Polymorphism-based luminescence and morphology-dependent optical waveguide properties in 1 : 1 charge transfer cocrystals

A polymorph is an ideal crystal model for studying the structures affect the optical waveguide behaviors of organic cocrystals, resulting from their multiple crystal structures with the same compositions. However, the polymorphism-based optical waveguide properties of cocrystals are rarely reported. Here, two polymorphic cocrystals of 9-acetylanthracene-1,2,4,5-tetracyanobenzene in a novel sheet-like form α and a known bulk form β were prepared by the evaporation of a donor (D)/acceptor (A) solution mixed in a ratio of 1 : 1 in a mixed acetonitrile and tetrahydrofuran solution. The two polymorphs present different molecular stacking structures due to the diverse intermolecular hydrogen bond interactions, causing differences in their intermolecular charge transfer (CT) interactions and photophysical properties. Structural and spectroscopic analyses revealed stronger CT transition in the form β than in the form α. Polymorphism-based luminescence properties and morphology- and color-dependent optical waveguide behaviors were observed in the polymorphic CT cocrystals via photoluminescence (PL) micro-imaging. The propagation losses of the form α were 0.37 and 0.6 times less than those of the form β in parallel and perpendicular direction with respect to the growth direction, respectively. Three-dimensional (3D) stereoscopic mode optical waveguide behaviors can be observed in the form β. The study provides a good method for the preparation and detection of polymorphic materials. The optical waveguide properties of the cocrystals were tuned by the crystal phase, which is helpful for a deep understanding of the structure-optical functional relation in organic cocrystals. The study will open new potential applications of cocrystal materials in 3D photonic and optoelectronic devices.