Molecular Scylla and Charybdis: Maneuvering between pH Sensitivity and Excited-State Localization in Ruthenium Bi(benz)imidazole Complexes

Bi(benz)imidazole Complexes

Article

Molecular Scylla and Charybdis: Maneuvering between pH

Sensitivity and Excited-State Localization in Ruthenium

†

Alexander K. Mengele, Carolin Muller, Djawed Nauroozi, Stephan Kupfer, Benjamin Dietzek,*

and Sven Rau *

Cite This: https://dx.doi.org/10.1021/acs.inorgchem.0c01022

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sı Supporting Information

ABSTRACT: Bi(benz)imidazoles (b(b)im) acting as N,N-chelates

in ruthenium complexes represent a unique class of ligands. They do

not harbor metal-to-ligand charge-transfer (MLCT) excited states in

ruthenium polypyridyl complexes upon visible-light excitation

provided that no substitution is introduced at the N atoms.

Hence, they can be used to steer light-driven electron-transfer

pathways in a desired direction. Nonetheless, the free N atoms are

susceptible to protonation and, hence, introduce highly pH-

dependent properties into the complexes. Previous results for

ruthenium complexes containing R bbim ligands with alkylic or

arylic N,N′-substitution indicated that, although pH insensitivity was

accomplished, unexpected losses of spectator ligand features incurred simultaneously. Here, we report the synthesis and

photophysical characterization of a series of diﬀerently N,N′-alkylated b(b)im ligands along with their corresponding

[

(tbbpy) Ru(R b(b)im)](PF ) complexes (tbbpy = 4,4′-tert-butyl-2,2′-bipyridine). The data reveal that elongation of a rigid

6 2

ethylene bridge by just one methylene group drastically increases the emission quantum yield, emission lifetime, and photostability

of the resultant complexes. Quantum-chemical calculations support these ﬁndings and allow us to rationalize the observed eﬀects

based on the energetic positions of the respective excited states. We suggest that N,N′-propylene-protected 1H,1′H-2,2′-biimidazole

(

prbim) is a suitable spectator ligand because it stabilizes suﬃciently long-lived MLCT excited states exclusively localized at auxiliary

bipyridine ligands. This ligand represents, therefore, a vital building block for next-generation photochemical molecular devices in

artiﬁcial photosynthesis.

INTRODUCTION

[(tbbpy) Ru(tpphz)PdCl ](PF ) (tbbpy = 4,4′-tert-butyl-2,2′-

6 2

■

bipyridine and tpphz = tetrapyridophenazine; Figure 1,

complex 1), localization of the electron density in the initially

populated excited state and subsequent relaxation path-

Although photosynthetic plants typically do not exceed a year-

averaged solar-to-biomass conversion eﬃciency of 1%, many

basic principles in the context of solar fuel generation can be

learned from nature. Not only the overall design of the

complete functional protein framework itself encouraged

4−16

ways

additionally determine the photocatalytic eﬃciency

of the system.

research in the ﬁeld of artiﬁcial photosynthesis but also the

Because contributions of higher excited states associated

with the tpphz bridging ligand lead to improved photon-to-

hydrogen conversion eﬃciencies, tbbpy-based excited states

seemed to oﬀer additional loss channels, although their

perfectly balanced initial electron transfer processes upon light

excitation, which allow plants to operate under up to 100%

quantum eﬃciency for the production of a stable initial charge-

separated photoproduct under optimized conditions. By

imitating the main functional properties of natural photosyn-

thesis on a molecular level, so-called supramolecular photo-

catalysts have been established within the past 14 years that

contain a photosensitive metal complex unit connected to a

relaxation in a tpphz-based sphere has been reported to

14,21,23

occur with a time constant of 0.8 ps.

Recently,

computational studies addressing the photoinduced electron-

−8

Received: April 9, 2020

metal-containing catalytic center via a bridging ligand.

Similar to natural photosynthesis, also in various artiﬁcial

heteroleptic metal-complex-based systems a wavelength

dependence of the initial excited-state localization on speciﬁc

10−13

oligopyridine ligands has been reported.

As, for example,

exempliﬁed for the heterodinuclear hydrogen-evolving catalyst

XXXX American Chemical Society

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

ligands in the MLCT excited state.

8−22

Figure 1. Molecular structures of diﬀerent heterodinuclear ruthenium−palladium (1) and ruthenium−platinum (2) complexes.

Ligands in red

are harboring the long-lived MLCT state, and the tetranuclear ruthenium−palladium complex (3) shows no involvement of the bibenzimidazole

Figure 2. Molecular structures of previously (left column) and herein (right column) investigated N,N′-protected bi(benz)imidazole ligands and

their corresponding ruthenium complexes.

transfer kinetics in [(tbbpy) Ru(tpphz)PdCl ] as well as in

question of which parameters are of crucial importance to

5,26

structurally closely related photocatalysts

showed that

generate highly eﬃcient photoredox catalysts.

7,29

eﬃcient light-driven charging of the catalytic center can be

achieved by unidirectional population of the tpphz bridging

ligand but only at low photon energies. Therefore, the

incorporation of spectator ligands has been proposed to steer

the photoinduced electron-transfer cascade toward the

catalytically competent metal center and thus to improve the

In this context, biimidazole (bim)

(bbim)

and bibenzimidazole

9,20,30

ligands constitute promising candidates for

spectator ligands in ruthenium polypyridine complexes because

of their high-lying lowest unoccupied molecular orbitals

(LUMOs). Although the excited-state localization on speciﬁc

ligands is not aﬀected by the protonation state of the bbim

ligands, drastic eﬀects on the absorption properties and

overall catalytic activity of these and similar photocatalysts.

In contrast to this design concept, recent studies showed that,

in heterobimetallic photocatalysts resorting on other bridging

ligands, the catalytic activity was increased upon enabling

initial electron transfer to the terminal ligands (Figure 1,

The identiﬁcation of nonreducible spectator

ligands might therefore lead to further insight into the basic

excited-state lifetimes were observed. Similarly, complexation

of other metal centers like palladium or copper to the

deprotonated bbim sphere induced profound changes in the

photophysical characteristics of the complexes. Full depro-

tonation of the bbim sphere leads to a complete loss of

emission and a 100 nm red shift (0.45 eV) of the metal-to-

2,28

complex 2).

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

ligand charge-transfer (MLCT) absorption maximum.

