Molecular Enantiomers Research Articles

Homochiral metalated tetraphenylethylene‐based organic cages: Unusual chiral and luminescent behavior depending on thermodynamic and kinetic aggregation

AbstractChirality and luminescence are important for both chemistry and biology, which are highly influenced by aggregation. In this work, a pair of metalated tetraphenylethylene(TPE)‐based organic cage enantiomers are reported, which feature a quadrangular prismatic cage structure. These homochiral cages exhibit concentration‐dependent chiral behaviors alongside a propensity for thermodynamic aggregation. Aggregation caused quench effect is found for these cages accompanying the increasing of the concentrations. When a poor solvent is added to produce a kinetical aggregation, the aggregation‐annihilation circular dichroism and aggregation‐induced emission behaviors are observed for these enantiomeric cages. By comparing these observations with the photophysical behaviors of a pair of structurally similar organic molecular enantiomers, the unique photophysical properties observed are intricately linked to the metal‐integrated TPE‐functionalized cage structures.

Chiral Assembly of Perovskite Nanocrystals: Sensitive Discrimination of Amino Acid Enantiomers.

Chirality is a widespread phenomenon in nature and in living organisms and plays an important role in living systems. The sensitive discrimination of chiral molecular enantiomers remains a challenge in the fields of chemistry and biology. Establishing a simple, fast, and efficient strategy to discriminate the spatial configuration of chiral molecular enantiomers is of great significance. Chiral perovskite nanocrystals (PNCs) have attracted much attention because of their excellent optical activity. However, it is a challenge to prepare perovskites with both chiral and fluorescence properties for chiral sensing. In this work, we synthesized two chiral fluorescent perovskite nanocrystal assembly (PNA) enantiomers by using l- or d-phenylalanine (Phe) as chiral ligands. PNA exhibited good fluorescence recognition for l- and d-proline (Pro). Homochiral interaction led to fluorescence enhancement, while heterochiral interaction led to fluorescence quenching, and there is a good linear relationship between the fluorescence changing rate and l- or d-Pro concentration. Mechanism studies show that homochiral interaction-induced fluorescence enhancement is attributed to the disassembly of chiral PNA, while no disassembly of chiral PNA was found in heterochiral interaction-induced fluorescence quenching, which is attributed to the substitution of Phe on the surface of chiral PNA by heterochiral Pro. This work suggests that chiral perovskite can be used for chiral fluorescence sensing; it will inspire the development of chiral nanomaterials and chiral optical sensors.

Probing Interactions between Chiral Plasmonic Nanoparticles and Biomolecules.

Chiral plasmonic nanoparticles (and their assemblies) interact with biomolecules in a variety of different ways, resulting in distinct optical signatures when probed by circular dichroism spectroscopy. These systems show promise for biosensing applications and offer several advantages over achiral plasmonic systems. Arguably the most notable advantage is that chiral nanoparticles can differentiate between molecular enantiomers and can, therefore, act as sensors for enantiomeric purity. Furthermore, chiral nanoparticles can couple more effectively to chiral biomolecules in biological systems if they have a matching handedness, improving their effectiveness as biomedical agents. In this article, we review the different types of interactions that occur between chiral plasmonic nanoparticle systems and biomolecules, and discuss how circular dichroism spectroscopy can probe these interactions and inform how to optimize systems for biosensing and biomedical applications.

Self-assembly and tiling of a prochiral hydrogen-bonded network: bi-isonicotinic acid on coinage metal surfaces

Submolecular resolution scanning tunnelling microscopy and qPlus atomic force microscopy reveal that, close to thermal equilibrium, bi-isonicotinic acid (4,4'-COOH-2,2'-bpy) assembles into extended molecular rows on both Au(111) and Ag(100) surfaces, driven primarily by the formation of OH··· N hydrogen bonds. Both the intermolecular separation and inter-row spacing for Au(111) and Ag(100) are identical within experimental uncertainty, highlighting that the assembly of bi-isonicotinic acid networks on both metal surfaces is predominantly driven by intermolecular hydrogen-bonding and that the potential energy variation due to the substrate has relatively little influence. Nonetheless, the surface plays a key role in molecular organisation: symmetry-breaking induces prochiral behaviour, which drives the molecular enantiomers to form a racemic mixture of rows of different handedness. We adapt a tiling model previously introduced to model the formation of 2D networks of tetracarboxylic derivatives [Blunt et al. Science 322, 1077 (2008)] to the bi-isonicotinic acid system, providing key insights into the growth kinetics and attaining good agreement with the molecular morphologies observed in experiment.

