C-70 TRIPLET EXCIMERS - EVIDENCE FROM TRANSIENT ABSORPTION KINETICS

Time-resolved absorption measurements on solutions of pure C-70 in toluene reveal complex kinetics that vary strongly with sample concentration and probe wavelength. These observations suggest efficient association between triplet and ground state C-70 molecules to form triplet excimers. Decay data measured with sample concentrations near and below 1 mu M show that the intrinsic lifetime of the C-70 tripler monomer is at least 12 ms. Data at higher sample concentrations are fit by a kinetic model in which triplet monomers rapidly preequilibrate with shorter-lived excimers, causing strong apparent self-quenching of the triplet excitation. The rate constants for excimer formation, dissociation, and deactivation in this model are deduced to be approximately 10(9) M(-1) s(-1), 10(5) s(-1), and 10(4) s(-1), respectively. Because the equilibrium constant far association lies near 10(4) M(-1), many photoexcited solutions of C-70 may be expected to contain not only monomer, as normally assumed, but also significant concentrations of excimer.