Quantitative chirality/enantioselectivity relations in large random supramolecular structures

We study the relationship between shape and enantioselectivity, employing quantitative geometric chirality measurements. The model we use comprises of the boundary surfaces of two-dimensional (2D) chiral, large, random selectors (diffusion limited aggregates), interacting with homologous series of small 2D-chiral S-shaped probes (the selectands). We show how the enantioselectivity of the selectors depends on the chirality of the selectands and report the following findings: I) The enantioselectivity of a chiral selector can switch preference from the "right" to the "left" enantiomer within a homologous series of selectands. II) At this switch point the chiral selector is functionally achiral. III) Within a homologous series of chiral selectands, there is a "resonance of recognition", namely, the classical key-lock concept is replaced by a picture of various degrees of recognition. IV) The degree of enantioselectivity and the switch in handedness preference are the outcome of a complex interplay between the details of the specific geometry of the selector and the selectand, and the global shape parameter of chirality measure. V) It is shown that isochiral selectands, namely selectands of the same chirality value, may be recognized differently by a chiral selector. VI) It is proposed that a more realistic way to treat the issue of minimal points needed for chiral interaction is resolution based. VII) It is shown how to attach handedness to purely random objects.     

Supramolecular chirality of sodium dodecylsulfate

The aggregative chirality of sodium dodecylsulfate has been discovered as a new kind of supra-molecular chirality.     

Supramolecular chirality of self-assembled systems in solution

Self-assembly plays an important role in the formation of many (chiral) biological structures, such as DNA, alpha-helices or beta-sheets of proteins. This process, which is the main tool of Supramolecular Chemistry (i.e. the chemistry of the molecular assemblies and of the intermolecular bonds), starts to play a significant role in nanotechnology for the construction of functional synthetic structures of nanometer size. The control of chirality in synthetic self-assembled systems is very important for applications of these systems e.g. in molecular recognition or mimicking of the catalytic activity of enzymes. This tutorial review deals with the most representative contributions in the field of supramolecular chirality. Specifically, the discussion centers on several examples that represent the control over chirality for self-assembled systems in solution.     

Amplification of chirality in dynamic supramolecular aggregates

The quest to understand the origin of chirality in biological systems has evoked an intense search for nonlinear effects in catalysis and pathways to amplify slight enantiomeric excesses in racemates to give optically pure molecules. The amplification of chirality in polymeric systems as a result of cooperative processes has been intensely investigated. Ten years ago, this effect was shown for the first time in noncovalent dynamic supramolecular systems. Since then, it has become clear that a subtle interplay of noncovalent interactions such as hydrogen-bonding, pi-pi stacking, and hydrophobic interactions is also sufficient to observe amplification of chirality in small molecules. Here we summarize the results obtained over the past decade and the general guidelines we can deduce from them. Predicting amplification of chirality is still impossible, but it appears to be a balance between different types of interactions, the formation of an intrinsically chiral object, and cooperative aggregation processes.     

Supramolecular interactions between fullerenes and porphyrins

Perdew-Burke-Ernzerhof density functional theory calculations have been carried out to investigate the host-guest interactions for several fullerene-porphyrin supramolecular complexes. The nature of the interactions has been discussed. The fullerene-porphyrin interaction energies for the complexes studied are found to be in the range from -16 to -18 kcal/mol.     

NMR evidence for planar chirality in natural porphyrins

The ¹H NMR spectra of several six-coordinate cobalt(III) porphyrins of general formula L₂Co(DPDME)CI, where DPDME=deuteroporphyrin dimethyl ester and L is an optically active ligand, have been measured at 250 or 400 MHz. In a few cases, two signals of equal intensity are observed for the L ligand protons. This magnetic non-equivalence, which never exceeds 0.04 ppm, is thought to arise from the planar chirality of the porphyrin ring, which makes a proton in the ligand L above the porphyrin ring and the corresponding proton below this ring diastereotopic.     

Supramolecular chirality formation of bisazobenzene-substituted polydiacetylene LB films

In order to investigate the exact effect of stereoregular packing of head group in the side chain on the helical structure formation of polydiacetylene backbone, the larger size of bisazobenzene-substituted diacetylene monomer, 4-(4-nitrophenylazo) azobenzene-10, 12-pentacosadiynoate (BNADA) was synthesized successfully. Owing to overcrowded packing of bisazobenzene chromophores, the BNADA Langmuir-Blodgett (LB) films showed macroscopic supramolecular chirality, although BNADA molecules were achiral. Under circularly polarized UV light (CPUL) irradiation, supramolecular helix of bisazobenzene chromophores always maintained, due to the large size and lower photo-isomerization rate of bisazeobenzene chromophores. While for polydiacetylene backbone, the helical direction of the polymer chain should be decided by the competition of the effect of stereoregular packing of bisazobenzene chromophores and the interaction between the CPUL and the diacetylene dimer.     

