Helical Superstructures Research Articles

Light-directing chiral liquid crystal nanostructures: from 1D to 3D.

Endowing external, remote, and dynamic control to self-organized superstructures with desired functionalities is a principal driving force in the bottom-up nanofabrication of molecular devices. Light-driven chiral molecular switches or motors in liquid crystal (LC) media capable of self-organizing into optically tunable one-dimensional (1D) and three-dimensional (3D) superstructures represent such an elegant system. As a consequence, photoresponsive cholesteric LCs (CLCs), i.e., self-organized 1D helical superstructures, and LC blue phases (BPs), i.e., self-organized 3D periodic cubic lattices, are emerging as a new generation of multifunctional supramolecular 1D and 3D photonic materials in their own right because of their fundamental academic interest and technological significance. These smart stimuli-responsive materials can be facilely fabricated from achiral LC hosts by the addition of a small amount of a light-driven chiral molecular switch or motor. The photoresponsiveness of these materials is a result of both molecular interaction and geometry changes in the chiral molecular switch upon light irradiation. The doped photoresponsive CLCs undergo light-driven pitch modulation and/or helix inversion, which has many applications in color filters, polarizers, all-optical displays, optical lasers, sensors, energy-saving smart devices, and so on. Recently, we have conceptualized and rationally synthesized different light-driven chiral molecular switches that have very high helical twisting powers (HTPs) and exhibit large changes in HTP in different states, thereby enabling wide phototunability of the systems by the addition of very small amounts of the molecular switches into commercially available achiral LCs. The light-driven chiral molecular switches are based on well-recognized azobenzene, dithienylcyclopentene, and spirooxazine derivatives. We have demonstrated high-resolution and lightweight photoaddressable displays without patterned electronics on flexible substrates. The wide tunability of the HTP furnishes reflection colors encompassing the whole visible spectrum and beyond in a reversible manner. Photomodulation of the helical pitch of the CLCs has been achieved by UV, visible, and near-infrared (NIR) light irradiation. NIR-light-induced red, green, and blue (RGB) reflections have been leveraged only by varying the power density of the IR laser. Some chiral switches are found to confer helix inversion to the cholesteric systems, which qualifies the CLCs for applications where circularly polarized light is involved. Dynamic and static primary RGB reflection colors have been achieved in a single film. LC BPs have been fabricated and investigated in the context of self-organized 3D photonic band gap (PBG) materials, and dynamic phototuning of the PBG over the visible region has been achieved. Omnidirectional lasing and tuning of the laser emission wavelength have also been attained in monodisperse photoresponsive CLC microshells fabricated by a capillary-based microfluidic technique. This Account covers the research and development in our laboratory starting from the design concepts and synthesis of photodynamic chiral molecular switches to their applications in the fabrication of photoresponsive CLCs and BPs. Potential and demonstrated practical applications of photoresponsive CLCs, microshells, and BPs are discussed, and the Account concludes with a brief forecast of what lies beyond the horizon in this rapidly expanding and fascinating field.

Helical supramolecular aggregates of sugar-based perylene dyes: the effect of core-substituted groups.

Here we have shown that it is possible to modulate π-π stacking, hydrogen bonds and hydrophobic and hydrophilic interactions to further adjust the supramolecular helicity by simply changing the core-substituted group. These modulated supramolecular interactions provide a new pathway to the controlled production of helical superstructures.

Self-assembly of magnetite nanocubes into helical superstructures.

Organizing inorganic nanocrystals into complex architectures is challenging and typically relies on preexisting templates, such as properly folded DNA or polypeptide chains. We found that under carefully controlled conditions, cubic nanocrystals of magnetite self-assemble into arrays of helical superstructures in a template-free manner with >99% yield. Computer simulations revealed that the formation of helices is determined by the interplay of van der Waals and magnetic dipole-dipole interactions, Zeeman coupling, and entropic forces and can be attributed to spontaneous formation of chiral nanocube clusters. Neighboring helices within their densely packed ensembles tended to adopt the same handedness in order to maximize packing, thus revealing a novel mechanism of symmetry breaking and chirality amplification.

