pubs.acs.org/Langmuir © 2010 American Chemical Society
Photocatalytic Hydrogen Evolution from Rhenium(I) Complexes to [FeFe] Hydrogenase Mimics in Aqueous SDS Micellar Systems: A Biomimetic Pathway Hong-Yan Wang, Wen-Guang Wang, Gang Si, Feng Wang, Chen-Ho Tung, and Li-Zhu Wu* Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry & Graduate University, the Chinese Academy of Sciences, Beijing 100190, P. R. China Received December 19, 2009 To offer an intriguing access to photocatalytic H2 generation in an aqueous solution, the hydrophobic photosensitizer, Re(I)(4,40 -dimethylbpy)(CO)3Br (1) or Re(I)(1,10-phenanthroline)(CO)3Br (2), and [FeFe] H2ases mimics, [Fe2(CO)6(μ-adt)CH2C6H5] (3) or [Fe2(CO)6(μ-adt)C6H5] (4) [μ-adt = N(CH2S)2], have been successfully incorporated into an aqueous sodium dodecyl sulfate (SDS) micelle solution, in which ascorbic acid (H2A) was used as a sacrificial electron donor and proton source. Studies on the reaction efficiency for H2 generation reveal that both the close contact and the driving force for electron transfer from the excited Re(I) complexes and [FeFe] H2ases mimics are crucial for efficient H2 generation with visible light irradiation. Steady-state and time-resolved investigations demonstrate that the electron transfer takes place from the excited Re(I) complex 1 or 2 to [FeFe] H2ases mimic catalyst 3, leading to the formation of the long-lived Fe(I)Fe(0) charge-separated state that can react with a proton to generate Fe(I)Fe(II) 3 H, an intermediate for H2 production. As a result, a reaction vessel for the photocatalytic H2 production in an aqueous solution is established.
Introduction Enzymes may bind substrates through multiple interactions in elaborate pockets to direct a specific reaction pathway under mild conditions.1 The high efficiency and selectivity have stimulated chemists to mimic enzyme-like reactions that proceed within restricted environments.2 Hydrogenases (H2ases), which are natural enzymes for hydrogen (H2) evolution, have recently gained much attention owing to the utilization of base metals, either Fe or Ni and Fe, to catalyze the reversible oxidation of hydrogen into protons and electrons: 2Hþ þ 2e T H2 with high efficiency under mild conditions.3 The high-resolution X-ray crystallographic structures reveal that the active site of [FeFe] H2ases is deeply embedded within the protein matrix, featuring a butterfly Fe2S2 subunit coordinated by cysteine-linked Fe4S4 cluster, carbon monoxide, and cyanide ligands, with a dithiolate bridging the two iron centers (Scheme 1).4 Over the past several years, much *To whom correspondence should be addressed. E-mail: lzwu@ mail.ipc.ac.cn. (1) (a) Breslow, R.; Dong, S. D. Chem. Rev. 1999, 98, 1997–2012. (b) Vincent, K. A.; Parkin, A.; Armstrong, F. A. Chem. Rev. 2007, 107, 4366–4413. (2) (a) Kang, J.; Rebek, J., Jr. Nature 1997, 385, 50–52. (b) Yoshizawa, M.; Tamura, M.; Fujita, M. Science 2006, 312, 251–254. (c) Wu, X.-L.; Luo, L.; Lei, L.; Liao, G.-H.; Wu, L.-Z.; Tung, C.-H. J. Org. Chem. 2008, 73, 491–494. (3) (a) Holm, R. H.; Kennepohl, P.; Solomon, E. I. Chem. Rev. 1996, 96, 2239– 2314. (b) Frey, M. ChemBioChem 2002, 3, 153–160. (4) (a) Nicolet, Y.; Piras, C.; Legrand, P.; Hatchikian, C. E.; Fontecilla-Camps, J. C. Structure 1999, 7, 13–23. (b) Pandey, A. S.; Harris, T. V.; Giles, L. J.; Peters, J. W.; Szilagyi, R. K. J. Am. Chem. Soc. 2008, 130, 4533–4540. (5) (a) Lawrence, J. D.; Li, H.; Rauchfuss, T. B. Chem. Commun. 2001, 16, 1482– 1483. (b) Gloaguen, F.; Rauchfuss, T. B. Chem. Soc. Rev. 2009, 38, 100–108. (6) Mejia-Rodriguez, R.; Chong, D.; Reibenspies, J. H.; Soriaga, M. P.; Darensbourg, M. Y. J. Am. Chem. Soc. 2004, 126, 12004–12014. (7) Capon, J.-F.; Gloaguen, F.; Petillon, F. Y.; Schollhammer, P.; Talarmin, J. Coord. Chem. Rev. 2009, 253, 1476–1494. (8) (a) Borg, S. J.; Behrsing, T.; Best, S. P.; Razavet, M.; Liu, X.; Pickett, C. J. J. Am. Chem. Soc. 2004, 126, 16988–16999. (b) Tard, C.; Liu, X. M.; Ibrahim, S. K.; Bruschi, M.; De Gioia, L.; Davies, S. C.; Yang, X.; Wang, L. S.; Sawers, G.; Pickett, C. J. Nature 2005, 433, 610–613. (c) Tard, C.; Pichett, C. J. Chem. Rev. 2009, 109, 2245– 2274.
