A PRECISE DETERMINATION OF THE TRIPLET ENERGY OF C60 BY PHOTOACOUSTIC CALORIMETRY

The relatively new technique of time-resolved, pulsed-laser photoacoustic calorimetry has been exploited to precisely determine the triplet-state energy of C60, the newly discovered spheroidal allotrope of carbon. Excitation at 510 nm, in the long-wavelength absorption band of C60, produces C60(T1) with unit efficiency; in the presence of dioxygen, triplet C60 is readily quenched by energy transfer. Photoacoustic waves were recorded for C60 in argon-saturated, air-saturated, and partially argon-saturated toluene solutions. Each experimental wave was then fit to a two-component model, the first component of which corresponds to production of C60(T1) and the second of which relates to its decay. The recovered heat-deposition parameters are phi-1 = 0.359 +/- 0.005 and phi-2 = 0.237 +/- 0.011; these correspond to the fraction of the absorbed photon energy that is released in forming C60(T1) and in the quenching of C60(T1) by dioxygen. Since the quantum yield for intersystem crossing of C60, from S1 to T1, is unity, phi-1 corresponds to a C60(T1) energy of 36.0 +/- 0.6 kcal mol-1. Since the energy of C60(T1) is defined by phi-1 and the energy of O2(1-DELTA) is known, phi-2 is used to calculate a singlet oxygen sensitization quantum yield of 1.01 +/- 0.03. The lifetime of C60(T1) in argon-saturated toluene is found to be > 10-mu-s, and in air-saturated toluene, to be 290 +/- 40 ns.