The photophysical and photochemical deactivation pathways of electronically excited methyl viologen (1,1′dimethyl-4,4′-bipyridinium, MV 2+ ) were studied in several polar solvents at room temperature using a variety of ultrafast time-resolved and steady-state spectroscopic techniques. The results highlight the very strong electron accepting character of the lowest singlet excited state of MV 2+ . Transient absorption was measured between 270 and 740 nm as a function of delay time after excitation of the strong π-π* transition of MV 2+ by a 150 fs, 265 nm pump pulse. In methanol, the radical cation of methyl viologen (MV •+ ) appeared within our time resolution, indicating that forward electron transfer from a nearby donor quenches electronically excited MV 2+ in < 180 fs. Identical dynamics within experimental uncertainty were observed for the chloride salt of MV 2+ and for the salt prepared with tetrafluoroborate counterions. This latter "superhalide" ion has a condensed-phase detachment threshold that is too high to permit oxidation by the excited state of MV 2+ . Thus, electron transfer does not take place within an associated MV 2+ -counterion complex in methanol but results instead from oxidation of a solvent molecule. Photoreduction of MV 2+ in methanol is a novel example of ultrafast electron-transfer quenching of a photoexcited acceptor in an electron-donor solvent. This is the first demonstration that a hydrogen-bonding solvent can serve as the electron donor in an ultrafast intermolecular ET reaction. Decay of the initial MV •+ population and simultaneous recovery of ground-state MV 2+ with a characteristic time constant of 430 ( 40 fs were observed immediately after the pump pulse and assigned to back electron transfer in the geminate radical pair. Despite the high rate of back electron transfer, a significant fraction of the initial radical pairs avoid recombination, and a finite yield (∼12%) of MV •+ ions is observed at delay times > 2 ps. There was no evidence of photoreduction when the solvent was acetonitrile or water. Both of these solvents have high gas-phase ionization potentials that prevent oxidation by excited MV 2+ . The transient absorption signals indicate, however, that very different excited-state decay channels exist in these two solvents. In aqueous solution, an unknown nonradiative decay process causes decay of excited MV 2+ with a time constant of 3.1 ps in H 2 O and 5.3 ps in D 2 O. In acetonitrile, on the other hand, the transient absorption decays hundreds of times slower and fluorescence is observed. This is the first report of an efficient radiative decay pathway for MV 2+ in fluid solution. The excited-state absorption spectrum (S 1 fS N spectrum) of MV 2+ was measured in acetonitrile and the fluorescence was characterized by time-correlated singlephoton counting and steady-state measurements. The fluorescence quantum yield is 0.03 ( 0.01 and the lifetime in acetonitrile at room temperature is 1.00 ( 0.04 ns. The fluorescence is efficiently quenched by electron transfer from a...

Articles you may be interested inPhotodetachment and electron reactivity in 1-methyl-1-butyl-pyrrolidinium bis(trifluoromethylsulfonyl)amide J. Chem. Phys. 137, 034512 (2012); 10.1063/1.4736569Collisional electron transfer to photoexcited acceptor radical anions Two-photon dissociation and ionization of liquid water studied by femtosecond transient absorption spectroscopyExcess electrons were studied in liquid acetonitrile at room temperature by femtosecond pumpprobe spectroscopy. Using Ϸ200 fs, 265 nm laser pulses, electrons were ejected into the liquid by photodetachment from iodide ions and, in separate experiments, by photoionization of indole. A strong and broad absorption band with a maximum near 1400 nm was observed in both systems. A second absorption band was observed at wavelengths below 620 nm for iodide solutions, but was not seen in photoexcited indole due to overlapping excited state absorption. The bands are in good agreement with ones seen previously in nanosecond pulse radiolysis experiments ͓I. P. Bell, M. A. J. Rodgers, and H. D. Burrows, J. Chem. Soc., Faraday Trans. 1 73, 315 ͑1977͔͒. Bell, Rodgers, and Burrows assigned the visible and IR bands to absorption by acetonitrile dimer and monomer anions, respectively. Our results strongly question this interpretation. Instead, we assign the short-wavelength absorption band to a solvent-bound valence anion consisting of one or two acetonitrile molecules and the IR band to a solvated or cavity electron. Low-level quantum chemical calculations indicate that valence anion formation is strongly correlated with CCN bending, but do not provide a clear indication of whether a monomer or dimer valence anion is favored. The highly mobile cavity electron is scavenged by added chloroform at a bimolecular reaction rate of (1.02 Ϯ0.03)ϫ10 11 M Ϫ1 s Ϫ1 . The appearance of both absorption bands within our time resolution suggests that the two forms of the excess electron are produced by prompt reaction with the iodide charge-transfer-to-solvent ͑CTTS͒ excited state. In support of this mechanism, strong static scavenging by chloroform was observed at both visible and IR wavelengths. For iodide in acetonitrile, the signal in the IR decays biexponentially due to competition between geminate recombination of the cavity electron with the parent iodine atom and its reaction with the solvent. Geminate recombination between the solvated electron and the parent iodine atom occurs with a characteristic time constant of Ϸ30 ps, while additional solvent anions are formed in a slow reaction with a time constant of Ϸ260 ps. Approximately 30% of the solvated electrons photodetached from iodide undergo geminate recombination. There is no evidence for geminate reaction between the promptly formed solvent anion and iodine, suggesting that these species are formed at larger initial separation than the IR-absorbing cavity electron/iodine atom pair. In indole, geminate recombination occurs on a slower time scale of Ϸ135 ps.