Ruthenium(II) photosensitizers of tridentate click-derived cyclometalating ligands: A joint experimental and computational study

DOI: 10.1002/chem.201103451

Ruthenium(II) Photosensitizers of Tridentate Click-Derived Cyclometalating

Ligands: A Joint Experimental and Computational Study

Benjamin Schulze,^{[a, b]}Daniel Escudero,^{[b, c]}Christian Friebe,^{[a, b]}Ronald Siebert,^{[b, d]}

Helmar Gçrls,^[e]Stephan Sinn,^{[a, b]}Martin Thomas,^{[b, c]}Sebastian Mai,^{[b, c]}Jꢀrgen Popp,^{[b, d]}

Benjamin Dietzek,*^{[b, d]}Leticia Gonzꢁlez,*^{[b, c, f]}and Ulrich S. Schubert*^{[a, b]}

Abstract: A systematic series of het-

eroleptic bis(tridentate)ruthenium(II)

complexes of click-derived 1,3-

radiationless deactivation mechanisms.

Furthermore, we provide a fully opti-

mized synthesis and systematic varia-

tions towards redox-matched, broadly

and intensely absorbing, cyclometalat-

ed ruthenium(II) complexes. Most of

them show a weak room-temperature

emission and a prolonged excited-state

lifetime. They display a broad absorp-

tion up to 700 nm and high molar ex-

ACHTUNGTRENNUNG

bis(1,2,3-triazol-4-yl)benzene N^C^N-

coordinating ligands was synthesized,

analyzed by single crystal X-ray diffrac-

tion, investigated photophysically and

electrochemically, and studied by com-

putational methods. The presented

comprehensive characterization allows

a more detailed understanding of the

tinction

coefficients

up

to

20000m^ꢀ1cm^ꢀ1of the metal-to-ligand

charge transfer bands, resulting in

a black color. Thus, the complexes

reveal great potential for dye-sensitized

solar-cell applications.

Keywords: click chemistry · compu-

tational methods · cyclometalation ·

photosensitizers · ruthenium

Introduction

[a] B. Schulze, C. Friebe, S. Sinn, Prof. Dr. U. S. Schubert

Laboratory of Organic and Macromolecular Chemistry (IOMC)

Friedrich-Schiller-University Jena

Ruthenium(II) polypyridyl complexes are highly prominent

in photochemistry, since they allow for a light-driven charge

separation in which the ligand becomes photoreduced while

the metal is photooxidized and both can undergo subse-

quent redox reactions in terms of artificial photosynthesis.

This metal-to-ligand charge transfer (MLCT) can be fine-

tuned by the ligand properties to optimize the photophysical

and electrochemical properties. To allow homogeneous, dif-

fusion-controlled photocatalysis, a long excited-state lifetime

is most important. A central dilemma is that, in contrast to

tris(bidentate) ruthenium(II) complexes, bis(tridentate) ones

are more stable and allow an isomer-free functionalization,

but typically show only short excited-state lifetimes.^[1]Vari-

ous optimization strategies to prolong the excited-state life-

time have been developed.^[2]The use of very strong, anionic

donors causes slightly prolonged lifetimes, and moreover, in-

teresting properties such as a broadened and red-shifted ab-

sorption of visible light and a directed MLCT transition.

An application for which these features become most im-

portant and the lifetimes are not that crucial, due to immo-

bilization of the complexes and fast electron injection into

the semiconductor, is the dye-sensitized solar cell (DSSC),

developed by OꢀRegan and Grꢁtzel in 1991.^[3]The DSSC ap-

plies the principles of natural photosynthesis, namely the

spatial separation of the basic functions that are light-driven

charge separation and charge transport, and, therefore,

allows for modular manipulations of the light-harvesting

dyes. Here, the almost pure, and thus predictable and tun-

Humboldtstr. 10, 07743 Jena (Germany)

Fax : (+49)3641948202

E-mail: ulrich.schubert@uni-jena.de

[b] B. Schulze, Dr. D. Escudero, C. Friebe, Dr. R. Siebert, S. Sinn,

M. Thomas, S. Mai, Prof. Dr. J. Popp, Prof. Dr. B. Dietzek,

Prof. Dr. L. Gonzꢂlez, Prof. Dr. U. S. Schubert

Jena Center for Soft Matter (JCSM)

Friedrich-Schiller-University Jena

Humboldtstr. 10, 07743 Jena (Germany)

[c] Dr. D. Escudero, M. Thomas, S. Mai, Prof. Dr. L. Gonzꢂlez

Laboratory of Theoretical Chemistry

Friedrich-Schiller-University Jena

Helmholtzweg 4, 07743 Jena (Germany)

[d] Dr. R. Siebert, Prof. Dr. J. Popp, Prof. Dr. B. Dietzek

Institute of Physical Chemistry and Abbe Center of Photonics

Friedrich-Schiller-University Jena

Helmholtzweg 4, 07743 Jena (Germany)

and Institute of Photonic Technology Jena

Albert-Einstein-Straße 9, 07745 Jena (Germany)

Fax : (+49)3641206399

E-mail: benjamin.dietzek@uni-jena.de

[e] Dr. H. Gçrls

Laboratory of Inorganic and Analytic Chemistry

Friedrich-Schiller-University Jena

Lessingstr. 8, 07743 Jena (Germany)

[f] Prof. Dr. L. Gonzꢂlez

Current address: Prof. Dr. L. Gonzꢂlez

Institute of Theoretical Chemistry, University of Vienna

Wꢁhringer Str. 17, 1090 Vienna (Austria)

Fax : (+43)1427752793

E-mail: leticia.gonzalez@univie.ac.at

AHCTUNGTRENNUNG

able MLCT and reversible redox behavior made Ru^IIpoly-

Supporting information for this article is available on the WWW

under http://dx.doi.org/10.1002/chem.201103451.

pyridyl complexes the most attractive candidates. In particu-

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FULL PAPER

lar, Ru^IIcomplexes featuring thiocyanate ligands like the

red (N3, N719) and black dyes (N749) still display the

benchmark with about 11% solar-cell efficiency.^[4]Their

anionic, strong s- and p-donating thiocyanate ligand enables

panchromatic absorption and efficient electron injection

into the semiconductor. However, at the same time the

main drawback of the classical Ru^IIdyes is the monodentate

thiocyanate ligand limiting their stability and prohibiting

further functionalization that could improve the light har-

vesting. Consequently, Ru^IIcomplexes possessing aromatic

carbanion donors that essentially adopt the function of the

thiocyanate have been employed in DSSCs with great suc-

cess.^[5]When embedded within a multidentate ligand, this

cyclometalation^[6]allows for higher stability and ligand func-

tionalization to optimize the photophysical and electrochem-

ical properties.

Furthermore, mesityl was chosen as substituent for further

reasons: 1) it enables both good solubility and good crystal-

lization behavior, 2) it is electronically decoupled due to its

orthogonality and therefore a reasonable electronic refer-

ence, 3) it allows for eased NMR interpretations, and 4) it is

readily available from mesityl amine through diazotization/

azidation and can be considered as a safe azide. The diethy-

nylbenzene building blocks were synthesized under standard

Sonogashira conditions with additional LiCl^[13]starting from

functionalized dibromobenzenes (Scheme 2).

Scheme 2. Exemplary synthesis of the cyclometalating ligands: a) [Pd-

AHCTUNGTRENNUNG

Recently, click-derived^[7]ligands have been successfully

used as analogues of polypyridyl ligands, in particular of

2,2’:6’,2’’-terpyridine (tpy).^[8]We were interested in extend-

ing this analogy to tridentate cyclometalating polypyridyl

2

CH₂Cl₂/EtOH/H₂O (1:2:1), 608C, 12 h, 90%.

ligACHTUNGTRENNUNG

ands, namely 1,3-dipyridylbenzene (dpbH).^[9]In this con-

text, we present a new and systematic series of click-derived,

tridentate, cyclometalated Ru^IIcomplexes^[10]that was stud-

ied in detail by experimental and computational methods to

elucidate the potential for dye-sensitized solar-cell applica-

tion.^[11]Thereby, the combination of theoretical investiga-

tions and photophysical as well as electrochemical studies

enables a consistent and emergent explanatory picture of

the new dyes.

In one case, s-accepting fluoro substituents replace the

methyl groups in the position meta to the carbanion to allow

blocking as well as electronic fine-tuning.^[5c,e,12]In the case

of 1,3,5-tribromobenzene, 2-methylbut-3-yn-2-ol was chosen

as protected alkyne to ease the chromatographic separa-

tion.^[14]After deprotection and cycloaddition, the according

5-bromo-1,3-bis(triazolyl)benzene allows further ligand-

functionalizations by cross-coupling methods in an impor-

tant position. The subsequent installation of a chromophore

at the para position of the cyclometalating ring, for example,

thiophene, would extend the conjugated system and increase

the light absorptivity. Similarly, the mesityl azide reference

was changed once to 9-(4-azidophenyl)-3,6-di-tert-butyl-9H-

carbazole to install an organic chromophore at the complex

periphery as light-harvesting antenna.^[15]In this case, the

conjugation through the triazole ring is not expected^[16]and

the increase of light harvesting would be additive only.

