A ferrocene-C60-dinitrobenzene triad:: Synthesis and computational, electrochemical, and photochemical studies

Synthesis and physicochemical characterization of a molecular triad comprised of ferrocene, C-60, and dinitrobenzene entities is reported. Electrochemical studies revealed multiple redox processes involving all three redox active, ferrocene, C-60, and dinitrobenzene entities. A total of eight reversible redox couples within the accessible potential window of o-dichlorobenzene containing 0.1 M (TBA)ClO4 are observed. A comparison between the measured redox potentials with those of the starting compounds revealed absence of any significant electronic interactions between the different redox entities. The geometric and electronic structure of the triad is elucidated by using ab initio B3LYP/3-21G(*) methods. In the energy optimized structure, as predicted by electrochemical studies, the first HOMO orbitals are found to be located on the ferrocene entity, whereas the first LUMO orbitals are mainly on the C-60 entity with small orbital coefficients on the dinitrobenzene entity. The subsequent LUMO's track the observed site of electrochemical reductions of the triad. The photochemical events in the triad are probed by both steady-state and time-resolved techniques. The steady-state emission spectra of the triad and the starting dyad, 2-(ferrocenyl)fulleropyrrolidine, are found to be completely quenched compared to fulleropyrrolidine bearing no redox active substituents. The subpicosecond and nanosecond transient absorption spectral studies reveal efficient charge separation in the triad. As suggested by the coefficients of the LUMO orbitals, the transient absorption spectrum of the triad revealed bands corresponding to the formation of the fulleropyrrolidine anion species. The estimated rise and decay rate constants are found to be 2.5 x 10(11) and 8.5 x 10(9) s(-1) for the triad, and these values compare with a rise and decay rate constants of 2.2 x 10(11) and 4.5 x 10(9) s(-1) for the ferrocene-C-60 dyad in benzonitrile. The observed faster rate constants are attributed to the close spacing of the redox entities of the triad to one another.