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.