Hence, these factors limit their applicability in systems for

solar fuel generation, in which drastic pH changes might occur

during catalytic turnover, but allow bbim-containing metal

complexes to act as luminescent ion sensors. Consequently,

in order to keep the favorable property of nonreducibility and

to combine it with an insensitivity to pH, substitution of the

acidic N and N′ protons for alkyl and aryl groups was

attempted. However, the resulting [(tbbpy) Ru(R bbim)]-

(

^P^F6⁾complexes entirely lacked the typical ruthenium

Figure 3. Solid-state structures of ﬁve N,N′-alkylated ligands: etbim,

dmbbim, prbim, etbbim, and prbbim (from top left to bottom right).

The thermal ellipsoids are drawn at the 50% probability level, and H

atoms as well as cocrystallized solvent molecules were omitted for

clarity.

polypyridine-based emission. Instead, the initially excited

state was also localized on the protected R bbim ligands; i.e.,

their favorable property of acting as spectator ligands is lost

upon protection with an ethylene or a phenylene group

(Figure 2).

Following the hypothesis that these rigid protection groups

might impair optimal ligand interaction with the Ru core, we

distance. These trends have also been observed for similar aryl-

substituted R bbim compounds synthesized in the group of

expanded the series of N,N′-alkylated R b(b)im ligands and

36,37

Siegel.

investigated the photophysical properties of the resulting

Upon the bridge is changed from ethylene to propylene, the

two (benz)imidazole spheres become planarized, as reﬂected

[

(tbbpy) Ru(R b(b)im)](PF ) complexes in detail. The

2 2 6 2

experimental results of this study are corroborated by

quantum-chemical calculations and allow for delineation of

structure−property relationships, leading to the establishment

of molecular design criteria and enabling the development of

optimized future photoredox catalysts. In this paper, we show

that an optimized design of the protected bbim ligand enables

ﬁne-tuning of both the MLCT and MLCT properties. The

presented results show that 1H,1′H-2,2′-biimidazole bearing a

N,N′-propylene bridge is an ideal innocent spectator ligand,

providing long-lived and photostable MLCT excited states

while at the same time not harboring the excess electron

density associated with formation of the MLCT states

themselves.

40,41

bending of the ligand scaﬀold toward the central metal upon

RESULTS AND DISCUSSION

45−47

■

coordination,

leading to metal-facing N−N distances of

Synthesis of the Ligands. To investigate the eﬀect of

diﬀerent alkyl protection patterns on the photophysics of the

corresponding bisheteroleptic [(tbbpy) Ru(R b(b)im)](PF )

263.8 (0.3) to 267.0 (0.4) pm in the case of (tbbpy) Ru-based

46,47

complexes.

Of the N,N′-alkylene bridges utilized, only the

propylene bridge induces a tilt of the ligand skeleton already

appropriate for coordination (Table 1). Hence, merely these

ligands may result in an eﬃcient coordinative interaction with

the Ru core.

6 2

complexes, six diﬀerent N,N′-alkylated R b(b)im ligands were

synthesized (Figure 2). The synthesis followed established

4−39

protocols

and started from either 1H,1′H-2,2′-biimidazole

(

bim), 1H,1′H-bibenzo[d]imidazole (bbim), or its respective

Synthesis of the Complexes. After structural character-

ization of the N,N′-alkylated ligands, they were utilized to

generate the bisheteroleptic [(tbbpy) Ru(R b(b)im)](PF )

,4′,5,5′-tetramethylated derivative (tmbbim). Full H and C

NMR spectroscopic characterization of the six ligands can be

found in Chapter 9 in the Supporting Information (SI). Except

for prtmbbim, all N,N′-alkylated ligands were very well soluble

in pure chlorinated organic solvents, from which they did

crystallize to yield crystals suitable for X-ray-diﬀraction-based

solid-state structure determination (Figure 3).

6 2

complexes using a standard synthesis method by reﬂuxing a

small excess of the ligand together with [(tbbpy) RuCl ] in an

ethanol/water mixture. Surprisingly, only ﬁve of the desired

complexes were obtained. The reaction with etbim did not

yield the corresponding ruthenium complex, with the ligand

acting in a bidentate fashion. Similar results have been

obtained with a diﬀerent ruthenium precursor, where binding

Structural Characterization of the Ligands. Table 1

summarizes the relevant structural parameters obtained from

crystal structure analysis: Variation of the bridge length does

not strongly alter the central C−C bond length but has a

profound impact on the N −N and N −N distances because

29,48

of etbim occurs in a monodentate fashion.

Structural Characterization of the Complexes. i. NMR

and Mass Spectrometry (MS) Data. After puriﬁcation, all

the alkyl protection group eﬀectively modulates the associated

bond angles within the central bim core. As a result of the

smaller length of the ethylene bridge compared to the

propylene group, the N−C−C bond angle facing the alkylene

complexes were characterized by combined H, C, and 2D

NMR spectroscopy as well as MALDI-MS (see Chapters 9 and

10 in the SI ). As for structurally analogous complexes, the H

NMR signal of the R b(b)im proton pointing most strongly

bridge (α ) is signiﬁcantly reduced in the case of etbim and

toward the tbbpy sphere is shifted to relatively low parts per

bim1

29,46,47,49

etbbim compared to prbim and prbbim. This leads to a

million values (Figure S1).

decreased N −N distance.

ii. Comparison of the Solid-State Structures of Ruprbim

and prbim and Literature-Known Ruthenium Complex

Derivatives. Contrary to the above-discussed rigid ethylene

In contrast, the N−C−C bond angle α

strongly increases

bim2

for the shorter alkylene bridge, leading to an elongated N −N

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

ray diﬀraction structure of Ruprbim (Figure 4 ). It indeed

Article

Table 1. Selected Atom Distances, Bond Lengths, Bond Angles, and Dihedral Angles of the N,N′-Alkylene-Bridged R b(b)im

Ligands (Standard Deviations in Parentheses)

ligand

r_N₁₋_N₃/pm

r_C₋_C/pm

r_N₂₋_N₄/pm

α_b_i_m₁/deg

α_b_i_m₂/deg

δ_b_i_m/deg

etbim

273.5 (0.1)

314.1 (0.4)

272.4 (0.3)

312.2 (0.2)

145.1 (0.2)

145.4 (0.5)

145.4 (0.4)

146.2 (0.2)

316.0 (0.1)

280.6 (0.4)

310.2 (0.4)

278.2 (0.2)

117.6 (0.1)

128.9 (0.3)

117.4 (0.2)

127.1 (0.1)

130.0 (0.1)

119.7 (0.3)

128.7 (0.3)

119.8 (0.1)

12.4 (0.2)

4.7 (0.4)

7.9 (0.5)

3.3 (0.2)

prbim

etbbim

prbbim

Because of the lack of crystallographic data or a diﬀerent molecular symmetry, prtmbbim and dmbbim are not covered in this table.

