Investigating Squaraine Dye Decolorization as a Method for Gaseous Amine Indication

Herein, we investigate structure–property relationships for the dye molecule bis(1,3,5-trihydroxyphenyl) squaraine (SQPH). Single-crystal X-ray diffraction analysis of crystals grown from various solvent media revealed a planar structure that assembles into solvent-dependent slip-stacked motifs. Complementary solution UV–visible spectroscopy establishes controllable monomer-to-J-aggregate equilibria, while acid–base titrations map a reversible J-aggregate-to-monomer-to-deprotonated progression with distinct optical features. Electrochemistry reveals low-lying frontier levels in line with electron-accepting organic π-conjugated materials. Green-solvent ink formulations were engineered and coated using a high-throughput technique to produce large-area, uniform, and robust films on flexible substrates. Exposing the SQPH-based films to gaseous amines results in a striking color change with eventual decolorization, demonstrating practical application potential.