PHOTOPHYSICS AND PHOTOINDUCED-ELECTRON-TRANSFER REACTIONS OF ZINC AND FREE-BASE OCTAETHYLPORPHYCENES

The photoexcited triplet states of free-base and zinc octaethylporphycenes (H2OEPC and ZnOEPC, respectively) were investigated by laser photolysis in the liquid phase and by time-resolved CW-EPR spectroscopy in partially oriented liquid crystal matrices. The triplet and fluorescence yields of H2OEPC are temperature dependent, and at room temperature they are lower by an order of magnitude than those of ZnOEPC. This temperature dependence is interpreted in terms of a thermally activated excited state in the singlet manifold, approximately 2.5 kcal mol-1 above S1. Frozen solutions of both porphycenes exhibit strong triplet EPR spectra, from which the magnetic and spin dynamics could be extracted. Electron transfer from the triplet state of H2OEPCT and ZnOEPCT to duroquinone (DQ) was studied by the Fourier transform EPR method. The two triplet porphycenes transfer an electron to produce a spin-polarized anion radical DQ.- with a dominating radical-pair mechanism and a negligible contribution of triplet mechanism, due to mixing of in-plane and out-of-plane active spin states.