Spectroscopic and theoretical studies of fluorescence effects in bio-active: 4-(5-(methyl-1,3,4-thiadiazol-2-yl))benzene-1,3-diol and 4-(5-(methylamino-1,3,4-thiadiazol-2-yl))benzene-1,3-diol compounds: Effect of molecular aggregation and amino group position

• Published in: Journal of luminescence 2018-09, Vol.201, p.44-56
• Main Authors: Matwijczuk, Arkadiusz, Karcz, Dariusz, Pustuła, Katarzyna, Makowski, Marcin, Górecki, Andrzej, Kluczyk, Dariusz, Karpińska, Monika M., Niewiadomy, Andrzej, Gagoś, Mariusz
• Format: Article
• Language: English
• Subjects: 1,3,4-thiadiazole; Dual fluorescence effects; Molecular aggregation; Molecular spectroscopy; Substituent effects (-NH- group)
• ISSN: 0022-2313; 1872-7883
• DOI: 10.1016/j.jlumin.2018.04.003

This work deals with the results of fluorescence studies of 4-(5-(methylamino-1,3,4-thiadiazol-2-yl))benzene-1,3-diol (MATB) and 4-(5-(methyl-1,3,4-thiadiazol-2-yl))benzene-1,3-diol (MTB) in an aqueous solution at various pH. The aqueous solutions of both compounds analysed exhibited interesting fluorescence effects: dual fluorescence for MTB and two clearly separated distinct (partially overlapping) fluorescence emissions for MATB. Such fluorescence effects were observed in the aqueous solutions at pH 1–7 for MTB and pH 1–5 for MATB. At higher pH values, single fluorescence emission band were observed for both compounds. Based on the results reported to date, it was concluded that the observed fluorescence effects were mainly influenced by aggregation factors (induced by concentration changes, conformational effects or other interactions, e.g. formation of hydrogen bonds) and charge transfer effects (pH-induced and structure (substituent)-dependent). The dual fluorescence effect was noted for the analysed MTB compound (which does not have the –NH– secondary amine group), whereas two distinctly separated fluorescence bands were observed for the MATB analogue (which has the secondary amine group). The results presented in this paper highlight the impact of the secondary amine moiety on the fluorescence effects observed a long the range of solvents tested. The experimental research is accompanied by density functional theory-based (DFT) molecular modelling, which provide some hypothesis of the microscopic origins of the phenomena observed. Upper left corner –MTB molecule (without the NH group – only with the CH3 group), lower left corner – fluorescence emission spectra for MTB in the pH range from 1 to 7 (dual fluorescence effect). Upper right corner –MATB molecule (with the NH and CH3 groups), lower right corner – fluorescence emission spectra for MATB at pH from 1 to 7 (two clearly overlapping fluorescence emissions). [Display omitted]