Spectroscopic study of the ground and excited state prototropic equilibria of 4-azaindole

摘要

The ground and singlet excited state prototropism of 4-azaindole, 4AI, in acid and basic aqueous solutions, inside and outside the pH range, has been systematically studied by using absorption and fluorescence spectroscopic techniques. These studies have thrown light on some interesting aspects on the nature and the photophysics of the 4-AI prototropic species. Thus, the changes of the 4AI absorption spectra reveal the existence of four ground state species; the pyridinic protonated cation, C (pKaC=7.5±0.1), the neutral molecule, N (pKaN=15.5±0.5), the pyrrolic deprotonated anion, A, and a previously unnoticed dication, DC (pKaDC=-4.6±0.4). Besides the emissions of these species, a new fluorescence profile appears in alkaline solutions at around 500 nm. This extra band has been ascribed to the neutral phototautomer, NT. What is more relevant to this study is the fact that the position and the intensity of the emission band assigned to the monoprotonated cation are very different from those observed for the normal cation of the 7-azaindole, 7-AI. This together with the fact that for the formation of the DC species a cationic precursor with a quinoid structure must be invoked, have prompted us to assign this cationic emission to the isomeric CI cations. Finally, the excited-state pK _as of the prototropic species of 4AI have been theoretically estimated by using the F?rster-Weller cycle.