Polymer–Inorganic Composites with Dynamic Covalent Mechanochromophore

Polymer inorganic composites with diaryl-bibenzofuranone (DABBF) moieties, dynamic covalent mechanochromophores, were prepared, and their mechanochromic behavior was systematically investigated. The central C-C bonds in DABBF moieties can be cleaved by mechanical force to form the corresponding stable blue radicals, which can be quantitatively evaluated by electron paramagnetic resonance (EPR) spectroscopy. One controversial issue but attractive property in the DABBF system is the equilibrium between the activated and deactivated states. Although the deactivation process decreases the sensitivity of some equilibrium mechanophores, the equilibrium has rarely been considered when establishing molecular and/or material design of these systems. Herein, a rational macromolecular design to suppress the deactivation of activated dynamic mechanophores and improve sensitivity by limiting their molecular motion is proposed. Polymer-inorganic composite materials with rigid networks prepared from DABBF alkoxysilane derivatives exhibited significant activation of the incorporated DABBF linkages by grinding, with sensitivities more than 50 times as high as that of DABBF monomers. The increased sensitivity is due to the effective transmission of mechanical force to the DABBF moieties in the network structures and suppression of the recombination of the generated radicals by the rigid frameworks. Furthermore, when the rigid frameworks were incorporated into elastomers as inorganic hard domains, the DABBF mechanophores at the interface between the organic and inorganic domains were preferentially activated by elongation.