Showing papers by "Bart Kahr published in 2022"

Charge Transport in Twisted Organic Semiconductor Crystals of Modulated Pitch

Abstract: 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.

Disrupting Crystal Growth through Molecular Recognition: Designer Therapies for Kidney Stone Prevention.

Abstract: ConspectusAberrant crystallization within the human body can lead to several disease states or adverse outcomes, yet much remains to be understood about the critical stages leading to these events, which can include crystal nucleation and growth, crystal aggregation, and the adhesion of crystals to cells. Kidney stones, which are aggregates of single crystals with physiological origins, are particularly illustrative of pathological crystallization, with 10% of the U.S. population experiencing at least one stone occurrence in their lifetimes. The human record of kidney stones is more than 2000 years old, as noted by Hippocrates in his renowned oath and much later by Robert Hooke in his treatise Micrographia. William Hyde Wollaston, who was a physician, chemist, physicist, and crystallographer, was fascinated with stones, leading him to discover an unusual stone that he described in 1810 as cystic oxide, later corrected to cystine. Despite this long history, however, a fundamental understanding of the stages of stone formation and the rational design of therapies for stone prevention have remained elusive.This Account reviews discoveries and advances from our laboratories that have unraveled the complex crystal growth mechanisms of l-cystine, which forms l-cystine kidney stones in at least 20 000 individuals in the U.S. alone. Although l-cystine stones affect fewer individuals than common calcium oxalate stones, they are usually larger, recur more frequently, and are more likely to cause chronic kidney disease. Real-time in situ atomic force microscopy (AFM) reveals that the crystal growth of hexagonal l-cystine is characterized by a complex mechanism in which six interlaced anisotropic spirals grow synchronously, emanating from a single screw dislocation to generate a micromorphology with the appearance of stacked hexagonal islands. In contrast, proximal heterochiral dislocations produce features that appear to be spirals but actually are closed loops, akin to a Frank-Read source. These unusual and aesthetic growth patterns can be explained by the coincidence of the dislocation Burgers vector and the crystallographic 61 screw axis. Inhibiting l-cystine crystal growth is key to preventing stone formation. Decades of studies of "tailor-made additives", which are imposter molecules that closely resemble the solute and bind to crystal faces through molecular recognition, have demonstrated their effects on crystal properties such as morphology and polymorphism. The ability to visualize crystal growth in real time by AFM enables quantitative measurements of step velocities and, by extension, the effect of prospective inhibitors on growth rates, which can then be used to deduce inhibition mechanisms. Investigations with a wide range of prospective inhibitors revealed the importance of precise molecular recognition for binding l-cystine imposters to crystal sites, which results in step pinning and the inhibition of step advancement as well as the growth of bulk crystals. Moreover, select inhibitors of crystal growth, measured in vitro, reduce or eliminate stone formation in knockout mouse models of cystinuria, promising a new pathway to l-cystine stone prevention. These observations have wide-ranging implications for the design of therapies based on tailor-made additives for diseases associated with aberrant crystallization, from disease-related stones to "xenostones" that form in vivo because of the crystallization of low-solubility therapeutic agents such as antiretroviral agents.

Twisted Tetrathiafulvalene Crystals

Abstract: Optically-active optoelectronic materials are of great interest for many applications, including chiral sensing and circularly polarized light emission. Traditionally, such applications have been enabled by synthetic strategies to design chiral...

Efficient Polymorph Screening through Crystallization from Bulk and Confined Melts

Abstract: Crystallization from the melt can allow the achievement of high driving force for crystallization accompanied by relatively slow growth, nucleation, and transformation rates, features that favor its use as an efficient polymorph screening method. Surprisingly, even though melt crystallization has a long history, it has been employed less often in the search for new polymorphs than solution crystallization. Applications of melt crystallization to 21 highly polymorphic, well-characterized compounds with at least five ambient polymorphs revealed that melt crystallization afforded more than half of the known polymorphs and in many cases revealed new polymorphs not detected by other screening methods. A statistical analysis revealed that polymorphs grown from the melt have a greater propensity for high Z′ values, which are not easily accessible by other crystallization protocols and are often not detectable by crystal structure prediction methods. Melt crystallization within nanopores (8–100 nm) performed for 19 of the 21 compounds mostly resulted in polymorphs that dominated crystallization from the bulk melt at similar temperatures. The total number of polymorphs observed in nanopores was less than that observed during crystallization from the bulk melt, however, and melt crystallization under confinement revealed new polymorphs not detected by other crystallization methods.

Extracting Pure Circular Dichroism from Hierarchically Structured CdS Magic Cluster Films.

