PROBING THE EXCHANGE INTERACTION THROUGH MICELLE SIZE .1. PROBABILITY OF RECOMBINATION OF TRIPLET GEMINATE RADICAL PAIRS

The probability of recombination (P(r)) of the primary geminate radical pairs derived from optically active methyldeoxybenzoin (MDB) and from diastereomerically pure 2,4-diphenylpentan-3-one (DPP) have been determined in alkyl sulfate micelles of different sizes. These probabilities have been measured by monitoring the extent of isomerization in the recovered ketone as a function of conversion. The P(r) values for these two ketones, as a function of micelle size, display disparate behavior: P(r) for MDB increases as the micelle size increases, while P(r) for DPP decreases as the micelle size increases. Simple kinetic models which neglect distance-dependent interactions fail, even qualitatively, in predicting this trend. A theoretical treatment which explicitly considers (1) a distance-dependent electron spin exchange interaction (ESE), (2) micelles with a permeable boundary and (3) a coefficient of mutual diffusion that is a function of the micelle size is presented. The permeability of the micelle boundary is treated by introduction of a boundary factor in an improved theoretical model. This adjustment allows us to model radical escape as only occuring from the boundary and does not force us to consider it as a site-independent monoexponential process. Experimental evidence for a micelle size-dependent coefficient of mutual diffusion is presented. Reasonable fits for MDB and DPP, at both the qualitative and quantitative levels, are obtained using this model; omission of any one of the three parameters during the fitting procedure results in an unacceptable deterioration in the quality of the match between the measured and the calculated values. The qualitative result is that there is an increase in the effectiveness of the ESE in suppressing intersystem crossing as the micelle size decreases. For smaller micelles this results in the rate of intersystem crossing becoming the rate-limiting step. The rate of intersystem crossing in these radical pairs is determined not by pure hyperfine interactions but rather by hyperfine interactions modulated by the distance-dependent ESE.