Influence of Ionic Liquids on Physico Chemical Reactions Reactions of Excited-State Benzophenone Ketyl Radical in a Room-Temperature Ionic Liquid

The photochemistry of benzophenone ketyl radical in the D1 excited state, BPH(D1), was studied by means of two-color dual-pulse laser flash photolysis (355 and 532 nm) in a room-temperature ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (Bmim-TFSA), and in methanol. Upon excitation with the 532-nm pulse, BPH(D1) emitted strong fluorescence. The transient absorption and fluorescence spectra of BPH(D1) were measured with nanosecond and sub-nanosecond time resolution, respectively. The observed Stokes shift was 1700 cm in Bmim-TFSA, and this shift was close to that in acetonitrile. The fluorescence lifetime of BPH(D1) was determined to be 5 ns in Bmim-TFSA, and again the value was close to that in acetonitrile. The rate constant of the reaction of BPH(D1) with CCl4 in Bmim-TFSA was determined to be (2.1 ± 0.4) × 10 M s, which was 10 times the rate constant calculated on the basis of the bulk viscosity of Bmim-TFSA. The results are discussed in terms of the effective microscopic viscosity of the ionic liquid that was recently reported for the cage effect.