Design and photochemical studies of zeolite-based artificial photosynthetic systems

The work described in this thesis describes steps towards building a zeolite-membrane based photochemical assembly, which can be used for developing hydrogen evolving artificial photosynthetic system. We adopted the membrane system of natural photosynthesis in our system to separate photochemically generated redox species. For photochemical applications, membranes without any inter crystal pinholes and grain boundaries are preferred since these defects introduce non-shape selective pathway for molecules to pass through the membrane. Also the membrane needs to be mechanically stable for assembly and operation of the system. To address these issues, novel secondary treatment method to prepare zeolitic membranes was developed. Positive-type photoresist was used to fill nano to micrometer size pinholes that are generated during zeolite membrane casting. With this method, membrane leaking was reduced to 0.05% while zeolitic surface and pores were still accessible to molecules. For photochemical studies, photoresist-coated zeolitic membrane was used as a host for electron acceptor molecules and provided a route for charge propagation by electron hopping across the membrane. Since acceptor molecules are separated from donor molecules by a membrane, back electron transfer is prohibited and permanent charge separation can be achieved. Ruthenium dyad molecules were utilized as photosensitizers in our artificial photosynthetic system. To improve the efficiency of synthesis and photo electron transfer reaction, we synthesized and developed new ruthenium dyad molecules, [(bpy)₂Ru(dmb-L or L′-4DQ)]4+, which have conjugated bridge L or L′ between the ruthenium donor and bipyridinium acceptor. Using modified “ship in a bottle” method, the dyad molecules were partially entrapped in pores of zeolite Y. Spectroscopic and photochemical studies were conducted to test the efficiency of photo electron transfer reactions using these dyads.; The zeolitic membranes and ruthenium dyad photosensitizers developed in this study can be utilized as a solar energy conversion assembly for photolytic splitting of water into hydrogen and oxygen with proper catalysts such as platinium (Pt) and/or ruthenium oxide (RuO₂).