Time-resolved absorption measurements on solutions of pure C-70 in tol
uene reveal complex kinetics that vary strongly with sample concentrat
ion and probe wavelength. These observations suggest efficient associa
tion between triplet and ground state C-70 molecules to form triplet e
xcimers. Decay data measured with sample concentrations near and below
1 mu M show that the intrinsic lifetime of the C-70 tripler monomer i
s at least 12 ms. Data at higher sample concentrations are fit by a ki
netic model in which triplet monomers rapidly preequilibrate with shor
ter-lived excimers, causing strong apparent self-quenching of the trip
let 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 photo
excited solutions of C-70 may be expected to contain not only monomer,
as normally assumed, but also significant concentrations of excimer.