7-Azaindole in the Gas Phase: Absorption, Luminescence, and the Mechanism of Long-Lived Luminescence

Absorption and luminescence spectra of 7-azaindole vapor were measured. The absorption spectrum in the range 240–300 nm had a broad band with a superimposed sequence of vibrational maxima. The luminescence spectrum exhibited two bands, one of which with λ_max = 305 nm belonged to fast fluorescence and the second of which with λ_max = 520 nm, to long-lived luminescence. The fluorescence spectrum with excitation near the 0–0-transition (λ_ex = 289 nm) was quasilinear. The fluorescence spectrum was diffuse at other excitation wavelengths. Long-lived luminescence decaying in the microsecond range was interpreted as luminescence of free radicals formed as a result of triplet–triplet annihilation. Excited states of free radicals were populated by nonradiative energy transfer from 7-azaindole triplet states into free-radical doublet states.