Interchain-solvent-induced chirality change of 1D helical chains: from achiral to chiral crystallization

Three helical supramolecular stereoisomers of meso-2, Delta-2, and Delta-3 with the formula of cis-[Ni(f-rac-L)][Ni(CN)4] were successfully constructed based on the [Ni(f-rac-L)2+ and [Ni(CN)4]2- building blocks (L = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane). In all three supramolecular stereoisomers, cis-[Ni(f-rac-L)]2+ cations are alternately bridged by [Ni(CN)4]2- anions through two cis (in meso-2 and Delta-2) or trans (in Delta-3) cyano groups to form one-dimensional (1D) helical chains of cis-[Ni(f-rac-L)][Ni(CN)4]. In meso-2, the right/left-handed chirality of the originally formed chain is transferred oppositely to adjacent chains through the interchain hydrogen-bonding interactions of hexameric water clusters, leading to the formation of meso-2 with a central symmetrical space group, P21/n, in which the 1D helical chains are packed in an alternating right- and left-handed chirality. In Delta-2 and Delta-3, the right/left-handed chirality of the original chain is transferred uniformly to adjacent chains through the zipper-like interchain hydrophobic interactions, resulting in the formation of Delta-2 and Delta-3 with chiral space groups of P212121 and P3121, respectively, in which all of the 1D helical chains are arranged in the same right/left-handed chirality.     

Self-assembly of spherical colloids into helical chains with well-controlled handedness

Physical constraint and capillary force have been combined to provide a generic approach to assemble achiral building blocks such as monodisperse spherical colloids into helical mesostructures. The key component of this process is an array of V-shaped grooves anisotropically etched in the surface of a Si(100) wafer. The structural arrangement among the spherical colloids is determined by the ratio between the width of the V-grooves and the diameter of the colloids. Double-layered structures with a helical morphology will be formed when this ratio falls between 2.70 and 2.85. The exact handedness of these helical structures could be controlled by varying the relative orientation of capillary force with respect to the longitudinal axis of the spirals. The processing of an achiral material into helical mesostructures having well-controlled handedness should allow us to explore new properties that this material otherwise does not exhibit. The self-assembly process may also provide valuable insights to improve our understanding on the mechanism(s) by which pure enantiomers with a particular sense of rotation evolved in nature.     

Amplification of chirality from molecules into morphology of crystals through molecular recognition

We have found a novel type of morphological chiral tuning on inorganic helical crystals through stereochemical recognition of organic molecules. Helical forms consisting of twisted twins emerged from triclinic crystals under diffusion-limited conditions. The proportion of the right- and left-handed helices was precisely tuned with the addition of a specified amount of chiral molecules, such as d- and l-glutamic acids. The chiral molecules recognized the enantiomeric surface of the triclinic crystal and then changed the growth behavior of the helical morphology. As a result, the microscopic chiral information, at a molecular level, was amplified into the macroscopic helices consisting of inorganic achiral components.     

Synthesis of cyclophanes with planar and helical chirality

Novel helical macrocyclic imines derived from planar chiral [2.2]paracyclophane were synthesized. The chiroptical properties of the enantiopure compounds were investigated and their absolute configurations were assigned.     

Detection and amplification of chirality by helical polymers

A unique feature of synthetic helical polymers for the detection and amplification of chirality is briefly described in this article. In sharp contrast to host-guest and supramolecular systems that use small synthetic receptor molecules, chirality can be significantly amplified in a helical polymer, such as poly(phenylacetylene)s with functional pendants, which enable the detection of a tiny imbalance in biologically important chiral molecules through a noncovalent bonding interaction with high cooperativity. The rational design of polymeric receptors can be possible by using chromophoric helical polymers combined with functional groups as the pendants, which target particular chiral guest molecules for developing a highly efficient chirality-sensing system. The chirality sensing of other small molecular and supramolecular systems is also briefly described for comparison.     

