Induction Of Supramolecular Chirality Research Articles

The More the Slower: Self-Inhibition in Supramolecular Chirality Induction, Memory, Erasure, and Reversion

Self-inhibition has been observed widely in hierarchical biochemical processes but has yet to be demonstrated in pure molecular physical rather than chemical or biological processes. Herein, we report an unprecedented example of self-inhibition during the supramolecular chirality induction, memory, erasure, and inversion processes of pillar[5]arene (P[5]) derivatives. The addition of chiral alanine ethyl ester to bulky substituent-modified P[5]s led to time-dependent chirality induction due to the shift in the equilibrium of the SP and RP conformers P[5]. Intriguingly, more chiral inducers led to more intensive final chiroptical properties but lower chiral induction rates. Thus, the chiral inducer plays the role of both activator and inhibitor. Such self-inhibition essentially arises from kinetics manipulation of three tandem equilibria. Moreover, the chiroptical properties could be memorized by replacing the chiral inducer with an achiral competitive binder, and the chiroptical signal could be erased and reversed by an antipodal chiral inducer, which also showed the self-inhibition property.

In Situ Controlled Construction of a Hierarchical Supramolecular Chiral Liquid‐Crystalline Polymer Assembly

AbstractHierarchical supramolecular chiral liquid‐crystalline (LC) polymer assemblies are challenging to construct in situ in a controlled manner. Now, polymerization‐induced chiral self‐assembly (PICSA) is reported. Hierarchical supramolecular chiral azobenzene‐containing block copolymer (Azo‐BCP) assemblies were constructed with π–π stacking interactions occurring in the layered structure of Azo smectic phases. The evolution of chirality from terminal alkyl chain to Azo mesogen building blocks and further induction of supramolecular chirality in LC BCP assemblies during PICSA is achieved. Morphologies such as spheres, worms, helical fibers, lamellae, and vesicles were observed. The morphological transition had a crucial effect on the chiral expression of Azo‐BCP assemblies. The supramolecular chirality of Azo‐BCP assemblies destroyed by 365 nm UV irradiation can be recovered by heating–cooling treatment; this dynamic reversible achiral–chiral switching can be repeated at least five times.

In Situ Controlled Construction of a Hierarchical Supramolecular Chiral Liquid-Crystalline Polymer Assembly.

Hierarchical supramolecular chiral liquid-crystalline (LC) polymer assemblies are challenging to construct in situ in a controlled manner. Now, polymerization-induced chiral self-assembly (PICSA) is reported. Hierarchical supramolecular chiral azobenzene-containing block copolymer (Azo-BCP) assemblies were constructed with π-π stacking interactions occurring in the layered structure of Azo smectic phases. The evolution of chirality from terminal alkyl chain to Azo mesogen building blocks and further induction of supramolecular chirality in LC BCP assemblies during PICSA is achieved. Morphologies such as spheres, worms, helical fibers, lamellae, and vesicles were observed. The morphological transition had a crucial effect on the chiral expression of Azo-BCP assemblies. The supramolecular chirality of Azo-BCP assemblies destroyed by 365 nm UV irradiation can be recovered by heating-cooling treatment; this dynamic reversible achiral-chiral switching can be repeated at least five times.

Reversing the Handedness of Self‐Assembled Porous Molecular Networks through the Number of Identical Chiral Centres

AbstractScanning tunnelling microscope observations at the 1‐phenyloctane/graphite interface reveal how chiral structural information at the molecular level is transferred and expressed structurally at the 2D supramolecular level for a porous system. The chirality of self‐assembled molecular networks formed by chiral dehydrobenzo[12]annulene (cDBA) derivatives having three chiral chains and three achiral chains, alternatingly, is compared with those of cDBAs having six chiral chains reported previously. While for all cDBAs homochiral surfaces are formed, their handedness is not simply a reflection of the absolute configuration of the stereogenic centres. Both the number of stereogenic centres as well as the length of the achiral chains determine the supramolecular handedness, providing a deep insight into the supramolecular chirality induction mechanisms at play. Moreover, these cDBAs act to induce chirality in porous networks formed by achiral DBAs.

Reversing the Handedness of Self-Assembled Porous Molecular Networks through the Number of Identical Chiral Centres.

