Electron-induced switching of the supramolecular chirality of optically active polythiophene aggregates

A chiral regioregular polythiophene (PT), poly[3-[4-((R)-4-ethyl-2-oxazolin-2-yl)phenyl]thiophene] (poly-1), forms chiral aggregates which exhibit a unique induced circular dichroism (ICD) in the pi-pi transition region derived from the supramolecular chirality in the presence of various poor solvents or metal salts in chloroform. We report here that the chirality of such supramolecular aggregates can be switched ("on" and "off") through electron transfer. We have found that upon the addition of copper(II) trifluoromethanesulfonate [Cu(OTf)(2)] to the chiral aggregates of poly-1 in a chloroform-acetonitrile mixture, the ICD disappears because of the oxidative doping of the poly-1 main chain, while a further addition of amines such as triethylenetetramine (TETA) induces undoping of the poly-1 which results in the reappearance of the ICDs. Therefore, the supramolecular chirality of the poly-1 assemblies was reversibly controlled by the addition or removal of an electron from the poly-1 main chain. This may be the first example of a reversible supramolecular chirality switch on chiral PT aggregates. We investigated the mechanism of the chirality switch through the doping and undoping process on the polymer main chain by means of absorption and CD spectroscopies, ESR, cyclic voltammetry, X-ray diffraction, and AFM measurements.     

Main-chain optically active riboflavin polymer for asymmetric catalysis and its vapochromic behavior

A novel optically active polymer consisting of riboflavin units as the main chain (poly-1) was prepared from naturally occurring riboflavin (vitamin B(2)) in three steps. The riboflavin residues of poly-1 were converted to 5-ethylriboflavinium cations (giving poly-2), which could be reversibly transformed into the corresponding 4a-hydroxyriboflavins (giving poly-2OH) through hydroxylation/dehydroxylation reactions. This reversible structural change was accompanied by a visible color change along with significant changes in the absorption and circular dichroism (CD) spectra. The nuclear Overhauser effect spectroscopy (NOESY) and CD spectra of poly-2 revealed a supramolecularly twisted helical structure with excess one-handedness through face-to-face stacking of the intermolecular riboflavinium units, as evidenced by the apparent NOE correlations between the interstrand riboflavin units and intense Cotton effects induced in the flavinium chromophore regions. The hydroxylation of poly-2 at the 4a-position proceeded in a diastereoselective fashion via chirality transfer from the induced supramolecular helical chirality assisted by the ribityl pendants, resulting in a 83:17 diastereomeric mixture of poly-2OH. The diastereoselectivity of poly-2 was remarkably higher than that of the corresponding monomeric model (64.5:35.5), indicating amplification of the chirality resulting from the supramolecular chirality induced in the stacked poly-2 backbones. The optically active poly-2 efficiently catalyzed the asymmetric organocatalytic oxidation of sulfides with hydrogen peroxide, yielding optically active sulfoxides with up to 60% enantiomeric excess (ee), whose enantioselectivity was higher than that catalyzed by the monomeric counterpart (30% ee). In addition, upon exposure to primary and secondary amines, poly-2 exhibited unique high-speed vapochromic behavior arising from the formation of 4a-amine adducts in the film.     

Synthesis and property of helical poly(phenylacetylene)s bearing chiral ruthenium complexes and real space imaging of meso- and nanoscopic structures by atomic force microscopy

Novel sets of helical poly(phenylacetylene)s bearing a chiral ruthenium (Ru) complex with opposite chirality (Δ and Λ forms) as a bulky pendant (poly- 1 and poly- 2 ) were synthesized through the polymerization of the corresponding optically pure phenylacetylenes with a rhodium catalyst, and their structures in solution and morphology on solid substrates were investigated with NMR, ultraviolet–visible, and circular dichroism (CD) spectroscopies and with atomic force microscopy (AFM), respectively. The obtained cis–transoidal polymers (poly- 1 and poly- 2 ) showed characteristic Cotton effects in the region of metal-to-ligand charge transfer of the chiral Ru pendants. Poly- 1 and poly- 2 were thought to have a predominantly one-handed helical conformation induced by the chiral pendants. However, the apparent Cotton effects derived from the helically twisted π-conjugated polymer backbone could not be observed, probably because of the strong chiral chromophoric pendants. However, in the AFM images, the helical polymers adsorbed on mica could be easily discerned as isolated strands, and the visualization and discrimination of the right- and left-handed helical structures of the chiral polymers were achieved by high-resolution AFM imaging. On the basis of the AFM observations together with the CD measurements and computational calculation results, possible structures of poly- 1 and poly- 2 were examined. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4621–4640, 2004     

