Laser photolysis of indole in cyclohexane

Abstract

Indole in cyclohexane was studied by nanosecond laser photolysis. The fourth harmonic (265 nm, 30 ns FWHM) of a Q-switched Nd 3+-doped glass laser was used for excitation; transient absorption changes were measured by kinetic spectrophotometry. Transient absorption spectra appearing after excitation were attributed to the indole triplet state and to the indole radical, respectively. Extinction coefficients of the indole triplet were determined using triplet naphthalene as reference. From the decay kinetics, a triplet lifetime of 16 μs was obtained at low triplet concentrations. TT annihilation was observed at higher triplet concentrations, with a rate constant of 7 X 10 9 s −1 M −1. It was shown that the radical is formed in a one-photon process from the excited singlet state. A complementary study of 1-methylindole indicated that the radical is created by dissociation at the NH bond. Extinction coefficients of the indole radical were also determined. A study of indole in methylcyclohexane as a function of temperature indicated that radiationless transition from the excited singlet state to the ground state is not significant.