0,41

protection groups,

the propylene bridge may allow a

Compared to Ruprbim and the above-discussed non-

alkylated derivatives, the previously reported phenylene-

certain degree of distortion of the bim scaﬀold. In order to

verify this hypothesis, we analyzed an obtained single-crystal X-

bridged R bbim ligand in the corresponding (tbbpy) Ru-based

complex revealed a much longer metal-facing N −N distance

of 278.9 (0.7) pm. This possibly results from a rather short

N −N distance of 282.9 (0.7) pm induced by the short C−C

distance of the bridging phenylene ring. As a consequence, to

adopt less eﬃciently to the Ru-oﬀered coordination site, the

Ru−N distances in this complex are signiﬁcantly elongated

(

214−216 pm) with respect to the ruthenium complexes of the

nonalkylated derivatives (typically between 209 and 211 pm)

and the Ru−N distances of Ruprbim [208 (1) pm].

properties, hence, point toward a high rigidity of the

33,47

These

phenylene-protected bbim ligand.

iii. Analysis of the Calculated Structural Data for All

Complexes. In order to correlate the obtained data for

Ruprbim with the other protected b(b)im ligand complexes,

detailed calculations at the density functional level of theory

(

DFT) including solvent eﬀects (CH CN) have been

Figure 4. Left: Solid-state structure of Ruprbim (thermal ellipsoids

are drawn at the 50% probability level; H atoms, PF₆⁻counterions,

and cocrystallized solvent molecules were omitted for clarity). Right:

Comparison of prbim bound to the Ru core (colored) and free prbim

performed to generate suﬃcient structural data for all

complexes. In good agreement with the X-ray structures

obtained for the pure ligands and the crystal structure of

Ruprbim, DFT predicts for all complexes a distortion in the

(gray faded) indicating slight bending of the ligand scaﬀold toward

vicinity of the central C−C single bond of the R b(b)im

the metal center upon coordination.

ligands (Table 2). This distortion originates from two

opposing eﬀects: coordination and alkylation. The propylene-

and (di)methyl-substituted ligands (prbim, prbbim,

prtmbbim, and dmbbim) promote a bending of the b(b)im

^s^c^a^ﬀ^o^l^d^t^o^w^a^r^d^t^h^e^r^u^t^h^eⁿⁱ^u^m^,^a^sⁱ^s^e^vⁱ^d^eⁿ^t^b^y^t^h^e^aⁿ^g^l^e^s^αbim2

ranging between 115° and 117°. Thus, propylene and

revealed slightly altered parameters of the complexed prbim

scaﬀold compared to the free ligand. Upon coordination, a

behavior similar to that for the structurally well-characterized

nonalkylated bbim-derived ruthenium complexes was ob-

(

di)methyl substitution stabilizes the Ru−R b(b)im coordina-

served; i.e., an additional bending of the ligand toward the

tion. In contrast, Ruetbbim exhibits reverse bending of the

^e^t^b^bⁱ^m^lⁱ^g^aⁿ^d⁽^αbim2 = 122.0°) caused by the tension induced

by the shorter bridging ethylene group. This variation of the

ligand structure results in an elongation of the Ru−N bonds

from about 210 pm for Ruprbim, Ruprbbim, Ruprtmbbim,

and Rudmbbim to roughly 218 pm for Ruetbbim. Further

ﬂexibility of the bbim scaﬀold is observed for Rudmbbim,

metal sphere occurred, which indicates ﬂexibility of the

propylene-bridged ligands. Consequently, an increased N −

N distance and a decreased N −N distance [262 (0.1) pm in

Ruprbim and 280.6 (0.4) pm in prbim] were observed for the

complexed prbim compared to the free ligand. These results

are in line with previously published reports on nonalkylated

[

(tbbpy) Ru(bbim)](PF ) complexes. Therefore, in addi-

2 6 2

tion to the already correct tilt of the b(b)im scaﬀold for

coordination due to the introduced propylene bridge, further

distortions to eﬀect eﬃcient metal−ligand interaction seem to

be possible using the ﬂexible propylene-protected systems.

which features, in contrast to all other R b(b)im ligands, a

slight nonplanar distortion (φ = 11°). The combined evidence

of NMR, single-crystal X-ray diﬀraction, and DFT-based

structural investigations draws the coherent picture that the

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Table 2. Selected Structural Parameters within the Fully Relaxed Singlet Ground-State Structure of the Five Ruthenium

Complexes Obtained at the DFT (B3LYP/def2-svp) including D3BJ Dispersion Correction and Solvent Eﬀects (CH CN) by a

Polarizable Continuum Model

complex

r_b_p_y_,_a_x/pm

r_b_p_y_,_e_q/pm

r_b_i_m/pm

α_b_p_y/deg

α_b_i_m₁/deg

α_b_i_m₂/deg

α_b_i_m₃/deg

δ_b_i_m/deg

Ruprbim

206.7

204.3 (0.9)

206.7

206.5

206.7

205.7

201 (1)

205.7

205.6

204.4

206.0

210.7

210 (1)

210.6

217.5

209.5

78.8

78.5 (0.4)

78.8

79.1

126.0

132 (1)

127.3

127.0

118.8

129.1

117.3

118 (1)

116.6

122.0

115.0

78.4

78.5 (0.4)

77.5

80.4

−1

−2 (2)

−1

−2

Ruprbim_e_x

Ruprbbim

Ruprtmbbim

Ruetbbim

Rudmbbim

207.0

78.6

76.8

The subscript “ex” stands for the experimentally determined values of Ruprbim ; standard deviations are given in parentheses.

nature of the alkyl-based N,N′ protection of the b(b)im ligand

scaﬀold has important consequences for their ability to act as a

ligand to Ru ions.

To evaluate whether the structural changes upon variation of

the N,N′-alkylation pattern aﬀect the visible-light absorption

properties and the properties of the usually long-lived emissive

MLCT states, the photophysical properties of the ligands and

corresponding ruthenium complexes were studied by steady-

state absorption and emission spectroscopy.

