Methyl viologen (N,N′-dimethyl-4,4′-bipyridine, abbreviated MV2+) is widely used in light-driven molecular devices, where it switches between the redox forms MV2+ and MV•+. Yet, little is known about the excited-state dynamics of the radical cationic form MV•+. Femtosecond pump-probe spectroscopy was used to investigate the excited state dynamics of the electrochemically generated MV̇+ in acetonitrile solution. Subpicosecond excitation of the D0-D1 transition at 730 nm led to rapid relaxation (700 fs), generating two intermediates in the transient absorption spectra. The longer-lived intermediate, with a lifetime of 16 ps, could be assigned to a vibrationally excited ground state of MV•+. Its absorption spectrum was very similar to the ground-state spectrum of MV•+ in both shape and extinction coefficients, but red-shifted by ca. 810 cm-1. This energy shift equal one quantum of the intraring C-C stretch mode. The shorter lived transient decayed with a time constant of 1.0 ± 0.1 ps and is possibly also a vibrationally excited ground state. Thus, our results show that the excited D1-state of MV•+ in acetonitrile solution relaxes on the subpicosecond time scale via at least one long-lived (τ = 16 ps) vibrationally excited ground state.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry