A fast acid-responsive fluorescent porous polymer for acid-modified gas detection and optical anti-counterfeiting

Acid-stimulated responsive POP materials have received increased attention because of their intelligent photophysical properties and ability to translate changing environmental conditions into a specific response in the material. However, designing POP materials with new structures, excellent stability, and multi-stimulus response (simultaneous color and fluorescence response) remains challenging. Herein, a novel multi-stimulus responsive cyano-sp²c conjugated backbone POP-2 was designed and synthesized based on the synergistic effect of material protonation and intramolecular charge transfer (ICT). For the first time, we combined 2,4,6-Tri-(4-formyl-biphenyl-4-yl)-1,3,5-triazine (TFBT), a triazine-based monomer with biphenyl structure, with CPTD to synthesize a novel multi-responsive POP via sp²-C linkage (POP-2). POP-2 showed a dual response of color and fluorescence under acidic conditions. The introduction of the biphenyl structure increased the conjugation length and pore size of POP-2, which significantly improved the acid response behaviour of POP-2 due to the extended conjugation length, the conjugation effect, and the increase in pore size. Moreover, POP-2 can display aggregation-induced emission (AIE). Compared to the sp²-C-linked model compound TT-POP, POP-2 has a more significant fluorescence/color redshift and a lower response acid concentration under the same acidic conditions. This finding highlights the synergistic effect between conjugate length and pore size. In addition, we found that POP-2 exhibited a red shift of nearly 100 nm in aqueous hydrochloric acid at a concentration of 1 M, which was significantly better than most reported POPs. Interestingly, the resulting test strips produce a similar fluorescence response to HCl/NH₃ gases with high sensitivity and reversibility. The POP material can be used as an intuitive and simple detection strategy for acidic and alkaline gases. In addition, its unique fluorescence response makes it valuable in optical anti-counterfeiting applications.