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4-[5-(Naphthalen-1-ylmethyl)-1,3,4-thiadiazol-2-yl]benzene-1,3-diol (NTBD) was extensively studied through stationary UV–vis absorption and fluorescence measurements in various solvents and solvent mixtures and by first-principles quantum chemical calculations. It was observed that while in polar solvents (e.g., methanol) only a single emission band emerged; the analyzed 1,3,4-thiadiazole derivative was capable of producing dual fluorescence signals in low polarity solvents (e.g., n-hexane) and certain solvent mixtures (e.g., methanol/water). As clearly follows from the experimental spectroscopic studies and theoretical modeling, the specific emission characteristic of NTBD is triggered by the effect of enol → keto excited-state intramolecular proton transfer (ESIPT) that in the case of solvent mixture is reinforced by aggregation of thiadiazole molecules. Specifically, the restriction of intramolecular rotation (RIR) due to environmental hindrance suppresses the formation of non-emissive twisted intramolecular charge transfer (TICT) excited keto* states. As a result, this particular thiadiazole derivative is capable of simultaneously producing both ESIPT and aggregation-induced emission (AIE). [ABSTRACT FROM AUTHOR]

In the presented study, advanced experimental techniques, including electronic absorption and fluorescence spectroscopies [with Resonance Light Scattering (RLS)], measurements of fluorescence lifetimes in the frequency domain, calculations of dipole moment fluctuations, quantum yields, and radiative and non-radiative transfer constants, were used to characterize a selected analogue from the group of 1,3,4-thiadiazole, namely: 4-[5-(naphthalen-1-ylmethyl)-1,3,4-thiadiazol-2-yl]benzene-1,3-diol (NTBD), intrinsically capable to demonstrate enol → keto excited-states intramolecular proton transfer (ESIPT) effects. The results of spectroscopic analyses conducted in solvent media as well as selected mixtures were complemented by considering biological properties of the derivative in question, particularly in terms of its potential microbiological activity. The compound demonstrated a dual fluorescence effect in non-polar solvents, e.g. chloroform and DMSO/H2O mixtures, while in polar solvents only a single emission maximum was detected. In the studied systems, ESIPT effects were indeed observed, as was the associated phenomenon of dual fluorescence, and, as demonstrated for the DMSO: H2O mixtures, the same could be relatively easily induced by aggregation effects related to aggregation-induced emission (AIE). Subsequently conducted quantum-chemical (TD-)DFT calculations supported further possibility of ESIPT effects. The following article provides a comprehensive description of the spectroscopic and biological properties of the analyzed 1,3,4-thiadiazole derivatives, highlighting its potential applicability as a very good fluorescence probes as well as a compound capable of high microbiological activity. [ABSTRACT FROM AUTHOR]

This work reports on biophysical insights into the excited state intramolecular proton transfer (ESIPT) processes taking place in three 1,3,4-thiadiazole derivatives that served as model compounds, on which electronic absorption, fluorescence, Fourier-transform infrared spectroscopy (FTIR), surface plasmon resonance (SPR) and electrochemical impedance spectroscopy (EIS) studies were performed. The fluorescence spectra recorded in various solvents revealed an interesting dual fluorescence effect. In molecules in their monomeric form, the effect is associated with the ESIPT phenomenon, and may be further enhanced by aggregation-related effects, such as aggregation-induced emissions. Other spectroscopic studies on the selected molecules in a liposomal medium as a model revealed that, in a biomimetic environment, they can exist in both monomeric and aggregated forms. In both cases, however, the effects observed are closely related to the lipid's main phase transition temperature and the structure of the molecule. Introduction of specific substituents to the phenyl moiety either allows or prevents proton transfer from occurring in the excited state. The hydrophobicity changes in a lipid environment may result in an emergence of specific molecular forms and therefore either facilitate or hinder ESIPT processes. SPR and EIS confirmed the significant hydrophobicity changes in the model lipid systems, while FTIR measurements revealed a notable influence of 1,3,4-thiadiazoles on the fluidity of liposomal membranes. The results obtained clearly show that the thiadiazole derivatives are very good model molecules for studying hydrophobic-hydrophilic environments, and particularly with polymers or liposomes used as drug delivery systems. [ABSTRACT FROM AUTHOR]

The paper presents the results of studies conducted with the use of stationary and time-resolved fluorescence spectroscopy for the new derivative 2-Hydroxy-N-(2-phenylethyl)benzamide (SAL-3) in aqueous solutions with various concentrations of hydrogen ions as well as in solvent mixtures (i.e. media with changing polarity/polarizability). For the compound selected for the study placed in aqueous solutions with varying concentrations of hydrogen ions, the fluorescence emission spectra revealed a single emission band within most of the pH range, however, at low pH (pH<3) a significant broadening (noticeable effect of dual fluorescence) and shifting of the band was observed. Whereas, for water and polar (protic) solvents, we observed a very interesting phenomenon of dual fluorescence never before reported for this particular group of analogues (with the specific substituent system). Based on the results of the experiments, it was observed that the presented effects may be related both with conformational effects (related to the possible positioning of the–OH group on the side of the carbonyl system, which facilitates the possibility of proton transfer) as well as, most importantly, the effects of excited state intramolecular proton transfer (ESIPT–Excited State Intramolecular Proton Transfer) related in this case with the necessary (new/previously unobserved in published literature) presence of ionic and non-ionic forms of the compound). Both the conducted quantum-mechanical [TD]DFT–Time-Dependent Density Functional Theory) calculations and excited state dipole moment change calculations for the analyzed molecule in solvents with varying pH confirmed the association between the observed fluorescence phenomena and the two aforementioned effects. [ABSTRACT FROM AUTHOR]

