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.