Quenching of triplet states of aromatic hydrocarbons by quinones due to charge-transfer interactions

Abstract

The rate constants for quenching of the triplet states of seven hydrocarbons by benzoquinone, toluquinone and xyloquinone have been measured in benzene solution at room temperature using the technique of nanosecond laser photolysis. The pairs of molecules were selected to exclude the possibility of electronic energy transfer and no transients were detected resulting from the quenching reaction. The quenching rate constants, k_Q, which for the most efficient cases reach 10¹⁰ dm³ mol^–1 s^–1, were shown to increase with the increasing electron affinity of the quinone and decreasing ionisation potential of the triplet state being quenched. Various free energy relationships are examined in order to explain the dependence of log k_Q on ΔG^⊖_ct, the standard free energy change expected for charge transfer in benzene as solvent.