Katrin
Peuntinger‡
a,
T. David
Pilz‡
bc,
Robert
Staehle‡
bd,
Markus
Schaub
bd,
Simon
Kaufhold
d,
Lydia
Petermann
d,
Markus
Wunderlin
e,
Helmar
Görls
c,
Frank W.
Heinemann
b,
Jing
Li
a,
Thomas
Drewello
a,
Johannes G.
Vos
f,
Dirk M.
Guldi
*a and
Sven
Rau
*bd
aFriedrich-Alexander-Universität Erlangen-Nürnberg, Department of Chemistry and Pharmacy and Interdisciplinary Center for Molecular Materials (ICMM), Egerlandstrasse 3, 91058 Erlangen, Germany. E-mail: dirk.guldi@chemie.uni-erlangen.de; Fax: (+49) 9131-852-8307
bFriedrich-Alexander-Universität Erlangen-Nürnberg, Department Chemie und Pharmazie, Egerlandstrasse 1, 91058 Erlangen, Germany
cInstitut für Anorganische und Analytische Chemie, Friedrich-Schiller-Universität Jena, August-Bebel-Strasse 2, 07743 Jena, Germany
dInstitut für Anorganische Chemie I, Universität Ulm, Albert-Einstein-Allee 11, 89081 Ulm, Germany. E-mail: sven.rau@uni-ulm.de; Fax: (+49) 731/50-23039; Tel: (+49) 731/50-22575
eInstitut für Organische Chemie II und Neue Materialien, Universität Ulm, Albert-Einstein-Allee 11, 89081 Ulm, Germany
fSRC for Solar Energy Conversion School of Chemical Sciences, Dublin City University, Dublin 9, Ireland
First published on 29th July 2014
Novel photocatalysts based on ruthenium complexes with NHC (N-heterocyclic carbene)-type bridging ligands have been prepared and structurally and photophysically characterised. The identity of the NHC-unit of the bridging ligand was established unambiguously by means of X-ray structural analysis of a heterodinuclear ruthenium–silver complex. The photophysical data indicate ultrafast intersystem crossing into an emissive and a non-emissive triplet excited state after excitation of the ruthenium centre. Exceptionally high luminescence quantum yields of up to 39% and long lifetimes of up to 2 μs are some of the triplet excited state characteristics. Preliminary studies into the visible light driven photocatalytic hydrogen formation show no induction phase and constant turnover frequencies that are independent on the concentration of the photocatalyst. In conclusion this supports the notion of a stable assembly under photocatalytic conditions.
This modular concept allows for a rational design and for the systematic optimization of the individual subunits. This is of significance for the development of new types of catalysts, which may overcome the drawbacks of the previous generations. Importantly, for the advancement of intramolecular systems several design considerations have to be taken into account.10 The photochemical properties as well as the redox properties of the photoexcited state have to allow the reduction of appropriate catalytic centres and these reduced centres must be capable of reducing the substrate, i.e. protons to hydrogen. This ability correlates also with the structure of the catalytic centre. In particular, the excited state energy of the chromophore should be tuneable and the bridging ligand should be electronically linked in a stable manner to the catalytic centre in all of its redox states leading to chemically stable compounds. The later point has been emphasized by Hammarström et al. for a dinuclear ruthenium–palladium catalyst.11 During photocatalysis, palladium colloids were detected, which implies their release from the reduced complex in the weakly stabilizing NN-chelating environment. Moreover, the ensuing investigations on platinum catalysts based on polypyridine ligands showed contradicting results. Eisenberg et al. could detect a photodecomposition of platinum catalysts, which led to the conclusion that the resulting colloids may serve as active catalysts.12 However, in contrast to this findings, Sakai et al. demonstrated that some of his platinum catalysts are stable.13 Consequently, the question, under which circumstances colloids are formed and whether they are the catalytically active species, cannot be answered satisfyingly yet for oligopyridine based palladium or platinum catalysts. It is therefore highly desirable to take alternative bridging ligands with more suitable donor sets into consideration. The problem of stability of the low valence state of a catalyst is addressed in great detail in organometallic catalysis. Hence, appropriate ligands such as phosphine and, more recently, NHC ligands have been favoured. Research in this area is increasing rapidly, showing the importance and advantages of these systems, in particular, the enhanced stability. Therefore, these NHC ligands are useful in gaining full control over the electronic and steric properties of the catalytic centre during particular reactions.14 Recently, Chung et al. showed, for instance, that a dinuclear (NHC)-ruthenium–iridium-complex can be used as a sensitizer for CO.15
Motivated by these observations, we present here the synthesis and characterisation of the first photocatalyst for the water reduction containing a NHC-NN-bridging ligand15–17 based on 1,3-(bisbenzyl)-1H-imidazo[4,5-f][1,10]phenanthrolinium bbip+. Starting from [Ru(bbip)]3+ [(tbbpy)2Ru(bbip)](PF6)3 as a building block the heterometallic intramolecular photocatalysts [(tbbpy)2Ru(μ-bbip){AgCl}]Cl2 ([Ru(bbip)Ag]2+), [(tbbpy)2Ru(μ-bbip){PdCl2X}]Cl2 (X = coordinated solvent; [Ru(bbip)Pd]2+), and [(tbbpy)2Ru(μ-bbip){Rh(cod)Cl}]Cl2 (cod = 1,5-cyclooctadiene; [Ru(bbip)Rh]2+) were prepared. A detailed structural and photophysical investigation corroborates photoinduced electron transfer reactions that are the key step for the overall photocatalytic activity.
