Supramolecular-Tilt-Chirality on Twofold Helical Assemblies

Pnictogen bonding in imide derivatives for chiral folding and self-assembly

Pnictogen bonding (PnB) is an attraction interaction that originates from the anisotropic distribution of electron density of pnictogen elements, which however has been rarely found in nitrogen atoms. In this work, for the first time, we unveil the general presence of N-involved PnB in aromatic or aliphatic imide groups and reveal its implications in chiral self-assembly of folding. This long-neglected interaction was consolidated by Cambridge structural database (CSD) searching as well as subsequent computational studies. Though the presence of PnB has limited effects on spectroscopic properties in the solution phase, conformation locking effects are sufficiently expressed in the chiral folding and self-assembly behavior. PnB anchors the chiral conformation to control the emergence and inversion of chiroptical signals, while intramolecular PnB induces the formation of supramolecular tilt chirality. It also enables the chiral folding of imide-containing amino acid or peptide derivatives, which induces the formation of unique secondary structural sequences such as β-sheets. Finally, the effects of PnB in directing folded helical structures were revealed. Examples of cysteine and cystine derivatives containing multiple N⋯O and N⋯S PnBs constitute an α-helix like secondary structure with characteristic circular dichroism. This work discloses the comprehensive existence of imide-involved PnB, illustrates its important role in folding and self-assembly, and sheds light on the rational fabrication of conformation-locked compounds and polymers with controllable chiroptical activities.

Elucidation of Substituent-Responsive Reactivities via Hierarchical and Asymmetric Assemblies in Crystalline p-Quinodimethane Derivatives

We propose a mechanism for substituent-responsive reactivities of p-quinodimethane derivatives with four ester groups through their hierarchical and asymmetric assembly modes. Four asymmetric 7,8,8-tris(methoxycarbonyl)-p-quinodimethanes with a 7-positioned ethoxycarbonyl (2 a(H)), 2'-fluoroethoxycarbonyl (2 b(F)), 2'-chloroethoxycarbonyl (2 c(Cl)), or 2'-bromoethoxycarbonyl (2 d(Br)) were synthesized and crystallized. 2 a(H), 2 b(F) and 2 d(Br) afforded only one shape crystal, while 2 c(Cl) did two polymorphic 2 c(Cl)-α and 2 c(Cl)-β. UV-irradiation induced topochemical polymerization for 2 a(H), no reactions for 2 b(F) and 2 c(Cl)-α, and [6+6] photocycloaddition dimerization for 2 c(Cl)-β and 2 d(Br). Such substituent-responsive reactivities and crystal structures were compared with those of the known symmetric 7,7,8,8-tetrakis(alkoxycarbonyl)-p-quinodimethanes such as 7,7,8,8-tetrakis(methoxycarbonyl)- (1 a(Me)-α and 1 a(Me)-β), 7,7,8,8-tetrakis(ethoxycarbonyl)- (1 b(Et)), and 7,7,8,8-tetrakis(bromoethoxycarbonyl)- (1 c(BrEt)). The comparative study clarified that the reactivities and crystal structures are classified into four types that link to each other. This linkage is understandable when we analyze the crystal structures through the following hierarchical and asymmetric assemblies; conformers, dimers, one dimensional (1D)-columns, two dimensional (2D)-sheets, and three dimensional (3D)-stacked sheets (3D-crystals). This supramolecular viewpoint is supported by intermolecular interaction energies among neighbored molecules with the density functional theory (DFT) calculation. Such research enables us to elucidate the substituent-responsive reactivities of the crystals, and reminds us of the selection of the right path in a so-called "maze game".

Highly-Active Chiral Organic Photovoltaic Catalysts with Suppressed Charge Recombination

Recombination of free charges in organic semiconductors reduces the available photo-induced charge-carriers and restricts photovoltaic efficiency. In this work, the chiral organic semiconductors (Y6-R and Y6-S with enantiopure R- and S- chiral alkyl sidechains) are designed and synthesized, which show effective aggregation-induced chirality through mainchain packing with chiral conformations in non-centrosymmetric space groups with tilt chirality. Based on the analysis of spin-injection, magnetic-hysteresis loop, and thermodynamics and dynamics of the excited state, we suggest that the aggregation-induced chirality can generate spin-polarization, which suppresses charge recombination and offers more available charge-carriers within Y6-R and Y6-S relative to the achiral counterpart (Y6). Then the chiral Y6-R and Y6-S show enhanced catalytic activity with optimal average hydrogen evolution rates of 205 and 217 mmol h-1  g-1 , respectively, 60-70 % higher than Y6, when they are employed as nanoparticle photocatalysts in photocatalytic hydrogen evolution under simulated solar light, AM1.5G, 100 mW cm-2 .

Mechanical twinning in organic crystals

Various kinds of organic crystals can deform beyond their elastic limit, show unique mechanical properties, and switch directions of anisotropic functions by mechanical twinning based on stress-induced molecular movements.

