Chirality Determination of Nanocrystals by Electron Crystallography

Chirality is a common phenomenon in nature and plays an important role in the properties of matter. The rational synthesis of chiral compounds and exploration of their applications in various fields require an unambiguous determination of their handedness. However, in many cases, determinations of the chiral crystal structure and chiral morphology have been a challenging task due to the lack of proper characterization methods, especially for nanosized crystals. Therefore, it is crucial to develop novel and efficient characterization methods. Owing to the strong interactions between matter and electrons, electron crystallography has become a powerful tool for structural analysis of nanomaterials. In recent years, methods based on electron crystallography, such as high-resolution electron microscopy imaging and electron diffraction, have been developed to unravel the chirality of nanomaterials. This brings new opportunities to the design, synthesis, and applications of versatile chiral nanomaterials. In this perspective, we summarize the recent methodology developments and ongoing research of electron crystallography for chiral structure and morphology determination of nanocrystals, including inorganic and organic materials, as well as highlight the potential and further improvement of these methods in the future.

Mechanically controlled multifaceted dynamic transformations in twisted organic crystal waveguides

This study introduces mechanically induced phenomena such as standing, leaning, stacking, and interlocking behaviors in naturally twisted optical waveguiding microcrystals on a substrate. The microscale twisted crystal self-assembled from 2,4-dibromo-6-(((2-bromo-5-fluorophenyl)imino)methyl)phenol is flexible and emits orange fluorescence. Mechanistic analysis reveals the strain generated by the intergrowing orientationally mismatched nanocrystallites is responsible for the twisted crystal growth. The crystal's mechanical flexibility in the perpendicular direction to (001) and (010) planes can be attributed to intermolecular Br···Br, F···Br, and π···π stacking interactions. Through a systematic process involving step-by-step bending and subsequent optical waveguiding experiments at each bent position, a linear relationship between optical loss and mechanical strain is established. Additionally, the vertical standing and leaning of these crystals at different angles on a flat surface and the vertical stacking of multiple crystals reveal the three-dimensional aspects of organic crystal waveguides, introducing light trajectories in a 3D space. Furthermore, the integration of two axially interlocked twisted crystals enables the coupling of polarization rotation along their long axis. These crystal dynamics expand the horizons of crystal behavior and have the potential to revolutionize various applications, rendering these crystals invaluable in the realm of crystal-related science and technology.

Coherence in Polycrystalline Thin Films of Twisted Molecular Crystals

Helicoidal crystallites in rhythmically banded spherulites manifest spectacular optical patterns in small molecules and polymers. It is shown that concentric optical bands indicating crystallographic orientations typically lose coherence (in-phase twisting) with growth from the center of nucleation. Here, coherence is shown to increase as the twist period decreases for seven molecular crystals grown from the melt. This dependence was correlated to crystallite fiber thickness and length, as well as crystallite branching frequency, a parameter that was extracted from scanning electron micrographs, and supported by numerical simulations. Hole mobilities for 2,5-didodecyl-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DPP-C12) measured by using organic field-effect transistors demonstrated that more incoherent boundaries between optical bands in spherulites lead to higher charge transport for films with the same twist period. This was rationalized by combining our growth model with electrodynamic simulations. This work illustrates the emergence of complexity in crystallization processes (spherulite formation) that arises in the extra variable of helicoidal radial twisting. The details of the patterns analyzed here link the added complexity in crystal growth to the electronic and optical properties of the thin films.

Leveling up Organic Semiconductors with Crystal Twisting

The performance of crystalline organic semiconductors depends on the solid-state structure, especially the orientation of the conjugated components with respect to device platforms. Often, crystals can be engineered by modifying chromophore substituents through synthesis. Meanwhile, dissymetry is necessary for high-tech applications like chiral sensing, optical telecommunications, and data storage. The synthesis of dissymmetric molecules is a labor-intensive exercise that might be undermined because common processing methods offer little control over orientation. Crystal twisting has emerged as a generalizable method for processing organic semiconductors and offers unique advantages, such as patterning of physical and chemical properties and chirality that arises from mesoscale twisting. The precession of crystal orientations can enrich performance because achiral molecules in achiral space groups suddenly become candidates for the aforementioned technologies that require dissymetry.

