Dynamics of Spin-Correlated Radical Pairs in Non-Ionic Surfactant Solutions

Abstract

Time-resolved (CW) electron paramagnetic resonance spectroscopy (TREPR) has been used to investigate the diffusional dynamics of radical pairs created in micelles made from nonionic, poly(ethylene glycol) (PEG)-based surfactants (Brij-35, Triton X-100, and Cremophor EL). Photoexcitation of solubilized perdeuterated benzophenone (BP) at 308 nm leads to hydrogen atom abstraction by triplet BP from the surfactant chains. Spectral simulation and comparison to other PEG radicals shows that H-atom abstraction is taking place in the PEG outer shell of Brij-35 and Triton X-100 micelles rather than in the alkyl chain core. The TREPR spectra exhibit a temperature-dependent superposition of chemically induced electron spin polarization (CIDEP) patterns. In Brij-35 solutions, radical pair mechanism (RPM) multiplet polarization is predominant at room temperature, but strong spin-correlated radical pair (SCRP) polarization is observed at temperatures above 40 °C. In Triton X-100, the triplet mechanism (TM) dominates at all temperatures and delay times, with some evidence of S-T - SCRP polarization at longer delay times. The results are discussed qualitatively in terms of a spectral exchange model that uses spin product operators. Using known micellar dimensions, the diffusion coefficient of the radicals in the micelle interior and their escape rates can be estimated. Radicals of identical structure produced from pure PEG and aqueous PEG solutions show ordinary RPM polarization at all temperatures, and these spectra are used to confirm the photochemical mechanism and to provide characterization data (hyperfine couplings) for the radicals. The BP/Cremophor EL system was TREPR silent most likely because of triplet quenching impurities.