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2005, 109, 6929-6932 Published on Web 03/26/2005
Zeolite Membrane-Based Artificial Photosynthetic Assembly for Long-Lived Charge Separation Yanghee Kim,† Amitava Das,‡ Haoyu Zhang,† and Prabir K. Dutta*,† Department of Chemistry, The Ohio State UniVersity, 120 West 18th AVenue, Columbus, Ohio 43210, and Central Salt and Marine Chemicals Research Institute, BhaVnagar, 364002, Gujarat, India ReceiVed: January 20, 2005; In Final Form: March 8, 2005
Photochemically generated long-lived charge separation is the key step in processes that aim for conversion of solar energy into chemical energy. In this study, we focus on a Ru polypyridyl complex [(bpy)2Ru(II)L, bpy ) bipyridine, L ) 1,2-bis[4-(4′-2,2′-bipyridyl) ethene] encapsulated on the surface of a pinhole-free zeolite membrane by quaternization of L and surrounded with intrazeolitic bipyridinium ions (N,N′-trimethyl2,2′-bipyridinium ion, 3DQ2+). Visible-light irradiation of the Ru complex side of the membrane in the presence of a sacrificial electron donor led to formation of PVS-• on the other side. Pore-blocking disilazane-based chemistry allows for Na+ to migrate through the membrane to maintain charge balance, while keeping the 3DQ2+ entrapped in the zeolite. These results provide encouragement that the zeolite membrane based architecture has the necessary features for not only incorporating molecular assemblies with long-lived charge separation but also for ready exploitation of the spatially separated charges to store visible light energy in chemical species.
Molecular assemblies with long-lived photochemical charge separation are necessary for conversion of solar energy into chemical energy.1,2 Adapting these structures to demonstrate function has rarely been done; an exception being proton transport in vesicles.3 Without proper support architecture, practical applications cannot be developed. Several groups have shown that zeolite encapsulation can promote long-lived charge separation,4-7 but the use of zeolite particles is impractical since the redox species can react with each other. In this study, a Ru polypyridyl complex (bpy)2Ru(II)L (bpy ) bipyridine, L ) 1,2bis[4-(4′-2,2′-bipyridyl)ethene]) was encapsulated on one surface of a pinhole free zeolite membrane filled with intrazeolitic bipyridinium ions (N,N′-trimethyl-2,2′-bipyridinium ion, 3DQ2+). Visible-light irradiation of the Ru complex led to electron transport through the membrane and donation to propyl viologen sulfonate (PVS) to form PVS-• on the other side of the membrane. Pore-blocking chemistry allows for Na+ to migrate through the membrane to maintain charge balance, while keeping the 3DQ2+ entrapped in the zeolite. Here we show that zeolite membranes have the necessary features for a practical system for solar to chemical energy conversion. The photochemical system of interest in this study is based on the molecule shown in Figure 1a. This molecule was chosen since a spectroscopic study of (dmb)2RuL (dmb ) 4,4′-dimethyl2,2′-bipyridine) demonstrated that the 3MLCT (metal-to-ligand charge transfer) state had long lifetimes with the electron localized on L.8 The bipyridine analogue used in this study [(bpy)2Ru(II)L] exhibits an intraligand absorption band at 340 * Corresponding author. E-mail:
[email protected]. Phone: 614-2924532. Fax 614-688-5402. † The Ohio State University. ‡ Central Salt and Marine Chemicals Research Institute.
