Planar Chirality Research Articles

Acetylene Derivatives of Cationic Diazaoxatriangulenes and Diaza [4]Helicenes ‐ Access to Red Emitters and Planar Chiral Stereochemical Traits

Cationic triangulenes, and related helicenes, constitute a rich class of dyes and fluorophores, usually absorbing and emitting light at low energy, in the orange to red domains. Recently, to broaden the scope of applications, regioselective late‐stage functionalizations on these core moieties have been developed. For instance, with the introduction of electron‐donating groups (EDGs), important bathochromic shifts are observed pushing absorptions towards or in the near‐infrared (NIR) spectral domain while emissive properties disappear essentially completely. Herein, to upset this drawback, acetylene derivatives of cationic diazaoxa triangulenes (DAOTA) and [4]helicenes are prepared (16 examples). Contrary to other EDG‐functionalized derivatives, C≡C− functionalized products remain broadly fluorescent, with red‐shifted absorptions (Δλ abs up to 25 nm) and emissions (Δλ em up to 73 nm, Φ PL up to 51 %). Quite interestingly, a general dynamic stereoisomerism phenomenon is evidenced for the compounds derived from achiral DAOTA cores. At low temperature in 1H NMR spectroscopy (218 K), N−CH2 protons become diastereotopic with chemical shifts differences (Δδ) as high as +1.64 ppm. The signal coalescence occurs around 273 K with a barrier of ∼12 kcal mol−1. This phenomenon is due to planar chiral conformations (S p and R p configurations), induced by the geometry of the alkyl (n‐propyl) side‐chains next to the acetylenic substituents. Ion pairing studies with Δ‐TRISPHAT anion not only confirm the occurrence of the chiral conformations but evidence a moderate but definite asymmetric induction from the chiral anion onto the cations. Finally, DFT calculations offer a valuable insight on the geometries, the corresponding stereodynamics and also on the very large difference in NMR for some of the diastereotopic protons.

Spectroscopic investigation on 1,2-substituted ferrocenes with only planar chirality: How chiroptical data are related to absolute configuration and to substituents

Single enantiomers of three 1,2-substituted ferrocene derivatives, i.e. 1-methoxymethyl-2-hydroxymethylferrocene (1), 1-formyl-2-hydroxymethylferrocene (2) and 1-iodo-2-hydroxymethylferrocene (3), sharing the common hydroxymethyl substituent and the presence of planar chirality only, were investigated for their spectroscopic (IR and UV) and chiroptical (VCD and ECD) properties. Both enantiomers of 1 were obtained for the first time in optically pure form by lipase-catalyzed kinetic resolution of the corresponding racemate (±)-1 and separately converted into formyl derivatives (+)-2 and (–)-2.The experimental spectroscopic and chiroptical data were compared with DFT calculated spectra and excellent correspondence was found for all compounds, allowing one to confirm the previously assigned absolute configurations. The common features in the VCD spectra of a doublet between 940 and 965 cm−1 and the short-wavelength (about 200 nm) doublet and the longest wavelength band in the ECD spectra were analyzed to test whether they may be taken as markers of the absolute configuration (AC). The predominance of conformers with intramolecular hydrogen bond for the first two investigated compounds is predicted by conformational analysis and also confirmed by NMR.

Self-Assembly of Chiral Ferrocene-Functionalized Polyoxotitanium Clusters for Photocatalytic Selective Sulfide Oxidation.

Here, we systematically studied the self-assembly behavior of chiral polyoxytitanium clusters for the first time. Through the cooperative assembly of ferrocenecarboxylic acid and ketoxime ligands, we successfully incorporated the planar chirality of ferrocene (Fc) into the layered {Ti5} building blocks. The resulting {Ti5Fc} clusters can be used as structural units to assemble into large ordered structures in various ways; either a pair of {Ti5Fc} enantiomers are bridged by organic adhesive to form sandwich structures or two homochiral {Ti5Fc} units participate in the assembly to form the large clusters. Depending on the assembly modes, the chirality of {Ti5Fc} can be transferred to large nanoclusters or disappear to form mesostructures. The difference of the assembly modes between the {Ti5Fc} units can also tune the photoelectric activity of the resulting clusters, which has been verified by using {Ti10Fc-6/7} as catalysts for photocatalytic selective sulfide oxidation. This work not only is an important breakthrough in the study of the self-assembly of chiral nanoclusters but also provides an important reference for understanding of chiral transfer on the nanoscale.

