DOI: 10.1002/chem.201404480
Communication
& Self-Assembly | Hot Paper |
Functionalized Membranes for Photocatalytic Hydrogen Production Stefan Troppmann and Burkhard Kçnig*[a]
Chem. Eur. J. 2014, 20, 14570 – 14574
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2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Communication Abstract: Functionalized vesicles for photocatalytic hydrogen production in water have been prepared by co-embedding of amphiphilic photosensitizers and a hydrogenevolving catalyst in phospholipid membranes. The self-assembly allows a simple two-dimensional arrangement of the multicomponent system with close spatial proximity, which gave turnover numbers up to 165 for the incorporated amphiphilic cobaloxime water reduction catalyst 3 b under optimized conditions in purely aqueous solution. Superior photocatalytic activity in fluid membranes indicates that mobility and dynamic reorganization of catalytic subunits in the membrane promote the visible-lightdriven hydrogen production. The functionalized membranes represent nanostructured assemblies for hydrogen production in aqueous solution mimicking natural photosynthesis.
Figure 1. Self-assembled functionalized membranes for photocatalytic hydrogen generation by co-embedding of ruthenium photosensitizer 1 and hydrogen evolving catalyst 3 b in phospholipid membranes.
The conversion of sunlight energy into chemical energy by means of photocatalytic production of hydrogen is one of the most promising approaches towards a sufficient and sustainable energy supply, but still a challenging task.[1] Photochemical hydrogen generation typically uses three-component systems consisting of a light-absorbing photosensitizer, a water-reduction catalyst, and a sacrificial electron donor. The electron transfer between the photosensitizer and the reduction catalyst can be enhanced by covalent connection or assembly through bridging ligands leading to a more efficient hydrogen generation. In contrast, the intermolecular electron transfer in multicomponent systems with isolated subunits is diffusion limited. However, their synthesis avoids bridging ligands, is much simpler and allows varying composition and ratio of photosensitizer and catalyst. A variety of single- and multicomponent systems for photocatalytic hydrogen generation have been developed,[2] and many of them use cobalt reduction catalysts.[3] Although water is the ideal medium for H2 production, almost all reported systems with Co catalysts use organic solvents or mixtures of organic solvents and water. It was demonstrated that increasing the water content resulted in a decreased catalytic activity,[4] and cobalt-based photocatalytic systems working in purely aqueous solution are rare.[5] Herein, we report the photocatalytic hydrogen generation in pure water using self-assembled functionalized vesicle membranes by co-embedding of amphiphilic photosensitizers in combination with an amphiphilic cobalt-based hydrogenevolving catalyst (Figure 1). Vesicle membranes have been reported as model systems to investigate electron transfer across membranes and energy conversion.[6] Using vesicles overcomes solubility problems and allows hydrogen production in aque[a] S. Troppmann, Prof. Dr. B. Kçnig Institute of Organic Chemistry University of Regensburg, 93040 Regensburg (Germany) Fax: (+ 49) 941-943-1717 E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201404480. Chem. Eur. J. 2014, 20, 14570 – 14574
www.chemeurj.org
ous solution without the need of any organic solvent. The incorporation of the two redox-active subunits into lipid bilayers leads to a two-dimensional arrangement resulting in a high local concentration of the embedded photosensitizer and catalyst at the surface. The structured assembly put the complexes in close spatial proximity without direct covalent linkage, and membrane fluidity allows the dynamic reorganization, both of which may accelerate efficient electron transfer and improve the catalytic activity. For incorporation into vesicular membranes, the amphiphilic photosensitizers 1, 2 b, and the amphiphilic cobalt catalyst 3 b were synthesized (Figure 2). The amphiphilic derivative 1 of
Figure 2. Structures of photosensitizers (1, 2 a, and 2 b) and hydrogen-evolving catalysts (3 a and 3 b) used for photocatalytic hydrogen production.
