nd [Zn(terpy)2](PF6)2 at a doping level of 0.2–1 mol% and comparehem to published results on [Ru(bpy)3]2+ doped into various hostattices.

Crystals of [Zn1−xRux(terpy)2](PF6)2, x = 0.2%, are dichroic, hav-ng an axis of stronger absorption and an axis of weaker absorption.

ig. 6 shows the polarised absorption spectrum of this system atoom temperature, with E parallel and perpendicular to the crys-al c-axis, which coincides approximately with the molecular C2

Reviews 282–283 (2015) 87–99

axis1. The band with the maximum at 478 nm has been attributedto the 1MLCT transition. It is substantially stronger for the polarisa-tion along the molecular C2 axis, which is readily explained by thefact that the apical metal-nitrogen bond length in bis-terpyridinecomplexes is always by approximately 0.1 A shorter than the distalones. For E perpendicular to the C2 axis, the absorption band is quitestructured, and for E parallel, there is a weak but distinct shoulderon the low-energy side of the main band. This can be attributedto the spin-forbidden 3MLCT transition based on the fact that thecorresponding weak luminescence recorded at 77 K shows only asmall Stokes shift. For [Ru(m-bpy)3]2+ it was not possible to growdoped crystals sufficient in size for absorption spectroscopy.

4.2.1. Temperature and pressure dependence of the luminescenceSimilar to acetonitrile solutions, at room temperature the two

complexes in the solid solutions are non-luminescent at the sen-sitivity of the fluorimeter. However, the luminescence can beswitched on by either applying an external pressure or by coolingdown to cryogenic temperatures. Fig. 7a and c shows the tempera-ture dependence of the luminescence of [Zn1−xRux(terpy)2](PF6)2,x = 0.5%, and of [Zn1−xRux(m-bpy)3](PF6)2, x = 1%, down to 3 K. Atthe lowest temperature, the luminescence is fairly intense, but,apart from the more complex behaviour at very low temperaturesreminiscent of the behaviour of [Ru(bpy)3]2+ as reported by Crosbyet al. [22] due to the splitting of the 3MLCT state into three sub-levels, at ambient pressure it decreases rapidly with increasingtemperature for both compounds. Fig. 8 shows the evolution ofthe integrated luminescence intensity as a function of tempera-ture for the two systems. For [Ru(terpy)2]2+ the luminescence isquenched to below 1% of its maximum value at ∼50 K. For [Ru(m-bpy)3]2+ the corresponding temperature is ∼80 K. At the same timethe observed luminescence lifetimes drop likewise. Under respec-tive pressures of approximately 4.2 and 5.2 GPa, the luminescencedecreases much less for both compounds with increasing temper-ature and persists all the way up to room temperature.

Fig. 9 depicts the pressure dependent luminescence of[Zn1−xRux(m-bpy)3](PF6)2, x = 1%, at room temperature. As statedabove at room temperature and ambient pressure, this system isnon-luminescent, but at an applied pressure of just above 2 GPaluminescence can be clearly discerned. As shown in Fig. 9, its inten-sity increases rapidly with increasing pressure up to 5 GPa and thenlevels off to a constant intensity. Due to local inhomogeneities inthe pressure of the DAC, the corresponding luminescence decaycurves deviate from single exponential behaviour. In such a case,the average lifetime can be determined via a stretched exponentialfit with the fit function [27]

f (t) = A exp (−kt)ˇ (13a)

and

⟨�obs

⟩=

�(

ˇ−1)

ˇk−1 (13b)

where � is the gamma function. The corresponding average life-time or rather the average decay rate constant kobs as a functionof pressure is displayed in Fig. 10. It follows the behaviour of theluminescence intensity, with an average lifetime of ∼600 ns above6 GPa. This is of the order of magnitude expected for the intrinsiclifetime of the 3MLCT state in ruthenium(II) tris-diimine complexes.

