Instantly Reversible Mechanofluorochromism in Benzocyclobutene Mechanophores via Mechanochemical Stabilization of an Extremely Short-Lived Intermediate

This study leverages the high instability of the ortho-quinodimethide (oQDM) intermediate generated by force-activated benzocyclobutene (BCB) to develop an instantly reversible mechanofluorochromic polyurethane material (P-BCB). The exceptionally low reverse activation energy (1.4 kJ/mol) renders oQDM short-lived at room temperature. However, oQDM can be stabilized under a sustained mechanical force. Due to extended conjugation, the stretched P-BCB sample exhibits significantly enhanced blue-green fluorescence. When force is removed, oQDM instantly reverts to the nonemissive BCB structure, enabling instantaneous and reversible switching of the fluorescence signal. The oQDM can be covalently captured through a [4 + 2] cycloaddition reaction with N-phenylanthracene maleimide, resulting in a 4-fold increase in fluorescence intensity. Thermally activated self-cross-linking capability is observed for P-BCB. Heating at 230 °C induces dimerization of oQDM, increasing the P-BCB fracture strength from 1.94 to 5.16 MPa. Given the instantaneous reversible nature of the BCB mechanophore, it can be utilized to visually monitor stress relaxation behavior within the P-BCB chains, enabling in situ optical monitoring of internal stress evolution. This work proposes a novel design strategy for instantaneous mechanofluorochromic materials based on the stabilization of extremely short-lived intermediates, providing a new mechanophore for intelligent sensing, biomimetic camouflage, and damage visualization materials.