the established [Ru(bpy)3]2 + (bpy = 2,2’-bipyridine) photosensitizer was prepared by modification of one bipyridine ligand with long alkyl chains.[7] As metal-free alternative, the xanthene photosensitizer 2 b was tested for light absorption. Catalyst 3 b for the photochemical proton reduction in vesicular membranes was obtained by attaching a hydrophobic hydrocarbon chain on the axial pyridine ligand of the literature-reported CoIII complex [Co(dmgH)2(py)Cl] 3 a (dmgH = dimethylglyoximate).[8]
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Communication Functionalized vesicles for photocatalytic hydrogen production were prepared by sonication of a lipid film comprising a phospholipid (4–8, Table 2), either photosensitizer 1 or 2 b and catalyst 3 b in aqueous solution. Clear vesicular solutions were obtained, and dynamic light scattering (DLS) analysis showed small unilamellar vesicles (Figure S1 in the Supporting Information). The incorporation of the amphiphilic photosensitizer and catalyst into vesicular membranes was proven by size-exclusion chromatography (Figures S2 and S3 in the Supporting Information), and the stability of vesicles was checked by DLS measurements (Figure S4 in the Supporting Information). The samples were degassed with nitrogen and irradiated at constant temperature with high-power light-emitting diodes (LEDs). The amount of evolved hydrogen was quantified by head-space gas chromatography. For detailed experimental setup and data, see the Supporting Information. Figure 3 shows the effect of the pH on the catalytic hydrogen production upon irradiation of self-assembled 1,2-dioleoyl-
15 %, respectively (Figure S5 in the Supporting Information). In the absence of TEOA, no hydrogen could be quantified by head-space gas chromatography.[10] As was already reported, the hydrogen evolution by using cobaloxime complexes as reduction catalysts depends on the stability of these complexes, which can be enhanced by the addition of free dimethylglyoxime ligand to the solution.[9a] Without free dmgH2, TONs of 23 and 93 for 3 b and 1, respectively, were achieved. In the presence of 32 equivalents of free dmgH2 (vs. catalyst 3 b), the catalytic activity increased to 40 and 159, respectively (Figure S6 in the Supporting Information). Next, the influence of the ratio of the two redox-active subunits was investigated. Performing the photochemical hydrogen generation in a 1:1 ratio of catalyst 3 b and photosensitizer 1 (126 mm of each component) embedded in DOPC (4) resulted in 76 TON (Table 1, entry 3). Decreasing the concentration
Table 1. Evolved hydrogen with different ratios of catalyst (cat.) 3 b and photosensitizer (PS) 1 co-embedded in DOPC (4) in aqueous solution, 15 % TEOA, 4 mm dmgH2, pH 8.3 and irradiation over 13 h at 19 8C.
Figure 3. Influence of the pH on photocatalytic hydrogen generation from a system composed of 1 (31.5 mm) and 3 b (126 mm) embedded in DOPC (4, 843 mm) in 15 % TEOA aqueous solution after 13 h of irradiation at 19 8C.
sn-glycero-3-phosphocholine (DOPC, 4) vesicles containing 12.6 mol % of 3 b and 3.15 mol % of 1 in a 15 % triethanolamine (TEOA) aqueous solution. The highest activity was observed at pH 8.3 with a maximum amount of H2 of 11.8 mmol, resulting in a turnover number (TON) of 23 and 93 based on 3 b and 1, respectively. The catalytic activity dropped with increasing or decreasing the pH value. At more basic conditions, the protonation of the reduced CoI catalyst is restricted, thus limiting hydrogen evolution.[9] At more acidic pH, the low concentration of nonprotonated electron donor TEOA reduces the catalytic activity. Investigating different electron donors, TEOA was found to be most efficient. Upon using TEOA as sacrificial electron donor, the turnover number for 1 rose from 50 to 93 with increasing the concentration of the tertiary amine from 5 to Chem. Eur. J. 2014, 20, 14570 – 14574
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Entry Ratio cat. 3 b/PS1
ccat3b [mm]
cPS1 [mm]
cDOPC [mm]
n (H2) [mmol]
TON cat. TON 3b PS1
1 2 3 4 5
126 126 126 15.8 3.9
15.8 31.5 126 126 126
858 843 748 858 866
4.6 20.0 38.4 7.3 2.6
9 40 76 115 165
8:1 4:1 1:1 1:8 1:32
72 159 76 14 5
of photosensitizer 1 to 31.5 mm gave an increased hydrogen evolution of 159 turnovers for 1 (entry 2), whereas a loss of activity was observed at lower concentrations of 1 (entry 1). Similarly, a decrease in the concentration of catalyst 3 b to 15.8 or 3.9 mm at a constant concentration of 126 mm for photosensitizer 1 resulted in higher TONs of 115 and 165 for the catalyst 3 b, respectively (entries 4 and 5).[11] These results indicate that the high local concentration of co-embedded catalytic subunits allows generation of hydrogen at comparable low catalyst concentration with high catalytic activity. Control experiments confirmed that every single component of the photocatalytic system is necessary. No hydrogen production could be observed in the absence of either photosensitizer 1, reducing catalyst 3 b, or light. To investigate the influence of the synthetic phospholipid on the catalytic activity, we co-embedded photosensitizer 1 (126 mm) and catalyst 3 b (31.5 mm) in vesicles prepared from different lipids, DOPC (4), 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC, 5), 1-stearoyl-2-myristoyl-sn-glycero-3-phosphocholine (SMPC, 6), 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC, 7), and 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC, 8). The different phospholipids have the same hydrophilic head group but different hydrophobic tails, changing the membrane properties significantly (Table 2). We observed a strong influence of the nature of the phospholipid and the membrane fluidity on the hydrogen production. The highest
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Communication Table 2. Chemical structure and phase-transition temperature of different phospholipids (4–8) used for membrane preparation (Tm is phase-transition temperature).
