TIME-RESOLVED EPR STUDY OF A 1.9-FLEXIBLE BIRADICAL DISSOLVED IN LIQUID CARBON-DIOXIDE - OBSERVATION OF A NEW SPIN-RELAXATION PHENOMENON - ALTERNATING INTENSITIES IN SPIN-CORRELATED RADICAL PAIR SPECTRA

X-band (9.5 GHz) time-resolved electron paramagnetic resonance (TREPR) spectra of a 1,9-acyl-alkyl biradical were obtained at room temperatu re in benzene and in liquid (950 psi) carbon dioxide (CO2) solutions. The spin exchange interaction (J) in this biradical is negative and la rger in magnitude than the hyperfine interaction (q). This leads to th e observation, in both solvents, of spin-correlated radical pair (SCRP ) spectra which are net emissive. Spectra obtained at later delay time s (>1.5 mu s) in CO2 exhibit alternating intensities of their SCRP tra nsitions due to spin relaxation but do not show any significant change in line width. The same effect is observed in benzene, but on a slowe r time scale. Q-band (35 GHz) experiments in benzene showed that the p henomenon was found to be both field and temperature dependent. It is also chain-length dependent, being much stronger in short biradicals ( (Clo). A Redfield theory analysis of the spin-state populations is pre sented and discussed that includes J modulation, electron dipole-dipol e interaction modulation, and uncorrelated relaxation mechanisms (hype rfine and g-factor anisotropies). Using this model, simulation of the Q-band time dependence at 64 degrees C, along with a careful considera tion of several relaxation parameters, leads to the conclusion that hy perfine-dependent J modulation relaxation, coupled with the dipolar me chanism and S-T- mixing, is responsible for the observed effects.