Flash Photolytic Triplet Sensitization and the Mechanism of Carbonylferroporphyrin Photodissociation

Triplet sensitization is used to examine the nature and energy of the electronic state(s) involved in photodissociation from a carbonylferroporphyrin and to study the sensitized production of the zinc porphyrin triplet state in parallel observations. Both processes are reversible over ordinary time scales, and thus we have employed a flash photolytic sensitization procedure in which the concentration of the “photoproduct” is measured in a time short compared to its lifetime, and the relative amounts generated by direct and sensitized excitation are assessed. Through the use of an appropriate suite of donors, we show that CO release occurs subsequent to triplet excitation transfer to a carbonylferroporphyrin state(s) with higher than singlet multiplicity and with energy no higher than 14 300 cm-1, most probably 3(π-π*). These measurements suggest the possibility that CO photodissociation occurs via a relaxation process which proceeds through the lowest lying porphyrin (π-π*) singlet (3Q) and triplet (3Q) states.