Synthesis and optical properties of (a)chiral terpyridine-ruthenium complexes

Chiral terpyridine ligands have been synthesized and characterized. By applying Ru(III)/Ru(II) chemistry, symmetrical as well as asymmetrical bis-terpyridine ruthenium(II) complexes were obtained. These materials were fully characterized and their optical properties investigated. While the chiral metal complexes revealed no Cotton effect in good solvents such as chloroform, CD-measurements in dodecane showed an effect in both ligand and MLCT regions, suggesting chirality transfer from the lateral alkyl chains to the complex core. This behavior points to the formation of supramolecular aggregates in dodecane. Furthermore, the analogous achiral ligand and its corresponding ruthenium(II) complexes were prepared.     

Solvent-induced switching of the macromolecular helicity of poly[(4-carboxyphenyl)acetylene] induced by a single chiral amino alcohol

Cis–transoidal poly[(4-carboxyphenyl)acetylene] (poly- 1 ) complexed with optically active amines and amino alcohols showed an induced circular dichroism (ICD) in the ultraviolet–visible region because of a predominantly one-handed helix formation in water and in dimethyl sulfoxide (DMSO). The Cotton effect signs of the poly- 1 /chiral amino alcohol complexes were inverted in the presence of water, whereas the ICD pattern of the poly- 1 /chiral amine complexes showed no change, regardless of the water content. These results demonstrated that the helix sense of poly- 1 induced by optically active amino alcohols through noncovalent acid–base interactions could be switched by changes in the solvent ratio in the DMSO–water mixtures. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3625–3631, 2003     

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.     

Mechanism of helix induction on a stereoregular Poly((4-carboxyphenyl)acetylene) with chiral amines and memory of the macromolecular helicity assisted by interaction with achiral amines

Cis-transoidal poly((4-carboxyphenyl)acetylene) (poly-1) is an optically inactive polymer but forms an induced one-handed helical structure upon complexation with optically active amines such as (R)-(1-(1-naphthyl)ethyl)amine ((R)-2) in DMSO. The complexes show a characteristic induced circular dichroism (ICD) in the UV-visible region of the polymer backbone. Moreover, the macromolecular helicity of poly-1 induced by (R)-2 can be "memorized" even after complete replacement of (R)-2 by various achiral amines. We now report fully detailed studies on the mechanism of the helicity induction and memory of the helical chirality of poly-1 by means of UV-visible, CD, and infrared spectroscopies. We have found that a one-handed helix is cooperatively induced on poly-1 upon the ion pair formation of the carboxy groups of poly-1 with optically active amines and that the bulkiness of the chiral amines plays a crucial role for inducing an excess of a single-handed helix. On the other hand, the free ion formation was found to be essential for the macromolecular helicity memory of poly-1 after the replacement of the chiral amine by achiral amines, since the intramolecular electrostatic repulsion between the neighboring carboxylate ions of poly-1 significantly contributes to reduce the atropisomerization process of poly-1. On the basis of the mechanism of helicity induction and the memory of the helical chirality drawn from the present studies, we succeeded in creating an almost perfect memory of the induced macromolecular helicity of poly-1 with (R)-2 by using 2-aminoethanol as an achiral chaperoning molecule to assist in maintaining the memory of helical chirality.     

Supramolecular helical assembly of an achiral cyanine dye in an induced helical amphiphilic poly(phenylacetylene) interior in water

A water-soluble amphiphilic poly(phenylacetylene) bearing the bulky aza-18-crown-6-ether pendants forms a one-handed helix induced by l- or d-amino acids and chiral amino alcohols through specific host-guest interactions in water. We now report that such an induced helical poly(phenylacetylene) with a controlled helix sense can selectively trap an achiral benzoxazole cyanine dye among various structurally similar cyanine dyes within its hydrophobic helical cavity inside the polymer in acidic water, resulting in the formation of supramolecular helical aggregates, which exhibit an induced circular dichroism (ICD) in the cyanine dye chromophore region. The supramolecular chirality induced in the cyanine aggregates could be further memorized when the template helical polymer lost its optical activity and further inverted into the opposite helicity. Thereafter, thermal racemization of the helical aggregates slowly took place.     

