Viscosity effect on the strategic kinetic overgrowth of molecular crystals in various morphologies: concave and octapod fullerene crystals.

A kinetic overgrowth allowing organic molecular crystals in various morphologies is induced by temperature-dependent viscosity change of crystallization solution. By this strategy, concave cube and octapod fullerene C 70 crystals were successfully obtained by antisolvent crystallization (ASC). The structural analysis of fullerene C 70 crystals indicates that the morphological difference is the result of kinetic processes, which reveals that viscosity, the only variable that can change dynamics of solutes, has a significant influence on determining the morphology of crystals. The effect of solvent viscosity in the stage of crystal growth was investigated through time-dependent control experiments, which led to the proposal of a diffusion rate-based mechanism. Our findings suggest morphology control of organic crystals by diffusion rate control, which is scarcely known compared to inorganic crystals. This strategic method will promote the morphology controls of various organic molecular crystals, and boost the morphology-property relationship study.