Electron- and Energy-Transfer Processes in a Photocatalytic System

Letter pubs.acs.org/JPCL

Electron- and Energy-Transfer Processes in a Photocatalytic System Based on an Ir(III)-Photosensitizer and an Iron Catalyst Antje Neubauer,*,† Gilbert Grell,‡ Aleksej Friedrich,† Sergey I. Bokarev,*,‡ Patrick Schwarzbach,† Felix Gar̈ tner,§ Annette-E. Surkus,§ Henrik Junge,§ Matthias Beller,§ Oliver Kühn,‡ and Stefan Lochbrunner† †

Institute for Physics, University of Rostock, Universitätsplatz 3, 18055 Rostock, Germany Institute for Physics, University of Rostock, Wismarsche Str. 43-45, 18057 Rostock, Germany § Leibniz-Institute for Catalysis, Albert-Einstein-Str. 29a, 18059 Rostock, Germany ‡

S Supporting Information *

ABSTRACT: The reaction pathways of bis-(2-phenylpyridinato-)(2,2′-bipyridine)iridium(III)hexafluorophosphate [Ir(ppy)2(bpy)]PF6 within a photocatalytic water reduction system for hydrogen generation based on an iron-catalyst were investigated by employing time-resolved photoluminescence spectroscopy and time-dependent density functional theory. Electron transfer (ET) from the sacrificial reagent to the photoexcited Ir complex has a surprisingly low probability of 0.4% per collision. Hence, this step limits the efficiency of the overall system. The calculations show that ET takes place only for specific encounter geometries. At the same time, the presence of the iron-catalyst represents an energy loss channel due to a triplet−triplet energy transfer of Dexter type. This loss channel is kept small by the employed concentration ratios, thus favoring the reductive ET necessary for the water reduction. The elucidated reaction mechanisms underline the further need to improve the sun light’s energy pathway to the catalyst to increase the efficiency of the photocatalytic system. SECTION: Spectroscopy, Photochemistry, and Excited States

F

pyridinato-) (2,2′-bipyridine)iridium(III)hexafluorophosphate [Ir(ppy)2(bpy)]PF6 (IrPS), [Co(bpy)3]2+ as electron relay, and triethanolamine as SR, revealed two quenching pathways.9 Therein, photoluminescence (PL) quenching rates due to the electron relay were described as about one order of magnitude higher than those due to the SR. For a similar system without electron relay and employing Ir complexes as PS and Pt colloids as catalyst, a reductive quenching mechanism is described.10 However, mechanistic insights for different pathways on a molecular level are still missing. Recently, some of us developed a homogeneous photocatalytic system based on IrPS as photosensitizer, [HFe3(CO)11][NEt3H] as catalyst (Fe-cat) replacing the noble-metal complexes, and triethylamine (TEA) as SR (Scheme 1).11−14 The catalytically active species and intermediates for this system have been studied by in situ electron paramagnetic resonance, in situ infrared spectroscopy and DFT, as well as CASSCF/CASPT2 calculations.12,15 For instance, the reduced IrPS− species has been observed under irradiation of IrPS in a THF/TEA/water mixture. However, in the presence of the Fe-cat, this species could not be detected due to limited time resolution.15,16 This paper addresses the primary photophysical reaction steps of the iridium photosensitizer [Ir(ppy)2(bpy)]PF6 with its

or the large-scale utilization of sunlight as a sustainable, greenhouse-gas-emission-neutral, and secure energy source, an efficient storage of solar energy in chemical fuels is essential.1 Therefore, water splitting as one possible way to achieve this goal has gained high scientific attention in recent years. A broad variety of heterogeneous and homogeneous photocatalytic systems for hydrogen generation has been investigated. (See refs 1−6 and references therein.) Because of the complexity of the processes involved, each of the half reactions, water oxidation and reduction, is often studied separately by adding a sacrificial oxidant or reductant, respectively. Thus, homogeneous photocatalytic systems for hydrogen generation contain a light-absorbing photosensitizer, optionally an electron relay to facilitate charge separation, a catalyst which reduces the aqueous protons to hydrogen, and a sacrificial reductant (SR). Surprisingly, despite the diversity of the investigated systems based on these components, detailed mechanistic studies are scarce.4,7−9 Eisenberg et al. 7,8 investigated systems with cobaloxime complexes as catalysts and Pt terpyridyl acetylide chromophores as photosensitizers. For these systems, oxidative quenching of the Pt photosensitizer by the cobaloximes are described, and rate constants between 4 × 107 and 2 × 109 M−1 s−1 were reported for this process. The latter rate constants are close to the diffusioncontrolled limit. Mechanistic studies of a homogeneous photocatalytic system with heteroleptic Ir complexes as photosensitizer, including the here studied bis-(2-phenyl© 2014 American Chemical Society

