Femtosecond Dynamics of the Photoexcited Methyl Viologen Radical Cation

Methyl viologen ( N, N ‘-dimethyl-4,4‘-bipyridine, abbreviated MV 2+ ) is widely used in light-driven molecular devices, where it switches between the redox forms MV 2+ 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 (τ = 16 ps) vibrationally excited ground state.