Optimization of the double-laser-pulse scheme for enantioselective orientation of chiral molecules.

We present a comprehensive study of enantioselective orientation of chiral molecules excited by a pair of delayed cross-polarized femtosecond laser pulses. We show that by optimizing the pulses' parameters, a significant degree (∼10%) of enantioselective orientation can be achieved at 0 and 5K rotational temperatures. This study suggests a set of reasonable experimental conditions for inducing and measuring strong enantioselective orientation. The strong enantioselective orientation and the wide availability of the femtosecond laser systems required for the proposed experiments may open new avenues for discriminating and separating molecular enantiomers.

Chiral light in single-handed Fabry-Perot resonators

Chirality is a universal phenomenon that is encountered on many different length scales in nature. Interaction of chiral matter with chiral light results in the effect of circular dichroism, which underlies many techniques of discriminating molecular enantiomers. Enhancing dichroic effects is typically achieved by interfacing chiral matter with various optical resonators. In this context it is important to understand how the eigenmodes of optical cavities relate to the field states with well-defined handedness. Here, we present the model of a single-handedness chiral optical cavity supporting only an eigenmode of a given handedness without the presence of modes of other helicity. Resonant excitation of the cavity with light of appropriate handedness enables formation of a helical standing wave with a uniform chirality density, while the opposite handedness does not cause any resonant effects. Our findings expand the set of tools for investigations of chiral matter and open the door towards studies of chiral electromagnetic vacuum states.

Clock It: Solving the 3-D Chirality Problem with a 2-D Method—A Simple Algorithm for Teaching and Determining Enantiomers in General Chemistry

One of the more difficult concepts introduced in the first-year undergraduate course, general chemistry, is that of chirality. Typically, left and right hands are the common, macroscopic objects of reference used to demonstrate the quality of being nonsuperimposable, followed by the use of a mirror and molecular models, to illustrate molecular enantiomers. While this approach works well as an introduction, and the handedness model becomes more useful in organic chemistry where absolute R- and S-configuration assignment becomes necessary, it does not help the general chemistry student to determine molecular chirality or “handedness” at this basic level. Much of the difficulty stems from the representation of 3-dimensional (3-D) structures on the 2-dimensional (2-D) paper plane. While detailed flowcharts exist for molecules of varying orbital hybridization and geometry, these are more suited to advanced students in inorganic chemistry and are not designed to be mentally retained or learned. Additionally, the process of determining R- and S-configurations for carbon tetrahedral centers in organic chemistry is more complex as ligands must be ranked according to Cahn–Ingold–Prelog (CIP) rules. Previous publications have addressed this topic thoroughly. A simple method is presented here (called clocking) which introduces the concept of chirality for general chemistry and primes the way for more complex chirality assignments in higher level courses.

Ir catalyst makes both enantiomers with time

Different molecular enantiomers can have different chemistry, both in the body and in industrial processes. Controlling the yield of the right one is a time-consuming yet critical step in organic a...

