Development of synthetic double helical polymers and oligomers

Synthesis and properties of helical polystyrene derivatives with amino acid side groups

A series of polystyrene derivatives with chiral amide groups with a controlled molecular weight and narrow molecular weight distribution were synthesized by reversible addition–fragmentation chain transfer (RAFT) radical polymerization.

Synthesis and properties of helically-folded poly(arylenediethynylene)s

Three arylenediethnylene-based helical foldamers having pyridine, naphthaleneimide and pyrene cores show unique conformational changes and photophysical properties in various organic solvents.

A new soft-matter material with old chemistry: Passerini multicomponent polymerization-induced assembly of AIE-active double-helical polymers with rapid visible-light degradability

Mimicking the superstructures and functions of natural chiral materials is beneficial to understand specific biological activities in living organisms and broaden applications in the fields of chemistry and materials sciences. However, it is still a great challenge to construct water-soluble, double-helical polymers with multiple responsiveness. Herein, we report for the first time a straightforward, general strategy to address this issue by taking advantage of Passerini multicomponent polymerization-induced assembly (PMPIA). The polymerization-induced generation of supramolecular interactions in chiral α-acyloxy amides drives the assembly of polymers and improves their stability in various solvents. This double-helical polymer is sensitive to metal ions, temperature, pH, and solvents, making both the superstructure and the AIE effect reversibly adjustable. Meanwhile, the hydrogen-bonding-assisted cyclization of photolabile α-acyloxy amides accelerates the degradation of helical polymers under visible-light irradiation. It is anticipated that this novel PMPIA strategy opens new horizons to inspire the design of advanced chiral/helical polymers with multiple functions.

Halogen Bonding Directed Supramolecular Quadruple and Double Helices from Hydrogen-Bonded Arylamide Foldamers

Halogen bonding has been used to glue together hydrogen-bonded short arylamide foldamers to achieve new supramolecular double and quadruple helices in the solid state. Three compounds, which bear a pyridine at one end and either a CF₂ I or fluorinated iodobenzene group at the other end, engage in head-to-tail N⋅⋅⋅I halogen bonds to form one-component supramolecular P and M helices, which stack to afford supramolecular double-stranded helices. One of the double helices can dimerize to form a G-quadruplex-like supramolecular quadruple helix. Another symmetric compound, which bears a pyridine at each end, binds to ICF₂ CF₂ I through N⋅⋅⋅I halogen bonds to form two-component supramolecular P and M helices, with one turn consisting of four (2+2) molecules. Half of the pyridine-bearing molecules in two P helices and two M helices stack alternatingly to form another supramolecular quadruple helix. Another half of the pyridine-bearing molecules in such quadruple helices stack alternatingly with counterparts from neighboring quadruple helices, leading to unique quadruple helical arrays in two-dimensional space.

Atomic structure and electronic properties of A2B2XY (A = Si-Pb, B = Cl-I, and XY = PN and SiS) inorganic double helices: first principles calculations

We study the structural stability and electronic properties of new classes of DNA-like inorganic double helices of the type A2B2XY (A = Si-Pb, B = Cl-I, and XY = PN and SiS) by employing first principles density functional theory (DFT) calculations including van der Waals interactions. In these quaternary double helices PN or SiS forms the inner helix while the AB helix wraps around the inner helix and the two are interconnected. We find that the bromides and iodides of Ge, Sn, and Pb as well as Pb2Cl2PN form structurally stable double helices while Ge2I2SiS as well as bromides and iodides of Sn and Pb have stable double helices. The atomic structures of different double helices have been analyzed in detail to understand the stability of these systems as there is up to about 80% difference in the interatomic distances in the two helices which is remarkable. Also in these new classes of double helices there is polar covalent bonding in the inner helix due to heteroatoms. We have calculated the DDEC6 partial atomic charges and bond orders which suggest strong covalent bonding in the inner helix. The electronic structure reveals that these double helices are semiconducting and in many cases the band gap is direct. Furthermore, we have studied the effects of doping and found that hole doping is the most appropriate way to tuning their electronic properties.