The formation of all ligands was further verified by ESI-HRMS, elemental analysis and X-ray crystallography (see ESI†). Selected bond lengths and angles are listed in Table 1. A detailed discussion is given below.
Angles [°] | ip | Bip | bbip+ | [Ru(ip)]2+ | [Ru(bip)]2+ | [Ru(bbip)]3+ | [Ru(bbip)Ag]2+ |
---|---|---|---|---|---|---|---|
a Refers to the protonated ip. | |||||||
C13–Ag1–Cl1 | — | — | — | — | — | — | 178.9 (2) |
N1–Ru1–N2 | — | — | — | 80.15 (17) | 79.43 (17) | 79.18 (16) | 79.32 (19) |
N3–C13–N4 | 110.1 (2) | 113.60 (16) | 110.9 (6) | 112.5 (5) | 113.5 (5) | 111.0 (5) | 105.7 (5) |
C13–N3–C5 | 107.9 (2) | 106.09 (16) | 108.1 (6) | 107.5 (5) | 105.9 (5) | 107.8 (5) | 110.6 (5) |
C13–N4–C6 | 107.8 (2) | 104.28 (16) | 107.7 (6) | 103.7 (5) | 104.1 (5) | 107.9 (5) | 111.0 (5) |
N3–C5–C6 | 106.9 (2) | 105.47 (15) | 105.8 (6) | 105.5 (5) | 105.4 (5) | 106.6 (5) | 106.4 (5) |
C5–C6–N4 | 107.2 (2) | 110.56 (16) | 107.5 (6) | 110.7 (5) | 111.1 (5) | 106.6 (5) | 106.4 (5) |
N1–C12–C11 | 117.7 (2) | 116.59 (15) | 116.2 (6) | 116.3 (5) | 115.1 (5) | 115.8 (5) | 116.0 (5) |
N2–C11–C12 | 117.0 (2) | 117.03 (16) | 116.5 (6) | 115.8 (5) | 117.4 (5) | 114.9 (5) | 115.3 (5) |
Ruthenium was chosen because of its favourable light harvesting properties. The syntheses of the ruthenium complexes were carried out by the reaction of stoichiometric reactions of [Ru(tbbpy)2Cl2] and ip, bip, or bbip+, in ethanol–water using a microwave reactor. After counter ion exchange with NH4PF6, [Ru(tbbpy)2(ip)][PF6]2 ([Ru(ip)]2+), [Ru(tbbpy)2(bip)][PF6]2 ([Ru(bip)]2+), and [Ru(bbip)]3+ were obtained in good yields. In the case of [Ru(bbip)]3+, a slightly longer reaction time of five hours was necessary to drive the reaction between the positively charged ruthenium and the positively charged ligand. Chromatographic purification yielded pure [Ru(ip)]2+, [Ru(bip)]2+, and [Ru(bbip)]3+ (for details regarding the synthesis see ESI†). 1H- and 13C-NMR spectroscopy revealed the typical imidazolium salt properties of [Ru(bbip)]3+, while the H/D-exchange in methanol-d4 was too fast for kinetic NMR-investigations (compared to bbip+, Fig. S2†). Most important is the pattern of the phenanthroline related 6/9-, 5/10- and 4/11-proton signals, which implies the presence of a symmetric ligand in the case of [Ru(ip)]2+ and [Ru(bbip)]3+ and an asymmetry ligand in the case of [Ru(bip)]2+ (Fig. 3; for numbering see Fig. 2).