Arbitrary-order three-point finite difference method for the modal analysis of chiral waveguides.

A three-point finite difference method with an arbitrary order of accuracy is proposed for the modal analysis of chiral planar waveguides. The elaborate application of a nonuniform grid, compact finite difference technique, and boundary conditions results in an efficient, easily implemented, and versatile tool for the modal analysis of chiral planar waveguides with an arbitrarily discontinuous profile of permittivity, permeability, and chirality. In particular, this method efficiently resolves the fine structures in plasmon and photonic crystal waveguides. For the test model of a chiral-metallic plasmon waveguide, stable convergence up to a sixteenth order of accuracy can be obtained, which produces a relative error on the effective index that approaches the machine precision with only eighty grid points.

Planar Chirality: A Mine for Catalysis and Structure Discovery.

Planar chirality is one of the most fascinating expressions of chirality, which is exploited by nature to lock three‐dimensional chiral conformations and, more recently, by chemists to create new chiral reagents, catalysts, and functional organic materials. Nevertheless, the shortage of procedures able to induce and secure asymmetry during the generation of these unique chiral entities has dissuaded chemists from exploiting their structural properties. This Minireview intends to illustrate the limited but remarkable catalytic methods that have been reported for the production of planar chirality in strained molecules and serve as a source of inspiration for the development of new unconventional procedures, which are expected to appear in the near future.

Planar Chirality: A Mine for Catalysis and Structure Discovery

AbstractPlanar chirality is one of the most fascinating expressions of chirality, which is exploited by nature to lock three‐dimensional chiral conformations and, more recently, by chemists to create new chiral reagents, catalysts, and functional organic materials. Nevertheless, the shortage of procedures able to induce and secure asymmetry during the generation of these unique chiral entities has dissuaded chemists from exploiting their structural properties. This Minireview intends to illustrate the limited but remarkable catalytic methods that have been reported for the production of planar chirality in strained molecules and serve as a source of inspiration for the development of new unconventional procedures, which are expected to appear in the near future.

Chiral Ferrocenyl−Iodotriazoles and −Iodotriazoliums as Halogen Bond Donors. Synthesis, Solid State Analysis and Catalytic Properties.

AbstractDespite the increasing number of applications based on halogen bond (XB), asymmetric catalysis purely based on such supramolecular interactions still remains a huge challenge. The first step toward its development is the design of appropriate XB chiral donor molecules with good catalytic properties. In this context, we report the synthesis of a series of iodinated compounds based on the triazole or triazolium ring and possessing the planar chirality of ferrocene. Their XB donor property was attested by X‐ray diffraction analysis, showing short I⋅⋅⋅N and I⋅⋅⋅F interactions in the triazole‐based derivatives and in the tetrafluoroborate salt of a idodotriazolium, respectively. The potential of these compounds to act as XB‐based catalysts was demonstrated in the aza‐Diels‐Alder reaction involving an imine and a diene. Whereas triazole‐based derivatives were inactive in this reaction, the triflate salts of iodotriazoliums delivered the expected cycloadduct with high yield.

Stereoselective Self-Assembly of Complex Chiral Radial [5]Catenanes Using Half-Sandwich Rhodium/Iridium Building Blocks.

Herein, we have successfully achieved the stereoselective synthesis of two chiral radial [5]catenanes in a single step through the self-assembly of bidentate ligands containing l-alanine residues and binuclear half-sandwich organometallic rhodium(III)/iridium(III) clips. Remarkably, these two chiral radial [5]catenanes exhibit complex stereochemical structures as revealed by single-crystal X-ray diffraction. The eight binuclear units and eight bidentate ligands in their solid-state structures all exhibit a single planar chirality, and the interlocking between molecular macrocycles exhibits a single co-conformational mechanical helical chirality. This indicates that the introduction of the point chirality in the ligands enables the efficient stereoselective construction of mechanically interlocked molecules. Furthermore, by using ligands containing d-alanine residues, radial [5]catenanes with the opposite planar chirality and opposite co-conformational mechanical helical chirality have also been obtained.