Chiral Liquid Crystals: Structures, Phases, Effects

The introduction of chirality, i.e., the lack of mirror symmetry, has a profound effect on liquid crystals, not only on the molecular scale but also on the supermolecular scale and phase. I review these effects, which are related to the formation of supermolecular helicity, the occurrence of novel thermodynamic phases, as well as electro-optic effects which can only be observed in chiral liquid crystalline materials. In particular, I will discuss the formation of helical superstructures in cholesteric, Twist Grain Boundary and ferroelectric phases. As examples for the occurrence of novel phases the Blue Phases and Twist Grain Boundary phases are introduced. Chirality related effects are demonstrated through the occurrence of ferroelectricity in both thermotropic as well as lyotropic liquid crystals. Lack of mirror symmetry is also discussed briefly for some biopolymers such as cellulose and DNA, together with its influence on liquid crystalline behavior.

Thermal‐ and Photo‐Induced Phase‐Transition Behaviors of a Tapered Dendritic Liquid Crystal with Photochromic Azobenzene Mesogens and a Bicyclic Chiral Center

A ribbon-shaped chiral liquid crystalline (LC) dendrimer with photochromic azobenzene mesogens and an isosorbide chiral center (abbreviated as AZ3 DLC) was successfully synthesized and its major phase transitions were studied by using differential scanning calorimetry (DSC) and linear polarized optical microscopy (POM). Its ordered structures at different temperatures were further identified through structure-sensitive diffraction techniques. Based on the experimental results, it was found that the AZ3 DLC molecule exhibited the low-ordered chiral smectic (Sm*) LC phase with 6.31 nm periodicity at a high-temperature phase region. AZ3 DLC showed the reversible photoisomerization in both organic solvents and nematic (N) LC media. As a chiral-inducing agent, it exhibited a good solubility, a high helical-twisting power, and a large change in the helical-twisting power due to its photochemical isomerization in the commercially available N LC hosts. Therefore, we were able to reversibly "remote-control" the colors in the whole visible region by finely tuning the helical pitch of the spontaneously formed helical superstructures.

Reversible Near-Infrared Light Directed Reflection in a Self-Organized Helical Superstructure Loaded with Upconversion Nanoparticles

Adding external, dynamic control to self-organized superstructures with desired functionalities is an important leap necessary in leveraging the fascinating molecular systems for applications. Here, the new light-driven chiral molecular switch and upconversion nanoparticles, doped in a liquid crystal media, were able to self-organize into an optically tunable helical superstructure. The resulting nanoparticle impregnated helical superstructure was found to exhibit unprecedented reversible near-infrared (NIR) light-guided tunable behavior only by modulating the excitation power density of a continuous-wave NIR laser (980 nm). Upon irradiation by the NIR laser at the high power density, the reflection wavelength of the photonic superstructure red-shifted, whereas its reverse process occurred upon irradiation by the same laser but with the lower power density. Furthermore, reversible dynamic NIR-light-driven red, green, and blue reflections in a single thin film, achieved only by varying the power density of the NIR light, were for the first time demonstrated.

Thermotropic Properties and Molecular Packing of Discotic Tristriazolotriazines with Rigid Substituents

Tristriazolotriazines with a threefold dialkoxyaryl substitution have been prepared by Huisgen reaction of cyanuric chloride and the corresponding tetrazoles. Although these dyes show a negative or inverted solvatochromism of the UV/Vis absorption, their fluorescence is strongly positive solvatochromic. These discotic fluorophores are also emissive in their solid state and in their broad liquid-crystalline mesophase. The structural study indicates that the thermotropic properties and organization of these systems can be well tuned by the steric demand of the aryl groups. Depending on the substituents, the compounds showed either a pure crystalline phase or a highly complex helical superstructure with a characteristic liquid-crystalline phase at elevated temperatures. Changing the steric demand of the attached aryls allowed controlling the discs arrangement within the columnar helix, which is of great importance for the molecular orbital overlap.

Complex superstructures in chiral liquid crystals: surface-induced helix destruction.