10.1021/jp0503664 CCC: $30.25
Figure 1. (a) Structures of (bpy)2RuL and (bpy)2RuLq held in a zeolite supercage, and (b) proposed scheme of photoinduced electron transport through a (bpy)2RuLq-3DQ2+- zeolite Y membrane to PVS across the membrane.
nm and the MLCT band 470 nm, with emission at 660 nm.9 We have reported the synthesis of zeolite membranes and measurement of the leaking properties using fluorescent dyes.10,11 Typically, 4-5 µm thick zeolite membranes are synthesized on porous alumina supports by a secondary growth process using nanosized seeds of zeolite Y.12 Figure 2a is a typical crosssection SEM micrograph of a membrane showing the dense zeolite layer on top of a porous alumina support. The (bpy)2Ru(II)L was anchored on the zeolite surface via quater© 2005 American Chemical Society
6930 J. Phys. Chem. B, Vol. 109, No. 15, 2005
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Figure 2. (a) Cross-section SEM micrograph of zeolite membrane on an alumina support. (b) Schematic representation of the two-compartment photochemical cell. The (bpy)2RuLq-zeolite is facing the lefthand compartment (LHC), and the right-hand compartment (RHC) has a 1 cm cuvette for absorption measurements.
nization of the bipyridine part of L by reaction with Br(CH2)4 Br, forming (bpy)2Ru(II)Lq-zeolite, as shown in Figure 1a. The reaction strategy is similar to a recent report with a stilbenetype L ligand.13 Prior to the reaction with Br(CH2)4Br, the complex (bpy)2Ru(II)L is adsorbed on the zeolite surface and the bipyridine part of the L ligand can penetrate into the zeolite supercages through the 7 Å windows. Upon quaternization, the (bpy)2Ru(II)Lq is formed and is not released into solution by ion exchange. The presence of the methyl group on the diquat part of Lq leads to entrapment; in its absence, the complex would be released from the zeolite surface by ion exchange. During the synthesis, the alumina support and the sides of the membrane were masked off with a polydimethoxysilane film. The results presented here are based on a study of eight different membranes. The Ru-loaded membranes were ion-exchanged with 3DQ2+ and mounted in the photochemical cell shown in Figure 2b. The (bpy)2Ru(II)Lq-zeolite surface faces the left-hand compartment (LHC) of the membrane. In all cases, the solution in the right-hand compartment (RHC) of the cell was 0.025 M PVS, with Ru(bpy)32+ (8 × 10-6 M) and a sacrificial electron donor, triethanolamine, or the tetrapropylammonium salt of EDTA (TPA-EDTA) (0.001 M). Prior to photolysis of LHC, the RHC is photolyzed to generate PVS-•, which scavenges the oxygen present in solution and oxygen slowly leaking into the cell through the septum. During the photolysis of LHC, the RHC is wrapped with aluminum foil to prevent formation of PVS-• by inadvertent light exposure. Visible light (420-650 nm) was incident on the LHC and the UV-visible spectrum of the solution in RHC of the cell was monitored to determine the change in PVS-• concentration. Figure 3a shows the change of intensity of the 600 nm PVS-• absorption band with 0.01 M NaCl in LHC. A decrease of PVS-• in RHC is noted at early times due to oxygen leaking into the cell, followed by growth of PVS-• due to charge migration through the membrane and eventual decay of the signal. After photolysis, the diffuse reflectance spectrum showed complete destruction of the (bpy)2Ru(II)Lq, as shown in the inset. Figure 3b shows the absorbance at 600 nm (PVS-•) with 0.01 M Na2EDTA in LHC, with and without 3DQ2+ in the zeolite.
Figure 3. Fate of PVS-• in right-hand compartment (RHC) during photolysis of the left-hand compartment (LHC). RHC in all cases has 0.025 M PVS with Ru(bpy)32+ (8 × 10-6 M) and the sacrificial electron donor triethanolamine (0.001 M) and is initially photolyzed to scavenge oxygen and then wrapped in Al foil. (a) Photolysis with 0.01 M NaCl(aq) in LHC: (bpy)2RuLq-3DQ2+ zeolite membrane. Inset shows diffuse reflectance spectra before and after 200 min of illumination of (bpy)2RuLq-zeolite surface. (b) Photolysis with 0.01 M Na2EDTA(aq) in LHC: (bpy)2RuLq zeolite membrane with and without 3DQ2+. Inset shows diffuse reflectance spectra before and after 200 min irradiation of (bpy)2RuLq-3DQ2+ zeolite membrane. (c) Photolysis with 0.01 M TPA-EDTA(aq) with and without 0.01 M NaCl in LHC: (bpy)2RuLq3DQ2+-zeolite membrane.
