Racemic Phase Research Articles

Molecular Recognition in Mechanochemistry: Insights from Solid‐State NMR Spectroscopy

AbstractMolecular‐recognition events are highly relevant in biology and chemistry. In the present study, we investigated such processes in the solid state under mechanochemical conditions using the formation of racemic phases upon reacting enantiopure entities as example. As test systems, α‐(trifluoromethyl)lactic acid (TFLA) and the amino acids serine and alanine were used. The effects of ball‐milling and resonant acoustic mixing (RAM) on the formation of racemic phases were probed by using solid‐state Nuclear Magnetic Resonance (NMR) spectroscopy. In a mixer mill, a highly efficient and fast racemic phase formation occurred for both TFLA and the two amino acids. RAM led to the racemic phase for TFLA also, and this process was facilitated upon employing pre‐milled enantiopure entities. In contrast, under comparable conditions RAM did not result in the formation of racemic phases for serine and alanine.

Molecular Recognition in Mechanochemistry: Insights from Solid-State NMR Spectroscopy.

Molecular-recognition events are highly relevant in biology and chemistry. In the present study, we investigated such processes in the solid state under mechanochemical conditions using the formation of racemic phases upon reacting enantiopure entities as example. As test systems, α-(trifluoromethyl)lactic acid (TFLA) and the amino acids serine and alanine were used. The effects of ball-milling and resonant acoustic mixing (RAM) on the formation of racemic phases were probed by using solid-state Nuclear Magnetic Resonance (NMR) spectroscopy. In a mixer mill, a highly efficient and fast racemic phase formation occurred for both TFLA and the two amino acids. RAM led to the racemic phase for TFLA also, and this process was facilitated upon employing pre-milled enantiopure entities. In contrast, under comparable conditions RAM did not result in the formation of racemic phases for serine and alanine.

Stereoisomeric Non-Fullerene Acceptors-Based Organic Solar Cells.

Chiral alkyl chains are ubiquitously observed in organic semiconductor materials and can regulate solution processability and active layer morphology, but the effect of stereoisomers on photovoltaic performance has rarely been investigated. For the racemic Y-type acceptors widely used in organic solar cells, it remains unknown if the individual chiral molecules separate into the conglomerate phase or if racemic phase prevails. Here, the photovoltaic performance of enantiomerically pure Y6 derivatives, (S,S)/(R,R)-BTP-4F, and their chiral mixtures are compared. It is found that (S,S) and (R,R)-BTP-4F molecule in the racemic mixtures tends to interact with its enantiomer. The racemic mixtures enable efficient light harvesting, fast hole transfer, and long polaron lifetime, which is conducive to charge generation and suppresses the recombination losses. Moreover, abundant charge diffusion pathways provided by the racemate contribute to efficient charge transport. As a result, the racemate system maximizes the power output and minimizes losses, leading to a higher efficiency of 18.16% and a reduced energy loss of 0.549 eV, as compared to the enantiomerically pure molecules. This study demonstrates that the chirality of non-fullerene acceptors should receive more attention and be designed rationally to enhance the efficiency of organic solar cells.

Crystal Engineering of Conglomerates: Dilution of Racemate-Forming Fe(II) and Ni(II) Congeners into Conglomerate-Forming [Zn(bpy)3](PF6)2

Conglomerate formation, where enantiomers within a racemic mixture self-segregate upon crystallization, is an advantageous property for obtaining chirally pure crystals and allows large-scale chiral resolution. However, the prevalence of conglomerates is low and difficult to predict. In this report, we describe our attempts to engineer conglomerates from racemate-forming compounds by integrating them into a conglomerate-forming matrix. In this regard, we found that Ni(II) and Fe(II) form molecular alloys with Zn(II) in [MxZn(1−x)(bpy)3](PF6)2 (where bpy = 2,2′-bipyridyl). Powder X-ray Diffraction (PXRD) and Energy-Dispersive X-ray spectroscopy (EDX) evidenced conglomerate crystallization with Ni(II) concentrations up to about 25%, while it was observed only for much lower concentrations of Fe(II). This can be attributed to the ability of [Ni(bpy)3](PF6)2 to access a metastable conglomerate phase, while no such phase has been detected in [Fe(bpy)3](PF6)2. Furthermore, the chiral phase appears to be favored in fast-growing precipitates, while the racemic phase is favored in slow re-crystallizations for both Ni(II) and Fe(II) molecular alloys. X-ray natural circular dichroism (XNCD) measurements on [Ni0.13Zn0.87(bpy)3](PF6)2 demonstrate the chirality of the nickel molecules within the zinc molecular matrix.

