Resorcinol crystallization from the melt: a new ambient phase and new “riddles”

Structures of the alpha and beta phases of resorcinol, a major commodity chemical in the pharmaceutical, agrichemical, and polymer industries, were the first polymorphic pair of molecular crystals solved by X-ray analysis. It was recently stated that "no additional phases can be found under atmospheric conditions" (Druzbicki, K. et al. J. Phys. Chem. B 2015, 119, 1681.). Herein, is described the growth and structure of a new ambient pressure phase, epsilon, through a combination of optical and X-ray crystallography evaluated by computational crystal structure prediction algorithms. alpha-Resorcinol has long been a model for mechanistic crystal growth studies from solution and the vapor because prisms extended along the polar axis grow much faster in one direction than in the opposite direction. Research has focused on identifying the absolute sense of the fast direction – the so-called ‘resorcinol riddle’ – with the aim of identifying how solvent controls crystal growth. Here, the growth velocity dissymmetry in the melt is analyzed for the ? phase. The epsilon phase only grows from the melt, concomitant with the beta phase, as polycrystalline, radially growing spherulites. If the radii are polar, the sense of the polar axis is an essential feature of the form. Here, this determination is made for spherulites of beta resorcinol (epsilon, point symmetry 222, does not have a polar axis) with additives that stereoselectively modify growth velocities. Both beta and epsilon have the additional feature that individual radial lamellae may adopt helicoidal morphologies. We correlate the appearance of twisting in beta and epsilon with the symmetry of twist-inducing additives

Synthetic asters as elastic and radial skeletons

The radial geometry with rays radiated from a common core occurs ubiquitously in nature for its symmetry and functions. Herein, we report a class of synthetic asters with well-defined core-ray geometry that can function as elastic and radial skeletons to harbor nano- and microparticles. We fabricate the asters in a single, facile, and high-yield step that can be readily scaled up; specifically, amphiphilic gemini molecules self-assemble in water into asters with an amorphous core and divergently growing, twisted crystalline ribbons. The asters can spontaneously position microparticles in the cores, along the radial ribbons, or by the outer rims depending on particle sizes and surface chemistry. Their mechanical properties are determined on single- and multiple-aster levels. We further maneuver the synthetic asters as building blocks to form higher-order structures in virtue of aster-aster adhesion induced by ribbon intertwining. We envision the astral structures to act as rudimentary spatial organizers in nanoscience for coordinated multicomponent systems, possibly leading to emergent, synergistic functions

Structure, Energetics, and Dynamics of Screw Dislocations in Even n-Alkane Crystals

Spiral hillocks on n-alkane crystal surfaces were observed immediately after Frank recognized the importance of screw dislocations for crystal growth, yet their structures and energies in molecular crystals remain ill-defined. To illustrate the structural chemistry of screw dislocations that are responsible for plasticity in organic crystals and upon which the organic electronics and pharmaceutical industries depend, molecular dynamics was used to examine heterochiral dislocation pairs with Burgers vectors along [001] in n-hexane, n-octane, and n-decane crystals. The cores were anisotropic and elongated in the (110) slip plane, with significant local changes in molecular position, orientation, conformation, and energy. This detailed atomic level picture produced a distribution of strain consistent with linear elastic theory, giving confidence in the simulations. Dislocations with doubled Burgers vectors split into pairs with elementary displacements. These results suggest a pathway to understanding the mechanical properties and failure associated with elastic and plastic deformation in soft crystals

Dislocation-Actuated Growth and Inhibition of Hexagonal L-Cystine Crystallization at the Molecular Level

Crystallization of L-cystine is a critical process in the pathogenesis of kidney stone formation in cystinuria, a disorder affecting more than 20 000 individuals in the United States alone. In an effort to elucidate the crystallization of L-cystine and the mode of action of tailored growth inhibitors that may constitute effective therapies, real-time in situ atomic force microscopy has been used to investigate the surface micromorphology and growth kinetics of the {0001} faces of L-cystine at various supersaturations and concentrations of the growth inhibitor L-cystine dimethylester (CDME). Crystal growth is actuated by screw dislocations on the {0001} L-cystine surface, producing hexagonal spiral hillocks that are a consequence of six interlacing spirals of anisotropic molecular layers. The high level of elastic stress in the immediate vicinity around the dislocation line results in a decrease in the step velocities and a corresponding increase in the spacing of steps. The kinetic curves acquired in the presence of CDME conform to the classical Cabrera–Vermilyea model. Anomalous birefringence in the {101̅0} growth sectors, combined with computational modeling, supports a high fidelity of stereospecific binding of CDME, in a unique orientation, exclusively at one of the six crystallographically unique projections on the {1010} plane