Abstract: Chiroptically active, hierarchically structured materials are difficult to accurately characterize due to linear anisotropic contributions (i.e., linear dichroism (LD) and linear birefringence (LB)) and parasitic ellipticities that produce artifactual circular dichroism (CD) signals, in addition to chiral analyte contributions ranging from molecular-scale clusters to micron-sized assemblies. Recently, we have shown that CdS magic-sized clusters (MSC) can self-assemble into ordered films that have a hierarchical structure spanning seven orders of length-scale. These films have a strong CD response, but the chiral origins are obfuscated by the hierarchical architecture and LDLB contributions. Here, we derive and demonstrate a method for extracting the "pure" CD signal (CD generated by structural dissymmetry) from hierarchical MSC films and identified the chiral origin. The theory behind the method is derived using Mueller matrix and Stokes vector conventions and verified experimentally before being applied to hierarchical MSC and nanoparticle films with varying macroscopic orderings. Each film's extracted "true CD" shares a bisignate profile aligned with the exciton peak, indicating the assemblies adopt a chiral arrangement and form an exciton coupled system. Interestingly, the linearly aligned MSC film possesses one of the highest g-factors (0.05) among semiconducting nanostructures reported. Additionally, we find that films with similar electronic transition dipole alignment can possess greatly different g-factors, indicating chirality change rather than anisotropy is the cause of the difference in the CD signal. The difference in g-factor is controllable via film evaporation geometry. This study provides a simple means to measure "true" CD and presents an example of experimentally understanding chiroptic interactions in hierarchical nanostructures.

Crystallography of Contemporary Contact Insecticides

Abstract: Simple Summary The efficacy of crystalline contact insecticides is dependent foremost on the uptake of insecticide molecules by insect tarsi contacting crystal surfaces. Insecticide molecules, however, may organize in more than one way in the crystalline state, resulting in more than one crystalline form (also known as polymorph). We recently discovered that the lethality of contact insecticides increases with decreasing thermodynamic stability of the crystalline forms; the most stable crystalline form is invariably the least lethal/slowest acting. Polymorphism in contact insecticides, and its importance to efficacy, was largely unknown to the vector control community. It is argued that the crystallographic characterization of contact insecticide solids should be systematic to identify more active solid forms. Herein, we report seven new crystal structures, mostly pyrethroid insecticides recommended by the WHO for indoor residual spraying, as well as a new form of a neonicotinoid insecticide. These results further highlight polymorphism in contact insecticides and the importance of solid-state chemistry in the search for more active crystal forms. Abstract The active forms of contact insecticides used for combatting mosquito-borne infectious diseases are typically crystalline solids. Numerous molecular crystals are polymorphic, crystallizing in several solid forms characterized by different physicochemical properties, including bioavailability. Our laboratory recently found that the activity of crystalline contact insecticides is inversely dependent on the thermodynamic stability of their polymorphs, suggesting that efficacy can be enhanced by the manipulation of the solid-state structure. This paper argues that crystallography should be central to the development of contact insecticides, particularly because their efficacy continues to be compromised by insecticide resistance, especially among Anopheles mosquito populations that spread malaria. Although insecticidal compounds with new modes of action have been introduced to overcome resistance, new insecticides are expensive to develop and implement. The repurposing of existing chemical agents in metastable, more active crystalline forms provides an inexpensive and efficient method for ‘evergreening’ compounds whose risks are already well-established. We report herein seven new single-crystal structures of insecticides used for controlling infectious disease vectors. The structures reported herein include pyrethroid insecticides recommended by the WHO for indoor residual spraying (IRS)-bifenthrin, β-cyfluthrin, etofenprox, α-cypermethrin, and λ-cyhalothrin as well as the neonicotinoid insecticide thiacloprid.

Gender and Library of Mineralogy

Abstract: Few would expect women to feature often in the literature on minerology from the 15th through the 19th centuries, the recorded history of science being what it is. Among the approximately 1500 scholars in a massive catalogue of authors of mineralogy texts from 1439 to 1919 complied by the independent scholar, Curtis P. Schuh, we count six women as primary entries and three others discussed secondarily. From the documents that Schuh left behind before his death, our database for this investigation, women wrote approximately 0.5% of the texts described. Only very unusual circumstances supported the life of a woman devoted to crystals in centuries past.

Disrupting Crystal Growth through Molecular Recognition: Designer Therapies for Kidney Stone Prevention

Abstract: proximal heterochiral dislocations produce features that appear to be spirals but actually are closed loops, akin to a Frank − Read source. These unusual and aesthetic growth patterns can be explained by the coincidence of the dislocation Burgers vector and the crystallographic 6 1 screw axis. Inhibiting L -cystine crystal growth is key to preventing stone formation. Decades of studies of “ tailor-made additives ” , which are imposter molecules that closely resemble the solute and bind to crystal faces through molecular recognition, have demonstrated their e ff ects on crystal properties such as morphology and polymorphism. The ability to visualize crystal growth in real time by AFM enables quantitative measurements of step velocities and, by extension, the e ff ect of prospective inhibitors on growth rates, which can then be used to deduce inhibition mechanisms. Investigations with a wide range of prospective inhibitors revealed the importance of precise molecular recognition for binding L -cystine imposters to crystal sites, which results in step pinning and the inhibition of step advancement as well as the growth of bulk crystals. Moreover, select inhibitors of crystal growth, measured in vitro , reduce or eliminate stone formation in knockout mouse models of cystinuria, promising a new pathway to L -cystine stone prevention. These observations have wide-ranging implications for the design of therapies based on tailor-made additives for diseases associated with aberrant from disease-related stones to “ xenostones ” that form in vivo because of the crystallization of low-solubility therapeutic agents such as antiretroviral agents. the (0001) surface because of specific binding, signaling the rational design of crystal growth inhibitors Molecular Recognition in L -Cystine Crystal Growth Inhibition. 3 L -Cystine mimics, particularly L -cystine diesters and diamides, inhibit L -cystine crystal growth through a common inhibition mechanism consistent with Cabrera − Vermilyea step pinning . crystallization inhibitors for cystinuria.