Received: February 28, 2014 Accepted: March 26, 2014 Published: March 26, 2014 1355

dx.doi.org/10.1021/jz5004318 | J. Phys. Chem. Lett. 2014, 5, 1355−1360

The Journal of Physical Chemistry Letters

Letter

Scheme 1. Scheme of the Investigated Photocatalytic System

reaction partners, that is, the SR and the catalyst in a THF or THF/water solution. It aims at mechanistic insights into the individual reaction steps on a molecular level and at identifying loss channels on relevant time scales for these processes. In particular, by means of steady-state and time-resolved PL spectroscopy as well as DFT/TD-DFT (density functional theory/time-dependent density functional theory) calculations, we focus on the following issues: (i) electron transfer (ET) from the SR TEA to the excited IrPS, (ii) excited-state quenching of the IrPS by the iron catalyst as loss channel in the photocatalytic system, and (iii) the dynamics of these reaction steps in the overall photocatalytic system. Finally, the relevance of the interplay of these reaction steps for the photocatalytic system is discussed. Quenching by Sacrif icial Reductant. The ET from the SR TEA to the excited IrPS was studied by PL quenching experiments in THF and THF/water mixture. The experimental conditions, for example, concentrations of the components, were chosen to be comparable to previous activity studies of the photocatalytic system12 but also to minimize spectroscopically problematic effects such as inner filter phenomena.17 The determined PL quantum yield, φPL, and lifetime, τ0, of IrPS in degassed THF are 0.15 (±0.03) and 370 ns (±30 ns), respectively. PL lifetimes reported in literature9,14,18−22 range from values of about 18019 to 565 ns,22 demonstrating the high sensitivity of the IrPS PL to solvatochromic and quenching effects. In fact, this is the lifetime of the lowest triplet excited electronic state, which has been proven to be relevant for photocatalysis according to spectroscopic9,14,23 and computational studies.24 By adding TEA as SR, the PL lifetime of IrPS decreases, for example, to 50 ns for a 5 vol% TEA concentration (0.4 M, Figure 1a). All measured PL decay signals exhibit a monoexponential behavior. Figure 1b shows the Stern−Volmer plot of the quantum yield and lifetime for quenching with TEA concentrations in the range of 0.3 to 2.6 M. The dependencies of lifetime and quantum yield ratios on concentration are linear within experimental accuracy and for the chosen concentration regime. Both plots of lifetime and corresponding yields coincide nicely, indicating the high reliability of the extracted information. Thus, the quenching rate constant kq was determined from the slope of the fit according to the Stern− Volmer equation (eq 1)25 φ τ0 = 0 = 1 + kqτ0[Q] τ φ (1)

Figure 1. (a) Photoluminescence decay curves and monoexponentially fitted time constants of IrPS (1.3 × 10−5 M) in degassed THF without TEA (black triangles) and with 5 vol% TEA (black circles) as well as in degassed THF/17 vol% water solution without TEA (red triangles) and with 5 vol% TEA (red circles). (b) Stern−Volmer plots: Ratios of luminescence lifetimes (triangles) and quantum yields (squares) versus TEA concentration in degassed THF solution without (open symbols) and with 17 vol% water (filled symbols). Mean quenching rate constants, determined by the slope of the linear fit curves for the lifetimes (solid line) and quantum yields (dashed line), are also given.

