ELECTRONIC-STRUCTURE OF TRIPLET-STATES OF ZINC(II) TETRAPHENYLPORPHYRINS

The effects of ligation and meso substitution on the energies of two tripler states of zinc(II) tetraphenylporphyrin (ZnTPP) were investigated. Phosphorescence and triplet-triplet absorption spectroscopy were used to determine the energies of the T-1 state and a higher energy T-s state, of a series of para-phenyl-substituted ZnTPPs in both a ligating and a nonligating solvent. The para-phenyl substituents were found to have a greater effect on the energy of the T-s state than on that of the T-1 state, The magnitude and direction of energy shifts caused by the various electron-donating/withdrawing substituents on the T-s state suggest a greater resonance interaction between the phenyl and porphyrin rings in the T-s state, possibly caused by a decrease in the average dihedral angle between the ring systems. Ligation by pyridine causes the energy of the porphyrin T-1 state to drop by about 300 cm(-1) on average and has a slightly lesser effect on the energy of the T-s state. Our results indicate that the electronic structures of the T-1 and T-s states of ZnTPP differ significantly around the meso-carbons and less so around the pyrrole nitrogens, The results of this phosphorescence/triplet-triplet absorption study and previously obtained time-resolved resonance Raman spectra of ZnTPP are combined to yield a description of the dynamics of these two triplet states.