Spectroscopic properties of a self-assembled zinc porphyrin tetramer I. Steady state optical spectroscopy

Aggregation of zinc mono-(4-pyridyl)-triphenylporphyrin (ZnPyP) in toluene and polystyrene/toluene mixtures has been investigated by steady-state optical spectroscopy. The Q-band absorption spectra, as well as the fluorescence spectra, show a temperature-dependent red shift as a result of ligation of the porphyrin zinc center. The smallest aggregate that can account for the optical spectra and their concentration dependence is a symmetrical tetramer in which each zinc atom is ligated to a pyridyl substituent of a neighboring porphyrin molecule. The Soret band shows a splitting, which can be explained by applying simple exciton theory to the tetramer. The equilibrium constant and thermodynamic parameters for the monomer-tetramer equilibrium have been determined as K = (6.2 ± 0.8) x 1013 M-3, H = -40 ± 2 kCalM-1, S = -150 ± 10 CalK-1M-1, in agreement with tetramer formation