Interaction of multiporphyrin systems with molecular oxygen in liquid solutions: extra-ligation and screening effects

Abstract

Steady-state and time-resolved studies indicate that for a sequence of porphyrin or chlorin chemical dimers Zn-cyclodimer→(ZnOEP) 2Ph→(ZnOEP) 2→(ZnHTPP) 2→(ZnOEChl) 2 with relative lowering of excited S 1- and T 1-states, the extra-ligation by pyridine (PYR) does not influence essentially on fluorescence parameters but leads to an increase of T 1-states non-radiative decay (the most pronounced for dimers with higher lying T 1-levels). For pyridinated dimers at 293 K T 1-states quenching by molecular oxygen depends on the spacer flexibility and donor–acceptor interactions with PYR. In self-assembled triads and pentads energy and electron transfer (within a few ps) takes place from Zn-dimers to pyridyl substituted porphyrin extra-ligand, H 2P, followed by the effective population of H 2P T 1-state. For these systems, bimolecular constants of H 2P T 1-states quenching by O 2 decrease by 1.4–1.8 times with respect to those found for individual monomeric porphyrins. This effect is explained by the screening action of a strongly quenched Zn–porphyrin dimer subunit limiting the access of oxygen molecule to the excited extra-ligand.