Enantiomeric separation of triacylglycerols containing fatty acids with a ring (cyclofatty acids)

Triacylglycerols (TAGs) containing cyclofatty acids (cycloFAs) from two oilseeds of Sterculia foetida and Hydnocarpus wightiana were analysed using both reversed-phase (RP18) and chiral phase columns. TAGs were identified using high-resolution electrospray ionization mass spectrometry in the positive ion mode. Fifty-five molecular species of TAGs have been identified in sterculic oil, 27 of which contained at least one cyclopropenyl-FA (e.g., malvalic or sterculic acids). The structures of regioisomers and enantiomers were determined for five major TAGs with cyclopropenyl-FAs.One hundred thirty-six TAGs were identified in chaulmoogra oil, 71 of which contained at least one cyclopentenyl-FA (e.g., gorlic, chaulmoogric, and hydnocarpic acids, etc.). Furthermore, in three molecular species, regioisomers and enantiomers were identified using HPLC on a chiral phase column. Eight molecular species of TAGs were prepared through organic synthesis to facilitate the identification of enantiomers. Retention times of fatty acid-containing triacylglycerols with one ring and one double bond are very similar to triacylglycerols with a dienoic fatty acid, but elution times are shorter. For example, dimalvaloylpalmitate elutes earlier than dilinoleylpalmitate.The order of elution of TAGs on the chiral column differs. In TAGs with 2 degrees of unsaturation (ring and double bond, e.g. PStP-StPP-PPSt), the order of elution is symmetric-asymmetric-asymmetric TAGs. TAGs with 4 degrees of unsaturation (one ring and three double bonds or two rings and two double bonds) present a different pattern. When TAGs contain two rings and two double bonds, the order of elution TAGs is asymmetric-symmetric-asymmetric (StStP-StPSt-PStSt); when TAGs contain a ring and 3 double bonds, the elution order is symmetric-asymmetric-asymmetric TAGs (OStO-StOO-OOSt). In species with a higher degree of unsaturation (e.g., 5), the elution order of the TAGs is asymmetric-asymmetric-symmetric (e.g. CCO-OCC-COC).

Laser control of molecular rotation: Expanding the utility of an optical centrifuge

Since its invention in 1999, the optical centrifuge has become a powerful tool for controlling molecular rotation and studying molecular dynamics and molecular properties at extreme levels of rotational excitation. This technique has been applied to a variety of molecular species, from simple linear molecules to symmetric and asymmetric tops, to molecular ions and chiral enantiomers. Properties of isolated ultrafast rotating molecules, the so-called molecular superrotors, have been investigated, as well as their collisions with one another and the interaction with external fields. The ability of an optical centrifuge to spin a particular molecule of interest depends on both the molecular structure and the parameters of the centrifuge laser pulse. An interplay between these two factors dictates the utility of an optical centrifuge in any specific application. Here, we discuss the strategy of assessing and adjusting the properties of the centrifuge to those of the molecular rotors and describe two practical examples of optical centrifuges with very different characteristics, implemented experimentally in our laboratory.

Observation of persistent orientation of chiral molecules by a laser field with twisted polarization

Molecular chirality is an omnipresent phenomenon of fundamental significance in physics, chemistry and biology. For this reason, search for novel techniques for enantioselective control, detection and separation of chiral molecules is of particular importance. It has been recently predicted that laser fields with twisted polarization may induce persistent enantioselective field-free orientation of chiral molecules. Here we report the first experimental observation of this phenomenon using propylene oxide molecules ($\mathrm{CH_{3}CHCH_{2}O}$, or PPO) spun by an optical centrifuge - a laser pulse, whose linear polarization undergoes an accelerated rotation around its propagation direction. We show that PPO molecules remain oriented on a time scale exceeding the duration of the centrifuge pulse by several orders of magnitude. The demonstrated long-time field-free enantioselective orientation opens new avenues for optical manipulation, discrimination, and, potentially, separation of molecular enantiomers.

Electrospinning chiral fluorescent nanofibers from helical polyacetylene: preparation and enantioselective recognition ability.

