TREPR spectra of micelle-confined spin correlated radical pairs: I. Molecular motion and simulations

Abstract

Radical pairs created by the photoreduction of benzophenone (BP) in sodium dodecyl sulfate (SDS) micelles exhibit strong asymmetry in the line shapes of their time-resolved electron paramagnetic resonance (TREPR) signals. The asymmetry is strongly dependent on the temperature from 16 °C to 66 °C. Simulations of the anti-phase structure (APS) line shape of these spin correlated radical pairs (SCRPs), based on a numerical solution of the Stochastic Liouville Equation with the spin exchange interaction depending exponentially on the distance between radicals, are presented and discussed. The proposed model takes into account the diffusive motion of the radicals along with the motion of the transverse magnetization and accounts satisfactorily and self-consistently for the asymmetry of the observed TREPR signals.

Investigation of Liquid‐Phase Inhomogeneity on the Nanometer Scale Using Spin‐Polarized Paramagnetic Probes

“…Moreover, it was not clear if the importance of the compartment size arises from the space dependence of the exchange and dipolar interactions or on other factors. It must be underscored that the line widths of radical pairs were found [82,84,85] to be noticeably larger than those of the "escaped" ones, despite both radicals, paired and escaped, being located in the same micellar phase. This difference in line widths was found to be dependent on micellar size as well.…”

“…For example, rather surprisingly, J eff was found to depend on temperature [81,82] and on the chemical structure [83] of the detergent making up the micelle. The model failed to explain the dependence of the width of the spectral lines on micelle's size [82,84,85] or on the length of the hydrocarbon tether [86] connecting two paramagnetic centers in biradicals. Moreover, it was not clear if the importance of the compartment size arises from the space dependence of the exchange and dipolar interactions or on other factors.…”

“…Despite some rather simple but necessary simplifications, the model is capable of reproducing all of the key features of the experimental spectra and all of the observed temperature effects. Specifically, these are [82,91]: (1) increasing the temperature causes broadening of the APS components, (2) the strong line shape asymmetry in the APS spectral pattern depends on the q values, (3) narrowing of the central portion of the spectra takes place with increased temperature, and (4) narrowing of the line widths of the escaped alkyl radicals is observed. The parameters used in the calculations are presented in Table 3.…”

“…Temperature dependence: photoreduction of benzophenone in SDS micelles [82,96] From the point of view of the CFN model and QSA, the temperature dependence of the APS is quite unexpected. Indeed, the QSA operates in terms of the distance dependent on exchange potential and always in regard to a particular distribution of radicals at the time of observation.…”

Electron Spin Exchange in Linked Phenothiazine–Viologen Charge Transfer Complexes Incorporated in “Through‐Ring” (Rotaxane) α‐Cyclodextrins

“…Simulation of the line positions gives the magnitude of the J coupling, and the time dependence of the signals can provide information about radical reaction rates, spin relaxation processes and the magnitude of J even when it exceeds the hyperfine value a I . The model is rigorously applicable to the rigid biradicaloids studied here . We use a home‐written visual BASIC program based on the CFN model .…”