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

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 S1- and T1-states, the extra-ligation by pyridine (PYR) does not influence essentially on fluorescence parameters but leads to an increase of T1-states non-radiative decay (the most pronounced for dimers with higher lying T1-levels). For pyridinated dimers at 293 K T1-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, H2P, followed by the effective population of H2P T1-state. For these systems, bimolecular constants of H2P T1-states quenching by O2 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.