A Versatile Benzimidazole- Substituted Spirolactam Mechanophore: When Excited State Intramolecular Proton Transfer Couples with Rhodamine

Exploration of multicolor mechanochromic bulk polymers based on a single mechanophore is a big challenge to date. Herein, we report a versatile benzimidazole- substituted spirolactam mechanophore where excited state intramolecular proton transfer (ESIPT) coupled with rhodamine. Themechanophore was facilely synthesized and then covalently linked to polyurethane (PU) chains. The PU film containing themechanophore (1@PU) showed cooperative photochromism upon irradiation involving simultaneous enhancement of normalized enol and rhodol emissions based on a cooperative ESIPT process and the ring-opening reaction of spirolactam. Moreover, the film exhibited dual-mode multicolor mechanochromism upon stretching and compression. The normalized intensity of enol emission increased and the fluorescence turned from light green to cyan after stretching, then red coloration appeared from colorless after compressing. Control experiments and density functional theory calculations confirmed that the stretch-induced increase of enol emission was attributed to torsion of the dihedral angle between xanthene and benzimidazole in the mechanophore via force-induced disaggregation and direct force action on the isolated mechanophore. Torsion of the dihedral angle and the ring-opening reaction of spirolactam in a single mechanophore occurred sequentially during compression, resulting in an observed red coloration. This study might provide a glimpse into the design of novel multicolor mechanochromic mechanophores.