Abstract
X-band (9.5 GHz) time-resolved electron paramagnetic resonance (TREPR) spectra of a 1,9-acyl−alkyl biradical were obtained at room temperature in benzene and in liquid (950 psi) carbon dioxide (CO2) solutions. The spin exchange interaction (J) in this biradical is negative and larger in magnitude than the hyperfine interaction (q). This leads to the observation, in both solvents, of spin-correlated radical pair (SCRP) spectra which are net emissive. Spectra obtained at later delay times (>1.5 μs) in CO2 exhibit alternating intensities of their SCRP transitions due to spin relaxation but do not show any significant change in line width. The same effect is observed in benzene, but on a slower time scale. Q-band (35 GHz) experiments in benzene showed that the phenomenon was found to be both field and temperature dependent. It is also chain-length dependent, being much stronger in short biradicals (<C10). A Redfield theory analysis of the spin-state populations is presented and discussed that includes J modulation, electron dipole−dipole interaction modulation, and uncorrelated relaxation mechanisms (hyperfine and g-factor anisotropies). Using this model, simulation of the Q-band time dependence at 64 °C, along with a careful consideration of several relaxation parameters, leads to the conclusion that hyperfine-dependent J modulation relaxation, coupled with the dipolar mechanism and S−T- mixing, is responsible for the observed effects.
