Large polarization of push–pull “Cruciforms” via coordination with lanthanide ions

“Bent” or “cruciform” shaped conjugated push–pull compounds exhibit striking differences in their supramolecular recognition of lanthanide cations, with in an outstanding supramolecular polarization.

Aggregation-Induced Circularly Polarized Luminescence: Chiral Organic Materials for Emerging Optical Technologies

Chirality is becoming increasingly important in the design of organic materials with functional properties, when bulk anisotropy is needed. In the past decades, a plethora of chiral organic materials have been studied and developed. Nanostructures have brought substantial advancement to the realization of organic-molecule-based devices, and the possibilities for solid-state light emission are very promising in view of potential applications. Scientific approaches to the realization of chiral emissive materials are indeed growing exponentially. The chiral nanostructures discussed are related both to the way in which luminescence is generated and the way in which it is detected. As to the former, the focus will be on organic chromophores with aggregation-induced emission properties, so that emission is present, or at least largely amplified, when the molecules are in the aggregated state. As to the latter, the focus will be on the ability and a quantitative comparison of organic nanostructures capable of circularly polarized emission.

Binaphthyl-Based Macrocycles as Optical Sensors for Aromatic Diphenols

The synthesis of several rigid, homochiral organic macrocycles possessing, respectively, average molecular D₂ and D₃ symmetries, is described. They have been obtained from aromatic dicarboxylic acids, in combination with an axially-chiral, suitable dibenzylic alcohol, derived from 1,1'-binaphthyl-2,2'-diol (BINOL) using one-pot esterification reactions in good isolated yields. NMR and circular dichroism (CD) spectroscopies detect the structural and shape variability in the scaffolds, reflected both in terms of the changes in chemical shifts and the shape of selected proton resonances, and in terms of the variation of the CD signature related to the dihedral angle defined by the binaphthyl units embedded in the rigid cyclic skeleton. The D₂ cyclic adducts are able to form stable complexes with aromatic diphenols, with binding strengths that are dependent on small variations in the spacing units, and therefore on the shapes of the internal cavities of the cyclic structures.

Structural effects of ditopic azoprobe–cyclodextrin complexes on the selectivity of guest-induced supramolecular chirality

Guest-induced supramolecular chirality: the guest ion selectivity was dramatically altered by a slight change in the spacer length of (15C5-Azo-n-dpa)₂–γ-CyD complexes in water.

Chiral Nanotubes

Organic nanotubes, as assembled nanospaces, in which to carry out host-guest chemistry, reversible binding of smaller species for transport, sensing, storage or chemical transformation purposes, are currently attracting substantial interest, both as biological ion channel mimics, or for addressing tailored material properties. Nature's materials and machinery are universally asymmetric, and, for chemical entities, controlled asymmetry comes from chirality. Together with carbon nanotubes, conformationally stable molecular building blocks and macrocycles have been used for the realization of organic nanotubes, by means of their assembly in the third dimension. In both cases, chiral properties have started to be fully exploited to date. In this paper, we review recent exciting developments in the synthesis and assembly of chiral nanotubes, and of their functional properties. This review will include examples of either molecule-based or macrocycle-based systems, and will try and rationalize the supramolecular interactions at play for the three-dimensional (3D) assembly of the nanoscale architectures.

Chiral nanostructuring of multivalent macrocycles in solution and on surfaces

We describe the design and synthesis of a novel functionality-rich, homochiral macrocycle, possessing the overall molecular D2 symmetry, in which multivalency is introduced into the covalent framework by means of four suitably positioned pyridine moieties. The macrocycle synthesis is carried out with functionalized, enantiopure 1,1'-binaphthyl synthons as the source of chirality by means of a room temperature esterification reaction as the cyclization procedure. Upon addition of Pd(2+), coordination of the pyridine moieties occurs both intra and intermolecularly, to afford chiral ordered mono and dimeric macrocycles or multimeric aggregates depending on the solvents and conditions used. The metal binding event takes place in combination with a significant macrocyclic conformational rearrangement detected by circular dichroism spectroscopy. When in combination with a third component (C60), the macrocycle-Pd(2+) hybrid undergoes surface-confined nanostructuring into chiral nanofibres.

Guest-induced supramolecular chirality in a ditopic azoprobe-cyclodextrin complex in water

We report a novel supramolecular chirality induced by the twisted structural change of two ditopic azoprobes (15C5-Azo-dpa) inside the chiral cavity of γ-cyclodextrin (γ-CyD) due to multi-point recognition of guest ions by 15C5-Azo-dpa molecules in water.

Synthesis of Binaphthyl-Based Push-Pull Chromophores with Supramolecularly Polarizable Acceptor Ends

We report on the design and synthesis of new enantiopure binaphthyl derivatives in which electron-donating and electron-withdrawing substituents are placed in direct conjugation, to create push-pull dyes potentially active for NLO applications. The dyes, unprecedentedly, extend theirπ-bridge from the 3,3′ positions of the binaphthyl units and incorporate as acceptors 1,3-dicarbonyl and tetrafluorobenzene units, useful for further supramolecular polarization of the chiral dyes.

