Tetraphenylethylene Cross-Linked Poly(2-isopropenyl-2-oxazoline) Networks as Basis for Strong Luminescent Hydrogels and Humidity Responsive Organogels

Strong and tough luminescent hydrogels provide a versatile platform for flexible optoelectronics, soft actuators, optical sensing, anticounterfeiting, and biointerfaces that demand simultaneous mechanical integrity and reliable emission. The challenge is to reinforce polymer networks while preserving fluorescence and processability, motivating designs that integrate robust covalent cross-linking with luminophores. Herein, we report TPE-cross-linked PiPOx networks that display humidity-dependent fluorescence in the organogel state, while upon solvent exchange with water the same networks form mechanically robust, highly fluorescent hydrogels. At first, organogels were prepared via chemical cross-linking of the 2-oxazoline side groups of poly(2-isopropenyl-2-oxazoline) (PiPOx) with the carboxylic acid groups of 4,4′-(1,2-diphenylethene-1,2-diyl)dibenzoic acid (TPE-DA) in an organic solvent. Subsequently, the organogel was transformed into a hydrogel by solvent exchange with water. The resulting hydrogel showed strong luminescence under ultraviolet (UV)-irradiation due to the aggregation-induced emission of the TPE-DA cross-linker in water, and excellent mechanical properties. By avarying the amount of TPE-DA, the fluorescent intensity and mechanical properties of the luminescent hydrogels could be finely tuned. The redistribution of the TPE-DA cross-linker into larger aggregates and the further shrinkage of the cross-linked network mesh size over time enhance the mechanical properties of the hydrogel. The luminescent PiPOx hydrogels exhibit compressive Young’s moduli up to 2.97 MPa and remarkable toughness, as demonstrated by loading–unloading tests (compressive strain of 40%, and dissipated energy recoveries ranging from 66 to 85%). Furthermore, in relative humidity (RH) tests, the fluorescence of the TPE-PiPOx organogels in N,N-dimethylacetamide (DMA) was found to strongly increase when the RH was increased from 25 to 90% RH resulting from water-sorption by DMA inducing TPE aggregation, demonstrating excellent humidity responsive luminescence that could serve toward future development of humidity sensors.