Self-Patterning Tetrathiafulvalene Crystalline Films

Tetrathiafulvalene (TTF) crystals grown from the melt are organized as spherulites in which helicoidal fibrils growing radially from the nucleation center twist in concert with one another. Alternating bright and dark concentric bands are apparent when films are viewed between crossed polarizers, indicating an alternating pattern of crystallographic faces exposed at the film surface. Band-dependent reorganization of the TTF crystals was observed during exposure to methanol vapor. Crystalline growth appears on bright bands at the expense of the dark bands. After a 24 h period of exposure to methanol vapor, the original spherulites were completely restructured, and the films comprise isolated, concentric circles of crystallites whose orientations are determined by the initial TTF crystal fibril orientation. While the surface of these outgrowths appears faceted and smooth, cross-sectional SEM images revealed a semiporous inner structure, suggesting solvent-vapor-induced recrystallization. Collectively, these results show that crystal twisting can be used to rhythmically redistribute material. Crystal twisting is a common and often controllable phenomenon independent of molecular or crystal structure and therefore offers a generalizable path to spontaneous pattern formation in a wide range of materials.

Twisting, untwisting, and retwisting of elastic Co-based nanohelices

The reversible transformation of a nanohelix is one of the most exquisite and important phenomena in nature. However, nanomaterials usually fail to twist into helical crystals. Considering the irreversibility of the previously studied twisting forces, the reverse process (untwisting) is more difficult to achieve, let alone the retwisting of the untwisted crystalline nanohelices. Herein, we report a new reciprocal effect between molecular geometry and crystal structure which triggers a twisting-untwisting-retwisting cycle for tri-cobalt salicylate hydroxide hexahydrate. The twisting force stems from competition between the condensation reaction and stacking process, different from the previously reported twisting mechanisms. The resulting distinct nanohelices give rise to unusual structure elasticity, as reflected in the reversible change of crystal lattice parameters and the mutual transformation between the nanowires and nanohelices. This study proposes a fresh concept for designing reversible processes and brings a new perspective in crystallography.

Self-assembly of chiral diketopyrrolopyrrole chromophores giving supramolecular chains in monolayers and twisted microtapes

Chiral diketopyrrolopyrroles appended with enantiomeric ethyl lactate functions through an ether linkage to the aryl backbone of the chromophore were synthesized via the Mitsunobu reaction. The molecules have good solubility and excellent optical properties, high molar absorption coefficients, and fluorescence quantum yields. Helical aggregates with circular dichroism arising from the supramolecular arrangement are seen in both solution and thin films, and the aggregates also display circularly polarized luminescence (glum  ≈ ±0.1). The molecules assemble to give monolayers on graphite and precipitate from solution forming supramolecular twisted tapes hundreds of microns long.

Charge Transport in Twisted Organic Semiconductor Crystals of Modulated Pitch

Many molecular crystals (approximately one third) grow as twisted, helicoidal ribbons from the melt, and this preponderance is even higher in restricted classes of materials, for instance, charge-transfer complexes. Previously, twisted crystallites of such complexes present an increase in carrier mobilities. Here, the effect of twisting on charge mobility is better analyzed for a monocomponent organic semiconductor, 2,5-bis(3-dodecyl-2-thienyl)-thiazolo[5,4-d]thiazole (BDT), that forms twisted crystals with varied helicoidal pitches and makes possible a correlation of twist strength with carrier mobility. Films are analyzed by X-ray scattering and Mueller matrix polarimetry to characterize the microscale organization of the polycrystalline ensembles. Carrier mobilities of organic field-effect transistors are five times higher when the crystals are grown with the smallest pitches (most twisted), compared to those with the largest pitches, along the fiber elongation direction. A tenfold increase is observed along the perpendicular direction. Simulation of electrical potential based on scanning electron microscopy images and density functional theory suggests that the twisting-enhanced mobility is mainly controlled by the fiber organization in the film. A greater number of tightly packed twisted fibers separated by numerous smaller gaps permit better charge transport over the film surface compared to fewer big crystallites separated by larger gaps.