In the absence of 3DQ2+, there was a slow decay of the PVS-•. With 3DQ2+ in the zeolite, there is a growth of PVS-•, reaching a maximum around 75-100 minutes, followed by a steady decrease. The (bpy)2Ru(II)Lq survived the photolysis, as shown in the inset. After photolysis, the electronic spectrum of the solution in LHC showed the presence of 3DQ2+, suggesting
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that the Na+ (from EDTA) ion exchanges 3DQ2+ out of the zeolite, and leads to the cessation of PVS-• growth. There is a considerable variation in the decay of PVS-• in the dark conditions, arising from the fact that the sealing of the photolytic cell by O-rings and septa is not readily reproduced. This results in different leak conditions for the eight different membrane experiments. Figure 3c shows traces with 0.01 M TPA-EDTA with and without 0.01 M NaCl in LHC. TPA cannot ion exchange 3DQ2+ out of the zeolite membrane, and no PVS-• growth is observed with TPA-EDTA, though EDTA can recycle the Ru(III). However, in the presence of NaCl, growth of PVS-• is observed, suggesting that cation transport through the membrane is necessary for electron transfer through the membrane. The requirement of having 3DQ2+ in the zeolite, as well as maintaining charge neutrality, necessitates partial pore-blocking. Most pore-blocking methods for zeolites occur at higher temperatures.14 However, considering that pore-blocking can be done only after Ru complexation and bipyridinum ion incorporation, a mild method is required. The use of disilazanes for pore blocking of mesoporous materials has been reported.15 To model this procedure, zeolite particles were treated with 1,1,3,3tetramethyldisilazane at room temperature. Infrared spectroscopy showed the disappearance of the 3700 cm-1 OH groups of zeolite and the appearance of bands at 2966 and 2874 cm-1 due to the CH3 groups of the product. In addition, the 13C MAS NMR spectrum showed a peak at 0 ppm due to the Si-CH3 group. The reaction with the surface OH groups of the zeolite can be described as
To check pore blocking, MV2+-exchanged zeolite (methyl viologen or 1,1′-dimethyl-4,4′-bipyridinium ion) was surface derivatized and the MV2+ release into solution in the presence of NaCl was monitored. For 0.01 M Na+, the equilibration time for ion-exchange was within 30 min for the unmodified zeolite, whereas, for the disilazane treated zeolite, the corresponding time was on the order of a week. A similar pore-blocking procedure was repeated with the (bpy)2RuLq-3DQ2+-zeolite Y membrane, and the photolysis experiments repeated.16 Figure 4a contrasts the PVS-• generation of a pore-blocked membrane with 0.01 M Na2EDTA in LHC with a non-poreblocked membrane, the latter from Figure 3b (note different absorbance scales for the two data sets). With pore blocking, there is a dramatic growth of PVS-• during the first 120 min, followed by decay over about 300 min. After the photolysis, diffuse reflectance spectroscopy showed that the Ru complex was destroyed and is the reason the growth of the PVS-• gradually decreases. We calculate a loading of 3.9 × 10-5 µmol of Ru complex on the surface of the zeolite membrane.17 At the peak of the PVS-• formation at 120 