Photoinduced intramolecular electron-transfer processes in [60]fullerene-(spacer)-N,N-bis(biphenylyl)aniline dyad in solutions

Intramolecular photoinduced charge-separation and charge-recombination processes in a covalently connected C-60-(spacer)-N,N-bis(biphenylyl)aniline (C-60-spacer-BBA) dyad, in which the center-to-center distance of the electron acceptor and electron donor is 15 angstrom, have been studied by time-resolved fluorescence and transient absorption methods. The observed low fluorescence intensity and the short fluorescence lifetime of the C-60 moiety of the dyad in PhCN and THF indicate that charge separation takes place via the excited singlet state of the C-60 moiety at a quite fast rate and a high efficiency. The nanosecond transient absorption spectra in PhCN and THF showed the broad absorption bands at 880 and 1100 nm, which were attributed to C(60)(center dot-)spacer-BBA(center dot+). The charge-separated state decays with a lifetime of 330-360 ns in PhCN and THF at room temperature. From temperature dependence of the charge-recombination rate constants, the reorganization energy was evaluated to be 0.77-0.87 eV, which indicates that the charge-recombination process is in the inverted region of the Marcus parabola. With lowering temperature, the contribution of charge separation via the excited triplet state of the C-60 moiety increases due to an increase in solvation of C(60)(center dot-)spacer-BBA(center dot+)..