New Strategy for Control of Electron Transfer in Photosynthetic Models -Local Electric Field and Hydrogen Bond

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1) Porphyrin (H) -pyromellitimide (I) dyads bearing a quaternary ammonium site have been prepared and their intramolecular electron transfer reaction has been studied by measuring their fluorescence spectra in a wide range of solvents. The charges site enhances the electron transfer, but its effects are relatively small in polar solvents such as DMF and THF but are very large in low polarity media such as benzene and hexane. Pico-second time resolved transient absorption spectra revealed the formation of a charge separated state, H+-I-, giving firm evidence of the electron transfer for the fluorescence quenching. Introduction of a charges site into a more elaborated photosynthetic models such as dimer-monomer-pyromellitimide triad has been achieved through multi-step synthesis. The electron transfer dynamics of these models are now in being pursued.2) A porphyrin host bearing 2,6-diacylaminopyridine binding site was prepared and was found to be complexed with naphthalene-diimide guest in nonpolar solvents. In benzene, the association constant is ca.10^3M.Intracomplex electron transfer has been studied by the picosecond time-resolved fluorescence lifetime and the transient absorption spectra. Electron transfer over hydrogen-bond is found to be quite effective, comparable to that of covalently-linked models having the same energy gap, but the ion-pair state was found to be rather short-lived in comparison to that in the covalently linked counterpart.