Photoinduced Charge Separation in a Ferrocene−Aluminum(III) Porphyrin−Fullerene Supramolecular Triad

Abstract

Light-induced electron transfer is investigated in a ferrocene-aluminum(III) porphyrin-fullerene supramolecular triad (FcAlPorC(60)) and the constituent dyads (AlPorC(60) and FcAlPorPh). The fullerene unit (C(60)) is bound axially to the aluminum(III) porphyrin (AlPor) via a benzoate spacer, and ferrocene (Fc) is attached via an amide linkage to one of the four phenyl groups in the meso positions of the porphyrin ring. The absorption spectra and voltammetry data of the complexes suggest that the ground state electronic structures of the Fc, AlPor, and C(60) entities are not significantly perturbed in the dyads and triad. Time-resolved optical and transient electron paramagnetic resonance (EPR) data show that photoexcitation of the AlPorC(60) dyad results in efficient electron transfer from the excited singlet state of the porphyrin to fullerene, producing the charge-separated state AlPor(•+)-C(60)(•-). The fluorescence and transient EPR data also suggest that some energy transfer from the porphyrin to fullerene may occur. The lifetime of the radical pair AlPor(•+)-C(60)(•-) measured by transient absorbance spectroscopy is found to be 39 ns in o-dichlorobenzene at room temperature. At 200 K, transient EPR experiments place a lower limit of 5 μs on the radical pair lifetime. In the triad, the data suggest that excitation of the porphyrin gives rise to the charge-separated state Fc(•+)-AlPor-C(60)(•-) in two electron transfer steps. Photocurrent measurements demonstrate that both dyads and the triad have good photovoltaic performance. However, when Fc is appended to AlPorC(60), the expected improvement of the radical pair lifetime and the photovoltaic characteristics is not observed.