Delayed luminescence of indole derivatives in the gas phase

Spectral and kinetic characteristics of delayed luminescence of indole derivatives in the gas phase have been studied. The mechanism for delayed luminescence has been shown to depend on the disposition of the two lowest excited singlet ππ*-states, ¹L_a and ¹L_b. For vapors of 3-methyl-and 2,3-dimethylindole having a small ¹L_a-²L_b energy gap ΔE (ΔE < 500 cm⁻¹), prolonged light emissions with maxima at 525 and 540 nm, respectively, have been observed and interpreted as luminescence of free radicals formed due to dissociation of an N-H bond. For vapors of 5-methoxyindole having ΔE ≈ 3700 cm⁻¹, annihilation of delayed luminescence has been observed. The lifetimes for triplet states of 3-methyl-, 2,3-dimethyl-, and 5-methoxyindole at T = 373 K equal to 30, 26, and 65 μsec, respectively, were calculated from the kinetics of the delayed luminescence. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 4, pp. 503–508, July–August, 2008.