Photoinduced electron transfer in a porphyrin dyad

A porphyrin dyad designed to facilitate vectorial interporphyrin electron transfer has been synthesized and studied using steady-state and time-resolved absorption and emission spectroscopies. The dyad features a zinc tetraarylporphyrin bearing electron-donating substituents linked by an amide bond to a free base porphyrin carrying electron-withdrawing groups. Excitation of the zinc porphyrin moiety in dichloromethane solution is followed by singlet energy transfer to the free base and concurrent electron transfer to the same moiety to yield a charge-separated state. The free base first excited singlet state decays by accepting an electron from the zinc porphyrin to form the same charge-separated state. Similar results are observed in butyronitrile. Transient absorption studies in butyronitrile verify the formation of a short-lived (8 ps) charge-separated state from the porphyrin first excited singlet states. The results support the suggestion that fluorescence quenching in related porphyrin dyads and carotenoid-diporphyrin triads is due to photoinduced electron transfer, rather than some other decay process.