Studies on the interaction of achiral cationic pseudoisocyanine with chiral metal complexes

The effect of chiral metal complexes ([Co(en)(3)]I(3)·H(2)O, cis-[CoBr(NH(3))(en)(2)]Br(2), K[Co(edta)]·2H(2)O and [Ru(phen)(3)](PF(6))(2)) on the polymer-bound J-aggregates in aqueous mixtures of pesudoisocyanine (PIC) iodine and poly(acrylic acid, sodium)(PAAS) have been studied by UV-vis absorption, circular dichroism (CD) and fluorescence spectra. At low concentration, the PIC monomers could self-assemble to form supermolecules by binding to each of the COO(-) groups on the polymer chains through electrostatic interactions. After the addition of chiral metal complexes to the formed PIC-PAAS J-aggregates, we found that only the chiral multiple π-conjugated phenanthroline metal complexes could transfer their metal-centered chiral information to the formed J-aggregates. The chiral J-aggregates showed a characteristic induced circular dichroism (ICD) in the visible region of J-band chromophore, and the ICD signals depend on the absolute configuration, concentration of the chiral multiple π-conjugated metal complexes, as well as temperature. More interestingly, the supramolecular chirality of the polymer supported PIC J-aggregates could be memorized even after the addition of an excess opposite chiral complex enantiomers. This is in sharp contrast to the behavior in the high concentrated NaCl induced PIC-J aggregates, in which the optical rotation of a mixture of two enantiomers varies linearly with their ratio.     

Mimicking [2,2]paracyclophane topology: molecular clips for the coordination-driven cofacial assembly of pi-conjugated systems

Self-assembly of CuI dimers, featuring a bridging phosphole ligand, and ditopic cyano-substituted chromophores affords nanosize supramolecular cationic rectangles. Due to the short Cu-Cu distance in the bimetallic clips, the coordinated chromophores are forced to participate in cofacial pi-pi interactions as evidenced by X-ray data. In addition, intermolecular pi-pi interactions in the solid state are observed leading to infinite columns of pi-stacked (para-phenylenevinylene)-based chromophores.     

Metal‐Ion‐Mediated Supramolecular Chirality of l‐Phenylalanine Based Hydrogels

For chiral hydrogels and related applications, one of the critical issues is how to control the chirality of supramolecular systems in an efficient way, including easy operation, efficient transfer of chirality, and so on. Herein, supramolecular chirality of l ‐phenylalanine based hydrogels can be effectively controlled by using a broad range of metal ions. The degree of twisting (twist pitch) and the diameter of the chiral nanostructures can also be efficiently regulated. These are ascribed to the synergic effect of hydrogen bonding and metal ion coordination. This study may develop a method to design a new class of electronically, optically, and biologically active materials.     

Intermolecular covalent pi-pi bonding interaction indicated by bond distances, energy bands, and magnetism in biphenalenyl biradicaloid molecular crystal

Density-functional theory (DFT) calculations were performed for energy band structure and geometry optimizations on the stepped pi-chain, the isolated molecule and (di)cations of the chain, and various related molecules of a neutral biphenalenyl biradicaloid (BPBR) organic semiconductor 2. The dependence of the geometries on crystal packing provides indirect evidence for the intermolecular covalent pi-pi bonding interaction through space between neighboring pi-stacked phenalenyl units along the chain. The two phenalenyl electrons on each molecule, occupying the singly occupied molecular orbitals (SOMOs), are participating in the intermolecular covalent pi-pi bonding making them partially localized on the phenalenyl units and less available for intramolecular delocalization. The band structure shows a relatively large bandwidth and small band gap indicative of good pi-pi overlap and delocalization between neighboring pi-stacked phenalenyl units. A new interpretation is presented for the magnetism of the stepped pi-chain of 2 using an alternating Heisenberg chain model, which is consistent with DFT total energy calculations for 2 and prevails against the previous interpretation using a Bleaney-Bowers dimer model. The obtained transfer integrals and the magnetic exchange parameters fit well into the framework of a Hubbard model. All presented analyses on molecular geometries, energy bands, and magnetism provide a coherent picture for 2 pointing toward an alternating chain with significant intermolecular through-space covalent pi-pi bonding interactions in the molecular crystal. Surprisingly, both the intermolecular transfer integrals and exchange parameters are larger than the intramolecular through-bond values indicating the effectiveness of the intermolecular overlap of the phenalenyl SOMO electrons.     