Scanning tunnelling microscope observations at the 1-phenyloctane/graphite interface reveal how chiral structural information at the molecular level is transferred and expressed structurally at the 2D supramolecular level for a porous system. The chirality of self-assembled molecular networks formed by chiral dehydrobenzo[12]annulene (cDBA) derivatives having three chiral chains and three achiral chains, alternatingly, is compared with those of cDBAs having six chiral chains reported previously. While for all cDBAs homochiral surfaces are formed, their handedness is not simply a reflection of the absolute configuration of the stereogenic centres. Both the number of stereogenic centres as well as the length of the achiral chains determine the supramolecular handedness, providing a deep insight into the supramolecular chirality induction mechanisms at play. Moreover, these cDBAs act to induce chirality in porous networks formed by achiral DBAs.

Chiral Crystal Packing Induces Enhancement of Vibrational Circular Dichroism.

We demonstrate that molecular vibrations with originally low or zero intensity in a vibrational circular dichroism (VCD) spectrum attain chirality in molecular crystals by coordinated motion of the atoms. Ab initio molecular dynamics simulations of anharmonic solid-state VCD spectra of l-alanine crystals reveal how coherent vibrational modes exploit the space group's chirality, leading to non-local, enhanced VCD features, most significantly in the carbonyl region of the spectrum. The VCD-enhanced signal is ascribed to a helical arrangement of the oscillators in the crystal layers. No structural irregularities need to be considered to explain the amplification, but a crucial point lies in the polarization of charge, which requires an accurate description of the electronic structure. Delivering a quantitative atomic conception of supramolecular chirality induction, our ab initio scheme is applicable well beyond molecular crystals, for example, to address VCD in proteins and related compounds.

Chirality Construction from Preferred π-π Stacks of Achiral Azobenzene Units in Polymer: Chiral Induction, Transfer and Memory.

The induction of supramolecular chirality from achiral polymers has been widely investigated in composite systems consisting of a chiral guest, achiral host, and solvents. To further study and understand the process of chirality transfer from a chiral solvent or chiral molecules to an achiral polymer backbone or side-chain units, an alternative is to reduce the components in the supramolecular assembled systems. Herein, achiral side-chain azobenzene (Azo)-containing polymers, poly(6-[4-(4-methoxyphenylazo) phenoxy] hexyl methacrylate) (PAzoMA), with different Mns, were synthesized by atom transfer radical polymerization (ATRP). Preferred chirality from supramolecular assembled trans-Azo units of PAzoMAs is successfully induced solely by the neat limonene. These aggregates of the polymers in limonene solution were characterized by circular dichroism (CD), UV-vis spectra, and dynamic light scattering (DLS) under different temperatures. The temperature plays an important role in the course of chiral induction. Meanwhile, supramolecular chirality can be constructed in the solid films of the achiral side-chain Azo-containing polymers that were triggered by limonene vapors. Also, it can be erased after heated above the glass transition temperature (Tg) of the polymer, and recovered after cooling down in the limonene vapors. A chiroptical switch can be built by alternately changing the temperature. The solid films show good chiral memory behaviors. The current results will facilitate studying the mechanism of chirality transfer induced by chiral solvent and improve potential application possibilities in chiral film materials.

The Breaking of Symmetry Leads to Chirality in Cucurbituril-Type Hosts

Cucurbituril-type hosts are highly symmetric, but there are means to break their symmetry. This review will present examples from three directions of induction of chirality in or by cucurbituril-type hosts: first, through the incorporation of stereogenic elements into host molecules; second, through complexation with achiral guests, which leads to axial supramolecular chirality and helical structures; third, through the formation of complexes with chiral guests in multi-molecule complexes and induction of supramolecular chirality. In addition, a list of chiral guests used in binding studies with cucurbiturils is collected. We would envision that encouraged by the outlined examples of outstanding applications of chiral cucurbituril-supramolecular systems, the boundaries of chiral applications of cucurbiturils would be widened.

Induction of supramolecular chirality by chiral solvation in achiral Azo polymers with different spacer lengths and push–pull electronic substituents: where will chiral induction appear?

Effects of achiral azobenzene polymers with different spacer lengths and push–pull electronic substituents on supramolecular chirality induced by chiral limonene are reported in detail.

Chiral metal-macrocycle frameworks: supramolecular chirality induction and helicity inversion of the helical macrocyclic structures.

