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

Abstract

Tailor-made additives (TMAs) have found a role in crystal morphology engineering and control through specific binding to crystal surfaces through stereochemical 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. Expected final online publication date for the Annual Review of Materials Research, Volume 53 is July 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Collimating the growth of twisted crystals of achiral compounds

“…Nevertheless, enantiomorphous helices can be correlated with stereogenic building components or the stereochemistry of twist‐inducing additives 20 . Heterochiral helices can also be correlated with enantiomorphous space groups 32 and/or enantiopolar growth directions 16,33,34 …”

“…[51][52][53] Though likely only marginally relevant to spherulitic growth of molecular crystals, helically twisted photonic bundles of silica fibers exploit their dissymmetry to control the orbital angular momentum of light. 54 To test waveguiding in our materials, we must switch from making optical chiroptical measurements of banded spherulites normal to the radial growth direction 22,34,55 to make measurements along the radial growth direction, for example as in Scheme 2. 53 We have previously shown that the CR directed along twisted fibrils of polymers can be especially high at the very center of the polycrystalline ensemble where the fibers are parallel to the wavevector of light.…”

“…To test waveguiding in our materials, we must switch from making optical chiroptical measurements of banded spherulites normal to the radial growth direction 22,34,55 to make measurements along the radial growth direction, for example as in Scheme 2 53 . We have previously shown that the CR directed along twisted fibrils of polymers can be especially high at the very center of the polycrystalline ensemble where the fibers are parallel to the wavevector of light 56 .…”

“…20 Heterochiral helices can also be correlated with enantiomorphous space groups 32 and/or enantiopolar growth directions. 16,33,34 4A displays a CR micrograph of an unconfined BDT spherulite bisected into two halves, one with a positive CR signal and the other with a negative CR signal corresponding to regions with heterochiral twists, respectively. The signal is not natural optical activity but has its origins in the splaying of anisotropic lamellae with a fixed sense.…”

Chiral Crystals, Jack, Conductivity and Magnetism

“…Typically, one side of a TTF spherulite spins out crystals that twist to the right while the other spins out crystals that twist to the left. This is ultimately determined by the sense of the enantiopolar faces of the crystal that served as the nucleus [70–72] …”