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Energy Challenge STUDIES OF ZEOLITE-BASED ARTIFICIAL PHOTOSYNTHETIC SYSTEMS DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By Haoyu Zhang ***** The Ohio State University 2008 Dissertation Committee: Approved by Professor Prabir K. Dutta, Advisor Professor Bern Kohler _________________________________ Professor Jon Parquette Advisor Professor Jeff Culbertson Graduate Program in Chemistry ABSTRACT Photosynthesis is the basis for life on earth and involves the transformation of carbon dioxide to sugars with the help of sunlight. There is considerable interest in developing artificial photosynthetic assemblies that can use sunlight to generate useful chemical compounds, e.g. generation of hydrogen and oxygen from water. In this dissertation, we provide a plan for assembly of an artificial photosynthetic system on a zeolite membrane that aims for conversion of solar energy into chemical energy. 2+ Two ruthenium polypyridyl compounds of structural formula [(bpy)2RuL] (RuL) and 4+ [(bpy)2RuLDQ] (RuLDQ) (where bpy = bipyridine, L = trans-1,2-bis-4-(4'-methyl)-2,2'-bipyridyl) ethane, LDQ = 1-[4-(4’-methyl)-2,2’-bipyridyl)]-2-[4-(4’-N,N’-tetramethylene-2,2’-bipyridinium)] ethene) were synthesized and purified. The structures were examined by Nuclear Magnetic Resonance (NMR) and Electrospray Ionization Mass Spectrometry (ESI-MS) and their MS fragmentation were discussed. The photochemical and electrochemical properties were investigated by UV-vis, Emission, Laser-flash-photolysis (LFP), and Cyclic Voltammetry (CV). It was found that the excited electron was delocalized over the π* framework of L ligand. The delocalization of excited electron not only increased the MLCT life time of RuL, but also enabled the transfer of the electron to secondary - ii - acceptors. The kinetics of electron transfer between ruthenium complexes and viologen acceptors in non aqueous solution were investigated. The photochemistry of RuL and RuLDQ in aqueous solution were examined. From * pH titrations, it was found that the Ru complex was a stronger base (pKa = 6) in the excited state than in the ground state (pKa = 4). Photolysis of the RuL complex in solutions at pH 7 and 12 led to formation of species with increased emission quantum yields, ~55 nm blue-shift of the emission maximum to 625 nm and disappearance of the absorption band at 330 nm, the latter arising from the olefinic bond of the L ligand. No spectral changes were observed in solutions at pH ≤ 4. With the help of chromatography, mass spectroscopy, Raman spectroscopy and NMR, photoproducts formed at neutral pH have been analyzed. It was found that the major product was a dimer of RuL, dimerizing around the double bond. Photoreactions did not occur in the dark or in the aprotic solvent acetonitrile. We proposed that a Ru(III) radical intermediate was formed by photoinduced excited-state electron and proton transfer, which initiated the dimerization. The radical intermediate also underwent photochemical degradative reductions. Below pH 4, the emission quenching was proposed to arise via protonation of the monoprotonated RuLH+ followed by electron transfer to the viologen-type moiety created by protonation. The products of photodegradation at pH > 12 were different from those of pH 7, but the mechanism of the degradation at pH > 12 was not elucidated. RuLDQ was stable under visible irradiation. We examined nanocrystalline zeolite as a host for light absorbing sensitizers (electron donors) and electron acceptors. Nanocrystalline zeolite Y (NanoY) with uniform particle size, pure phase was prepared. NanoY was obtained by periodically removing - iii - nanocrystals from the mother liquor and recycling the unused reagents. The nanoparicles were characterized by XRD and TEM. Optically clear colloidal solutions of NanoY were obtained. The Ru complexes were anchored on the surface of zeolites via ion-exchange or “ship-in-bottle” synthesis. The spectroscopic properties of the NanoY-entrapped species including methyl viologen (MV2+), RuL were measured via transmission techniques. The zeolite-encapsulated species were found to have red-shift absorption and emission bands and longer MLCT life times. By incorporating both donors Ru complexes and acceptors MV2+ in NanoY, electron transfer kinetics was examined. LFP study showed a slower back-electron-transfer rate as compared to forward electron transfer. Photochemically generated long-lived charge separation is the key step in processes that aim for conversion of