X⋅⋅⋅X Halogen Bond-Induced Supramolecular Helices

The halogen bond is the attractive interaction between the electrophilic region of a halogen atom and the nucleophilic region of another molecular entity, emerging as a favorable manner to manipulate supramolecular chirality in self-assemblies. Engineering halogen bonded helical structures remains a challenge due to its sensitivity to solvent polarity and competitive forces like hydrogen bonds. Herein, we report a X⋅⋅⋅X (X=Cl, Br, I) type weak halogen bond that induces the formation and evolution of supramolecular helical structures both in solid and solution state. The π-conjugated phenylalanine derivatives with F, Cl, Br and I substitution self-assembled into 2₁ helical packing driven by hydrogen bond and halogen bond, respectively. The specific molecular geometries of π-conjugated amino acids gave rise to multiple noncovalent forces to stabilize the X⋅⋅⋅X halogen bond with small bond energies ranging from -0.69 to -1.49 kcal mol^-1 . Halogen bond induced an opposite helicity compared to the fluorinated species, accompanied by the inversed circularly polarized luminescence.

Inverse Evolution of Helicity from the Molecular to the Macroscopic Level Based on N-Terminal Aromatic Amino Acids

Precise control of the emergence of macroscopic helicity with specific handedness is promising in rationally designing chiral nanomaterials, but it is rather challenging. Herein, we present a protocol to address the transmission of helicity at a molecularly resolved level to a macroscopically resolved level, in which process supramolecular chirality undergoes an inversion. A series of N-terminal aromatic amino acids could self-assemble in water, enabling the occurrence of helicity at the molecularly resolved scale, evidenced by the single crystal structure and chiroptical responses. While it failed to transmit the helicity to the macroscopic scale for individual self-assembly, the coassembly with small organic binder through hydrogen bonding interactions allows for the emergence of helical structures at the nano/micrometer scale. Experimental and theoretical results demonstrate that the introduction of extra hydrogen bonds enables a moderate crystallinity of coassemblies with remaining one-dimensional orientation to enhance the helical growth. The transmission of helicity to higher levels by coassembly is accompanied by the helicity inversion, resulting from the exchange of hydrogen bonds. This study presents a rational protocol to precisely control the emergence of macroscopic helicity from molecularly resolved helicity with finely tailored handedness, providing a deeper understanding of the chirality origin in the assembled systems in order to facilitate the design and construction of functional chiral nanomaterials.

Characterization of Hidden Chirality: Two-Fold Helicity in β-Strands

A β-strand is a component of a β-sheet and is an important structural motif in biomolecules. An α-helix has clear helicity, while chirality of a β-strand had been discussed on the basis of molecular twists generated by forming hydrogen bonds in parallel or non-parallel β-sheets. Herein we describe handedness determination of two-fold helicity in a zig-zag β-strand structure. Left- (M) and right-handedness (P) of the two-fold helicity was defined by application of two concepts: tilt-chirality and multi-point approximation. We call the two-fold helicity in a β-strand, whose handedness has been unrecognized and unclarified, as hidden chirality. Such hidden chirality enables us to clarify precise chiral characteristics of biopolymers. It is also noteworthy that characterization of chirality of high dimensional structures like a β-strand and α-helix, referred to as high dimensional chirality (HDC) in the present study, will contribute to elucidation of the possible origins of chirality and homochirality in nature because such HDC originates from not only asymmetric centers but also conformations in a polypeptide chain.

Supramolecular synthons in the gamma-hydroxybutenolides

The supramolecular organization in the solid state of five novel gamma-hydroxybutenolides is described.

Aza-capped cyclodextrins for intra-cavity metal complexation

N,N-Chelators based on aza-capped cyclodextrins were designed to produce complexes with an encapsulated metal unit.

Asymmetric Strecker Reaction Arising from the Molecular Orientation of an Achiral Imine at the Single-Crystal Face: Enantioenriched l- and d-Amino Acids

Strecker synthesis has long been considered one of the prebiotic reactions for the synthesis of α-amino acids. However, the correlation between the origin of chirality and highly enantioenriched α-amino acids through this method remains a puzzle. In the reaction, it may be conceivable that the handedness of amino acids has been determined at the formation stage of the chiral intermediate α-aminonitrile, that is, the enantioselective addition of hydrogen cyanide to an imine. Herein, an enantiotopic crystal surface of an achiral imine acted as an origin of chirality for the enantioselective formation of α-aminonitriles by the addition of HCN. In conjunction with the amplification of the enantiomeric excess and multiplication of enantioenriched aminonitrile, a large amount of near enantiopure α-amino acids, with the l- and d-handedness corresponding to the molecular orientation of the imine, is reported.

