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

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-NH2-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-NH2-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-NH2-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.