Chiral Lanthanum Metal–Organic Framework with Gated CO2 Sorption and Concerted Framework Flexibility

A metal–organic framework (MOF) CTH-17 based on lanthanum(III) and the conformationally chiral linker 1,2,3,4,5,6-hexakis(4-carboxyphenyl)benzene, cpb6–: [La2(cpb)]·1.5dmf was prepared by the solvothermal method in dimethylformamide (dmf) and characterized by variable-temperature X-ray powder diffraction (VTPXRD), variable-temperature X-ray single-crystal diffraction (SCXRD), and thermogravimetric analysis (TGA). CTH-17 is a rod-MOF with new topology och. It has high-temperature stability with Sohncke space groups P6122/P6522 at 90 K and P622 at 300 and 500 K, all phases characterized with SCXRD and at 293 K also with three-dimensional (3D) electron diffraction. VTPXRD indicates a third phase appearing after 620 K and stable up to 770 K. Gas sorption isotherms with N2 indicate a modest surface area of 231 m2 g–1 for CTH-17, roughly in agreement with the crystal structure. Carbon dioxide sorption reveals a gate-opening effect of CTH-17 where the structure opens up when the loading of CO2 reaches approximately ∼0.45 mmol g–1 or 1 molecule per unit cell. Based on the SCXRD data, this is interpreted as flexibility based on the concerted movements of the propeller-like hexatopic cpb linkers, the movement intramolecularly transmitted by the π–π stacking of the cpb linkers and helped by the fluidity of the LaO6 coordination sphere. This was corroborated by density functional theory (DFT) calculations yielding the chiral phase (P622) as the energy minimum and a completely racemic phase (P6/mmm), with symmetric cpb linkers representing a saddle point in a racemization process.

Chirality recognition and separation of 4-ethynyltriphenylamine induced by chiral Kagomé network on Cu (1 1 1)

Chirality Recognition and Separation induced by chiral molecular template with chiral cavities. • EPTA molecules tend to form achiral nanostructures on bare Cu (1 1 1). • ABA molecules form two Kagomé chiral structures with different symmetries. • Chiral recognition of EPTA molecules is realized by introducing Kagomé templates. Chirality recognition and separation on surfaces became a hot research topic in recent years for generation of homochiral nanostructures. Previously, chirality of the assembled nanostructures is determined by the molecule–molecule or molecule–substrate interaction which is uncontrolled to some extent and some organic molecules are prone to bind to its enantiomer to form racemic phase rather than separating into different homochiral ones. In this paper, 4-ethynyltriphenylamine (EPTA) molecules form achiral nanostructures on bare Cu (1 1 1). Interestingly, by introducing a chiral molecular template with chiral cavities, the chirality recognition and separation of EPTA molecules are realized, and finally chiral aggregations formed by EPTA molecules with the same chirality are achieved. This discovery provides a feasible approach for the construction of chiral nanomaterials, chirality recognition and separation.

Highly Distorted Chiral Two-Dimensional Tin Iodide Perovskites for Spin Polarized Charge Transport.

Incorporating chiral organic molecules into organic/inorganic hybrid 2D metal-halide perovskites results in a novel family of chiral hybrid semiconductors with unique spin-dependent properties. The embedded chiral organic moieties induce a chiroptical response from the inorganic metal-halide sublattice. However, the structural interplay between the chiral organic molecules and the inorganic sublattice, as well as their synergic effect on the resulting electronic band structure need to be explored in a broader material scope. Here we present three new layered tin iodide perovskites templated by chiral (R/S-)methylbenzylammonium (R/S-MBA), i.e., (R-/S-MBA)2SnI4, and their racemic phase (rac-MBA)2SnI4. These MBA2SnI4 compounds exhibit the largest level of octahedral bond distortion compared to any other reported layered tin iodide perovskite. The incorporation of chiral MBA cations leads to circularly polarized absorption from the inorganic Sn-I sublattice, displaying chiroptical activity in the 300-500 nm wavelength range. The bandgap and chiroptical activity are modulated by alloying Sn with Pb, in the series of (MBA)2Pb1-xSnxI4. Finally, we show that vertical charge transport through oriented (R-/S-MBA)2SnI4 thin films is highly spin-dependent, arising from a chiral-induced spin selectivity (CISS) effect. We demonstrate a spin-polarization in the current-voltage characteristics as high as 94%. Our work shows the tremendous potential of these chiral hybrid semiconductors for controlling both spin and charge degrees of freedom.

