Superflexible Fluorescent π-Conjugated Organic Crystals: Synergistic Mechanical-Optical Properties and Molecular Engineering Insights

Molecular crystals exhibit broad application prospects in the field of flexible functional materials due to their unique elastic behavior. This study designed and synthesized four structurally similar flexible molecular crystals with fluorescence properties. Among them, three crystals contain −C═N–NH– groups (named Bhn, Chn, Pthn), while the fourth incorporates −N═N– group (termed Modn). The mechanical response characteristics of these crystals were systematically explored through a comprehensive series of analyses. Experimental results demonstrate that, under external force stimulation, the four crystals exhibit differentiated bending performance, among which Pthn and Modn crystals can achieve complete 180° bending, showcasing excellent flexibility. Through three-point bending tests and nanoindentation techniques, the mechanical properties of these two large-sized single crystals were quantitatively characterized. It was found that they can withstand ultralarge strains of 80% and 120% respectively under extremely low stress conditions before fracturing, while also possessing low elastic moduli (EPthn = 3.17 ± 0.11 GPa, EModn = 3.75 ± 0.13 GPa) and hardness values (HPthn = 0.13 ± 0.02 GPa, HModn = 0.23 ± 0.01 GPa), confirming their outstanding mechanical flexibility and ductility. Furthermore, under ultraviolet excitation, all four crystals emit orange-red fluorescence (λ > 600 nm), but with significant differences in fluorescence intensity. This synergistic regulation effect of mechanical-optical properties provides an important theoretical foundation for developing novel intelligent flexible optoelectronic materials. Further optimization of crystal packing modes through molecular engineering strategies is expected to achieve precise control of force-light coupling characteristics, which holds significant guiding importance for promoting practical applications of such materials in fields like flexible optoelectronic sensors and adaptive optical devices.