Chirality is ubiquitous in nature and closely related to the pharmacological effects of chiral drugs. Therefore, chiral recognition of molecular enantiomers becomes an important research theme. Fluorescence detection is highly sensitive and fast but has achieved only limited success in enantiomeric detection due to the lack of powerful chiral fluorescence detection materials. In this paper, a novel chiral fluorescent probe material, i.e. a chiral fluorescent nanofiber membrane, is prepared from chiral helical substituted polyacetylene by the electrospinning technique. The SEM images demonstrate the success in fabricating continuous, uniform nanofibers with a diameter of about 100 nm. Circular dichroism spectra show that the nanofibers exhibit fascinating optical activity. One of the enantiomeric chiral fluorescent membranes has chiral fluorescence recognition effects towards alanine and chiral phenylethylamine, while the other enantiomeric membrane does not. The prepared chiral fluorescent nano-materials are expected to find various applications in chirality-related fields due to their advantages such as chirality, fluorescence, and a high specific surface area. The established preparation approach also promises a potent and versatile platform for developing advanced nanofiber materials from conjugated polymers.

Enhanced Chiral Sensing with Dielectric Nanoresonators.

Chiro-sensitive molecular detection is highly relevant as many biochemical compounds, the building blocks of life, are chiral. Optical chirality is conventionally detected through circular dichroism (CD) in the UV range, where molecules naturally absorb. Recently, plasmonics has been proposed as a way to boost the otherwise very weak CD signal and translate it to the visible/NIR range, where technology is friendlier. Here, we explore how dielectric nanoresonators can contribute to efficiently differentiate molecular enantiomers. We study the influence of the detuning between electric (ED) and magnetic dipole (MD) resonances in silicon nanocylinders on the quality of the CD signal. While our experimental data, supported by numerical simulations, demonstrate that dielectric nanoresonators can perform even better than their plasmonic counterpart, exhibiting larger CD enhancements, we do not observe any significant influence of the optical chirality.

Field-Induced Diastereomers for Chiral Separation

A novel approach for the state-specific enantiomeric enrichment and the spatial separation of enantiomers is presented. Our scheme utilizes techniques from strong-field laser physics-specifically an optical centrifuge in conjunction with a static electric field-to create a chiral field with defined handedness. Molecular enantiomers experience unique rotational excitation dynamics, and this can be exploited to spatially separate the enantiomers using electrostatic deflection. Notably, the rotational-state-specific enantiomeric enhancement and its handedness are fully controllable. To explain these effects, the conceptual framework of field-induced diastereomers of a chiral molecule is introduced and computationally demonstrated through robust quantum-mechanical simulations on the prototypical chiral molecule propylene oxide (C_{3}H_{6}O), for which ensembles with an enantiomeric excess of up to 30% were obtained.

Chalcogens-Induced Ag6Z4@Ag36 (Z = S or Se) Core-Shell Nanoclusters: Enlarged Tetrahedral Core and Homochiral Crystallization.

Control over core structure is much more challenging than that over shell structure in core-shell silver nanoclusters. Herein, two isostructural chalcogen-mediated [Ag6Z4@Ag36] (Z = S or Se) nanoclusters (SD/Ag42a and SD/Ag42b) caging tetrahedral [Ag6Z4] as cores were synthesized by introducing Ph3CSH or Ph3PSe as slow-release source of S2- or Se2-, respectively, and characterized by single-crystal X-ray diffraction (SCXRD). As compared to the previously reported [AgS4@Ag36] cluster (Ag37), we found that introducing additional S2- or Se2- ions can effectively enlarge the inner core from tetrahedral AgS4 to Ag6Z4, which is a regular octahedron of silver with four Z2- capping on one tetrahedral set of four faces. More interestingly, the molecular enantiomers of SD/Ag42a and SD/Ag42b segregate into different crystals (P212121), while those of Ag37 form racemic crystals (I41/acd). The larger Ag6Z4 core in Ag42 clusters also extends their emission to the near-infrared region (∼760 nm). The study confirms that chalcogenide can enlarge the nuclearity of nanoclusters by altering the inner core structure and affords a new strategy to synthesize chiral core-shell silver nanoclusters of higher-order in controlled fashion.

Selective Synthesis of Interlocked Molecules with Topological Chirality

Mechanically interlocked molecules can be chiral if each of the rings has no bilateral symmetry. Although chiral catenanes have been isolated from racemic mixtures via chromatographic techniques, these optically active molecules have not been directly synthesized until now. In this issue of Chem, Goldup and co-workers describe an auxiliary approach for the stereoselective synthesis of the topological enantiomers of chiral [2]catenanes. This methodology enables exploration of the potential applications of these chiral molecules in areas such as medicine and materials science.

