Dynamic Molecular Crystal Triggered by Near-Ambient Intramolecular Twisting for Switchable Luminescence and Two-Photon Absorption

Dynamic molecular crystals capable of undergoing reversible transitions near-ambient temperature are highly promising for practical applications, such as in biological systems where mild operating conditions are essential, yet their intramolecular twisting design remains challenging. Herein, we report the fabrication of dynamic rod-shaped crystals based on a donor-acceptor-donor molecular architecture that enables rapid and reversible switching of optical properties through mild thermal stimulation. The rod-shaped crystals exhibit reversible thermofluorochromic behavior upon thermal cycling between approximately 20 and 40 °C. Specifically, heating to 40 °C induces a fluorescence redshift from 594 to 633 nm accompanied by a 5-fold enhancement in quantum yield, while cooling to 20 °C restores the original emission profile. Single-crystal X-ray diffraction (SCXRD) analysis reveals that this switching behavior is initiated through thermal disruption of key intermolecular Se···π interactions (chalcogen bonding), which releases the constrained molecular conformation and allows relaxation into a more planar structure with a reduced donor-acceptor dihedral angle (from 77.5° to 34.5°). This conformational planarization enhances intramolecular conjugation, resulting in a more than 4-fold increase in the two-photon absorption cross-section (to ∼6700 GM at 810 nm). Furthermore, the system maintains excellent reversibility and stability over at least 20 operational cycles, underscoring its potential for advanced applications in smart optical materials and biorelated technologies.