Quenching of the triplet state of Pd-porphyrins by Cu-etioporphyrin in dimethyl formamide

Quenching of phosphorescence of Pd-tetrabenzoporphyrin (Pd-TBP) and Pd-etioporphyrin I (Pd-EP) by the Cu-EP molecules was studied using pulsed photoexcitation. The rate constant of quenching (Kq) by the Cu-EP molecules was found to be 2.3×109 1/(mol s) for the low lying triplet state (TS) of Pd-Ep and 5.8×108 1/(mol s) for the TS of Pd-TBP. It is concluded that the quenching was caused by the intermolecular energy transfer, and the low value of Kq for Pd-TBP can be explained by the fact that the level of Cu-EP (X), to which the energy is transferred, lies above the TS of Pd-TBP (ET=12 900 cm−1). Based on the results obtained and the published data, it is assumed that the energy transfer proceeds via the formation of long-lived excited complexes of interacting molecules. With the Kq value for Pd-TBP and the results of kinetic analysis of the adopted quenching mechanism, the difference between the energy of states, involved in energy transfer, and the X state energy are determined (EX=13 520 cm−1 and ET=14 500 cm−1 for Cu-EP). Some specific features of photophysical and luminescence properties of Cu-porphyrins are explained by the presence of such a state related to the excitation of the copper ion.