Visualizing B-N Bond Breaking in Silicone Elastomer Networks through Turn-On Mechanochromic Luminescence

Mechanochromic luminescence (MCL) plays important roles in sensors, encryption, as impact indicators, and in real-time stress mapping. Boron-nitrogen Lewis pairs (LPs) are promising as mechanophores because both the interaction energy and the luminescence color can be readily fine-tuned. However, elastomers based on luminescent organoboron polymers have yet to be explored. Mechanofluorochromic elastomers were prepared by cross-linking pyridyl-substituted poly(dimethylsiloxane) (PDMS) with luminescent polystyrene (PS)-based organoborane polymeric Lewis acids. Apart from enabling the full compatibilization of otherwise immiscible PS/PDMS matrices, supramolecular B-N bond formation quenched the blue emission of PS1 via complexation of its pendant bithiophenyl-substituted arylborane chromophore units with pyridyl-functionalized PDMS. Upon stretching of the polymer film, a fluorescence turn-on, corresponding to uncomplexed boron chromophores, was observed by the naked eye under UV irradiation, indicating that the mechanical force was effectively translated to cleave the dynamic B-N bond. Aided by the low Tg of the PDMS segments, the "frustrated" mechanofluorochromic Lewis pairs (MFLPs) recombined instantly upon stress relief, rendering the material nonfluorescent again. Use of a yellow-luminescent donor-π-acceptor fluorophore that turns blue-emissive after pyridyl binding in PS2 led to a ratiometric fluorescence response. Tensile tests and oscillatory rheology studies showed that network F1 was relatively stronger than network F2, consistent with the relative Lewis acid strengths and ligand exchange kinetics derived from studies on molecular model systems. These results demonstrate the potential of tunable B-N Lewis pairs as versatile mechanofluorochromic cross-links in polymer networks.