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effort has been undertaken to mimic the structure and functionality of the H2ases and to understand the mechanism of stepwise proton reduction in the active site of [FeFe] H2ases analogues as well.5-12 These achievements have provided invaluable information on designing biomimetic catalysts. Along this line of consideration, developing a new system capable of simulating the functionality of [FeFe] H2ases under aqueous conditions becomes an important objective to pursue because water would provide the advantage serving as both the solvent and the proton source under physiological conditions. More importantly, an understanding of the chemistry of [FeFe] H2ases mimics in water is essential not only for designing artificial assembling catalyst but also for providing mechanistic insights into a broad array of natural oxidoreductases. However, in this regard, there were only a few scattered references to the synthetic water-soluble [FeFe] H2ases mimics reported so far. In 2004, for example, Darensbourg and co-workers6 incorporated substituent ligand PTA (PTA = 1,3,5-triaza-7-phosphaadamantane) into a [FeFe] H2ases model to improve the solubility of synthetic model analogues in water. Later on, Weigand et al. used functionalized sugars as ligands toward water-soluble [FeFe] H2ases mimics.13 Nevertheless, these systems finally finish their electrocatalytic H2 production in a mixture of H2O and CH3CN. Very recently, Jones et al. designed a [FeFe] H2ases mimic that is (9) Song, L.-C.; Tang, M.-Y.; Su, F.-H.; Hu, Q.-M. Angew. Chem., Int. Ed. 2006, 45, 1130–1133. (10) (a) Si, G.; Wang, W.-G.; Wang, H.-Y.; Tung, C.-H.; Wu, L.-Z. Inorg. Chem. 2008, 47, 8101–8111. (b) Wang, W.-G.; Wang, H.-Y.; Si, G.; Tung, C.-H.; Wu, L.-Z. Dalton Trans. 2009, 2712–2720. (c) Si, G.; Wu, L.-Z.; Wang, W.-G.; Ding, J.; Shan, X.-F.; Zhao, Y.-P.; Tung, C.-H.; Xu, M. Tetrahedron Lett. 2007, 48, 4775–4779. (11) (a) Lomoth, R.; Ott, S. Dalton Trans. 2009, 9952–9959. (b) Ekstr€om, J.; Abrahamsson, M.; Olson, C.; Bergquist, J.; Kaynak, F. B.; Eriksson, L.; Sun, L.; Becker, H.-C.; Åkermark, B.; Hammarstr€om, L.; Ott, S. Dalton Trans. 2006, 4599–4606. (12) (a) Na, Y.; Pan, J.; Wang, M.; Sun, L. Inorg. Chem. 2007, 46, 3813–3815. (b) Na, Y.; Wang, M.; Pan, J.; Zhang, P.; Åkermark, B.; Sun, L. Inorg. Chem. 2008, 47, 2805–2810. (13) Apfel, U.-P.; Halpin, Y.; Gottschaldt, M.; G€orls, H.; Vos, J. G.; Weigand, W. Eur. J. Inorg. Chem. 2008, 5112–5118.