Synergic extraction of metals with α-acyl-d-camphor and optically active lewis bases

Solvent extraction of europium (III), zinc (II) cobalt (II) with α-acyl-d-camphor and optically active isomers of quinine and quinidine was studied in order to obtain information on chirality recognition based on adduct formation between a chiral metal chelate and optically active isomers. It was possible to differentiate clearly between the adduct formation of quinine and that of quinidine in the synergic extraction of cobalt and europium with 3-heptafluorobutyryl- d-camphor and cobalt with 3-trifluoroacetyl-d-camphor.     

Detection and amplification of a small enantiomeric imbalance in alpha-amino acids by a helical poly(phenylacetylene) with crown ether pendants

We have designed a novel stereoregular poly(phenylacetylene) bearing the bulky crown ether as the pendant (poly-1) for the amino acid binding site. The polymer forms a one-handed helix upon complexation with l-amino acid perchlorates, and the complexes exhibit an induced circular dichroism (ICD) with the same Cotton effect signs in the polymer backbone region through a significant cooperative interaction. Poly-1 is highly sensitive to the amino acid chirality and can detect an extremely small enantiomeric imbalance in alpha-amino acids (less than 0.005% enantiomeric excess of alanine, for example).     

Co-Assembly of Chiral Amines and a Four-Armed Cyano-Substituted Luminophore through Hydrogen Bonds for Potential Development of Smart Chiroptical Materials

Chirality transfer from chiral molecules to assemblies is of vital importance to the design of functional chiral materials. In this work, selective co-assembly behaviors between chiral molecules and an achiral luminophore, potentially driven by the intermolecular salt-bridge type hydrogen bonds are reported. Cyano-substituted tetrakis(arylthio)benzene carboxylic acid ( TA ) served as the luminophore and hydrogen bond donors, which underwent co-assembly with different chiral amines. It was found that structures of chiral amines affect the chirality transfer and the properties of co-assemblies due to effects on hydrogen bonds and stacking pattern. Only in specific co-assemblies, the chiroptical properties occurred at both ground state and excited states based on the emerged Cotton effects and circularly polarized luminescence (CPL) signals, revealing that the chirality was successfully transferred from molecular level to supramolecular level. In addition, accurate quantitative examination of chiral amines was realized by circular dichroism (CD) spectra. This work demonstrates the characteristic chirality response and transfer through co-assembly, providing a potential method to develop smart chiroptical materials.     

Isolable chiral aggregates of achiral π-conjugated carboxylic acids

The induced aggregation of achiral building blocks by a chiral species to form chiral aggregates with memorized chirality has been observed for a number of systems. However, chiral memory in isolated aggregates of achiral building blocks remains rare. One possible reason for this discrepancy could be that not much is understood in terms of designing these chiral aggregates. Herein, we report a strategy for creating such isolable chiral aggregates from achiral building blocks that retain chiral memory after the facile physical removal of the chiral templates. This strategy was used for the isolation of chiral homoaggregates of neutral achiral π-conjugated carboxylic acids in pure aqueous solution. Under what we have termed an "interaction-substitution" mechanism, we generated chiral homoaggregates of a variety of π-conjugated carboxylic acids by using carboxymethyl cellulose (CMC) as a mediator in acidic aqueous solutions. These aggregates were subsequently isolated from the CMC templates whilst retaining their memorized supramolecular chirality. Circular dichroism (CD) spectra of the aggregates formed in the acidic CMC solution exhibited bisignated exciton-coupled signals of various signs and intensities that were maintained in the isolated pure homoaggregates of the achiral π-conjugated carboxylic acids. The memory of the supramolecular chirality in the isolated aggregates was ascribed to the substitution of COOH/COOH hydrogen-bonding interaction between the carboxylic acid groups within the aggregates for the hydrogen-bonding interactions between the COOH groups of the building blocks and the chiral templates. We expect that this "interaction-substitution" procedure will open up a new route to isolable pure chiral aggregates from achiral species.     

