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

Abstract

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.

Badania spektroskopowe efektów podwójnej fluorescencji w wybranym związku z grupy 1,3,4-tiadiazoli w rozpuszczalnikach organicznych oraz roztworach wodnych

“…Moreover, it has been reported that these changes are associated with the deprotonation of resorcinyl ortho -OH (low pH) and protonation of the thiadiazole nitrogen (high pH) (Fig. 1A and 1C) [19]. These results are consistent with UV-Vis spectroscopic data obtained in this work.…”

“…The transition dipole moment values calculated for the FABT monomer in selected solvents are given in Table 1. The calculated distance between FABT dimers in an aqueous solution equals 3.71 Å [19], in Pr-OH, it is 3.79 Å. The exciton splitting theory-based calculations are consistent with the crystallographic data.…”

“…The Uv-Vis spectroscopic studies carried out in the pH range of 1 to 12 reveal that the most notable shifts in the positioning of the absorption maxima occur in the pH range similar to that of physiological pH [19]. Also, a comparison of the absorption spectrum of FABT recorded at pH = 7, with those recorded at both acidic and basic conditions shows a respective hipsochromic and batochromic shifts of the absorption maxima.…”

“…1C represents the cationic form of FABT, which according to crystallographic studies consists of the protonated thiadiazole ring, with the protonation occurring at the nitrogen atom (N3), which resides in close proximity to the resorcinyl -OH group [17]. Although the rotation of resorcinyl moiety is possible and may result in a conformation in which the -OH group resides near the thiadiazole sulphur atom, the crystallographic data suggests that the form 1C is more favourable [17,19] and occurs in acidic solutions (low pH values). All forms of FABT presented in Scheme 1 are planar and do not occur in other conformations.…”

“…Furthermore, through interaction with lipids, they influence the molecular organisation and other structural properties of biomembranes [18]. Various excited state-related effects, including the nature of the fluorescence emission properties of 1,3,4-tiadiazoles, were extensively studied, pointing at the differences between the dual fluorescence and double fluorescence phenomena [19,[41][42]. The most recent reports refer to 1,3,4-tiadiazoles as ligands in the synthesis of transition metal complexes with an implication for the antineurodegenerative activity [20,21].…”