min, the total amount of PVS-• (Figure 4a) is calculated to be 1.8 µmol,17 suggesting that the Ru2+ f Ru2+* f Ru3+ f Ru2+ cycle has occurred over ∼4 × 104 times. The inset of Figure 4a shows the absorbance of PVS-• for photolysis of a solution (10 mL) of 3 × 10-5 µmol of Ru(bpy)32+, EDTA (0.01 M), and PVS (0.025 M), simulating a homogeneous version of the membrane experiment. At the peak of PVS-• generation, the Ru(bpy)32+ has cycled 3 × 104 times, indicating comparable efficiencies for the solution and membrane experiments. The subsequent decrease in PVS-• (as shown in the inset) possibly arises from the photodecomposition of the Ru(bpy)32+ complex.18
Figure 4. Fate of PVS-• in the right-hand compartment (RHC) during photolysis of the left-hand compartment (LHC). RHC in all cases has 0.025 M PVS, with Ru(bpy)32+ (8 × 10-6 M) and the sacrificial electron donor EDTA (TPA-EDTA) (0.001 M) and is initially photolyzed to scavenge oxygen and then wrapped in Al foil. (a) Photolysis with 0.01 M Na2EDTA in LHC: pore-blocked and not-pore-blocked (bpy)2RuLq3DQ2+-zeolite membrane (note two different y-axis scales and poreblocked sample same as Figure 3b). Inset shows growth of PVS-• during photolysis of a 10 mL solution of 3 × 10-9 M Ru(bpy)32+, 0.025 M PVS, and 0.01 M Na2EDTA solution. (b) Photolysis with 0.025 M PVS and 0.01 M NaCl in LHC: pore-blocked (bpy)2RuLq-zeolite with and without 3DQ2+. Inset shows excitation (monitored at 530 nm) and emission (λexc.: 390 nm) spectra of the solution recovered from LHC after 225 min of photolysis.
The steps leading to charge separation across the membrane (Figure 1b) are hν
(bpy)2Ru2+Lq-3DQ2+-Z 98 (bpy)2Ru3+Lq-•-3DQ2+-Z 3+
-•
2+
(bpy)2Ru Lq -3DQ -Z f (bpy)2Ru
3+
+•
Lq-3DQ -Z
(1) (2)
(bpy)2Ru3+Lq-3DQ+•-Z + PVS(s, RHC) f (bpy)2Ru3+Lq-3DQ2+-Z + PVS-•(s, RHC) (3) (bpy)2Ru3+Lq-3DQ2+-Z + EDTA f (bpy)2Ru2+Lq-3DQ2+-Z + EDTA products (4) hν
(bpy)2Ru3+Lq-3DQ2+-Z + H2O 98 (bpy)2Ru3+ (H2O) Lq-3DQ2+-Z (5) hν
(bpy)2 Ru3+ (H2O) Lq- 3DQ2+-Z 98 photodecomposition of the Ru complex (6) Reaction 2 is the forward electron transfer from the electron on the Lq ligand (partially in the zeolite) to 3DQ2+ in neighboring
6932 J. Phys. Chem. B, Vol. 109, No. 15, 2005 cages. The back electron transfer from 3DQ+• to Ru3+ involves the dπ (A1) orbital of the Ru3+ center. We have suggested, for the intrazeolitic system, that immobilization of Ru(bpy)32+ in the supercages causes poor overlap of the metal and viologen orbitals and slows the back electron transfer.19 To mimic the intrazeolitic case, ligand L with the double bond was chosen so as to constrain the motion of the (bpy)2 Ru moiety on the zeolite membrane surface. A charge hopping mechanism involving densely packed 3DQ2+ can move the photogenerated electron into the zeolite, leading to a long-lived charge separated state.5-7 Electron transport within zeolite by charge hopping has also been observed with MV2+-zeolite Y.20 The reducing agent [Mn(CO)4P(OPh3)]cannot penetrate into the zeolite, yet reduces all of MV2+ in MV2+ zeolite Y almost instantaneously and quantitatively. The proposed mechanism involved reduction of MV2+ at the zeolite periphery followed by self-exchange that moved the charge into the zeolite.20 Packed viologen