Two supramolecular isomers of molecular squares and 1D helical chains with alternating right- and left-handed chirality

The reaction of [Ni(alpha-rac-L)](ClO4)2 with K2[Ni(CN)4] gives a cyanide bridged [2+2] type of molecular square, {cis-[Ni(f-rac-L)][Ni(CN)4]}2 (1). By slightly changing the reaction conditions, the reaction of [Ni(alpha-rac-L)](ClO4)2 with KCN leads to a metastable compound, cis-[Ni(f-rac-L)(CN)2] (2), and an unexpected 1D helical chain, {cis-[Ni(f-rac-L)][Ni(CN)4]}n (3). In 3, the 1D helical chains are packed in an alternating right- and left-handed chirality due to the oppositely twisted arrangements of two adjacent [Ni(CN)4]2- anions. The metastable compound 2 can be converted to 3 in a CH3CN/CH3OH solution. Compounds 1 and 3 are classified as supramolecular isomers, and isomer 3 can be considered to be formed by the ring-opening polymerization of the square precursor 1. Magnetic susceptibility measurements of 1 and 3 show that the adjacent six-coordinated Ni(II) atoms are antiferromagnetically coupled through the bent -NC-Ni-CN- bridges of the diamagnetic [Ni(CN)4]2- anions, with g = 2.08 and J = -0.426 cm(-1) for 1 and g = 2.08 and J = -0.278 cm(-1) for 3. The correlation between the structures and the J values is discussed.     

Structure and dynamics of a molecular hydrogel derived from a tripodal cholamide

Tripodal cholamide 1 is a supergelator of aqueous fluids. A variety of physical techniques, including cryo-transmission electron microscopy (TEM), circular dichroism (CD), steady-state fluorescence, time-resolved fluorescence, and dynamic light-scattering, were employed to understand the structure and dynamics of the gel. Fluorescent probes [ANS (8-anilinonaphthalene-1-sulfonic acid) and pyrene] reported two critical aggregation concentrations (CAC(1) and CAC(2)) of 1 in predominantly aqueous media, with the minimum gel concentration (MGC) being close to CAC(2). Fluorescence lifetime measurements with pyrene revealed ineffective quenching of pyrene fluorescence by oxygen, possibly caused by slower Brownian diffusion due to the enhanced viscosity in the gel phase. The study of the gelation kinetics by monitoring the ultrafast dynamics of ANS revealed a progressive increase in the aggregate size and the microviscosity of the aqueous pool encompassed by the self-assembled fibrillar network (SAFIN) during the gelation. The striking difference between microviscosity and bulk (macroscopic) viscosity of the gel is also discussed.     

Crystallization of helical oligomers with chirality selection. I. A molecular dynamics simulation for bare helix

Helical polymers often exhibit pronounced chirality recognition during crystallization. By molecular dynamics simulation, we have already shown that the helical polymers crystallize with or without marked chirality selection depending on structural details of the polymer molecules. We have there classified the helical polymers into two categories: the bare helices made of only backbone atoms which show rather tolerant chirality selection, and the general helices with large side groups showing strict chirality recognition. Polymer crystallization is in general largely hampered and retarded by slow dynamics of the entangled chains, and therefore short helical oligomers are very suitable models for studying the chiral crystallization. We here report on molecular simulations of crystallization in the bare helical oligomer molecules by the use of Monte Carlo and molecular dynamics simulations. First we confirm the low temperature chiral crystal phase and the reversible order-disorder transition. We also observe frequent inversions of the helical sense, and the helix reversal defects propagating along the chains. Then we investigate crystallization from the melt into the chiral crystal phase. We find that the crystallization rate depends very sensitively on the degree of undercooling. The crystallization is found to be the first order transition that conforms well to the traditional picture of crystal growth in small molecules. Even when the crystallization directly into the chiral crystal phase is conducted, marked chirality selections are not observed at the early stage of crystallization; the chains adhere to the crystal surfaces selecting their helical senses rather at random resulting in racemic crystallites. The isothermal crystallization for a sufficiently long time, however, yields lamellar crystals composed of well-developed chiral domains, the growth of which seems to be accomplished through the transition back into the ordered chiral crystal phase.     