kq in THF. The here-derived PL quenching rate of 1.3 × 107 M−1 s−1 for the excited state of IrPS in THF/water is in good agreement with the ones determined by Bernhard et al. in acetonitrile/water as solvent with TEA and triethanolamine as quencher, where kq was determined to be 1.9 × 107 and 6 × 106 M−1 s−1, respectively.9,23 The reduced quenching rate in THF/ water mixture compared with pure THF can probably be attributed to the reaction of TEA as a weak base with water or shielding by water molecules surrounding the positively charged IrPS from interaction with the quencher TEA. Additionally, a shorter excited-state lifetime can also reduce the evaluated quenching rate constant. However, the impact of this effect is very low here due to a relatively long measured PL lifetime of 230 ns in THF/water mixture (Figure 1a). These quenching experiments also suggest that increasing the amount of SR leads to a higher performance of the photocatalytic system. However, previous hydrogen evolution experiments in a solvent mixture of THF/water showed a maximum activity at 33 vol% TEA (∼2.4 M), which was due to the limited solubility of TEA in the THF/water mixture.12 The measured rates can be compared with the diffusion limit (Supporting Information, section B). The calculated diffusion rate constant kD for IrPS and TEA in THF as solvent is 1.4 × 1010 L mol−1 s−1 and hence ∼200 times higher than the observed quenching rate constant.

where τ0 and φ0 are the PL lifetime and quantum yield in the absence of the quencher and τ and φ are PL lifetime and quantum yield at the quencher concentration [Q]. The following discussion is based on the mean values for kq derived from the lifetime and quantum yield ratios. Interestingly, the quenching rate constant in a THF/water mixture (17 vol% water content), kq,water, is reduced by a factor of ∼5 (Figure 1b) compared with the quenching rate constant 1356



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This indicates that the quenching efficiency per encounter is surprisingly low with about 0.4 and 0.1% in THF and THF/ water mixture, respectively. To elucidate the origin of the low quenching efficiencies on a molecular level, we performed TDDFT calculations. According to the oxidation and reduction potentials (Scheme SI-1 in the Supporting Information) and the zero-zero energy E00 of IrPS with 2.36 eV (525 nm), as determined by the interception of absorption and luminescence spectra, the triplet excited state of IrPS (T1) with an redox potential of +1.27 V versus NHE is capable of oxidizing TEA (+1.09 V vs NHE), thus being reduced to IrPS−, where the spin density is predominantly located on the bipyridine ligand.16 Hence, the reductive luminescence quenching of IrPS in the triplet excited state by TEA corresponds to an ET from TEA in the ground state (S0) to IrPS (T1), resulting in radical species in their doublet ground states (D0), as given in eq 2 (conservation of the total spin is allowed for)

The charge-transfer (CT) behavior of the system IrPS/TEA and the binding energies of the corresponding collision complexes were investigated at the DFT/TD-DFT level. (See the Supporting Information for computational details.) The energy of the D0+D0 state should be lower than that of the T1+S0 state to make the reaction (eq 2) thermodynamically possible. For analysis of the CT behavior, the lowest five triplet states of the joint IrPS/TEA system were calculated. In Figure 2a, the angular dependence of the binding energy between TEA and IrPS is plotted for a distance of R = 7 Å between the nitrogen atom of TEA and the iridium atom approximately corresponding to the potential energy minimum of the joint system. (For results with R = 8 Å, see the Supporting Information.) The plot represents a spherical surface and regions, where the polar angles are φ = 0 and π contract into one point. The spherical grid is represented by black dots, and the projections of ligand atoms onto the angular coordinates are represented by red dots for the 2,2-bipyridine ligand and green or blue dots for the two 2-phenylpyridine ligands. The grid points, where the energy of the CT (D0+D0) state is lower than the energy of the localized one (T1+S0), are marked with large orange circles in Figure 2a. The number of favorable orientations is only about 5−10% of the total 152 points for distances of 7 to 8 Å between IrPS and TEA. The CT process is favored by a closer distance and the inclusion of solvent effects but in general only possible for a very limited number of configurations. In addition, the calculated binding energies for IrPS and TEA are of the same order of magnitude as the binding energies between the solvent molecules calculated with the same method (Table SI-1 and Figure SI-5 in the Supporting Information). Hence, no stable complexes of TEA and IrPS are expected to form in the multicomponent reaction mixture. Furthermore, the orientations favorable for CT do not systematically correspond to the regions with the highest binding energy, but the CT state is lower in energy when TEA is close to the ligands. Weak bonding and the small number of orientations favoring CT thus explain the relatively low rates for reductive quenching observed in the experiments because only a small fraction of collisions leads to the products.