1 Space group of [Zn(bpy)3](PF6)2: P421c at 295 K (P. Pattison, unpublished).

Q. Sun et al. / Coordination Chemistry Reviews 282–283 (2015) 87–99 95

Fig. 7. Luminescence spectra as a function of temperature for [Zn1−xRux(terpy)2](PF6)2, x = 0.5%, at ambient pressure (a) and at ∼4.2 GPa (b), and for [Zn1−xRux(m-bpy)3](PF6)2,x = 1% at ambient pressure (c) and ∼5.2 GPa (d), excitation at 473 nm, not corrected for spectral response of experimental setup.

Fig. 8. Integrated luminescence intensities and observed lifetimes as function of temperature of (a) [Zn1−xRux(terpy)2](PF6)2, x = 0.5%, at ambient pressure and ∼4.2 GPa, (b)[Zn1−xRux(m-bpy)3](PF6)2, x = 1% and at ambient pressure and ∼5.2 GPa.

96 Q. Sun et al. / Coordination Chemistry Reviews 282–283 (2015) 87–99

Lum

ines

cenc

e in

tens

ity [

a.u.

]

800750700650600550

Wavelength [nm]

p [GPa] 0.16 0.4 0.8 1.1 1.3 1.5 1.8 1.9 2.3 2.7 2.8 2.9 3.1

3.2 3.5 3.9 4.1 4.2 4.4 4.7 4.9 5.1 5.5 5.8 5.9 6.7

Fig. 9. Luminescence spectra as a function of pressure for [Zn1−xRux(m-bpy)3](PF6)2,x = 1%, at room temperature; inset: shift of the emission maximum as a function ofpressure. The black line indicates the shift of the emission maximum with increasingpp

aW3

sailttb

⟨

wofpB3iaDeofmtmiltiecft

106

2

3

45678

107

2

3

45678

108

k obs

[s-1

]

86420p [GPa]

0.01

2

3

456780.1

2

3

456781 norm

alised Intensity

Experimental kobs Int

Calculated with kMLCT = 2x10

6 s

-1

kdd = 5x109 s

-1

B = 0.2 = 400 cm

-1GPa

-1

Fig. 10. Luminescence intensity ( ) and average relaxation rate constant (•) as

3

ressure. The sharp peaks are due to luminescence from Sm2+:BaFCl used as in situressure sensor.

nd/or the quencher state is destabilised by the external pressure.ith regard to the former, inspection of Fig. 9 reveals a shift of the

MLCT maximum to lower energies by ∼85 cm−1 GPa−1. This is notufficient for the very rapid increase of the intensity and the lifetimes a function of pressure at around 4 GPa. Assuming that the plateaun lifetime and intensity above 5 GPa corresponds to the intrinsicuminescence of the 3MLCT state and that at room temperature thehermal equilibration between the 3MLCT and the 3dd states is fast,he increase in the difference in energy between the two states cane estimated using Eq. (14) derived from Eq. (9b)

�obs

⟩−1 =⟨

kobs

⟩= kMLCT + kddge−�E0(p)/kBT

1 + ge−�E0(p)/kBT

= kMLCT + kddBe−˛p/kBT

1 + Be−˛p/kBT(14)

here �E0(p) = �E0(0) + ˛p. A least-squares fit using fixed valuesf kMLCT and kdd of 2 × 106 s−1 from the plateau and 5 × 109 s−1

rom the ultrafast transient spectroscopy in solution at ambientressure, respectively, results in values of ≈ 400 cm−1 GPa−1 and

