Femtosecond dynamics of the photoexcited methyl viologen radical cation

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 MV.+ in acetonitrile solution. Subpicosecond excitation of the D-0-D-1 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 equals 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 D-1-state of MV.+ in acetonitrile solution relaxes on the subpicosecond time scale via at least one long-lived (tau = 16 ps) vibrationally excited ground state.