However, although click chemistry provides facile function-

alization within the ligand formation, leading to modular

and higher functionalized complexes, we kept the mesityl

moiety as reference in all other cases to discuss the more

pronounced influences of substituents directly attached to

the cyclometalating phenyl ring or the opposed ligand

(Scheme 3). In addition, it is questionable if the overall

device efficiency profits from the increased absorptivity due

to the carbazoles or if it drops due to lowered dye coverage

on the semiconductor surface.

Results and Discussion

Syntheses: A fully optimized synthetic procedure is present-

ed for the new cyclometalated complexes as well as for

a non-cyclometalated model complex.^[8d]The optimization,

the design strategy and an exemplary synthetic procedure

are explained in the following. For synthetic details, the

reader is referred to the Supporting Information.

The ligands were obtained from aryl azides and diethynyl-

benzene building blocks in good yields using standard click

conditions. For the sake of blocking alternative, bidentate

coordinations that were observed in initial attempts, methyl

groups were placed at strategic positions when possible and

reasonable.^[12]Therefore, o-xylene was chosen as the central

ring as well as mesityl moieties for the clicked-on functional-

ities (Scheme 1).

To facilitate the coordination and cyclometalation, the

common [Ru^III

ACTHUNTRGNE(NUG tpy)Cl₃] precursor can be activated in situ by

halide abstraction with a silver(I) salt in a weakly coordinat-

ing solvent. However, it is known that silver(I) can oxidize

the product yielding a homocoupled dimer^[9a]and therefore

needs to be filtered off after the activation step. Still, appli-

cation of a Ru^IIIprecursor includes a reduction step towards

Ru^IIafter coordination that is normally achieved by alcohols

or amines. Since the cyclometalated complexes are oxidized

Scheme 1. Schematic illustration of the optimization strategy.

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U. S. Schubert, B. Dietzek, L. Gonzꢂlez et al.

Scheme 3. Design strategy and overview of the synthesized ruthenium(II) complexes.

easily, the product is achieved either as Ru^IIIcomplex or it

already underwent side reactions in the position para to the

cyclometalation that has significant radical character within

the Ru^IIIcomplex. This drawback can be overcome by the

under oxygen-free conditions in a closed vial using an alco-

hol as solvent and microwave heating to 1608C for 30 min.

Isolation of the product by a combination of column chro-

matography and crystallization afforded the desired com-

plexes, in most cases as X-ray-quality crystals (Figure 1 and

the Supporting Information) and in reasonable yields vary-

ing from 40 to 70% (Scheme 4 and the Supporting Informa-

tion), depending, amongst others, on whether all strategic

methyl groups were present.

use of [Ru^II

ACHTUNGTRENNUNG(tpy)AHCNUTRGTE(GNNUN CH₃CN)₃]CAHTNUGRTNE[UNGN PF₆]₂as precursor (see

Scheme 4 for a representative example).^[17]In fact, it is

Since cyclometalated complexes are very electron-rich in

the position para to the carbanion, they enable targeted ho-

mocoupling and post-complexation functionalizations in the

presence of oxidants, electrophiles, or both.^[18]This allowed

the introduction of a nitro group under Menke conditions

and, thereby, the respective manipulation of the carbanion

donation by a s- and p-accepting group in turn.^[5d]

The installation of the anchoring carboxylic acid functions

for the DSSC was achieved simply by using ester functional-

ized ligands and saponification^[5e,f]subsequent to the com-

plexation. Thus, the intermediate, highly soluble, ester-func-

tionalized complexes could be purified and studied, since

they are seen as models for the final complexes adsorbed to

TiO₂.^[5k]

Scheme 4. Exemplary synthesis of the cyclometalated ruthenium(II) com-

plexes: a) HNCN, methanol, microwave, 30 min., 1608C, 50%.

easily synthesized from [Ru^III

nol/water using AgNO₃and, in contrast to [Ru^III

AHCTUNGTRENNUNG

ACHTUNGTRENNUNG

can be purified completely, thus simplifying the subsequent

complexation. In more detail, after removal of the AgCl by

filtration over celite, the product can be isolated either by

column chromatography or directly by vapor diffusion of di-

ethyl ether into a concentrated acetonitrile solution yielding

large, even X-ray-quality crystals (see the Supporting Infor-

mation). The subsequent cyclometalation was performed

Crystal structures: Single crystals of the ligands HNCN and

HNCN-F as well as of the three Ru^IIprecursors and of

RuNNN, RuNCN, RuNCN-NO₂, RuNCN-F and RuNCN-

Tph could be grown and characterized successfully by X-ray

diffraction (Figure 1 and the Supporting Information). The

systematic variation allows for comparison although only

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Ruthenium(II) Photosensitizers

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Figure 1. Solid-state structures of RuNCN (top left), RuNCN-NO₂(top right), RuNCN-F (bottom left) and RuNCN-Tph (bottom right) (50% probabili-

ty level; counterions, hydrogen atoms and solvent molecules omitted for clarity).

the most pronounced changes are discussed to beware of

packing effects (note the strong distortion within RuNCN-

F).

The mesityl–triazole torsion angle of the complex series

varies between 608 and 908. The thiophene–phenyl torsion

angle in RuNCN-Tph was determined to be 30.78, thus

being in good agreement with the calculated value of 30.28

(see Scheme S9 in the Supporting Information) and allowing

for partial extension of the conjugation into the thiophene

ring.

ꢀ

The replacement of a dative Ru N bond of the poly-

pyridyl-type complex RuNNN by a covalent, organometallic

ꢀ

Ru C bond within the cyclometalated RuNCN complex

leads to a bond shortening from 2.02 to 1.98 ꢄ, caused by

the very good s donation and additional p donation as well

as by electrostatic interactions with the anionic, aromatic

ꢀ

Figure 2. Selected bond lengths (ꢄ) and angles (8) of RuNNN, RuNCN,

RuNCN-NO₂and RuNCN-F.

carbon donor. Furthermore, the adjacent triazole N Ru

bonds are slightly elongated, most likely due to a declined

s orbital overlap by the smaller bite angle. As a consequence

of the good electron donation ability of the carbanion, the

2

3

ꢀ

opposed Ru N bond becomes elongated from 1.97 to

Scheme 5). Also within the triazole ring, the N N double

bond is elongated as a consequence of the increased p back

donation into p* orbitals.

2.01 ꢄ which is well-known as trans influence. Furthermore,

ꢀ

the outer pyridine N Ru bonds are shortened as result of

increased p back donation from the more electron-rich Ru^II

For RuNCN-NO₂, upon installing an electron-withdrawing

group, namely a nitro group that is capable of withdrawal

metal center in the cyclometalated complex (Figure 2 and

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U. S. Schubert, B. Dietzek, L. Gonzꢂlez et al.

DFT calculations: As a basis for a deeper understanding of

the photophysical and electrochemical properties of the pre-

sented Ru^IIcomplexes, namely to gain insight into detailed

structure–property relations, density functional theory

(DFT) calculations, and time-dependent (TD) DFT calcula-

tions have been performed.

Scheme 5. Schematic representation of the electronic consequences of

the cyclometalation and an electron-withdrawing group.

Whilst the description of the UV/Vis characteristics of

these complexes is nowadays close to routine, the descrip-

tion of non-adiabatic events occurring after light excitation

is more troublesome. Their description would in principle

require the use of multiconfigurational methods in combina-

tion with a proper description of spin-orbit coupling (SOC)

effects. Unfortunately, these methods are practically unaf-

fordable for Ru^II–polypyridyl dyes.^[20]Therefore, D-SCF-

DFT (SCF=self-consistent field) and TDDFT methods

remain as valuable alternatives to obtain qualitative and

even quantitative information about Ru^IIcomplexes and

many examples are found in the literature.^[21]DFT calcula-

tions provide the geometries and energies of the ground and

lowest excited states of each symmetry and spin, whilst in-

formation on the higher excited states (i.e., energies, oscilla-

tor strengths and associated character of the excitations) can

be obtained with the help of TDDFT calculations.

through the s and p system, most of the consequences of

the cyclometalation are less pronounced than for RuNCN.

Even though the p-accepting capability might be reduced

due to the dihedral angle of 51.88 (51.68 calculated, see

Scheme S9 in the Supporting Information) with the central

phenyl ring due to repulsion with the ortho-methyl groups,

there is a distinct influence on the p system that strongly re-

duces the p-donation ability of the cyclometalating carban-

ꢀ

ion in the para position. Consequently, the Ru C bond is

elongated to 2.00 ꢄ (Figure 2 and Scheme 5). In particular

in comparison to RuNCN-F, the fluoro substituents that are

strongly s-accepting, but whose moderate p-donation ability

does not affect the carbanion because they are in meta posi-

ꢀ

tions, still allow a very short Ru C bond of 1.98 ꢄ. Also the

changes in bond lengths within the central phenyl ring are

consistent with a participation of a chinoid resonance struc-

ture in RuNCN-NO₂. The successive reduction of the elec-

tron donation of the carbanion by the fluoro and nitro sub-

stituents is demonstrated by the shortening of the triazole

N²=N³double bond due to decreased p back donation. Also

In order to understand the deactivation mechanisms after

light excitation for RuNNN and RuNCN, their most rele-

vant structures involved, namely the singlet ground state

3

(S₀) as well as the most stable MLCT and triplet metal-cen-

tered (³MC) excited state, were optimized. As known for

1

Ru^II–polypyridyl complexes, after excitation of the MLCT

ꢀ

the Ru N bond trans to the carbanion is further elongated

for the same reason.

manifold, ultrafast inter-system crossing (ISC) occurs within

less than 100 fs, leading to the formation of the ³MLCT

states with near-unity quantum yield. Among the subsequent

radiative and non-radiative processes, radiationless deactiva-

Apparently, the fluoro substituent mostly influences the

s donation and might lower the energy of the p system indi-

rectly (inductive effect), while the nitro group causes an ad-

ditional polarization of the p system that strongly weakens

the p donation (mesomeric effect) but to a less extent the

s donation. This is consistent with the electrochemical data:

for RuNCN-F only the HOMO is stabilized, located on the

Ru^IImetal center and the fluoro-substituted cyclometalating

phenyl ring as well, while RuNCN-NO₂shows an additional

LUMO stabilization that is mediated through the aromatic

p system/Ru^IId orbitals, since the LUMO is located on the

opposed tpy ligand.

Interestingly, within all investigated solid-state structures

of triazole-containing ruthenium(II) complexes (see Fig-

ure S143 in the Supporting Information), short-contact inter-

actions of the triazole with either the counterions or the sol-

vent are present. Triazoles and triazolium salts are known to

allow hydrogen bonding as well as electrostatic interac-

tions.^[19]Similar to triazolium salts, a ruthenium-coordinated

triazole is expected to be more polarized than a free tri-

3

tion through thermal population of MC states is supposed

to determine the ³MLCT lifetime.^[1,2]Thus, in addition to

3

the location of the MLCT and MC states, crossing points

3

between the S₀and the MC potential energy surface deter-

mine the non-adiabatic population transfer, as has been re-

cently stated by Boggio-Pasqua et al. for similar Ru^II–poly-

pyridyl complexes.^[22]

The electronic nature of the lowest-energy triplet excited

states of RuNCN has been confirmed by analysis of the spin

density distributions (Figure 3). The most stable ³MLCT

state indeed displays unpaired electrons within a Ru 4d_yzor-

bital and a p* orbital of the tpy ligand, while only Ru-based

3

4d orbitals are involved in the MC state. The main geomet-

3

rical features of both the optimized MLCT and MC struc-

tures for RuNNN and RuNCN are given in Scheme S9 and

S10 in the Supporting Information. In comparison to the S₀

3

geometry, the MLCT and also the MC geometries of each

complex show a weakening of the coordination, attributed

to the population of antibonding orbitals, either p* or “e_g*”,

as well as to the weakened p back bonding with the formally

ACHTUNGTRENNUNGazole. Preliminary results indicate an interaction of the ruth-

enium(II)-coordinated triazole with iodide (see Figure S144

in the Supporting Information). The question, if hydrogen

bonds/electrostatic interactions might allow the preorganiza-

tion of the redox mediator in a position favorable for

3

oxidized Ru “t_2g” orbitals. In the MC structures the tpy co-

3

ordination is even distorted (see exemplarily the MC struc-

ture of RuNCN in Figure 3) due to the weakening, not only

ruthenium

N

of the p back donation but also of the s donation by the

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Ruthenium(II) Photosensitizers

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3

gies (see below and Figure 4), while in the MC geometry, in

which these orbitals are occupied, the destabilizing effects

are less pronounced (see the photophysical model section

below). Furthermore, while the tridentate ligand is only dis-

torted, a monodentate ligand, such as thiocyanate, can be

cleaved off. For all other compounds, only the S₀and the

3

lowest MLCT states were optimized and the main geomet-

rical features are given in the Supporting Information.

To understand the substituent effects on the photophysical

properties, the relevant frontier Kohn–Sham orbitals are

plotted in Figure 4. For RuNCN, p donation destabilizes the

HOMO that is composed of Ru d_yzand NCN p orbitals. In

contrast, for RuNNN the HOMO is less destabilized and

almost of pure Ru d_xzcharacter; only a weak p donation

contributes to the HOMOꢀ1, which is therefore lower in

energy. In both complexes, the LUMO is formed by the

same p* orbital of the tpy ligand; however, the strongly de-

stabilized HOMO of RuNCN causes an additional indirect

LUMO destabilization through the p back donation. Be-

cause the LUMO destabilization is less pronounced than for

the HOMO, the resulting energy gap is much smaller for

RuNCN. A further effect of the strong electron donation

within RuNCN is the strongly destabilized “e_g*” orbitals in

terms of a strong ligand field. Thus, the d_z₂orbital is the

LUMO+8 in RuNNN, being 1.9 eV higher in energy than

the LUMO, while in RuNCN it is the LUMO+14 with an

energy difference of 2.3 eV. This demonstrates that cyclome-

talation indeed enables destabilization of orbitals that are

Figure 3. Spin density distribution of the energy optimized ³MLCT (top)

and MC (bottom) geometries of RuNCN.

3

population of the “e_g*” levels. Thus, repulsive interactions

are avoided by ligand distortion, hence lowering the energy

3

populated in MC states and that are relevant for the radia-

3

of the MC state. This is important, since in the MLCT ge-

ometry, due to the strong effective s donation, the unoccu-

pied, antibonding metal d orbitals are located at high ener-

tionless deactivation. However, the actual electronic situa-

3

tion at the MC geometry might be different as mentioned

3

above. Therefore, although the MC stabilization might be

Figure 4. Selected PCM-B3LYP/6-31G* Kohn–Sham orbitals and energy level scheme for the Kohn–Sham orbitals of the Ru^IIcomplexes.

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U. S. Schubert, B. Dietzek, L. Gonzꢂlez et al.

helpful, it does not necessarily cause a suppression of the

formation. Firstly, the comparison of the parent cyclometa-

lated complex RuNCN with its non-cyclometalated counter-

part RuNNN reveals the strong influence of the carbanion

donor on the UV/Vis absorption properties. A significant

bathochromic shift of the MLCT maxima from 428 to

532 nm, corresponding to 4500 cm^ꢀ1, is observed upon cyclo-

metalation and well reproduced by the performed calcula-

tions. Additionally, an extension of the absorption from 550

to 650 nm is observed that can be explained by destabiliza-

tion of the Ru-4d orbitals in the RuNCN complex. Indeed,

the electronic excitations responsible for these bands involve

mainly these orbitals (Table S4 in the Supporting Informa-

tion). Furthermore, since RuNCN possesses an organome-

tallic, covalent bond, the HOMO is composed of Ru-d orbi-

tals as well as p orbitals of the cyclometalating ligand, while

the LUMO (and higher unoccupied molecular orbitals) is

p*-tpy-based. Thus, if the anchoring groups are installed at

the tpy acceptor ligand, the transition dipole moment is di-

rected towards the semiconductor surface by the distinct

push–pull effect.^[5a]Since these transitions exhibit partial

ligand-to-ligand charge-transfer (LLCT) character, they can

be described as metal/ligand-to-ligand charge-transfer

(MLLCT) excitations. This underlines the feasibility of di-

rectly influencing the HOMO by manipulation of the cyclo-

metalating ligand, although usually the MLCT declaration is

kept in literature.^[5f,j]Furthermore, the MLCT bands are

broadened and even split because of the electronic asymme-

try that breaks the orbital degeneracy. Thus, the shorter

wavelength transitions around 400 nm exhibit MLCT,

MLLCT, and admixed MC character (see S₆, S₁₄and S₁₇in

Table S4 in the Supporting Information). In the UV region,

the high-energy transitions are mainly of p–p* character

(see S₃₄and S₄₁). However, after thermal relaxation in terms

of Kashaꢀs rule, the transferred charge will reside on the ac-

ceptor ligand. As a further result of the strong anionic

carbon donor, a weak room-temperature emission at 751 nm

(F_PL: 0.006%) was observed for RuNCN (see the photo-

physical model below).^[6a]

3

thermal population of the MC from the MLCT states (this

issue will be discussed in more detail in the photophysical

model section below).

