Relevance of the Second Harmonic Generation to Characterize Crystalline Samples

Revisiting Homochiral versus Heterochiral Interactions through a Long Detective Story of a Useful Azobis-Nitrile and Puzzling Racemate

This paper documents and reinvestigates the solid-state and crystal structures of 4,4'-azobis-4-cyanopentanoic acid (ACPA), a water-soluble azobis-nitrile of immense utility as a radical initiator in living polymerizations and a labile mechanophore that can be embedded within long polymer chains to undergo selective scission under mechanical activation. Surprisingly, for such applications, both the commercially available reagent and their derivatives are used as "single initiators" when this azonitrile is actually a mixture of stereoisomers. Although the racemate and meso compounds were identified more than half a century ago and their enantiomers were separated by classical resolution, there have been confusing narratives dealing with their characterization, the existence of a conglomeratic phase, and fractional crystallization. Our results report on the X-ray crystal structures of all stereoisomers for the first time, along with further details on enantiodiscrimination and the always intriguing arguments accounting for the stability of homochiral versus heterochiral crystal aggregates. To this end, metadynamic (MTD) simulations on stereoisomer molecular aggregates were performed to capture the incipient nucleation events at the picosecond time scale. This analysis sheds light on the driving homochiral aggregation of ACPA enantiomers.

Noncentrosymmetric γ-Cs2I4O11 Obtained from IO4 Polyhedral Rearrangements in the Centrosymmetric β-Phase

We report the synthesis and optical properties of noncentrosymmetric (NCS) γ-Cs₂I₄O₁₁ that was obtained through IO₄ polyhedral rearrangements from centrosymmetric (CS) β-Cs₂I₄O₁₁. Trifluoroacetic acid (TFA) acts as a structure-directing agent and plays a key role in the synthesis. It is suggested that the function of TFA is to promote rearrangement reactions found in the organic synthesis of stereoisomers. γ-Cs₂I₄O₁₁ crystallizes in the NCS monoclinic space group P2₁ (No. 4) and exhibits a strong second-harmonic-generation (SHG) response of 5.0 × KDP (KH₂PO₄) under 1064 nm laser radiation. Additional SHG experiments indicate that the material is type I phase matchable. First-principles calculations show that SHG intensity mainly comes from its d₃₄, d₂₁, and d₂₃ SHG tensor components. The synthetic strategy of discovering γ-Cs₂I₄O₁₁ provides a new way for designing novel NCS SHG materials.

Polymorphs with Remarkably Distinct Physical and/or Chemical Properties

Polymorphism in crystals is known since 1822 and the credit goes to Mitscherlich who realized the existence of different crystal structures of the same compound while working with some arsenate and phosphate salts. Later on, this phenomenon was observed also in organic crystals. With the advent of different technologies, especially the easy availability of single crystal XRD instruments, polymorphism in crystals has become a common phenomenon. Almost 37 % of compounds (single component) are polymorphic to date. As the energies of the different polymorphic forms are very close to each other, small changes in crystallization conditions might lead to different polymorphic structures. As a result, sometimes it is difficult to control polymorphism. For this reason, it is considered to be a nuisance to crystal engineering. It has been realized that the property of a material depends not only on the molecular structure but also on its crystal structure. Therefore, it is not only of interest to academia but also has widespread applications in the materials science as well as pharmaceutical industries. In this review, we have discussed polymorphism which causes significant changes in materials properties in different fields of solid-state science, such as electrical, magnetic, SHG, thermal expansion, mechanical, luminescence, color, and pharmaceutical. Therefore, this review will interest researchers from supramolecular chemistry, materials science as well as medicinal chemistry.

Simultaneous modal phase and group velocity matching in microstructured optical fibers for second harmonic generation with ultrashort pulses

Optical fibers provide a favorable medium for nonlinear optical processes owing to the small mode field size and concurrently high optical intensity combined with the extended interaction lengths. Second harmonic generation (SHG) is one of those processes that has been demonstrated in silica glass optical fibers. Since silica is centrosymmetric, generating SHG in an optical fiber requires poling of the glass. In addition and when one wants to use ultrashort pulses for SHG, achieving both phase and group velocity matching is crucial. Although fibers that feature either modal phase velocity or group velocity matching for SHG have been reported, the possibility of simultaneous modal phase and group velocity matching was never reported before. In this paper we address this challenge, and for the first time to our knowledge, we show that it is feasible to do so with silica microstructured optical fibers featuring at least one ring of air holes in the cladding and a heavily Germanium doped core (above 25 mol.%) by exploiting the LP₀₁(ω) and LP₀₂(2ω) modes at 1.06 µm pump and 0.53 µm second harmonic wavelengths. This finding can greatly impact applications requiring waveguide based SHG generation with ultrashort pulses, including microscopy, material characterization and nonlinear imaging.

SHG in a centrosymmetric crystal - can it be possible?

The present report lists selected publications on centrosymmetric compounds that manifest second harmonic generation responses in a laser, along with a few publications that dispute the laser outcomes. Two studies provide a plausible explanation for this apparent contradiction between second-order nonlinear susceptibility and inversion symmetry: the crystals are noncentrosymmetric and are twinned by inversion. If crystal structures of SHG-active compounds are presented in centrosymmetric settings, the authors of the publications may consider stipulating that the true space group is likely to be one of the noncentrosymmetric sub-space groups.