DOPC (4) DMPC (5) SMPC (6) DPPC (7) DSPC (8)
R1
R2
D9-cis-C17H35 C13H27 C17H35 C15H31 C17H35
D9-cis-C17H35 C13H27 C13H27 C15H31 C17H35
Tm [8C] 21 24 30 41 55
Figure 4. Effect of the phospholipid (4–8) and the reaction temperature on the photocatalytic hydrogen generation with co-embedded chromophore 1 (31.5 mm) and catalyst 3 b (126 mm), 4 mm dmgH2 and 15 % TEOA aqueous solution at pH 8.3 upon irradiation for 12 h.
hydrogen generation activity at 19 8C was observed with a turnover number of 159 for chromophore 1 by using DOPC (4; Figure 4, for kinetic measurements, see Figure S7 in the Supporting Information). Replacing the unsaturated DOPC (4) by saturated phospholipids (5–8), the catalytic performance of the vesicular systems decreased significantly. In DMPC (5), SMPC (6), DPPC (7), and DSPC (8), the TON dropped to 91, 42, 31, and 20, respectively. We explain this by the fluidity of the membrane and the mobility of the embedded photosensitizer 1 and water-reduction catalyst 3 b in the membrane. In DOPC (4) vesicles, with a gel to liquid-crystalline phase transition temperature (Tm) of 21 8C, the membrane in its liquid-crystalline state allows the diffusion of the embedded catalytic subunits in the fluid membrane. The dynamic self-organization results in high activity with a beneficial distribution of photosensitizer and catalyst in the membrane. Vesicles made from saturated phospholipids (5–8) have a higher phase-transition temperature and the diffusion in the rigid gel membrane is restricted at 19 8C, thus, the hydrogen production decreases. However, the H2 evolution enhanced for each phospholipid above the phase-transition temperature of the respective phospholipid. For DMPC (5) and DPPC (7) in the liquid-crystalline state, the overall hydrogen evolution was comparable to Chem. Eur. J. 2014, 20, 14570 – 14574
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DOPC (4). Nevertheless, the overall performance also depends on the nature of the lipid. For SMPC (6) and DSPC (8) vesicles an increase in the TON above the transition temperature was observed, but the overall performance was comparatively low. To get an insight into the mechanism of the photoinduced intermolecular electron transfer, the redox potentials of photosensitizer 1 and catalyst 3 b were measured (Figure S8 in the Supporting Information). The redox potentials indicate that oxidative quenching of the excited photosensitizer 1 is feasible from a thermodynamic point of view. The calculated free energy DG for the oxidation of the excited photosensitizer 1 by catalyst 3 b is slightly negative, and the emission quenching of 1 upon addition of 3 b confirms the possibility of an oxidative quenching (Figures S9 and S10 in the Supporting Information). However, the reductive quenching of the excited photosensitizer 1 by the sacrificial electron donor TEOA is expected to be the major pathway. The concentration of the electron donor in solution is much higher than the catalyst concentration, and reductive quenching yields RuI, which reduces CoII to CoI inducing hydrogen generation upon formation of a CoIII hydride. The concept of co-embedding was extended to the amphiphilic EosinY photosensitizer 2 b in an attempt to reduce the metal content of the catalytic system. Vesicular systems consisting of DOPC (4), photosensitizer 2 b (126 mm), catalyst 3 b (126 mm), and 4 mm dmgH2 in 10 % TEOA aqueous solution pH 7.5 gave 13 TON after 18 h of light irradiation with 535 nm. Thus, the TON under optimized conditions by using 2 b as photosensitizer decreased to one-sixth compared with photosensitizer 1 (76 TON) for a 1:1 ratio of the catalytic subunits. A significant decrease in the fluorescence intensity for embedded photosensitizer 2 b compared to homogeneous solution indicates clustering and self-quenching of the xanthene dye 2 b in the membrane (Figure S11 in the Supporting Information). We assume that the close proximity of 2 b in the lipid bilayer leads to stacking of the photosensitizer in patches preventing an efficient intermolecular electron transfer to the catalyst. TONs up to 251 for a homogeneous system by using 2 b and 3 a were comparable to literature-reported hydrogen production based on EosinY 2 a and 3 a,[9a] thus excluding the synthetic modification of 2 a as a reason for low hydrogen production (Table S1 in the Supporting Information). In conclusion, self-assembled functionalized vesicles for photocatalytic hydrogen production in purely aqueous solution have been developed. The membrane co-embedded amphiphilic ruthenium photosensitizer and cobalt-based water-reduction catalyst evolve hydrogen upon irradiation in the presence of TEOA as sacrificial electron donor giving TONs up to 165 for the cobalt catalyst under optimal conditions in water. Membrane fluidity affects the assembly structure of the incorporated catalytic subunits and allows the control of the photoinduced hydrogen generation. Best catalytic activity was observed in fluid membranes with high mobility of the photosensitizer and the catalyst. Embedding of an amphiphilic EosinY as photosensitizer reduced the metal content of the system but resulted in a significant self-quenching of the chromophores with decreased hydrogen production. The membrane embed-
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Communication ding allows the functionalization of soft surfaces and may provide an approach for overall water splitting with membrane separated half reactions and transmembrane electron transfer mimicking natural photosynthesis.
