PHOTOINDUCED ELECTRON-TRANSFER IN A PORPHYRIN DYAD

A porphyrin dyad designed to facilitate vectorial interporphyrin elect ron transfer has been synthesized and studied using steady-state and t ime-resolved absorption and emission spectroscopies. The dyad features a zinc tetraarylporphyrin bearing electron-donating substituents link ed by an amide bond to a free base porphyrin carrying electron-withdra wing groups. Excitation of the zinc porphyrin moiety in dichloromethan e solution is followed by singlet energy transfer to the free base and concurrent electron transfer to the same moiety to yield a charge-sep arated state. The free base first excited singlet state decays by acce pting an electron from the zinc porphyrin to form the same charge-sepa rated state. Similar results are observed in butyronitrile. Transient absorption studies in butyronitrile verify the formation of a short-li ved (8 ps) charge-separated state from the porphyrin first excited sin glet states. The results support the suggestion that fluorescence quen ching in related porphyrin dyads and carotenoid-diporphyrin triads is due to photoinduced electron transfer, rather than some other decay pr ocess.