Figure 2. (a) Angular dependence (θ and φ are azimuthal and polar angles in radians, as described in panel b) of the binding energy (in electronvolts, contour plot). TEA was placed on a spherical grid (black points), centered at the iridium atom with a distance of R = 7 Å between the iridium atom of IrPS and the nitrogen atom of TEA. Red, green, and blue dots correspond to projections of ligand atoms onto the sphere. Orange circles denote orientations energetically favorable for CT reaction (eq 2). (c) Density difference plot of the CT state corresponding to n(TEA) + dx2−y2(Ir) + π(ppy) → π*(bpy) excitation.

Quenching by Iron Catalyst. The PL of IrPS is also quenched by the presence of Fe-cat in the reaction solution. For instance, the PL lifetime of 370 ns for IrPS in pure degassed THF is reduced to 280 ns for an Fe-cat concentration of 1.6 × 10−5 M (Table 1). To investigate the reaction between IrPS and Fe-cat, Table 1. PL Lifetimes of IrPS (c = 1.3 × 10−5 M) in Different Solvent Mixtures (degassed) and without and with Fe-cat (c = 1.6 × 10−5 M)a solvent components

τPL [ns] without Fe-cat

τPL [ns] with Fe-cat

THF THF/TEA (5/1) THF/water (5/1) THF/TEA/water (4/1/1)

370 13 230 42

280 14 210 42

a

If added, the amount of TEA or water was kept constant at a proportion of ∼17 vol %.

we performed corresponding luminescence quenching experiments (Figure 3a). All measured PL decay curves exhibit a monoexponential behavior. Figure 3b shows the steady-state absorption spectra of IrPS and Fe-cat as well as the normalized luminescence spectrum of IrPS. Because of the spectral overlap of the absorption spectrum of the iron catalyst and the luminescence spectrum of the iridium photosensitizer, the 1357



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× 1010 M−1 s−1. Thus, in degassed THF, the quenching by the Fe-cat is 1000 times more efficient than quenching by TEA. Because of the charges of each partner of this quenching pair, the Debye equation25,26 has to be applied for the theoretical diffusion rate kD,i, resulting in a value of 1.5 × 1011 M−1 s−1. Hence, quenching by the Fe-cat seems to be nearly diffusionlimited. ET that would be favorable for photocatalysis can be excluded due to the reduction potentials. The reduction potential of the excited state of the IrPS is with −0.82 V versus NHE (Scheme SI-1 in the Supporting Information), not large enough to reduce the Fe-cat species [HFe3(CO)11]− (−1.09 V vs NHE). The calculations, applying the same scheme as before for the IrPS/Fe-cat system at a distance of R = 10 Å, also suggest that the Fe-cat is neither oxidized nor reduced by the excited IrPS, because local states are always lower in energy than the CT ones. Thus, in accordance with other photocatalytic systems based on IrPS as PS,10 the oxidative ET mechanism does not play a role in our investigated system. Note that the reduced IrPS, in contrast with the oxidized form discussed here, is capable of reducing the Fe-cat, which is a prerequisite for catalytic water splitting in our system. Most likely, the efficient quenching of the PL of IrPS* by the Fe-cat is due to an energy transfer, which is known for quite a few similar iridium complexes in multicomponent systems.27−29 According to eq 3 IrPS(T1) + Fe‐cat(S0) → IrPS(S0) + Fe‐cat(Tx)

(3)

and to the calculated energies of the involved species, the energy of the lowest singlet−triplet transition for IrPS is larger than the energies of several of the lowest singlet−triplet transitions of the Fe-cat with a maximal energy difference of 1.2 eV (Figure 3c). Hence, the spectral overlap between the absorption of the Fe-cat and the PL of the IrPS, the energies of the possibly involved states, and the quenching rate close to diffusion limit are strong indications for a triplet−triplet energy transfer of Dexter type.30 In consequence, quenching of IrPS* by the Fe-cat is an energy-loss process in this photocatalytic system. Photocatalytic Half-Cell. The PL lifetimes τPL of IrPS obtained for different combinations of the components in Scheme 1 with concentration ratios similar to the photocatalytic system11 are summarized in Table 1. For comparison, a typical catalysis experiment in ref 11 was performed by employing 7.5 × 10−4· M IrPS. The IrPS/Fe3-species concentration ratio was varied between 0.4 and 4.8, and a ratio of 4:1:1 for the THF/TEA/ water solvent mixture. The highest turnover number for the chosen Fe3-species was observed at a concentration ratio of 0.62 of IrPS/Fe3-species. In our experiments, we chose comparable concentration ratios by employing an 1.3 × 10−5 M IrPS concentration, an IrPS/Fe-cat ratio of 0.8, and a ratio of 4:1:1 for the THF/TEA/water solvent mixture. At a TEA concentration of 17 vol %, PL lifetime is only marginally affected by adding the Fe-cat, for example, 13 versus 14 ns in THF/TEA and 42 ns in THF/TEA/water for both the absence and presence of the Fe-cat. Thus, the loss process due to the energy transfer from the IrPS* to the Fe-cat plays only a minor role in the overall photocatalytic system. Considering the concentration ratios within this system with a huge excess of SR, this is not very surprising. Even though the ET probability per collision with TEA is very low, this process outruns the energy transfer to the Fe-cat due to the high collision rate. Furthermore, the PL lifetime of IrPS in presence of Fe-cat is increased from 14 to 42 ns by the addition of water in THF/