Fig. 3 Aromatic region of the 1H-NMR spectra for [Ru(ip)]2+ (top), [Ru(bip)]2+ (middle) and [Ru(bbip)]3+ (bottom) in acetonitrile-d3 with corresponding peak assignment. |
The benzylic CH-signals further corroborate these observations (δRu(bip)4HCH2 = 5.99 ppm and δRu(bbip)4HCH2 = 6.17 ppm). Significant is also the shift of the singlet signals of the 2-protons due to the +I-effect (δRu(ip)H2-ip = 8.44 ppm, δRu(bip)H2-ip = 8.48 ppm, and δRu(bbip)H2-ip = 9.08 ppm) due to increasing deshielding of the NCHN-proton after stepwise alkylation and salt formation ([Ru(bbip)]3+).
ESI-HRMS mass spectrometry and elemental analysis further confirm the formation of the complexes. Suitable crystals for X-ray crystallography were obtained for all of the monometallic ruthenium complexes (Table 1 and ESI†). A detailed discussion is given below.
Fig. 4 Synthesis of [Ru(bbip)Ag]2+ from [Ru(bbip)]3+. Displayed are the crystal structures of both complexes; counter ions, solvent molecules, and protons are omitted for clarity. |
The presence of the NHC unit was corroborated by the loss of the H2 signal in the 1H-NMR and the typical high field shift of the C2 signal (146.82→199.72 ppm) in the 13C-NMR, the X-ray crystal structure confirm the successful NHC formation (Fig. 4, Table 1 and ESI†) and by ESI-HRMS analyses of [Ru(bbip)Ag]2+ (see ESI†).
As the first fully characterised bbip-carbene complex, [Ru(bbip)Ag]2+ was subject of detailed investigations into its fragmentation behaviour under MS and MS/MS conditions, using ESI as the ionisation method.
A 1 mM MeOH solution of [Ru(bbip)Ag]2+ ([(tbbpy)2Ru(μ-bbip)AgCl]Cl2) was electrosprayed and the resulting MS spectrum is depicted in Fig. 5a (top trace). The most abundant ion corresponds to the singly charged [Ru(bbip)Ag]2+[Cl]− ([[(tbbpy)2Ru(μ-bbip)AgCl]Cl]+) ion (m/z = 1216.8), which represents the quasi-molecular ion, the identity of which has been verified by its isotope pattern and by ESI-HRMS. Interestingly, there are additional signals for [Ru(bbip)Ag]2+ at m/z = 591.1 and [Ru(bbip)]2+ at m/z = 519.2 (Fig. 5 and S3†). Since we have no other indications of impurities present in the sample, these fragments most probably originate from dissociations occurring during the electrospray process of the sample. The ESI-MS (Fig. 5a) differs from the MS/MS (Fig. 5b), which is not unusual since the ESI-MS may result from a multitude of processes occurring in the spray, while the MS/MS reveals the true fragmentation behaviour of a selected precursor ion.
Fig. 5 (a) MS spectrum of [Ru(bbip)Ag]2+. (b) MS2 spectrum of m/z = 1217, which shows the [Ru(bbip)Ag]2+[Cl]− cation. |
In the MS/MS experiment, the [Ru(bbip)Ag]2+[Cl]− ion was selected and submitted to collision-induced dissociations (CID) using helium as the collision gas. The resulting daughter ion spectrum is shown in Fig. 5b.
As far as the fragmentation pattern is concerned, a statistical loss of the tbbpyversusbbip+ ligand would lead to a 2:1 ratio of the respective signals. However, the loss of the bbip+ ligand is virtually absent from the MS/MS spectrum and only loss of the tbbpy ligand is observed as shown in Fig. 5b. This indicates a much stronger bond between bbip+ and ruthenium compared to tbbpy. Similar findings were reported in earlier investigations related to the competitive complexation of 1,10-phenanthroline (phen) and 2,2′-bipyridine (bpy) to Mn2+ ions.25 In this study, bpy replaces tbbpy, and phen replaces bbip+. The weaker complexation of bpy ligands by Mn2+ was attributed to its higher flexibility.25 While phen is frozen in the cis configuration, bpy has the ability to rotate into the trans form which may lead to monodentate coordination of the ligands, followed by ligand loss as has also been observed in photochemical experiments with ruthenium polypyridyl complexes. This aspect might be even more relevant for tbbpy with its two tert-butyl groups.