Acknowledgements We thank the German Science Foundation (DFG) for financial support. Keywords: photocatalysis · photosynthesis · self-assembly · supramolecular chemistry · vesicles [6] [1] a) N. S. Lewis, D. G. Nocera, Proc. Natl. Acad. Sci. USA 2006, 103, 15729 – 15735; b) T. R. Cook, D. K. Dogutan, S. Y. Reece, Y. Surendranath, T. S. Teets, D. G. Nocera, Chem. Rev. 2010, 110, 6474 – 6502; c) R. Eisenberg, D. G. Nocera, Inorg. Chem. 2005, 44, 6799 – 6801. [2] a) W. T. Eckenhoff, R. Eisenberg, Dalton Trans. 2012, 41, 13004 – 13021; b) M. Wang, Y. Na, M. Gorlov, L. Sun, Dalton Trans. 2009, 6458 – 6467; c) E. S. Andreiadis, M. Chavarot-Kerlidou, M. Fontecave, V. Artero, Photochem. Photobiol. 2011, 87, 946 – 964; d) L. L. Tinker, N. D. McDaniel, S. Bernhard, J. Mater. Chem. 2009, 19, 3328 – 3337. [3] a) V. Artero, M. Chavarot-Kerlidou, M. Fontecave, Angew. Chem. Int. Ed. 2011, 50, 7238 – 7266; Angew. Chem. 2011, 123, 7376 – 7405; b) S. Losse, J. G. Vos, S. Rau, Coord. Chem. Rev. 2010, 254, 2492 – 2504; c) J. L. Dempsey, B. S. Brunschwig, J. R. Winkler, H. B. Gray, Acc. Chem. Res. 2009, 42, 1995 – 2004. [4] a) J. Hawecker, J. M. Lehn, R. Ziessel, New J. Chem. 1983, 7, 271 – 277; b) C. V. Krishnan, B. S. Brunschwig, C. Creutz, N. Sutin, J. Am. Chem. Soc. 1985, 107, 2005 – 2015; c) P. Du, J. Schneider, G. Luo, W. W. Brennessel, R. Eisenberg, Inorg. Chem. 2009, 48, 4952 – 4962; d) P. Zhang, M. Wang, J. Dong, X. Li, F. Wang, L. Wu, L. Sun, J. Phys. Chem. C 2010, 114, 15868 – 15874; e) T. M. McCormick, Z. Han, D. J. Weinberg, W. W. Brennessel, P. L. Holland, R. Eisenberg, Inorg. Chem. 2011, 50, 10660 – 10666; f) A. Call, Z. Codol, F. AcuÇa-Pars, J. Lloret-Fillol, Chem. Eur. J. 2014, 20, 6171 – 6183. [5] a) G. M. Brown, B. S. Brunschwig, C. Creutz, J. F. Endicott, N. Sutin, J. Am. Chem. Soc. 1979, 101, 1298 – 1300; b) C. V. Krishnan, N. Sutin, J. Am. Chem. Soc. 1981, 103, 2141 – 2142; c) B. Probst, M. Guttentag, A. Rodenberg, P. Hamm, R. Alberto, Inorg. Chem. 2011, 50, 3404 – 3412; d) M. Guttentag, A. Rodenberg, R. Kopelent, B. Probst, C. Buchwalder, M. Brandsttter, P. Hamm, R. Alberto, Eur. J. Inorg. Chem. 2012, 2012, 59 – 64; e) M. Guttentag, A. Rodenberg, C. Bachmann, A. Senn, P. Hamm, R. Alberto, Dalton Trans. 2013, 42, 334 – 337; f) C. Bachmann, M. Guttentag, B. Spingler, R. Alberto, Inorg. Chem. 2013, 52, 6055 – 6061; g) M. Nippe, R. S. Khnayzer, J. A. Panetier, D. Z. Zee, B. S. Olaiya, M. Head-Gordon, C. J. Chang, F. N. Castellano, J. R. Long, Chem. Sci. 2013, 4, 3934 – 3945; h) Y. Sun, J. Sun, J. R. Long, P. Yang, C. J. Chang, Chem. Sci. 2013, 4, 118 –