Figure 3. (a) Stern−Volmer plots: Ratios of IrPS luminescence lifetimes (triangles) and quantum yields (squares) in dependence of Fe-cat concentration in degassed THF solution and linear fit curves for the lifetimes (solid line) and quantum yields (dashed lines). The determined quenching rate constant is based on the mean value for these two slopes according to eq 1. (b) UV/vis absorption spectra of IrPS (c = 1.3 × 10−5 M, black line), Fe-cat (c = 4 × 10−5 M, blue line), and normalized luminescence spectrum of IrPS in degassed THF (black dashed line, λexc = 388 nm). (c) Schematic representation of energetic positions of IrPS and Fe-cat local triplet states as predicted by theoretical calculations. The rectangles correspond to the variation of excitation energies depending on the mutual orientation of IrPS and Fe-cat.

procedure for determining the Stern−Volmer constant was slightly changed. Instead of determining the intensity ratios for the whole luminescence spectrum, the considered spectral range was limited to 650−750 nm, where the spectral overlap is negligible. Additionally, the obtained intensity values were corrected for the absorption of incident light by the Fe-cat (Figure SI-1 in the Supporting Information). The concentration of IrPS was 1.3 × 10−5 M in degassed THF as in the experiments with TEA (see previous). The Fecat concentration was varied between 3.7 × 10−6 M and 1 × 10−4 M. The obtained Stern−Volmer plots of the quantum yield and the lifetime are again in quite good agreement (Figure 3a) yielding values of (7.8 and 5.3) × 1010 M−1 s−1 for kq, respectively. Hence, the averaged quenching rate constant is 6.6 1358



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TEA and decreased from 280 to 210 ns by the addition of water in pure THF (Table 1). This is in accordance with the Stern− Volmer constants determined in the TEA quenching experiments with and without water (Figure 1b). In conclusion, the presented results on PL quenching show that the ET from TEA to the photoexcited IrPS with a yield of only 0.4% per collision is surprisingly inefficient. This result is explained well by the weak binding between TEA and IrPS compared with the binding energies of the solution components water, THF and TEA with each other. In addition, the DFT/TD-DFT calculations show that an ET from TEA to IrPS is, in general, improbable. For distances of 7 to 8 Å, ET occurs only in 5−10% of all investigated geometries. It only happens if the molecules accidentally collide in the right orientation. Hence, replacement of the sacrificial reagent has two positive effects: (i) the ET step from the excited photosensitizer to the electron donor has large potential for improvement and (ii) substituting the sacrificial electron donor by the oxidation reaction of the water splitting is essential for a large-scale application. Photoluminescence quenching of the photoexcited IrPS by the Fe-cat can be a loss channel for the photocatalytic hydrogen-generating system. This is in accordance with previous experimental studies on this photocatalytic system, which showed a decrease in turnover numbers for both Fe-cat and IrPS with increasing Fe-cat amounts.11 The absence of CT between the photoexcited IrPS and the Fe-cat does not allow an ET between these two species. Instead, triplet−triplet energy transfer by a Dexter-type mechanism takes place, as indicated by the high quenching efficiency and the spectral overlap of the emission and absorption spectra of IrPS and Fecat. In the overall photocatalytic system, this loss channel is minimized by the chosen conditions for the photocatalytic hydrogen generation, for example, the concentration ratios. In addition, modified photocatalytic systems with employed phosphines and different ligand substituents of the IrPS reach incident photon to hydrogen yields up to 16%,12,13 which are rather high for organometallic PS despite the unfavorable reaction steps. Our results show that for further efficiency improvement, the pathway of the absorbed light to the catalyst has to be modified. Hence, keeping the here elucidated energy loss channel ineffective might be an issue in future designs.