Porous molecular solids composed of discrete macrocycles/cages have great potential for catalysis, separation and sensing techniques. Dynamic structural transformation of the host building blocks, especially a helicity inversion responsive to chemical triggers, is central to upgrading the spatial functions. Here we have achieved the syntheses of homochiral porous molecular solids composed of helical metal macrocycles through supramolecular chirality induction to both enantiomorphic forms with the aid of two different enantiopure sugar-derived lactones in the crystallization process. Moreover, we found that the helicity of the macrocyclic skeletons can be inverted in the crystalline state only by changing the type of solvent. This finding would lead to dynamic control of space chirality in connection with optical resolution, chiral amplification and asymmetric reactions.

Stoichiometry-controlled supramolecular chirality induction in magnesium(II) porphyrin dimer by amino alcohols : Mechanistic insights and effect of ligand bulkiness

Stoichiometry-controlled supramolecular chirality induction in magnesium(II) porphyrin dimer by amino alcohols : Mechanistic insights and effect of ligand bulkiness

Application of Octaethylporphyrin Structural Motif for Supramolecular Chirogenesis

Supramolecular chirogenesis is a smart combination of supramolecular chemistry and chirality associated with the processes of asymmetry induction, amplification, modulation in non-covalently linked multicomponent assemblies.1Porphyrin stuctures are of particular interest to study this phenomen owing to their unique spectral, physico-chemical, and synthetic properties. Octaethylporphyrin (OEP) is one of the simplest porphyrins, yet being easily accessible via the standard synthetic procedure or commercially available from many reagent suppliers. Therefore, monomeric ZnOEP was firstly tested in the process of supramolecular chirality induction. Hence, upon interation with chiral amines (L) the induced optical activity in inherently circular dichroism (CD) silent ZnOEP was almost neglidgible that indicated inefficiency of the chirality transfer process in these supramolecular systems.2 However, in the case of solid state (a glassy KBr matrix) there was the time-dependent formation of (ZnOEP.L)n chiral aggregates posessing a high degree of optical activity with the anisotropy factor of 0.015.3 The chirality transfer mechanism was based upon the unidirectional arrnagement of ZnOEP molecules. In contrast to monomeric ZnOEP, dimeric (ZnOEP)2 exhibited considerable optical activity upon interaction with chiral guests.2,3 Thus, monodentate ligands resulted in formation of the stable 1:2 supramolecular chiral complexes and induction of the noticeable (moderate-to-strong) exciton couplet CD signals in the absorption region of porphyrin. It was established that the mechanism of asymmetry transfer from chiral guests to achiral (ZnOEP)2 consists of two major steps. At the first step there is formation of the host-guest complex owing to abilities of chiral guests to interact with (ZnOEP)2 resulting in the initial syn-to-anti conformational switching. The second step is another type of structural deformation in (ZnOEP)2 induced by the size difference of ligand’s substituents at the chirogenic center resulting in asymmetric screw formation in the resulting complexes. For example, when the bulkiness order of the guest substituents around the chiral center coincides with the substituents’ priority rule, the (R)-enantiomer yields a negative first Cotton effect and a positive second Cotton effect, while the guest with the (S) absolute configuration produces CD signal of the opposite signs, thus making this sensing protocol to be effectively used for determining the absolute configuration of different classes of organic molecules. In the solid state the situation is different; (R)-ligands upon interaction with (ZnOEP)2 induce a positive first Cotton effect, whilst the corresponding (S)-ligands produce a mirror image CD spectra.3 Remarkably, the induced chirality in the solid state exhibits enhanced intensity, with an inverted sign in comparison to that in the solution phase owing to formation of chiral aggregates of regular structure, in which the intermolecular screw sense is opposite to the intramolecular screw sense of the single molecule. The generated CD sign unambiguously correlates with the induced helicity allowing straightforward determination of the absolute configuration of various chiral guests and thus making it possible to apply (ZnOEP)2 as effective and universal chirality sensors for different types of optically active compounds as in solution and in the solid state. For investigation of the weak host-guest interaction mode a special tetrameric (NiOEP)4 host has been designed.4 This host satisfies the major following requirements: (1) total inertness towards most of the conventional host-guest binding modes, (2) possession of an open cavity between the two bis-porphyrin subunits owing to the syn-orientation in (NiOEP)4, which allows chiral guest molecules to be incorporated into it and thus to interact with the host in a stereospecific manner, and (3) presence of a spectrally separated B|| electronic transitions aligned along the diacetylene bridges and located at the low energy region of the porphyrin B band due to the enhanced interporphyrin interactions through the unsaturated linkage. The chirogenic process in (NiOEP)4 was monitored upon interaction with various chiral solvents and resulted in induction of optical activity in the region of B|| electronic transitions due to the corresponding host-guest interactions. In particular, (R)-solvents yield a positive first and negative second Cotton effects, while the (S)-enantiomers gives the opposite signs. The apparent mechanism of chirality induction in (NiOEP)4 includes penetration of the solvent molecules inside the interporphyrin cavity to enhance the solvent-solute interactions and thus transfer the chiral information to an achiral host by inducing a unidirectional screw in (NiOEP)4 and following the guest’s absolute configuration. V. V. Borovkov, Y. Inoue, Top. Curr. Chem. 2006, 265, 89.V. V. Borovkov, J. M. Lintuluoto, Y. Inoue, J. Am. Chem. Soc. 2001, 123, 2979V. V. Borovkov, G. A. Hembury, Y. Inoue, Acc. Chem. Res. 2004, 37, 449.V. V. Borovkov, T. Yamamoto, H. Higuchi, Y. Inoue, Org. Lett. 2008, 10, 1283.

