ELECTRON-SPIN POLARIZATION IN RADICAL TRIPLET PAIRS - SIZE AND DEPENDENCE ON DIFFUSION

Using laser flash irradiation and time-resolved EPR spectroscopy, the chemically induced electron polarization (CIDEP) is investigated, which is generated by interaction of persistent 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radicals with triplet excited benzophenone in 1,2-epoxypropane solution. Net emission superposed by a weak emission/absorption multiplet type polarization in the TEMPO radical system is observed and analyzed quantitatively at temperatures between 193 and 298 K. With decreasing temperature the relative diffusion coefficient of the particles decreases from 5.9 x 10(-5) to 0.9 x 10(-5) cm2/s, and the absolute values of net and multiplet CIDEP increase from 0.6 to 9.0 and from 0.07 to 0.26, respectively, in units of Boltzmann polarization. The CIDEP is attributed to mixing and splitting of doublet and quartet spin states in radical-triplet pairs. A theoretical model is outlined, which quantitatively describes the size of polarizations as well as their dependence on diffusion. It indicates that the CIDEP is generated predominantly in regions where the exchange interaction is smaller than the Zeeman energy and that the multiplet polarization is diminished effectively by fast T2 relaxation of the triplet spin. For an exchange interaction J = J0 exp(-alpha(r - d)), decaying exponentially with interparticle distance r, \J0\d/alpha = 4.4 x 10(-6) cm2/s is determined from the net polarization, d being the distance of closest approach. The magnitude of the multiplet polarization allows an estimation of J0 almost-equal-to 5 X 10(9) rad/s and alpha almost-equal-to 8 X 10(7) cm-1, i.e., a surprisingly small doublet-quartet splitting at distance d.