Unique Crystallization of Fullerenes: Fullerene Flowers

Jungah Kim, Chibeom Park, Intek Song, Minkyung Lee, Hyungki Kim and Hee Cheul Choi*

We made highly symmetrical flower-shaped fullerene crystals starting from mesitylene/ethanol solutions of mixed C₆₀ and C₇₀. The crystals consisted of both C₆₀ and C₇₀, with solvation by mesitylene. The main factor behind the unique crystal growth was the solubility difference between C₆₀ and C₇₀, which led to the two-stage growth: first C₇₀ formed the center of the crystal, and then C₆₀-C₇₀ co-crystallized to form petals. The poor solvent, ethanol in our case, guided the crystallization mechanism to yield complex flower crystals, which is surprising because of the role of the poor solvent in affecting the crystallization pathway. By manipulating the morphology of the crystals on which the petal formation occurs, various fullerene flowers could be obtained: single-deck, double-deck, and multi-deck fullerene flowers. In addition, we synthesized more complex crystals by adopting a two-step growth process. The high surface energy of the tips of C₆₀ hexagonal tubes resulted in a pair of fullerene flowers grown on each tip of a stem. The use of different types of solutions can effectively alter pre-grown fullerene flowers into larger fullerene flowers or edge-grown fullerene flowers. We believe that our findings provide insights into the solution-phase growth of molecular crystals and illuminate new aspects of crystallization chemistry.