Helical mesoscopic crystals based on an achiral charge-transfer complex with controllable untwisting/breaking

The development of synthetic helical structures from achiral molecules and stimulus-responsive shape transformations are vital for biomimetics and mechanical actuators. A stimulus regarded as the force to induce chirality modulation plays a significant role in the helical supramolecular structure design through symmetry breaking. Herein, we synthesized a metastable complex Form 1 crystal composed of pyrene and (4,8-bis(dicyanomethylene)-4,8-dihydrobenzo[1,2-b:4,5-b']-dithiophen-e) DTTCNQ components with a torsional backbone by C-H⋯N hydrogen bonds via a quick cooling method. The helix motion kinetics of Form 1 depends on the intrinsic factor (crystal thickness) and external stimuli (polar solvents). The self-assembled helical microstructures grow into needle-like crystals in liquid media via an untwistingprocess. Furthermore, they undergo predictable deformation of untwisting or breaking under a stimulus-responsive strain-relaxing phase transformation. This work illustrates a new approach in the mediated formation of helical morphologies from achiral binary supramolecules and dynamic motion, which is vital for biomimetics and mechanical actuators.

Chiroptical anisotropy of crystals and molecules

Optical activity, a foundational part of chemistry, is not restricted to chiral molecules although generations have been instructed otherwise. A more inclusive view of optical activity is valuable because it clarifies structure-property relationships however, this view only comes into focus in measurements of oriented molecules, commonly found in crystals. Unfortunately, measurements of optical rotatory dispersion or circular dichroism in anisotropic single crystals have challenged scientists for more than two centuries. New polarimetric methods for unpacking the optical activity of crystals in general directions are still needed. Such methods are reviewed as well as some of the 'nourishment' they provide, thereby inviting to new researchers. Methods for fitting intensity measurements in terms of the constitutive tensor that manifests as the differential refraction and absorption of circularly polarized light, are described, and examples are illustrated. Single oriented molecules, as opposed to single oriented crystals, can be treated computationally. Structure-property correlations for such achiral molecules with comparatively simple electronic structures are considered as a heuristic foundation for the response of crystals that may be subject to measurement.

Eshelby untwisting

The concept of Eshelby untwisting, the effect of an axial screw dislocation driving an intrinsically twisted nanocrystal towards a straighter configuration more consistent with long-range translational symmetry, is introduced here. Force-field simulations of nanorods built from the enantiomorphous (space groups, P3₁21 and P3₂21) crystal structures of benzil (C₆H₅-C(O)-C(O)-C₆H₅) were previously shown to twist in opposite directions, even in the absence of dislocations. Here, both right- and left-handed screw dislocations were introduced into benzil nanorods in silico. For rods built from the P3₂21 enantiomorph, dislocations with negative Burgers vectors increased the right-handed twisting already present in the intrinsically twisted structures without dislocations, whereas dislocations with positive Burgers vectors drove the twisted structure back towards a straight configuration, untwisting. In the dynamic simulations, the P3₂21 helicoid endowed with a positive Burgers vector ultimately twisted back through the straight configuration, until a helicoid of opposite sense from that of the starting structure, was obtained. The bearing of these observations on the propensity of small crystals to adopt non-polyhedral morphologies is discussed.

Growth and inhibition of monohydrate sodium urate banded spherulites

The growth and inhibition of banded monosodium urate spherulites are explored in detail.

Crystals of Benzamide, the First Polymorphous Molecular Compound, Are Helicoidal

The growth of spontaneously twisted crystals is a common but poorly understood phenomenon. An analysis of the formation of twisted crystals of a metastable benzamide polymorph (form II) crystallizing from highly supersaturated aqueous and ethanol solutions is given here. Benzamide, the first polymorphic molecular crystal reported (1832), would have been the first helicoidal crystal observed had the original authors undertaken an analysis by light microscopy. Polymorphism and twisting frequently concur as they are both associated with high thermodynamic driving forces for crystallization. Optical and electron microscopies as well as electron and powder X-ray diffraction reveal a complex lamellar structure of benzamide form II needle-like crystals. The internal stress produced by the overgrowth of lamellae is shown to be able to create a twist moment that is responsible for the observed non-classical morphologies.

