Luminescent Mesoporous Squaraine Copolymers: Probing the Structural, Optical, and Sensing Behaviors

Abstract

Herein, we have designed and developed four luminescent mesoporous squaraine copolymers (PSQ-X) based on substituted diaryl-1,4-diamine moiety with various heteroatom “X” such as CH₂, O, S, and SO₂ to demonstrate the effect of heteroatom on structural, thermal, electrochemical, and optical properties of copolymers. Synthesis has been achieved through a facile single step metal-free poly-condensation reaction using commercially available low-cost monomers. All polysquaraines (PSQs) possess good thermal stability, crystallinity and low bandgap energy (2–3 eV). Utilizing the fluorescent nature of the PSQs, we have explored them as a “turn-off” sensor for the selective and sensitive detection of picric acid among various nitro-aromatic explosives via photoinduced electron transfer (PET) and inner filter effect (IFE) mechanism. The Stern–Volmer constant (K_SV) for PA has estimated as 1.93 × 10⁴, 1.92 × 10⁴, 0.68 × 10⁴, and 3.57 × 10⁴ M⁻¹ for PSQ-CH₂, PSQ-O, PSQ-S, and PSQ-SO₂, respectively, indicating that PA can quench the emission intensity of PSQ-SO₂ most efficiently with the detection limit in the nano-molar range (LOD = 5.96 nM). Contact mode detection of PA has been also accomplished using economical and transportable fluorescent paper test strip devices for on-site sensing, which can detect a minimum of 6.9 fg of PA.