Highlighting the DCO‐SCF 2020 Award Winners – A Valuable Collaboration with EurJOC

Highlighting the DCO‐SCF 2020 Award Winners – A Valuable Collaboration with EurJOC

Highly Phosphorescent Planar Chirality by Bridging Two Square-Planar Platinum(II) Complexes: Chirality Induction and Circularly Polarized Luminescence.

Chiral organometallic complexes have demonstrated many potential and practical applications. However, building metal-induced chirality for square-planar complexes still remains a big challenge, because their 2D planar molecular structures are usually superimposable on their mirror images. Herein, we report a straightforward and efficient way to achieve a novel kind of planar chirality by constructing 3D double-layer molecular structures. When the achiral ligand 1,3,4-oxadiazole-2-thiol (OXT) was used to bridge two square-planar complexes, a pair of racemic R/S planar-chiral binuclear Pt(II) complexes was obtained, which could be separated by chiral high-performance liquid chromatography (HPLC). Moreover, enantiopure R,R,R or S,S,S complexes could be prepared by the use of chiral (R)-/(S)-binaphthalene-derived OXT ligands in 99% diastereoselectivity without the use of chiral HPLC. The binaphthalene groups help to ensure good solubility and a smooth amorphous thin film morphology but have little effect on the photophysical properties. The resultant complexes display strong orange-red and near-infrared phosphorescence with quantum yields of up to 83.4% and can be applied as emitters in highly efficient solution-processed organic light-emitting diodes to achieve luminance, luminance efficiency, external quantum efficiency, and an asymmetry factor of up to 3.22 × 104 cd m-2, 28.7 cd A-1, 14.3%, and 2.0 × 10-3, respectively. With a comprehensive consideration of EL efficiency and the asymmetry factor, this is the best performance among Pt(II) complex based circularly polarized OLEDs. Therefore, this work provides a new and simple strategy to build planar chirality for chiroptical and circularly polarized luminescence applications.

Dynamic inversion of planar-chiral response of terahertz metasurface based on critical transition of checkerboard structures

Abstract Dynamic inversion of the planar-chiral responses of a metasurface is experimentally demonstrated in the terahertz regime. To realize this inversion, the critical transition of the checkerboard-like metallic structures is used. Resonant structures with planar chirality and their complementary enantiomeric patterns are embedded in the checkerboard. Using vanadium dioxide as a variable resistance, the metasurface is implemented in the terahertz regime. The responses of the metasurface to circularly polarized waves are then characterized by terahertz time-domain spectroscopy. Further, the sign of the circular conversion dichroism, which is closely related to the handedness of the planar chirality of the metasurface, is observed to be inverted at 0.64 THz by varying the temperature. Such invertible planar-chiral responses can be applied practically to the handedness-invertible chiral mirrors.

Controlled helical senses of twisting in two-, three- and four-layer cyclophanes with planar chirality.

We synthesized planar chiral cyclophanes with multiple pairs of helical conformations that were generated by twisting at the interlayer. Three- and four-layer cyclophanes preferred a homochiral form with a single sense of twisting through intramolecular transmission of planar chirality. Alternatively, a heterochiral form was dominant in a two-layer cyclophane consisting of two achiral planes that were stacked orthogonally.

The concise synthesis and resolution of planar chiral [2.2]paracyclophane oxazolines by C-H activation.

Planar chiral [2.2]paracyclophanes are resolved through the direct C–H arylation of enantiopure oxazolines, providing a convenient route to ligands and chiral materials. Preliminary results show that hydrolysis followed by decarboxylative phosphorylation leads to enantiopure [2.2]paracyclophane derivatives that are otherwise challenging to prepare.

