PDT-related photophysical properties of conformationally distorted palladium(II) porphyrins

The photodynamic therapy (PDT) related photophysical properties of five palladium(II) porphyrins with increasing steric demand of the meso-substituents Were investigated by steady state and time-resolved absorption and luminescence spectroscopy. The steric strain of the meso-substituents is reflected in a bathochromic shift of the B-band in the absorption spectra. In contrast to free-base porphyrins. the palladium(II) complexes exhibit only a very weak fluorescence with Phi(FI) similar to 10(-4). Instead, the corresponding triplet states of the porphyrins are formed via inter system crossing (ISC) with a quantum yield (Disc of nearly unity. A phosphorescence quantum yield (Disc of 10-4 was determined assuming dynamic quenching. The triplet state of all compounds is efficiently quenched by molecular oxygen, generating singlet molecular oxygen with a quantum yield of Phi(Delta) similar to0.85. The various substitution patterns exerted no significant influence on the ISC rates or sin-let oxygen quantum yields. Significant differences, on the other hand, were observed with respect to the triplet lifetimes, with a tendency of longer lifetimes for the porphyrins with sterically more demanding substituents (in air-saturated solution). All porphyrins were successfully incorporated into liposomes. The phototoxic activity of three compounds was investigated in cell suspensions. The use of the new oporphyrinato-palladium(II) in PDT applications is discussed. Copyright (C) 2001 John Wiley Sons, Ltd.