Energy Transfer Processes in Electronically Coupled Porphyrin Hetero-Dyads Connected at the beta Position

Energy transfer in a series of hetero-dyads of zinc porphyrin and free-base porphyrin connected at the b position by p conjugated bridges has been determined. The dyads have been characterized and compared with the homo dyads, excellent models for the donor and the acceptor porphyrins in the electronically conjugated system. The homo dyads provide reliable parameters for the determination of the energy transfer rate calculated according to the Fo¨ rster theory. This model was inadequate to account for the experimental findings and an electron exchange mechanism was shown to contribute. A favorable coplanar arrangement of the bridge and the tetrapyrroles facilitates the energy transfer process, which displays a very low distance dependence and an efficiency >98%.