It is interesting to note, that the fragmentation of [Ru(bbip)Ag]2+ under ESI-MS conditions (Fig. 5a) leads to the dication [Ru(bbip)]2+ (m/z = 519.23), an ion that most probably contains a free carbene structure within the imidazolium unit. The formation of imidazolium-based carbenes is not unexpected under ESI-MS conditions, but the resulting carbene is in most cases a neutral species and such reactions cannot be followed by mass spectrometry which relies on the detection of ions. However, in the present case the reaction product can be detected as the ligand is firmly attached to the charge-carrying central metal cation, which acts as a charge tag for the free carbene.26,27 This analysis provides the firm basis for the characterization of further and more complex μ-bbip bridged complexes by mass spectroscopy. However, the ease by which fragmentation can occur, allows that only imprecise predictions can be made of the actual stability of these compounds.
Fig. 6 Numbering of the atoms in Table 1. |
All distances and angles are in good agreement compared to similar compounds.15,16,28–30 No or only small changes of the bond distances (N1–C12, N2–C11, C11–C12) and bond angles (N1–C12–C11, N2–C11–C12) in the phenanthroline part are observed between the free ligands and their corresponding metal complexes. The phenanthroline part remains also widely unaffected by changes at the imidazole part with respect to bond lengths and bond angles within the series of the free ligands and the corresponding complexes. In contrast, the imidazole part changes with the stepwise introduction of benzyl moieties. Although, the imidazole rings exhibit almost identical N3–C5, N4–C6, and C5–C6 bond lengths, a significant asymmetry by means of shortening and elongating of the N4–C13 and N3–C13 bond is discernable in bip and [Ru(bip)]2+. This is attributed to the partial double bond character of N4–C13 (bip) and N4–C13 ([Ru(bip)]2+). Various twist angles regarding the benzyl groups were found, which are, however, due to crystal packing distortions and indicate a free rotation in solution, which is in good agreement with 1H-NMR experiments (e.g. singlet for the benzylic CH2 group; see ESI†). The Ag1–C13 (2.080(6) Å) and Ag1–Cl1 (2.3214(16) Å) bond distances are in good agreement with the expected values for [(NHC)AgCl]-type complexes. The bond angle C13–Ag1–Cl1 (178.9(2)°) is almost linear and no indications for other coordination geometries were observed, which are known for [(NHC)AgX]-type complexes (X = Cl, Br, I).31–33
By reacting [Ru(bbip)Ag]2+ with Pd(CH3CN)2Cl2 in dichloromethane (Fig. 7) an insoluble AgCl-precipitate was formed. From this observation we infer the formation of the palladium-NHC complex [Ru(bbip)Pd]2+. The removal of the precipitate and the evaporation of the remaining solvent yielded [Ru(bbip)Pd]2+. The corresponding rhodium complex was synthesized analogously (for details regarding the synthesis see ESI†). The formation of the target compound was confirmed by means of 1H-NMR- and ESI-HRMS experiments (Fig. 8 and ESI†). The splitting of the CH2-signals (δRu(bbip)PdHCH2 = 7.14 ppm (d, J = 17.7 Hz) and 6.96 ppm (d, J = 17.6 Hz) of [Ru(bbip)Pd]2+ into two sets may be interpreted in two ways. Firstly, a dimeric complex [[Ru(bbip)Pd]2]4+ may be formed in solution.37 Secondly, the splitting could be caused by hindered rotation of the benzyl groups38 at room temperature. Similar effects could be seen for [Ru(bbip)Rh]2+.
