Strong and pH dependent fluorescence in unprecedented anthra[2,3-d]imidazole derivatives

We here report the synthesis and characterization of three new molecules based on an unprecedented anthra [2,3-d]imidazole unit. The new heterocycle was unexpectedly formed while attempting to obtain 2,3-diacetamidoanthracene by reducing the corresponding anthraquinone molecules by sodium borohydride. The same reaction was successfully performed using two further different 2,3-diamidoanthraquinone derivatives, thus confirming its generality. The chemical identity of the novel molecules was also proved by realizing single crystals suitable for XRD diffraction and solving the crystalline structure of one of the prepared systems. All the molecules are strong blue emitters in ethanol, with fluorescence quantum yields around 0.40. Because of the acid-base properties of anthraimidazole ring, the optical response was investigated at different pH values. While slight differences were observed in acidic conditions, in the alkaline pH range we observed a dramatic change of the fluorescence spectra in a narrow pH range (from 11 to 13), with the color of the emission turning from blue to green. This feature makes the new class of derivatives here reported very promising as pH sensors working in strongly alkaline conditions.