Published on Web 05/14/2010
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Article Scheme 1. [FeFe] H2ases in Nature
coordinated to a simple, R-helical peptide via two cysteine residues, opening the chemical door for the synthesis of more sophisticated peptides containing second coordination sphere residues.14 In this contribution, we started to construct a self-assembling system for photocatalytic H2 evolution in water. Although threecomponent intermolecular systems containing a ruthenium Ru(II) complex as photosensitizer, a [FeFe] H2ases mimic, and a sacrificial electron donor have been studied in organic solvents,12b it is rather difficult to conduct such reactions in water because either the photosensitizer or the [FeFe] H2ases mimic is hardly soluble. Herein, it is intended to assemble the active components from the light-harvesting photosensitizer and the [FeFe] H2ases mimic as an alternative to increase the solubility in water, thus rendering the possibility to achieve photocatalytic H2 evolution in an aqueous solution by visible light irradiation. The sodium dodecyl sulfate (SDS) micelle, containing a negatively charged surface and a hydrophobic interior microheterogeneous environment, was selected in this work because it can interact with water-insoluble transition-metal complexes to remarkably enhance the solubility of compounds in water.15 With respect to the subtle changes in the molecular structure of Re(I)(4,40 -dimethylbpy)(CO)3Br (1), Re(I)(1,10-phenanthroline)(CO)3Br (2) and [Fe2(CO)6(μ-adt)CH2C6H5] (3), [Fe2(CO)6(μ-adt)C6H5] (4) [μ-adt = N(CH2S)2], it is anticipated that spectroscopic study and efficiency for H2 generation could provide valuable information on to what extent they are included into the SDS micelle that is reminiscent of a [FeFe] H2ases enzyme buried in a heterogeneous protein matrix. Our findings on the assembly of water-insoluble Re(I) complexes 1, 2 and [FeFe] H2ases mimics 3, 4 in the SDS micelles for photocatalytic H2 evolution in water are presented below.
Experimental Section Materials. CH2Cl2 (Fisher Chemicals, HPLC grade), CH3OH (Fisher Chemicals, HPLC grade), sodium dodecyl sulfate (SDS, Alfa Aesar), ascorbic acid (H2A, Aldrich), and dehydroascorbic acid (A, Aldrich) were used directly as received. Deionic water was employed to prepare the sample for photocatalytic reactions. CH3CN (Fisher Chemicals, HPLC grade) for electrochemical performance was distilled under argon atmosphere from CaH2, P2O5 in a stepwise manner followed by distillation from CaH2 again before use. Instrumentation and Methods. 1H NMR spectra were run on a Bruker-400 spectrometer with tetramethylsilane (1H) as internal standard. HR-ESI-MS was performed on a Bruker APEX III 7.0 T FTICR mass spectrometer combined with an Apollo ESI source. Infrared spectra were recorded on a Nicolet NEXUS 670 FT-IR spectrophotometer. The hydrogen production experiment in a Pyrex tube was performed by irradiation with a 500 W high-pressure Hanovia mercury lamp. A glass filter was (14) Jones, A. K.; Lichtenstein, B. R.; Dutta, A.; Gordon, G.; Dutton, P. L. J. Am. Chem. Soc. 2007, 129, 14844–14845. (15) (a) Thorp, H. H.; Kumar, C. V.; Turro, N. J.; Gray, H. B. J. Am. Chem. Soc. 1989, 111, 4364–4368. (b) Wu, L.-Z.; Cheung, T.-C.; Che, C.-M.; Cheung, K.-K.; Lam, M. H. W. Chem. Commun. 1998, 1127–1128.
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used to cut off light below 400 nm and thus guarantee the irradiation by visible light. Hydrogen detection was carried out on a Shimadzu GC-14B instrument with methane as an internal standard. The response factor for H2/CH4 determined by calibration with known amounts of H2 and CH4 was 3.0 under the experimental conditions. The UV-Vis absorption spectra were recorded using a Shimadzu 1601 PC spectrophotometer. The emission spectra were determined on a Hitachi 4500 spectrophotometer. Time-resolved emission and transient absorption spectroscopy were carried out on an Edinburgh LP 920 instrument. A three-electrode system, a 3 mm glass carbon working electrode, a platinum wire counter electrode, and a nonaqueous Ag/Agþ reference electrode, was used to measure the cyclic voltammograms. The working electrode was polished with a 0.05 μm alumina paste and sonicated for 15 min before use. The electrolyte solution, 0.1 M n-Bu4NPF6 used as electrolyte, was degassed with argon for 30 min before measurement. Experimental Procedure. All reactions and operations were carried out in dry argon atmosphere with standard Schelenk techniques. According to the reported methods in the literature,5a,16 the photosensitizers Re(I) complexes 1, 2 and the [FeFe] H2ases mimics 3, 4 were synthesized and then characterized by using 1H NMR, IR, and HR-ESI-MS spectrometers. The sample of the self-assembling system was prepared as follows: (1) Re(I) complexes 1, 2 and [FeFe] H2ases mimics 3, 4 were dissolved in CH2Cl2 or CH3OH, (2) the organic solvents were removed by N2 current, then (3) 0.166 M SDS aqueous solution (20 times its CMC17b) was added and stirred under N2 in darkness, and finally (4) the obtained sample was filtered by a colander with d = 0.22 um before irradiation. The concentration of each component was ascertained by a job plot (Supporting Information Figure S1).