= ge−�E0(0)/kBT ≈ 0.2. The latter gives a value of �E0(0) of around00 cm−1, that is, despite the efficient quenching �E0(0) is still pos-

tive albeit close to zero. At first sight, a positive value for �E0(0)nd the corresponding value of B, which is equal to the ratio of/M in the steady state approximation, seems puzzling with ref-rence to the TA discussed above suggesting �E0 < 0 and a valuef B larger than 1. However, the pressure experiments were per-ormed for the ruthenium complex doped into a crystalline solid

atrix and the TA experiments were performed in solution. Givenhat in the 3dd state of the ruthenium complex the equilibrium

etal-nitrogen bond length is not only substantially larger thann the ground state but also larger than the one of the zinc hostattice, the latter actually destabilises the 3dd state with respecto solution via a chemical pressure. This destabilisation can eas-ly reach several hundred wavenumbers [45,46]. In fact, this also

xplains the luminescence and the comparatively long lumines-ence lifetimes for [Ru(m-bpy)3]2+ [84] and [Ru(terpy)2]2+ [40] inrozen solutions at 77 K, because frozen solutions constitute a veryight fitting environment known to destabilise excited ligand-field

a function of pressure at room temperature for [Zn1−xRux(m-bpy)3](PF6)2, x = 1%;calculated curves according to Eqs. (14) and (15).

states with larger equilibrium bond lengths than the ground state[85].

The above is corroborated by the luminescence intensity of[Zn1−xRux(m-bpy)3](PF6)2, x = 1%, as function of pressure includedin Fig. 10. The calculated luminescence intensity using Eq. (15)

I∼ kMLCT

kMLCT + kddge−�E0(p)/kBT= kMLCT

kMLCT + kddBe−˛p/kBT(15)

as derived from Eq. (6) in the limit of a fast pre-equilibriumand using the above parameter set reproduces the experimentaldependence reasonably well.

The value of of 400 cm−1 GPa−1 is substantially larger than the-85 cm−1 GPa−1 shift of the 3MLCT emission. Thus the external pres-sure must destabilise the 3dd by approximately 300 cm−1 GPa−1.Assuming that this shift is entirely due to a work term of the formp�V, a value for the difference in molecular volumes between the3dd and the 3MLCT state of approximately 7 A3 can be estimated.This seems somewhat on the low side compared to values knownfor a t2g to eg promotion in spin-crossover compounds [55]. How-ever, the 4d orbitals are strongly covalent, and the nephelauxeticeffect, which increases with increasing pressure, counteracts thepure work term. Thus the 7 A3 are to be considered as lower limitfor �V.

4.3. DFT calculations

As mentioned in the introduction, today quantum mechanicalapproaches, in particular based on DFT, often allow a more quan-titative understanding of the observed phenomena, as for instancethe mechanism of ligand substitution [62]. Table 2 gives the DFToptimised equilibrium Ru N bond lengths of the 1A1 ground state,the 3MLCT state and the 3dd state for some of the key complexespresented above. All minima correspond to true minima based onthe absence of imaginary frequencies in the corresponding vibra-tional analyses. For direct comparison, experimental bond lengthsin the ground state are included in Table 2. The calculated groundstate values are in very good agreement with these. As qualitatively

expected, the equilibrium geometries of the MLCT states are notvery much different to the ones of the ground state with respect tothe Ru N bond length. This is no longer the case for the 3dd states.As expected, with an average increase of 0.12 A the Ru N bond

Q. Sun et al. / Coordination Chemistry Reviews 282–283 (2015) 87–99 97

Table 2Experimental and DFT optimised Ru N bond lengths (Å) of [Ru(L)3]2+, (L = bpy, m-bpy, dm-bpy, terpy) in the 1A1 ground state, the 3dd state and the 3MLCT state, andcalculated excited-state/ground-state zero-point energy differences �E0 (cm−1), luminescence lifetimes �em (�s) and emission quantum yields (˚).

bpy m-bpy dm-bpy terpy*

d(Ru N)a,b 1A1/exp 2.07(6)c 2.12(3)/2.06(3)d 2.13(6)d 2.07(4)/1.98(2)e

1A1/calc 2.09(6) 2.16(3)/2.09(3) 2.15(6) 2.10(4)/2.00(2)3dd/calc 2.43(2)/2.13(4) 2.44(2)/2.13(4) 2.90(1)/2.19(5) 2.43(2)/2.12(4)3MLCT/calc 2.09(6) 2.15(3)/2.09(3) – 2.02(2)/2.10(4)

