Photoinduced electron transfer in Langmuir-Blodgett monolayers of double-linked phthalocyanine-fullerene dyads

Photoinduced electron transfer (ET) in films of the double-linked free-base phthalocyanine-fullerene dyads (Pc-C-60) was studied in solid Langmuir-Blodgett (LB) films using femtosecond pump-probe and microsecond flash-photolysis methods. Excitation of the phthalocyanine (Pc) moiety of the dyad results in a rapid (0.8 ps) intramolecular ET from phthalocyanine to fullerene. The formed charge separated (CS) state relaxes in 35 ps, yielding an intermolecular CS state, which relaxes to the ground state in the microsecond time domain. The surface organization of the Pc-C-60 dyad monolayers was studied by atomic force microscopy for both pure Pc-C-60 films and Pc-C-60 mixed films with matrix molecules, octadecylamine (ODA). This study together with the estimations done based on surface pressure-mean molecular area isotherms reveals that the dyads tend to aggregate rather than form a homogeneous mixture with the ODA molecules. This enforces intermolecular interactions and leads to the formation of a long-lived intermolecular CS state.