Photocontrollable induction of supramolecular chirality in achiral side chain Azo-containing polymers through preferential chiral solvation

The preferred supramolecular chirality in aggregates of achiral azobenzene-containing polymers by limonene as a chiral transducer is achieved for the first time.

Autoamplification of Molecular Chirality through the Induction of Supramolecular Chirality

The novel concept for the autoamplification of molecular chirality, wherein the amplification proceeds through the induction of supramolecular chirality, is presented. A solution of prochiral, ring-open diarylethenes is doped with a small amount of their chiral, ring-closed counterpart. The molecules co-assemble into helical fibers through hydrogen bonding and the handedness of the fibers is biased by the chiral, ring-closed diarylethene. Photochemical ring closure of the open diarylethene yields the ring-closed product, which is enriched in the template enantiomer.

Autoamplification of Molecular Chirality through the Induction of Supramolecular Chirality

AbstractThe novel concept for the autoamplification of molecular chirality, wherein the amplification proceeds through the induction of supramolecular chirality, is presented. A solution of prochiral, ring‐open diarylethenes is doped with a small amount of their chiral, ring‐closed counterpart. The molecules co‐assemble into helical fibers through hydrogen bonding and the handedness of the fibers is biased by the chiral, ring‐closed diarylethene. Photochemical ring closure of the open diarylethene yields the ring‐closed product, which is enriched in the template enantiomer.

Induction of Supramolecular Chirality in the Self-Assemblies of Lipophilic Pyrimidine Derivatives by Choice of the Amino Acid-Based Chiral Spacer

A new family of supramolecular organogelators, based on chiral amino acid derivatives of 2,4,6-trichloro-pyrimidine-5-carbaldehyde, has been synthesized. L-alanine was incorporated as a spacer between the pyrimidine core and long hydrocarbon tails to compare the effect of chirality and hydrogen bonding to that of the achiral analogue. The role of aromatic moiety on the chiral spacer was also investigated by introducing L-phenyl alanine moieties. The presence of intermolecular hydrogen-bonding leading to the chiral self-assembly was probed by concentration-dependent FTIR and UV/Vis spectroscopies, in addition to circular dichroism (CD) studies. Temperature and concentration-dependent CD spectroscopy ascribed to the formation of β-sheet-type H-bonded networks. The morphology and the arrangements of the molecules in the freeze-dried gels were examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), and X-ray diffraction (XRD) techniques. Calculation of the length of each molecular system by energy minimization in its extended conformation and comparison with the small-angle XRD pattern reveals that this class of gelator molecules adopts a lamellar organization. Polarized optical microscopy (POM) and differential scanning calorimetry (DSC) indicate that the solid state phase behavior of these molecules is totally dependent on the choice of their amino acid spacers. Structure-induced aggregation properties based on the H-bonding motifs and the packing of the molecule in three dimensions leading to gelation was elucidated by rheological studies. However, viscoelasticity was shown to depend only marginally on the H-bonding interactions; rather it depends on the packing of the gelators to a greater extent.