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124; i) R. S. Khnayzer, V. S. Thoi, M. Nippe, A. E. King, J. W. Jurss, K. A. El Roz, J. R. Long, C. J. Chang, F. N. Castellano, Energy Environ. Sci. 2014, 7, 1477 – 1488; j) W. M. Singh, T. Baine, S. Kudo, S. Tian, X. A. N. Ma, H. Zhou, N. J. DeYonker, T. C. Pham, J. C. Bollinger, D. L. Baker, B. Yan, C. E. Webster, X. Zhao, Angew. Chem. Int. Ed. 2012, 51, 5941 – 5944; Angew. Chem. 2012, 124, 6043 – 6046; k) B. Shan, T. Baine, X. A. N. Ma, X. Zhao, R. H. Schmehl, Inorg. Chem. 2013, 52, 4853 – 4859; l) S. Varma, C. E. Castillo, T. Stoll, J. Fortage, A. G. Blackman, F. Molton, A. Deronzier, M.-N. Collomb, Phys. Chem. Chem. Phys. 2013, 15, 17544 – 17552; m) C. Gimbert-SuriÇach, J. Albero, T. Stoll, J. Fortage, M.-N. Collomb, A. Deronzier, E. Palomares, A. Llobet, J. Am. Chem. Soc. 2014, 136, 7655 – 7661; n) F. Lakadamyali, E. Reisner, Chem. Commun. 2011, 47, 1695 – 1697; o) L. Tong, R. Zong, R. P. Thummel, J. Am. Chem. Soc. 2014, 136, 4881 – 4884; p) M. Natali, A. Luisa, E. Iengo, F. Scandola, Chem. Commun. 2014, 50, 1842 – 1844; q) J. Xie, Q. Zhou, C. Li, W. Wang, Y. Hou, B. Zhang, X. Wang, Chem. Commun. 2014, 50, 6520 – 6522. a) T. Mizushima, A. Yoshida, A. Harada, Y. Yoneda, T. Minatani, S. Murata, Org. Biomol. Chem. 2006, 4, 4336 – 4344; b) S. Bhosale, A. L. Sisson, P. Talukdar, A. Frstenberg, N. Banerji, E. Vauthey, G. Bollot, J. Mareda, C. Rçger, F. Wrthner, N. Sakai, S. Matile, Science 2006, 313, 84 – 86; c) A. Perez-Velasco, V. Gorteau, S. Matile, Angew. Chem. Int. Ed. 2008, 47, 921 – 923; Angew. Chem. 2008, 120, 935 – 937; d) G. Steinberg-Yfrach, P. A. Liddell, S.-C. Hung, A. L. Moore, D. Gust, T. A. Moore, Nature 1997, 385, 239 – 241; e) J. J. Grimaldi, S. Boileau, J.-M. Lehn, Nature 1977, 265, 229 – 230; f) K. Watanabe, S.-y. Takizawa, S. Murata, Chem. Lett. 2011, 40, 345 – 347; g) R. F. Khairutdinov, J. K. Hurst, Nature 1999, 402, 509 – 511; h) L. Zhu, R. F. Khairutdinov, J. L. Cape, J. K. Hurst, J. Am. Chem. Soc. 2005, 127, 825 – 835; i) D. Hvasanov, J. R. Peterson, P. Thordarson, Chem. Sci. 2013, 4, 3833 – 3838; j) J. N. Robinson, D. J. Cole-Hamilton, Chem. Soc. Rev. 1991, 20, 49 – 94. M. Hansen, F. Li, L. Sun, B. Konig, Chem. Sci. 2014, 5, 2683 – 2687. W. C. Trogler, R. C. Stewart, L. A. Epps, L. G. Marzilli, Inorg. Chem. 1974, 13, 1564 – 1570. a) T. Lazarides, T. McCormick, P. Du, G. Luo, B. Lindley, R. Eisenberg, J. Am. Chem. Soc. 2009, 131, 9192 – 9194; b) X. Hu, B. S. Brunschwig, J. C. Peters, J. Am. Chem. Soc. 2007, 129, 8988 – 8998. With ascorbic acid (200 mm, pH 2–4) as sacrificial electron donor, just negligible amounts of hydrogen were produced (TON = 3 based on the photosensitizer). A direct comparison of the catalytic activity of our vesicular system with literature-reported systems is not possible, because photocatalytic systems with cobalt water-reduction catalysts working in water use different cobalt complexes and compositions (see Ref. [5]). Furthermore, the insolubility of the modified photosensitizer and the catalyst in water did not allow performing the hydrogen generation in water in the absence of phospholipid. R. Koynova, M. Caffrey, Biochim. Biophys. Acta Biomembr. 1998, 1376, 91.