Results and Discussion Micelles formed by surfactants in water have long been used to simulate a water-membrane interface found in biological systems.17 Like a cell membrane to some extent, it contains a negatively charged surface and a hydrophobic interior microheterogeneous environment in water. Herein, we employed one of the most popular micelles, SDS micelles, to incorporate lightharvesting rhenium Re(I) complexes 1 or 2 as photosensitizers and [FeFe] H2ases mimics 3 or 4 as catalysts in water, where ascorbic acid (H2A) was used as a sacrificial electron donor and proton source (Scheme 2). Although a three-component system containing a ruthenium Ru(II) complex, a [FeFe] H2ases mimic, and H2A was studied in organic solvents,12b reports on Re(I) complex as a photosensitizer are fairly rare. The difference between the Re(I) complex16 and other reported Ru(II) or Ir(III) complexes is that the excited Re(I) complex is sufficient to drive an electron transfer to a [FeFe] H2ases mimic, whereas Ru(II) or Ir(III) complexes are not. Considering that the N-bridged adt [FeFe] H2ases [μ-adt = N(CH2S)2] model complexes, closely related to the active site of H2ases in nature,18 could be reduced more easily than C-bridged pdt models [μ-pdt = C(CH2S)2],7 the combination of Re(I) complexes and adt [FeFe] H2ases mimics is therefore believed to provide a powerful driving force for the photoinduced electron transfer, the key process in light-driven H2 generation.11a The interaction of the SDS micelles with Re(I) complexes 1, 2 and [FeFe] H2ases mimics 3, 4 in water was clearly observed, as shown in Figure 1. No absorbance could be detected from (16) Schanze, K. S.; MacQueen, D. B.; Perkins, T. A.; Cabana, L. A. Coord. Chem. Rev. 1993, 122, 63–89. (17) (a) Cui, X.; Mao, S.; Liu, M.; Yuan, H.; Du, Y. Langmuir 2008, 24, 10771– 10775. (b) Bezzobotnov, V. Y.; Borbely, S.; Cser, L.; Farago, B.; Gladkih, I. A.; Ostanevich, Y. M.; Vass, S. J. Phys. Chem. 1988, 92, 5738–5743. (c) Tulumello, D. V; Deber, C. M. Biochemistry 2009, 48, 12096–12103. (18) Ryde, U.; Greco, C.; Gioia, L. D. J. Am. Chem. Soc. 2010, 132, 4512-4513.
DOI: 10.1021/la101322s
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Wang et al. Scheme 2. Photocatalytic H2 Evolution in an Aqueous SDS Micelle Solution
complexes 1-4 in pure water, while with progressive addition of an aqueous SDS ([SDS] = 0.166 M) micelle solution their solubility and absorbance were remarkably enhanced accompanying the formation of a colored solution (Figure 1, Supporting Information Figure S2). The observations suggest that the water-insoluble Re(I) complexes 1, 2 and [FeFe] H2ases mimics 3, 4 are incorporated into the aqueous SDS micelle solution. Similar to the concentrated organic solutions, complexes 1 and 3 in the aqueous SDS micelle solution are yellow and orange, respectively. In contrast, the system containing 2 or 4 is light-colored, close to the SDS micelle itself. Evidently, the incorporation by the SDS micelles is selective. With respect to the subtle change in molecular structure of the individual complex, the more rigid frameworks of 2 and 4 than those of the corresponding 1 and 3 might affect their interaction and movement in the SDS micelle, thus giving rise to the extent of each complex included in the SDS micelle is distinctly different. Subsequently, the concentration of each complex was determined by job plot after the above-mentioned solution was filtrated with a d = 0.22 um colander (Supporting Information Figure S1). For the Re(I) complexes 1 and 2, the saturated concentration was determined as 1.2 10-3 and 1.1 10-4 M, while for the [FeFe] H2ases mimics 3 and 4 the concentration was 5.9 10-4 M and 5.3 10-5 M, respectively. In addition, the UV-Vis absorption spectral changes of the [FeFe] H2ases mimics 3 and 4 were monitored for several hours to examine their stability both in darkness and lightness. The slower decay of the mimics 3 and 4 in the aqueous SDS micelle solution than that in CH3CN implies that