PREPARATION AND PHOTOPHYSICAL STUDIES OF PORPHYRIN C-60 DYADS

Porphyrin-C-60 dyads in which the two chromophores are linked by a bicyclic bridge have been synthesized using the Diels-AIder reaction. The porphyrin singlet lifetimes of both the zinc (P-Zn-C-60) and free base (P-C-60) dyads, determined by time-resolved fluorescence measurements, are less than or equal to 7 ps in toluene. This substantial quenching is due to singlet-singlet energy transfer to C-60 The lifetime of P-zn-C-1(60) is similar to 5 Ps in toluene, whereas the singlet lifetime of an appropriate C-60 model compound is 1.2 ns. This quenching is attributed to electron transfer to yield P-Zn(.+)-C-60(.-). In toluene, P-C-1(60) is unquenched; the lack of electron transfer is due to unfavorable thermodynamics. In this solvent, a transient state with an absorption maximum at 700 nm and a lifetime of similar to 10 mu s was detected using transient absorption methods. This state was quenched by oxygen, and is assigned to the C-60 triplet. In the more polar benzonitrile, P-C-1(60) undergoes photoinduced electron transfer to give P-.+-C-60(.-). The electron transfer rate constant is similar to 2 x 10(11)-s(-1).