Deprotonation-Induced Large Fluorescence Turn ON in a 2,4-Bis(benzo[<i>d</i>]thiazol-2-yl)phenol (HBT-BT) Derivative

2-(2'-Hydroxyphenyl)benzothiazole (HBT) is an interesting candidate that has been used to develop fluorescent probes with large Stokes' shifts (Δλ > 100 nm) by utilizing excited-state intramolecular proton transfer (ESIPT). The efficiency of the ESIPT process in HBT is dependent on the acidity of the central phenolic group. The possible deprotonation of the <i>enol</i> form of HBT (PhOH → PhO^- + H⁺) can generate <i>anionic</i> form in solution that exhibits a noticeable hypsochromic effect in emission (λ_em ≈ 475). By introduction of an appropriate substituent to the central phenolic ring, the ESIPT efficiency of the HBT can be controlled by modulating <i>enol</i> to <i>anionic</i> dynamic equilibrium. In this work, we extensively studied the solvent-dependent equilibrium of the <b>2-<i>enol</i></b>, <b>2-<i>keto</i></b>, and <b>2-<i>anoinic</i></b> species of the HBT derivative 2,4-bis(benzo[<i>d</i>]thiazol-2-yl)phenol (<b>2</b>) by steady-state and time-resolved fluorescence spectroscopy. Probe <b>2</b> was synthesized by attaching a second benzothiazole unit via a <i>para</i> phenylene linkage and was found to be significantly acidic. In comparison to parent HBT (<b>1</b>), probe <b>2</b> produced a highly emissive (λ_em ≈ 475) <b>2-<i>anionic</i></b> species in solution by deprotonation. The <b>2-<i>anionic</i></b> was dominated in polar aprotic solvents such as DMF and DMSO and much weakly observed in nonpolar to moderately polar solvents. The solvent-dependent equilibrium was studied by ¹H NMR spectroscopy in different solvents. The formation of <b>2-<i>anionic</i></b> was found to be dependent on (a) the type of solvent and (b) the presence of basic anionic species (i.e., fluoride). The photophysical properties (absorbance and emission) of probe <b>2</b> were investigated in different solvent environments and in the presence of different anionic species to understand the dynamic equilibrium that leads to the generation of <b>2-<i>anionic</i></b> in solution. The fluorescence lifetime measurements of <b>2</b> in different solvents revealed the existence of <b>2-<i>enol</i></b>, <b>2-<i>keto</i></b>, and <b>2-<i>anionic</i></b> species. Also, the fluoride-induced transformation of <b>2-<i>enol</i></b> to <b>2-<i>anionic</i></b> was studied by fluorescence lifetime measurements. The density functional theory (DFT)-based computational calculations were performed to evaluate the electronic nature of the dynamic equilibrium that produced <b>2-<i>anionic</i></b>. Computational studies, employing density functional theory (DFT) and time-dependent density functional theory (TD-DFT), confirmed that the <b>2-<i>enol</i></b> species is stable in the ground state, while upon excitation, <b>2-<i>enol</i></b> species undergoes ESIPT spontaneously. Computed absorption and emission wavelengths showed good agreement with the experimental results.