Multiple competing pathways for chemical reaction: drastic reaction shortcut for the self-catalytic double-helix formation of helicene oligomers

Competition among multiple pathways in a chemical reaction exhibits notable kinetic phenomena, particularly when amplification by self-catalysis is involved. A pseudoenantiomeric 1 : 1 mixture of an aminomethylene helicene (P)-tetramer and an (M)-pentamer formed enantiomeric hetero-double helices B and C in solution when random coil A was cooled. When a solution of A at 70 °C was directly cooled to 25 °C, the A-to-B reaction was predominant, then B was slowly converted to C over 60 h. The slow conversion in the A-to-B-to-C reaction was due to the formation of the hetero-double helix B, which was an off-pathway intermediate, and the slow B-to-C conversion. In contrast, when a solution of A at 70 °C was snap-cooled to -25 °C before then maintaining the solution at 25 °C, the A-to-C reaction predominated, and the formation of C was complete within 4 h. The reactions involve competition between the self-catalytic A-to-B and A-to-C pathways, where B and C catalyze the A-to-B and A-to-C reactions, respectively. Subtle differences in the initial states generated by thermal pretreatment were amplified by the self-catalytic process, which resulted in a drastic reaction shortcut.

Supramolecular Double-Helix Formation by Diastereoisomeric Conformations of Configurationally Enantiomeric Macrocycles

Solid-state superstructures, resulting from assemblies programmed by homochirality, are attracting considerable attention. In addition, artificial double-helical architectures are being investigated, especially in relation to the ways in which homochiral small molecules can be induced to yield helical forms as a result of chiral induction. Herein, we report the highly specific self-assembly upon crystallization of a double-helical superstructure from an enantiopure macrocyclic dimer which adopts two diastereoisomeric conformations in a molar ratio of 1.5:1 in dimethyl sulfoxide. These two conformational diastereoisomers self-organize-and self-sort-in the crystalline phase in equimolar proportions to form two single-handed helices which are complementary to each other, giving rise to the assembly of a double helix that is stabilized by intermolecular [C-H···O] and π-π stacking interactions. The observed self-sorting phenomenon occurs on going from a mixed-solvent system containing two equilibrating conformational diastereoisomers, presumably present in unequal molar proportions, into the solid state. The diastereoisomeric conformations are captured upon crystallization in a 1:1 molar ratio in the double-helical superstructure, whose handedness is dictated by the choice of the enantiomeric macrocyclic dimer. The interconversion of the two conformational diastereoisomers derived from each configurationally enantiomeric macrocycle was investigated in CD₃SOCD₃ solution by variable-temperature ¹H NMR spectroscopy (VT NMR) and circular dichroism (VT CD). The merging of the resonances for the protons corresponding to the two diastereoisomers at a range of coalescence temperatures in the VT NMR spectra and occurrence of the isosbestic points in the VT CD spectra indicate that the two diastereoisomers are interconverting slowly in solution on the ¹H NMR time scale but rapidly on the laboratory time scale. To the best of our knowledge, the self-assembly of such solid-state superstructures from two conformational diastereoisomers of a homochiral macrocycle is a rare, if not unique, occurrence.

Supramolecular Helical Systems: Helical Assemblies of Small Molecules, Foldamers, and Polymers with Chiral Amplification and Their Functions

In this review, we describe the recent advances in supramolecular helical assemblies formed from chiral and achiral small molecules, oligomers (foldamers), and helical and nonhelical polymers from the viewpoints of their formations with unique chiral phenomena, such as amplification of chirality during the dynamic helically assembled processes, properties, and specific functionalities, some of which have not been observed in or achieved by biological systems. In addition, a brief historical overview of the helical assemblies of small molecules and remarkable progress in the synthesis of single-stranded and multistranded helical foldamers and polymers, their properties, structures, and functions, mainly since 2009, will also be described.