The influence of surface anchoring in thin chiral ferroelectric liquid crystals on helical superstructures imposed onto smectic layers is studied by means of a simple model allowing surface anchoring through the self-depolarization effect. It is shown that, for narrow temperature ranges close to temperatures of tilting phase transitions, these interactions can destroy helices, leading to the emergence of complex, intertwined undulatory, and helical substructures with different, mainly short, pitches. Regions with both undulated and helical orderings are demonstrated to reveal different sizes and random distributions along the smectic layer normal. Assuming values of material model parameters, typical for liquid crystals being close to tilting phase transitions, the free energy of thin helical smectic liquid crystal systems is determined as a function of two variables. One of them specifies the helix period of the appropriate bulk system (unperturbed by surfaces), while the second describes the interplay between the surface anchoring and the liquid crystal elasticity. It is shown that the free energy exhibits a very complicated behavior within variable regions for which the regular helices or complex superstructures occur. The resulting sensitivity of the free energy on changes of these variables corresponds to the appearance of a large variety of complex superstructures with coexisting helices of different periods. This gives an explanation of the occurrence of intricate superstructures and provides new insight into the interplay between molecular and surface interactions near the tilting phase transitions.

Self-Assembly of an Amphiphilic Designer-Peptide into Double Helical Superstructures

Self-assembling amphiphilic designer-peptides are used as building blocks for the development of controllable, tailor-made biomaterials. In solution, they self-assemble above a critical aggregation concentration into supramolecular structures, like vesicles, bilayers, twisted tapes, fibers or tubes. In this study we investigated the concentration- and time-dependent self-assembly of an 8-residue amphiphilic designer-peptide. We observed structural transitions from peptide monomers to elongated pairwise aligned tapes. Highly concentrated samples assembled into the first double helix superstructure that was observed within the class of amphiphilic designer-peptides so far. Synchrotron small angle X-ray scattering provided a detailed insight into the internal organization of the double helix. The obtained electron-density-profile suggested a 3-shell-model, mirrored at the central axis. Shell 1 (∼1 nm) and shell 2 (∼3.5 nm) together account for the peptide containing region, where the hydrophobic parts of the peptide monomers were interdigitated and tightly packed. In the innermost region we confirmed antiparallel stacking due to intermolecular hydrogen bonding by circular dichroism and infrared spectroscopy. Shell 3 spans around 12 nm and was assigned to a hydration shell where negatively charged trifluoroacetate counter ions preferably attach to the positively charged peptide headgroups. The total diameter of the double helix was 24 nm, with a repeating pitch distance of ∼60 nm. Cryo transmission electron microscopy supported the double helix morphology and revealed that their lengths extended to several hundreds of nanometers. The double helices were intertwined into a tight network. The resulting hydrogel properties may lead to promising future applications.

Photodynamic Chiral Molecular Switches with Thermal Stability: From Reflection Wavelength Tuning to Handedness Inversion of Self‐Organized Helical Superstructures

A good turn: Three compounds that bear two axially chiral bridged binaphthyl units were developed as photodynamic chiral dopants for nematic liquid crystals. For compounds with suitable bridge lengths, a change in the dihedral angle induced a switch of the binaphthyl units from the cisoid to the transoid form upon UV irradiation, which led to an inversion of the handedness of the helices.

Photodynamic Chiral Molecular Switches with Thermal Stability: From Reflection Wavelength Tuning to Handedness Inversion of Self-Organized Helical Superstructures

Photodynamic Chiral Molecular Switches with Thermal Stability: From Reflection Wavelength Tuning to Handedness Inversion of Self-Organized Helical Superstructures

Single-Handed Helical Wrapping of Single-Walled Carbon Nanotubes by Chiral, Ionic, Semiconducting Polymers