Fig. 8 Aromatic region of the 1H-NMR-spectra of [Ru(bbip)Ag]2+, [Ru(bbip)Pd]2+, and [Ru(bbip)Rh]2+ in acetonitrile-d3 with signal assignments. |
Complex | L3 | L2 | L1 | RuII/III |
---|---|---|---|---|
[Ru(bbip)]3+ | −2.25 | −1.98 | −1.55 | 0.86 |
[Ru(bbip)Ag]2+ | −2.22 | −1.96 | −1.64 | 0.87 |
[Ru(tbbpy)2(phen)]2+ (ref. 40) | −2.25 | −1.98 | −1.77 | 0.74 |
Cyclic voltammograms of [Ru(bbip)Pd]2+ and [Ru(bbip)Rh]2+ were also measured. However, the spectra show several irreversible peaks, which make it impossible to assign the redox potentials. It is important to note that the first reduction potentials of the bbip compounds are between 130 and 220 mV more positive than observed for the phen based analogue as shown in Table 2. This suggests that bbip+ ligands are easier to reduce and that therefore the lowest energy 3MLCT state is based in this ligand.
Complex | λ max, abs [nm] | λ sh, abs [nm] | λ max, em [nm] | Φ | τ a1 [ns] | τ a2 [ns] |
---|---|---|---|---|---|---|
a For [Ru(bbip)Rh]2+ a third lifetime was observed (see ESI Table S3a). | ||||||
[Ru(bbip)]3+ | 434 | 481 | 659 | 0.25 | 930 | 2142 |
[Ru(bbip)]3+ + TEA | 440 | 476 | 624 | 0.40 | 890 | 2340 |
[Ru(bbip)Ag]2+ | 440 | 474 | 647 | 0.34 | 1447 | 2309 |
[Ru(bbip)Pd]2+ | 440 | 475 | 646 | 0.39 | 717 | 1096 |
[Ru(bbip)Pd]2+ + TEA | 440 | 472 | 645 | 0.47 | 1271 | 1769 |
[Ru(bbip)Rh]2+ | 440 | 473 | 625 | 0.17 | 220 | 2103a |
[Ru(bpy)3]2+ | 451 | 423 | 607 | 0.0642 | 870 | — |
[Ru(bpy)3]2+ + TEA | 850 |
Very notable are higher emission quantum yields, and longer 3MLCT lifetimes (Fig. 9, Fig. 10, and Table 3, S3a, S3b, and Fig. S7†) compared to [Ru(bpy)3]2+. Striking is also the impact that the nature of the NHC bound metal exerts on the emission quantum yields. Here, the following order has been observed Pd > Ag > metal free > Rh. Very interestingly, even for the weakest emitting complex, that is, [Ru(bbip)Rh]2+, a significantly higher quantum yield relative to that reported for [Ru(bpy)3]2+ was observed. Further proof of the 3MLCT nature of the emitting state is derived from singlet oxygen evolution detected in steady state emission measurements.
The emission lifetime data are more complex than expected. In dichloromethane single exponential decays are observed in aerated as well as deaerated solutions for the [Ru(bbip)]3+ precursor and [Ru(bbip)Ag]2+. For [Ru(bbip)Pd]2+ and [Ru(bbip)Rh]2+ biexponential decays are observed (Table 3 and S3c–d†). The same is observed for aerated acetonitrile solution (Table S3b†). However in deaerated acetonitrile solution double exponential decay is observed for all compounds (Table S3a†).
At this stage this behaviour is not fully understood, but similar results were obtained for structurally related compounds.15,29
A potential explanation is the presence of impurities. However taking into account the detailed characterisation of the compounds with ESI-HRMS and NMR which do not show any evidence for impurities this does not look likely, although we cannot exclude this possibility totally. It is also possible that the presence of different conformers in solution is responsible, but no direct evidence for this is obtained from NMR studies. This important issue is at present under further investigation.
Taking into account the above discussion we tentative suggest that the data can be explained by the presence of two emitting states, one based on the peripheral tbbpy ligands and one based on the carbene moiety. The presence of two emitting states suggests a week coupling between the two states which may be surprising since they are based on the absorption spectra of the compounds expected to be close in energy. Further studies involving time resolved resonance Raman in combination of partial deuteration of the compounds are needed to better understand this behaviour.43
We can make a number of observations from the obtained data. Surprisingly the long components of the emission lifetimes together with the emission quantum yields are considerably larger than observed for [Ru(bpy)3]2+ as shown in Table 3. In addition, the excited state lifetimes of the heteronuclear compounds are not quenched by the introduction of the second metal centre and are increased in some cases.44 This may suggest week coupling between the photosensitiser and the catalytic centre and/or a strongly localised carbene excited state.