Figure 4 also shows the HOMO orbitals of RuNCN-F,

RuNCN-NO₂and RuNCN-Tph. The introduction of an elec-

tron-withdrawing fluoro or nitro group directly attached to

the HOMO site leads to HOMO stabilization, since the

electronic repulsion and electron donation of the carbanion

donor is tempered, but also because the aromatic system,

which forms a part of the HOMO itself, is stabilized. In case

of RuNCN-Tph, the HOMO and LUMO are slightly desta-

bilized due to electron donation from the thiophene moiety,

while the energy gap remains constant. Importantly, the con-

jugation of the HOMO is extended onto the thiophene ring,

which should give rise to an increased light absorptivity (see

the photophysical properties). Apparently, stabilization due

to extension of the conjugation is overcompensated by elec-

tron donation of the thiophene. For complexes of ester-func-

tionalized tpy ligands, the HOMO is slightly stabilized be-

cause of the increased p acidity of the ligand. Since they are

directly attached to the LUMO site, the LUMO level is

strongly stabilized by their electron withdrawal, resulting in

smaller energy gaps, in particular for RuNCN-(COOMe)₃.

Furthermore, the LUMO, which is not shown for these com-

plexes, is the same orbital throughout the whole series and

differs only in energy. As an exception, in RuNCN-

(COOMe)₃the LUMO is a different orbital that is however

located on the tpy ligand.

Photophysical properties: A key feature of designated

photo-redoxactive Ru^IIcomplexes, in particular when

aiming at a potential application in dye-sensitized solar cells,

is their photophysical behavior. Thus, UV/Vis absorption

and emission spectrum measurements as well as photolumi-

nescence quantum yield (F_PL) and lifetime determinations

were executed. Additionally, PCM-TD-B3LYP (PCM=po-

larizable continuum model) vertical excitations were com-

puted for all the complexes except for RuNCN-Cbz (see the

Supporting Information for computational details).

First of all, the free cyclometalating ligands were charac-

terized. Their UV/Vis spectra show a strong absorption

peak at around 240 nm with extinction coefficients of

36000–140000m^ꢀ1cm^ꢀ1. Additional bands are located at

about 295 nm with weak intensities of 1100 and

4600m^ꢀ1cm^ꢀ1for HNCN and HNCN-F, respectively. In con-

trast, HNCN-Cbz and HNCN-Tph, possessing additional

chromophores, exhibit strong absorption peaks beyond

290 nm, with e values of 58000 and 46300m^ꢀ1cm^ꢀ1for

HNCN-Cbz and 15400m^ꢀ1cm^ꢀ1for HNCN-Tph. All ligands

are fluorescent, showing emission bands at 325 (HNCN,

HNCN-F), 367 (HNCN-Tph), and 404 nm (HNCN-Cbz)

(see the Supporting Information).

To understand the influence of the triazole moiety, a com-

parison referring to the corresponding Ru^IIcomplexes of

pyridine analogues, namely 2,2’:6’,2’’-terpyridine and 1,3-di-

pyridylbenzene, is helpful. When compared to [Ru

ACHTUNGTRENNUNG(tpy)₂]-

AHCTUNGTRENNUNG

a broadened and blue-shifted absorption. Also the emission

at 77 K, which is similar in shape for both, is blue-shifted

from 603 to 574 nm. According to the calculations, the emit-

ting state is of ³MLCT character and tpy-based (see the

DFT calculation above). Additionally, the computed emis-

sion maxima (adiabatic emission energies obtained with D-

SCF approach, see the Supporting Information for details),

are given in Table 1 and correlate well with the experimen-

tal data. The absorption spectra of [RuACHTNUGRTNENUG(tpy)ACHTUNGTRENN(UGN dpb)]PF₆and

RuNCN are similar,^[5j]except for a slight hypsochromic shift

that is observed in the absorption and emission spectra of

RuNCN. Interestingly, although still weak, the emission is

slightly increased for RuNCN, most likely because of the

higher emission energy in accordance with the energy-gap

The absorption and emission features as well as the com-

puted transitions of the studied Ru^IIcomplexes are shown in

Figure 5 and Table 1. For the assignment of the PCM-TD-

B3LYP excitations, see Tables S4–S10 in the Supporting In-

4016

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Chem. Eur. J. 2012, 18, 4010 – 4025

Ruthenium(II) Photosensitizers

FULL PAPER

Figure 5. UV/Vis absorption and emission spectra of the investigated ruthenium(II) complexes (10^ꢀ6m in CH₃CN). For RuNCN and RuNNN, the PCM-

TD-DFT/6-31G* computed vertical excitations are superimposed. Solid lines represent the measured curve and symbols are only used for assignment.

law. Furthermore, the extinction coefficients are lowered for

RuNCN, attributed to the shorter conjugation that is only

partially extended into the triazole.^[8a]

Evidently, this is due to extension of the HOMO and, thus,

expansion of the optical cross section (see DFT calculations

and Figure 4).^[5d]In contrast, the attachment of the carbazole

moiety provides an additional but separated chromophore

that is not in conjugation with the cyclometalated phenyl

ring.^[16]Thereby, the extinction coefficients below 450 nm

double with respect to RuNCN, because the carbazole par-

ticipates in LC transitions, while the absorption bands

beyond 450 nm, in analogy to RuNCN assigned mainly to

To allow for HOMO fine-tuning (see the electrochemical

properties below), electron-withdrawing groups, namely

nitro and fluoro substituents, were installed at the central

phenyl ring. Consequently, a slight hypsochromic shift

(700 cm^ꢀ1) of the MLCT features in the UV/Vis absorption

spectrum was observed due to HOMO stabilization. Ac-

cordingly, a room-temperature emission can be observed for

d_Ru/p_NCN!p*_tpytransitions, remain unchanged in shape and

RuNCN-NO₂that is blue-shifted by 870 cm^ꢀ1(F_PL

:

intensity. Furthermore, the room-temperature emission of

RuNCN was preserved, thus no additional quenching path-

ways are introduced; instead, the emission intensity was

even slightly increased (Table 1).

0.010%). In contrast, RuNCN-F features no measurable

photoluminescence (see the temperature-dependent lifetime

measurements below). Besides, the p-accepting nitro group,

ꢀ

para to the Ru C bond, leads to a decrease of the extinction

For the immobilization on the semiconductor surface in

DSSCs, carboxylic groups on the acceptor ligand are neces-

sary. Beside their function as anchoring groups, they also

strongly influence the photophysical properties as additional

electron-withdrawing groups. Here, the ester-functionalized

complexes were seen as models for the TiO₂-bound dyes.^[5k]

The introduction of a single carboxylic ester at the 4’-posi-

coefficient by a third compared to the parent RuNCN com-

plex, attributed to interference with the push–pull polariza-

tion.

To increase the extinction coefficients, additional chromo-

phores were attached^{[5d–g,15,23]}either directly to the central

phenyl ring or as clicked-on antennas. The thiophene moiety

ꢀ

that was installed para to the Ru C bond increases the ex-

tion of the terpyridine causes a stabilization of the p*_tpy-

tinction coefficients over the whole UV/Vis absorption spec-

trum, including the highest wavelength absorption that

grows from 7300m^ꢀ1cm^ꢀ1for RuNCN to 16500m^ꢀ1cm^ꢀ1.

based LUMO (Figure 4) and an enhanced transition dipole

moment, thus leading to a slight bathochromic shift by

about 500 cm^ꢀ1as well as a tripled extinction coefficient in

Chem. Eur. J. 2012, 18, 4010 – 4025

ꢃ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemeurj.org

4017

U. S. Schubert, B. Dietzek, L. Gonzꢂlez et al.

Table 1. Photophysical data of the complexes.