Quantified chirality, molecular similarity, and helical twisting power in lyotropic chiral nematic guest/host systems

Lyotropic liquid crystals can exhibit phase chirality. The mechanism behind the transfer of chirality between a chiral dopant and a liquid crystalline host phase is still under discussion. Our own recent results and proposals are the following. Lyotropic phase chirality can exist even at very low concentrations of chiral dopants, with less than 1 chiral dopant per 50 micelles. There is evidence for an intramicellar double twist which could be due to the induction of chiral conformations in the achiral surfactant chains. The chirality of arbitrary molecules can be quantified by means of the 'Hausdorff distance'. Increasing chirality of a dopant does not necessarily imply increasing helical twisting power, and molecular similarity between chiral guest and achiral host is essential for effective chirality transfer.     

Self-assembly of folic acid derivatives: induction of supramolecular chirality by hierarchical chiral structures

Hierarchical chiral structures made up of dendritic oligo(L- or D-glutamic acid) moieties of folic acid derivatives induce supramolecular chirality in the self-assembled columnar structures of the folic acids. These folic acids self-assemble through the intermolecular hydrogen bonds of the pterin rings to form disklike tetramers. In the neat states, the stacked tetramers form thermotropic hexagonal columnar phases over wide temperature ranges, including room temperature. Addition of alkali metal salts induces chirality in the columnar phases. In dilute solution states in a relatively polar solvent (chloroform), the folic acid derivatives form non-chiral, self-assembled structures. In the presence of sodium triflate, the folic acid forms chiral columnar assemblies through the oligo(L-glutamic acid) moiety, similar to those formed in the liquid-crystalline (LC) states. The enantiomer of the folic acid induces columnar assemblies with reversed helicity. In the case of the diastereomer, no induced helicity is observed. Application of an apolar solvent (dodecane) drives the folic acid derivatives to form chiral assemblies in the absence of ions. In this case, lipophilic interactions promote nanophase segregation, which enhances the formation of chiral columns. Interestingly, the chiral supramolecular structure of the diastereomer induces the most intense circular dichroism. In both cases, the molecular chirality in the oligo(glutamate) moieties yields supramolecular chirality of the folic acids that self-assemble through cooperative molecular interactions.     

Local symmetry and chirality of molecular faces

The concept of local symmetry has been applied to faces of planar sites such as carbon–carbon double bonds and aromatic rings with the principal results being as follows. The two faces of a planar site must have the same local symmetry group. This local symmetry group is limited to the polar point groups. For cyclic compounds, directed cycles must have chirotopic faces although the reverse is not necessarily true: chirotopic faces are possible for both directed and undirected cycles. A number of examples are provided to illustrate these results. Received: 30 June 1999 / Accepted: 4 October 1999 / Published online: 19 April 2000     

Self-assembly of amphiphiles with terthiophene and tripeptide segments into helical nanostructures

We previously reported a class of tripeptide amphiphiles known as peptide lipids that self-assemble into one-dimensional nanostructures with superhelical twisting. The pitch of this supramolecular twisting is controlled directly through sterics in the molecular structure of hydrophobic segments. In this work we study the supramolecular behavior of these nanoscale helices by substituting with a terthiophene conjugated segment of potential electronic interest and also through variations in the stereochemistry of the tripeptide. This terthiophene peptide lipid was shown to self-assemble into one-dimensional helical nanofibers with a regular diameter of 9±1 nm and helical pitch of 65±6 nm, and also found to form hierarchical double- and triple-stranded helices, which could be associated with terthiophene J-aggregate interactions among fibers. For stereochemical effects, we compared four diastereomers in the tripeptide sequence using l-glutamic acid and l- and d-alanine residues to probe their ability to control supramolecular organization. Interestingly, we found by atomic force microscopy that the LLD diastereomers formed cylindrical nanofibers without any twisting, whereas LDD diastereomeric segments self-assembled into helical nanofibers with a pitch of 40±6 nm. LDL diastereomeric segments formed, on the other hand, aggregates without any regular shape. We propose that these profound effects of chirality with amino acid sequence are related to changes in the β-sheet sub-structure within the nanofibers.     