Absorption Properties of the Complexes. As a result of

the larger π system of the bbim-derived pure ligands, their

absorption and emission spectra are bathochromically shifted

with respect to the smaller R bim systems (etbim and prbim;

Figure S2 ). Furthermore, the R bbim ligands etbbim, prbbim,

and prtmbbim exhibit vibrationally resolved absorption

features in CHCl , whereas dmbbim shows an unstructured

absorption band like the R bim ligands due to free rotation of

the bbim moieties around the central C−C axis. However,

for all R bbim ligands, including dmbbim, vibrationally

resolved emission bands were observed (

E = 0.17 eV),

which most likely stems from the planar conﬁguration of the

ligands (Figure S2 ).

Upon complexation, the resulting ruthenium complexes

reveal a new absorption feature between 370 and 520 nm (see

Figure 5). In contrast to the free ligands, the [(tbbpy) Ru-

(

R bbim)](PF ) complexes exhibit all structured and strong

2 6 2

absorption band between 300 and 370 nm. Interestingly, the

rather broad absorbance of free dmbbim (Figure S2) is

changed upon coordination, now displaying two sharp bands

between 340 and 360 nm in Rudmbbim (Figure 5E). This is

assigned to the chelating binding mode that directs the

coordinatively available N atoms in a rigid, metal-interlocked,

syn position. In contrast, this spectral region remains

featureless upon complexation of prbim (Figure 5C).

In contrast to the non-N,N′-alkylated complex [(tbbpy) Ru-

(

bbim)](PF ) , the addition of a base does not change the

6 2

absorption spectra of the ruthenium complexes depicted in

Figure 5 (Figure S3). Also for similar non-N-alkylated N−H

acidic ruthenium complexes bearing unsymmetric quinoline−

pyrazole ligands, bathochromic shifts of the absorption

Figure 5. Experimental and simulated (black lines) absorption spectra

of Ruprtmbbim (A), Ruprbbim (B), Ruprbim (C), Ruetbbim (D),

and Rudmbbim (E) in CH CN. The electronic character of

excitations contributing to the absorption in the visible region is

indicated by charge-density diﬀerences; charge transfer takes place

from red to blue.

maximum upon the addition of a base have been reported.

This clearly indicates the pH insensitivity of these

(tbbpy) Ru(R b(b)im)](PF ) complexes due to the sub-

[

6 2

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

stitution of acidic N−H protons by the introduced alkylene

1,52

groups.

Time-dependent DFT (TD-DFT) simulations were per-

formed to assign the observed absorption features in the range

between 300 and 520 nm of Ruprbim, Rudmbbim, Ruetbbim,

Ruprbbim, and Ruprtmbbim within the Franck−Condon

region. Time-dependent B3LYP predicts for all investigated

ruthenium complexes two dominant MLCT states mainly

localized on the tbbpy sphere as visible absorption maxima

(

Figure 5 and Tables S1 and S2). According to the

computational results, the weak red-sided absorption features

longer than 460 nm) can be assigned to excitation into weakly

(

absorbing MLCT states with contributions of both ligand

spheres: MLCT_b_b_i_m, MLCT_t_b_b_p_y_/_b_b_i_m, and MLCT_t_b_b_p_y. In

contrast, the excitation energies of MLCT states involving

contributions of the N,N′-alkylated ligand are considerable

lowered upon an increase of the π system from the bim to

bbim scaﬀold: for Ruprbim, the ﬁrst MLCT_b_i_mcontribution is

predicted in the UV region at around 360 nm (S ). This is in

Figure 6. Comparison of the experimental (colored lines) and

calculated rR spectra (gray line) of Rudmbbim (A), Ruetbbim (B),

Ruprbim (C), Ruprbbim (D), and Ruprtmbbim (E) recorded in

CH Cl at an excitation wavelength of 458 nm and overlay of the

agreement with the experimental results obtained from the

homoleptic ruthenium complexes [Ru(bim) ](PF ) and

[

Ru(prbim) ](PF ) , which show unusually blue-shifted

experimental rR spectra of the ﬁve ruthenium complexes at 458 nm

excitation (F).

6 2

29,53

MLCT maxima shorter or slightly longer than 400 nm.

In agreement with the experimental data, the quantum-

chemical simulations predict intraligand (IL) states localized

(

Tables S3 −S7 and Figure S4). In the case of Ruetbbim, an

on the R bbim ligands for the [(tbbpy) Ru(R bbim)](PF )

6 2

intense characteristic bbim mode (213) is predicted at about

Because the experimentally recorded bands longer than 1600

cm (excitation at 458 nm) are modulated on the blue side of

the ﬂuorescence band, those intensities are not comparable.

Electrochemical Characterization of the Complexes.

b(b)im ligands are known to inﬂuence the electrochemical

properties of metal complexes signiﬁcantly; i.e., relatively low

oxidation potentials of, e.g., Ru /Ru

−

−1

complexes between 300 and 350 nm. In the case of

Ruprbim, such IL states are predicted at lower wavelengths

because of the smaller π system, i.e., between 270 and 280 nm

685 cm , which is experimentally obtained at 1683 cm .

−

Figure S2 ). Moreover, strong ligand-to-ligand charge-transfer

excitations contribute to the absorption of all [(tbbpy) Ru-

(

R b(b)im)](PF ) complexes in the UV.

2 6 2

In summary, quantum-chemical calculations suggest that

III33,53,55,56

only Ruprbim shows MLCT formation upon absorption of

visible light without involving the LUMO localized on the

biimidazole ligand framework (Figure 5C).

or Os /

III57−59

redox pairs were found. In order to verify this

general trend and to delineate the eﬀect of the N,N′-alkylation

pattern on the electrochemical properties of the ruthenium

complexes, cyclic voltammetry was measured in degassed

Experimental Localization of the Franck−Condon

Region. To investigate the localization of the initially

populated excited states within the ligand sphere, resonance

Raman (rR) spectra were recorded at a set of three diﬀerent

CH CN containing 0.1 M Bu NPF as the conducting salt.

Pleasingly, except for the third reduction of Ruprbim and the

fourth reduction of Rudmbbim , all potentials were considered

to behave reversibly (Figure S5), as reported for structurally

similar complexes. For the diﬀerent [(tbbpy) Ru(R b(b)-

im)] complexes, peak separation potentials between 66 and

86 mV were observed, which were close to the 76 mV obtained

for the Fc /Fc reference couple using our setup (Table S8).

