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

Abstract

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...

Real Time Quantification of Ultrafast Photoinduced Bimolecular Electron Transfer Rate: Direct Probing of the Transient Intermediate

“…This is to note that MV 2+ also gets excited by 266 nm pump pulse; however, its excited state dynamics in 460−720 nm region does not reflect the formation of any new state (i.e., no rise time has been observed as can be seen in Figure S4 of the Supporting Information), which has also been reported earlier. 70 A comparison between the kinetics of Py, MV 2+ , and a mixture of Py and 20 mM MV 2+ at 600 nm ( Figure S5 of the Supporting Information) clearly shows the existence of the growth in transient only for the last case. Thus, the assignment of the rise component observed in the Py−MV 2+ mixtures to the electron transfer process is further supported.…”

Fluorescence as a Tool to Understand Changes in Photosynthetic Electron Flow Regulation

“…5) (Dan Hess 2000). MV 2+ is an extremely powerful electron acceptor, due to the nature of the bipyridinium salts, which temporarily become a stable radical with the addition of an electron, neutralising the positive charge of the cation (Moreland 1980;Peon et al 2001). MV 2+ oxidizes the primary acceptor (ferrodoxin) of linear electron transport, allowing a DpH to become established.…”

Synthesis, Structure, and Photochemical Behavior of [5]Heli-viologen Isomers

“…25 The fluorescence quantum yields for both heli-viologens were 2.1 times smaller than for their heli-bipyridine precursor (Table 4). On the other hand, the measured lifetime for 2 2+ , 10.2 ± 0.6 ns, is substantially larger than the lifetime measured for 1 2+ (5.8 ± 0.6 ns).…”