One-Handed Covalent Helical Ladder Polymers: The Dawn of a Tailorable Class of Chiral Functional Materials.

In the last decades, chemists have developed methods to synthesize helical molecular architectures using a combination of covalent and non-covalent interactions. Very recently, the new class of completely covalent, one-handed helical ladder polymers has vigorously emerged. Such polymers can be rationally and programmably obtained through an approach guided by the principles of chirality-assisted-synthesis (CAS) and making use synergically of two disciplines that have so far rarely interacted: non-planar chiral π-conjugated synthons and ladder polymer chemistry. The precise programmability of the 3D structure and new mechanical and chiroptical properties will lead to potential applications in areas such as enantiorecognition, catalysis, spintronics and chiral-related optoelectronics. This minireview examines the emerging field of one-handed helical ladder polymers, analyzing their synthesis, applications, and limitations.

Triptycene-based diiron(II) mesocates: spin-crossover in solution.

Triptycene-based diiron(II) and dizinc(II) mesocates were obtained using a novel rigid ligand with two pyridylbenzimidazole chelating units fused into the triptycene scaffold. Studies on the diiron(II) assembly in solution showed that the complex undergoes thermal-induced one-step spin-crossover with T1/2 at 243 K (Evans method).

Homoconjugation and Tautomeric Isomerism in Triptycene-Fused Pyridylbenzimidazoles.

The facile, metal-free synthesis and characterization of three new series of triptycene-fused pyridylbenzimidazoles are reported; such compounds possess an imidazole moiety fused within the benzene rings of the trypticene and a pyridine ring installed at position 2 of the imidazole rings. The position of the nitrogen atom of the pyridyl moiety linked to position 2 of the fused benzimidazole scaffold is systematically changed from the ortho to para position. The number of substituted blades bearing the pyridyl-substituted fused benzimidazole scaffolds has been increased from one to three. Such a library of compounds allowed us to evaluate the enhancement of two main effects: tautomeric isomerism and homoconjugation. The characteristic dynamic equilibrium between different isomers induced by prototropic tautomerization was examined by 1H nuclear magnetic resonance spectroscopy. By comparison of the photophysical properties of the new compounds with those of classical planar pyridylbenzimidazoles, the presence of the homoconjugation effect between the different triptycene blades was demonstrated. Fine details of the electronic structure of the new derivatives were unraveled by a computational analysis. The novel compounds can be employed for the construction of intriguing self-assembled supramolecular architectures.

Enabling Stereochemical Communication and Stimuli-Responsive Chiroptical Properties in Biphenyl-Capped Cyclodextrins.

Chiroptical materials are gaining increasing interest due to their innovative character and their applications in optoelectronics and data encryption technologies. Fully harnessing the potential of building blocks from the "chiral pool", such as native cyclodextrins (CDs), as they often lack chromophores suitable for the construction of materials with significant chiroptical properties. Here, we present the synthesis and characterization of a two-level molecular stack consisting of a point-chiral element (CD) and an axially chiral element (biphenyl), capable of effectively translating the overall stereochemical information contained in CDs into stimuli-responsive chiroptical properties. α- and ß-permethylated CDs were efficiently capped with two different 2,2'-difunctionalized 1,1'-biphenyl units. In CD derivatives containing the rigid 2,2'-dihydroxy-1,1'-biphenyl cap, two intramolecular hydrogen bonds act synergistically as stereoselective actuators, enabling effective communication between the two levels and the transfer of nonchromophoric stereochemical information from the cyclic-oligosaccharide to the atropoisomeric cap. The chiroptical properties can be finely tuned by external stimuli such as temperature and solvent. The way chirality is transferred from the CD platform to the biphenyl cap was revealed thanks to crystallographic and computational analyses, together with electronic circular dichroism (ECD) studies.

Activation of Solid-State Emission and Photostability through Molecular Confinement: The Case of Triptycene-Fused Quinacridone Dyes.

We report the facile, metal-free convergent synthesis and the characterization of novel quinacridone dyes in which two triptycene units end-cap and sterically confine the quinacridone chromophore. A precise comparison of the confined dyes with their known homologues reveals that the reduction of π-π interactions in triptycene-fused quinacridone dyes compared to classical quinacridone results not only in an increase of solubility and processability but also in an enhancement of fluorescence quantum yield and photostability in the solid state.

Chiroptical sensing of perrhenate in aqueous media by a chiral organic cage.

A chiral cage is proposed as an effective chiroptical sensor for perrhenate (surrogate for 99TcO4-) in water, fruit juice and artificial urine media. The key mechanism for the chiroptical sensing resides in the change of dihedral angle of the binaphthyl unit and H-bonds with the guest, resulting in ample changes of the CD signal as a consequence of the binding event.

Triptycene derivatives as chiral probes for studying the molecular enantiorecognition on sub-2-µm particle cellulose tris(3,5-dimethylphenylcarbamate) chiral stationary phase.

Two chiral triptycene derivatives were analyzed on the Chiralpak IB-U column packed with cellulose tris(3,5-dimethylphenylcarbamate)-based sub-2-µm diameter particles. Under normal-phase conditions, sub-minute baseline enantioseparations were obtained. Differences in structural elements and chromatographic behavior of the investigated compounds were evaluated to identify the interactions that drive the chiral discrimination process. From the evaluation of the experimental chromatographic data, it was found that hydrogen bond formation is essential for the separation of enantiomers.

Chiral Triptycenes in Supramolecular and Materials Chemistry.

Triptycenes are an intriguing class of organic molecules with several unusual characteristics, such as a propeller-like shape, saddle-like cavities around a symmetrical scaffold, a rigid π-framework. They have been extensively studied and proposed as key synthons for a variety of applications in supramolecular chemistry and materials science. When decorated with an appropriate substitution pattern, triptycenes can be chiral, and, similarly to other popular chiral π-extended synthons, can express chirality robustly, efficiently, and with relevance to chiroptical spectroscopies. This minireview highlights and encompasses recent advances in the synthesis of chiral triptycenes and in their introduction as molecular scaffolds for the assembly of functional supramolecular materials.