Bart Kahr

Growth actuated bending and twisting of single crystals.

Crystals of a variety of substances including elements, minerals, simple salts, organic molecular crystals, and high polymers forgo long-range translational order by twisting and bending as they grow. These deviations have been observed in crystals ranging in size from nanometers to centimeters. How and why so many materials choose dramatic non-crystallographic distortions is analyzed, with an emphasis on crystal chemistries that give rise to stresses operating either on surfaces of crystallites or within the bulk.

Model for Photoinduced Bending of Slender Molecular Crystals

The growing realization that photoinduced bending of slender photoreactive single crystals is surprisingly common has inspired researchers to control crystal motility for actuation. However, new mechanically responsive crystals are reported at a greater rate than their quantitative photophysical characterization; a quantitative identification of measurable parameters and molecular-scale factors that determine the mechanical response has yet to be established. Herein, a simple mathematical description of the quasi-static and time-dependent photoinduced bending of macroscopic single crystals is provided. This kinetic model goes beyond the approximate treatment of a bending crystal as a simple composite bilayer. It includes alternative pathways for excited-state decay and provides a more accurate description of the bending by accounting for the spatial gradient in the product/reactant ratio. A new crystal form (space group P21/n) of the photoresponsive azo-dye Disperse Red 1 (DR1) is analyzed within the constraints of the aforementioned model. The crystal bending kinetics depends on intrinsic factors (crystal size) and external factors (excitation time, direction, and intensity).

Lock-arm supramolecular ordering: A molecular construction set for cocrystallizing organic charge transfer complexes

Organic charge transfer cocrystals are inexpensive, modular, and solution-processable materials that are able, in some instances, to exhibit properties such as optical nonlinearity, (semi)conductivity, ferroelectricity, and magnetism. Although the properties of these cocrystals have been investigated for decades, the principal challenge that researchers face currently is to devise an efficient approach which allows for the growth of high-quality crystalline materials, in anticipation of a host of different technological applications. The research reported here introduces an innovative design, termed LASO-lock-arm supramolecular ordering-in the form of a modular approach for the development of responsive organic cocrystals. The strategy relies on the use of aromatic electronic donor and acceptor building blocks, carrying complementary rigid and flexible arms, capable of forming hydrogen bonds to amplify the cocrystallization processes. The cooperativity of charge transfer and hydrogen-bonding interactions between the building blocks leads to binary cocrystals that have alternating donors and acceptors extending in one and two dimensions sustained by an intricate network of hydrogen bonds. A variety of air-stable, mechanically robust, centimeter-long, organic charge transfer cocrystals have been grown by liquid-liquid diffusion under ambient conditions inside 72 h. These cocrystals are of considerable interest because of their remarkable size and stability and the promise they hold when it comes to fabricating the next generation of innovative electronic and photonic devices.

Resorcinol Crystallization from the Melt: A New Ambient Phase and New "Riddles".

Structures of the α and β 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, ε, through a combination of optical and X-ray crystallography and by computational crystal structure prediction algorithms. α-Resorcinol has long been a model for mechanistic crystal growth studies from both solution and 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 β phase. The ε phase only grows from the melt, concomitant with the β phase, as polycrystalline, radially growing spherulites. If the radii are polar, then the sense of the polar axis is an essential feature of the form. Here, this determination is made for spherulites of β resorcinol (ε, point symmetry 222, does not have a polar axis) with additives that stereoselectively modify growth velocities. Both β and ε have the additional feature that individual radial lamellae may adopt helicoidal morphologies. We correlate the appearance of twisting in β and ε with the symmetry of twist-inducing additives.

Complete polarimetry on the asymmetric transmission through subwavelength hole arrays

Dissymmetric, periodically nanostructured metal films can show non-reciprocal transmission of polarized light, in apparent violation of the Lorentz reciprocity theorem. The wave vector dependence of the extraordinary optical transmission in gold films with square and oblique subwavelength hole arrays was examined for the full range of polarized light input states. In normal incidence, the oblique lattice, in contrast to square lattice, showed strong asymmetric, non-reciprocal transmission of circularly polarized light. By analyzing the polarization of the input and the output with a complete Mueller matrix polarimeter the mechanisms that permits asymmetric transmission while preserving the requirement of electromagnetic reciprocity is revealed: the coupling of the linear anisotropies induced by misaligned surface plasmons in the film. The square lattice also shows asymmetric transmission at non-normal incidence, whenever the plane of incidence does not coincide with a mirror line.

