Time-resolved ESR examination of a simple supramolecular guest-host system. Electron spin exchange interaction in micellized spin-correlated radical pairs

Abstract

The electron spin exchange interaction between micellized spin-correlated radical pairs (SCRP) is investigated by time-resolved electron spin resonance (TRESR) spectroscopy. On the basis of the SCRP model proposed independently by Closs and McLauchlan, an effective exchange interaction (Jeff) can be extracted either from a direct reading of the observed TRESR spectra or through spectral simulation. The results for a series of micellized radical pairs (of similar electronic and magnetic structure) show that the Jvalues are a function of the effective micelle size experienced by a given micellized radical pair and also a function of the radical pair molecular structure for a given micelle size. The values of Jeff can be systematically manipulated by varying the size of the micelles and/or the structure of the radical pairs. The results are shown to be consistent with a simple model of the structure of the micelle which includes the relationship between the magnitude of the exchange interaction between the fragments of the micellized radical pair and the micellar parameters such as the frequency of geminate pair reencounters, the relative diffusion coefficient of the fragments in the micelles, and the effective volume of the micelle explored by the pair during the time of observation. It is shown that a micellized radical pair, because of interactive spin and molecular dynamics, constitutes a simple and useful model of a guest-host supramolecular structure so that variations of the micelle size and the radical