Impact of Polymer Matrix on Polymer Mechanochromism from Excited State Intramolecular Proton Transfer†

Comprehensive Summary Mechanochromic polymers based on non‐covalent changes have attracted much attention recently. Herein, we report the impact of inter/intramolecular hydrogen bonds on polymer mechanochromism from the excited state intramolecular proton transfer (ESIPT) process. PhMz‐NH 2 ‐OH and PhMz=2A are designed and obtained by simple and high‐yield synthesis, and are connected into polyurethane and poly(methyl acrylate‐ co ‐2‐ethylhexyl acrylate), respectively. In the initial state, the PhMz‐NH 2 ‐OH@PU sample shows blue fluorescence from the excited enol form (E*) excitons, owing to intermolecular hydrogen bonds that interrupt the ESIPT reactions but the PhMz=2A@PMA‐2‐EA sample expresses cyan fluorescence belonging to the excited keto form (K*) emission, implying that the intramolecular hydrogen bonds matter. Furthermore, under stretching, external force can tune the emission of the PhMz=2A@PMA‐2‐EA sample from K* to E* state. Though external force can putatively still promote a bond rotation, ESIPT reactions remain equivalently interrupted in both the relaxed and stressed states in a hydrogen‐bond donating environment. DFT calculation confirms the force‐induced increase in dihedral angle for the transition of ESIPT‐on/off. Thus, PhMz‐NH 2 ‐OH@PU and PhMz=2A@PMA‐2‐EA showed disparate initial ESIPT states and further different responses/sensitivity to force. This study reports a novel and efficient strategy for enriching mechanochromic investigation and extending the applications of ESIPT reactions.