their stability toward light increases to some extent in the SDS micelle solution (Supporting Information Figure S3). As both the complexes were found stable either in CH3CN or in the aqueous SDS micelle solution in the dark, this may have resulted from the fact that CH3CN is a good ligand leading to complexes 3 and 4 being photochemically unstable. The photochemical reaction for H2 evolution was carried out in the presence of H2A in a degassed SDS micelle solution at room temperature, where H2A was used as a sacrificial electron donor and proton source12b,19 because the interaction of the SDS micelle and H2A allows for the incorporation of a large amount of H2A in
the micelle.20 Typically, a solution of Re(I) complex 1 (1.8 10-4 M) and [FeFe] H2ases mimic 3 (1.8 10-4 M) in the presence of H2A (0.1 M) in a Pyrex tube was irradiated with a 500 W highpressure Hanovia mercury lamp. A glass filter was used to cut off light below 400 nm and thus guarantee irradiation by visible light. The generated H2 was collected and analyzed by GC with methane as the internal standard. As shown in Figure 2, the timedependence indicates that the amount of H2 evolution is increased linearly in the first 50 min. Upon 1 h of irradiation, the amount of H2 reached to 5.5 μL for the mixture of 1 and 3 with a turnover of 0.13 in the aqueous SDS micelle solution. However, prolonged irradiation over 1 h would decompose the [FeFe] H2ases mimics and slow down the rate of H2 production, similar to reported [FeFe] H2ases cases.11,12 The other three systems, the mixtures of 1 and 4, 2 and 3, 2 and 4, also show H2 generation upon irradiation, but the efficiencies are much lower than that of 1 and 3 (Supporting Information Table S1). The controlled experiment shows that (i) no H2 formation was observed when the reaction of the systems was carried out in darkness; (ii) omission of any of H2A, Re(I) complexes and [FeFe] H2ases mimics did not result in detectable H2 evolution; (iii) owing to the much lower solubility of [FeFe] H2ases mimic 4 in the aqueous SDS micelle solution, the H2 evolution efficiencies from the mixtures of 1 and 4, 2 and 4 are small (Supporting Information Table S1). All of the results suggest that with light, Re(I) complex, [FeFe] H2ases mimic, and H2A are required for the photocatalytic H2 production in aqueous SDS micelle solutions. As discussed above, the concentration of each complex included in the aqueous SDS micelle solution was different. For systematic comparison, we performed the photocatalytic reaction of the mixtures of 1 and 3, 2 and 3 at identical concentrations. The amount of H2 evolution from the mixture of 1 and 3 (2.3 μL) was higher than that of 2 and 3 (1.2 μL) under the same experimental condition (Figure 2b, c). Since the extinction coefficients above λ > 400 nm for complexes 1 and 2 are identical in both cases, and the higher concentration of complexes 1 and 3 included in the SDS micelle bears higher efficiency (Figure 2a), it is considered that the close contact between the excited Re(I) complexes and the [FeFe]
(19) (a) Tabushi, I.; Yazaki, A. J. Org. Chem. 1981, 46, 1899–1901. (b) Brown, G. M.; Brunschwig, B. S.; Creutz, C.; Endicott, J. F.; Sutin, N. J. Am. Chem. Soc. 1979, 101, 1298–1300.
(20) Szymula, M.; Szczypa, J.; Friberg, S. E. J. Dispersion Sci. Technol. 2002, 23, 789–797.
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Figure 1. UV-Vis absorption spectra of 1 in pure water (black dash-dot-dot-dashed line) and in an aqueous SDS micelle solution (red solid line); 3 in pure water (blue dashed line) and in an aqueous SDS micelle solution (green dash-dot-dashed line). The concentration is 6.9 10-5 M for 1 and 5.7 10-5 M for 3 in the aqueous SDS micelle solution.
Figure 2. Time dependence of H2 evolution and turnover number of the mixtures of (a) [1]=1.8 10-4 M, [3]=1.8 10-4 M, [H2A]= 0.1 M; (b) [1] = 7.4 10-5 M, [3] = 1.8 10-4 M, [H2A] = 0.1 M; (c) [2] = 7.4 10-5 M, [3] = 1.8 10-4 M, [H2A] = 0.1 M in an aqueous SDS micelle solution.
