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Studies of zeolite-based artificial photosynthetic systems

by Zhang, Haoyu

Abstract (Summary)
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. Two ruthenium polypyridyl compounds RuL and RuLDQ were synthesized and purified. The structures were examined and the photochemical and electrochemical properties were investigated. The photochemistry of RuL and RuLDQ in aqueous solution was also 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 spectroscopic changes in both absorption and emission. No spectral changes were observed in solutions at pH <= 4. It was found that the major product was a dimer of RuL, dimerizing around the double bond. We proposed a Ru(III) radical intermediate mechanism. 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. 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 (MV^2+), 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 MV^2+ in NanoY, electron transfer kinetics was examined. LFP study showed a slower back-electron-transfer rate as compared to forward electron transfer. We incorporated RuL complex on the surface of a pinhole-free zeolite membrane by quaternization of L and surrounded with intrazeolitic bipyridinium ions. Visible-light irradiation of the Ru complex side of the membrane in the presence of a sacrificial electron donor led to formation of PVS radial on the other side. Pore-blocking disilazane-based chemistry allows for Na^+ to migrate through the membrane to maintain charge balance, while keeping the 3DQ^2+ 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. Methyl viologen (MV^2+) was chosen as the guest molecule, since it is widely used as an herbicide and its release is of interest in agricultural applications. A MV^2+ 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 MV^2+-loaded zeolites encapsulated the guest molecules in the zeolite cages and release of MV^2+ by ion-exchange with sodium ions was studied. In the absence of surface modification, equilibration occurred within 20 minutes, whereas with surface modification, the equilibration time was extended to 7 days.
Bibliographical Information:

Advisor:

School:The Ohio State University

School Location:USA - Ohio

Source Type:Master's Thesis

Keywords:zeolite artifical photosynthesis eletron transfer

ISBN:

Date of Publication:01/01/2008

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