Energiesa �E0MLCT 15,010 14,520 – 14,860

�E0dd

17,930 14,100 11,220 16,935�E0 2920 −420 – 2080

�em (�s) 77 K/glass 5.3f 4.1f 2.5f 8.3g

10 K/Zn-host 56h 31 – 82˚ 77 K/glass 0.37f 0.097f 0.018f –

a Computational geometries with CP2K according to Ref. [48].b Number in brackets = number of bonds with this bond length in the coordination octahedron.c Refs [48,87].d Ref. [72].e Ref. [88].f Ref. [82].

l[eJFlntqttfirIteltbHcafTs3

FslisotsswimcTwoio

g Ref. [40].h Ref. [45].* This work.

engths are substantially longer in the respective 3dd states. ForRu(bpy)3]2+ and [Ru(m-bpy)3]2+ the increase occurs with a distinctlongation along one axis. This is a direct manifestation of a strongahn–Teller effect due to the population of an eg orbital [65,86].or [Ru(m-bpy)3]2+ with its meridional coordination of the threeigands, the Jahn–Teller axis goes through the two nitrogen atomsext to the methyl groups in trans position. For [Ru(dm-bpy)3]2+

he situation is less evident. Already the ground-state geometry isuite distorted because of the steric hindrance brought about byhe methyl groups. The even more severe and asymmetric distor-ion with one Ru N bond becoming much longer than the otherve observed in the dd state may be viewed as a way to release theesulting strain. For [Ru(terpy)2]2+ the situation is different again.n the ground state the apical Ru N bond lengths are shorter thanhe distal ones, but in the 3dd state, the two longer bonds are cis toach other and belong to two of the distal pyridine rings on eachigand (it should be noted that in Ref. [63] the equilibrium struc-ure of the 3dd state is described very differently, with the two longonds belonging to the distal pyridine moieties of the same ligand.owever, this could be an artefact due to the constrained distortionoordinates assumed by these authors). Likewise very importantre the energetic considerations. DFT may not be the best methodor these, but it can reproduce correct trends semi-quantitatively.hus, with reference to Table 2, the zero-point energy of the 3ddtate in [Ru(bpy)3]2+ is predicted to be higher in energy than theMLCT state by �E0 = 2900 cm−1, in agreement with experiment.or [Ru(m-bpy)3]2+ on the other hand the calculations using theame functional and basis sets predict the 3dd state to be slightlyower in energy than the 3MLCT state, that is �E0 = −420 cm−1. Thiss mostly due to a lowering of the zero-point energy of the 3ddtate by around 4000 cm−1. Again, this is in line with experimentalbservations. For [Ru(dm-bpy)3]2+ it was not possible to localisehe 3MLCT state, that is, irrespective of the starting geometry theystem always converged to the 3dd state, indicating that 3MLCTtate is at substantially higher energy than the 3dd state. However,ith respect to [Ru(m-bpy)3]2+, the 3dd state of [Ru(dm-bpy)3]2+

s lowered in energy by another 3000 cm−1, so that the best esti-ate for this complex is �E0 ≈ −4000 cm−1. For the [Ru(terpy)2]2+

omplex the corresponding calculated value of �E0 ≈ 2000 cm−1.his is in accordance with the fact that this complex luminesces

ith quite a high quantum yield at 77 K in a frozen glass, but shows

nly minimal luminescence at higher temperatures, with an exper-mentally determined activation energy for the quenching processf 1500 cm−1 in ethanol/methanol (4:1) solution [40].