The ﬁrst oxidation, which can be ascribed to oxidation of the

central metal, occurs at less positive potentials in the

heteroleptic complexes compared to [Ru(tbbpy) ](PF )

excitation wavelengths, namely, 458, 476, and 488 nm. In

addition, rR spectra were simulated by considering contribu-

tions of the low-lying MLCT states S −S in the corresponding

energy range.

Independent of the excitation wavelength, the rR spectra of

the [(tbbpy) Ru(R b(b)im)](PF ) complexes are dominated

6 2

by intense vibrational modes localized at 1615, 1535, 1480,

−

315, and 1030 cm , while only R bbim and not the prbim-

containing complexes shows minor intense modes at around

6 2

−

605, 1280, 1265, and 1249 cm , additionally (Figure 6). In

(Table 3). This indicates that the introduction of N,N′-

agreement with the rR spectra of the homoleptic reference

complex [Ru(tbbpy) ](PF ) , excited at 458 nm, the strong

alkylated R b(b)im ligands results in an increased electron

6 2

density at the Ru center. In agreement with the literature, the

vibrational modes are assigned to the vibrational modes on the

tbbpy sphere. The corresponding calculated wavenumbers of

ca. 1618, 1548, 1481, 1308, and 1026 cm⁻are in excellent

agreement with the experimental results. In accordance with

the experimental data, the calculations predict exclusively

tbbpy modes to contribute to the rR spectrum of Ruprbim due

to the higher-lying MLCT_b_i_mwith respect to the MLCT_b_b_i_m

states (Figure 5).

eﬀect is strongest for prbim. While a comparison of

Ruprbbim and Ruprtmbbim highlights the inﬂuence of the

four methyl groups on the easier oxidation of the Ru center,

contraction from the propylene (Ruprbbim) to ethylene

(Ruetbbim) bridge leads to a metal-based oxidation at higher

potentials, reﬂecting the less eﬃcient metal−ligand interaction

and therefore a less pronounced electron density shift toward

the Ru core.

Thus, the minor rR features are assigned to the contribution

of bbim associated modes, which is further supported by

The ﬁrst two ligand-based reductions (E₁_/₂^r^e^d¹and E₁_/₂^r^e^d²)

occur at more negative potentials for the heteroleptic

complexes compared to the homoleptic reference compound.

−

calculated wavenumbers at 1602, 1287, 1268, and 1265 cm

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Table 3. Electrochemical Potentials of the Five New

Ruthenium Complexes Compared to [Ru(tbbpy) ](PF₆)₂

Used as a Homoleptic Reference Compound (the

Electrochemical Properties of the Latter Fit to Previously

61a

Published Results)

complex

E₁_/₂^o^x

E₁_/₂^r^e^d¹

E₁_/₂^r^e^d²

E₁_/₂^r^e^d³

E₁_/₂^r^e^d⁴

[

Ru(tbbpy) ](PF )

6 2

0.73

0.54

0.60

0.56

0.66

0.60

−1.80

−1.94

−1.88

−1.89

−1.88

−1.85

−2.01

−2.17

−2.07

−2.11

−2.06

−2.03

−2.28

−2.82

−2.30

−2.38

−2.28

−2.29

Ruprbim

Ruprbbim

Ruprtmbbim

Ruetbbim

−2.52

−2.74

Rudmbbim

Measurements were performed in degassed CH CN using Bu NPF

(

0.1 M) as the conducting salt at a scan rate of 100 mV/s. All

potentials are given in volts and are referenced versus Fc /Fc.

This can be explained by an overall increased electron density

in the heteroleptic complexes resulting from the N,N′-alkylated

R b(b)im ligand-induced electron density increase at the metal

center and a concomitant electron-density redistribution onto

the tbbpy ligands. However, no particular trend was found for

the diﬀerence between the ﬁrst oxidation and ﬁrst reduction

potentials because these vary only slightly between 2.43 and

.48 V for the diﬀerent [(tbbpy) Ru(R b(b)im)] complexes.

2 2

Also, no clear correlation between the optical and electro-

chemical properties was found (Figure S6 ).

As for the third reduction, merely Ruprbim displays a very

negative potential, the prbim ligand seems to be much harder

to reduce compared to the R bbim ligands. Hence, the

electrochemistry of ruthenium complexes is in complete

agreement with the quantum-chemical simulations and the

results of rR spectroscopy (Figures S7 and 6): The prbim-

localized MLCT state was found at much higher energy

compared to the MLCT state residing on the tbbpy ligands

Figure 7. Left: Comparison between normalized UV−vis spectra

(

solid line) and the corresponding normalized excitation spectra

dotted line; recorded at maximum emission intensity) of

Ruprtmbbim (blue, A), Ruprbbim (dark red, B), Ruprbim (red,

C), Ruetbbim (yellow, D), and Rudmbbim (green, E) and emission

spectra of those ﬁve ruthenium complexes, which were recorded upon

excitation in the MLCT maximum. The corresponding excitation

wavelengths and obtained emission maxima, lifetimes, and quantum

yields as well as the nonradiative decay rate are highlighted in the

graphs (for details, see Chapter 4 in the SI ). All spectra were recorded

Table S1 ). Therefore, prbim did not participate in the

formation of the initially populated excited state using

excitation wavelengths longer than 450 nm (Figure 6). In

contrast, the N,N′-alkylated R bbim-containing complexes

exhibit three reduction processes that resemble [Ru(tbbpy) ]-

in degassed CH CN under an atmosphere of argon (c = 10 μM).

(

PF ) much more. This is in line with the absence of a

6 2

Right: (i) Relationship between ln(k ) and the energy diﬀerences

wavelength-selective MLCT state localization.

obtained from the electrochemical data (triangles, HOMO−LUMO

gap), visible absorption (squares), and emission maxima (circles).

Lines are included as guides to the eye. (ii) Jablonski diagram for the

Characterization of the Luminescence Properties.

