Photoinduced electron- and energy-transfer processes of [60]fullerene covalently bonded with one and two zinc porphyrin(s): effects of coordination of pyridine and diazabicyclooctane to Zn atom

Abstract

C60–zinc porphyrin (ZnP) dyad (ZnP–C60) and triad (ZnP–C60–ZnP) were synthesized to probe energy-transfer and electron-transfer processes in the absence and presence of pyridine and diazabicyclooctane (DABCO). The syntheses of C60–ZnP and ZnP–C60–ZnP were carried out by Diels–Alder cycloaddition between sulfolene moiety-containing porphyrin and C60. The photoinduced electron-transfer processes between the spatially positioned C60 and ZnP in the dyad and triad systems were investigated by time-resolved transient absorption and fluorescence measurements with changing solvent polarity. Upon excitation of the ZnP moiety, charge separation via an excited singlet state of ZnP takes place competitively with energy transfer to C60 generating the excited singlet state of C60, from which charge-separated states (ZnP˙+–C60˙− and ZnP˙+–C60˙−–ZnP) are also generated in polar solvents. Rates and efficiencies of energy transfer and charge separation for the triad are higher than those of the dyad. The generated charge-separated species recombine with lifetimes in the rage of 240–330 ns in polar solvents such as DMF, PhCN, and THF for both dyad and triad. In o-dichlorobenzene, although the lifetimes of charge-separated states are very short (<20 ns), coordination of DABCO and pyridine to ZnP in the dyad and triad producing relatively stable coordinated complexes gives rise to prolongation of the charge-separated states up to 460 ns.