An important feature that should be central to the design of intramolecular systems is that upon excitation of the photosensitiser vectorial electron transfer to the catalytic centre is taking place via the bridging ligand. It is this process that makes these compounds intra- rather than intermolecular photocatalysts. Insights into such intramolecular electron transfer processes can be obtained from time resolved techniques. Femtosecond (150 fs) pump probe measurements were carried out using 480 nm excitation. In this study the photophysical properties of [Ru(bbip)]3+ and [Ru(bbip)Pd]2+ were investigated. The results gathered for [Ru(bbip)]3+ are in line with those obtained for similar polypyridyl compounds and indicate rapid intersystem crossing of the initially populated 1MLCT excited state. The population of the correspondingly 3MLCT excited state from the associated singlet state for [Ru(bbip)]3+ is about 150 fs similar to that obtained for [Ru(bpy)3]2+ (∼100–300 fs)41 (Fig. 11; lower part). Characteristic features observed for [Ru(bbip)]3+ include minima and maxima at 443 and 560 nm, respectively, and these compare well to those of [Ru(bpy)3]2+ (ref. 41) (Fig. 11, upper part). Fig. S8† shows differential absorption changes during the 8 ns timescale of our femtosecond instrumentation showed no appreciable decay. To further investigate this feature pump probe measurements on the nanosecond timescale were carried out. A notable difference in the spectra of [Ru(bbip)]3+ relative to the aforementioned spectra on the shorter time scale is that the region from 610 to 780 nm is masked by 3MLCT centred emission. However, in the differential absorption spectra of [Ru(bbip)]3+, a minimum was observed at 440 nm as well as maxima at 560 and 820 nm (Fig. 12, upper part) correlating either with a transition that is centred on a reduced ligand or with a ligand-to-metal charge-transfer (LMCT) transition to the formally oxidised Ru(III) ion of the 3MLCT state.41 In the absence of molecular oxygen, all 3MLCT excited state features decay, in agreement with the time-resolved emission experiments, quantitatively back to the ground state following the same kinetics, which indicates that the 3MLCT decay and the ground state recovery go hand in hand.45 At delay times beyond 20 μs, no residual transitions neither in the form of bleaching nor positive absorption remain. From the decay profiles 3MLCT excited state lifetimes were determined to be around 1000 ns. Similar results were obtained by investigating [Ru(bbip)Pd]2+ (Fig. S9a, S9b, S10a, and S10b†) with related transitions; a minimum at 480 nm and two maxima at 560 and 820 nm these have been assigned in the same spectroscopic features as discussed for [Ru(bbip)]3+.41 In contrast, the region from 600 to 760 nm reflects the 3MLCT centred emission.
The emission properties of [Ru(bbip)]3+ and [Ru(bbip)Pd]2+ in the presence of various amounts of triethylamine (TEA) a known sacrificial electron donor, were probed. The addition of TEA caused no appreciable changes to the absorption spectra regardless of the concentration of the quencher. On the other hand, the 3MLCT emission spectra reveal an intensification by a factor of 1.62 and 1.21 for [Ru(bbip)]3+ and [Ru(bbip)Pd]2+, respectively, rather than a quenching. Furthermore a blue shift from 659 to 624 nm is observed (Fig. S11,†Table 3). These data suggest that TEA interacts strongly with both [Ru(bbip)]3+ and [Ru(bbip)Pd]2+ by stabilizing the emissive 3MLCT excited state (Table 3 and S3a†). A striking observation is that in the presence of 0.108 M TEA the emission lifetimes of the [Ru(bbip)]3+ and the corresponding Pd compounds increase rather than decrease as would be expected if reductive quenching was taking place. For [Ru(bpy)3]2+ a small decrease was observed indicating the rather weak ability of TEA to quench most ruthenium polypyridyl complexes. The observed increase in excited state lifetimes for the carbene compounds may be due to preferential solvation of the complexes by TEA or possibly by reductive quenching forming a long-lived reduced ruthenium complex, see below. Hereby, tbbpy and bbip+ ligands are involved. In the presence of TEA, the relative amplitudes of the longer lifetime (about 2 μs) increases, while that of the shorter one (about 1 μs) decreases (Table S3a†).§
As such, the emission enhancement might be due to the transition to the bridging ligand, while a potential quenching is due to the transition to tbbpy. This is further corroborated by the decrease of the shorter lifetime from 930 to 890 ns and the increase of the longer lifetime from 2142 to 2340 ns, for instance, in the case of [Ru(bbip)]3+ (Table S3a†).
