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.