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REFERENCES

(1) Teets, T. S.; Nocera, D. G. Photocatalytic Hydrogen Production. Chem. Commun. 2011, 47, 9268−9274. (2) Maeda, K.; Domen, K. Photocatalytic Water Splitting: Recent Progress and Future Challenges. J. Phys. Chem. Lett. 2010, 1, 2655− 2661. (3) Osterloh, F. E. Inorganic Materials as Catalysts for Photochemical Splitting of Water. Chem. Mater. 2008, 20, 35−54. (4) Eckenhoff, W. T.; McNamara, W. R.; Du, P.; Eisenberg, R. Cobalt Complexes as Artificial Hydrogenases for the Reductive Side of Water Splitting. Biochim. Biophys. Acta 2013, 1827, 958−973. (5) Losse, S.; Vos, J. G.; Rau, S. Catalytic Hydrogen Production at Cobalt Centres. Coord. Chem. Rev. 2010, 254, 2492−2504. (6) Esswein, A. J.; Nocera, D. G. Hydrogen Production by Molecular Photocatalysis. Chem. Rev. 2007, 107, 4022−4047. (7) Du, P.; Schneider, J.; Luo, G.; Brennessel, W. W.; Eisenberg, R. Visible Light-Driven Hydrogen Production from Aqueous Protons Catalyzed by Molecular Cobaloxime Catalysts. Inorg. Chem. 2009, 48, 4952−4962. (8) Du, P.; Knowles, K.; Eisenberg, R. A Homogeneous System for the Photogeneration of Hydrogen from Water Based on a Platinum(II) Terpyridyl Acetylide Chromophore and a Molecular Cobalt Catalyst. J. Am. Chem. Soc. 2008, 130, 12576−12577. (9) Goldsmith, J. I.; Hudson, W. R.; Lowry, M. S.; Anderson, T. H.; Bernhard, S. Discovery and High-Throughput Screening of Heteroleptic Iridium Complexes for Photoinduced Hydrogen Production. J. Am. Chem. Soc. 2005, 127, 7502−7510. (10) Tinker, L. L.; McDaniel, N. D.; Curtin, P. N.; Smith, C. K.; Ireland, M. J.; Bernhard, S. Visible Light Induced Catalytic Water Reduction without an Electron Relay. Chem.Eur. J. 2007, 13, 8726− 8732. (11) Gärtner, F.; Sundararaju, B.; Surkus, A.-E.; Boddien, A.; Loges, B.; Junge, H.; Dixneuf, P. H.; Beller, M. Light-Driven Hydrogen Generation: Efficient Iron-Based Water Reduction Catalysts. Angew. Chem., Int. Ed. 2009, 48, 9962−9965. (12) Gärtner, F.; Boddien, A.; Barsch, E.; Fumino, K.; Losse, S.; Junge, H.; Hollmann, D.; Brückner, A.; Ludwig, R.; Beller, M. Photocatalytic Hydrogen Generation from Water with Iron Carbonyl Phosphine Complexes: Improved Water Reduction Catalysts and Mechanistic Insights. Chem.Eur. J. 2011, 17, 6425−6436. (13) Gärtner, F.; Cozzula, D.; Losse, S.; Boddien, A.; Anikulmar, G.; Junge, H.; Schulz, T.; Marquet, N.; Spannenberg, A.; Gladiali, S.; et al. Synthesis, Characterization and Application of Iridium(III)-Photosensitizers for Catalytic Water Reduction. Chem.Eur. J. 2011, 17, 6998−7006. (14) Gärtner, F.; Denurra, S.; Losse, S.; Neubauer, A.; Boddien, A.; Anikulmar, G.; Spannenberg, A.; Junge, H.; Lochbrunner, S.; Blug, M.; et al. Synthesis and Characterization of New Iridium Photosensitizers for Catalytic Hydrogen Generation from Water. Chem.Eur. J. 2012, 18, 3220−3225. (15) Hollmann, D.; Gärtner, F.; Ludwig, R.; Barsch, E.; Junge, H.; Blug, M.; Hoch, S.; Beller, M.; Brückner, A. Insights into the Mechanism of Photocatalytic Water Reduction by DFT-Supported insitu EPR/Raman Spectroscopy. Angew. Chem., Int. Ed. 2011, 50, 10246−10250. (16) Bokarev, S. I.; Hollmann, D.; Pazidis, A.; Neubauer, A.; Radnik, J.; Kühn, O.; Lochbrunner, S.; Junge, H.; Beller, M.; Brückner, A. Spin Density Distribution after Electron Transfer from Triethylamine to an [Ir(ppy)2(bpy)]+ Photosensitizer during Photocatalytic Water Reduction. Phys. Chem. Chem. Phys. 2014, 16, 4789−4796. (17) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, 3rd ed.; Springer: New York, 2006. (18) Lowry, M. S.; Hudson, W. R.; Pascal, R. A.; Bernhard, S. Accelerated Luminophore Discovery through Combinatorial Synthesis. J. Am. Chem. Soc. 2004, 126, 14129−14135. (19) Wilde, A. P.; King, K. A.; Watts, R. J. Resolution and Analysis of the Components in Dual Emission of Mixed-Chelate/Ortho-Metalate Complexes of Iridium(III). J. Phys. Chem. 1991, 95, 629−634.

