Chiral Polymers Research Articles

Chiral Polyureas Derived Cinchona Alkaloids: Highly Efficient Bifunctional Organocatalysts for the Asymmetric Michael Addition Reaction

AbstractRepetitive Mizoroki‐Heck (MH) coupling polymerization was used to create novel chiral polyureas with cinchona alkaloids in the main chain. Apart from two‐component polycondensation of cinchona urea dimers with diiodobenzene via MH reaction conditions, some chiral polyureas were easily polymerized under self polycondensation to yield the chiral polymers. The asymmetric Michael addition reaction was successfully carried out using chiral polymers derived cinchona urea. Interestingly, phenolic‐based chiral polymers served as heterogeneous catalysts for the enantioselective Michael reaction, with high catalytic activity and excellent enantioselectivities (up to 99 % ee). These polymeric catalysts were also stable, and they could be recycled and reused.

Stacked Reticular Frame Boosted Circularly Polarized Luminescence of Chiral Covalent Organic Frameworks

AbstractChiral covalent organic frameworks (COFs) with circularly polarized luminescence (CPL) are intriguing as advanced chiroptical materials but have not been reported to date. We constructed chiroptical COF materials with CPL activity through the convenient Knoevenagel condensation of formyl‐functionalized axially chiral linkers and C3‐symmetric 1,3,5‐benzenetriacetonitrile. Remarkably, the as‐prepared chiral COFs showed high absorption and luminescent dissymmetric factors up to 0.02 (gabs) and 0.04 (glum), respectively. In contrast, the branched chiral polymers from the same starting monomers were CPL silent. Structural and spectral characterization revealed that the reticular frame was indispensable for CPL generation via confined chirality transfer. Moreover, reticular stacking boosted the CPL performance significantly due to the interlayer restriction of frame. This work demonstrates the first example of a CPL‐active COF and provides insight into CPL generation through covalent reticular chemistry, which will play a constructive role in the future design of high‐performance CPL materials.

Stacked Reticular Frame Boosted Circularly Polarized Luminescence of Chiral Covalent Organic Frameworks.

Chiral covalent organic frameworks (COFs) with circularly polarized luminescence (CPL) are intriguing as advanced chiroptical materials but have not been reported to date. We constructed chiroptical COF materials with CPL activity through the convenient Knoevenagel condensation of formyl-functionalized axially chiral linkers and C3-symmetric 1,3,5-benzenetriacetonitrile. Remarkably, the as-prepared chiral COFs showed high absorption and luminescent dissymmetric factors up to 0.02 (gabs ) and 0.04 (glum ), respectively. In contrast, the branched chiral polymers from the same starting monomers were CPL silent. Structural and spectral characterization revealed that the reticular frame was indispensable for CPL generation via confined chirality transfer. Moreover, reticular stacking boosted the CPL performance significantly due to the interlayer restriction of frame. This work demonstrates the first example of a CPL-active COF and provides insight into CPL generation through covalent reticular chemistry, which will play a constructive role in the future design of high-performance CPL materials.

Novel chiral multi-arm liquid crystal polymers containing azobenzene mesogens and cholic acid

ABSTRACT Design and synthesis a series of azo multi-arm liquid crystal monomers (M1-M3) with cholic acid as the chiral core, followed by side chain graft copolymerisation with polymethylhydrosiloxane (PMHS) to synthesise the liquid crystal polymer PM series. All liquid crystal monomers and polymers were conformed to the molecular design in structure by FT-IR and NMR. The liquid crystal performance was tested by DSC and POM, and the results revealed that M1 and M2 were thermotropic enantiotropic cholesteric phase liquid crystals. Liquid crystal monomer M3 has no liquid crystal behaviour in both the up and down temperature processes. Polymers of the PM series exhibited different liquid crystal phases depending on the monomer content, with Grand-Jean texture at lower percentage and oil filament texture at higher percentage. The glass transition temperature and melting temperature decrease progressively with the increase in the liquid crystal monomer content, as opposed to the isotropic temperature.