CPL on/off control of an assembled system by water soluble macrocyclic chiral sources with planar chirality

Herein, we report the synthesis and planar chiral properties of a pair of water-soluble cationic pillar[5]arenes with stereogenic carbons. Interestingly, although units of the molecules were rotatable, only one planar chiral diastereomer existed in water in both cases. As a new type of chiral source, these molecules transmitted chiral information from the planar chiral cavities to the assembly of a water-soluble extended π-conjugated compound, affording circularly polarized luminescence (CPL). The chirality transfer process and resulting CPL were extremely sensitive to the feed ratio of the chiral pillar[5]arenes owing to the combined action of their planar chirality, bulkiness, and strong binding properties. When a limited amount of chiral source was added, further assembly of the extended π-conjugated compound into helical fibers with CPL was triggered. Unexpectedly, larger amounts of chiral source destroyed the helical fiber assemblies, resulting in elimination of the chirality and CPL properties from the assembled structures.

Preparation of Planar Chiral Oxazolinyl Hydroxyl Ferrocene Silyl Ethers and Their Desilylation Derivatives

Preparation of Planar Chiral Oxazolinyl Hydroxyl Ferrocene Silyl Ethers and Their Desilylation Derivatives

An atom economical approach for enantioselective cross dehydrogenative coupling

Asymmetric C−H/C−H cross coupling offers an efficient strategy for direct synthesis of enantioriched molecules. In this issue of Chem Catalysis, You and coworkers disclosed a Pd catalyzed approach for enantioselective cross coupling between indolizines and dialkylaminomethylferrocenes. Using this protocol, a range of chiral planar ferrocenes have been accessed under mild condition.

Synthesis and Chiroptical Properties of One-Handed Helical Oligo-o-phenylene-ethynylenes Using Planar Chiral [2.2]Paracyclophane

Abstract Both enantiomers of a one-handed helical molecule consisting of an o-arylene-ethynylene skeleton were prepared using a planar chiral [2.2]paracyclophane. The helicity of the molecule was controlled by its planar chirality. Excellent chiroptical properties were observed; in particular, the molecule emitted intense circularly polarized luminescence with a high |glum| value of the order of 10−2.

Cover Feature: Stimuli‐Responsive Chiroptical Switching (ChemPlusChem 2/2022)

The cover feature image shows chiroptical switching upon the application of light. If we traverse across the rim via the light bulb, we encounter the same molecule but with opposite chirality, which highlights that the change of chirality is induced by light. The two vintage cartoon characters epitomize their mirror image relationship. The pillar[5]arene, helix and biphenyl exemplify the planar, helical and axial chirality, respectively. The middle bulb symbolizes one of the stimuli that can be used to induce chiroptical switching. More information can be found in the Review by Soumen De, and co-workers.

Optically Active Cyclic Oligomers Based on Planar Chiral [2.2]Paracyclophane.

Optically active cyclic dimer, trimer, and tetramer, in which the p-arylene-ethynylenes were stacked, were prepared using enantiopure 4,7,12,15-tetrasubstituted [2.2]paracyclophane as the chiral building block. All molecules exhibited clear Cotton effects in their absorption bands with mirror image spectra, and the signals of the longest wavelengths of the (Rp )- and (Sp )-isomers were positive and negative, respectively. Their circularly polarized luminescence (CPL) signs corresponded with the those of the first Cotton effect. For all molecules, molecular orbitals were localized in one of the stacked p-arylene-ethynylenes in the excited states, resulting in a similar photoluminescence behavior. Although the cyclic dimer did not emit CPL, the cyclic trimer and tetramer exhibited intense CPL emissions with a relatively high dissymmetry factor in the order of 10-3 . Their optical and chiroptical properties were reproduced by time-dependent density functional theory calculations.

Planar Chiral Analogues of PRODAN Based on a [2.2]Paracyclophane Scaffold: Synthesis and Photophysical Studies.

We describe the synthesis and photophysical characterization of differently substituted planar chiral analogues of PRODAN based on a [2.2]paracyclophane scaffold. This experimental and theoretical study highlights that the (chir)optical properties of the new "phane" compounds, which incorporate an electron-withdrawing propionyl moiety and an electron-donating dimethylamino group at their para or pseudo-para positions, strongly depend on their substitution patterns. In particular, for this series of molecules, a more pronounced solvatochromism and clear chiroptical behaviors are observed when the two substituents are placed on the two rings of the pCp core in a non-"co-planar" arrangement (pseudo-para derivative). This observation may help design new pCp-based luminophores with finely tuned photophysical properties.