Visible Light-Induced Collapse of Molecular Microcrystals into Droplets by E-to-Z Photoisomerization and Melting Phase Transition

Harnessing photoresponsive behavior in organic crystals, such as shape and morphology deformation, on multiple scales is intriguing with respect to precise spatiotemporal operation, because dynamic deformations are promising in fabricating future intelligent devices. Here, we synthesized a new photo responsive molecular crystal composed of dimethyl (E)-2-(3-(naphthalen-1-yl)allylidene)malonate ((E)-DNAM) that undergoes E-to -Z photoisomerization accompanied by photoinduced crystal-to-liquid melting phase transition when exposed to visible light. Instead of expanding in bulk polycrystals, single-crystal (E)-DNAM rectangular microplates in an aqueous solution rapidly collapsed into liquid droplets, showing a remarkable drastic reduction in surface area of more than 97% after light illumination. We deduce that the unique microscopic morphology of microplates with high aspect ratios of length-to thickness reaching up to 1000 is mainly responsible for the photoinduced melting leading to drastic collapse. In addition, the cohesive forces of liquid photoproduct facilitate the overall deformations and transformation. We have shown a facile way to achieve the remarkable collapse of molecular crystals into droplets by photochemical reaction and phase transition.