Received: July 20, 2014 Published online on October 5, 2014
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Communication
& Self-Assembly | Hot Paper |
Functionalized Membranes for Photocatalytic Hydrogen Production Stefan Troppmann and Burkhard Kçnig*[a]
Chem. Eur. J. 2014, 20, 14570 – 14574
14570
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Communication Abstract: Functionalized vesicles for photocatalytic hydrogen production in water have been prepared by co-embedding of amphiphilic photosensitizers and a hydrogenevolving catalyst in phospholipid membranes. The self-assembly allows a simple two-dimensional arrangement of the multicomponent system with close spatial proximity, which gave turnover numbers up to 165 for the incorporated amphiphilic cobaloxime water reduction catalyst 3 b under optimized conditions in purely aqueous solution. Superior photocatalytic activity in fluid membranes indicates that mobility and dynamic reorganization of catalytic subunits in the membrane promote the visible-lightdriven hydrogen production. The functionalized membranes represent nanostructured assemblies for hydrogen production in aqueous solution mimicking natural photosynthesis.
Figure 1. Self-assembled functionalized membranes for photocatalytic hydrogen generation by co-embedding of ruthenium photosensitizer 1 and hydrogen evolving catalyst 3 b in phospholipid membranes.
The conversion of sunlight energy into chemical energy by means of photocatalytic production of hydrogen is one of the most promising approaches towards a sufficient and sustainable energy supply, but still a challenging task.[1] Photochemical hydrogen generation typically uses three-component systems consisting of a light-absorbing photosensitizer, a water-reduction catalyst, and a sacrificial electron donor. The electron transfer between the photosensitizer and the reduction catalyst can be enhanced by covalent connection or assembly through bridging ligands leading to a more efficient hydrogen generation. In contrast, the intermolecular electron transfer in multicomponent systems with isolated subunits is diffusion limited. However, their synthesis avoids bridging ligands, is much simpler and allows varying composition and ratio of photosensitizer and catalyst. A variety of single- and multicomponent systems for photocatalytic hydrogen generation have been developed,[2] and many of them use cobalt reduction catalysts.[3] Although water is the ideal medium for H2 production, almost all reported systems with Co catalysts use organic solvents or mixtures of organic solvents and water. It was demonstrated that increasing the water content resulted in a decreased catalytic activity,[4] and cobalt-based photocatalytic systems working in purely aqueous solution are rare.[5] Herein, we report the photocatalytic hydrogen generation in pure water using self-assembled functionalized vesicle membranes by co-embedding of amphiphilic photosensitizers in combination with an amphiphilic cobalt-based hydrogenevolving catalyst (Figure 1). Vesicle membranes have been reported as model systems to investigate electron transfer across membranes and energy conversion.[6] Using vesicles overcomes solubility problems and allows hydrogen production in aque[a] S. Troppmann, Prof. Dr. B. Kçnig Institute of Organic Chemistry University of Regensburg, 93040 Regensburg (Germany) Fax: (+ 49) 941-943-1717 E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201404480. Chem. Eur. J. 2014, 20, 14570 – 14574
www.chemeurj.org
ous solution without the need of any organic solvent. The incorporation of the two redox-active subunits into lipid bilayers leads to a two-dimensional arrangement resulting in a high local concentration of the embedded photosensitizer and catalyst at the surface. The structured assembly put the complexes in close spatial proximity without direct covalent linkage, and membrane fluidity allows the dynamic reorganization, both of which may accelerate efficient electron transfer and improve the catalytic activity. For incorporation into vesicular membranes, the amphiphilic photosensitizers 1, 2 b, and the amphiphilic cobalt catalyst 3 b were synthesized (Figure 2). The amphiphilic derivative 1 of
Figure 2. Structures of photosensitizers (1, 2 a, and 2 b) and hydrogen-evolving catalysts (3 a and 3 b) used for photocatalytic hydrogen production.
the established [Ru(bpy)3]2 + (bpy = 2,2’-bipyridine) photosensitizer was prepared by modification of one bipyridine ligand with long alkyl chains.[7] As metal-free alternative, the xanthene photosensitizer 2 b was tested for light absorption. Catalyst 3 b for the photochemical proton reduction in vesicular membranes was obtained by attaching a hydrophobic hydrocarbon chain on the axial pyridine ligand of the literature-reported CoIII complex [Co(dmgH)2(py)Cl] 3 a (dmgH = dimethylglyoximate).[8]
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Communication Functionalized vesicles for photocatalytic hydrogen production were prepared by sonication of a lipid film comprising a phospholipid (4–8, Table 2), either photosensitizer 1 or 2 b and catalyst 3 b in aqueous solution. Clear vesicular solutions were obtained, and dynamic light scattering (DLS) analysis showed small unilamellar vesicles (Figure S1 in the Supporting Information). The incorporation of the amphiphilic photosensitizer and catalyst into vesicular membranes was proven by size-exclusion chromatography (Figures S2 and S3 in the Supporting Information), and the stability of vesicles was checked by DLS measurements (Figure S4 in the Supporting Information). The samples were degassed with nitrogen and irradiated at constant temperature with high-power light-emitting diodes (LEDs). The amount of evolved hydrogen was quantified by head-space gas chromatography. For detailed experimental setup and data, see the Supporting Information. Figure 3 shows the effect of the pH on the catalytic hydrogen production upon irradiation of self-assembled 1,2-dioleoyl-
15 %, respectively (Figure S5 in the Supporting Information). In the absence of TEOA, no hydrogen could be quantified by head-space gas chromatography.[10] As was already reported, the hydrogen evolution by using cobaloxime complexes as reduction catalysts depends on the stability of these complexes, which can be enhanced by the addition of free dimethylglyoxime ligand to the solution.[9a] Without free dmgH2, TONs of 23 and 93 for 3 b and 1, respectively, were achieved. In the presence of 32 equivalents of free dmgH2 (vs. catalyst 3 b), the catalytic activity increased to 40 and 159, respectively (Figure S6 in the Supporting Information). Next, the influence of the ratio of the two redox-active subunits was investigated. Performing the photochemical hydrogen generation in a 1:1 ratio of catalyst 3 b and photosensitizer 1 (126 mm of each component) embedded in DOPC (4) resulted in 76 TON (Table 1, entry 3). Decreasing the concentration
Table 1. Evolved hydrogen with different ratios of catalyst (cat.) 3 b and photosensitizer (PS) 1 co-embedded in DOPC (4) in aqueous solution, 15 % TEOA, 4 mm dmgH2, pH 8.3 and irradiation over 13 h at 19 8C.