Sustainable sorbitol-derived compounds for gelation of the full range of ethanol-water mixtures

During the development of soft material systems inspired by green chemistry, we show that naturally occurring starting materials can be used to prepare mono- and di-benzylidene sorbitol derivatives. These compounds gelate a range of organic, aqueous (including with mono and divalent metal salt solutions) and ethanolic (ethanol-water) solutions, with the equimolar mixture of two of the gelators gelling all compositions from 100% ethanol to 100% water (something neither of the individual components do). We explored the influence of modifications to the acetal substituents on the formation of the compounds as well as the impact of steric bulk on self-assembly properties of the gelators. The effect of solvent on the self-assembly, morphology, and rheology of the 1,3:2,4-di(4-isopropylbenzylidene)-d-sorbitol (DBS-iPr), 2,4(4-isopropylbenzylidene)-d-sorbitol (MBS-iPr) and the equimolar multicomponent (DBS-MBS-iPr) gels have been investigated. DBS-iPr gelates polar solvents to form smooth flat fibres, whereas in non-polar solvents such as cyclohexane helical fibres grow where the chirality is determined by the stereochemistry of the sugar. Oscillatory rheology revealed that MBS-iPr gels have appreciable strength and elasticity, in comparison to DBS-iPr gels, regardless of the solvent medium employed. Powder X-ray diffraction was used to probe the arrangement of the gelators in the xerogels they form, and two single crystal X-ray structures of related MBS derivatives give the first precise structural information concerning layering and hydrogen bonding in the monobenzylidene compounds. This kind of layering could explain the apparent self-sorting behaviour of the DBS-MBS-iPr multicomponent gels. The combination of sorbitol-derived gelators reported in this work could find potential applications as multicomponent systems, for example, in soft materials for personal care products, polymer nucleation/clarification, and energy technology.

Twist renormalization in molecular crystals driven by geometric frustration

Symmetry considerations preclude the possibility of twist or continuous helical symmetry in bulk crystalline structures. However, as has been shown nearly a century ago, twisted molecular crystals are ubiquitous and can be formed by about 1/4 of organic substances. Despite its ubiquity, this phenomenon has so far not been satisfactorily explained. In this work we study twisted molecular crystals as geometrically frustrated assemblies. We model the molecular constituents as uniaxially twisted cubes and examine their crystalline assembly. We exploit a renormalization group (RG) approach to follow the growth of the rod-like twisted crystals these constituents produce, inquiring in every step into the evolution of their morphology, response functions and residual energy. The gradual untwisting of the rod-like frustrated crystals predicted by the RG approach is verified experimentally using silicone rubber models of similar geometry. Our theory provides a mechanism for the conveyance of twist across length-scales observed experimentally and reconciles the apparent paradox of a twisted single crystal as a finite size effect.

Dislocations in molecular crystals

Dislocations in molecular crystals remain terra incognita. Owing to the complexity of molecular structure, dislocations in molecular crystals can be difficult to understand using only the foundational concepts devised over decades for hard materials. Herein, we review the generation, structure, and physicochemical consequences of dislocations in molecular crystals. Unlike metals, ceramics, and semiconductors, molecular crystals are often characterized by flexible building units of low symmetry, thereby limiting analysis, complicating modeling, and prompting new approaches to elucidate their role in crystallography from growth to mechanics. Such considerations affect applications ranging from plastic electronics and mechanical actuators to the tableting of pharmaceuticals.

Organization of Twisting Lamellar Crystals in Birefringent Banded Polymer Spherulites: A Mini-Review

In this mini-review, we summarize the evidences of lamellar twisting in the birefringent banded polymer spherulites demonstrated by various characterization techniques, such as polarized optical microscopy, real-time atomic force microscopy, micro-focus wide angle X-ray diffraction, etc. The real-time observation of lamellar growth under atomic force microscopy unveiled the fine details of lamellar twisting and branching in the banded spherulites of poly(R-3-hydroxybutyrate-co-17 mol% R-3-hydroxyhexanoate). Organization of the twisting lamellar crystals in the banded spherulites was revealed as well. The lamellar crystals change the orientation via twisting rather than the macro screw dislocations. In fact, macro screw dislocation provides the mechanism of synchronous twisting of neighboring lamellar crystals. The driving force of lamellar twisting is attributed to the anisotropic and unbalanced surface stresses. Besides molecular chirality, variation of the growth axis and the chemical groups on lamellar surface can change the distribution of the surface stresses, and thus may invert the handedness of lamellar twisting. Thus, based on both experimental results and physical reasoning, the relation between crystal chirality and chemical molecular structures has been suggested, via the bridge of the distribution of surface stresses. The factors affecting band spacing are briefly discussed. Some remaining questions and the perspective of the topic are highlighted.

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.