Tailor-Made Auxiliaries for Polar Growth from Melts

Tailor-made auxiliaries are generally less stereochemically discriminating of crystals grown from the melt than crystals grown from solution. However, it is possible to make supramolecular inferences using well-chosen additives in the manner taught to us by Lahav and Leiserowitz. Spherulites manifesting needle-like growth and small angle branching grow frequently from supercooled melts under high crystallization driving forces, along one principal crystallographic direction. If this direction is polar, it is important to establish its absolute sense as a basis for understanding growth at the interface between crystals and melt. This assignment, for polycrystalline ensembles, is beyond the capabilities of X-ray crystallography. Two examples of this discrimination are described with tailor-made additives, one for the β form of resorcinol and another for form I of L-malic acid. Assigning the absolute sense of a polar axis with molecular additives is a problem that resembles both the science and style of previous experiments from the Weizmann Institute.