H2ases mimics is crucial for efficient H2 generation with visible light irradiation (Scheme 2). To provide insight into the light-driven H2 production from the aqueous SDS micelle solution, we examined the photophysical properties of the Re(I) complexes with the addition of [FeFe] H2ases mimics in a deaerated solution. Re(I) complex 1 exhibits broad absorption bands between 250 and 550 nm with ε on the order of 103 dm3 mol-1 cm-1 (Figure 1). With reference to previous spectroscopic work on Re(I) complexes,16 the absorption bands at 380-500 nm are ascribed to the dπ(Re)fπ*(N-N) MLCT state. On the other hand, no emission could be observed from complex 1 in pure water due to water insolubility. Initial introduction of SDS into an aqueous solution of 1 led to a broad emission band at 550 nm, similar in energy to most reported 3 MLCT emissions of related Re(I) complexes.16 Figure 3 shows the emission spectra of Re(I) complex 1 in the absence and presence of the [FeFe] H2ases mimic 3 in aqueous SDS solutions. It was obvious that the luminescence of 1 was quenched dramatically when complex 3 was introduced into the solution, while the absorption spectrum of their mixture resembles the superposition of the spectra of its components (Supporting Information Figure S4). These results suggest that there is no significant electronic interaction between Re(I) complex 1 and [FeFe] H2ases mimic 3 in the ground state, but indeed in the excited state. Similarly, the remarkable quenching trend of 2 was also observed with the addition of [FeFe] H2ases mimic 3 into the SDS micelle solution (Supporting Information Figure S5). Furthermore, the timeresolved luminescence measurement provides another piece of evidence for the interactions between the Re(I) complexes and the [FeFe] H2ases mimics in the excited state. In the absence of 3, the Langmuir 2010, 26(12), 9766–9771
Figure 3. Emission spectra of 1 in pure water (purple dot-dashed line) and in aqueous SDS solution (5 10-5 M) without 3 and with 3 (0-6.2 10-5 M) (solid lines).
Figure 4. Transient absorption spectra of 1 (1.0 10-4 M) (a), and 1 (1.0 10-4 M) and 3 (1.2 10-4 M) (b) upon laser pulse by 355 nm light in aqueous SDS solution under argon atmosphere and the kinetic traces recorded at 400 nm.
luminescence lifetime τ0 was 38.2 ns for 1 and 78.0 ns for 2, while with the addition of 3 the lifetime was shortened to be τ1 = 25.1 ns for 1 and 64.2 ns for 2. Analysis of the decays suggests that the decay is monoexponential following Stern-Volmer kinetics (Supporting Information Figure S6). The shortened lifetime and luminescence quenching of 1 or 2 may be attributed to electron transfer from the excited Re(I) complex 1 or 2 to the [FeFe] H2ases mimic 3. According to eqs 1 and 2, the quenching constant (kET) and quenching efficiency (ΦET) can be subsequently determined as kET =2.3 1011 M-1 s-1 , 4.1 1010 M-1 s-1 and ΦET = 35.3%, 15.5% for the cases of 1 and 3, 2 and 3, respectively (Supporting Information Figure S6), where τ0 and τ1 refer to the lifetimes of the Re(I) complexes in the absence and presence of the [FeFe] H2ases mimic in the aqueous SDS micelle solution. The larger rate constant and higher efficiency for the mixture of 1 and 3 than that of 2 and 3 are consistent with the difference in the H2 evolution efficiency with visible light irradiation, indicating that the photoinduced electron transfer from the Re(I) complexes to the [FeFe] H2ases mimics is closely related to the H2 evolution in the aqueous SDS micelle solution. kET ½3 ¼ ð1=τ1 Þ - ð1=τ0 Þ
ð1Þ
ΦET ¼ kET ½3=ð1=τ0 - kET ½3Þ
ð2Þ
The photoinduced electron transfer process was further evidenced by a flash photolysis study at room temperature. Figure 4 DOI: 10.1021/la101322s
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Wang et al. Scheme 3
displays the time-resolved absorption difference spectra for complex 1, and the mixture of 1 and 3 in the degassed aqueous SDS micelle solution as well. Upon laser pulse by 355 nm light, strong transient 3MLCT absorption for 1 emerged immediately, the decay of which throughout the absorption region and the recovery of the bleach occurred on the same time scale and could be well described by a monoexponential function. The lifetime (τ0) of 38.2 ns obtained from the decays of the absorption transients as well as from the bleach recoveries was in quantitative agreement with that of the luminescence. When the [FeFe] H2ases mimic 3 was introduced into the solution of Re(I) complex 1 ([1] = 1.0 10-4 M, [3] = 1.2 10-4 M), the 3MLCT absorption of 1 was replaced by a series of new absorptions throughout the near-UV and visible