New insights on the 7-azaindole photophysics: the overlooked role of its non phototautomerizable hydrogen bonded complexes.

In this paper we explore the formation and the photophysical properties of the scarcely studied open hydrogen bonded aggregates of 7-Azaindole, 7AI. Thus, we have analyzed the influence that the increase of the 7AI concentration and the decrease of the temperature have on the 7AI photophysics. To help the interpretation of the results, the 7AI-Pyridine system has been used as the model for the analysis of the photophysical properties attributable to the open N(pyrrolic) - H · · · N(pyridinic) hydrogen bonded aggregates. Also, the hydrogen bond interactions have been studied by means of the atom in molecule approach from the Bader theory. Experimental and theoretical results support that the formation of open hydrogen bonded aggregates, (-7AI-)n with n ≥ 2 can efficiently compete with that of the profusely studied centro-symmetric cyclic dimer (7AI)2. Moreover, these aggregates suffer a proton-driven electron transfer process that strongly quenches their room temperature fluorescence and, therefore, masks their presence in the 7AI solutions. Therefore, because most of the studies on the 7AI photophysics have been interpreted without considering the existence of such aggregates and, more important, ignoring its quenching process, many conclusions obtained from these studies should be carefully revised.