Magnetic Field Effect on a Radical Pair Reaction as a Probe of Microviscosity

Abstract

The magnetic field effects (MFEs), caused by the ∆g mechanism, on the photoinduced hydrogen abstraction reaction of benzopheneone with thiophenol were investigated in alcoholic solutions of varying viscosities (η ) 0.55 to 59.2 cP) by a nanosecond laser flash photolysis technique. The escape yield of benzophenone ketyl radicals (Y) gradually decreased with increasing magnetic field strength (B) from 0 to 1.6 T. The relative yield observed at 1.6 T, R(1.6 T) ) Y(1.6 T)/Y(0 T), decreased with increasing η in the range of 0.55 cP e η e 5 cP, and then increased with increasing η in the range of 5 cP < η e 55.3 cP. When η was higher than 55.3 cP, the R(1.6 T) value became 1.0, and MFEs were completely quenched. The observed η dependence of the MFEs was analyzed by the stochastic Liouville equation (SLE), in which the effects of spin-orbit coupling by a heavy atom such as sulfur were taken into account. The observed MFEs were reproduced fairly well by the SLE analysis. The diffusion coefficients of the radicals obtained by the SLE were about three times smaller than those expected from the macroscopic solvent viscosities. One can probe the microviscosity in the vicinity of the radical pairs by observing MFEs on the present photochemical reaction system. IntroductionMagnetic field effects (MFEs) on photochemical reactions through radical pairs (RPs) and biradicals have received considerable attention during the past three decades. 1-3 Magnetic fields interact with the electron spins of RPs, and thus the spin conversion in the RPs is influenced by the fields. The lifetime of the RPs and the yield of the escaped radicals consequently show appreciable MFEs. In solution, the MFEs on the reactions of RPs have been interpreted in terms of the radical pair and triplet mechanisms (RPM and TM, respectively). According to the RPM, MFEs are observed during sequential steps as follows:(1) formation of close RPs through photochemical reactions with singlet (S) or triplet (T n , n ) 0, (1) spin multiplicity, (2) spin conversion between S and T n states in separated RPs, and (3) spin state selective recombination of the close RPs competing with the escape of radicals from the pairs. 2 These steps are promoted by the diffusion of the radicals by which the RPs are separated and re-encountered. Radical diffusion, therefore, has a pronounced effect on the magnitude of the observed MFEs. Large MFEs have been observed in confined and inhomogeneous systems such as micellar solutions, 2,3 highly viscous solutions, 4,5 nanotubes, 6 and very recently ionic liquids. 7 Molecular and radical diffusions are important processes that control chemical reactions in solutions. 8 Radical diffusion motion has been studied with the several techniques, including transient grating (TG), 9 photochemical space intermittency, 10,11 and electron paramagnetic resonance (EPR). 12,13 These techniques provide valuable information on radical diffusion in homogeneous solutions. Using the TG technique, for example, Terazima et al. found that photochemically genera...

Nanoscale Heterogeneous Structure of Ionic Liquid as Revealed by Magnetic Field Effects

“…In our previous paper, we estimated that the viscosity in the cage should be smaller than 5 cP from the results of the MFEs observed for the same reaction in alcoholic solutions. 17,23 These values are 1 order of magnitude smaller than the macroscopic viscosity of TMPA TFSA. Wishart and Castner et al reported that there are several microviscosities for ILs ranging from orders of magnitude smaller than the macroscopic viscosities to viscosities similar to or larger than the macroscopic viscosities.…”

Primary Photochemical Process of Thiobenzophenone as Studied by Laser Flash Photolysis

“…This value is ten times smaller than that observed for the similar reaction between 3 BP Ã and PhSH (1:2 Â 10 8 s À1 mol À1 dm 3 ) although the viscosity is slightly higher (37.3 cP) than that of the present study. 10 Such a small rate constant of 3 TBP Ã may be due to the lower triplet energy of 3 TBP Ã (166 kJ mol À1 ) than that of benzophenone (290 kJ mol À1 ).…”

“…In the present study, the self-quenching of 3 TBP Ã could successfully be suppressed in a viscous alcoholic solution of i-BuOH/c-HexOH, a SDS micellar solution, and an ionic liquid of TMPA TFSA. The reasons for suppressing the self-quenching of 3 TBP Ã may be as follows: high viscosity of the alcoholic solution, 10 cage effects with the high microviscosity of SDS micellar solution, [14][15][16][17] and cage effects with the low microviscosity of ionic liquid. 7,18,19 To elucidate the mechanism for suppressing the self-quenching, it is desirable to clarify the reaction field as a nanoscale structure.…”

Magnetic Field Effects on Radical Pairs in Homogeneous Solution

“…27 Studies on RIPs in mixed solvents have found evidence for the role of microheterogeneity in the solvent influencing the RP dynamics 28-31 and indeed MFEs have been used to measure solvent microviscosity in a range of solvents. 32 …”