Triphenylamine-Based\nPush–Pull Dyes for Chromogenic\nDetection of HSO<sub>4</sub><sup>–</sup> Ion in Water: The\nRole of Anion in the Formation of Fluorescent Organic Nanoparticles

Optical detection of the HSO<sub>4</sub><sup>–</sup> ion\nin pure aqueous medium is rare, owing to the very high Gibbs free\nenergy of hydration and ambiguity to distinguish with the isostructural\nH<sub>2</sub>PO<sub>4</sub><sup>–</sup> ion. Herein, a pair\nof triphenylamine-based push–pull dyes with different numbers\nof terminal pyridine fragments, connected via an acetylenic linker,\nwere synthesized by Sonogashira cross-coupling reaction. These two\ndyes displayed highly selective (LOD = 15.1/8.3 ppb), dual-mode color-changing\nresponses toward the HSO<sub>4</sub><sup>–</sup> ion in pure\naqueous medium without any interference. Despite the halochromic behavior,\nboth compounds exhibited very distinct optical responses with the\nHSO<sub>4</sub><sup>–</sup> ion. The mechanistic investigation\nindicated that HSO<sub>4</sub><sup>–</sup> was engaged in a\nbifurcated intermolecular hydrogen bonding interaction (leading to\nproton transfer) with pyridine nitrogen atoms that altered the extent\nof intramolecular charge transfer (ICT). The self-assembly of such\nprotonated species was found to be prominent when sulfate was present\nas the counteranion. The extent of self-assembly was found to be more\nprominent for the trisubstituted, Y-shaped quadrupolar derivative\nthan that observed for the linear, monosubstituted one. Furthermore,\nthe present system was utilized for the detection of HSO<sub>4</sub><sup>–</sup> ions in commercially available samples with satisfactory\nresponses.