Self-assembly and molecular dynamics of oligoindenofluorenes.

The optical properties, self-assembly mechanism, thermal properties, and the associated molecular dynamics of a series of stable blue-emitting oligoindenofluorenes up to the polymer were investigated by photoluminescence, wide-angle X-ray scattering, polarizing optical microscopy, differential scanning calorimetry, and dielectric spectroscopy. Oligomers of indenofluorenes possess high structural order and form smectic mesophases, as opposed to the nematic mesophases formed in the corresponding oligofluorenes. The alpha process associated with the liquid-to-glass transition displays an asymmetric broadening of relaxation times (non-Debye process), with a non-Arrhenius temperature dependence of relaxation times and a strong molecular weight dependence. The low dielectric loss and low ionic conductivity revealed the absence of ketone structural defects, which is consistent with the observed stable blue emission.     

Self-assembly of tris(phenylisoxazolyl)benzene and its asymmetric induction of supramolecular chirality

Tris(phenylisoxazolyl)benzene stacks in a columnar fashion to form helical fibers that act as an organogelator, and the supramolecular chirality is asymmetrically induced in the presence of a tiny amount of a chiral source in solution.     

Chelation-controlled molecular morphology: aminal to imine rearrangements

Reactions between the tripodal hydroxytriamine, 2,2-bis(aminomethyl)-3-aminopropan-1-ol, "hytame", and the isomeric pyridine aldehydes generate in all cases the tris(aminal) species based on a 1,3,5-triaza-adamantane skeleton. In all cases also, the product from water under basic conditions consists of an approximately 1:9 mixture of the triequatorial and monoaxial-diequatorial isomers. While all these tripyridyltriaza-adamantanes appear capable of acting as Lewis bases, in particular cases metal ion binding leads to a radical structural rearrangement. These cases involve the pyridine-2-aldehyde derivatives only and certain transition metal ions (notably Fe(II)), and result in the conversion of the tris(aminal) into its isomeric tris(imine) form. This is apparently favoured because it can act as a hexadentate ligand towards a single metal ion, although kinetic influences are clearly important in this chemistry because template reactions of the triamine, pyridine-2-aldehyde and several metal ions give much better yields of the tris(imine) complex than do analogous rearrangement reactions. For the low-spin, kinetically inert Fe(II) complex of the tris(imine), its formation is apparently so favourable that it is generated via aldehyde unit exchange when the aza-adamantanes derived from pyridine-3- and -4-aldehyde are heated with a mixture of Fe(II) and pyridine-2-aldehyde. When the kinetically labile Zn(II) complex is treated with EDTA, the metal ion is extracted but the released ligand does not undergo valence tautomerisation to what would be expected to be the triaxial isomer of the tripyridyltriaza-adamantane but instead rapidly undergoes partial hydrolysis before slowly forming the mixture of triequatorial and monoaxial-diequatorial isomers.     

Self-assembly of helical supramolecular channels from chiral aminopyrimidine hydrogen bonding motifs in the solid state

The H-bond mediated self-assembly of the chiral C₂-symmetric bis-(2-amino-4-chloro-pyrimidines) 3 and 4 allows for the molecular recognition directed generation of helical superstructures. In the former case, unoccupied channel structures defined by the cylindrical interior of the derived supramolecular helix result, as revealed by X-ray crystallographic analysis using a synchrotron source. Upon crystallization, racemic 3 spontaneously resolves to form homochiral crystals exhibiting a helical packing motif identical to that determined for optically pure 3. The data provide insight into the interplay of the different structural and interactional features of the molecular components to the generation of the channel structure and suggest design strategies toward porous organic molecular solids of variable size.