Quenching of triplet states of aromatic hydrocarbons by quinones due to favourable charge-transfer interactions. Part 2.—Solvent effects

Abstract

Rate constants for quenching of the triplet states of 1,2-benzanthracene and/or anthracene by 2,5-dimethyl-p-benzoquinone, 2-methyl-p-benzoquinone and p-benzoquinone have been measured in eight different solvents at room temperature. For those solvents with low dielectric constants, (< 10), i.e., toluene, cyclohexane, ethyl acetate and tetrahydrofuran, rate constants for quenching k_Q, relative to those for diffusion controlled reactions k_d, have been shown to depend on ΔG_CT, the free energy change expected for charge transfer to give a dielectrically non-relaxed CT state, in the same manner and to show the same correlation between log (k_Q/k_d) and ΔG_CT as found previously in similar studies for benzene as solvent. Quenching is relatively more efficient in polar solvents (acetonitrile, N,N-dimethyl-formamide and formamide) when ΔG_CT > –0.1 eV but again there is no correlation with static dielectric constant or with solvent reorientation energies, and it is suggested that quenching may result in these solvents from only partial charge transfer.