ASSOCIATED CONTENT

* Supporting Information S

Experimental and computational details. Calculation of diffusion rate constants and determination of oxidation and reduction potentials. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interests.

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ACKNOWLEDGMENTS We acknowledge the financial support for the project “Light2Hydrogen” by the BMBF program “Spitzenforschung & Innovation in den Neuen Ländern”. 1359



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(20) Ohsawa, Y.; Sprouse, S.; King, K. A.; DeArmond, M. K.; Hanck, K. W.; Watts, R. J. Electrochemistry and Spectroscopy of OrthoMetalated Complexes of Iridium(III) and Rhodium(III). J. Phys. Chem. 1987, 91, 1047−1054. (21) Ladouceur, S.; Fortin, D.; Zysman-Colman, E. Enhanced Luminescent Iridium(III) Complexes Bearing Aryltriazole Cyclometallated Ligands. Inorg. Chem. 2011, 50, 11514−11526. (22) Costa, R. D.; Monti, F.; Accorsi, G.; Barbieri, A.; Bolink, H. J.; Ortí, E.; Armaroli, N. Photophysical Properties of Charged Cyclometalated Ir(III) Complexes: A Joint Theoretical and Experimental Study. Inorg. Chem. 2011, 50, 7229−7238. (23) Curtin, P. N.; Tinker, L. L.; Burgess, C. M.; Cline, E. D.; Bernhard, S. Structure-Activity Correlations Among Iridium(III) Photosensitizers in a Robust Water-Reducing System. Inorg. Chem. 2009, 48, 10498−10506. (24) Bokarev, S. I.; Bokareva, O. S.; Kühn, O. Electronic Excitation Spectrum of the Photosensitizer [Ir(ppy)2(bpy)]+. J. Chem. Phys. 2012, 136, 214305−214310. (25) Balzani, V.; Moggi, L.; Manfrin, M. F.; Bolletta, F.; Laurence, G. S. Quenching and Sensitization Processes of Coordination Compounds. Coord. Chem. Rev. 1975, 15, 321−433. (26) Debye, P. Reaction Rates in Ionic Solutions. Trans. Electrochem. Soc. 1942, 82, 265−272. (27) Flamigni, L.; Barbieri, A.; Sabatini, C.; Ventura, B.; Barigelletti, F. Photochemistry and Photophysics of Coordination Compounds: Iridium. Top. Curr. Chem. 2007, 281, 143−203. (28) Dixon, I. M.; Collin, J.-P.; Sauvage, J.-P.; Flamigni, L.; Encinas, S.; Barigelletti, F. A Family of Luminescent Coordination Compounds: Iridium(III) Polyimine Complexes. Chem. Soc. Rev. 2000, 29, 385− 391. (29) You, Y.; Nam, W. Photofunctional Triplet Excited States of Cyclometalated Ir(III) Complexes: Beyond Electroluminescence. Chem. Soc. Rev. 2012, 41, 7061−7084. (30) Turro, N. J. Modern Molecular Photochemistry; The Benjamin/ Cummings Publishing Company, Inc: Menlo Park, CA, 1978.

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Electron- and Energy-Transfer Processes in a Photocatalytic System

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