Figure 3. Influence of the pH on photocatalytic hydrogen generation from a system composed of 1 (31.5 mm) and 3 b (126 mm) embedded in DOPC (4, 843 mm) in 15 % TEOA aqueous solution after 13 h of irradiation at 19 8C.
sn-glycero-3-phosphocholine (DOPC, 4) vesicles containing 12.6 mol % of 3 b and 3.15 mol % of 1 in a 15 % triethanolamine (TEOA) aqueous solution. The highest activity was observed at pH 8.3 with a maximum amount of H2 of 11.8 mmol, resulting in a turnover number (TON) of 23 and 93 based on 3 b and 1, respectively. The catalytic activity dropped with increasing or decreasing the pH value. At more basic conditions, the protonation of the reduced CoI catalyst is restricted, thus limiting hydrogen evolution.[9] At more acidic pH, the low concentration of nonprotonated electron donor TEOA reduces the catalytic activity. Investigating different electron donors, TEOA was found to be most efficient. Upon using TEOA as sacrificial electron donor, the turnover number for 1 rose from 50 to 93 with increasing the concentration of the tertiary amine from 5 to Chem. Eur. J. 2014, 20, 14570 – 14574
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Entry Ratio cat. 3 b/PS1
ccat3b [mm]
cPS1 [mm]
cDOPC [mm]
n (H2) [mmol]
TON cat. TON 3b PS1
1 2 3 4 5
126 126 126 15.8 3.9
15.8 31.5 126 126 126
858 843 748 858 866
4.6 20.0 38.4 7.3 2.6
9 40 76 115 165
8:1 4:1 1:1 1:8 1:32
72 159 76 14 5
of photosensitizer 1 to 31.5 mm gave an increased hydrogen evolution of 159 turnovers for 1 (entry 2), whereas a loss of activity was observed at lower concentrations of 1 (entry 1). Similarly, a decrease in the concentration of catalyst 3 b to 15.8 or 3.9 mm at a constant concentration of 126 mm for photosensitizer 1 resulted in higher TONs of 115 and 165 for the catalyst 3 b, respectively (entries 4 and 5).[11] These results indicate that the high local concentration of co-embedded catalytic subunits allows generation of hydrogen at comparable low catalyst concentration with high catalytic activity. Control experiments confirmed that every single component of the photocatalytic system is necessary. No hydrogen production could be observed in the absence of either photosensitizer 1, reducing catalyst 3 b, or light. To investigate the influence of the synthetic phospholipid on the catalytic activity, we co-embedded photosensitizer 1 (126 mm) and catalyst 3 b (31.5 mm) in vesicles prepared from different lipids, DOPC (4), 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC, 5), 1-stearoyl-2-myristoyl-sn-glycero-3-phosphocholine (SMPC, 6), 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC, 7), and 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC, 8). The different phospholipids have the same hydrophilic head group but different hydrophobic tails, changing the membrane properties significantly (Table 2). We observed a strong influence of the nature of the phospholipid and the membrane fluidity on the hydrogen production. The highest
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Communication Table 2. Chemical structure and phase-transition temperature of different phospholipids (4–8) used for membrane preparation (Tm is phase-transition temperature).
