PREPARATION AND PHOTOPHYSICAL STUDIES OF PORPHYRIN‐C₆₀ DYADS

Abstract Porphyrin‐C₆₀ dyads in which the two chromophores are linked by a bicyclic bridge have been synthesized using the Diels‐Alder reaction. The porphyin singlet lifetimes of both the zinc (Pz_n‐C₆₀) and free base (P‐C₆₀) dyads, determined by time‐resolved fluorescence measurements, are ≦17 ps in toluene. This substantial quenching is due to singlet‐singlet energy transfer to C₆₀ The lifetime of Pzn‐¹C₆₀ is ‐5 ps in toluene, whereas the singlet lifetime of an appropriate C₆₀ model compound is 1.2 ns. This quenching is attributed to electron transfer to yield P_zn^bull;+‐C₆₀^bull;‐. In toluene, P‐¹C₆₀ is unquenched; the lack of electron transfer is due to unfavorable thermodynamics. In this solvent, a transient state with an absorption maximum at 700 ran and a lifetime of‐10 μs was detected using transient absorption methods. This state was quenched by oxygen, and is assigned to the C₆₀ triplet. In the more polar benzonitrile, P‐¹C₆₀ underoes photoinduced electron transfer to give P^•+‐C₆₀^bull;‐. The electron transfer rate constant is −2 × 10¹¹ s⁻¹.