region with characteristic absorptions at 400 and 580 nm. The generated new absorptions were different from the profiles of the Re(I) complex (Figure 4, Supporting Information Figure S7), but quite similar to that of the reported Fe(I)Fe(0) species generated by the electrochemical reduction of a [FeFe] H2ases mimic.8b,12 The decay of the absorption occurred on the same time scale and could be well described by a monoexponential function with a long-lived charge-separated Fe(I)Fe(0) species of 619 and 697 μs for the mixtures of 1 and 3, 2 and 3, respectively. Prolonged irradiation of the mixtures of 1 and 3, 2 and 3 in the aqueous SDS micelle solution led to no permanent change, indicating that the Fe(I)Fe(0) species formed by the photoinduced electron transfer was quite stable. If we assume the generated new absorptions originated from the Fe(I)Fe(0) species of the reduced [FeFe] H2ases mimic alone, the rate (kCR) for the back electron transfer was determined to be 1.62 103 and 1.43 103 M-1 s-1 for the mixtures of 1 and 3, 2 and 3, respectively. Since the Fe(I)Fe(0) species is important for the generation of a Fe(I)Fe(II) 3 H intermediate in the proton reduction process,6,7 the long-lived charge separation of Fe(I)Fe(0) species is therefore believed to be responsible for the performance on the light-driven H2 production in the aqueous SDS micelle solution. In previously reported three-component systems,11,12 the excited state of a photosensitizer is not sufficient to deliver an electron to the [FeFe] H2ases mimic. However, the potential of the Re(I) complexes is on the contrary. To confirm this is indeed the case, we examined the driving force ΔG° of the photoinduced electron transfer between the Re(I) complexes and the [FeFe] H2ases mimics. Since the electrochemical window of the aqueous SDS micelle solution is too narrow to directly give the redox potential of the complexes, a cyclic voltammetric study of complexes 1-4 was performed in a CH3CN solution (0.1 M n-Bu4NPF6 as electrolyte) under argon atmosphere at room temperature. On the basis of the determined oxidation potential Eox of the Re(I) complexes, the reduction potential Ered of the [FeFe] H2ases mimics, and the excited state energy E00 of Re(I) complexes as well (Supporting Information Table S2), the free energy change (ΔG°) was calculated to be -0.15 eV for 1 and 3, -0.05 eV for 2 and 3, -0.18 eV for 1 and 4, and -0.08 eV for 2 and 4 (Supporting 9770 DOI: 10.1021/la101322s
Information Table S2). From the thermodynamically favorable free energy change (ΔG°), we believe that the electron transfer process from the excited Re(I) complexes to the [FeFe] H2ases mimics occurs. The excited state of Re(I) complex 1 is easier to be oxidized [Eox* = -1.66 eV] than that of complex 2 [Eox* = -1.56 eV], and thus, the driving force (ΔG°) for 1 and 3 (-0.15 eV) is greater than that of 2 and 3 (-0.05 eV). With the combination of the steady-state and time-resolved studies, it could be speculated that the initial step is oxidative quenching of the excited Re(I) complex by the [FeFe] H2ases mimic as an electron acceptor (Scheme 3). The formed Re(II)• is subsequently regenerated by electron transfer from the sacrificial electron donor HA-, while the long-lived Fe(I)Fe(0) species is reacted with a proton for H2 evolution. The resultant HA• radical, considered as the predominant reactive species, rapidly converts to dehydrogenated ascorbic acid A.19b,21 This process is well manifested by monitoring the formation of H2 and dehydrogenated ascorbic acid A in the photocatalytic system containing the Re(I) complex, the [FeFe] H2ases mimic, and H2A in a mixed solvent of CH3CN/H2O = 7:4. The reason for the presence of water here is that H2A is not soluble in pure CH3CN. As the irradiation at λ > 400 nm went on for 1 h, H2 evolution was clearly observed by GC and dehydrogenated ascorbic acid A, the major product when H2A loses its electron and proton,22 was detected by 1H NMR analysis (Supporting Information Figure S8). Therefore, it is reasonable to think that the photoinduced electron transfer from the Re(I) complexes to the [FeFe] H2ases mimics occurs in the aqueous SDS micelle solution even though the electrochemical window of the SDS micelle is too narrow to detect the redox potential of the complexes directly. The better performance for 1 and 3 than 2 and 3 implies that both the driving force (ΔG°) for the photoinduced electron transfer and the close contact between the excited Re(I) complexes and the [FeFe] H2ases mimics in the aqueous SDS solution are important for H2 evolution with visible light irradiation.