Complex

298 K

77 K

k_r+k₁

k₂

[s^ꢀ1

k₃

[s^ꢀ1

DE₂

[cm^ꢀ1

DE₃

[cm^ꢀ1

]

abs

max

em

max

[f]

l

F_PL

t

l

t

G

]

G

]

E

]

U

]

A

max

[nm] (e [10³m^ꢀ1·cm^ꢀ1

]

[nm]^[a,c,d]

10^ꢀ5[a,e][ns]

[nm]^[d][ms]

[a,b]

RuNNN

325 (sh), 428 (10.6),

500 (1.8)

308 (63.4), 475 (14.7)

371 (14.5), 488 (7.3),

532 (6.3)

– (545)

–

574

14

–

[5j,30]

[Ru

N

[PF₆]₂

–

6.1

0.25

4.1

603

719

–

4.1

6.5ꢅ10⁴

2.0ꢅ10¹³

2.1ꢅ10⁷

1700

720

350

RuNCN

751 (827)

2.44ꢅ10⁵1.1ꢅ10¹²

3.11ꢅ10⁸1830

[5j]

[g]

[Ru

U

(dpb)]PF₆

424 (9.6), 499 (14.4),

ꢁ540 (ꢁ10)

781

0.9

–

(dpb)]PF₆

504 (10.8), 550 (8.3)

363 (14.3), 473 (7.2),

507 (6.5)

784

–

4.5

–

4.5

752

0.48

5.8

–

1290

–

RuNCN-F

0.5^[h]661

1.72ꢅ10⁵9.74ꢅ10¹¹

RuNCN-NO₂

RuNCN-Cbz

RuNCN-Tph

RuNCN-COOEt

RuNCN-COOH

365 (9.0), 483 (7.4),

511 (6.8)

384 (26.2), 485 (7.3),

523 (6.3)

350 (36.4), 482 (16.5),

518 (13.8)

372 (36.8), 495 (19.9),

546 (18.2)

373 (22.0), 491 (10.5),

532 (10.5)

413 (17.9), 500 (9.4),

574 (9.8), 641 (5.3)

398 (8.7), 497 (5.0),

572 (5.3), 641 (3.2)

705 (759)

750

10.0

25.0

5.3

667

5.2

4.5

4.3

1.92ꢅ10⁵6.63ꢅ10¹¹

–

1395

–

6.7^[h]712

4.1^[h]722

2.22ꢅ10⁵2.04ꢅ10¹¹1.33ꢅ10⁸1570

2.33ꢅ10⁵1.89ꢅ10¹¹1.42ꢅ10⁸1452

270

240

–

745 (802)

– (941)^[d]

–

[d]

–

12.3

745

5.7

1.75ꢅ10⁵2.02ꢅ10¹⁰

1135

–

[d]

RuNCN-(COOMe)₃

RuNCN-(COOH)₃

– (1032)^[d]

–

[d]

–

[a] Measured 10^ꢀ6m in deaerated CH₃CN. [b] sh=shoulder. [c] In brackets: Adiabatic emission energy values (DSCF-PCM-DFT/6-31G*). [d] The detec-

tor limit is at 800 nm. [e] Determined using [Ru(dqp)₂]

G

ACHTUNGTRENNUNG

3

cal of the 77 K lifetime. [g] ttpy=4’-tolyl-tpy, from reference [32], note that the 4’-tolyl substituent stabilizes the MLCT and thereby prolongs the excited

state lifetime.^[33][h] Extrapolated from the temperature-dependent phosphorescence lifetime measurements.

the visible region. When carboxylic ester groups are at-

tached to the para positions of all three pyridine moieties of

the terpyridine ligand, namely for RuNCN-(COOMe)₃, the

absorption is again significantly red-shifted (by 3000 cm^ꢀ1)

and additionally broadened, hence covering almost the

whole visible spectrum and causing a black color of the

complex. The main visible absorption features are slightly

more intense and separated, but they reflect the same transi-

tions as in RuNCN, which becomes evident in comparison

to a 77 K excitation spectrum of RuNCN (see Figure S118

in the Supporting Information). A photoluminescence of the

complexes bearing carboxylic ester groups was not observed,

most likely due to the low energy gap according to the

energy-gap law (see below) or the reduced spectrometer

sensitivity at low emission energies.^[24]The drop in extinction

and the slight blue shift of the saponified complexes,

RuNCN-COOH and RuNCN-(COOH)₃, are attributed to

the lowered electron acception that causes a decreasing po-

larization and LUMO stabilization.

All the investigated complexes, except RuNCN-COOEt,

RuNCN-(COOH)₃and RuNCN-(COOMe)₃, are emissive at

77 K and show bandshapes typical for ruthenium coordina-

tion compounds, namely an intense 0–0 transition that is ac-

companied by a weaker vibronic satellite (Figure 6, see Fig-

77 K emission spectroscopy: As all presented coordination

compounds show either no or only a weak emission at room

temperature, owing to the presence of several non-radiative

deactivation pathways that will be discussed in detail later,

the exact energy of the lowest-lying excited state is challeng-

ing to determine. Emission spectroscopy at low tempera-

tures can enable the determination of these energies if the

dominant non-radiative channels are thermally activated.

^II(tpy)₂]

!

Figure 6. Emission spectra of the complexes: RuNNN ( ), [Ru

^

*

(

), RuNCN-F (3), RuNCN-NO₂("), RuNCN-Cbz ( ), RuNCN (&),

~

RuNCN-Tph ( ) and RuNCN-COOH (<) in n-butyronitrile glass at

77 K. The spectral resolution decreases at higher wavelengths due to a de-

creasing spectrometer sensitivity (spectrometer response is given as

a dashed line). Solid lines represent the measured curve and symbols are

only used for assignment, a color figure is given in the Supporting Infor-

mation (Figure S119).

4018

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ꢃ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Chem. Eur. J. 2012, 18, 4010 – 4025

Ruthenium(II) Photosensitizers

FULL PAPER

ure S118 in the Supporting Information for excitation spec-

tra). The emission maxima are summarized in Table 1. We

note that the emission at 77 K is blue-shifted compared to

the respective room-temperature emission, because the rigid

solvent matrix at low temperatures prevents solvent reor-

ganization and thus avoids the stabilization of the more

polar charge-separated excited state (rigidochromic

effect).^[25]Still, the comparison of the emission spectra at

77 K reveals a strong bathochromic shift of 3520 cm^ꢀ1rela-

tive to the non-cyclometalated RuNNN caused by the

strong HOMO destabilization in RuNCN. The functionaliza-

tion of the NCN ligand with the slightly electron-donating

thiophene causes no further HOMO destabilization, which

might be attributed to the dihedral angle of around 308

which diminishes the conjugation.^[26]In contrast, the func-

tionalization of the NCN ligand with electron-withdrawing

groups results in a HOMO stabilization, and thus a blue

shift of the emission maximum of about 1130 cm^ꢀ1with re-

spect to RuNCN.

Functionalization of the tpy ligand with carboxylic acid

esters or free carboxylic acids causes a LUMO stabilization

and, as a consequence, a red-shifted absorption and emis-

sion. However, it was only possible to measure the emission

spectra at 77 K for RuNCN-COOH, which is red-shifted by

490 cm^ꢀ1relative to RuNCN. We note that the detector is

less sensitive in the near-infrared region as demonstrated by

the response function in Figure 6. A theoretically predicted

emission of RuNCN-(COOMe)₃above 1000 nm (Table 1)

would thus not be detectable with our measurement setup.

Another important excited-state parameter is its lifetime,

which, in contrast to the emission quantum yield, truly re-

flects the stability of the excited state. Therefore, emission

lifetimes were determined at both room temperature and

77 K (Table 1). As a main result, the emission lifetime de-

creases with decreasing emission energy. Assuming that

thermally activated radiationless deactivation pathways are

frozen at 77 K, this can be explained by the energy-gap

law.^[27]Usually, it can be observed only within a series of

very similar complexes or in different solvents,^[28]since other

effects, such as delocalization and rigidity, may interfere so

that long excited-state lifetimes and small energy gaps do

not exclude each other.^[29]Accordingly, RuNCN-COOH has

a longer lifetime than RuNCN (Table 1), which is attributed

to the modification of the acceptor ligand.

Figure 7. Temperature-dependent emission lifetimes for the complexes:

*

RuNCN-F (³), RuNCN-NO₂("), RuNCN-Cbz

(

), RuNCN (&),

~

RuNCN-Tph ( ) and RuNCN-COOH (<) in n-butyronitrile. Symbols

correspond to measured lifetimes and solid lines represent a non-linear

fit according to equation 1 (RuNCN-COOH, RuNCN-F and RuNCN-

NO₂) or equation 2 (RuNCN, RuNCN-Cbz and RuNCN-Tph). A color

figure is given in the Supporting Information (Figure S120).

and RuNCN-NO₂show a different behavior compared to

RuNCN, RuNCN-Cbz, and RuNCN-Tph. In detail, for the

last three complexes the lifetime starts to increase at higher

temperatures and shows a reduced slope than for RuNCN-F

and RuNCN-NO₂, thus being shorter-lived at 77 K. In ac-

cordance with the literature, the excited-state lifetime at

higher temperatures is determined by thermal deactivation

via ³MC states.^[1,2]Evidently, the electron-withdrawing

groups reduce the donor strength of the carbanion and

3

therefore lower the MC states. Consequently, thermal deac-

tivation is facilitated, which can be quantified by fitting an

Arrhenius expression to the experimental data [Eqs. (1) or

(2)]. Thus, fundamental information about thermally activat-

ed, non-radiative deactivation channels can be obtained, for

example, their rate constants and activation energies.

