Spectroscopic and Theoretical Studies of Fluorescence Effects in 2-Methylamino-5-(2,4-dihydroxyphenyl)-1,3,4-thiadiazole Induced by Molecular Aggregation

Abstract

The article presents the results of fluorescence analyses of 2-methylamino-5-(2,4-dihydroxyphenyl)-1,3,4-thiadiazole (MDFT) in an aqueous environment. MDFT dissolved in aqueous solutions with a pH value in the range from 1 to 4.5 yielded an interesting effect of two clearly separated fluorescence emissions. In turn, a single fluorescence was observed in MDFT dissolved in water solutions with a pH value from 4.5 to 12. As it was suggested in the previous investigations of other 1,3,4-thiadiazole compounds, these effects may be associated with conformational changes in the structure of the analysed molecule accompanied by aggregation effects. Crystallographic data showed that the effect of the two separated fluorescence emissions occurred in a conformation with the –OH group in the resorcyl ring bound on the side of the sulphur atom from the 1,3,4-thiadiazole ring. The hypothesis of aggregation as the mechanism involved in the change in the spectral properties at low pH is supported by the results of (Time-Dependent) Density Functional Theory calculations. The possibility of rapid analysis of conformational changes with the fluorescence spectroscopy technique may be rather important outcome obtained from the spectroscopic studies presented in this article. Additionally, the presented results seem to be highly important as they can be easily observed in solutions and biologically important samples.

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.…”

“…Based on our earlier deliberations pertaining to the phenomenon of dual fluorescence, we put forward a research hypothesis regarding the association between the dual fluorescence observed in the analyzed compounds and a number of contributing factors [ 2 , 4 , 5 , 17 , 20 ]. We noticed that for the phenomenon of dual fluorescence to emerge in the relevant analogues, a number of conditions have to be met.…”

“…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.…”

“…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. Contemporary researchers working in the fields of biophysics and photophysics or photochemistry and utilizing the methods of molecular spectroscopy are faced with the extremely difficult task, posed by medical and pharmaceutical sciences, of determining the molecular properties of continuously developed new compounds [ 1 , 2 , 14 , 15 , 16 , 17 , 18 ].…”

“…In our earlier studies on various 1,3,4-thiadiazole analogues, we were able to associate the effect of dual fluorescence observed in the same with two main factors [ 1 , 2 , 18 ]. The first is the presence of a specific conformation of the molecule which must accept the conformation with the –OH group from the resorcylic ring, in the ortho position, on the side of the nitrogen atom [ 4 ].…”