Conclusion By virtue of intermolecular interactions, hydrophobic photosensitizer, Re(I) complexes 1, 2, and [FeFe] H2ases mimics 3, 4, as well as ascorbic acid (H2A) as a sacrificial electron donor and a proton source have been incorporated into an aqueous SDS micelle solution. Studies on the efficiency of the photocatalytic reaction for H2 production reveal that the concentration of the complexes within a SDS micelle and the driving force of the excited Re(I) complexes and [FeFe] H2ases mimics are crucial during irradiation, which depends on the characteristic of the individual complexes very much. The fact that (1) the higher (21) Bielski, B. H. J.; Comstock, D. A.; Bowen, R. A. J. Am. Chem. Soc. 1971, 93, 5624–5629. (22) (a) Nishikawa, Y.; Kurata, T. Biosci., Biotechnol., Biochem. 2000, 64, 476– 483. (b) Wang, Y.-N.; Lau, K.-C.; Lam, W. W. Y.; Man, W.-L.; Leung, C.-F.; Lau, T.-C. Inorg. Chem. 2009, 48, 400–406.
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Wang et al.
concentration of complexes 1 and 3 included in the SDS micelle, and (2) the better performance for 1 and 3 than 2 and 3 under identical conditions bears higher efficiency indicates that the driving force (ΔG°) of the photoinduced electron transfer and the close contact between the Re(I) complexes and the [FeFe] H2ases mimics are necessary for efficient H2 generation with visible light irradiation (λ > 400 nm). Steady-state and timeresolved investigations demonstrate that the electron transfer takes place from the excited Re(I) complex 1 or 2 to the [FeFe] H2ases catalyst 3, leading to the formation of the long-lived charge separated state of Fe(I)Fe(0) that can react with a proton to generate Fe(I)Fe(II) 3 H, an intermediate for H2 production. Compared with the reports in an organic solution, the present work not only offers an ideal approach for the photocatalytic H2 production in an aqueous solution, but also provides valuable information as to what extent Re(I) complexes and [FeFe] H2ases mimics are included into aqueous SDS micelle solutions. This is helpful in understanding the activity and mechanism of the catalytic cluster where in nature is buried in a heterogeneous site. Acknowledgment. We are grateful for financial support from Solar Energy Initiative of the Knowledge Innovation Program
Langmuir 2010, 26(12), 9766–9771
Article
of the Chinese Academy of Sciences (KGCXZ-YW-389), the National Science Foundation of China (20732007 and 50973125), the Ministry of Science and Technology of China (2006CB806105, 2007CB808004, and 2009CB22008), and the Bureau for Basic Research of the Chinese Academy of Sciences. Supporting Information Available: The job plot of 1-4 in an aqueous SDS micelle solution; UV-Vis spectra of 2, 4 in pure water and in an aqueous SDS micelle solution; UV-Vis spectra of 3 and 4 at room circumstance in an aqueous SDS micelle solution and in CH3CN; photocatalytic H2 evolution from the mixtures of 1 and 3, 1 and 4, 2 and 3, 2 and 4 in an aqueous SDS micelle solution with 1 h irradiation; UV-Vis spectrum for the mixture of 1 and 3 in an aqueous SDS micelle solution; the emission spectra of 2 in pure water and in an aqueous SDS micelle solution with various concentration of 3; the lifetimes of 1 and 2 in the presence of various concentrations of 3 in an aqueous SDS micelle solution; the transient absorption spectra of 2 and the mixture of 2 and 3 in an aqueous SDS micelle solution; the 1H NMR analysis for H2A, A, and the residue of the photocatalytic H2 evolution system. This material is available free of charge via the Internet at http://pubs.acs.org
DOI: 10.1021/la101322s
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