Macromolecular helicity induction on a poly(phenylacetylene) with C2-symmetric chiral [60]fullerene-bisadducts

Three chiral N-methylfulleropyrrolidine bisadducts were prepared, isolated, and completely resolved into each enantiomer using a chiral HPLC column, which were then converted to the corresponding optically active, cationic C(60)-bisadducts to investigate if they could act as a macromolecular helicity inducer in a poly(phenylacetylene) bearing an anionic monoethyl phosphonate pendant (poly-1) in aqueous solution. Upon complexation with the chiral C(60)-bisadducts, only the trans-3 bisadduct exhibited the characteristic induced circular dichroism (ICD) in the UV-visible region of the polymer backbone in dimethyl sulfoxide-water mixtures due to the predominantly one-handed helix formation of poly-1, while the trans-2 and cis-3 bisadducts induced almost no apparent CD in the same region. These results indicate that the helicity induction on poly-1 is highly sensitive to the structure and geometry of the cationic C(60)-bisadducts with a different distance between the separated charges.     

Chiral induction from ligands to metal centres. A copper(II)–nitroxide complex

Copper(II) complexes of a racemic mixture of a chiral nitronyl-nitroxide are characterised. One, A, is a centro-symmetrical species where two enantiomers are coordinated to a metal centre. The second, B, is a 1D compound comprising bridging ligands through the oxyl and pyridyl donor sites. One observes that, although the crystals are racemic, within a chain, all ligands and metal centres have respectively the same chirality. Possibilities of obtaining optically active extended structures through chiral induction from nitroxide ligands to metal ions are discussed in relation with molecular spin transition species.     

Control of Metal Arrays Based on Heterometallics Masquerading in Heterochiral Aggregations of Chiral Clothespin‐Shaped Complexes

Heterometal arrays in molecular aggregations were obtained by the spontaneous and ultrasound‐induced gelation of organic liquids containing the chiral, clothespin‐shaped trans‐bis(salicylaldiminato) d₈ transition‐metal complexes 1 . Heterometallic mixtures of complexes 1 a (Pd) and 1 b (Pt) underwent strict heterochiral aggregation entirely due to the organic shell structure of the clothespin shape, with no effect of the metal cores. This phenomenon provides an unprecedented means of generating highly controlled heterometallic arrangements such as alternating sequences [(+)‐Pd(?)‐Pt(+)‐Pd(?)‐Pt ??? ] as well as a variety of single metal‐enriched arrays (e.g., [(+)‐Pt(?)‐Pd(+)‐Pd(?)‐Pd(+)‐Pd(?)‐Pd ??? ] and [(+)‐Pd(?)‐Pt(+)‐Pt(?)‐Pt(+)‐Pt(?)‐Pt ??? ]) upon the introduction of an optically active masquerading unit with a different metal core in the heterochiral single‐metal sequence. The present method can be applied to form various new aggregates with optically active Pd and Pt units, to allow 1) tuning of the gelation ultrasound sensitivity based on the different hearing abilities of the metal units; 2) aggregation‐induced chirality transfer between heterometallic species; and 3) aggregation‐induced chirality enhancement. A mechanistic rationale is proposed for these molecular aggregations based on the molecular structures of the units and the morphologies of the aggregates.     

D-，L-苯丙氨酸诱导非手性菁染料的手性组装

超分子手性与分子自组装是生命体中非常重要和有趣的现象．报道了D,L-苯丙氨酸等氨基酸在氯化钠溶液中通过非共价键相互作用诱导非手性菁染料（Pseudoisocyanine,PIC）J-聚集体超分子手性的形成．实验结果表明,诱导的手性菁染料PIC聚集体发色团在π-π^＊跃迁区域产生了特征的镜像圆二色性,其圆二色信号和强度强烈地依赖于氨基酸的绝对构型、浓度、侧链基团和溶液温度．原子力显微镜照片清楚地表明,^聚集体由相互交联的纳米纤维组成,诱导的圆二色性可能来源于纤维状聚集体的宏观螺旋排列．