Switching handedness in optically active polysilanes

Supramolecular Chirality in Azobenzene-Containing Polymer System: Traditional Postpolymerization Self-Assembly Versus In Situ Supramolecular Self-Assembly Strategy

Recently, the design of novel supramolecular chiral materials has received a great deal of attention due to rapid developments in the fields of supramolecular chemistry and molecular self-assembly. Supramolecular chirality has been widely introduced to polymers containing photoresponsive azobenzene groups. On the one hand, supramolecular chiral structures of azobenzene-containing polymers (Azo-polymers) can be produced by nonsymmetric arrangement of Azo units through noncovalent interactions. On the other hand, the reversibility of the photoisomerization also allows for the control of the supramolecular organization of the Azo moieties within polymer structures. The construction of supramolecular chirality in Azo-polymeric self-assembled system is highly important for further developments in this field from both academic and practical points of view. The postpolymerization self-assembly strategy is one of the traditional strategies for mainly constructing supramolecular chirality in Azo-polymers. The in situ supramolecular self-assembly mediated by polymerization-induced self-assembly (PISA) is a facile one-pot approach for the construction of well-defined supramolecular chirality during polymerization process. In this review, we focus on a discussion of supramolecular chirality of Azo-polymer systems constructed by traditional postpolymerization self-assembly and PISA-mediated in situ supramolecular self-assembly. Furthermore, we will also summarize the basic concepts, seminal studies, recent trends, and perspectives in the constructions and applications of supramolecular chirality based on Azo-polymers with the hope to advance the development of supramolecular chirality in chemistry.

The helix-inversion mechanism in double-stranded helical oligomers bridged by rotary cyclic boronate esters

Attracted by the numerous regulatory functions of double-helical biopolymers such as DNA, many researchers have synthesized various double-helical systems. A recently synthesized double-stranded helical oligomer covalently bridged by rotary boronate esters (BBDD) was shown to undergo helix-inversion that might serve as platform to design rotor systems. However, the detailed helix-inversion mechanism could not be investigated experimentally. Direct molecular dynamics simulations based on density-functional tight-binding energies and gradients computed on-the-fly reveal that disentanglement to the unraveled form and following exchange of the twisted terminal trimethylsilyl (TMS) groups are prerequisites for the observed helix-inversion. The potential of mean force confirms that the originally assumed "concurrent" rotation of the boronate esters and the helix-inversion involves shorter time scale "step-wise" processes, triggered by the disentanglement and exchange of the TMS groups. These results indicate that inversion dynamics of double-helical molecules such as BBDD may be controllable by chemical fine-tuning of the terminal groups. © 2019 Wiley Periodicals, Inc.

Effect of backbone conformation and its defects on electronic properties and assessment of the stabilizing role of π-π interactions in aryl substituted polysilylenes studied by DFT on deca[methyl(phenyl)silylene]s

BACKGROUND

Recent efforts in the field of mesoscale effects on the structure and properties of thin polymer films call to revival interest in conformational structure and defects of a polymer backbone which has a crucial influence on electronic properties of the material. Oligo[methyl(phenyl)silylene]s (OMPSi) as exemplary molecules were studied theoretically by DFT in the form of optimal decamers and conformationally disrupted decamers (with a kink).

RESULTS

We proved that transoid backbone conformation is true energy minimum and that a kink in the backbone causes significant hypsochromic shift of the absorption maximum (λ max ), while backbone conformation altering from all-eclipsed to all-anti affects λ max in the opposite way. π-π stacking was investigated qualitatively through optimal geometry of OMPSi and mutual position of their phenyls along the backbone and also quantitatively by an evaluation of molecular energies obtained from single point calculations with functionals, which treat the dispersion effect in the varying range of interaction.

CONCLUSIONS

The kink was identified as a realistic element of the conformational structure that could be able to create a bend in a real aryl substituted polysilylene chain because it is stabilized by attractive π-π interactions between phenyl side groups.Graphical abstract.

