Triphenylamine-Based Push–Pull Dyes for Chromogenic Detection of HSO₄^– Ion in Water: The Role of Anion in the Formation of Fluorescent Organic Nanoparticles

Optical detection of the HSO₄^- ion in pure aqueous medium is rare, owing to the very high Gibbs free energy of hydration and ambiguity to distinguish with the isostructural H₂PO₄^- ion. Herein, a pair of triphenylamine-based push-pull dyes with different numbers of terminal pyridine fragments, connected via an acetylenic linker, were synthesized by Sonogashira cross-coupling reaction. These two dyes displayed highly selective (LOD = 15.1/8.3 ppb), dual-mode color-changing responses toward the HSO₄^- ion in pure aqueous medium without any interference. Despite the halochromic behavior, both compounds exhibited very distinct optical responses with the HSO₄^- ion. The mechanistic investigation indicated that HSO₄^- was engaged in a bifurcated intermolecular hydrogen bonding interaction (leading to proton transfer) with pyridine nitrogen atoms that altered the extent of intramolecular charge transfer (ICT). The self-assembly of such protonated species was found to be prominent when sulfate was present as the counteranion. The extent of self-assembly was found to be more prominent for the trisubstituted, Y-shaped quadrupolar derivative than that observed for the linear, monosubstituted one. Furthermore, the present system was utilized for the detection of HSO₄^- ions in commercially available samples with satisfactory responses.