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.