Positive/Negative Phototropism: Controllable Molecular Actuators with Different Bending Behavior

Herein, a series of molecular actuators based on the crystals of (E)-2-(4-fluorostyryl)benzo[d]oxazole (BOAF4), (E)-2-(2,4-difluorostyryl)benzo[d]oxazole (BOAF24), (E)-2-(4-fluorostyryl)benzo[d]thiazole (BTAF4), and (E)-2-(2,4-difluorostyryl)benzo[d]thiazole (BTAF24) showed unique bending behavior under UV irradiation. The one-dimensional (1D) crystals of BOAF4 and BTAF4 bent toward light, whereas those of BOAF24 and BTAF24 bent away from light. Although the chemical structures of these compounds are similar, the authors found that F center dot center dot center dot H-C interaction played a key role in the different molecular packing in structures crystals, which led to the positive/negative phototropism of the actuators. Moreover, theoretical calculations were carried out to reveal the mechanical properties of the crystals. Taking advantage of these photomechanical properties, the authors achieved the potential application in pushing objects, as well as enriching and removing pollutants. Hence, the molecular actuators with different bending behavior could be fabricated by introducing different number of F atom, which may open a novel gate for crystal engineering.