Protonation-induced reversible photochromism of a red fluorescent organic material with D-π-A structure for sensing ultra-acidic pH.

An organic material 4-(4-dimethylaminostyryl) methylbenzothiazole 4-methylbenzenesulfonate (DABT) with a D-π-A structure was prepared, and its luminescent properties and pH-driven fluorescence switching were investigated. The dimethylamino group in its cation contains a nitrogen (N) atom with a lone pair of electrons, serving as the electron donor for sensing H+ ions. Protonation of this N atom under acid stimulation modulates the intramolecular charge transfer (ICT) effect and the resultant light absorption and photoluminescence of DABT, thereby realizing the recognition and detection of H+. These characteristics endow DABT with photochromism in both liquid and solid states, and the feasibility of sensing ultra-acidic pH: (i) Both acid (trifluoroacetic acid, TFA) and base (triethylamine, TEA) additions can be efficiently sensed by three forms of reversible conversions, including color, light absorption, and fluorescence response of DABT in its dichloromethane (DCM) solution; (ii) TFA or TEA vapor in air can also be rapidly sensed by solid DABT powders or DABT-polyvinyl alcohol (PVA) composite films; (iii) In the strong acidic pH range of 1.0-3.5, a good linear relationship between the fluorescence emission of DABT and pH value was measured, with a high fitting R2 of 0.9925. This work reveals the peculiar reversible photochromism of organic DABT under acid or base treatment, and particularly, it discloses novel highly-sensitive fluorescent organic probes for efficiently sensing ultra-acidic pH.