SPIN DYNAMICS OF C-60 TRIPLETS

A study of the photoexcited triplet state of C-60 (C-3(60)) in liquid solution (toluene, methylcyclohexane) was made using pulsed-EPR techni ques. The spin-lattice relaxation time (T-I), phase-memory time (T-m), and line width (Delta v(1/2)) were measured as function of temperatur e (170-360 K). The results were used to analyze some aspects of the sp in dynamics of C-3(60). The data confirm earlier findings that the rel axation toward Boltzmann equilibrium is responsible for the initial gr owth of the signal amplitude when FT-EPR spectra are measured as a fun ction of delay time between laser excitation and pi/2 microwave pulse. Also, it is found that Delta v(1/2)approximate to 1/pi T-m in both so lvents over the whole temperature range. This establishes that there i s a negligible contribution to the line width from unresolved C-13 hyp erfine structure. The contribution of the electron spin-spin interacti on modulated by rotational motion to the relaxation rates is analyzed. The analysis shows that the value of the zero-field splitting paramet er D, which determines the magnitude of this contribution, is about an order of magnitude less than that found in rigid matrices at low temp erature. A significant residual, temperature-independent, relaxation c ontribution is found which may stem from the rapid interconversion bet ween triplet states that causes the reduction in D upon going from sol id state to liquid solution.