Photoinduced processes in a dendritic Zn porphyrin structure with a free-base porphyrin core

The photophysical characterization of a nonameric porphyrin assembly made up of a central free-base and eight peripheral zinc porphyrins connected by flexible nucleosidic linkers, and the identification of the photoinduced processes taking place in the array, is reported. Both singlet-singlet and triplet-triplet energy transfer from the peripheral zinc porphyrins to the free-base porphyrin core is detected. The kinetic parameters of singlet-singlet energy transfer are interpreted by assuming the existence of several nonequilibrated conformations in the array due to the flexibility of the linkages. A fast quenching of the donor beyond the time detection limit of the instrument is also hypothesized and assigned to strong interactions between the zinc chromophores in close contact which can give self-quenching phenomena. For the zinc porphyrin triplet excited state, a triplet-triplet annihilation is detected at moderate photon flux as a consequence of multi-excitation of the same array. Drastic reduction of the photon flux allows us to determine the triplet energy transfer rate from zinc porphyrin to free-base porphyrin, which is the final recipient of the energy collected by the array. The decay of the triplet state of the core free-base porphyrin is prolonged with respect to the model, indicating a shielding effect of the peripheral groups. Complexation experiments with monodentate and didentate bases have been performed in order to gain insight into the spatial arrangement of the zinc porphyrin chromophores and the proposed mechanism of self-quenching. Association of didentate bases increases the rigidity of the multi-porphyrin structure and improves the energy collection ability.