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 st udied in several polar solvents at room temperature using a variety of ultr afast time-resolved and steady-state spectroscopic techniques. The results highlight the very strong electron accepting character of the lowest single t 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* tra nsition of MV2+ by a 150 fs, 265 nm pump pulse. In methanol, the radical ca tion of methyl viologen (MV.+) appeared within our time resolution, indicat ing that forward electron transfer from a nearby donor quenches electronica lly excited MV2+ in < 180 fs. Identical dynamics within experimental uncert ainty were observed for the chloride salt of MV2+ and for the salt prepared with tetrafluoroborate counterions. This latter "superhalide" ion has a co ndensed-phase detachment threshold that is too high to permit oxidation by the excited state of MV2+. Thus, electron transfer: does not take place wit hin an associated MV2+-counterion complex in methanol but results instead f rom 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 simul taneous recovery of ground-state MV2+ with a characteristic time constant o f 430 +/- 40 fs were observed immediately after the pump pulse and assigned to back electron transfer in the geminate radical pair. Despite the high r ate of back electron transfer, a significant fraction of the initial radica l pairs avoid recombination, and a finite yield (similar to 12%) of MV.+ io ns is observed at delay times > 2 ps. There was no evidence of photoreducti on 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 e xcited-state decay channels exist in these two solvents. In aqueous solutio n, 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 f luorescence is observed. This is the first report of an efficient radiative decay pathway for MV2+ in fluid solution. The excited-state absorption spe ctrum (S-1-->S-N spectrum) of MV2+ was measured in acetonitrile and the flu orescence was characterized by time-correlated single-photon counting and s teady-state measurements. The fluorescence quantum yield is 0.03 +/- 0.01 a nd the lifetime in-acetonitrile at room temperature is 1.00 +/-4 0.04 ns. T he fluorescence is efficiently quenched by electron transfer from added que nchers with gas-phase ionization potentials lower than about 10.8 eV. Using the-measured emission spectrum, the excited-state reduction potential is d etermined to be E degrees (MV2+*/MV.+) = 3.65 V, confirming the highly oxid izing character of this photoexcited dication.