Tuning Light-Driven Motion and Bending in Macroscale-Flexible Molecular Crystals Based on a Cocrystal Approach.

Flexible molecular crystals with stimuli-responsive properties are highly desirable; however, uncovering them is still a challenging goal. Herein, we report a cocrystal approach to obtain elastic molecular crystals that exhibit light-induced fluorescence changes and dynamic mechanical responses at the macroscale level. Cocrystals of naphthylvinylpyridine and tetrafluoroterephthalic acid were fabricated in different stoichiometry ratios (2:1 and 1:1), which present different shapes [two-dimensional (2D) and one-dimensional (1D) morphologies], photoemission, and mechanical properties (rigidity and flexibility). Moreover, obviously different photomechanical energy conversions (light-driven cracking/popping and bending/motion) occur for the 2D and 1D cocrystals, respectively. Nuclear magnetic resonance (NMR) spectra show the occurrence of photoinduced [2 + 2] cycloaddition in both cocrystals, which is the primary mechanism for their photoactuating behaviors. Crystal structure analysis and theoretical calculation further reveal that protonation and the hydrogen-bonding network play important roles in light-stimulus-bendable 1D cocrystal. Thus, the transformation from rigidity to flexibility based on cocrystallization with different stoichiometry may offer an effective means to tune the dynamic light-driven responses for smart crystalline materials.