An ESIPT-ICT steered naphthylthioic-based ionic probe with dual emissive channels exhibiting CHEF and CHEQ effects

A naphthylthioic-based emissive probe (M) bearing a hydroxyl and amine group was designed and synthesized via a one-step Schiff base reaction process. The probe was characterized spectroscopically using ¹H NMR, UV-Vis and fluorescence spectrophotometers. The probe turned out to be spectroscopically and colorimetrically selective and sensitive to multiple cations and anions. Interestingly, the probe displayed characteristics of excited-state intramolecular proton transfer (ESIPT)-driven dual emissive channels; experiencing fluorescence enhancement upon the molar additions of Al³⁺ as well as the anions used, events presumably ascribed to chelation fluorescence enhancement (CHEF), hydrogen bonding and deprotonation effects. Moreover, the fluorometric titration with Hg²⁺ resulted in ratiometric spectral behaviors of M, with the disappearance of the peak at 450 nm, concomitant with the appearance of a new peak at 520 nm, distinguished by a clear isosbestic point, the same behaviors exhibited by Sn²⁺ and Ag⁺ analytes towards M. The introduction of all other cations used, resulted in fluorescence quenching, attributable to chelation enhanced fluorescence quenching (CHEQ), thereby inhibiting the ESIPT process. The experiments were all carried out in the aqueous environment medium of DMSO-H₂O (9 : 1) at ambient temperature. Theoretical density functional theory calculations were carried out to gain insight into the interaction of M with cations and anions, and their influence on the HOMO-LUMO energy gaps.