Isotropic reorientations of fullerene C₇₀ triplet molecules in solid glassy matrices revealed by light-induced electron paramagnetic resonance Available to Purchase

Continuous-wave X-band electron paramagnetic resonance (EPR) of fullerene C₇₀ molecules excited to a triplet state by continuous light illumination was studied in molecular glasses of o-terphenyl and cis/trans-decaline and in the glassy polymers polymethylmethacrylate (PMMA) and polystyrene (PS). Above ∼100 K, a distinct narrowing of EPR lineshape of the triplet was observed, which was very similar for all systems studied. EPR lineshape was simulated reasonably well within a framework of a simple model of random jumps, which implies that the C₇₀ molecule performs isotropic orientational motion by sudden jumps of arbitrary angles. In simulations, a single correlation time τ_c was used, varying in the range of 10⁻⁷−10⁻⁸ s. Near and below 100 K electron spin echo (ESE) signals were also obtained which were found to decay exponentially. Correlation times τ_c obtained from simulation of the EPR spectra in the slow-motion limit (τ_c close to 10⁻⁷ s) turned out to be in good agreement with the phase memory times T_M of the ESE decay, which additionally supports the employed simple model. The observed motional effects provide evidence that the nanostructure of the solid glassy media of different origins is soft enough to allow a large asymmetric C₇₀ molecule to reorient rapidly. Except for the EPR spectra of the triplet, in the center of the spectra, a small admixture of a narrow line was also observed; its possible nature is briefly discussed.