Study of the translational diffusion of the benzophenone ketyl radical in comparison with stable molecules in room temperature ionic liquids by transient grating spectroscopy

Abstract

Transient grating (TG) spectroscopy has been applied to the photoinduced hydrogen-abstraction reaction of benzophenone (BP) in various kinds of room temperature ionic liquids (RTILs). After the photoexcitation of BP in RTILs, the formation of a benzophenone ketyl radical (BPK) was confirmed by the transient absorption method, and the TG signal was analyzed to determine the diffusion coefficients of BPK and BP. For comparison, diffusion coefficients of carbon monoxide (CO), diphenylacetylene (DPA), and diphenylcyclopropenone (DPCP) in various RTILs were determined by the TG method using the photodissociation reaction of DPCP. While the diffusion coefficients of the stable molecules BP, DPA, and DPCP were always larger than those predicted by the Stokes-Einstein (SE) relation in RTILs, that of BPK was much smaller than those of the stable molecules and relatively close to that predicted by the SE relation in all solvents. For the smallest molecule CO, the deviation from the SE relation was evident. The diffusion coefficients of stable molecules are better represented by a power law of the inverse of the viscosity when the exponent was less than unity. The ratios of the diffusion coefficient of BP to that of BPK were larger in RTILs (2.7-4.0) than those (1.4-2.3) in conventional organic solvents. The slow diffusion of BPK in RTILs was discussed in terms of the fluctuation of the local electric field produced by the surrounding solvent ions.

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

“…The spectra in Bmim-TFSA are consistent with the spectrum recently measured in Bmim-PF 6 by Nishiyama et al 38 Therefore, as previously suggested, 18,19 hydrogen abstraction is slow in TMPA-TFSA.…”

“…However, studies of the reaction of BP(T 1 ) in ILs are limited. 18,19,38 In TMPA-TFSA, the lifetime of BP(T 1 ) is reported to be 19 μs, which is substantially longer than the lifetimes in cyclohexane (300 ns), 2-propanol (5 ns), and benzene (3 μs). 19 In contrast, the lifetime of BP(T 1 ) in Bmim-TFSA has been reported to be 3 μs.…”

“…Several research groups have suggested that the faster translational diffusion in ILs is due to a large void volume produced by cations and anions. 18,34 Recently, Kimura's group 38,39 For spectral measurements, a system consisting of a 300-mm monochromator (ACTON, SpectraPro 2358i) and a gated intensified charge-coupled device camera (Princeton Instruments, PI-MAX-II, UNIGENII) was used. The wavelength response of the detector was corrected by comparing the fluorescence spectrum of Coumarin 153 with a reported reference spectrum.…”

Spotlight on ionic liquids

“…58 Self-diffusion coefficients of photochemically generated CO were reported in both molecular solvents, including alkanes and alcohols, as well as in typical ILs including ͓bmim͔ + cations paired with ͓NTf 2 ͔ − , ͓BF 4 ͔ − and ͓PF 6 ͔ − . 59 Figure 6 plots the self-diffusion of CO following photodissociation. Fractional Stokes-Einstein behavior is observed with the exponent ␣ = 0.68.…”

Transport Properties of Binary Mixtures of Carbon Dioxide and 1-Butyl-3-methylimidazolium Hexafluorophosphate Studied by Transient Grating Spectroscopy

“…E-mail: ykimura@kuchem.kyoto-u.ac.jp coefficients of these molecules in various RTILs and found that the viscosity dependence of the smaller molecule such as CO is smaller than those of DPCP and DPA. 29 It is interesting to study how the existence of CO2 in RTILs affects the diffusivity of these molecules.…”