Reactions of excited-state benzophenone ketyl radical in a room-temperature ionic liquid

Abstract

The photochemistry of the benzophenone ketyl radical in 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⁻¹ 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) × 10⁹ M⁻¹ s⁻¹, 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.