Seeing Molecular Configuration in Twisted Crystal Form

Abstract

Structural chemistry began with Louis Pasteur, who first saw the configuration of molecules manifest in the shapes of crystals. Here is reviewed forgotten, century‐old efforts to make molecular configuration vivid in polycrystalline ensembles. Resorcinol, and a variety of other simple molecular crystals, when grown from the melt in the presence of resolved chiral additives, such as tartaric acids, form spectacular bull's‐eye patterns evident in the petrographic microscope. Concentric, rhythmic optical bands in so‐called banded spherulites are tell‐tale signs of the twisting of helicoidal crystalline lamellae. The sense of twisting was assayed by correlating displacements of extinction bands on rotation with additive configurations. Thus, the arrangements of atoms in space were made manifest in crystal form – not in the appearance of hemihedral facets, but rather, in the chirality of nonclassical morphologies with Gaussian curvature. The far reaching consequences of these experiments for molecular crystal growth and form, long ago foreseen, are evaluated with the hindsight of an extra century of experience and sophistication. The narrative aims to draw a continuous thread through the work of Pasteur, Grigorii Vyroubov, Frédéric Wallerant, John Monteath Robertson, Hélène Metzger, and Jack Dunitz.

Mechanical properties and peculiarities of molecular crystals

“…A large proportion (up to one-third) of molecular crystals can be made to twist as they grow. 38,[110][111][112][113] In some families of crystals, such as binary charge transfer complexes, the proportion can be greater than one-half. 114 This group includes many common materials that have been studied for decades or more.…”

“…Many twisted crystals display their nonclassical morphologies in the absence of impurities (in evidence), while in other cases, additives appear to be crucial to the process of twisted crystal growth. Impurities [110] to [210] and then to [310] with respect to the incident electron beam. Adapted with permission from ref.…”

“…Lower part: convergent beam electron diffraction patterns taken from the locations marked by black arrows. They demonstrate the change of the crystal orientation along the wire from[110] to[210] and then to[310] with respect to the incident electron beam. Adapted with permission from ref 158.…”

Poincaré and his polarization sphere

“…Chiroptics remained at the center of French physical optics 72 since the work of Arago, 73 Biot, Fresnel, and Pasteur (1822-1895) 74 in the first part of the 19th century. Arago first observed the consequences of optical rotatory dispersion in quartz, Biot first observed that solutions of some organic molecules can rotate the plane of polarization, and Pasteur established the first correlation between macroscopic chirality in the shapes of crystals and microscopic chirality in the action of molecules liberated from dissymmetric crystals on the azimuth of linearly polarized light.…”

Coherence in Polycrystalline Thin Films of Twisted Molecular Crystals

“…The spherulitic crystal growth morphology becomes more complex if crystallites spontaneously twist as they grow. − Such so-called banded spherulites display oscillating linear birefringence (LB) as a consequence of continuous changes in crystallite orientation and refractivity. , In general, up to one-third of molecular crystals can be made to twist as they grow from the melt. − In some families of crystals, such as binary charge transfer complexes (CTCs), the proportion can be greater than one-half . Due to the anisotropy of crystal cross section shape and branching frequency, twisting substantially complicates fiber organization in spherulites.…”