Versatile Switching in Substrate Topicity: Supramolecular Chirality Induction in Di‐ and Trinuclear Host Complexes

Supramolecular chirality effects have been achieved both for ditopic and monotopic substrates by using a programmable bis-salphen scaffold that incorporates either two or three Zn nuclei. The dinuclear host shows preferential chirogenesis in the presence of ditopic systems, whereas effective chirality transfer to the trinuclear complex is realized through monotopic binding. The mode of binding in the trinuclear host has been investigated through X-ray crystallography, CD measurements, UV/Vis spectroscopy, and DFT analysis. The bis-salphen scaffold holds promise for the development of substrate-specific host systems useful for determination of the absolute configuration of various types of organic molecules.

Influence of the Solvent and Metal Center on Supramolecular Chirality Induction with Bisporphyrin Tweezer Receptors. Strong Metal Modulation of Effective Molarity Values

We describe the synthesis of a bisporphyrin tweezer receptor 1·H(4) and its metalation with Zn(II) and Rh(III) cations. We report the thermodynamic characterization of the supramolecular chirality induction process that takes place when the metalated bisporphyrin receptors coordinate to enantiopure 1,2-diaminocyclohexane in two different solvents, toluene and dichloromethane. We also performed a thorough study of several simpler systems that were used as models for the thermodynamic characterization of the more complex bisporphyrin systems. The initial complexation of the chiral diamine with the bisporphyrins produces a 1:1 sandwich complex that opens up to yield a simple 1:2 complex in the presence of excess diamine. The CD spectra associated with the 1:1 and 1:2 complexes of both metalloporphyrins, 1·Zn(2) and 1·Rh(2), display bisignate Cotton effects when the chirogenesis process is studied in toluene solutions. On the contrary, in dichloromethane solutions, only 1·Zn(2) yields CD-active 1:1 and 1:2 complexes, while the 1:2 complex of 1·Rh(2) is CD-silent. In both solvents, porphyrin 1·Zn(2) features a stoichiometrically controlled chirality inversion process, which is the sign of the Cotton effect of the 1:1 complex is opposite to that of the 1:2 complex. In contrast, porphyrin 1·Rh(2) affords 1:1 and 1:2 complexes in toluene solutions with the same sign for their CD couplets. Interestingly, in both solvents, the signs of the CD couplets associated with the 1:1 sandwich complexes of 1·Zn(2) and 1·Rh(2) are opposite. The amplitudes of the CD couplets are higher for 1·Zn(2) than for 1·Rh(2). This observation is in agreement with 1·Rh(2) having a smaller extinction coefficient than 1·Zn(2). We performed DFT-based calculations and assigned molecular structures to the 1:1 and 1:2 complexes that explain the observed signs for their CD couplets. Unexpectedly, the quantification of the thermodynamic stability of the two metallobisporphyrin/diamine 1:1 sandwich complexes revealed the existence of interplay between effective molarity values (EM) and the strength of the intermolecular interaction (K(m); N···Zn or N···Rh) used in their assembly. The EM for the N···Rh(III) intramolecular interaction is 3 orders of magnitude smaller than that for the N···Zn(II) interaction, both of which are embedded in the same scaffold of the 1·M(2) bisporphyrin receptor.

Induction of supramolecular chirality in di-zinc(ii) bisporphyrin via tweezer formation: synthesis, structure and rationalization of chirality

Two new supramolecular complexes consisting of an achiral bisporphyrin host and a chiral diamine guest are reported. One shows a remarkably high amplitude bisignate CD signal while the other one shows a very low value. X-ray structure and other spectroscopic investigations of the tweezer complexes clearly rationalize the origin of the optical activity that has so far remained an unresolved issue.

Induction of supramolecular chirality in self-assembled nanofibers triggered by environmental change

A carbazole end-capped rod amphiphile was synthesized and observed to self-assemble into non-chiral fibers in both methanol and water solutions even though the molecule contains a chiral oligoether dendron. Remarkably, a mixture of water and methanol induces supramolecular chirality of the nanofibers caused by a conformational change of hydrophobic aromatic rods and reduction in the hydrodynamic volume of the ethylene oxide chains. The helicity induction of the nanofibers also takes place by heating the water solution. Furthermore, the helical fibers in aqueous solution were observed to further aggregate into a rigid gel at higher temperature by enhanced hydrophobic interactions between the dehydrated oligo(ethylene oxide) dendrons.