Computational Modeling of Chromatin Fiber to Characterize Its Organization Using Angle-Resolved Scattering of Circularly Polarized Light.

Understanding the structural organization of chromatin is essential to comprehend the gene functions. The chromatin organization changes in the cell cycle, and it conforms to various compaction levels. We investigated a chromatin solenoid model with nucleosomes shaped as cylindrical units arranged in a helical array. The solenoid with spherical-shaped nucleosomes was also modeled. The changes in chiral structural parameters of solenoid induced different compaction levels of chromatin fiber. We calculated the angle-resolved scattering of circularly polarized light to probe the changes in the organization of chromatin fiber in response to the changes in its chiral parameters. The electromagnetic scattering calculations were performed using discrete dipole approximation (DDA). In the chromatin structure, nucleosomes have internal interactions that affect chromatin compaction. The merit of performing computations with DDA is that it takes into account the internal interactions. We demonstrated sensitivity of the scattering signal’s angular behavior to the changes in these chiral parameters: pitch, radius, the handedness of solenoid, number of solenoid turns, the orientation of solenoid, the orientation of nucleosomes, number of nucleosomes, and shape of nucleosomes. These scattering calculations can potentially benefit applying a label-free polarized-light-based approach to characterize chromatin DNA and chiral polymers at the nanoscale level.

Chiral TADF‐Active Polymers for High‐Efficiency Circularly Polarized Organic Light‐Emitting Diodes

AbstractA strategy of chiral donor–acceptor copolymerization is proposed to develop chiral nonconjugated polymers with thermally activated delayed fluorescence (TADF). Based on this strategy, two pairs of chiral polymers (R,R)‐/(S,S)‐pTpAcDPS and (R,R)‐/(S,S)‐pTpAcBP were synthesized. The alternating copolymerization of the chiral donors and acceptors could effectively separate the frontier molecular orbitals, which made the polymers show small ΔEST of 0.01–0.03 eV and efficient TADF properties. Moreover, the polymers also showed the quantum yield of up to 92 % and the circularly polarized luminescence. The solution‐processed circularly polarized organic light‐emitting diodes showed circularly polarized electroluminescence signals with high external quantum efficiencies of up to 22.1 % and maximum luminance of up to 34350 cd m−2. This is the first report of CP‐OLEDs based on chiral TADF polymer, which provides a useful and valuable guidance for the development of high‐efficiency CPEL polymers.

Chiral TADF-Active Polymers for High-Efficiency Circularly Polarized Organic Light-Emitting Diodes.

A strategy of chiral donor-acceptor copolymerization is proposed to develop chiral nonconjugated polymers with thermally activated delayed fluorescence (TADF). Based on this strategy, two pairs of chiral polymers (R,R)-/(S,S)-pTpAcDPS and (R,R)-/(S,S)-pTpAcBP were synthesized. The alternating copolymerization of the chiral donors and acceptors could effectively separate the frontier molecular orbitals, which made the polymers show small ΔEST of 0.01-0.03 eV and efficient TADF properties. Moreover, the polymers also showed the quantum yield of up to 92 % and the circularly polarized luminescence. The solution-processed circularly polarized organic light-emitting diodes showed circularly polarized electroluminescence signals with high external quantum efficiencies of up to 22.1 % and maximum luminance of up to 34350 cd m-2 . This is the first report of CP-OLEDs based on chiral TADF polymer, which provides a useful and valuable guidance for the development of high-efficiency CPEL polymers.

Synthesis of a polyacetylene derivative bearing aspirin with chiral charge carrier “chiralions”

The synthesis of a polyacetylene-bearing aspirin was conducted. Gas-phase iodine doping of the polymer provided evidence of chemical interaction between the polyene and iodine, which acted as an electron acceptor. The polymer in a chiral aspirin derivative shows entire helical structure with optical activity. The electronic properties of the π-conjugated chiral liquid crystal polymer blend with a helical structure were evaluated.

