Electronic states and electron transfer processes in porphyrins and metalloporphyrins

The time-resolved and steady-state optical spectroscopic methods are employed in the investigation of the photophysical behavior of several porphyrin based systems. Photoinduced electron transfer is monitored from the lowest excited singlet state of a zinc porphyrin to a covalently linked quinone. The rate of charge transfer is found to be exponentially dependent on the donor-acceptor separation. In a series of osmium(II) porphyins, the ordering of lowest excited states is deduced from optical data. The effects of non-planarity on the excited state deactivation processes in a series of sterically crowded free-base porphyrins is investigated. Enhanced non-radiative rates are found in several distorted complexes. A novel porphyrin tautomer is investigated and found to relax rapidly to the ground state following photo-excitation. Mechanisms underlying these observations and their relationship to the function of porphyrins in biological systems are discussed.