5. Discussion and conclusions

The experimental evidence for the fast population of an inter-mediate state with the disappearance of the 3MLCT ESA and themuch slower ground state recovery in [Ru(m-bpy)3]2+, [Ru(m-phen)3]2+, [Ru(dm-bpy)3]2+ and [Ru(tm-bpy)3]2+ is compelling.The question is, why despite efforts of many researchers over thepast years, this has not been evidenced before. Well, there aretwo reasons for this. The first one is technical: up to recently TAspectroscopy with sub-picosecond time resolution below 400 nmwas not routine, and it is the disappearance of the 3MLCT ESA bandbelow 400 nm typical for the bpy− radical that unambiguouslyidentifies the fast process and therefore yields the lifetime of thisstate. The second one is more chemical: previously, mostly com-plexes with rather large negative values of �E0, that is case (ii) inSection 2, were investigated. For these, both the internal conversionfrom the 3MLCT to the 3dd state and the subsequent intersystemcrossing back to the ground state become ultrafast, the latter evenbecoming faster than the former. Only when �E0 is negative butnot too much so, do we have case (iii) and can we expect a sizeabletransient population of an intermediate state. The ligands m-bpyand m-phen result indeed in a negative value of only a few hundredwavenumbers, and it’s for these that the intermediate state has thelongest lifetime. With the ligands dm-bpy and tm-bpy, the absolutevalue of �E0 is already quite large, and we are approaching case(ii). Finally, with terpy as ligand, �E0 ≥ 0, so that quenching isefficient at room temperature, but, except at very short times, froma thermal pre-equilibrium between the 3MLCT and the 3dd states.However, there is a seeming contradiction to be resolved. Why iskdd for the bis-terpy complex larger than for the m-bpy complexdespite the fact that for the latter in solution we proposed �E0 < 0?Well in the intermediate coupling regime the width and the heightof the energy barrier are both very much dependent not only onthe differences in geometry and zero-point energy but also on theanharmonicity of the potential energy surfaces of the two statesalong the reaction coordinate [89]. For the more rigid tridentateterpy ligand, the normal coordinates involving the Ru N bondlength are certainly less anharmonic than the ones for the asym-metric m-bpy ligand with the forced elongation of some bonds.This effect becomes important for the crossing point of the ground

3

state potential energy surface with the one of the dd state, movingit to substantially lower energy. Of course this is almost impossibleto quantify, but such subtle differences are the reason for the largerange of kdd proposed in Fig. 2. What about photoinduced ligand

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8 Q. Sun et al. / Coordination Chem

ubstitution? In the present case of polydentate ligands, ligandubstitution in the acetonitrile is not very efficient or even absent.t first sight surprisingly, this is also the case for [Ru(dm-bpy)3]2+

nd [Ru(tm-bpy)3]2+, despite their large predicted distortion in thedd state. However, our DFT calculations yield true minima for thistate, indicating that even in this case ligand ejection is a thermallyctivated process competing with the decay to the ground state.s the ground state recovery for these two complexes is very fast,

his is in line with the experimentally determined low quantumield for ligand ejection [48].

Finally, with our article we hope to have contributed to theeneral understanding of the photophysical properties of ruthe-ium(II) complexes and in particular of the role of the low-lying

igand field state, thus helping in the design of complexes havingither maximum luminescence [90] and charge transfer proper-ies [10] or optimum quantum efficiencies for ligand substitution14–18,62,64] for applications in phototherapy.

cknowledgements

We thank the Swiss National Science Foundation (grant num-er 200020-125175) for financial support. We thank H. A. GoodwinUniversity of New South Wales) for samples of some of the com-lexes and P. Pattison (EPFL and ESRF) for the determination of therystal structure of [Zn(terpy)2](PF6)2. This work was supportedy a grant from the Swiss National Supercomputing Centre (CSCS)nder projects ID s103 and s296, and by a grant from the Center fordvanced Modeling Science (CADMOS) under project ID CTESIM.he financial support for CADMOS and for the BlueGene/Q system isrovided by the Cantons of Geneva and Vaud, the “Fondation Hansilsdorf, the “Fondation Louis-Jeantet”, the University of Geneva,

he University of Lausanne, and the Ecole Polytechnique Fédéraleausanne.

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