Dual-Functional Triphenylamine-Based Conjugated Porous Polymer for Visible-Light-Driven Photocatalysis and Rapid Reversible HCl Sensing

Abstract Conjugated porous organic polymers (CPPs) have emerged as promising photocatalysts because of their high surface area and efficient ability to collect and transport light energy through their delocalized backbone. Nevertheless, their synthesis methods require expensive monomers, stringent experimental conditions, including inert atmosphere, elevated polymerization temperature, and the use of metals as catalysts. To overcome these limitations, a triphenylamine-based conjugated porous polymer (TPA-TTA) has been prepared through a one-pot Schiff base reaction in ambient conditions in the absence of a metal catalyst. In this synthesis, 5’,5,5″-(benzene-1,3,5-triyl)tris(thiophene-2-carbaldehyde) (TTA), a stiff aromatic aldehyde, was condensed with tris(4-aminophenyl) amine (TPA). Diffuse reflectance spectroscopy (DRS) analysis revealed an optical band gap of 2.01 eV, confirming its semiconducting behavior. Electron paramagnetic resonance (EPR) analysis exhibited a pronounced radical signal at g = 2.00, suggesting the efficient formation of free radicals upon visible-light exposure which can promote the photocatalytic degradation of organic contaminants. The Brunauer–Emmett–Teller (BET) analysis indicates that TPA-TTA has a porous framework with a surface area of 109.3 m2 g–1, while thermogravimetric analysis (TGA) demonstrates its good thermal stability up to 333 °C. Photocatalytic studies further show that TPA-TTA achieves effective degradation of 88.38, 91, 90.4, 94, 79.8, 82 and 95% for TC, HQ, PCM, MO, AMX, NR, and RHB upon visible-light exposure, respectively. Owing to the protonation of imine moieties in the polymeric framework upon exposure to HCl vapors, TPA-TTA exhibits a pronounced “turn-off” fluorescence response along with a rapid (2s) visible color change to the naked eye. The color change of TPA-TTA triggered by HCl vapors is fully reversible upon exposure to NH3 vapors, demonstrating its strong reversibility and reusability. The protonation–deprotonation mechanism was also supported by Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) studies. These insights underscore the feasibility of large-scale production of metal-free conjugated porous polymers as dual-functional materials, serving as photocatalysts and reversible naked-eye solid-state HCl sensors.