Halogen-Tunable Mechanical Properties of the Photomechanical Molecular Crystals Based on Pyrrolo[2,3- b ]pyridineylpropenones

Abstract

Photomechanical molecular crystals, which convert light energy into mechanical motion, have attracted considerable attention because of their potential applications in actuators, soft robotics, and wearable electronic devices. Herein, we report a series of pyrrolo[2,3-b]pyridine-containing diarylpropenone derivatives that undergo highly regioselective and stereospecific solid-state [2 + 2] cycloaddition reactions. Each topochemical reaction affords a single photodimer, providing a green and efficient strategy for the stereospecific synthesis of cyclobutane derivatives. Upon irradiation with 365 nm light, the molecular crystals exhibit pronounced photomechanical bending driven by the photoinduced [2 + 2] cycloaddition. More importantly, the mechanical properties of the crystals can be systematically regulated through the position of halogen substitution. While the unsubstituted PEO forms brittle crystals, PEO-Cl3 and PEO-Br3 give rise to pronounced plasticity, whereas PEO-Cl5 and PEO-Br5 switch the mechanical response to excellent elasticity. Notably, the mechanically bent crystals retain their photomechanical responsiveness and continue to exhibit significant photoinduced bending, demonstrating the successful integration of mechanical reconfiguration and photomechanical actuation within a single crystalline material. Single-crystal structural analyses reveal that these distinct mechanical behaviors originate from different intermolecular interaction patterns and crystal packing modes induced by halogen substitution. This work demonstrates that positional halogen engineering provides an effective strategy for simultaneously tailoring crystal mechanics and photomechanical performance, opening new opportunities for the rational design of multifunctional photomechanical molecular crystals.