The emission properties of the [(tbbpy) Ru(R b(b)im)]-

(

PF ) complexes were investigated by steady-state and time-

formation of emissive (long-lived) MLCT versus MC states upon

photoexcitation.

resolved experiments in CH CN. Quantum yields and

emission lifetimes were obtained (as speciﬁed in the

experimental section), and the results are summarized in

Figure 7.

complexes do not obey this law because the nonradiative decay

rates increase with increasing energy gap (Figure 7). This

clearly demonstrates the impact of thermally accessible

deactivation pathways most likely via metal-centered (MC)

states. According to the nonradiative decay rates, it can be

assumed that such MC states are more accessible with a

decreasing alkylene bridging unit (Figure 7). This can be

related to a decreased d-orbital splitting induced by the rigid

protection groups (e.g., ethylene), which impair optimal

metal−ligand interactions. This is consistent with the ﬁndings

on previously reported ethylene- and phenylene-bridged

As for the pure ligands, the visible absorption and emission

energies as well as the phosphorescence lifetimes (τ) and

quantum yields (Φ) follow the order Ruprtmbbim >

Ruprbbim > Ruprbim > Ruetbbim ≈ Rudmbbim. Thus,

elongation of the alkylene bridging unit causes a decrease of

the absorption and emission energies but an increase of the

phosphorescence eﬃciencies and lifetimes by up to two orders

of magnitude. Besides the energy diﬀerences between the

ground and excited states inter alia, the energy-gap law

determines the phosphorescence lifetime. The energy-gap law

states that the nonradiative decay rates exponentially increase

ruthenium complexes.

Photoinduced Dynamics of the Complexes. Because

the structural and electronic properties within the Franck−

Condon region pronouncedly depend on the nature of N,N′-

alkylation, we aimed to further understand the inﬂuence of the

62−64

as the energy gap or emission energy decreases.

However,

the optical gaps (absorbance/emission) and highest occupied

molecular orbital (HOMO)−LUMO gaps of the investigated

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

00 ps) to zero (Figure S34 ).

Article

N,N′-alkylation pattern of the b(b)im ligands on the

photoinduced dynamics. Hence, the excited-state dynamics

of the complexes upon excitation at 403 nm were monitored

by femtosecond and nanosecond transient absorption (TA)

spectroscopy and complemented by TD-DFT simulations. In

addition, the spin-allowed triplet−triplet excitations were

investigated within the equilibrium structures of the (excited)

Upon excitation at 403 nm, the TA signals of Ruprbim,

Ruprbbim, and Ruprtmbbim reveal instantaneous (temporal

resolution of 0.3 ps) ground-state bleach (GSB), reﬂecting the

formally oxidized Ru center and excess charge density localized

on either the tbbpy sphere (Ruprbim) or both the R bbim

and tbbpy ligands (Ruprbbim and Ruprtmbbim). The GSB is

accompanied by strong excited-state absorption (ESA) peaking

at 370 nm and broad, structured ESA longer than 500 nm.

When a global ﬁt is employed, the decay of the femtosecond

and nanosecond TA signals of Ruprbim are characterized by

triplet MLCT states of interest. Depending on the R b(b)im

coordination sphere, the 403 nm pump pulse populates either

a MLCT_t_b_b_p_y(Ruprbim) or a mixed MLCT_t_b_b_p_y_/_b_b_i_m

two characteristic time components, namely, τ = 18 ps and

^τem = 48 ns, while global analysis yields three characteristic

(Ruetbbim, Ruprbbim, and Ruprtmbbim) excited state. In

time constants in the case of Ruprbbim (τ = 7.0 ps, τ = 2.5

the following, we focus on the photoinduced dynamics of

Ruprbim, Ruprbbim, and Ruprtmbbim (Figure 8). In the

case of Ruetbbim , the TA signal decays rapidly (within ca.

ns, and τ_e_m= 93 ns) and Ruprtmbbim (τ = 2.0 ps, τ = 0.5 ns,

and τ_e_m= 104 ns).

The spectral changes of Ruprbim associated with the ﬁrst

time constant, τ = 18 ps, characterize a long-lived MLCT

bpy

state with excess charge density localized on the tbbpy sphere.

Thus, the kinetic component indicates contributions from

19,65−68

intersystem crossing (ISC) and vibrational cooling.

This is manifested in the rise of ESA centered at 370 nm and

between 550 and 750 nm, which are typically assigned to ππ*

transitions on the reduced bpy-ligand (IL_b_p_y) and excited

9−71

singlet and triplet transitions of LMCT

character.

The

tbbpy

long-lived MLCT state, in turn, decays monoexponentially

bpy

with a lifetime of 48 ns.

For Ruprbbim and Ruprtmbbim, the spectral changes

associated with τ = 7.0 and 2.0 ps contain contributions of

ISC, vibrational cooling, and interligand hopping from the

MLCT_b_b_i_mto MLCT_b_p_ysphere. The latter process is

manifested in the decay of ESA centered at around 550 nm,

accompanied by the buildup of a signal at around 370 nm.

Because the ESA centered at around 550 nm is exclusively

obtained for the [(tbbpy) Ru(R bbim)] complexes, it can be

attributed to a MLCT

state with excess charge density

bbim

localized on the R bbim ligand, which is supported by TD-

DFT simulations on the MLCT

state (see Chapter 4 in

bbim

the SI ). Thus, the LMCT_b_b_i_mtransitions appear blue-shifted

compared to the LMCT_t_b_b_p_ystates, which is reasonable

because the R bbim ligands show more positive reduction

potentials than the tbbpy and bim ligands. The kinetic

component τ is assigned to relaxation of the MLCT

state

bbim

in the case of Ruprbbim (τ = 2.5 ns) and nonradiative decay

in the MLCT

sphere for Ruprtmbbim (τ = 0.5 ns),

yielding for both compounds a long-lived MLCT state

bpy

(

Figure S32 ). This is manifested in the overall decay of the TA

signal, while the species-associated spectra of Ruprbbim shows

a higher amplitude in the region of the bbim-centered ESA

between 530 and 560 nm with respect to Ruprtmbbim (cf. the

second species in parts A and B of Figure 9). TD-DFT

simulations further support that both complexes undergo

competitive relaxation pathways along both ligand spheres via

the MLCT

and MLCT states (cf. Figure 8). In the case

bbim

bpy

of Ruprbbim, MLCT_b_b_i_mrelaxation is slightly favored (by

.03 eV), whereas relaxation of Ruprtmbbim via the bpy

Figure 8. Left: TA- and decay-associated spectra of Ruprbim (A1),

sphere is preferred (by 0.08 eV). Consequently, the ESA

maximum of Ruprtmbbim is localized at a similar wavelength

compared to Ruprbim, which exclusively relaxes via the

Ruprbbim (A2), and Ruprtmbbim (A3) in CH CN upon photo-

excitation at 403 nm (0.4 mW, optical density at λ ≈ 0.3). Right:

exc

Energy levels of MLCT singlet states (in black) involved in the initial

absorption as well as of MLCT_b_i_mand MLCT_b_p_ytriplet states

orange and violet, respectively) involved in the subsequent excited-

MLCT_t_b_b_p_ysphere.

