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 AbstractUpper 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. Electronic supplementary materialThe online version of this article (doi:10.1007/s10895-017-2053-y) contains supplementary material, which is available to authorized users.

ESIPT-Related Origin of Dual Fluorescence in the Selected Model 1,3,4-Thiadiazole Derivatives

“…In polar solvents, the dual fluorescence emission may be induced by the aggregation, which in such conditions seems mandatory for the said dual fluorescence effects to occur. This hypothesis is supported by our previous findings from investigations into the behavior of the thiadiazole derivatives in polar solvents with varied “apparent pH” [ 1 , 2 , 3 , 4 , 5 ]. Protonation of the molecules due to changes in the environment resulted in the emergence of an additional band by blocking the possibility of a new hydrogen bond forming in the analyzed molecule.…”

“…The significance of compounds from the 2-amino-1,3,4-thiadiazole group [ 1 , 2 , 3 , 4 , 5 ] stems from their potential value in the fight against some of the most challenging diseases of the 21st century, including, e.g., cancer, neurodegenerative diseases, or various continuously mutating mycoses. These compounds show a wide spectrum of pharmacological properties, including neuroprotective [ 6 , 7 ], anti-cancer [ 8 ], antibacterial [ 9 ], antioxidative [ 10 , 11 ], and antimycotic [ 12 , 13 ] effects.…”

Spectroscopic and theoretical studies of fluorescence effects induced by the ESIPT process in a new derivative 2-Hydroxy-N-(2-phenylethyl)benzamide – Study on the effects of pH and medium polarity changes

“…The effects related to the dual nature of fluorescence [17,18] may be induced by changes in the solvent's polarity, pH [17], temperature, or concentration of the analyzed compound [19,20]. The most prevalent explanations offered for this type of effects include the emergence of intramolecular CT (charge transfer) states [19], particularly the twisted intramolecular charge transfer (TICT) [21,22]. A characteristic feature of molecules showing this type of effects is the observed high excited state dipole moment and relatively fast rate of stabilization with the increase in the medium's polarity.…”

“…Another explanations can be made based on effects related to the presence of excimer systems [23,24], the processes of excited-state intramolecular proton transfer (ESIPT) [25][26][27][28][29] or breaking Kasha's rule for polyatomic molecules (Brancato et al [30]). The most recent studies on the group of 1,3,4-thiadiazoles with the 2,4-dihydroxyphenyl substituent, where the effects of dual fluorescence are induced by processes related to the phenomenon of molecular aggregation [31][32][33], which in turn lead to the emergence of CT processes in certain analogues [17][18][19].…”

Microwave-Assisted Facile Synthesis, Anticancer Evaluation and Docking Study of N-((5-(Substituted methylene amino)-1,3,4-thiadiazol-2-yl)methyl) Benzamide Derivatives

“…Thiadiazoles (e.g., 1,3,4-thiadiazoles) also exhibit a number of extremely interesting spectroscopic properties. One of the very interesting spectroscopic properties of thiadiazoles is the dual fluorescence effect [ 16 , 17 , 18 , 19 ]. In addition, 1,3,4-thiadiazole compounds are excellent ligands and a good skeleton for metallo-complexes [ 20 ].…”