The paper presents the results of spectroscopic studies of 4-([1,2,4]triazolo[4,3- a ]pyridin-3-yl)-6-methylbenzene-1,3-diol (TPBD), which indicate that the molecular organization of the compound is altered in different solvents and at different concentrations. The results were obtained from two solvents: methanol and propan-2-ol. Methanol represents a group of solvents in which monomers are the predominant molecular form of TPBD. Propan-2-ol belongs to a group of solvents in which TPBD additionally forms aggregates (dimers or N-aggregates). At concentration C 0 = 0.05 × 10 −3 M, TPBD has a monomeric form in Mt-OH and an additional dimeric form in Pr-OH. Circular dichroism (CD) and resonance light scattering (RLS) spectra revealed predominance of aggregated TPBD forms already at low concentrations of the compound in the methanol solvent. In turn, monomeric and dimeric forms of the compound dominated at low concentrations in propan-2-ol. Formation of additional dimers or N-aggregates in propan-2-ol was observed only at high concentrations of the compound (C 10 = 1.36 × 10 −3 M-C 20 = 4.90 × 10 −3 M). The presence of monomers and dimers in the Pr-OH solution yielded two bands (at 408 and 502 nm, respectively) in the fluorescence emission spectrum. Additionally, it was found that dimeric molecular form was characterized by reduction of the amplitude-weighted average fluorescence lifetimes in comparison with the monomeric form. [ABSTRACT FROM AUTHOR]

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In our previous work, we discussed the emergence of the dual fluorescence phenomenon in selected compounds from the group of 1,3,4-thiadiazoles. The results obtained in a number of experimental studies, supported by [TD]DFT calculations, clearly indicated that the phenomenon of dual fluorescence stemmed from an overlap of several factors, including the correct conformation of the analyzed molecule and, very significantly in this context, aggregation effects. Where those two conditions were met, we could observe the phenomenon of intermolecular charge transfer (CT) and the emergence of electronic states responsible for long wave emissions. However, in light of the new studies presented in this paper, we were able, for the first time, to provide a specific theory for the effect of dual fluorescence observed in the analyzed group of 1,3,4-thiadiazoles. We present the results of spectroscopic measurements conducted for two selected analogues from the 1,3,4-thiadiazole group, both in polar and non-polar solvents, which clearly evidence (as we have already suspected in the past, albeit have not shown in publications to date) the possibility of processes related to emission from the tautomer formed in the process of excited state intramolecular proton transfer, which is responsible for the long-wavelength emissions observed in the selected analogues. The presented results obtained with the use of UV-Vis, fluorescence (stationary and time-resolved), FTIR, and Raman spectroscopy, as well as from calculations of dipole moment changes between the ground and excited state with the use of two derivatives with different structures of the resorcylic system, corroborated our standing hypothesis. At the same time, they excluded the presence of ground state keto forms of the analyzed analogues unless necessitated by the structure of the molecule itself. In this case, aggregation factors enhance the observed effects related to the dual fluorescence of the analyzed compounds (by way of AIE—aggregated induced emissions). [ABSTRACT FROM AUTHOR]

The article presents the results of spectroscopic studies conducted on three compounds from the 1,3,4-thiadiazole group: 2-amino-5-phenyl-1,3,4-thiadiazole (TB), 2-amino-5-(2-hydroxyphenyl)-1,3,4-thiadiazole (TS), 2-amino-5-(2-hydroxy-5-sulfobenzoyl)-1,3,4-thiadiazole (TSF). The fluorescence emission spectra of the previously reported TS analogue revealed an interesting dual fluorescence effect in butanol. Depending on the structural composition of the resorcylic system, for the other analogues two partially overlapping emission bands or single emission bands were observed. Interpretation of the UV–Vis spectra, stationary fluorescence, RLS, as well as fluorescence lifetimes allowed us to correlate said effects with the phenomenon of specific molecular aggregation which, depending on the substituent, can induce the process related to charge transfer within the given molecule. The conducted calculations of dipole moment changes for the ground and excited state, as well as the [TD] DFT calculations confirmed our assumptions with regard to the experimental results. The described study was also the first to analyse fluorescence effects, for this particular group of analogues, relative to primarily the structural composition of the first substituent by the thiadiazole ring. The correlation between said fluorescence effects and earlier measurements conducted on analogues in from this group of compounds in various systems (liposomes, micelles, or crystals) also suggests the possibility of charge transfer (CT) dependant on the appropriate conformation of the molecule (with the -OH group on the side of the nitrogen atom in the thiadiazole ring). It should be emphasised that this type of fluorescence effects can be relatively easily identified with the use of fluorescence methods, which means that fluorophores from this group can be used as ideal fluorescence probes which continue to be in high demand in the context of biology/molecular medicine. Top left – monomeric TB molecule. Top right – the possible aggregate of TB. Bottom – Fluorescence emission spectra of TB recorded in butanol (the maxima emission originating from the respective monomeric and aggregated forms is marked with arrows). Unlabelled Image • Selected 1,3,4-thiadiazole derivatives demonstrate dual fluorescence effects. • The dual fluorescence observed result from molecular aggregation and CT processes. • CT processes involvement was evidenced by the excited state dipole moment values. • Quantum-mechanical study confirmed the contribution from molecular aggregation. [ABSTRACT FROM AUTHOR]