Abstract: Vertebrate vision is based on the unique conversion of light into nerve impulses. Detailed understanding of the primary photochemical events is important with respect to potential applications in molecular optoelectronic devices. In our studies we apply modern theory, computer simulations, and experimental knowledge to develop new understanding of the photochemical processes and ultimately devise means to control their outcome. We use a combination of quantum theory and quasiclassical calculations to model excitation with laser pulses, excited state dynamics, internal conversion, and relaxation to photoproducts. In rhodopsin, the primary photochemical event involves an 11-cis to 11- trans photoisomerization. Recent time-resolved measurements provide the time-scales for disappearance of the reactant and the appearance of products. We find microscopical details of the internal conversion which are in accordance with the experimental results. This new perspective to the microscopic mechanism reveals the optimization of the electronic structure achieved in evolutionary development in nature. Cis-Stilbene is studied as a model system for which a double resonance laser excitation can be used for influencing the reaction. UV excitation leads to two photoproducts, trans-stilbene and dihydrophenanthrene (DHP). We show that IR excitation can be used to prepare ground sate cis-stilbene in an appropriate vibrational state before electronic excitation. We find that excitation of some modes leads to up to 4.5 times increase in the quantum yield of DHP or up to 4 times increase in trans-stilbene. Because of the shifted absorption spectra of the products, if such system is realized experimentally it can be used as switching bistable device. !17
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