Synthesis and helical structures of poly(ω-alkynamide)s having chiral side chains: effect of solvent on their screw-sense inversion

New ω-alkynamides, (S)-HC≡CCH2CONHCH2CH(CH3)CH2CH3 (1) and (S)-HC≡CCH2CH2CONHCH(CH3)CH2CH2CH2CH2CH3 (2) were synthesized and polymerized with a rhodium catalyst in CHCl3 to obtain cis-stereoregular poly(ω-alkynamide)s (poly(1) and poly(2)). Polarimetric, CD, and IR spectroscopic studies revealed that in solution the polymers adopted predominantly one-handed helical structures stabilized by intramolecular hydrogen bonds between the pendent amide groups. This behavior was similar to that of the corresponding poly(N-alkynylamide) counterparts (poly(3) and poly(4)) reported previously, whereas the helical senses were opposite to each other. The helical structures of the poly(ω-alkynamide)s were stable upon heating similar to those of the poly(N-alkynylamide)s, but the solvent response was completely different. An increase in MeOH content in CHCl3/MeOH resulted in inversion of the predominant screw-sense for poly(1) and poly(2). Conversely, poly(3) was transformed into a random coil, and poly(4) maintained the predominant screw-sense irrespective of MeOH content. The solvent dependence of predominant screw-sense for poly(1) and poly(2) was reasonably explained by molecular orbital studies using the conductor-like screening model (COSMO).

Theoretical study of polaron binding energy in conformationally disrupted oligosilanes

Density functional theory was used for a quantum chemical study of oligo[methyl(phenyl)silylene] structures containing a conformational defect: a kink in the silicon backbone. Oligomers were studied in the neutral state as well as in the form of positive (P(+)) and negative (P(-)) polaron quasiparticles. Computations performed using the B3LYP model and the 6-31G(d) basis set revealed that the charge distribution is not influenced by the presence of the kink, but the positive charge on the Si backbone differs slightly in P(+) and P(-) quasiparticles. On the other hand, the spin density is significantly shifted away from the chain part that contains the kink, and this effect is more intense in P(-) polarons. Changes in electron density are also evident from the frontier molecular orbital distribution. The deformation energy (which is associated with the relaxation of polarons) decreases with the number of atoms in the oligomer backbone in P(+) but shows the opposite behavior for P(-) quasiparticles.

Electronic transitions in conformationally controlled tetrasilanes with a wide range of SiSiSiSi dihedral angles

Unlike π-electron chromophores, the peralkylated n-tetrasilane σ-electron chromophore resembles a chameleon in that its electronic spectrum changes dramatically as its silicon backbone is twisted almost effortlessly from the syn to the anti conformation (changing the SiSiSiSi dihedral angle ω from 0 to 180°). A combination of UV absorption, magnetic circular dichroism (MCD), and linear dichroism (LD) spectroscopy on conformationally controlled tetrasilanes 1-9, which cover fairly evenly the full range of angles ω, permitted a construction of an experimental correlation diagram for three to four lowest valence electronic states. The free chain tetrasilane n-Si4 Me10 (10), normally present as a mixture of three enantiomeric conformer pairs of widely different angles ω, has also been included in our study. The spectral trends are interpreted in terms of avoided crossings of 1B with 2B and 2A with 3A states, in agreement with SAC-CI calculations on the excited states of 1-7 and conformers of 10.

Complementary Induction of Right- and Left-Handed Helical Structures by the Positioning of Chiral Groups on the Monomer Units: Introduction of (-)-Menthol as Side Chains of Poly(quinoxaline-2,3-diyl)s

Poly(quinoxaline-2,3-diyl) bearing menthyloxymethyl side chains derived from (-)-menthol at the 6- and 7-positions of the quinoxaline ring showed a single, right-handed helical structure in chloroform. Upon introduction of the same (-)-menthol-derived side chains into the 5- and 8-positions, a single, left-handed helical structure was formed in chloroform. The former poly(quinoxaline-2,3-diyl)s showed solvent-dependent inversion of the helical sense in 1,1,2-trichloroethane to form a left-handed helical structure with high screw sense purity. Copolymers bearing both menthyloxymethyl and o-(diphenylphosphino)phenyl groups in their side chains served as highly enantioselective chiral ligands in the palladium-catalyzed hydrosilylation of styrene.

