Synopsis of Research in Shanghai (July 7 - July 25):
Professor Zhihua An’s research project studies the crystal bending in melt-processed coumarin derivatives. Coumarin 102 is a coumarin derivative with a donor, bridge and acceptor moieties synthesized as an excellent low-cost alternative to conventional photovoltaics. Growing melt-processed thin crystal films of coumarin 102, we have observed the toroidal bent crystals formed on the spherulitic, or radially grown, straight crystals by polarized optical microscopy. The crystal structure was determined as P21/c by X-ray powder diffraction. The temperature dependence of the crystal structure and the size of the toroid was investigated. The growth behavior and the optical properties were also observed under polarized optical microscope and Mueller matrix microscope. These toroidal bent structures of coumarin 102 exhibit opposite bending directions with 50% going clockwise and counterclockwise randomly. Moreover, we have discovered that the fluorescence intensity of the bending crystal fibers is nearly twice stronger than the straight crystal fibers. These observations suggest that studying crystal bending beyond a single crystal may be another strategy to reveal crystallization mechanisms and develop new functional materials with structures that can improve the performance of optoelectronic devices.