Orienting Asymmetric Molecules by Laser Fields with Twisted Polarization.

We study interaction of generic asymmetric molecules with laser fields having twisted polarization, using a pair of strong time-delayed short laser pulses with crossed linear polarizations as an example. We show that such an excitation not only provides unidirectional rotation of the most polarizable molecular axis, but also induces a directed torque along this axis, which results in a transient orientation of the molecules. The asymmetric molecules are chiral in nature and different molecular enantiomers experience the orienting action in opposite directions causing out-of-phase oscillations of their dipole moments. The resulting microwave radiation was recently suggested to be used for analysis or discrimination of chiral molecular mixtures. We reveal the mechanism behind this laser-induced orientation effect, show that it is classical in nature, and envision further applications of light with twisted polarization.

Selective Orientation of Chiral Molecules by Laser Fields with Twisted Polarization.

We explore a pure optical method for enantioselective orientation of chiral molecules by means of laser fields with twisted polarization. Several field implementations are considered, including a pair of delayed, cross-polarized laser pulses, an optical centrifuge, and polarization-shaped pulses. We show that these schemes lead to out-of-phase time-dependent dipole signals for different enantiomers, and we also predict a substantial permanent molecular orientation persisting long after the laser fields are over. The underlying classical orientation mechanism common to all of these fields is discussed, and its operation is demonstrated for a range of chiral molecules of various complexity: hydrogen thioperoxide (HSOH), propylene oxide (CH3CHCH2O), and ethyl oxirane (CH3CH2CHCH2O). The presented results demonstrate generality, versatility, and robustness of this optical method for manipulating molecular enantiomers in the gas phase.

Electromagnetic trapping of chiral molecules: orientational effects of the irradiating beam

The photonic interaction generally responsible for the electromagnetic trapping of molecules is forward-Rayleigh scattering, a process that is mediated by transition electric dipoles connecting the ground electronic state and virtual excited states. Higher order electric and magnetic multipole contributions to the scattering amplitude are usually negligible. However, on consideration of chiral discrimination effects (in which an input light of left-handed circular polarization can present different observables compared to right-handed polarization, or molecules of opposite enantiomeric form respond differently to a set circular polarization), the mechanism must be extended to specifically accommodate transition magnetic dipoles. Moreover, it is important to account for the fact that chiral molecules are necessarily nonspherical, so that their interactions with a laser beam will have an orientational dependence. Using quantum electrodynamics, this article quantifies the extent of the energetic discrimination that arises when chiral molecules are optically trapped, placing particular emphasis on the orientational effects of the trapping beam. An in-depth description of the intricate ensemble-weighted method used to incorporate the latter is presented. It is thus shown that, when a mixture of molecular enantiomers is irradiated by a continuous beam of circularly polarized light, a difference arises in the relative rates of migration of each enantiomer in and out of the most intense regions of the beam. As a consequence, optical trapping can be used as a means of achieving enantiomer separation.

Environmental-confinement-induced stability enhancement of chiral molecules.

We computationally study the transition process of a chiral difluorobenzo[c]phenanthrene (DFBcPh) molecule within non-polar fullerene C(260) to explore the confinement effect. We find blue-shifts in the infrared and Raman spectra of the molecule inside the fullerene relative to those of isolated systems. Six types of spectrum features of the molecule appear in the 0-60 cm(-1) band. Interestingly, the energy barrier of the chiral transformation of the molecule is elevated by 15.88 kcal mol(-1) upon the confinement by the fullerene, indicating improvement in the stability of the enantiomers. The protection by C(260) lowers the highest occupied molecular orbital energy level and lifts the lowest unoccupied molecular orbital energy level of the chiral molecule such that the chiral molecule is further chemically stabilized. We concluded that the confinement environment has an impact at the nanoscale on the enantiomer transformation process of the chiral molecule.