Combining Incompatible Processes for Deracemization of a Praziquantel Derivative under Flow Conditions

An efficient deracemization method for conversion of the racemate to the desirable (R)-enantiomer of Praziquantel has been developed by coupling incompatible racemization and crystallization processes. By a library approach, a derivative that crystallizes as a conglomerate has been identified. Racemization occurs via reversible hydrogenation over a palladium on carbon (Pd/C) packed column at 130 °C, whereas deracemization is achieved by alternating crystal growth/dissolution steps with temperature cycling between 5-15 °C. These incompatible processes are combined by means of a flow system resulting in complete deracemization of the solid phase to the desired (R)-enantiomer (98 % ee). Such an unprecedented deracemization by a decoupled crystallization/racemization approach can readily be turned into a practical process and opens new opportunities for the development of essential enantiomerically pure building blocks that require harsh methods for racemization.

Spontaneous and Controlled Macroscopic Chiral Symmetry Breaking by Means of Crystallization

In this paper, macroscopic chiral symmetry breaking refers to as the process in which a mixture of enantiomers departs from 50–50 symmetry to favor one chirality, resulting in either a scalemic mixture or a pure enantiomer. In this domain, crystallization offers various possibilities, from the classical Viedma ripening or Temperature Cycle-Induced Deracemization to the famous Kondepudi experiment and then to so-called Preferential Enrichment. These processes, together with some variants, will be depicted in terms of thermodynamic pathways, departure from equilibrium and operating conditions. Influential parameters on the final state will be reviewed as well as the impact of kinetics of the R ⇔ S equilibrium in solution on chiral symmetry breaking. How one can control the outcome of symmetry breaking is examined. Several open questions are detailed and different interpretations are discussed.

Stochastic Differential Scanning Calorimetry by Nonlinear Optical Microscopy

Stochastic phase transformations within individual crystalline particles were recorded by integration of second harmonic generation (SHG) imaging with differential scanning calorimetry (DSC). The SHG activity of a crystal is highly sensitive to the specific molecular packing arrangement within a noncentrosymmetric lattice, providing access to information otherwise unavailable by conventional imaging approaches. Consequently, lattice transformations associated with dehydration/desolvation events were readily observed by SHG imaging and directly correlated to the phase transformations detected by the DSC measurements. Following studies of a model system (urea), stochastic differential scanning calorimetry (SDSC) was performed on trehalose dihydrate, which has a more complex phase behavior. From these measurements, SDSC revealed a broad diversity of single-particle thermal trajectories and direct evidence of a "cold phase transformation" process not observable by the DSC measurements alone.

A Novel Mechanism of Preferential Enrichment Phenomenon Observed for the Cocrystal of (RS)-2-{4-[(4-Chlorophenoxy)methyl]phenoxy}propionic Acid and Isonicotinamide

A new entry of chiral anti-hyperlipoproteinemia drug is reported, showing an excellent preferential enrichment (PE) phenomenon which is not caused by a polymorphic transition during crystallization, but is proposed to occur by a novel mechanism involving partially irregular stacking of R and S homochiral two-dimensional (2D) sheets with a large dipole moment, followed by selective redissolution of one homochiral 2D sheet into the mother liquor during crystallization. The cocrystal composed of (RS)-2-{4-[(4-chlorophenoxy)methyl]phenoxy}propionic acid (CPPPA) and achiral isonicotinamide exhibited a substantial enrichment in the mother liquor up to 93 % ee by simply repeating recrystallization under nonequilibrium conditions using high supersaturation. Furthermore, the deposited crystals with low ee values obtained at the end of PE experiment were second harmonic generation (SHG)-positive, indicating the formation of homochiral domains in the deposited crystals, which reflects the proposed mechanism of PE.

A Viedma ripening route to an enantiopure building block for Levetiracetam and Brivaracetam

A simple route to enantiomerically pure (S)-2-aminobutyramide - the chiral component of the anti-epileptic drugs Levetiracetam and Brivaracetam has been developed. This approach is based on the rational design and application of a Viedma ripening process. The practical potential of the process is demonstrated on a large scale.

The Strecker reaction coupled to Viedma ripening: a simple route to highly hindered enantiomerically pure amino acids

The Strecker reaction is broadly used for the preparation of α-amino acids. However, control of enantioselectivity remains challenging. We here couple the Strecker reaction to Viedma ripening for the absolute asymmetric synthesis of highly sterically hindered α-amino acids. As proof-of-principle, the enantiomerically pure α-amino acids tert-leucine and α-(1-adamantyl)glycine were obtained.

Third-harmonic generation Stokes-Mueller polarimetric microscopy

An experimental implementation of the nonlinear Stokes-Mueller polarimetric (NSMP) microscopy in third-harmonic generation modality is presented. The technique is able to extract all eight 2D-accessible χ⁽³⁾ components for any sample from 64 polarization measurements, and can be applied to noninvasive ultrastructural characterization. The polarization signature of an isotropic glass coverslip is presented, and carotenoid crystallites in the root of orange carrot (Daucus carota) are investigated, showing complex χ⁽³⁾ components with a significant chiral contribution.

Rapid Discrimination of Polymorphic Crystal Forms by Nonlinear Optical Stokes Ellipsometric Microscopy

The use of nonlinear optical Stokes ellipsometric (NOSE) microscopy for rapid discrimination of two polymorphic forms of the small molecule d-mannitol is presented. Fast (8 MHz) polarization modulated beam-scanning microscopy and a recently developed iterative, nonlinear least-squares fitting algorithm were combined to allow discrimination of orthorhombic and monoclinic crystal structures of d-mannitol with data acquisition times of <7 s per field of view with a signal-to-noise ratio (SNR) of ∼300. Discrimination between polymorphic forms within the 99.99% confidence interval was achieved by standard statistical tests of the recovered probability density functions for the measured observables following two class linear discriminant analysis. These measurements target bottlenecks in small-volume, high-throughput solid form screening experiments for polymorph discovery in the development of emerging active pharmaceutical ingredients.