units assembled on a micellar surface have also shown rapid electron migration by selfexchange.21 Polypyrrole films consisting of Ru(bpy)32+ and bipyridinium ions also exhibit photocurrents with electron transport to the electrodes occurring by electron-transfer hopping on the packed bipyridinium ions.22 The charge hopping and directional electron transfer to PVS (driving force 0.15 V) in solution is noted in reaction 3 and is accompanied with Na+ transport. The EDTA acts as a sacrificial electron donor to regenerate the Ru(II) (reaction 4). The photodissociation of the (bpy)2Ru(II)Lq complex in the absence of Na2EDTA (Figure 3a) suggests that Ru(III) is being formed, since Ru(III) species is expected to undergo visible light promoted photoaquation reaction 5 and result in eventual decomposition of the complex (reaction 6).23 We have shown that intrazeolitic Ru(bpy)33+ reacts with water to form covalent hydrates and hydroxylated species, which give rise to reactive oxygen species that react with bipyridinium ions to form pyridones.24 The pyridones are characterized by typical emission peaks, e.g., for methyl viologen, the 2- and 3-one emit at 516 and 528 nm.25 Thus, if photaquated Ru(III) species are forming, they should also react with bipyridinium ions to form pyridones. The photolysis was repeated with PVS (0.025 M) and NaCl (0.01 M) in LHC, with and without intrazeolitic 3DQ2+, and the results are compared in Figure 4b. With 3DQ2+, growth of the PVS-• in RHC is observed, and the excitation and emission spectra of the solution recovered from LHC showed absorption and emission peaks at 360 and 510 nm, respectively, characteristic of pyridones (inset of Figure 4b). In our study with the zeolite powder-surface derivatized ruthenium polypyridyl complex using the ligand 1,4-bis[2-(4′-methyl-2,2′-bipyrid-4-yl)ethenyl]benzene, we noted pyridone absorption and emission bands at 366 and 517 nm13 (note that in ref 13, the text lists these bands incorrectly at 390 and 530 nm and the assignment to 3-pyridone, though Figure 6 in ref 13 showing the spectrum is correct). The pyridones formed are likely a mixture of both the 2- and 3-forms. Pyridone formation confirms the formation of longlived Ru(III), which is photoaquated and regenerated with PVS in LHC (reactions 5 and 7).
(bpy)2Ru3+ (H2O)Lq-3DQ2+-Z + PVS(s, LHC) f (bpy)2Ru2+Lq-3DQ2+-Z +pyridone (s, LHC) (7)
Letters The zeolite membrane provides all the necessary elements for an artificial photosynthetic assembly: (1) attaching supramolecular assemblies, (2) providing a spatial arrangement for electron transport through charge hopping, (3) pore blocking of the membrane to keep species entrapped, and (4) providing entry of small ions to maintain charge neutrality. The demonstration of long-lived charge separation across a sturdy membrane provides an opportunity for the first time to access the photochemically generated redox species for reactions, such as the conversion of water to O2 and H2. Acknowledgment. We acknowledge the help of Dr. Hyunjung Lee, who did the (bpy)2RuL synthesis on zeolite particles, and Dr. R. Ramamoorthy for his help with the SEM. Funding was provided by NASA and NSF International program. References and Notes (1) Du¨rr, H.; Bossmann, S. Acc. Chem. Res. 2001, 34, 905. (2) Wasielewski, M. R.; Niemczyk, M. P.; Svec, W. A.; Pewitt, E. B. J. Am. Chem. Soc. 1985, 107, 5562. (3) Gust, D.; Moore, T.; Moore, A. Acc. Chem. Res. 2001, 34, 40. (4) Krueger, J. S.; Mayer, J. E.; Mallouk, T. E. J. Am. Chem. Soc. 1988, 110, 8232. (5) Borja, M.; Dutta, P. K. Nature 1993, 362, 43. (6) Sykora, M.; Kincaid, J. R. Nature 1997, 387, 162. (7) Park, Y.; Lee, E. J.; Chun, Y. S.; Yoon, Y. D.; Yoon, K. B. J. Am. Chem. Soc. 2002, 124, 7123. (8) Strouse, G. F.; Schoonover, J. R.; Duesing, R.; Boyde, S.; Jones, W. E., Jr.; Meyer, T. J. Inorg. Chem. 1995, 34, 473. (9) Ligand L (1,2-bis[4-(4′-methyl-2,2′-bipyridyl)ethene]) was synthesized according to the literature method.8 1H NMR (CDCl3): 2.42 (6H,s); 7.15 (2H,d); 7.38 (2H,d); 7.42 (2H,s); 8.23 (2H,s); 8.53 (2H,d); 8.56 (2H,s); 8.65 (2H,d). Ligand L (100 mg, 0.27 mmol) was dissolved in 30 mL of EtOH-H2O (3:1, v/v), and [(bpy)2RuCl2, 2H2O] (0.044 g, 0.09 mmol) was added and refluxed for 8 h. The product was precipitated with aqueous solution of KPF6 as [(bpy)2RuL] (PF6)2 and purified by column chromatography on neutral alumina (grade III) using acetonitrile-toluene mixtures (6:4, v/v) as eluent. Anal. Found: C, 50.1; H, 3.3; N, 10.4. Calcd: C, 49.50; H, 3.40; N, 10.49. 1H NMR (CD2Cl2): 8.75-7.16 (30H), 2.67 (s,3H), 2.49 (s,3H). MS: molecular ion peak at m/z ((bpy)2RuL(PF6)+) 923.17 (10) Kim, Y.; Dutta, P. K. Res. Chem. Intermed. 2004, 30, 147. (11) Lee, H.; Dutta, P. K. J. Phys. Chem. B 2002, 106, 11898. (12) Nair, S.; Tsapatsis, M. In Handbook of Zeolites and Layered Materials; Auerbach, S. M., Carrado, K. A., Dutta, P. K., Eds.; Marcel Dekker: New York, 2003; pp 867-920. (13) Kim, Y.; Lee, H.; Das, A.; Dutta, P. K. Inorg. Chem. 2003, 42, 4215. (14) Impens, N. R. E. N.; van der Voort, P.; Vansant, E. F. Microporous Mesoporous Mater. 1999, 28, 217. (15) Anwander, R.; Nagl, I.; Widenmeyer, M. J. Phys. Chem. B 2000, 104, 3532. (16) (bpy)2RuLq-3DQ2+-zeolite membrane was dehydrated under vacuum (10-4 Torr) and treated with 5.75 mM 1,1,3,3-tetramethyldisilazane in n-hexane over a 2 min period, left for 5 min, followed by washing with n-hexane. (17) The surface coverage of (bpy)2RuLq on zeolite particles was measured to be 1.4 × 10-6 mol/g. Assuming the same coverage on the 1 cm (diameter) zeolite membrane, we calculate a loading of 3.9 × 10-5 µmol. The peak absorbance of 1.23 of PVS-• at 120 min (Figure 4a) corresponds to 1.8 µmol of PVS-•. (18) Vaidyalingam, A.; Dutta, P. K. Anal. Chem. 2000, 72, 5219. (19) Vitale, M.; Castagnola, N. B.; Ortinis, N. J.; Brooke, J. A.; Vaidyalingam, A.; Dutta, P. K. J. Phys. Chem. B 1999, 103, 2408. (20) Yoon, K. B.; Park, Y. S.; Kochi, J. K. J. Am. Chem. Soc. 1996, 118, 12710. (21) Takuma, K.; Sakamoto, T.; Nagamura, T.; Matsuo, T. J. Phys. Chem. 1981, 85, 619. (22) Downard, A. J.; Surridge, N. A.; Gould, S.; Meyer, T. J.; Deronzier, A.; Moutet, J. J. Phys. Chem. 1990, 94, 6754. (23) Ghosh, P. K.; Brunschwig, B. S.; Chou, M.; Creutz, C.; Sutin, N. J. Am. Chem. Soc. 1984, 106, 4772. (24) Das, S. K.; Dutta, P. K. Langmuir 1998, 14, 5121. (25) Bahnemann, D.; Herbert-Fischer, C.; Janata, E.; Henglein, A. J. Chem. Soc., Faraday Trans 1 1987, 83, 2559.