The TA data underline the impact of the diﬀerent b(b)im

ligands on the ultrafast intramolecular charge-transfer dynam-

(

state relaxation cascades as obtained by TD-DFT. The relative

energies are shown within the Franck−Condon region (S ) and

ics. Depending on the nature of the R b(b)im ligand and the

within both fully optimized MLCT states.

N,N′-alkylation pattern, the MLCT

and MC states are

bbim

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Figure 10. (A and B) UV−vis spectroscopic assessment of the

photostability of the newly synthesized ruthenium complexes (ﬁlled

circles) and the homoleptic reference compounds (empty circles and

squares), measured in CH CN (c = 10 μM) under air (irradiation

with one blue-light-emitting diode stick from the bottom of the

0 mm × 10 mm quartz cuvette; 45 mW/cm and λ = 463 nm). H

max

NMR spectroscopic analysis of the selective ligand loss of Ruetbbim

C) and Rudmbbim (D) in CD CN (c = 3 mM). From bottom to

(

top, (i) an intact complex prior to irradiation, (ii) a product mixture

of the completely photolyzed ruthenium complex, and (iii)

independently measured pure etbbim (C) or dmbbim (D) are

displayed. The expanded spectra highlight the regions in which the

peaks of the pure ligands appear.

Figure 9. Species-associated spectra of Ruprtmbbim (A and D),

Ruprbbim (B and E), and Ruprbim (C and F) in CH CN and

Jablonski diagrams of the proposed relaxation models.

accessible in the relaxation pathway of the photoexcited

corresponding ruthenium complex is strongly increased if the

complexes: A high electron density in the R b(b)im ligand

ethylene group of Ruetbbim is elongated by only one CH −

sphere [higher reduction potential than the tbbpy ligands:

group, yielding Ruprbbim, which is no longer suﬀering from

E (b(b)im) > E (tbbpy)] leads to a preferential relaxation

low-lying MC states that provoke ligand dissociation (Figure

red

within the tbbpy sphere, whereas weaker donor ligands

E_r_e_d(b(b)im) < E_r_e_d(tbbpy)] show relaxation via the

7). As a result of the high photolability, the synthesis and

puriﬁcation of Ruetbbim and Rudmbbim had to be performed

in the dark to obtain a pure product. Furthermore, from UV−

vis spectroscopic data, it is inferred that the photostability of

Ruetbbim and Rudmbbim increases strongly if the solvent is

changed from CH CN to CH Cl (Figures S10 and S11)

[

MLCT_b₍_b₎_i_mmanifold. Additionally, rigid b(b)im ligand

structures yield highly accessible MC states, which cause a

rapid decay of the photoexcited complexes. Thus, the

investigated complexes provide a toolbox for the design of

optimized photosensitizers with directional electron-transfer

pathways originating from the excited states.

because CH CN acts as a much better ligand and thereby

increases the eﬃciency for ligand substitution.

Photostability of the Complexes. Besides optimized

optical properties, a high photostability of the ruthenium-based

photosensitizers is essential for their potential use in

chromophore−catalyst assemblies. Such photostability is

accessed by, e.g., in situ UV−vis absorption spectroscopy of

Although these large diﬀerences of the photolability among

the various ruthenium complexes indicated that the fast ligand

loss in the case of Ruetbbim and Rudmbbim correlates with

the presence of the N,N′-alkylated R bbim ligands, H NMR

spectroscopic investigations were performed to verify the

nature of the dissociated ligand as this cannot be

unambiguously predicted. Indeed, by a comparison of the

spectra of the photolyzed Ruetbbim and Rudmbbim with the

corresponding free ligands, selective cleavage of etbbim and

the complexes in CH CN under irradiation at 463 nm (Figure

0). The extinction-coeﬃcient-weighted rate of decrease of the

MLCT absorption maximum (detailed spectra are depicted in

Figure S8 ) qualitatively reﬂects the photostability of the

corresponding ruthenium complexes. This rate of decom-

position diﬀers strongly among the complexes. Whereas

Ruprtmbbim and Ruprbim display higher photostability

than the two homoleptic reference compounds

dmbbim accompanied by the formation of a C -symmetric

ruthenium complex, putatively [(tbbpy) Ru(CD CN) ] , was

observed (Figures 10C,D and S12 −S16 ; see also the UV−vis

absorption spectrum of independently synthesized

49,74

[

Ru(bpy) ](PF ) and [Ru(tbbpy) ](PF ) , Ruprbbim exhib-

[(tbbpy) Ru(CH CN) ] in Figure S17).

Additionally,

6 2

its comparable photostability.

In contrast, Ruetbbim and Rudmbbim are extremely

from the CD CN solution of photolyzed Rudmbbim, colorless

crystals of the N,N′-alkylated ligand could be isolated, proving

that dmbbim is selectively lost upon irradiation. The structural

parameters of these photochemically obtained dmbbim

crystals (Figure S18) did not diﬀer within the error of the

experiment compared to the data presented above (Figure 2

photolabile in CH CN, reaching a constant absorbance at

the former MLCT maximum after only 200 s of irradiation

Figure S9 ). Hence, not only the emission quantum yield and

excited-state lifetime but also the photostability of the

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

a conclusive picture (Figure 11 ). First, spectator ligand features

Article

and text therein), where crystals of dmbbim were obtained by

of the pH-insensitive N,N′-alkylated R b(b)im ligands can

slow evaporation of CHCl from the solution. Moreover, by a

only be obtained if the size of the π system is decreased from

the bbim to bim scaﬀold, as previously predicted by theoretical

change of the NMR solvent from CD CN to CD Cl , the rate

of photoinduced ligand loss becomes strongly reduced

studies. Second, when the N,N′-alkylation pattern is altered

using the bim framework, merely the synthetic accessibility of

the corresponding ruthenium complexes (etbim vs prbim) is

observed; for the bbim-based systems, their excited-state

properties, including photostabilities but not the initial

localization of the excited states on speciﬁc ligands, can be

tuned by varying the investigated N,N′-alkylation patterns. For

Ruetbbim and Rudmbbim, drastically lower emission

quantum yields, excited-state lifetimes, and photostabilities

compared to those of their propylene-bridged counterparts are

observed. On the basis of quantum-chemical simulations, these

properties might be explained by relatively easily accessible

MC states; elucidating the MC relaxation pathways within the

excited states will be the focus of future computational studies.

