Reversible pH-Responsive Red Fluorescent Switch Based on a Water-soluble Pillar[5]arene-Triphenylamine Assembly for Cellular Imaging

Abstract

Herein, we report a water-compatible, reversibly switchable fluorescent assembly engineered from a pyridinium-functionalized triphenylamine (TPA) and a water-soluble pillar[5]arene (WP5). UV-Vis titration confirmed a strong 1:1 host-guest stoichiometry with a high association constant (2.95×10⁶ L/mol). Supramolecular confinement induces significant bathochromic shifts in both absorption (453→472 nm) and emission (607→635 nm), alongside a nearly tenfold fluorescence enhancement. Mechanistically, protonation of the carboxylate rim at low pH reconfigures interfacial electrostatic and hydrophobic interactions, triggering a further redshift to 639 nm and elevating the absolute quantum yield from 1.04% (free TPA) to 5.20% (acidified complex). Crucially, TPA@WP5 exhibits negligible cytotoxicity (94%–100% viability) in A549 cells at concentrations up to 80 µmol/L and enables real-time intracellular imaging of acidic microenvironments. This work establishes a robust platform for pH-gated fluorescence amplification, offering a paradigm for designing responsive supramolecular probes.

Keywords

Pillar[5]arene / Host-guest assembly / pH-responsive / Triphenylamine / Cellular imaging

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