Access to Main-Chain Polymeric Tetracyanobutadiene Chromophores for Turn-On Wide-Range Fluoride Detection

Tetracyanobutadiene (TCBD)-appended main-chain polymers offer superior chromophore properties compared to side-chain polymers, but their development is hindered by challenges in complex synthesis and customization. We present a solution using a urea-appended alkyne polymer p-alkyne for easier customization and sensing applications. Our method involves synthesizing main-chain polymers containing TCBD and extended-TCBD (eTCBD) units via p-alkyne, which, on treatment with TCNE and TCNQ, yields p-TCBD and p-eTCBD polymers, respectively. These polymers were characterized by NMR, FT-IR, TGA, XPS, and SEC, with molecular weights ranging from 18 700 to 32 500 Da. These polymeric TCBDs form nonfluorescent rod-like aggregates with lengths ranging several micrometers due to hydrogen bonding-mediated self-assembly. However, exposure to fluoride (F–) ions disrupts these aggregates to form dideprotonated species into smaller-sized nanoparticles (∼90 nm) via mono- and dideprotonation, thus enhancing emission intensity and quantum yield. Their turn-on response to F– ions allows them to detect concentrations ranging from 30 μM to 7.2 mM. Notably, there was a 16-fold luminescence response of the p-eTCBD polymer with TCNQ functionalization when compared to other polymers (1.4–5.7 fold), suggesting that functionalization could be tailored to tune the sensitivity and performance. Mechanistic studies reveal a 1:1 urea/F– adduct formation, followed by a two-step deprotonation. Morphological studies have confirmed the disintegration of polymeric nanorods into luminescent nanoparticles upon F– addition. Our developed methodology paves the way for a wide variety of applications given its simplicity for facile access to urea-functionalized TCBD-based main-chain polymers.