Structures of N-acetyl-DL-isoleucine, N-acetyl-DL-alloisoleucine and their ammonium salts; role of ammonium ions in crystal structure formation

The crystal structures of N-acetyl-DL-isoleucine, N-acetyl-DL-alloisoleucine and their ammonium salts show that these four compounds exist as racemic compounds around room temperature. The two ammonium salts are arranged around a 21 screw axis, forming a helical column which consists of ammonium ions and single enantiomeric anions similar to the crystals of the ammonium salts of optically active N-acetyl-L-isoleucine and N-acetyl-D-alloisoleucine. The ammonium ion and the carboxylate ion in the helix are connected by three hydrogen bonds, the fourth hydrogen bond being formed between the ammonium ion and an external acetyl amino group of the neighboring helical column. The fourth hydrogen bond is formed between the ammonium ion and an external acetyl amino group of the neighboring 21 column. Ammonium N-acetyl-DL-alloisoleucinate was revealed to exist as an unstable racemic compound due to conformational similarity between the racemic and optically active compounds in the solid state and was optically resolved by fractional crystallization at 293 K.

Construction and Systematical Symmetric Studies of a Series of Supramolecular Clusters with Binary or Ternary Ammonium Triphenylacetates

Functions of clusters in nano or sub-nano scale significantly depend on not only kinds of their components but also arrangements, or symmetry, of their components. Therefore, the arrangements in the clusters have been precisely characterized, especially for metal complexes. Contrary to this, characterizations of molecular arrangements in supramolecular clusters composed of organic molecules are limited to a few cases. This is because construction of the supramolecular clusters, especially obtaining a series of the supramolecular clusters, is difficult due to low stability of non-covalent bonds compare to covalent bonds. From this viewpoint, utilization of organic salts is one of the most useful strategies. A series of the supramolecules could be constructed by combinations of a specific organic molecule with various counter ions. Especially, primary ammonium carboxylates are suitable as typical examples of supramolecules because various kinds of carboxylic acids and primary amines are commercially available, and it is easy to change their combinations. Previously, it was demonstrated that primary ammonium triphenylacetates using various kinds of primary amines specifically construct supramolecular clusters, which are composed of four ammoniums and four triphenylacetates assembled by charge-assisted hydrogen bonds, in crystals obtained from non-polar solvents. This study demonstrates an application of the specific construction of the supramolecular clusters as a strategy to conduct systematical symmetric study for clarification of correlations between molecular arrangements in supramolecules and kinds and numbers of their components. In the same way with binary salts composed of triphenylacetates and one kind of primary ammoniums, ternary organic salts composed of triphenylacetates and two kinds of ammoniums construct the supramolecular clusters, affording a series of the supramolecular clusters with various kinds and numbers of the components.

Exocyclic self-assembly behavior of carboxylic acid and lariat ether macrocyclic hosts: regulation by pendent arm

An exocyclic supramolecular building block through O–H⋯N hydrogen bonding interaction for the assembly of di-sidearm dibenzo-diazacrown ethers bearing a flexibility sidearm with PTA acid.

Right-handed 2/1 helical arrangement of benzene molecules in cholic acid crystal established by experimental and theoretical circular dichroism spectroscopy

Right handed 2/1 helical arrangement of benzene included in cholic acid (CA) crystals was directly established by experimental and theoretical circular dichroism spectral studies.

Biomimetic molecular organization of naphthalene diimide in the solid state: tunable (chiro-) optical, viscoelastic and nanoscale properties

Biomimetic molecular organization of naphthalene diimide in the solid state: tunable (chiro-) optical, viscoelastic and nanoscale properties.

Development of structural complexity by liquid-crystal self-assembly

Since the discovery of the liquid-crystalline state of matter 125 years ago, this field has developed into a scientific area with many facets. This Review presents recent developments in the molecular design and self-assembly of liquid crystals. The focus is on new exciting soft-matter structures distinct from the usually observed nematic, smectic, and columnar phases. These new structures have enhanced complexity, including multicompartment and cellular structures, periodic and quasiperiodic arrays of spheres, and new emergent properties, such as ferroelctricity and spontaneous achiral symmetry-breaking. Comparisons are made with developments in related fields, such as self-assembled monolayers, multiblock copolymers, and nanoparticle arrays. Measures of structural complexity used herein are the size of the lattice, the number of distinct compartments, the dimensionality, and the logic depth of the resulting supramolecular structures.

Linkage control between molecular and supramolecular chirality in 2₁-helical hydrogen-bonded networks using achiral components

Chiral molecules preferentially form one-handed supramolecular assemblies that reflect the absolute configuration of the molecules. Under specific conditions, however, the opposite-handed supramolecular assemblies are also obtained because of flexibility in the bond length and reversibility of non-covalent interactions. The mechanism of the handedness selectivity or switching phenomenon remains ambiguous, and most phenomena are observed by chance. Here we demonstrate the construction of chiral hydrogen-bonded twofold helical assemblies with controlled handedness in the crystalline state based on crystallographic studies. Detailed investigation of the obtained crystal structures enabled us to clarify the mechanism, and the handedness of the supramolecular chirality was successfully controlled by exploiting achiral factors. This study clearly reveals a connection between molecular chirality and supramolecular chirality in the crystalline state.