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

Abstract

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).

Novel Coumarin-Thiadiazole Hybrids and Their Cu(II) and Zn(II) Complexes as Potential Antimicrobial Agents and Acetylcholinesterase Inhibitors

“…The wavelength of high energy emission corresponds well with the enol-tautomeric form of the 1,3,4-thiadiazole derivatives [ 39 , 40 ]. The lower energy band is similar to that of the various substituted coumarins and conjugates [ 49 , 50 , 51 ] but may also be assigned to the excited keto form of the hybrid resulting from the excited-state proton transfer (ESIPT) processes [ 45 ]. Moreover, the molecular aggregation evidenced by the UV-Vis spectroscopy ( Section 2.6 ) may contribute to the emission characteristics via aggregation-induced emission (AIE) [ 52 , 53 ], and enhance the possible ESIPT-related effects in coumarin-thiadiazole hybrids 2 – 5 .…”

“…Moreover, the interplay between keto-and enol-like tautomers may also affect the energy of electronic transitions and contribute to the broadening of the main absorption band in hybrids 2-5 [29,39]. Moreover, the presence of the -OH group at the coumarin phenyl ring enables the intramolecular hydrogen bonding with the nearby thiadiazole nitrogen [29], which to some extent may alter the absorption characteristics and may affect the excited-state proton transfer (ESIPT) [45]. recording the spectra of more concentrated solutions (not shown) but did not reveal the d → d* transitions in Cu(II) complexes, supporting the hypothesis that the octahedral coordination mode is not likely.…”

Insight into dual fluorescence effects induced by molecular aggregation occurring in membrane model systems containing 1,3,4-thiadiazole derivatives

“…However, the most interesting effects confirming our earlier premise can be observed in Panels B and C. For instance, Panel C reveals (for TS) in the region of ~ 300 nm that the excitation spectra are slightly shifted relative to each other in ethanol and toluene (chloroform), which evidences the existence in the ground state of cis- and trans -conformers of the enol form, as already observed in the literature (Rodembusch et al 2007 ). The visible shift towards longer wavelengths in the cis- form relative to the trans- conformer is naturally due to the existence of an intramolecular hydrogen bond (Czernel et al 2020 ). Moreover, as corroborated by our studies on monocrystalline systems conducted for similar analogues (Hoser et al 2013 , 2018 ), the cis- form is more stable than the trans- form as it allows for molecule stabilisation via the intramolecular hydrogen bond (Scheme S1 in the SM).…”

“…Based on our previous studies, the differences at about 315 nm (for TB) or about 300 nm (for TS and TSF) are related to the presence of cis - and trans -conformers, and consistent with results reported elsewhere (Bhattacharyya et al 2020 ; Heyer et al 2017 ; Rodembusch et al 2007 ). On the other hand, the longwave differences with a maximum of about 350 nm (for TS and TSF) are associated with the aggregation of these compounds (Czernel et al 2020 ; Tong et al 2015 ).…”

Modern Theoretical Approaches to Modeling the Excited-State Intramolecular Proton Transfer: An Overview

“…Among many chemical processes induced with photon absorption, the excited-state intramolecular proton transfer (ESIPT) stands out with its ultrashort timescale and strong impact on the molecular electronic structure, which is often manifested with large emission Stokes shifts. Relying on a proton exchange between two electronegative centers along a pre-existing intramolecular hydrogen bond, the ESIPT process is recognized to provide a mechanism of excellent photostability to natural and artificial molecular systems [ 1 , 2 , 3 ], finds applications in fluorescent probes and imaging agents [ 4 , 5 , 6 ], governs characteristic emission of the green fluorescent protein and its analogs [ 7 , 8 , 9 ], and opens rich possibilities for multicolor emission in organic light-emitting diodes (OLEDs) [ 10 , 11 , 12 , 13 , 14 ]. Last but not least, ESIPT may also activate other excited-state reaction channels, facilitating the design of complex molecular photo-devices [ 15 , 16 , 17 , 18 , 19 ].…”