title: Spectroscopic Studies of Fluorescence Effects in Bioactive 4-(5-Heptyl-1,3,4-Thiadiazol-2-yl)Benzene-1,3-Diol and 4-(5-Methyl-1,3,4-Thiadiazol-2-yl)Benzene-1,3-Diol Molecules Induced by pH Changes in Aqueous Solutions.
abstract: This paper presents the results of stationary fluorescence spectroscopy and time-resolved spectroscopy analyses of two 1,3,4-thiadiazole analogues, i.e. 4-(5-methyl-1,3,4-thiadiazol-2-yl)benzene-1,3-diol (C1) and 4-(5-heptyl-1,3,4-thiadiazol-2-yl)benzene-1,3-diol (C7) in an aqueous medium containing different concentrations of hydrogen ions. An interesting dual florescence effect was observed when both compounds were dissolved in aqueous solutions at pH below 7 for C1 and 7.5 for C7. In turn, for C1 and C7 dissolved in water at pH higher than the physiological value (mentioned above), single fluorescence was only noted. Based on previous results of investigations of the selected 1,3,4-thiadiazole compounds, it was noted that the presented effects were associated with both conformational changes in the analysed molecules and charge transfer (CT) effects, which were influenced by the aggregation factor. However, in the case of C1 and C7, the dual fluorescence effects were visible in a higher energetic region (different than that observed in the 1,3,4-thiadiazoles studied previously). Measurements of the fluorescence lifetimes in a medium characterised by different concentrations of hydrogen ions revealed clear lengthening of the excited-state lifetime in a pH range at which dual fluorescence effects can be observed. An important finding of the investigations presented in this article is the fact that the spectroscopic effects observed not only are interesting from the cognitive point of view but also can help in development of an appropriate theoretical model of molecular interactions responsible for the dual fluorescence effects in the analysed 1,3,4-thiadiazoles. Furthermore, the study will clarify a broad range of biological and pharmaceutical applications of these compounds, which are more frequently used in clinical therapies. Graphical Abstract Upper left corner - C7 molecule at high pH, right upper corner - fluorescence emission spectrum for C7 dissolved in H2O at high pH (7-12) - single fluorescence. Bottom left corner - C7 molecule at low pH (1-7), lower right corner - fluorescence emission spectrum for C7 dissolved in water at low pH - two fluorescence emissions. The circles indicate the group related to dissociation of molecules at low and high pH and the additional long circles indicate C1 or a molecule with a shorter acyl chain.
authors.affiliation: Department of Biophysics, University of Life Sciences in Lublin, Akademicka 13, 20-950, Lublin, Poland. arkadiusz.matwijczuk@up.lublin.pl.
authors.forename: Arkadiusz
authors.initials: A
authors.lastname: Matwijczuk
authors.affiliation: Department of Cell Biology, Institute of Biology, Maria Curie-Skłodowska University, 20-033, Lublin, Poland.
authors.forename: Dariusz
authors.initials: D
authors.lastname: Kluczyk
authors.affiliation: Department of Physical Biochemistry, Faculty of Biochemistry, Biophysics and Biotechnology of the Jagiellonian University, Gronostajowa 7, 30-387, Krakow, Poland.
authors.forename: Andrzej
authors.initials: A
authors.lastname: Górecki
authors.affiliation: Institute of Industrial Organic Chemistry, Annopol 6, 03-236, Warsaw, Poland.|Department of Chemistry, University of Life Sciences in Lublin, 20-950, Lublin, Poland.
authors.forename: Andrzej
authors.initials: A
authors.lastname: Niewiadomy
authors.affiliation: Department of Cell Biology, Institute of Biology, Maria Curie-Skłodowska University, 20-033, Lublin, Poland. mariusz.gagos@poczta.umcs.lublin.pl.
authors.forename: Mariusz
authors.initials: M
authors.lastname: Gagoś
keywords: 1,3,4-thiadiazole; Dual fluorescence effects; Molecular aggregation; Molecular spectroscopy; Substituent effects