Dyeing crystals since 2000

Progress in the supramolecular chemistry of dyeing crystals in the 21st century is reviewed.

Circular Birefringence of Banded Spherulites

Crystal optical properties of banded spherulites of 21 different compounds--molecular crystals, polymers, and minerals--with helically twisted fibers were analyzed with Mueller matrix polarimetry. The well-established radial oscillations in linear birefringence of many polycrystalline ensembles is accompanied by oscillations in circular birefringence that cannot be explained by the natural optical activity of corresponding compounds, some of which are centrosymmetric in the crystalline state. The circular birefringence is shown to be a consequence of misoriented, overlapping anisotropic lamellae, a kind of optical activity associated with the mesoscale stereochemistry of the refracting components. Lamellae splay as a consequence of space constraints related to simultaneous twisting of anisometric lamellae. This mechanism is supported by quantitative simulations of circular birefringence arising from crystallite twisting and splaying under confinement.

Manipulating crystallization with molecular additives

Given the importance of organic crystals in a wide range of industrial applications, the chemistry, biology, materials science, and chemical engineering communities have focused considerable attention on developing methods to control crystal structure, size, shape, and orientation. Tailored additives have been used to control crystallization to great effect, presumably by selectively binding to particular crystallographic surfaces and sites. However, substantial knowledge gaps still exist in the fundamental mechanisms that govern the formation and growth of organic crystals in both the absence and presence of additives. In this review, we highlight research discoveries that reveal the role of additives, either introduced by design or present adventitiously, on various stages of formation and growth of organic crystals, including nucleation, dislocation spiral growth mechanisms, growth inhibition, and nonclassical crystal morphologies. The insights from these investigations and others of their kind are likely to guide the development of innovative methods to manipulate crystallization for a wide range of materials and applications.

Measurement of transmission and reflection from a thick anisotropic crystal modeled by a sum of incoherent partial waves.

Formulas for modeling ellipsometric measurements of bianisotropic crystals assume perfectly coherent plane wave illumination. As such, the finite coherence of typical spectroscopic ellipsometers renders such formulas invalid for crystals thicker than a few micrometers. Reflection measurements of thick crystalline slabs show depolarization. Researchers have proposed strategies for the full accounting for multiply reflected incoherent waves in anisotropic, arbitrarily oriented crystals [Appl. Opt.41, 2521 (2002).APOPAI0003-693510.1364/AO.41.002521], but to the best of our knowledge these methods have not been tested by explicit measurements. It is shown that by a summation of multiply reflected incoherent waves, transmission and reflection measurements of thick quartz slabs can be interpreted in terms of the constitutive material parameters.

Well Plate Circular Dichroism Reader for the Rapid Determination of Enantiomeric Excess

Circular dichroism (CD) spectropolarimeters typically employ one photoelastic modulator. However, spectropolarimeters employing two or even four modulators are more versatile and can be used to subvert common measurement errors arising from imperfectly isotropic samples or sample holders. Small linear anisotropies that can cause large errors in CD measurement can be associated with multi-well sample holders. Thus, high-throughput CD analyses in multi-well plates have not yet been demonstrated. One such application is the determination of enantiomeric excess of a library of reaction products. Herein, a spectropolarimeter employing four photoelastic modulators and a translation stage was used to determine the enantiomeric excess of a family of chiral amine complexes much more rapidly than could be achieved with a robotic fluid injection system. These experiments are proof of concept for high-throughput CD analysis. In practice, commercially available glass bottomed well plates are sufficiently strain free such that a simple instrument with just one photoelastic modulator and a vertical optical train should be able to deliver the CD without special considerations given herein. On the other hand, polystyrene well plates cannot be used in this way.