Deprotonation-Induced Large Fluorescence Turn ON in a 2,4-Bis(benzo[d]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 enol form of HBT (PhOH → PhO^- + H⁺) can generate anionic 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 enol to anionic dynamic equilibrium. In this work, we extensively studied the solvent-dependent equilibrium of the 2-, 2-, and 2- species of the HBT derivative 2,4-bis(benzo[d]thiazol-2-yl)phenol (2) by steady-state and time-resolved fluorescence spectroscopy. Probe 2 was synthesized by attaching a second benzothiazole unit via a para phenylene linkage and was found to be significantly acidic. In comparison to parent HBT (1), probe 2 produced a highly emissive (λ_em ≈ 475) 2- species in solution by deprotonation. The 2- 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 2- 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 2 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 2- in solution. The fluorescence lifetime measurements of 2 in different solvents revealed the existence of 2-, 2-, and 2- species. Also, the fluoride-induced transformation of 2- to 2- 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 2-. Computational studies, employing density functional theory (DFT) and time-dependent density functional theory (TD-DFT), confirmed that the 2- species is stable in the ground state, while upon excitation, 2- species undergoes ESIPT spontaneously. Computed absorption and emission wavelengths showed good agreement with the experimental results.