Thiol-functionalized ESIPT dye in rubber matrices: Mechanically robust, fluorescent, and reversible solid-state pH...

Rubbers are versatile materials used across numerous industries, yet their functionalization for sensing applications remains limited by challenges in dye compatibility and processability. Herein, we report the development of solid-state, pH-responsive rubber composites via covalent incorporation of a thiol-functionalized excited state intramolecular proton transfer (ESIPT) dye during sulfur vulcanization. For this purpose, we used 2-(2-hydroxyphenyl)benzothiazole (HBT) core modified with free thiol as a chemical handle (HBT-SH). Using butadiene (BR), isoprene (IR), and isobutylene isoprene rubber (IIR) as matrices, we demonstrate that the dye is stably embedded in the network, as confirmed by infrared spectroscopy and solvent-release studies. Despite minor modifications in thermal and mechanical behavior, especially in BR, the composites retain robust mechanical properties. Importantly, all dyed rubbers exhibit strong fluorescence with emission in the green region and distinct responses to acidic and basic vapors. Reversible fluorescence color changes are observed in BR and IR samples, indicating potential for use in solid-state optical pH sensors. This work demonstrates a scalable strategy to integrate ESIPT-based functionality into industrially relevant elastomers without compromising their core properties.