Mechanism of formation and spectral characteristics of triplet excimers of Pd-porphyrins in liquid solutions

Abstract

Concentration quenching of the triplet state (CQTS) of a number of Pd-porphyrins in toluene is investigated. Based on an analysis of experimental results, a conclusion is drawn that such quenching proceeds via formation of exciting complexes from molecules in the ground and triplet states (TS). From a small value of decay rate constants of the complexes it follows that these complexes are triplet excimers but not complexes due to collision. It is established that by increasing the concentration of porphyrins the maxima of their phosphorescence bands shift toward large wavelengths by a value of up to 3 nm and the bands, as a consequence, broaden by 5–7 nm. The formation of excimers is explained by enhancement of solvophobic dispersion interactions of molecules in the TS. Relatively small changes in the phosphorescence spectra upon formation of excimers are attributable to these interactions. The magnitude of dipole-dipole interaction of molecules in the TS is insignificant since the dipole moment of the T↭S transition is small. On the basis of consideration of the oxidation-reduction and energy characteristics of the compounds, it is shown that the interaction with charge transfer in the case of the CQTS of Pd-porphyrins is inefficient.