Tailor-Made Additives for Melt-Grown Molecular Crystals: Why or Why Not?

Tailor-made additives (TMAs) have found a role in crystal morphology engineering and control by specific binding to crystal surfaces through stereo-chemical recognition. The utility of TMAs, however, has been largely limited to crystal growth from solutions. In this review, we illustrate examples where TMAs have been used to influence the growth of crystals during cooling of their melts. In solution, the crystal growth driving force is governed by solute supersaturation, which corresponds to the deviation from equilibrium. In growth from melts, however, undercooling is the important thermodynamic parameter responsible for crystallization outcomes, a key difference that can influence the manner in which TMAs affect growth kinetics, crystal morphology, nucleation, enantioselective surface recognition, and the determination of the absolute sense of polar axes. When the crystallization driving force in a melt is small and diffusion is comparatively high, TMAs can exert their influence on well-faceted single crystals with the stereochemical richness observed in solution growth. Under high supercooling, where the driving force is large, ensembles of crystals can grow radially, masking stereochemical information and requiring new optical tools for understanding the influence of TMAs on emerging crystals.