Pulse-shift measurement: a direct observation of radical pair dynamics in a micelle

Abstract

The hydrogen abstraction reactions of benzophenone in a micellar solution in the absence and presence of 1,4-cyclohexadiene are compared with time-resolved electron spin resonance (ESR) and optically detected (OD) ESR. “Pulse shift” measurement by OD-ESR, which observes the effect of a resonant microwave pulse at different delay times after laser excitation, reveals that the lifetime of the radical pair becomes much shorter in the presence of 1,4-cyclohexadiene. This explains the change of chemically induced dynamic electron polarization from spin-correlated radical pair polarization in the absence of 1,4-cyclohexadiene to conventional, E*/A polarization in its presence. The rate constants determined by this technique indicate that the escaping rate of cyclohexadienyl radical is ten times larger than those of alkyl and benzophenone ketyl radicals.