Comparative investigations of the optical absorption, fluorescence, and phosphorescence of indole and its biologically active derivatives, L-tryptophan (Trp) and 6H-indolo2,3-bquinoxaline (IQX), were carried out. The spectral properties of these compounds in ethanol solution at room (300 K) and low (78 K) temperatures were analyzed. The positions of the first excited singlet (S1) and triplet (T1) electronic energy levels were determined. It was shown that the main center of optical absorption and emission in Trp corresponds to the indole π-electron system. For the tetracyclic compound IQX, the formation of the new optical center associated with the conjugation of the indole and quinoxaline fragments was observed.
Tkach et al. (Thu,) studied this question.