Racemic Monomer-Based One-Handed Helical Polymer Recognizes Enantiomers through Auto-Evolution of Its Helical Handedness Excess

A racemic monomer-based optically inactive polyacetylene folds into a one-handed helix assisted by a nonracemic alcohol, which can separate various enantiomers as a chiral stationary phase in chromatography. The chiral-resolving power is virtually identical to that of the enantiopure monomer-based one-handed helical polyacetylene. Because of its unique static memory of the induced helicity, the original racemic polyacetylene expresses an auto-evolution of its helical handedness over time, and at the same time, chirality of the nonracemic alcohol is discriminated accompanied by successive enhancement of its optical purity enantioselectively adsorbed on the helical polyacetylene owing to the chiral filter effect as directly monitored by NMR, which contributes to further enhancing the helix-sense-excess of the helical polyacetylene.

Spin Filtering Along Chiral Polymers

Spin-dependent conduction and polarization in chiral polymers were studied for polymers organized as self-assembled monolayers with conduction along the polymer backbone, namely, along its longer axis. Large spin polarization and magnetoresistance effects were observed, showing a clear dependence on the secondary structure of the polymer. The results indicate that the spin polarization process does not include spin flipping and hence it results from backscattering probabilities for the two spin states.

Raman Optical Activity (ROA) as a New Tool to Elucidate the Helical Structure of Poly(phenylacetylene)s

Poly(phenylacetylene)s are a family of helical polymers constituted by conjugated double bonds. Raman spectra of these polymers show a structural fingerprint of the polyene backbone which, in combination with its helical orientation, makes them good candidates to be studied by Raman optical activity (ROA). Four different well-known poly(phenylacetylene)s adopting different scaffolds and ten different helical senses have been prepared. Raman and ROA spectra were recorded and allowed to establish ROA-spectrum/helical-sense relationships: a left/right-handed orientation of the polyene backbone (M_helix /P_helix ) produces a triplet of positive/negative ROA bands. Raman and ROA spectra of each polymer exhibited the same profile, and the sign of the ROA spectrum was opposite to the lowest-energy electronic circular dichroism (ECD) band, indicating a resonance effect. Resonance ROA appears then as an indicator of the helical sense of poly(phenylacetylene)s, especially for those with an extra Cotton band in the ECD spectrum, where a wrong helical sense is assigned based on ECD, while ROA alerts of this misassignment.

Ambidextrous α,γ-Hybrid Peptide Foldamers

Molecular chirality is ubiquitous in nature. The natural biopolymers, proteins and DNA, preferred a right-handed helical bias due to the inherent stereochemistry of the monomer building blocks. Here, we are reporting a rare co-existence of left- and right-handed helical conformations and helix-terminating property at the C-terminus within a single molecule of α,γ-hybrid peptide foldamers composed of achiral Aib (α-aminoisobutyric acid) and 3,3-dimethyl-substituted γ-amino acid (Adb; 4-amino-3,3-dimethylbutanoic acid). At the molecular level, the left- and right-handed helical screw sense of α,γ-hybrid peptides are representing a macroscopic tendril perversion. The pronounced helix-terminating behaviour of C-terminal Adb residues was further explored to design helix-Schellman loop mimetics and to study their conformations in solution and single crystals. The stereochemical constraints of dialkyl substitutions on γ-amino acids showed a marked impact on the folding behaviour of α,γ-hybrid peptides.

2 H and 13 C NMR-Based Enantiodetection Using Polyacetylene versus Polypeptide Aligning Media: Versatile and Complementary Tools for Chemists

In this work, the practical/analytical potential of an L-valine-derived polyacetylene (PLA) lyotropic liquid crystal (LLC) is examined to spectrally discriminate enantiomers (racemic mixture) or enantiotopic directions of a large collection (23) of (pro)chiral model compounds (from rigid to flexible and polar to apolar ones), thus covering various important aspects of enantiomorphism. Experimental ² H-{¹ H} (deuterated analytes and at natural abundance level) and ¹³ C-{¹ H} NMR results are discussed in terms of the difference of ² H-RQCs or ¹³ C-RCSAs and compared to those obtained in polypeptide-type LLCs (PBLG). The analysis of the NMR results provides an overview of the enantiodifferentiation capabilities of PLA and gives useful/practical hints for the chemist to select the most appropriate chiral oriented system. Astonishing NAD NMR results were obtained in the case of one of the simplest, chiral alkanes, 3-methylhexane. From a theoretical viewpoint, the data collected highlight the key molecular factors involved in orientation/discrimination processes, as a basis for optimizing computational prediction (molecular dynamics simulation), as well as designing novel helically chiral polymers as new enantiodiscriminating aligning media. In addition, a new, robust and efficient protocol to synthesize PLA and its enantiomer (PDA) on a large scale and with small polydispersities is proposed.

Hydrogen-bonding-directed helical nanofibers in a polythiophene-based all-conjugated diblock copolymer

One-dimensional (1D) helical nanofibers were prepared through the self-assembly of an achiral all-conjugated diblock copolymer, poly(3-hexylthiophene)-b-poly[3-(6-hydroxyl)hexylthiophene] (P3HT-b-P3HHT) in an aged pyridine solution. Such helical nanofibers were formed by the π-π interaction between planar rigid polythiophene backbones cooperating with the hydrogen-bonding interactions between the polar hydroxyl groups of the side chains of polythiophenes. Intriguingly, the Young's modulus of such helical fibers is as high as ∼5.16 GPa, which is about twice that of P3HT films characterized by the peak force quantitative nanomechanical (PF-QNM) method. Furthermore, for the first time, we report that such helical fibers based on all-conjugated polythiophenes exhibited a relatively high field-effect mobility of 0.03472 cm2 V-1 s-1. This work provides a promising approach to craft crystalline helical nanostructures based on polythiophenes possessing both superior mechanical and good charge transport properties, which has great potential for application in other π-conjugated systems or building blocks for complex superstructures, and mechanical and optoelectronic devices.

Influence of a hydrodynamic environment on chain rigidity, liquid crystallinity, absorptivity, and photoluminescence of hydrogen-bonding-assisted helical poly(phenylacetylene)

The chain rigidity, liquid crystallinity, absorptivity, and photoluminescence of a helical poly(phenylacetylene) derivative varied significantly depending on the solvent, owing to the conformational changes based on intramolecular hydrogen bonds.