Excited State Dynamics of Methyl Viologen. Ultrafast Photoreduction in Methanol and Fluorescence in Acetonitrile

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 →S N spectrum) of MV 2+ was measured in acetonitrile and the fluorescence was characterized by time-correlated single-photon 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 added quenchers with gas-phase ionization potentials lower than about 10.8 eV. Using the measured emission spectrum, the excited-state reduction potential is determined to be E ° (MV 2+ */MV •+ ) = 3.65 V, confirming the highly oxidizing character of this photoexcited dication.