Photoinduced charge separation and stabilization in clusters of a fullerene-aniline dyad

Abstract

Fullerene-bridge-aniline dyad and the model fulleropyrrolidine compound form stable, optically transparent clusters in mixtures (3:1) of acetonitrile and toluene. Ground- and excited-state properties of the clusters of the dyad and the model compound are compared with their corresponding monomeric forms. Clustering of the dyad as well as the model compound exhibits a red-shifted emission maximum (λ_max 738 nm) compared to their monomeric forms (λ_max 714 nm). The electron transfer from the appended electron donor moiety to the parent fullerene core in the dyad cluster is evident from the decreased (~80%) fluorescence yield. The formation of fullerene radical anion (absorption maximum at 1010 nm) with a lifetime of several hundreds of microseconds was further confirmed using nanosecond laser (337 nm) flash photolysis experiments. In contrast, the dyad molecules in their monomeric form did not yield any detectable yield of C₆₀ radical anion following laser pulse excitation. The failure to observe any charge-transfer intermediates following laser pulse excitation, even in polar solvents such as benzonitrile or nitromethane, suggested that fast back-electron-transfer process must be operative in the monomeric dyad system. On the other hand, clustering of the fullerene-based dyads in a mixed-solvent system can provide a unique way to decrease the rate of back electron transfer, thus stabilizing the electron-transfer products.