Supramolecular stereocontrol of octahedral metal-centered chirality. Ligand modulation

By associating chiral labile [FeL3](2+) complexes with TRISPHAT anions, a stereocontrol of the metal-centered chirality is feasible; the sense of the stereoselective induction and its magnitude strongly depends upon the structure of the diimine ligands (L: bpy, phen).     

Solvent-tunable inversion of chirality transfer from carbon to copper

A change of solvent causes an inversion of the stereochemistry at copper of the chiral Cu(I) complex described herein.     

Supramolecular ensembles based on the donor-acceptor interactions of porphyrins

Dimeric and trimeric supramolecular structures were synthesized on the basis of donor-acceptor interactions of manganese(III) and tin(IV) complexes of 5,15-diphenyl-3,7,13,17-tetramethyl-2,8,12,18-tetrabutylporphyrin with 5-(4′-oxyphenyl)-15-phenyl-3,7,13,17-tetramethyl-2,8,12,18-tetrabutylporphyrin. The structure of the compounds was studied by NMR spectroscopy and TLC.     

Tetrakis-heteroleptic complexation at porphyrins: a convenient route to diversely functionalized aggregates

[structure: see text] Using a novel tetraphenanthrolinated porphyrin building block and heteroleptic bisphenanthroline complexation, copper-instructed multicomponent assemblies exhibiting distinct electrochemical and photoactive behavior were accessed.     

Dynamic chirality, chirality transfer and aggregation behaviour of dithienylethene switches

The synthesis and characterisation of a series of chiral and achiral low molecular weight organogelators (LMWGs) based on bis-amide substituted dithienylethene photochromic switches is reported. The LMWGs gelate a range of solvents depending on the specific functionalisation of the hydrogen bonding amide groups. In mixtures of chiral and achiral LMWGs the stereochemical outcome of the chiral aggregation is determined by the chiral LMWG molecules in most cases. However, for the first time we demonstrate that the stereochemical outcome of the aggregation can be influenced by the achiral LWMG molecules in some cases. Furthermore specific π-π (and/or van der Waals) interactions of chiral LMWGs 1-3o with the solvent allow the solvent to influence the control of chirality of aggregation. This influence of the solvent has a dramatic effect on whether four- or two-gel states are available.     

Role of the hydrophobic effect in the transfer of chirality from molecules to complex systems: from chiral surfactants to porphyrin/surfactant aggregates

The interaction between the achiral sulfonated porphyrin 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin, H 2TPPS 4 (4-), and two chiral cationic surfactants has been studied by optical absorption, fluorescence, and circular dichroism (CD) spectroscopies. At surfactant concentrations above the critical micellar concentration (cmc) the porphyrin is included in the micellar aggregates, but it is CD silent. Below the cmc at a definite porphyrin/surfactant stoichiometry the formation of heteroaggregates with transfer of chirality to the porphyrin chromophore occurs. The preferred surfactant/porphyrin stoichiometry is 3:1, which suggests a structure driven by electrostatic and hydrophobic interactions between porphyrin and surfactant and dipolar and ionic interactions with the water solution. At surfactant concentrations above the cmc, depending on the protocol of preparation of the samples, the formation of the two kinds of aggregates can be observed, reversible for the simple surfactant micelles incorporating the porphyrin, but irreversible for the heteroaggregates.     

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.     

Supramolecular chirality and reversible chiroptical switching in new chiral liquid-crystal azopolymers

The synthesis of chiral liquid-crystalline polymers of well-controlled structure (linear and three-armed star-shaped) with distinct average chain lengths and low polydispersity was achieved by atom transfer radical polymerisation (ATRP) of a new optically active monomer (S)-4-[6-(2-methacryloyloxypropanoyloxy)hexyloxy)]-4'-ethoxyazobenzene [(S)-ML6A], containing the L-lactic residue of one absolute configuration in the side-chain. All the obtained polymeric samples, characterised by differential scanning calorimetry (DSC), X-ray diffraction (XRD) and polarised optical microscopy (POM), exhibit a smectic A(1/2) (fully interdigitated) liquid-crystalline phase and high cleaning points, with transition temperatures dependent on the average polymerisation degree and the macromolecular structure. The chirality originated at the molecular level by the asymmetric functionality of the L-lactic acid residue provides the polymers, in the smectic phase, of highly homogeneous conformations with a prevailing chirality related to the presence of H-aggregates having conformational dissymmetry of one prevailing screw-sense. By irradiating with circularly polarised light (CPL), it is possible to photomodulate the chiroptical properties of these intrinsically chiral polymeric thin films. Upon irradiation with left-handed CPL (l-CPL), the circular dichroism (CD) spectra of the films show enhancement of ellipticity and a net inversion of sign. The effect is reversible and the mirror image of the CD spectrum can be restored by pumping with right-handed CPL radiation (r-CPL). The results show the ability of l-CPL to invert the supramolecular chirality of the materials and demonstrate the essential role of azoaromatic aggregates.