A precise determination of the triplet energy of carbon (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 C{sub 60}, the newly discovered spheroidal allotrope of carbon. Excitation at 510 nm, in the long-wavelength absorption band of C{sub 60}, produces C{sub 60}(T{sub 1}) with unit efficiency; in the presence of dioxygen, triplet C{sub 60} is readily quenched by energy transfer. Photoacoustic waves were recorded for C{sub 60} 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 C{sub 60}(T{sub 1}) and the second of which relates to its decay. The recovered heat-deposition parameters are {phi}{sub 1} = 0.359 {plus minus} 0.005 and {phi}{sub 2} = 0.237 {plus minus} 0.011; these correspond to the fraction of the absorbed photon energy that is released in forming C{sub 60}(T{sub 1}) and in the quenching of C{sub 60}(T{sub 1}) by dioxygen. Since the quantum yield for intersystem crossing of C{sub 60}, from S{sub 1} to T{sub 1}, is unity, {phi}{sub 1} corresponds to a C{sub 60}(T{sub 1}) energy of 36.0 {plus minus} 0.6 kcal mol{sup {minus}1}. Since the energy of C{sub 60}(T{sub 1}) is defined bymore » {phi}{sub 1} and the energy of O{sub 2}({sup 1}{Delta}) is known {phi}{sub 2} is used to calculate a singlet oxygen sensitization quantum yield of 1.01 {plus minus} 0.03. The lifetime of C{sub 60}(T{sub 1}) in argon-saturated toluene is found to be > 10 {mu}s, and in air-saturated toluene, to be 290 {plus minus} 40 ns.« less