Time-Resolved Electron Paramagnetic Resonance Study of Photoinduced Electron Transfer in Pd Porphyrin–Quinone and Zn Porphyrin–Quinone Dyads with a Cyclohexylene Spacer

Abstract

Peculiarities of the light induced intramolecular electron transfer processes in two ensembles where Pd porphyrin and Zn porphyrin donors with similar peripheral substituents are covalently linked via cyclohexylene spacer with a quinone acceptor, were studied by time-resolved electron paramagnetic resonance spectroscopy in different phases of the magnetically oriented nematic liquid crystal E-7. In the photoexcited PdP-Q the net absorptive signal was observed and ascribed to the thermally equilibrated spectrum of (3)*(PdP(•+)-Q(•-)). In ZnP-Q photoinduced intramolecular electron transfer was also found. It was demonstrated that the multiplet spectrum of the charge-separated state (3)*(ZnP(•+)-Q(•-)) consists of two signals with different widths and decay times. The signals were assigned to two spin-polarized triplets of the radical pairs formed in "stretched" and "folded" ensemble conformers, corresponding to different configurations of the cyclohexylene spacer. These findings were discussed in terms of differences in the properties of the porphyrin metal cores, macrocycle peripheral substituents and geometry of the donor-acceptor cyclohexylene spacer.