(Figures S19 and S20 ), which shows that the qualitative

judgment of decreasing the MLCT absorbance in pure

CH CN truly can be interpreted as decomposition of the

corresponding ruthenium complex by the loss of one chelating

ligand.

Further insight into the photodecomposition process was

obtained by analyzing the irradiation-induced UV−vis

spectroscopic changes in the presence of 1 mM tetra-n-

butylammonium chloride or the corresponding hydroxide salt

(

TBACl or TBAOH) using CH CN as the solvent. Compared

to the pure CH CN solutions discussed above, the presence of

−

Cl ions led to the evolution of a new band at ca. 550 nm,

which was assigned to the formation of [(tbbpy) RuCl ] by

The detailed investigations of all synthesized R b(b)im-

comparison with the UV−vis absorption spectrum of

based ligands and their corresponding ruthenium complexes,

independently synthesized [(tbbpy) RuCl ] (Figure S21). In

5,76

hence, clearly identify prbim as the sole spectator ligand

agreement with reports by Meyer and co-workers,

for the

stabilizing suﬃciently long-lived MLCT excited states located

labile complexes Ruetbbim and Rudmbbim, a two-step

process was observed, whereas for the more photostable

Ruprbim, Ruprbbim, Ruprtmbbim, and [Ru(tbbpy) ](PF ) ,

on a bipyridine ligand while at the same time not being

involved in the formation of the π*-based MLCT state. As

6 2

expected, all N,N′-protected ruthenium complexes display

insensitivity toward the solvent pH in their UV−vis properties,

which is a key requirement for their application in light-driven

catalysis utilizing basic electron donors such as triethanol-

amine.

isosbestic points were found. The eﬃcient Ru−N bond

cleavage in Ruetbbim and Rudmbbim, hence, produces a

detectable amount of an intermediate species, presumably

bearing a monodentate R bbim and one chloride ligand.

Similar eﬀects were observed under basic conditions using

TBAOH as the additive (Figure S22 ). However, compared to

the presence of Cl⁻ions, excess OH⁻ions led to faster

photodegradation of all ruthenium complexes (Figure S23). In

contrast to these N,N′-alkylated [(tbbpy) Ru(R b(b)im)]

CONCLUSION

■

In conclusion, a series of N,N′-alkylated R

b(b)im ligands and

Ru-based com-

their corresponding bisheteroleptic (tbbpy)

complexes, only marginal UV−vis spectroscopic changes

were observed for unmodi ﬁ ed [(tbbpy) Ru(bbim)](PF )

6 2

plexes were synthesized and thoroughly characterized with

respect to their structural and electronic properties. The

photophysical properties as well as photostability of the

under basic conditions (Figure S24).

Summary. The combination of all obtained data sets draws

obtained [(tbbpy) Ru(R b(b)im)](PF ) complexes can be

2 2 6 2

tuned by minor alterations in the N,N′-alkylation pattern of the

diﬀerent R b(b)im ligands such as elongation of a bridging

ethylene group by only one CH unit. The more ﬂexible

propylene bridges result in improved metal−ligand inter-

actions, giving rise to higher excited-state lifetimes and higher

photostability.

Beyond rationalizing previous ﬁndings on rigid phenylene-

and ethylene-bridged bbim ligands, we show that only prbim

fulﬁlls all of the requirements for implementation of the

designed construction of photosensitizer units in light-driven

photocatalysis, where light-induced electron-transfer reactions

proceed through the bridging ligand. For all larger R bbim-

based ruthenium complexes, participation of the N,N′-

alkylated ligand in the formation of the initially excited state

was observed. Having this important information in hand, the

synthesis and characterization of an intramolecular model

system for artiﬁcial photosynthesis with a selective light-

induced electron-transfer reaction pathway involving only one

feasible route will be possible by replacing the commonly used

77−80

(

tbbpy) Ru building block.

Such a degree of control over

excited-state dynamics might place our ability to control

electron-transfer pathways on par with the highly optimized

molecular architectures in biological photosynthesis or other

2−85

well-investigated diads and triads.

The H NMR spectroscopic investigations of photolysis

experiments yielded important insights into the selective light-

induced dissociation of these new ligands from the Ru core.

Figure 11. Graphical classiﬁcation of [(tbbpy) Ru((R )b(b)im)]

complexes resulting from the presence or alterations in the N,N′-

alkylation pattern and the reduced size of the π system.

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

The Supporting Information is available free of charge at

Article

This observation is of signiﬁcance not just for artiﬁcial

photosynthesis because it would allow the light-driven

formation of coordinatively highly reactive ruthenium com-

plexes with predictable structures. This could oﬀer the

opportunity to use structurally similar compounds in medical

applications such as cancer treatment, exploiting this eﬃcient

Research Foundation (DFG) under Project ID 390874152

(POLiS Cluster of Excellence).

Notes

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

phototriggered and very selective R bbim ligand loss.

All calculations were performed at the Universitat

srechenzen-

trum of the Friedrich-Schiller University Jena. We additionally

thank Nicolas Meitinger and Sebastian Knoll from the Institute

of Inorganic Chemistry I at Ulm University for providing UV−

ASSOCIATED CONTENT

sı Supporting Information

■

https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c01022.

vis absorption data of [(tbbpy) Ru(CH CN) ] .

Experimental and computational details, synthetic

procedures, crystallographic data, and NMR and

MALDI-MS spectra (PDF)

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Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.inorgchem.0c01022

Author Contributions

These authors contributed equally.

(12) Zedler, L.; Guthmuller, J.; Rabelo de Moraes, I.; Kupfer, S.;

†

Krieck, S.; Schmitt, M.; Popp, J.; Rau, S.; Dietzek, B. Resonance-

Raman spectro-electrochemistry of intermediates in molecular

artificial photosynthesis of bimetallic complexes. Chem. Commun.

Funding

A.K.M. and C.M. thankfully acknowledge ﬁnancial support

from the Fonds der Chemischen Industrie by a Chemiefonds

Stipendium (A.K.M.) and a Kekule Stipendium (C.M.).

Furthermore, funding by the Deutsche Forschungsgemein-

schaft (German Research Foundation) via the TRR CATA-

LIGHT, Projektnummer 364549901, TRR234, A1 and C5, is

gratefully acknowledged. This work contributes to the research

performed at CELEST (Center for Electrochemical Energy

Storage Ulm-Karlsruhe) and was funded by the German

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