Synthesis, chiroptical and SHG properties of polarizable push–pull dyes built on π-extended binaphthyls

Novel binaphthyl push–pull dyes extend their π-bridge and incorporate as acceptors pyridine units, useful for further supramolecular polarization.

Nanostructuring with chirality: binaphthyl-based synthons for the production of functional oriented nanomaterials

Chirality is a powerful tool for the generation of order, directionality, and, as such, of function, in assembled nanoscale chemical devices. Axially chiral binaphthyls have been widely used in organic synthesis; the stability of the enantiomers enables their use as robust chirality inducers and catalysts in asymmetric reactions, and they are nowadays industrially applied in a variety of organic transformations. Applications of these compounds in the field of nanosciences are more recent, and not yet fully explored. The integration of such a robust class of chiral compounds, capable of efficient transfer of stereochemical information, into functional aggregates and nanoarchitectures is of great current interest. We will discuss preeminent examples of applications of these synthons in several fields of nanoscience, such as reticular chemistry, non-linear optical materials and imaging, and liquid crystals.

Homochiral BINOL-based macrocycles with π-electron-rich, electron-withdrawing or extended spacing units as receptors for C60

The “one-pot” synthesis of several homochiral macrocycles has been achieved by using π-electron-rich, electron-deficient or extended aromatic dicarboxylic acids in combination with an axially-chiral dibenzylic alcohol, derived from enantiomerically-pure BINOL. Two series of cyclic adducts with average molecular D₂ and D₃ molecular symmetries, respectively, have been isolated in pure forms. Their yields and selectivities deviate substantially from statistical distributions. NMR and CD spectroscopic methods are efficient and functional in order to highlight the variability of shapes of the covalent macrocyclic frameworks. The larger D₃ cyclic adducts exhibit recognition properties towards C₆₀ in toluene solutions (up to log K_a = 3.2) with variable stoichiometries and variable intensities of the charge-tranfer band upon complexation.

Stereospecific generation of homochiral helices in coordination polymers built from enantiopure binaphthyl-based ligands

The novel enantiopure spacer 2,2′-dimethoxy-1,1′-binaphthyl-3,3′-bis(4-pyridyl-amido) has been designed to prepare helical coordination polymers here investigated by means of experimental and theoretical data.

Crystal structure analyses facilitate understanding of synthesis protocols in the preparation of 6,6′-dibromo-substituted BINOL compounds

A combination of crystallographic and spectroscopic techniques has been used in order to address thorough purification protocols for a series of atropisomeric 1,1′-binaphthalene-2,2′-diol (BINOL) derivatives to be used as building blocks for chiral nanoscale constructs.

The click reaction as an efficient tool for the construction of macrocyclic structures

The Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC, known as the click reaction) is an established tool used for the construction of complex molecular architectures. Given its efficiency it has been widely applied for bioconjugation, polymer and dendrimer synthesis. More recently, this reaction has been utilized for the efficient formation of rigid or shape-persistent, preorganized macrocyclic species. This strategy also allows the installment of useful functionalities, in the form of polar and function-rich 1,2,3-triazole moieties, directly embedded in the macrocyclic structures. This review analyzes the state of the art in this context, and provides some elements of perspective for future applications.

Anthranilamides as bioinspired molecular electrets: experimental evidence for a permanent ground-state electric dipole moment

As electrostatic equivalents of magnets, organic electrets offer unparalleled properties for impacting energy conversion and electronic applications. While biological systems have evolved to efficiently utilize protein α-helices as molecular electrets, the synthetic counterparts of these conjugates still remain largely unexplored. This paper describes a study of the electronic properties of anthranilamide oligomers, which proved to be electrets based on their intrinsic dipole moments as evident from their spectral and dielectric properties. NMR studies provided the means for estimating the direction of the intrinsic electric dipoles of these conjugates. This study sets the foundation for the development of a class of organic materials that are de novo designed from biomolecular motifs and possess unexplored electronic properties.

Synthesis and anion recognition properties of shape-persistent binaphthyl-containing chiral macrocyclic amides

We report on the synthesis and characterization of novel shape-persistent, optically active arylamide macrocycles, which can be obtained using a one-pot methodology. Resolved, axially chiral binol scaffolds, which incorporate either methoxy or acetoxy functionalities in the 2,2' positions and carboxylic functionalities in the external 3,3' positions, were used as the source of chirality. Two of these binaphthyls are joined through amidation reactions using rigid diaryl amines of differing shapes, to give homochiral tetraamidic macrocycles. The recognition properties of these supramolecular receptors have been analyzed, and the results indicate a modulation of binding affinities towards dicarboxylate anions, with a drastic change of binding mode depending on the steric and electronic features of the functional groups in the 2,2' positions.