Tautomerism, electronic structures, and electronic spectra of indolo[2,3-b]quinoxaline and its derivatives

Abstract

The tautomerism, electronic spectra, and electronic structures of the ground and excited states of indolo [2,3-b]quinoxaline and its 2,3- and 1,4-dibutoxy derivatives were investigated by the Pariser-Parr-Pople (PPP) method. It is shown that these compounds exist primarily in the 6H form; the long-wave sππ* transition is due to transfer of π charge from the indole fragment to the quinoxaline fragment, and in the first excited state many of the bonds in the quinoxaline fragment are loosened significantly. The effect of butoxy substituents on the first sππ* transition of indolo [2,3-b] quinoxaline was analyzed by means of perturbation theory within the framework of the Huckel MO method.