DOPC (4) DMPC (5) SMPC (6) DPPC (7) DSPC (8)
R1
R2
D9-cis-C17H35 C13H27 C17H35 C15H31 C17H35
D9-cis-C17H35 C13H27 C13H27 C15H31 C17H35
Tm [8C] 21 24 30 41 55
Figure 4. Effect of the phospholipid (4–8) and the reaction temperature on the photocatalytic hydrogen generation with co-embedded chromophore 1 (31.5 mm) and catalyst 3 b (126 mm), 4 mm dmgH2 and 15 % TEOA aqueous solution at pH 8.3 upon irradiation for 12 h.
hydrogen generation activity at 19 8C was observed with a turnover number of 159 for chromophore 1 by using DOPC (4; Figure 4, for kinetic measurements, see Figure S7 in the Supporting Information). Replacing the unsaturated DOPC (4) by saturated phospholipids (5–8), the catalytic performance of the vesicular systems decreased significantly. In DMPC (5), SMPC (6), DPPC (7), and DSPC (8), the TON dropped to 91, 42, 31, and 20, respectively. We explain this by the fluidity of the membrane and the mobility of the embedded photosensitizer 1 and water-reduction catalyst 3 b in the membrane. In DOPC (4) vesicles, with a gel to liquid-crystalline phase transition temperature (Tm) of 21 8C, the membrane in its liquid-crystalline state allows the diffusion of the embedded catalytic subunits in the fluid membrane. The dynamic self-organization results in high activity with a beneficial distribution of photosensitizer and catalyst in the membrane. Vesicles made from saturated phospholipids (5–8) have a higher phase-transition temperature and the diffusion in the rigid gel membrane is restricted at 19 8C, thus, the hydrogen production decreases. However, the H2 evolution enhanced for each phospholipid above the phase-transition temperature of the respective phospholipid. For DMPC (5) and DPPC (7) in the liquid-crystalline state, the overall hydrogen evolution was comparable to Chem. Eur. J. 2014, 20, 14570 – 14574
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DOPC (4). Nevertheless, the overall performance also depends on the nature of the lipid. For SMPC (6) and DSPC (8) vesicles an increase in the TON above the transition temperature was observed, but the overall performance was comparatively low. To get an insight into the mechanism of the photoinduced intermolecular electron transfer, the redox potentials of photosensitizer 1 and catalyst 3 b were measured (Figure S8 in the Supporting Information). The redox potentials indicate that oxidative quenching of the excited photosensitizer 1 is feasible from a thermodynamic point of view. The calculated free energy DG for the oxidation of the excited photosensitizer 1 by catalyst 3 b is slightly negative, and the emission quenching of 1 upon addition of 3 b confirms the possibility of an oxidative quenching (Figures S9 and S10 in the Supporting Information). However, the reductive quenching of the excited photosensitizer 1 by the sacrificial electron donor TEOA is expected to be the major pathway. The concentration of the electron donor in solution is much higher than the catalyst concentration, and reductive quenching yields RuI, which reduces CoII to CoI inducing hydrogen generation upon formation of a CoIII hydride. The concept of co-embedding was extended to the amphiphilic EosinY photosensitizer 2 b in an attempt to reduce the metal content of the catalytic system. Vesicular systems consisting of DOPC (4), photosensitizer 2 b (126 mm), catalyst 3 b (126 mm), and 4 mm dmgH2 in 10 % TEOA aqueous solution pH 7.5 gave 13 TON after 18 h of light irradiation with 535 nm. Thus, the TON under optimized conditions by using 2 b as photosensitizer decreased to one-sixth compared with photosensitizer 1 (76 TON) for a 1:1 ratio of the catalytic subunits. A significant decrease in the fluorescence intensity for embedded photosensitizer 2 b compared to homogeneous solution indicates clustering and self-quenching of the xanthene dye 2 b in the membrane (Figure S11 in the Supporting Information). We assume that the close proximity of 2 b in the lipid bilayer leads to stacking of the photosensitizer in patches preventing an efficient intermolecular electron transfer to the catalyst. TONs up to 251 for a homogeneous system by using 2 b and 3 a were comparable to literature-reported hydrogen production based on EosinY 2 a and 3 a,[9a] thus excluding the synthetic modification of 2 a as a reason for low hydrogen production (Table S1 in the Supporting Information). In conclusion, self-assembled functionalized vesicles for photocatalytic hydrogen production in purely aqueous solution have been developed. The membrane co-embedded amphiphilic ruthenium photosensitizer and cobalt-based water-reduction catalyst evolve hydrogen upon irradiation in the presence of TEOA as sacrificial electron donor giving TONs up to 165 for the cobalt catalyst under optimal conditions in water. Membrane fluidity affects the assembly structure of the incorporated catalytic subunits and allows the control of the photoinduced hydrogen generation. Best catalytic activity was observed in fluid membranes with high mobility of the photosensitizer and the catalyst. Embedding of an amphiphilic EosinY as photosensitizer reduced the metal content of the system but resulted in a significant self-quenching of the chromophores with decreased hydrogen production. The membrane embed-
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Communication ding allows the functionalization of soft surfaces and may provide an approach for overall water splitting with membrane separated half reactions and transmembrane electron transfer mimicking natural photosynthesis.
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Received: July 20, 2014 Published online on October 5, 2014
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