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

Abstract

The paper presents the results of studies conducted with the use of stationary and timeresolved 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

Understanding the Excited State Intramolecular Proton Transfer Phenomena of 2‐Hydroxy‐3‐Naphthaldehyde Thiosemicarbazone

“…[9][10][11] Over the last few years, there has been a spike in attention in the phenomenon of ESIPT. [10][11][12][13] The original pioneers in this field of research are Weber, [14] Forster, [15] and Weller [16] which opened a whole new outlook of widespread study in this field who primarily applied the findings in the area of fluorescence probe. [17] ESIPT is often associated with large Stokes' shift as molecules that exhibit ESIPT tend to avoid self-absorption and can give more precise fluorometric measurements depending on the solvent polarity in which the experimentation is carried out.…”

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

“…Naturally, molecules showing TICT fluorescence do not always produce dual fluorescence; there have recently been an increasing number of literature reports discussing this type of processes and systems leading to non-radiative deactivation of the excited state. Moreover, the molecules may also show the presence of multi-stage phototautomerization processes, ESIPT (excited state intramolecular proton transfer) [ 32 , 33 , 34 , 35 , 36 ]. The properties characterizing ESIPT molecules include dual fluorescence emission, a relatively significant Stokes shift, exceptional process dynamics, as well as high sensitivity of emission spectra to changing properties of the medium [ 37 , 38 , 39 , 40 ].…”