Raman Scattering in Chiral-Pure and Racemic Phases of Tryptophan and Tyrosine Polycrystals

Raman spectra of tryptophan and tyrosine polycrystals have been analyzed in a wide spectral range by fiber-optic spectroscopy. The Raman spectra have been recorded with a BWS465-785H spectrometer in the spectral range of 0–2700 cm–1 using a 785-nm cw laser as an excitation source. Parameters of the Raman spectra are compared for three crystalline phase modifications of aromatic amino acids: left-handed, right-handed, and racemic phase. The presence of strong Raman satellites, the characteristics of which change depending on the type of the chiral phase state of amino acid, is found in the low-frequency Raman spectra of tryptophan and tyrosine amino acid lattices. The results obtained can be used for monitoring the chiral purity of bioactive preparations containing amino acids.

Ferroelectric Alkylamide-Substituted Helicene Derivative with Two-Dimensional Hydrogen-Bonding Lamellar Phase.

Alkylamide (-CONHC nH2 n+1)-substituted benzene and its pyrene derivatives have shown a discotic hexagonal columnar liquid-crystalline phase through a one-dimensional (1D) intermolecular N-H···O═ hydrogen-bonding interaction, the direction of which is inverted through the application of an alternate current voltage. The polar hydrogen-bonding chains and dipole inversion reveal a ferroelectric polarization-electric field ( P- E) hysteresis curve. Non-π-planar helicene derivatives bearing two -CONHC14H29 chains also indicate a ferroelectric response. The racemic helicene derivative shows a bilayer lamellar liquid-crystal phase within a temperature range of 330-420 K, whereas there is no liquid crystallinity for the optically active derivative because of the different molecular assembly structure. The racemic phase is constructed through a two-dimensional (2D) N-H···O═ hydrogen-bonding network, which shows ferroelectric P- E hysteresis curves at above 340 K. The collective dipole inversion in the 2D layer contributes to the ferroelectricity in the lamellar phase. The remanent polarization ( Pr) of 11.1 μC cm-2 is about 6 times higher than those of the π-planar benzene- and pyrene-based 1D ferroelectrics. Both the density of the hydrogen-bonding site and the domain orientation in the 2D system are higher than those of the 1D columnar system.

Комбинационное рассеяние света в хирально чистых и рацемической фазах поликристаллов триптофана и тирозина

AbstractRaman spectra of tryptophan and tyrosine polycrystals have been analyzed in a wide spectral range by fiber-optic spectroscopy. The Raman spectra have been recorded with a BWS465-785H spectrometer in the spectral range of 0–2700 cm^–1 using a 785-nm cw laser as an excitation source. Parameters of the Raman spectra are compared for three crystalline phase modifications of aromatic amino acids: left-handed, right-handed, and racemic phase. The presence of strong Raman satellites, the characteristics of which change depending on the type of the chiral phase state of amino acid, is found in the low-frequency Raman spectra of tryptophan and tyrosine amino acid lattices. The results obtained can be used for monitoring the chiral purity of bioactive preparations containing amino acids.

Photoluminescence Spectra in Polycrystals of Chirally Pure and Racemic Phases of Tryptophan

The photoluminescence spectra of three crystalline phase modifications of aromatic amino acid tryptophan are compared: the left phase of L-tryptophan, the right phase of D-tryptophan, and the racemic phase of DL-tryptophan. The photoluminescence spectra were recorded using a fiber-optic procedure, a small-size FSD-8 spectrometer, and a data processing system that enables comparison of the analyzed spectrum with the spectrum of a reference substance. Photoluminescence was excited by a pulsed-periodic ultraviolet laser at a wavelength of 266 nm.

One-Dimensionally Disordered Chiral Sorting by Racemic Tiling in a Surface-Confined Supramolecular Assembly of Achiral Tectons.