Stereochemistry and Twisted Crystals

AbstractThis review article discusses a link between stereochemistry and structural and morphological chirality. We present the remarkably frequent, but less often reported, growth of twisted crystals and discuss the reasons for their formation and ways to favor their growth. The more commonly encountered conglomerates and racemic compounds can be formed by achiral molecules, but the emergence of morphological chirality is often not considered in this context and perhaps deserves attention. Twisted crystals are known for a wide variety of materials, molecular and macromolecular, complexes and oxides, and so on. In growing crystals, twisting of lamellae because of strain generated during growth eventually unwinds in larger objects, and may not be detected. This kind of chiral morphology arising from low symmetry growth, and the challenges and opportunities for research in the area, are highlighted. In particular, the link between structural bending in supramolecular aggregates in solution or at interfaces, chiral nanofilaments in liquid crystals, and chiral polymers, is discussed.

Polymer stabilized isotropic phase liquid crystals with large Kerr constant

ABSTRACT In this paper, we reported a polymer stabilised isotropic phase liquid crystal (PSILC). The influences of different content of chiral dopant and polymer monomer on PSILC were studied. Adding the proper proportion and total content of polymer monomer TMPTA and C12A into liquid crystal with 1.0 wt.% chiral dopant R5011, and UV photo-polymerisation at the isotropic phase, we obtain the Kerr constant greater than 10 nm/V2, hysteresis less than 2.0% and fast response in PSILC.

Preparation and Chiral Applications of Optically Active Polyamides.

Chirality is omnipresent in nature and plays vital roles in living organism, and has become a hot research topic across multidisciplinary fields including chemistry, biology, physics, and material science. Meanwhile, polyamides constitute an important class of polymers and have received significant attention owing to their outstanding properties and wide-ranging applications in many areas. Judiciously introducing chirality into polyamides will undoubtedly obtain attractive chiral polymers, namely, optically active polyamides. This review describes the preparation methods of chiral polyamides, including solution polycondensation, interfacial polycondensation, ring-open polymerization, and others; the newly emerging categories of chiral polyamides, i.e., helical polyamides, chiral polyamide-imides, are also presented. The applications of optically active polyamides in chiral research fields including asymmetric catalysis, membrane separation, and enantioselective crystallization are also summarized. In addition, current challenges in chiral polyamides are further presented and future perspectives in the field are proposed.

Chiral Supramolecular Polymers Assembled by Amphiphilic Oligopeptide-Perylene Diimides and High Electrochemical Sensing.

Various secondary structures, for example, β-sheet hydrogen bonds formed by oligopeptides exhibiting high directionality and selectivity provide a new avenue to regulate optoelectronic performances of supramolecular assemblies constructed by π-conjugated chromophores. In this work, oligopeptide-perylene diimides (AUPDIs) are synthesized to generate β-sheet strands which guide the formation of chiral supramolecular polymers with a diversity of morphologies in combination with the π-π stacking even in aqueous media. Complex morphology transitions are successfully controlled by simply adjusting the water volume fraction in the binary solvent of water and tetrahydrofuran from spherical hollow aggregates to long helical nanowires and to short nanofibers. The mechanism of the assembly changes from cooperative to the isodesmic model relying on AUPDI concentrations. This originates from the transformation in the β-sheet that regulates profoundly the arrangement of the AUPDI molecules. Prominently efficient and positive electronic sensing to triethylamine for highly helical nanowires engenders due to the highly ordered helical arrangement within the nanowires, fourfold of the short nanofibers.

Multistate Switching of Spin Selectivity in Electron Transport through Light-Driven Molecular Motors.

It is established that electron transmission through chiral molecules depends on the electron's spin. This phenomenon, termed the chiral‐induced spin selectivity (CISS), effect has been observed in chiral molecules, supramolecular structures, polymers, and metal‐organic films. Which spin is preferred in the transmission depends on the handedness of the system and the tunneling direction of the electrons. Molecular motors based on overcrowded alkenes show multiple inversions of helical chirality under light irradiation and thermal relaxation. The authors found here multistate switching of spin selectivity in electron transfer through first generation molecular motors based on the four accessible distinct helical configurations, measured by magnetic‐conductive atomic force microscopy. It is shown that the helical state dictates the molecular organization on the surface. The efficient spin polarization observed in the photostationary state of the right‐handed motor coupled with the modulation of spin selectivity through the controlled sequence of helical states, opens opportunities to tune spin selectivity on‐demand with high spatio‐temporal precision. An energetic analysis correlates the spin injection barrier with the extent of spin polarization.

