Controlling the π‑Stack Growth Direction in Organic π‑Conjugated Microcrystals

Organic π-conjugated microcrystals (PiMCs) can be used as waveguides, photodetectors, chemical sensors, solid-state lasers, logic operators, and multiplexers. The π-conjugated molecular arrangement along different crystal growth directions determines the waveguiding direction, emission intensity, lasing threshold, and optical losses in the PiMCs. Thus, it is paramount to gain control over the microcrystal growth process and obtain PiMCs of diverse morphologies. Most of the reported crystal growth processes involve crystal growth along the π-stacking direction due to strong π-stacking interactions in organic π-conjugated molecules. So, in order to control the microcrystal growth process and obtain PiMCs of diverse morphologies, there is a need to control the microcrystal facet growth, especially along the π-stacking direction. In this perspective, we discuss the use of conventional alkyl amphiphiles and polymer stabilizers along with novel aryl amphiphiles developed by us as morphology directors to control the PiMC growth morphology. We discuss a few examples of PiMC growth using conventional morphology directors and their limitations in controlling the growth along the π-stack direction. Then we introduce and discuss the use of aryl amphiphiles as morphology directors to control PiMC growth along the π-stack direction and the unique optical and waveguiding properties displayed by these novel growth morphologies.