Triplet state kinetics of buckminsterfullerene and carbon(70) in solution

Lifetimes of the lowest triplet electronic states of C{dollar}sb{lcub}60{rcub}{dollar} and C{dollar}sb{lcub}70{rcub}{dollar} have been studied in room temperature benzene solution. The lowest energy triplet state (T{dollar}sb1{dollar}) of each species was formed by intersystem crossing from a laser-excited singlet state. Transient triplet-triplet absorption spectroscopy was then used to measure the decay kinetics of the triplet state. The data were analyzed using an expanded kinetic model of C{dollar}sb{lcub}60{rcub}{dollar} and C{dollar}sb{lcub}70{rcub}{dollar} triplet relaxation in solution. This model includes not only unimolecular {dollar}rm Tsb{lcub}1{rcub}rightarrow Ssb0{dollar} radiationless relaxation, but also three bimolecular quenching processes: triplet-triplet annihilation, oxygen quenching and self-quenching. Under experimental conditions that suppress these bimolecular quenching processes, the exponential lifetimes of T{dollar}sb1{dollar} are found to be {dollar}rm 133 mu s for Csb{lcub}60{rcub} and >2.2 ms for Csb{lcub}70{rcub}{dollar}. Triplet C{dollar}sb{lcub}60{rcub}{dollar} and C{dollar}sb{lcub}70{rcub}{dollar} are self-quenched with rate constants of {dollar}rm 1.5 x 10sp{lcub}7{rcub} Msp{lcub}-1{rcub}ssp{lcub}-l{rcub} and 8.5 x 10sp{lcub}7{rcub} Msp{lcub}-1{rcub}ssp{lcub}-l{rcub}{dollar}, respectively. Triplet-triplet annihilation occurs with bimolecular rate constants of {dollar}rm 5.4 x 10sp{lcub}9{rcub} Msp{lcub}-l{rcub}ssp{lcub}-l{rcub} for Csb{lcub}60{rcub} and 1.2 x 10sp{lcub}10{rcub} Msp{lcub}-l{rcub}ssp{lcub}-l{rcub} for Csb{lcub}70{rcub}{dollar}. Mixed solutions of C{dollar}sb{lcub}60{rcub}{dollar} and C{dollar}sb{lcub}70{rcub}{dollar} are found to exhibit unusual kinetics. A kinetic model featuring efficient and reversible electronic excitation transfer between nearly isoenergetic triplet states is quantitatively successful in accounting for the observed behavior. Energy transfer rate constants for each energy transfer step have been determined from data spanning a range of sample compositions. The deduced values are {dollar}rm 2.4 x 10sp{lcub}9{rcub} Msp{lcub}-l{rcub}ssp{lcub}-l{rcub}{dollar} for energy transfer from {dollar}rm Csb{lcub}60{rcub}(Tsb{lcub}1{rcub}) to Csb{lcub}70{rcub}(Ssb{lcub}0{rcub}) and 2.1 x 10sp{lcub}9{rcub} Msp{lcub}-1{rcub}ssp{lcub}-1{rcub}{dollar} for energy transfer from {dollar}rm Csb{lcub}70{rcub}(Tsb{lcub}1{rcub}) to Csb{lcub}60{rcub}(Ssb{lcub}0{rcub}).{dollar} These values imply that T{dollar}sb1{dollar} excitation requires only 103 {dollar}pm{dollar} 30 cm{dollar}sp{lcub}-1{rcub}{dollar} more energy for C{dollar}sb{lcub}60{rcub}{dollar} than for C{dollar}sb{lcub}70{rcub}{dollar}. The long unimolecular lifetimes and rapid energy pooling may permit more efficient scheme for fullerene photochemistry and optical limiting.