Spectroscopic Studies of Dual Fluorescence in 2‑((4-Fluorophenyl)amino)-5-(2,4-dihydroxybenzeno)-1,3,4-thiadiazole

The paper presents results of fluorescence analysis of ionic and nonionic 2-((4-fluorophenyl)­amino)-5-(2,4-dihydroxybenzeno)-1,3,4-thiadiazole (FABT) in monocrystals and solutions. We found a single fluorescence band in the case of FABT crystals grown in methanol and dual fluorescence for FABT crystals grown in an aqueous environment. The effect of dual fluorescence was preserved for FABT dissolved in aqueous solutions with pH ranging from 7.5 to 1. In contrast, FABT dissolved in methanol exhibited a single fluorescence band. The dual fluorescence effect is associated with conformational changes in the FABT molecule, which can be induced by aggregation effects. On the basis of crystallographic data, two types of FABT crystal molecule conformations were distinguished. In methanol, FABT molecules are in conformation “S” (the −OH group from the resorcyl ring oriented toward the sulfur atom from the 1,3,4-thiadiazole ring), which a gives single fluorescence band. In water, FABT in conformation “N” (the −OH group from the resorcyl ring oriented toward the nitrogen atom from the 1,3,4-thiadiazole ring due to 180° rotation) has two fluorescence bands. This significant finding implies the possibility of performing a rapid analysis of conformational changes in FABT molecules using fluorescence spectroscopy both in solutions and in biological samples