The aggregation of (pro)chiral/achiral molecules into crystalline structures at interfaces forms conglomerates, racemates, and solid solutions, comparable to known bulk phases. Scanning tunneling microscopy and Monte Carlo simulations were employed to uncover a distinct racemic phase, expressing 1D disordered chiral sorting through random tiling in surface-confined supramolecularly assembled achiral 4,4''-diethynyl-1,1':4',1''-terphenyl molecules. The configurational entropy of the 1D disordered racemic tiling phase was verified by analytical modeling, and found to lie between that of a perfectly ordered 2D racemate and a racemic solid solution.

One‐Dimensionally Disordered Chiral Sorting by Racemic Tiling in a Surface‐Confined Supramolecular Assembly of Achiral Tectons

AbstractThe aggregation of (pro)chiral/achiral molecules into crystalline structures at interfaces forms conglomerates, racemates, and solid solutions, comparable to known bulk phases. Scanning tunneling microscopy and Monte Carlo simulations were employed to uncover a distinct racemic phase, expressing 1D disordered chiral sorting through random tiling in surface‐confined supramolecularly assembled achiral 4,4′′‐diethynyl‐1,1′:4′,1′′‐terphenyl molecules. The configurational entropy of the 1D disordered racemic tiling phase was verified by analytical modeling, and found to lie between that of a perfectly ordered 2D racemate and a racemic solid solution.

Laser spectroscopy and dynamics of crystal lattices of chirally pure and racemic phases of amino acids

Strong sharp lines due to the librational modes characterized by a pseudoscalar symmetry type have been found in the low-frequency Raman spectra of the lattices of glycine and tyrosine amino acids. The intensities of these lines exceed those for Raman scattering in the region of intramolecular vibrations. The spectra of chirally pure and racemic phases of amino acids differ significantly. The results obtained can be used to observe stimulated Raman scattering from the librational modes of crystalline amino acids and monitor the chiral purity of bioactive preparations containing amino acids.

Lattice modes of the chirally pure and racemic phases of tyrosine crystals

High-Q librational modes have been found to be present in the infrared absorption and Raman spectra of chirally pure L-tyrosine. Such modes can serve as terahertz radiation detectors and generators in chirally pure biostructures.

Chiral discrimination for a system of tetrahedral molecules on a triangular lattice

A tetrahedral molecule consisting of a central atom attached to four different groups exists as two non-superimposable mirror images (enantiomers). We consider a model system of such molecules adsorbed on a triangular lattice and containing an equimolar mixture of each enantiomer. By rewriting the Hamitonian in more than one way, we can obtain an m-potential in the two regions of parameter space within which the ground state configurations are finitely degenerate. We apply the rigorous Pirogov–Sinai theory to prove that at low temperatures the system in one of these regions will undergo enantiomeric phase separation into two homochiral phases, each containing a single enantiomer, and in the other region will form a racemic phase in which the two enantiomers form alternating rows in the crystal.

Chirality Driven Metallic versus Semiconducting Behavior in a Complete Series of Radical Cation Salts Based on Dimethyl-Ethylenedithio-Tetrathiafulvalene (DM-EDT-TTF)

Enantiopure (S,S) and (R,R) dimethyl-ethylenedithio-tetrathiafulvalene (DM-EDT-TTF) 1 donors are synthesized by cross coupling followed by decarboxylation reactions. In the solid state the methyl groups are arranged in axial positions within sofa-type conformation for the six-membered rings. Crystalline radical cation salts formulated as [(S,S)-1]2PF6, [(R,R)-1]2PF6, and [(rac)-1]2PF6 are obtained by electrocrystallization. When the experiment is conducted with enantioenriched mixtures both enantiopure and racemic phases are obtained. The monoclinic enantiopure salts, containing four independent donors in the unit cell, show semiconducting behavior supported by band structure calculations of extended Hückel type. The racemic salt contains only one independent donor in the mixed valence oxidation state +0.5. Under ambient pressure the racemic material is metallic down to 120 K, while an applied pressure of 11.5 kbar completely suppresses the metal-insulator transition. Band structure calculations yield an open Fermi surface, typical for a pseudo-one-dimensional metal, with unperfected nesting, thus ruling out the possibility of charge or spin density modulations to be at the origin of the transition. Raman spectroscopy measurements, in agreement with structural analysis at 100 K, show no indication of low-temperature charge ordering in the racemic material at ambient pressure, thus suggesting Mott-type charge localization for the observed metal-insulator transition.

