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 ' -bipyddinium, MV2+) 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 MV2+. Transient absorption was measured between 270 and 740 nm asa function of delay time after excitation of the strong pi-pi* transition of MV2+ 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 MV2+ in < 180 fs. Identical dynamics within experimental uncertainty were observed for the chloride salt of MV2+ 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 MV2+. Thus, electron transfer: does not take place within an associated MV2+-counterion complex in methanol but results instead from oxidation of a solvent molecule. Photoreduction of MV2+ 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 MV2+ 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 (similar to 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 MV2+. 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 MV2+ with a time constant of 3.1 ps in H2O and 5.3 ps in D2O. 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 MV2+ in fluid solution. The excited-state absorption spectrum (S-1-->S-N spectrum) of MV2+ 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 +/-4 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 degrees (MV2+*/MV.+) = 3.65 V, confirming the highly oxidizing character of this photoexcited dication.