Effects of hydrogen bonding interaction and solvent polarity on the competition between excitation energy and photo-induced electron transfer processes in hydroxy(1-pyrenebutoxy)phosphorus(V)porphyrin

The solvent effects on an excitation energy transfer and a photo-induced electron transfer processes were examined using synthesized hydroxy(1-pyrenebutoxy)phosphorus(V)porphyrin. In the photoexcited state of the pyrene moiety, the intramolecular energy transfer to the porphyrin competed with the electron transfer from the pyrene to the porphyrin. The quantum yield of energy transfer in non-alcoholic solvents decreased with an increase of the solvent polarity due to the enhancement of electron transfer. However, the energy transfer was predominant process in alcoholic solvents with high polarity. The energy transfer yield increased with an increase in the ratio of methanol in the mixture of acetonitrile and methanol, of which solvent polarity is almost the same as that of acetonitrile. The redox potential measurements and ab initio molecular orbital calculation at Hartree-Fock 6-31G* level have shown that the electron affinity of the porphyrin moiety decreased through the hydrogen bonding with alcoholic solvents, resulting in that the electron transfer is suppressed and the excitation energy transfer becomes the predominant process. In conclusion, the hydrogen bonding interaction with alcoholic solvent contributes to the competition between energy and electron transfer by the changing of the energy level of charge transfer state rather than the effect of solvent polarity.