solar energy into chemical energy. We incorporated RuL complex on the surface of a pinhole-free zeolite membrane by quaternization of L and surrounded with intrazeolitic bipyridinium ions (N,N’-trimethyl-2,2’-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−y 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 provided 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. The pore-narrowing strategy applied under mild conditions can be used in control-release of active substances such as drug, pesticides, and herbicides. Methyl - iv - viologen (MV2+) was chosen as the guest molecule, since it is widely used as an herbicide and its release is of interest in agricultural applications. To explore the controlled-release capability of the surface-modified zeolite, MV2+-encapsulated zeolite Y particles were used as a model system. A MV2+-loaded zeolite was treated with disilazane reagents under ambient conditions and the grafting of siloxy functionality on the zeolite was confirmed by infrared, NMR spectroscopy and elemental analysis. Surface modification of MV2+-loaded zeolites encapsulated the guest molecules in the zeolite cages and release of MV2+ by ion-exchange with sodium ions was studied. The total amount of MV2+ released was dependent on the concentration of Na+ in solution, and was similar for the derivatized and underivatized samples. In the absence of surface modification, equilibration occurred within 20 minutes, whereas with surface modification, the equilibration time was extended to 7 days. These kinetics are reflected in the effective diffusion coefficients (D) of MV2+, with D = 1.2 × 10-15 cm2 s-1 for derivatized zeolite Y and D = 0.2 -1.1 × 10-7 cm2 s-1 for the underivatized sample. - v - Dedicated to my family - vi - ACKNOWLEDGMENTS I would like to express my sincere gratitude and respect to my advisor, Professor Prabir Dutta, for his guidance and encouragement during my studying at The Ohio State University. His broad knowledge, creativity and enthusiasm for scientific research have set up an example worthy of emulation. I deeply thank him for his important inspiration and encouragement for this work. I wish to thank Dr. Bern Kohler and Dr. Jon Parquette for serving on my dissertation committee. They gave me valuable advices for my dissertation. I am deeply indebted to Dr. Cheruvallil Rajesh, Dr. Yanghee Kim for their invaluable assistance on research and willingness to share their insight and wisdom. In addition, I want to extend my appreciation to those who provided me various helps during my study including Dr. Bob Kristovich, Dr. Joe Trimboli, Dr. John Doolittle, Dr. Ramamoorthy Ramasamy, Dr. Dipankar Sukul, Dr. Radha Vippagunta, Dr. Kefa Onchoke, Dr. Joe Obirai, Dr. Toni Ruda, Dr. John Spirig, Dr. Jiun-Chan Yang, Dr. Xiaogan Li, Dr. Supriya Sabbani, Mariela Oyola, Bill Schumacher, Jeremy White, Brian Peebles, Dedun Adeyemo, Kevin Cassidy, Julia Rabe. Finally, I want to thank my family for their self-giving support. I can not complete my study without their support and encouragement. - vii - VITA July 21, 1976. Born – Beijing, China June, 1999. .B.S., Chemistry Fudan University, China. June, 2002. .M.S., Chemistry Fudan University, China. 2003 – 2005. Graduate Teaching Associate The Ohio State University. 2005 – present. .Graduate Research Associate The Ohio State University. PUBLICATIONS Research Publications 1. Zhang, H.; Rajesh, C. S.; Dutta, P. K. Visible-Light-Driven Photoreactions of [(Bpy)2Ru(II)L]Cl2 in Aqueous Solutions (Bpy = Bipyridine, L = 1,2-Bis(4-(4'-Methyl)-2,2'-Bipyridyl) Ethene). Journal of Physical Chemistry A 2008, 112, 808-817. 2. Zhang, H.; Kim, Y.; Dutta, P. K. Controlled Release of Paraquat from Surface-Modified Zeolite Y. Microporous and Mesoporous Materials 2006, 88, 312-318. 3. Kim, Y.; Das, A.; Zhang, H.; Dutta, P. K. Zeolite Membrane-Based Artificial Photosynthetic Assembly for Long-Lived Charge Separation. J Phys Chem B 2005, 109, 6929-6932. 4. Zhang, H.; Chen, Z.; Weng, L.; Zhou, Y.; Zhao, D. Hydrothermal Synthesis of New I I + + + Berylloborophosphates M BeBPO (M = K , Na and NH4 ) with Zeolite ANA Framework Topology. Microporous and Mesoporous Materials 2003, 57, 309-316. - viii - 5. Chen, Z.; Zhou, Y.; Weng, L.; Zhang, H.; Zhao, D. Hydrothermal Synthesis of Two Layered Indium Oxalates with 12-Membered Apertures. Journal of Solid State Chemistry 2003, 173, 435-441. 6. Chen, Z.; Weng, L.; Zhou, Y.; Zhang, H.; Zhao, D. Synthesis and Structure of A New Three-Dimensional Microporous
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