Thermal Energy-Driven Solid-State Molecular Rotation Monitored by Real-Time Emissive Color Switching

Abstract

Inspired by nature's molecular machines, the scientific research on solid-state molecular rotors is of great interest yet remains largely unexplored. Herein, we report a unique example of a thermal energy-driven stimuli-responsive solid-state molecular rotor, which features an o-carborane moiety as a rotor that directly transduces the surrounding thermal energy into molecular rotations in the crystalline state. Its rotation is confirmed by X-ray diffraction, low-temperature emission, and time-dependent density functional theory (TD-DFT), and so on, which are responsible for the dynamic conformation changes in the excited states, leading to the expression of twisted intramolecular charge transfer (TICT) emission. TICT states are further identified by temperature-dependent emissions, which was regulated by electronic communications between the electron-donating carbazole unit and the electron-withdrawing o-carborane moiety. As a result, significantly, the molecular rotations of o-carborane-based crystal and its fluorescence reversible processes can be conveniently monitored in real time and visually through emissive color switching from blue to green to yellow-green emission.