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

The effects of ligation and meso substitution on the energies of two t ripler states of zinc(II) tetraphenylporphyrin (ZnTPP) were investigat ed. Phosphorescence and triplet-triplet absorption spectroscopy were u sed 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 ligati ng 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 o f the T-1 state, The magnitude and direction of energy shifts caused b y the various electron-donating/withdrawing substituents on the T-s st ate suggest a greater resonance interaction between the phenyl and por phyrin rings in the T-s state, possibly caused by a decrease in the av erage dihedral angle between the ring systems. Ligation by pyridine ca uses 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 a nd less so around the pyrrole nitrogens, The results of this phosphore scence/triplet-triplet absorption study and previously obtained time-r esolved resonance Raman spectra of ZnTPP are combined to yield a descr iption of the dynamics of these two triplet states.