Effect of micelle size and intermicellar distance on the chirality transfer in the intrinsic lyotropic cholesteric phases

Lyotropic nematic and cholesteric liquid crystalline phases were produced using the racemic amphiphiles, potassium N-dodecanoyl-DL-alaninate (DL-KDDA) and DL-serinate (DL-KDDS) and their l-enantiomers, potassium N-dodecanoyl- L-alaninate (L-KDDA) and L-serinate (l-KDDS), respectively. The different racemic nematic and cholesteric phases were characterised by a polarising light microscope. In order to calculate the helical twisting powers (htp) of the cholesteric phases, their pitches were determined as a function of concentrations of the L-enantiomers. The results showed that the L-KDDA system had a higher helical twisting power than the L-KDDS system. Small-angle X-ray scattering (saxs) techniques were performed for determination of the structural differences between the racemic micelles and the chiral micelles of the racemic and intrinsic cholesteric phases, respectively. Saxs results showed that the intrinsic cholesteric micelles are more compact than their corresponding racemic micelles. These results were interpreted in terms of the different molecular arrangements of the chiral amphiphiles in micelles compared to the amphiphiles in racemic micelles. According to our experimental results and the results on induced cholesteric phases in the literature, we proposed that there must be (1) a crucial distance and (2) a critical micelle size in order to obtain chiral induction, supporting some theoretical studies reported in the literature. In addition, we also showed that chiral induction does not take place completely as was suggested by pairwise interaction or sterical interaction models in the literature, but through anisotropic chiral interactions via distorted micelles for which a crucial distance between the micelles and a critical micelle size are required.

Crystal Structures, Dehydration Mechanism, and Chiral Discrimination in the Solid State of a Hydantoin Derivative

(±) 5-Methyl-5-(4′-ethylphenyl) hydantoin (18H) crystallizes in usual organic solvents as a conglomerate in the P212121 space group. A new monohydrated racemic compound of 18H could however be obtained by antisolvent induced crystallization. The structure of this first hydrate of a hydantoin was successfully resolved by single crystal X-ray diffraction. It crystallizes in space group P21/c, and the crystal packing shows an association of homochiral molecular ribbons resulting from strong hydrogen bonds involving the water molecule and the hydantoin rings. At 30 °C and under low relative humidity (in the range 0–30%), the monohydrate exhibits an efflorescent behavior, with a reversible evolution toward a metastable anhydrous racemic phase through a cooperative zip-like mechanism. But under specific conditions (40–60% R.H., 30 °C), dehydration also leads to the nucleation and growth of the stable conglomerate, as confirmed by dynamic vapor sorption and X-ray powder diffraction analyses. The detailed analyses of solid–solid transformations between a hydrated racemic phase, an anhydrous racemic phase, and/or two anhydrous enantiomerically pure phases provide an opportunity to investigate examples of chiral recognition mechanisms in the solid state. The way the water molecules are released from the racemic monohydrate at 30 °C plays a decisive role in the nucleation and growth of the conglomerate.

Chiral segregation, an unusual racemic phase, and a residual entropy for a lattice system ofmodel chiral molecules

We study low temperature phases of a 2D model molecular system that contains anequimolar mixture of a chiral molecule and its mirror image. The molecules lie on a thinfilm in the close-packed regime, occupying the sites of a honeycomb lattice, and neitherenantiomer is externally favored. We show that, in one range of interactions, chiralsegregation into two ordered homochiral phases containing a single enantiomer occurs. Anordered racemic phase occurs in which one sublattice is occupied by one enantiomer andthe other sublattice is occupied by the other enantiomer. When second-closest groupinteractions are relaxed, the ground states of the homochiral and the associated racemicphase have the same energy; in fact, the total number of ground state configurationsbecomes infinite and yields a residual entropy. This residual entropy is calculated exactly.In a third range of interactions, an unusual ordered racemic phase occurs in which the twoenantiomers are assembled in alternating infinite rows, thus forming a patternindependent of the sublattice structure. In order to prove these results, we apply thePirogov–Sinai theory, a powerful generalization of the Peierls-type approach of statisticalmechanics.