Controllability of dynamic double helices: quantitative analysis of the inversion of a screw-sense preference upon complexation

We describe a quantitative analysis of the complexation-induced inversion of a screw-sense preference based on a conformationally dynamic double-helix structure in a macrocycle. The macrocycle is composed of two twisting units (terephthalamide), which are spaced by two strands (1,3-bis(phenylethynyl)benzene), and is designed to generate a double-helix structure through twisting about a C2 axis in a conrotatory manner. The attachment of chiral auxiliaries to the twisting units induces a helical preference for a particular sense of (M)- or (P)-helicity through the intramolecular transmission of chirality to dynamic double helices. The twisting unit can also act as a binding site for capturing a guest molecule, and, in a complexed state, the preferred screw sense of the dynamic double-helix structure is reversed to exhibit the contrary preference. We quantitatively monitored the complexation-induced inversion of the screw-sense preference using 1H NMR spectroscopy, which enabled us to observe independently two species with (M)- or (P)-helicity in both the absence and presence of a guest molecule. Inversion of the screw-sense preference was induced upon complexation with an achiral guest as well as a chiral guest.

Racemic and conglomerate 1-(4-haloaryl)ethylammonium tetrachlorocobaltate salts: formation of helical structures

Whereas rac-1-(4-fluorophenyl)ethylamine (rac-fpea) reacts with CoCl₂ and HCl forming rac-[(R)-fpeaH]₂[CoCl₄], the chloro analogue rac-1-(4-chlorophenyl)ethylamine (rac-cpea) yields a conglomerate of [(R)-cpeaH]₃[CoCl₄]Cl and [(S)-cpeaH]₃[CoCl₄]Cl.

Chiral tether-mediated stabilization and helix-sense control of complementary metallo-double helices

A series of novel PtII-linked double helices were prepared by inter- or intrastrand ligand-exchange reactions of the complementary duplexes composed of chiral or achiral amidine dimer and achiral carboxylic acid dimer strands joined by trans-PtII-acetylide complexes with PPh3 ligands using chiral and achiral chelating diphosphines. The structure and stability of the PtII-linked double helices were highly dependent on the diphosphine structures. An interstrand ligand exchange took place with chiral and achiral 1,3-diphosphine-based ligands, resulting in trans-PtII-bridged double helices, whose helical structures were quite stable even in dimethyl sulfoxide (DMSO) due to the interstrand cross-link, whereas a 1,2-diphosphine-based ligand produced non-cross-linked cis-PtII-linked duplexes, resulting from an intrastrand ligand-exchange that readily dissociated into single strands in DMSO. When enantiopure 1,3-diphosphine-based ligands were used, the resulting trans-PtII-bridged double helices adopted a preferred-handed helical sense biased by the chirality of the bridged diphosphines. Interestingly, the interstrand ligand exchange with racemic 1,3-diphosphine toward an optically-active PtII-linked duplex, composed of chiral amidine and achiral carboxylic acid strands, was found to proceed in a diastereoselective manner, thus forming complete homochiral trans-PtII-bridged double helices via a unique chiral self-sorting.

Synthesis and self-association of double-helical AADD arrays

The design and syntheses of four self-complementary oligomers that contain an underlying AADD hydrogen bond sequence are presented, and their self-association was examined in the solution and solid state. The molecular recognition between the two strands is highly sensitive to substitutions of their component heterocycles. Substitution with electron-donating and -withdrawing groups and the influence of preorganization has a large effect on the overall stabilities of the complexes studied. In particular, a wide range (>10(5) M(-1)) of stabilities with respect to substitutions at various positions in the AADD oligomers was demonstrated. In the most extreme case, the dimerization constant measured (K(dimer) ≥4.5×10(7) M(-1)) is comparable to the most stable homodimers of neutral AADD arrays reported to date.