‘Excimer’ fluorescence - VIII. Lifetime studies of 1,6-dimethyl naphthalene

Observations were made by the pulsed light source method of the fluorescence lifetimes of 1, 6-dimethyl naphthalene solutions of 10 -4 to 6⋅4 M (pure liquid) in n -heptane from 200 to 295°K, and in cyclohexane and phenylcyelohexane at 295°K. The design and characteristics of discharge lamps containing various gases are described. For dilute solutions up to ca . 1M the lifetime data are consistent with the established monomer/excimer reaction scheme with rate parameters which are independent of concentration. For more concentrated solutions the rate parameters, notably those of internal quenching, are concentration dependent. In dilute n -heptane solution (and in the pure liquid, for which the values are in parentheses) the respective monomer and excimer rate parameters are k fM = 5x10 6 s -1 and k fD = 1⋅4 x 10 6 s -1 (2⋅2 x 10 6 s -1 ) for fluorescence, and k 0 iM = 11⋅6 x 10 6 s -1 and k 0 iD = 5⋅1 x 10 6 s -1 (4⋅2 x 10 6 s -1 ) for the temperature-independent component of intersystem crossing. The respective monomer and excimer temperature-dependent intersystem crossing rate parameters, which are influenced by solvent-solute interactions, have frequency factors of k ' iM = 2⋅7 x 10 8 s -1 and k ' iD = 6⋅0 x 10 9 s -1 (3⋅2x10 10 s -1 ) and activation energies of W iM = 0⋅11 eV and W iD = 0⋅16 eV (0⋅19 eV). Evidence is presented that k fM / k 0 iM = k fD / k 0 iD and that the excimer triplet state is dissociated. The excimer binding energy of B = 0⋅26 ± 0⋅05 eV agrees with previous spectral data.