1

P

tðTÞ ¼

¼

ð1Þ

ð2Þ

k_rþ k_nrk_rþ k₁þ k₂expðꢀDE₂=k_BTÞ

1

P

¼

k_rþ k_nr

1

k_rþ k₁þ k₂expðꢀDE₂=k_BTÞ þ k₃expðꢀDE₃=k_BTÞ

Temperature-dependent lifetime measurements: To gain de-

tailed insight into the deactivation dynamics of the lowest-

lying excited state and the stabilizing/destabilizing effects of

different substitution patterns, temperature-dependent life-

time measurements were carried out between 300 and

160 K.^[34]The results of these experiments are depicted in

Figure 7.

In general, the investigated cyclometalated complexes

reveal a steady rise of the emission lifetime with decreasing

temperature. However, depending on the specific substitu-

tion pattern, the slope of the lifetime increase varies. Similar

to the emission spectra at 77K, the complexes RuNCN-F

For RuNCN, RuNCN-Cbz, and RuNCN-Tph, two ther-

mally activated (k₂, DE₂and k₃, DE₃) and one non-activated

decay channel (k₁),^[35]in addition to the radiative one (k_r),

are necessary to fit the equation to the data [Eq. (2)]. In

contrast, for RuNCN-F and RuNCN-NO₂a model of three

channels (a radiative, a non-activated, and a thermally acti-

vated non-radiative one) is sufficient to reproduce the data

[Eq. (1)]. In principle, there should be a third dark channel

for the last two complexes, but due to its low activation

energy it is not visible in the experimental temperature

Chem. Eur. J. 2012, 18, 4010 – 4025

ꢃ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemeurj.org

4019

U. S. Schubert, B. Dietzek, L. Gonzꢂlez et al.

range between 300 and 160 K. The results obtained by ana-

lyzing the temperature-dependent lifetime data are summar-

ized in Table 1.

ture emission might be due to experimental limitations and

does not necessarily mean short excited-state lifetimes.

The second activated decay channel (k₃, DE₃) can be at-

tributed to internal conversion (IC) to an energetically

slightly higher-lying MLCT state of increased singlet charac-

ter (MLCT’), which is also a common feature for ruthenium

polypyridyl dyes.^[30,37]

The first activated decay channel (k₂, DE₂) is assigned to

the transition from the emitting ³MLCT to the S₀via the

ACTHNUTRGNEUNG

³MC excited state.^[36]Compared to [Ru(tpy)₂]²⁺and other

complexes of functionalized terpyridines, this activation

3

energy is remarkably low (see the discussion of the MC ge-

ometry in the DFT section).^[30,34]We postulate that the

room-temperature emission and prolonged excited-state life-

times, which were observed despite similar or lower activa-

Electrochemistry: Crucial for the potentially photo-redoxac-

tive Ru^IIcomplexes, in particular with respect to photovolta-

ic applications, are their electrochemical properties. Thus,

the reversibility of the redox processes and the location of

the oxidation and reduction potentials in comparison to the

3

tion energies for the MLCT–³MC internal conversion than

3

in [Ru

(tpy)₂]²⁺, are caused by a weaker coupling of the MC

ꢀ

and the ground state.^[36]This is substantiated by small k₂

I₃/I^ꢀcouple and the TiO₂conduction band, respectively,

rate constants (10¹¹–10¹²vs. 1.7ꢅ10¹³s^ꢀ1in case of [Ru-

are highly important. Consequently, cyclic voltammetry

(CV) measurements were carried out and related results are

presented in Figures 8 and 9, Table 2, and the Supporting In-

formation.

ACHTUNGTRENNUNG

(tpy)₂]²⁺). To the best of our knowledge, such a tempera-

ture-dependent excited-state lifetime measurement has been

performed the first time for cyclometalated ruthenium(II)

complexes. In principle, we would expect a similar behavior

Analyzing the influence of cyclometalation by comparing

RuNCN to RuNNN shows a strong cathodic shift of the oxi-

dation potential of 900 mV due to the strong s and p dona-

tion as well as electronic repulsion caused by the carban-

for [RuACHTUNGTRENNUNG(tpy)ACHTUNGTRENNUNG

(dpb)]⁺or analogous complexes.

Furthermore, for RuNCN-F and RuNCN-NO₂lower DE₂

values were obtained, supporting the assumption of a de-

3

creased MC destabilization, but k₂is again small and even

ion.^[6a]Based on the calculations (see above), the first oxida-

ACHTUNGTRENNUNG

smaller for RuNCN-NO₂. When comparing RuNCN-F and

tion of RuNCN is not only metal-, but also ligand-based,

3

+

RuNCN-NO₂, the fluoro substituent mainly lowers the MC

and corresponds to a transition from d_Ru/p_NCNto d_Ru/

3

2+

C

energy, which is therefore closer to the MLCT state. In con-

p_NCN

transition. Also the first reduction process, located

trast, the p-accepting nitro group para to the carbanion also

on the terpyridine ligand, is shifted towards lower potentials

by 260 mV, owing to increased p back donation from the

more electron-rich Ru^IIcenter.^[5j]Both oxidation and reduc-

tion process of the RuNCN complex are fully reversible

under cyclic voltammetric conditions. Nevertheless, reversi-

bility was investigated in a more detailed fashion by UV/Vis

spectroelectrochemical means (see below).

Again, a comparison of the triazole-containing complexes

with their pyridine counterparts allows for a relative classifi-

cation of electronic properties of the ligands. In comparison

affects the ³MLCT energy (see the LUMO energy in

3

Table 2) resulting in a larger observed MLCT–³MC barrier

that allows for room-temperature emission. This is also re-

flected by the displacement of the RuNCN-F curve to lower

temperatures (Figure 7).

Despite this good correlation between structure and excit-

ed-state dynamics, the temperature-dependent emission

properties of RuNCN-COOH need to be discussed sepa-

rately. Within the whole series, its activation energy for the

³MLCT–³MC internal conversion is the least. Nevertheless,

a room-temperature lifetime of 12.3 ns could be measured,

which is remarkably high in comparison with the other com-

plexes discussed herein. This can only be explained by the

relatively low transition rate for this process, which is one

order of magnitude smaller than in the other complexes.

Apparently, here the absence of a detectable room-tempera-

to the RuNNN-analogous [Ru

of a terpyridine ligand by the click-derived 2,6-bis(1,2,3-tri-

azol-4-yl)pyridine leads to a metal-based HOMO of lower

ACHTUNGTRENGU(N tpy)₂]ACHUTGNTREN[NUGN PF₆]₂, the substitution

AHCTUNGTRENNUNG

energy and tpy-based LUMO of higher energy, indicating

a weaker s-donor and p-acceptor strength of the triazole-

containing ligand that would allow the tpy to predominate

the p back donation.^[8a–d]In contrast, when comparing

Table 2. Electrochemical data of the complexes.

ox

1=2

red

1=2

Complex

E

[V] (i_pa/i_pc,DE_p[mV])^[a]

E

[V] (i_pa/i_pc,DE_p[mV])^[a]E_S*(ox) [V]^[d]E_HOMO[eV]^[c]E_LUMO[eV]^[c]E_gap,el[eV] E_gap,opt[eV]

RuNNN

0.98 (1.1, 74)

0.89 (64)

0.08 (1.0, 67)

0.12 (62)

0.31 (1.0, 74)

0.26 (1.0, 76)

0.10 (1.0, 83)

0.07 (1.0, 69)

0.16 (1.0, 70)

ꢀ1.72 (0.9, 80)

ꢀ1.66 (63)

ꢀ1.22

–

ꢀ5.78

–

ꢀ3.22

–

2.56

–

1.97

–

2.15

1.96

1.89

1.94

1.82

1.69

2.20

–

1.91

–

1.98

2.03

1.94

2.00

1.90

1.77

[5j]

[Ru

RuNCN

[Ru(tpy)ACHTUNGTRENNUNG

ACHTUNGTRENNUNG(tpy)₂]ACHTUNGTRENNUNG

ꢀ1.98 (1.0, 71)

ꢀ1.95 (63)

ꢀ1.83

ꢀ4.88

ꢀ2.91

[5j]

T

–

RuNCN-F

ꢀ1.95 (1.0, 79)

ꢀ1.82 (1.0, 88)

ꢀ1.97 (irrev.)^[b]

ꢀ1.79 (1.1, 80)

ꢀ1.56 (1.0, 71)

ꢀ1.67

ꢀ1.77

ꢀ1.84

ꢀ1.93

ꢀ1.74

ꢀ1.51

ꢀ5.12

ꢀ5.07

ꢀ4.89

ꢀ4.87

ꢀ4.96

ꢀ5.06

ꢀ2.97

ꢀ3.11

ꢀ3.00

ꢀ2.93

ꢀ3.14

ꢀ3.37

RuNCN-NO₂

RuNCN-Cbz

RuNCN-Tph

RuNCN-COOEt

RuNCN-(COOMe)₃0.26 (1.0, 71)

[a] Measured in CH₃CN with 0.1m Bu₄NPF₆; with respect to Fc/Fc⁺as a reference. [b] Irreversible process; E_1/2received from DPP. [c] Calculated by

Fc=Fc^þ

onset

[3b]

using E_LUMO/HOMO=[ꢀ(E_o^re_n^d_s⁼_e^o_t^xꢀE

)ꢀ4.8] eV. [d] Calculated using E_S=E^o₁₌^x₂ꢀE_gap,opt

.

