DYNAMICS OF C-3(60) PROBED BY ELECTRON-PARAMAGNETIC RESONANCE - MOTIONAL ANALYSIS IN ISOTROPIC AND LIQUID-CRYSTALLINE MATRICES

Detailed analysis of time-resolved electron paramagnetic resonance (EP R) line shapes of photoexcited triplet states of C60 in isotropic and anisotropic matrices has been carried out. Two main motional models ha ve been tested over a very wide temperature range. For most temperatur es, the best fit to the experimental line shapes was obtained with a m odel of discrete jumps between two configurations, which can be formal ly defined as an exchange between the longitudinal (Z) dipolar axis an d one of the two transverse axes (X,Y). This exchange process is proba bly related to two distorted Jahn-Teller (JT) configurations of the mo lecule. For very high temperatures, the best fit was obtained with a r ecently developed model of Brownian rotational diffusion. In the latte r cases, relating to the liquid phase of the solvent, the triplets app ear to be in thermal spin equilibrium, and their kinetics can be descr ibed by a special form of transient nutations. In the former cases, re lated to the glass and amorphous phases (toluene) or nematic phase (E- 7), the triplets are clearly spin polarized. For very low temperatures the motional (jump) rate is practically temperature independent. This seems to be due to the nature of the motion, i.e., exchange between J T configurations, rather than actual molecular motion.