Indole molecules are chromophores of biologically important organic compounds. Nowadays, problems on determining the lifetime of triplet states of indole and its derivatives in different media and on studying the influence of the microenvironment on the relaxation processes of triplet states are important. The objectives of the present work were to reveal the mechanisms for the delayed luminescence and to measure the lifetime of the triplet states of these molecules in the gas phase.
Spectral and kinetic characteristics of delayed luminescence of N-methylindole and indole in the gas phase have been studied. The mechanisms of the delayed luminescence of indole and N-methylindole in the gas phase are essentially different. N-methylindole delayed luminescence represents P-type delayed fluorescence in the spectral region at λmax= 310 nm. Its spectrum approximately coincides with a fluorescence spectrum. Indole P-type delayed fluorescence has not been recorded, however, its delayed luminescence in the spectral region with a maximum at λmax = 535 nm has been observed and interpreted as luminescence of free radicals to be formed due to the N-H bond dissociation. It is supposed that the excited states of free radicals are populated due to nonradiative energy transfer from the triplet states of indole to the doublet states of free radicals. Calculated from the kinetics of the delayed luminescence lifetimes of the triplet states of N-methylindole and indole at Т = 373 К are 2.5 and 1 ms, respectively.