| Abstract
| - Methyl viologen (N,N‘-dimethyl-4,4‘-bipyridine, abbreviated MV2+) is widely used in light-driven moleculardevices, 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 toinvestigate the excited state dynamics of the electrochemically generated MV•+ in acetonitrile solution.Subpicosecond excitation of the D0−D1 transition at 730 nm led to rapid relaxation (700 fs), generating twointermediates 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 similarto the ground-state spectrum of MV•+ in both shape and extinction coefficients, but red-shifted by ca. 810cm-1. This energy shift equals one quantum of the intraring C−C stretch mode. The shorter lived transientdecayed 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 timescale via at least one long-lived (τ = 16 ps) vibrationally excited ground state.
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