Role of Chain Entanglements in the Stereocomplex Crystallization between Poly(lactic acid) Enantiomers.

Stereocomplex (SC) crystallization between polymer enantiomers has opened a promising avenue for preparing high-performance materials. However, high-crystallinity SCs are difficult to achieve for high-molecular-weight (HMW) enantiomeric blends of chiral polymers [e.g., poly(lactic acid)]. Despite extensive studies, why HMW enantiomeric blends have difficulty in SC crystallization has not been clarified. Herein, we chose the HMW poly(l-lactic acid)/poly(d-lactic acid) (PLLA/PDLA) 1/1 blend as the model system and demonstrated the crucial role of chain entanglement in regulating SC crystallization. PLLA/PDLA blends with various entanglement degrees were prepared by freeze-drying. We observed that disentangling promoted not only the crystallization rate but also the crystallinity of SCs in both the nonisothermal and isothermal processes. The less-entangled samples crystallized exclusively as the high-crystallinity SCs at different temperatures, in contrast to the predominant homocrystallization that occurred in the common entangled samples. This study provides deep insight into the SC crystallization mechanism of polymers and paves the way for future research attempting to prepare SC materials.

Supramolecular Polymerization of C3-Symmetric, Triphenylene-Cored Aza-Polycyclic Aromatic Hydrocarbons with Excellent and Switchable Circularly Polarized Luminescence Performance

A new type of C3-symmetric aza-polycyclic aromatic hydrocarbon (PAH) discs attached with chiral alkoxyl chains is developed and explored as circularly polarized luminescence (CPL)-active materials with selective control and switching of chiroptical properties. Supramolecular polymerization of these PAHs is studied experimentally and theoretically. With the aid of weak hydrogen bonding and CH···π interactions, these chromophores have strong tendency to form anisotropic chiral aggregates both in solutions and films. The well-controlled noncovalent interactions are critical for their excellent CPL performance. The resultant supramolecular polymers display a polarized emission band with the maximum glum value up to 10–2. Benefited from the chiral supramolecular polymerization process and N-doping structure feature of the core, on/off CPL switching from these molecules can be reversibly realized by external stimuli including the change of temperature, solvent, and pH conditions. Our work shows that chiral supramolecular polymers from concise yet highly rigid and planar PAH cores are superbly suitable for exceptional CPL emission both in dilute solutions and bulk materials, with a promising switchable CPL feature, thereby unlocking new design possibilities.

Bipolar Electrochemical Measurement of Enantiomeric Excess with Inherently Chiral Polymer Actuators.

Straightforward enantioselective analytical methods are very important for drug safety, considering that in certain cases one of the two enantiomers of a chiral molecule might be harmful for humans. In this work, we propose a simple system for the direct and easy read-out of the enantiomeric excess of 3,4-dihydroxyphenylalanine (DOPA) as a model analyte. A conducting oligomer, i.e. oligo-(3,3′-dibenzothiophene), bearing inherently chiral features, is electrogenerated on a polypyrrole film. The resulting freestanding hybrid material is used as a wireless enantioselective actuator in a bipolar electrochemical cell. Combining in a single setup two individual actuators with opposite chiral features allows a direct visual read-out of enantiomeric excess, as the bending amplitude of each of the two actuators is directly correlated with the concentration of the corresponding stereoisomer of the analyte. Optimization of the experimental parameters results in efficient bending, giving access to the percentage values of the enantiomeric excess in mixtures containing different ratios of the antipodes, thus opening the way to potential applications for chiral in situ analysis.

Strategies to Achieve a Ferrocene-Based Polymer with Reversible Redox Activity for Chiral Electroanalysis of Nonelectroactive Amino Acids.

In the past, various chiral isomers accompanied by electroactive units have been distinguished using electrochemical techniques, which can produce electrochemical signals by themselves. However, it is still difficult to use an electrochemical technique to detect nonelectroactive samples. To address this bottleneck, an electroactive chiral polymer (S,S)-p-CVB-Fc that contains one redox-active ferrocene unit was designed and synthesized in this study. The electroactive polymer can give electrochemical signals as an alternative to the tested chiral samples, regardless of whether the isomers have electroactive units. Then, it was fixed on the surface of a glassy carbon electrode as an electrochemical chiral sensor. When nonelectroactive amino acids including proline, threonine, and alanine were examined by the sensor, clear discrimination in the response of peak current could be observed toward l- and d-isomers at pH 6.5. The peak current ratios (IL/ID) for proline and alanine were 1.47 and 1.48, respectively. In contrast, for threonine, the d-isomer exhibited a higher peak current than the l -isomer with a ratio of 2.59. In summary, the results ensure that the current work can enlarge the testing scope of chiral samples in the field of chiral electroanalysis using an electroactive sensor.

Hydrogen-Bond-Driven Synergistically Enhanced Hyperpolarizability: Chiral Coordination Polymers with Nonpolar Structures Exhibiting Unusually Strong Second-Harmonic Generation.

Four chiral coordination polymers (CPs), M[(S,S)-C14 H14 N2 O6 ] and M[(R,R)-C14 H14 N2 O6 ] (M=Zn or Cd), have been exclusively synthesized in high yields with the aid of newly designed chiral ligand under hydrothermal condition. The CPs crystallizing in the orthorhombic nonpolar space group, C2221 , reveal three-dimensional framework structures composed of MO4 tetrahedra and the corresponding homochiral linkers. Powder second-harmonic generation (SHG) measurements indicate that the nonpolar CPs reveal very strong SHG efficiency of ca. 5-9 times that of KH2 PO4 and exhibit type-I phase-matching behavior. Density functional theory calculations suggest that the unusually large SHG efficiency found from the nonpolar CPs should be attributable to the synergistic effect of polarizable metal cations and enhanced hyperpolarizability in the donor-acceptor system originating from the hydrogen bonding in the coordinated linkers.

Hydrogen‐Bond‐Driven Synergistically Enhanced Hyperpolarizability: Chiral Coordination Polymers with Nonpolar Structures Exhibiting Unusually Strong Second‐Harmonic Generation

AbstractFour chiral coordination polymers (CPs), M[(S,S)‐C14H14N2O6] and M[(R,R)‐C14H14N2O6] (M=Zn or Cd), have been exclusively synthesized in high yields with the aid of newly designed chiral ligand under hydrothermal condition. The CPs crystallizing in the orthorhombic nonpolar space group, C2221, reveal three‐dimensional framework structures composed of MO4 tetrahedra and the corresponding homochiral linkers. Powder second‐harmonic generation (SHG) measurements indicate that the nonpolar CPs reveal very strong SHG efficiency of ca. 5–9 times that of KH2PO4 and exhibit type‐I phase‐matching behavior. Density functional theory calculations suggest that the unusually large SHG efficiency found from the nonpolar CPs should be attributable to the synergistic effect of polarizable metal cations and enhanced hyperpolarizability in the donor‐acceptor system originating from the hydrogen bonding in the coordinated linkers.

Influence of molecular weight on helical twisting power of oligomer chiral dopants

Chiral mesogenic monomers have been widely used in applications based on chiral liquid crystals, where polymerization is normally required to fulfill the device function. The change of molecular weight has a notable impact on the properties of the formed polymers. Herein, we systematically investigate the influence of molecular weight on the helical twisting power (HTP) of chiral oligomers when used as chiral dopants. Two series of side-chain and main-chain liquid crystal oligomers are synthesized and examined in terms of phase behavior, HTP value and dependence on temperature. The relationship between the HTP value and molecular weight of the oligomer is established, which is markedly dependent on the molecular structure. The insights obtained from the current study could help develop methods to in-situ analyze functional devices based on chiral liquid crystal polymers.