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Ruthenium(II) Photosensitizers

FULL PAPER

RuNCN with the analogous [RuACHTUNTRGNENUG(tpy)ACHTUGNTREN(NUGN dpb)]PF₆, for the

click-derived complex the oxidation and reduction potentials

are cathodically shifted. Evidently, the triazole-containing

cyclometalating ligand is a stronger p donor increasing the

electron density on the Ru^II/NCN-based HOMO and,

through increased p back donation from the more electron-

rich Ru^IIto the tpy ligand, the energy of the tpy-based

LUMO. This is most likely due to weaker stabilization of

the carbanion by the triazole in terms of its electron excess

and shorter conjugation length.^[8a]Additionally, for the same

reason as for RuNNN, the lower p acceptor strength of the

triazole-containing cyclometalating ligand, when compared

to its pyridine analogue, might cause the HOMO and tpy-

based LUMO of higher energy. Consequently, the more neg-

ative excited-state oxidation potential (Figure 9, Table 2)

should increase the driving force for the electron injection

into the TiO₂conducting band or would allow for a higher

TiO₂conducting band, which can be achieved by a different

electrolyte composition,^[38]and therefore higher cell voltag-

es. At the same time, the lower oxidation potential would

lower the driving force for the regeneration of the photooxi-

dized dye (Figure 9). In consistence with the blue shift of ab-

sorption and emission, which correspond to the optical gap,

the electrochemical HOMO–LUMO gap of RuNCN is in-

creased in comparison with [RuACHTUNTRGNENGU(tpy)ACHTUTGNREN(NUGN dpb)]PF₆.

To still allow efficient dye regeneration, a fine-tuning of

the oxidation potential was achieved by installing electron-

withdrawing fluoro and nitro groups on the cyclometalated

phenyl ring.^[5c–e]Thus, the HOMO is stabilized and the oxi-

dation shows an anodic shift by 230 and 180 mV, respective-

ly, to be about 0.5 V more positive than the I^ꢀ/I₃redox

ꢀ

Figure 8. Cyclovoltammetric spectra of the cyclometalated ruthenium(II)

complexes and RuNNN as a reference (10^ꢀ5m in CH₃CN with 0.1m

Bu₄NPF₆). Solid lines represent the measured curve and symbols are only

used for assignment.

couple and, thereby, ensure enough driving force for the dye

reduction.^{[3b,5c,e,i,39]}In the case of RuNCN-F, the reduction

potential remains nearly unchanged, while for RuNCN-NO₂

a distinct anodic shift from ꢀ1.98 to ꢀ1.82 V is observed.

Most likely, the strong p-accepting nitro group weakens the

p donation of the para-carbanion and, thereby, the p back

donation to the terpyridine. A more detailed discussion of

the electronic effects of nitro and fluoro substituents on a cy-

clometalated phenyl ring depending on their positions can

be taken from the literature.^[5d,40]

Introduction of the carbazole and thiophene moieties af-

fects the oxidation and reduction potentials only marginally,

but leads to irreversibility of the reduction process under

CV conditions in both cases. However, only the dye oxida-

tion and subsequent reduction is the operative process in

DSSCs and this process still is reversible. We note that a stra-

tegic methyl group was placed in the 5-position of the thio-

phene to avoid any following reactions, such as radical dime-

rizations.^[41]

Electrochemical investigations on the ester-substituted

complexes RuNCN-COOEt and RuNCN-(COOMe)₃

showed significant anodic shifts of the reduction potentials

about 190 and 430 mV, respectively, due to stabilization of

the LUMO, which is tpy-based. Still, enough driving force

for a fast electron injection would be given. Furthermore,

since the p-accepting esters are in para position, they in-

Figure 9. Comparison of the excited-state and ground-state oxidation po-

ꢀ

tentials with the TiO₂conducting band and the I^ꢀ/I₃redox couple, re-

spectively. The grey line indicates a potential that ensures enough driving

force for the dye regeneration.^[3b,5i]The actual TiO₂conducting band

edge depends on the electrolyte composition and is therefore drawn dif-

fusely.^[38]

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www.chemeurj.org

4021

U. S. Schubert, B. Dietzek, L. Gonzꢂlez et al.

crease the overall p-acceptor strength of the polypyridyl

ligand, causing a small anodic shift of the oxidation of 80

and 180 mV, respectively. Thus, the oxidation potential of

RuNCN-(COOMe)₃would enable efficient regeneration.

However, the strongly electron-withdrawing carboxylic ester

can only be seen as approximation of TiO₂-adsorbed carbox-

ylic acids^[5k]and the actual electronic situation depends on

the protonation state of the adsorbed complex (see pK_ade-

terminations in the Supporting Information).^[42]Therefore,

although electron-withdrawing, the anchoring carboxylic

acids most likely will have to be combined with above-men-

tioned strategies to lower the oxidation potential direct-

RuNCN-NO₂the arising transition is very weak. In contrast,

the thiophene-containing complex RuNCN-Tph exhibits the

appearance of two intense absorption peaks around 450 and

900 nm (Figure S126 in the Supporting Information), which

can be likely assigned to a mixed MC/MLCT (MMLCT,

C

d_Ru!d_Ru/p_NCN), MLCT (d_Ru!p*_NCN), or LMLCT transitions

that would possess large orbital contributions of the thio-

phene. Remarkably, the reductions of all oxidized species

recreate the original spectra almost completely, thus con-

firming that the oxidation processes are fully reversible even

under these demanding conditions under which the com-

plexes are oxidized for a long time.

A

The first reductions (studied only for the complexes show-

ing reversible reduction under CV conditions, see the Sup-

porting Information), being located on the terpyridine

applicable in established DSSCs.

C^ꢀ

UV/Vis spectroelectrochemical analysis: To obtain a more

detailed insight into the electrochemistry of the presented

cyclometalated Ru^IIsystems, mainly with regard to reversi-

bility and redox stability, UV/Vis spectroelectrochemical ex-

periments were performed (see Figure 10 for RuNCN and

Figures S122–S128 in the Supporting Information for the re-

maining complexes).

ligand (tpy!tpy ), reveal a less-defined spectral change in

spectroelectrochemical measurements. Again, an absorbance

decrease in the MLCT/MLLCT region can be observed,

caused by the population of a p*_tpyorbital that acts as the

acceptor within the longest-wavelength transition processes.

Additionally, the absorbance also increases at around

450 nm and several changes occur in the UV region of the

spectrum, both originating from appearing, disappearing, or

shifted LC and LLCT transitions. In contrast to the oxida-

tion described above, recreation of the initial complex is not

successful in most cases, which is likely due to following re-

actions. As an exception, RuNCN-(COOMe)₃, which pos-

sesses three electron-withdrawing ester groups at the terpyr-

idine ligand that enable an enhanced stabilization of the

C^ꢀ

electron-rich tpy moiety, allows the nearly full regenera-

tion by re-oxidation.

Photophysical model: Cyclometalated polypyridyl Ru^IIcom-

plexes have been known for some time,^[6,9a]but it was only

quite recently that they have been applied to the field of

dye-sensitized solar cells.^[5]Although there has been elabo-

rated research on photoactive electron-transfer assemblies,

such as homo- and heteronuclear dyads, for the prototypical

bis(tridentate), heteroleptic Ru^IIcomplex of terpyridine and

its cyclometalated analogue 1,3-dipyridylbenzene, a detailed

investigation on the excited-state processes is missing up to

date.^[32,43]Only a simplified, qualitative explanation of its

photophysical properties by relative energies of the S₀, the

lowest ³MLCT state and the ³MC state has been report-

Figure 10. UV/Vis spectroelectrochemical investigation on the oxidation

process of RuNCN (voltage varied between 400 and 1000 mV vs. Ag/

AgCl; 10^ꢀ5m in CH₃CN with 0.1m Bu₄NPF₆).

