Photoinduced energy transfer in highly conjugated porphyrinic dyads.

Whereas photoinduced processes in meso-meso bridged porphyrin dyads are well documented, very few reports on porphyrinic dyads connected at the ?-pyrrole are present in the literature. It is well known that the efficiency of energy transfer in porphyrinic assemblies depends both on the position of the bridge connection and on the nature of the bridge. In this report photoinduced energy transfer has been studied in a series of newly synthesized porphyrinic dyads where the individual components, a free-base and a zinc porphyrin, are connected at the ?-pyrrole position by highly conjugated bridges, namely a divinyl, and two p-phenylenevinylene groups with increasing length. The absorption spectra showed perturbation of the properties of the constituent components, indicating electronic coupling between the porphyrin units due to the nature of the connectors. To overcome the difficulties in the identification of the photoinduced processes in these coupled systems, homo-dyads with the same type of connections were synthesized and found to be excellent models. Steady state and time resolved experiments allowed to detect very efficient energy transfer from the Zn porphyrin moiety to the free-base porphyrin unit in the examined dyads (energy-transfer efficiencies of the order of 98% - 99%). The experimental rates were compared with the ones calculated according to the current models. In all cases the energy transfer process cannot be entirely explained by a dipole-dipole (Förster) scheme and a contribution by electron exchange (Dexter) mechanism must be included.