Additionally, in nanosecond pump probe measurements the decay kinetics of the 3MLCT centred emission, which spans from 610 to 780 nm, shows a pronounced effect upon addition of TEA (Fig. 13 and S12†).
In the case of [Ru(bbip)]3+ the lifetime increases from 1018 to 1551 ns in the absence and presence of 0.108 M TEA. This correlates to an increase by a factor of 1.52. A similar observation was recently made for another ruthenium polypyridyl complex.46 Commencing with the decay of the excited state a transient evolves that exhibits a minimum at 460 nm and a maximum at 500 nm. Notable is also the broadening of the maximum (Fig. 12 and S12a†). In line with our spectroelectrochemical investigations – vide supra – and previous studies on [Ru(bpy)3]2+ (ref. 47 and 48) we ascribe this observed process to the reductive quenching of an excited state. As a matter of fact the reduced complexes [Ru(bbip)]2+ and [Ru(bbip)Pd]+ and the oxidized TEA, i.e. the radical cation are formed. Spectroscopic evidence for the reduction is the 500 nm maximum, while the broadening around 475 nm refers to the oxidation of TEA (Fig. S12a†).47 Based on this analysis we could determine the lifetime of the reduced complex [Ru(bbip)]2+. Within the time window of 9 ms a lifetime of 3.3 ms was derived from the 500 nm fingerprint of [Ru(bbip)]2+ (Fig. S12b†).
In contrast to other intramolecular photoredox catalysts, like [(tbbpy)2Ru(tpphz)PdCl2]2+,9 no induction period is observed for any of the new complexes, since hydrogen is detected already after a few minutes of irradiation. Several important conclusions can be drawn from these observations. Firstly, [Ru(bbip)]3+ serves as a photosensitizer for the three different catalytic centres. Secondly, the lack of an induction phase, the concentration independence of catalytic activity, and the constant turn over frequency, especially for [Ru(bbip)Pd]2+, support an increased chemical stability of the whole photoactive catalyst by using NHC-bridging ligands. The influence of potential dimeric structures in the catalytic mixtures is currently under investigations.
To investigate whether the [Ru(bbip)M]2+ complexes are stable during irradiation DLS measurements were performed under catalytic conditions. For [Ru(bbip)Ag]2+ a linear correlation between particle growth and irradiation time was observed (Fig. S13†). A possible explanation for this could be the known formation of complex silver halogenido anions during synthesis49,50 or the formation of silver particles over the known photographic effect. Since DLS is not a quantitative method of analysis very small amounts of impurities of a silver halogenide may generate over the known photographic effect a detectible silver particle. Similar experiments were performed for [Ru(bbip)Pd]2+. However, no clear conclusion could be drawn from the obtained results. A potential explanation may be the presence of silver based impurities which may occur also during the synthesis of [Ru(bbip)Pd]2+ and [Ru(bbip)Rh]2+ alternative methods of investigation such as TEM may be needed. Further investigations into this direction are in progress.
This evidence provides the opportunity to further optimize the catalytic performance of these systems for instance along the lines described for the [(tbbpy)2Ru(tpphz)PdCl2][PF6]2.9 All aspects observed so far suggest a high stability of the new NN-NHC bridged heterodinuclear complexes, which support the assumption that the NHC-donor set has a favourable influence on catalytic centres. Further investigations aimed at improving catalytic activity by optimizing the conditions like solvent composition and water concentration are under way. This new type of photoredox active dinuclear complexes bridged by a more stabilizing NN-NHC ligand showed a promising way for further synthesis of photoactive catalysts.
Footnotes |
† Electronic supplementary information (ESI) available: Synthesis, crystal data, NMR H/D exchange, mass spectrometry, electrochemistry, spectroelectrochemistry, and photophysical data. CCDC 893729–893733, 765499 and 796734. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt01546k |
‡ K.P. and T.D.P. and R.S. contributed equally to the manuscript. |
§ Alternative explanations for this phenomenon could be the appearance of two different conformers in solution due to the benzyl groups attached to the bbip+ ligand, deprotonation events for the imidazolium C–H function in the excited state and ligand rearrangements at the carbene bound metal complexes. |
¶ COLLECT, Data Collection Software; Nonius B.V., Netherlands, 1998. |
|| SADABS 2.10, Bruker-AXS Inc., 2002, Madison, WI, U.S.A. |
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