In general, the oxidation processes show several isosbestic

points, indicating the temporary presence of only two spe-

cies to ultimately form the singly oxidized complex in

a well-defined reaction. The most evident changes during

oxidation are the decrease of MLCT and MLLCT bands be-

AHCTUNGTRENNUNG

ed.^[6a,32]According to that, the lifetime of the charge-sepa-

rated excited-state is determined by the ³MLCT–³MC

energy difference, since the metal-centered excited state

shows a strong coupling to the ground state and therefore

3

tween 350 and 600 nm, caused by depopulation of the d_Ru

/

causes a rapid relaxation once the MC state is populated.

p_NCNHOMO, and the appearance of additional, broad

peaks between 600 and 850 nm (up to 1000 nm in case of

RuNCN-(COOMe)₃), most likely attributed to emerging

This is plausible because antibonding orbitals are occupied

in the MC state, which shows a displacement that typically

3

matches the ground-state geometry at high-energy vibra-

tions; in other words, the transition is highly probable be-

cause of a large Franck–Condon factor (strong coupling case

of displaced oscillators).^[44]Alternatively, the fast decay to

the ground state can be explained in a classical picture as-

suming the surfaces show a nearly barrierless crossing.^[45]

C

LMCT (p_NCN!d_Ru) or LMLCT (p_NCN!d_Ru/p_NCN) transi-

tions. Here, the fluoro-substituted RuNCN-F represents an

exception that shows no changes beyond 600 nm (Fig-

ure S123 in the Supporting Information), probably because

of a very low transition dipole moment. Accordingly, for

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Ruthenium(II) Photosensitizers

FULL PAPER

3

However, we emphasize that the MC–S₀intersystem cross-

isoenergetic for RuNCN, while for RuNNN the MC mini-

3

ing not only depends on the MC, but also on the S₀poten-

mum is lower in energy than the ³MLCT one. This is in

agreement with a destabilized ³MC state for RuNCN as

a result of the cyclometalation. As an additional conse-

tial energy surface, which itself is strongly influenced by the

electronic nature of the ligand. Thus, several Ru^IIcomplexes

3

have been reported, for example, Ru

A

quence, the S₀is destabilized as well and both MLCT and

³MC states appear at lower energies relative to the S₀. How-

3

N

ever, for the thermal MLCT–³MC internal conversion, the

3

A

U

energy barrier and the respective MLCT–³MC conversion

uted to the ³MC destabilization by the carbanion so far.

Nonetheless, temperature-dependent emission lifetime

measurements reveal a similar and even lowered activation

barrier for the population of the ³MC state within the

rate (see DE₂and k₂in the temperature-dependent lifetime

measurements) are determining. Usually, the subsequent

³MC–S₀intersystem crossing rate is the limiting rate. Thus,

referring to the experimental MLCT–³MC energy barrier,

3

RuNCN series compared to [Ru

the non-radiative deactivation rate constant of RuNCN is

orders of magnitude smaller than for [Ru

(tpy)₂]⁺.^[30]We

expect a similar behaviour for the analogous [Ru(tpy)-

(dpb)]⁺complex.

Consequently, D-SCF calculations were performed to gain

(tpy)₂]²⁺. At the same time,

which is lower for RuNCN than for [RuACTHNGUTERNNUG

(tpy)₂]²⁺, and the

nonetheless prolonged excited-state lifetimes, we postulate

3

E

a weaker MC–S₀coupling. In agreement with previous re-

3

T

ports,^[46]we conclude that the MC–S₀intersystem crossing

A

occurs at high energies on the potential energy surfaces for

3

RuNCN, while for RuNNN and [Ru

(tpy)₂]²⁺this MC–S₀in-

a deeper understanding of the photophysics. In Figure 11

the schematic potential energy surfaces for the complexes

RuNCN and RuNNN are depicted. The diabatic energies

(DE) are obtained as the energetic differences between the

energy minima of the optimized geometries, while the adia-

batic energies (AE) are obtained as the actual energy differ-

tersystem crossing point is at low energies and thus readily

accessible. This is plausible, since the covalent binding of

the cyclometalating ligand has a significant influence on

both electronic structure and geometry already of the S₀af-

3

fecting also the MC–S₀coupling.

Still, the lifetime of cyclometalated complexes is relatively

short and the quantum yield is low in comparison to [Ru-

3

ences at the MLCT and the MC optimized geometries. As

3

shown in Figure 11, the MLCT and MC minima are almost

(bpy)₃]²⁺, for example, because of the S₀destabilization.

ACHTUNGTRENNUNG

The resulting small S₀–³MLCT energy gap leads to a more

probable thermally non-activated, radiationless deactivation

due to an increased Franck–Condon overlap of the S₀and

³MLCT vibrational wave functions. The observed decrease

of the excited-state lifetime with decreasing emission energy

is in accordance with the already mentioned energy-gap

law.^[27]

Conclusion

A systematically modified series of new ruthenium(II) com-

plexes of click-derived tridentate cyclometalating ligands

aimed towards the application in dye-sensitized solar cells

was investigated. An optimized synthetic route was estab-

lished. The presented cyclometalated ruthenium(II) poly-

pyridyl complexes feature all benefits of established Ru^IIthi-

ocyanate dyes:

1) The HOMO is raised in energy causing a small energy

gap and, therefore, a strongly red-shifted absorption.

3

2) The strong electron donation destabilizes MC states and

thus offers prolonged excited-state lifetimes.

3) The HOMO is extended to the cyclometalating ligand

that facilitates the dye regeneration.

4) The LUMO is located on the opposite, anchoring ligand.

Consequently, the charge transfer is directed towards the

semiconductor surface.

5) At the same time, the anchoring groups, namely the car-

boxylic acid functions, strongly lower the LUMO energy,

resulting in a panchromatic shift and intense absorption

Figure 11. Proposed potential energy surfaces (the MLCT’ is omitted for

clarity) including adiabatic (AE) and diabatic energies (DE) at the

DSCF-PCM-DFT/6-31G* level of theory for the complexes RuNCN

(top) and RuNNN (bottom).

Chem. Eur. J. 2012, 18, 4010 – 4025

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www.chemeurj.org

4023

U. S. Schubert, B. Dietzek, L. Gonzꢂlez et al.

kindly acknowledged. B.S., C.F., and R.S. are grateful to the Fonds der

Chemischen Industrie for Ph.D. scholarships. D.E. is grateful to the

DAAD for financial support. B.D. and J.P. thank the Fonds der Chemi-

schen Industrie. We also thank M. Jꢁger, E. Altuntas, W. Gꢆnther and G.

Sentis for discussions and performing experiments. Additionally, we

thank D. Schrader, S. Ziemann, and J. Tꢆrker for experimental assis-

tance.

due to the pronounced push–pull effect that heightens

the oscillator strengths and the extinction coefficients.

Additionally, the cyclometalated complexes offer further

advantages that are essentially absent in thiocyanate com-

plexes:

6) The electronic functions of the monodentate thiocyanate

ligands are adopted by a multidentate ligand thus pre-

venting photochemical ligand loss and offering higher

long-term stability.

7) Since the HOMO is extended to the cyclometalating

ligand, the optoelectronic properties can be optimized by

ligand functionalization. Thus, redox-matching with the

electrolyte and improvement of the light harvesting are

enabled.

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8) The stronger effective electron donation allows for

longer excited-state lifetimes.

9) Their ligands can be readily and modularly functional-

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A potential drawback might be the lower extinction coef-

ficients, although strategies to increase them have been

demonstrated. Still, the determined optoelectronic proper-

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triazole interaction shall be investigated in the future.

As a result of the combined efforts of experimental and

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photophysical properties is provided, in particular of the

crucial radiationless deactivation process of cyclometalated

ruthenium(II) complexes.

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These include synthetic procedures, UV/Vis absorption and emission,

CV, NMR and ESI-Tof MS spectra, further solid-state structures and

a more detailed discussion thereof, as well as computational details.

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G

E

G

ACHTUGNERTN(NUNG CH₃CN)₃]-

E

T

N

ACHTUNGTRENNUNG

848611 (RuNNN), CCDC-848612 (RuNCN), CCDC-848613 (RuNCN-

NO₂), CCDC-848614 (RuNCN-F), and CCDC-848615 (RuNCN-Tph)

contain the supplementary crystallographic data for this paper. These

data can be obtained free of charge from The Cambridge Crystallograph-

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Published online: February 29, 2012

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Ruthenium(II) photosensitizers of tridentate click-derived cyclometalating ligands: A joint experimental and computational study

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