Coordination chemistry strategies for dynamic helicates: time-programmable chirality switching with labile and inert metal helicates

'Chirality switching' is one of the most important chemical processes controlling many biological systems. DNAs and proteins often work as time-programmed functional helices, in which specific external stimuli alter the helical direction and tune the time scale of subsequent events. Although a variety of organic foldamers and their hybrids with natural helices have been developed, we highlight coordination chemistry strategies for development of structurally and functionally defined metal helicates. These metal helicates have characteristic coordination geometries, redox reactivities and spectroscopic/magnetic properties as well as complex chiralities. Several kinds of inert metal helicates maintain rigid helical structures and their stereoisomers are separable by optical resolution techniques, while labile metal helicates offer dynamic inversion of their helical structures via non-covalent interactions with external chemical signals. The latter particularly have dynamically ordered helical structures, which are controlled by the combinations of metal centres and chiral ligands. They further function as time-programmable switches of chirality-derived dynamic rotations, translations, stretching and shape flipping, which are useful applications in nanoscience and related technology.

Polymerization of substituted acetylenes and features of the formed polymers

This review summarizes the recent advances of substituted polyacetylene chemistry, including new polymerization catalysts and the properties and functions of the polymers.

Double helix formation of oligoresorcinols in water: thermodynamic and kinetic aspects

We previously reported that the oligoresorcinols formed double-stranded helices in neutral water through interstrand aromatic interactions. In the present study, we synthesized a new series of oligomers from the 2mer to the 15mer to explore the thermodynamics, kinetics, and mechanism of the double helix formation of the oligoresorcinols in water. The double helix formation was dependent on the chain length of the oligomers and significantly affected by solvent, pH, salt, and temperature. The free energy change (-DeltaG) for the double helix formation linearly increased with the chain length from the 4mer to the 11mer (DeltaDeltaG = -0.94 kcal mol(-1) unit(-1)), whereas it did not change for the oligomers longer than the 11mer. The van't Hoff analysis of the 9mer revealed that the double helix formation was an enthalpically driven process (DeltaH = -27 +/- 1.5 kcal mol(-1) and DeltaS = -70 +/- 5 cal mol(-1) K(-1)), which was consistent with the upfield shifts in the (1)H NMR spectra and the hypochromicity of the absorption spectra as a result of the interstrand aromatic interactions in water. Furthermore, the kinetic analysis of the chain exchange reaction between the double helices of the optically active and optically inactive 11mers revealed a small DeltaS(double dagger), suggesting that the chain exchange proceeds not via the dissociation-association pathway, but via the direct exchange pathway.

Sequence- and chain-length-specific complementary double-helix formation

The artificial sequential strands consisting of two, three, or four m-terphenyl groups joined by diacetylene linkers with complementary binding sites, either the chiral amidine (A) or achiral carboxyl (C) group, were synthesized in a stepwise manner. Using circular dichroism and (1)H NMR spectroscopies along with liquid chromatography, we showed that, when three dimeric molecular strands (AA, CC, and AC) or six trimeric molecular strands (AAA, CCC, AAC, CCA, ACA, and CAC) were mixed in solution, the complementary strands were sequence-specifically hybridized to form one-handed double-helical dimers AA.CC and (AC) 2 or trimers AAA.CCC, AAC.CCA, and ACA.CAC, respectively, through complementary amidinium-carboxylate salt bridges. Upon the addition of CCA to a mixture of AAA, AAC, and ACA, the AAC.CCA double helix was selectively formed and then isolated from the mixture by chromatography. Moreover, the homo-oligomer mixtures of amidine or carboxylic acid from the monomers to tetramers (A, AA, AAAA, C, CC, and CCCC) assembled with a precise chain length specificity to form A.C, AA.CC, and AAAA.CCCC, which were separated by chromatography.

Double-stranded helical polymers consisting of complementary homopolymers

Two complementary homopolymers of chiral amidines and achiral carboxylic acids with m-terphenyl-based backbones were synthesized by the copolymerization of a p-diiodobenzene derivative with the diethynyl monomers bearing a chiral amidine group and a carboxyl group using the Sonogashira reaction, respectively. Upon mixing in THF, the homopolymer strands assembled into a preferred-handed double helix through interstrand amidinium-carboxylate salt bridges, as evidenced by its absorption, circular dichroism, and IR spectra. In contrast, when mixed in less polar solvents, such as chloroform, the complementary strands kinetically formed an interpolymer complex with an imperfect double helical structure containing a randomly hybridized cross-linked structure, probably because of strong salt bridge formations. This primary complex was rearranged into the fully double helical structure by treatment with a strong acid followed by neutralization with an amine. High-resolution atomic force microscopy revealed the double-stranded helical structure and enabled the determination of the helical sense.