Tuning Organic Microcrystal Morphologies through Crystal Engineering Strategies toward Anisotropic Optical Waveguide

The construction of organic optoelectronic materials with desirable size and morphology remains a challenge now. Crystal engineering strategies (polymorphs and cocrystals) provide convenience for tailoring molecular packing and further controlling the growth morphology and photofunctionality of materials. Herein, we prepare polymorphic 2D plate crystals and 3D microhelixes by assembly of a cyanostilbene derivative (2-(3',5'-bis(trifluoromethyl)-biphenyl-4-yl)-3-(4-(pyridin-4-yl)phenyl)acrylonitrile, CF₃-CN-Py). The former emits blue emission, while the latter emits green emission. Different crystallization environments contribute to the adjustable morphologies. Then, novel cocrystals are fabricated with the introduction of 1,4-diiodotetrafluorobenzene (FDIB) to CF₃-CN-Py. Both molecular conformation and packing are totally changed in the cocrystal system. Such cocrystal displays a 1D sky-blue emissive rod shape on account of a long-range ordered π-stacking of molecules. In addition, the 2D plate crystal and 1D rod cocrystal are further applied to optical waveguides. In the plate crystal, a packing of transition dipole moment (μ) inclined to the upper surface leads to an anisotropic optical waveguide. In the cocrystal, owing to the nearly horizontal μ orientation, the cocrystal exhibits light propagation along the primary growth direction and a low optical loss coefficient. The present study supplies an effective way to construct materials with controlled morphology and optical waveguide.