Dichroism in helicoidal crystals

Accounting for the interactions of light with heterogeneous, anisotropic, absorbing, optically active media is part of the characterization of complex, transparent materials. Stained biological structures in thin tissue sections share many of these features, but systematic optical analyses beyond the employ of the simple petrographic microscopes have not be established. Here, this accounting is made for polycrystalline, spherulitic bundles of twisted d-mannitol lamellae grown from melts containing light-absorbing molecules. It has long been known that a significant percentage of molecular crystals readily grow as helicoidal ribbons with mesoscale pitches, but a general appreciation of the commonality of these non-classical crystal forms has been lost. Helicoidal crystal twisting was typically assayed by analyzing refractivity modulation in the petrographic microscope. However, by growing twisted crystals from melts in the presence of dissolved, light-absorbing molecules, crystal twisting can be assayed by analyzing the dichroism, both linear and circular. The term "helicoidal dichroism" is used here to describe the optical consequences of anisotropic absorbers precessing around radii of twisted crystalline fibrils or lamellae. d-Mannitol twists in two polymorphic forms, α and δ. The two polymorphs, when grown from supercooled melts in the presence of a variety of histochemical stains and textile dyes, are strongly dichroic in linearly polarized white light. The bis-azo dye Chicago sky blue is modeled because it is most absorbing when parallel and perpendicular to the radial axes in the respective spherulitic polymorphs. Optical properties were measured using Mueller matrix imaging polarimetry and simulated by taking into account the microstructure of the lamellae. The optical analysis of the dyed, patterned polycrystals clarifies aspects of the mesostructure that can be difficult to extract from bundles of tightly packed fibrils

Timoshenko Bending and Eshelby Twisting Predicted in Molecular Nanocrystals

Well-formed crystals are polyhedral with flat facets and sharp edges. Nevertheless, a remarkable number of molecular crystals can bend and twist during growth. Many others can be distorted by applying external forces or creating heterogeneities by temperature gradient or photochemical reaction. As part of an effort to identify the forces that so commonly deform molecular crystals and to characterize their consequences, a force field is evaluated for its ability to predict mechanical distortions in nanocrystals. Macroscopic materials provide estimates of the expected responses that were tested here in silico for "molecular bimetallic strips" created from rods of iodoform and bromoform in smooth contact and nanocrystalline rods of iodoform with left and right screw dislocations. It was demonstrated that an optimized force field based largely on AMBER parameters matches expectations for elastic and plastic distortions, despite the fact that these mechanical responses are far removed from the force field parametrization set

Homochirality in biomineral suprastructures induced by assembly of single-enantiomer amino acids from a nonracemic mixture

© 2019, The Author(s). Since Pasteur first successfully separated right-handed and left-handed tartrate crystals in 1848, the understanding of how homochirality is achieved from enantiomeric mixtures has long been incomplete. Here, we report on a chirality dominance effect where organized, three-dimensional homochiral suprastructures of the biomineral calcium carbonate (vaterite) can be induced from a mixed nonracemic amino acid system. Right-handed (counterclockwise) homochiral vaterite helicoids are induced when the amino acid l-Asp is in the majority, whereas left-handed (clockwise) homochiral morphology is induced when d-Asp is in the majority. Unexpectedly, the Asp that incorporates into the homochiral vaterite helicoids maintains the same enantiomer ratio as that of the initial growth solution, thus showing chirality transfer without chirality amplification. Changes in the degree of chirality of the vaterite helicoids are postulated to result from the extent of majority enantiomer assembly on the mineral surface. These mechanistic insights potentially have major implications for high-level advanced materials synthesis