We establish the requisite design for aryleneethynylene polymers that give rise to single-handed helical wrapping of single-walled carbon nanotubes (SWNTs). Highly charged semiconducting polymers that utilize either an (R)- or (S)-1,1'-bi-2-naphthol component in their respective conjugated backbones manifest HRTEM and AFM images of single-chain-wrapped SWNTs that reveal significant preferences for the anticipated helical wrapping handedness; statistical analysis of these images, however, indicates that ∼20% of the helical structures are formed with the "unexpected" handedness. CD spectroscopic data, coupled with TDDFT-based computational studies that correlate the spectral signatures of semiconducting polymer-wrapped SWNT assemblies with the structural properties of the chiral 1,1'-binaphthyl unit, suggest strongly that two distinct binaphthalene SWNT binding modes, cisoid-facial and cisoid-side, are possible for these polymers, with the latter mode responsible for inversion of helical chirality and the population of polymer-SWNT superstructures that feature the unexpected polymer helical wrapping chirality at the nanotube surface. Analogous aryleneethynylene polymers were synthesized that feature a 2,2'-(1,3-benzyloxy)-bridged (b)-1,1'-bi-2-naphthol unit: this 1,1'-bi-2-naphthol derivative is characterized by a bridging 2,2'-1,3 benzyloxy tether that restricts the torsional angle between the two naphthalene subunits along its C1-C1' chirality axis to larger, oblique angles that facilitate more extensive van der Waals contact of the naphthyl subunits with the nanotube. Similar microscopic, spectroscopic, and computational studies determine that chiral polymers based on conformationally restricted transoid binaphthyl units direct preferential facial binding of the polymer with the SWNT and thereby guarantee helically wrapped polymer-nanotube superstructures of fixed helical chirality. Molecular dynamics simulations provide an integrated picture tying together the global helical superstructure and conformational properties of the binaphthyl units: a robust, persistent helical handedness is preferred for the conformationally restricted transoid binaphthalene polymer. Further examples of similar semiconducting polymer-SWNT superstructures are reported that demonstrate that the combination of single-handed helical wrapping and electronic structural modification of the conjugated polymer motif opens up new opportunities for engineering the electro-optic functionality of nanoscale objects.

Origins of the Helical Wrapping of Phenyleneethynylene Polymers about Single-Walled Carbon Nanotubes

The highly charged, conjugated polymer poly[p-{2,5-bis(3-propoxysulfonicacidsodiumsalt)}phenylene]ethynylene (PPES) has been shown to wrap single-wall carbon nanotubes (SWNTs), adopting a robust helical superstructure. Surprisingly, PPES adopts a helical rather than a linear conformation when adhered to SWNTs. The complexes formed by PPES and related polymers upon helical wrapping of a SWNT are investigated using atomistic molecular dynamics (MD) simulations in the presence and absence of aqueous solvent. In simulations of the PPES/SWNT system in an aqueous environment, PPES spontaneously takes on a helical conformation. A potential of mean force, ΔA(ξ), is calculated as a function of ξ, the component of the end-to-end vector of the polymer chain projected on the SWNT axis; ξ is a monotonic function of the polymer's helical pitch. ΔA(ξ) provides a means to quantify the relative free energies of helical conformations of the polymer when wrapped about the SWNT. The aqueous system possesses a global minimum in ΔA(ξ) at the experimentally observed value of the helical pitch. The presence of this minimum is associated with preferred side chain conformations, where the side chains adopt conformations that provide van der Waals contact between the tubes and the aliphatic components of the side chains, while exposing the anionic sulfonates for aqueous solvation. The simulations provide a free energy estimate of a 0.2 kcal/mol/monomer preference for the helical over the linear conformation of the PPES/SWNT system in an aqueous environment.

Organotitanium phosphates with free P–OH groups: Synthesis, spectroscopy and solid state structures

Titanocene dichloride reacts with 2,6-diisopropylphenyl phosphate (dippH2) in tetrahydrofuran to yield a functionalized mononuclear titanium phosphate molecule [Cp2Ti(dippH)2] (1) in 70% yield. A similar reaction with titanium bis(acetylacetonate)diisopropoxide [Ti(acac)2(OiPr)2] yields a tetranuclear cluster [Ti4(μ2-OH)2(OiPr)2(dipp)4(dippH)4(THF)2] (2) in 70% yield. Owing to the presence of free P–OH groups in 1, its reactivity towards aluminum alkyls has been investigated. Compounds 1 and 2 have been characterized with the aid of elemental analysis, mass, IR and NMR (1H and 31P) spectroscopy. In spite of the poor quality of the crystals obtained, the solid-state structures of compounds 1 and 2 have been determined by single crystal X-ray diffraction studies to a satisfactory level. Compound 1 crystallizes as hydrogen bonded chiral helical superstructures in the monoclinic non-centrosymmetric P21 space group. Compound 2 is a discrete tetranuclear titanophosphate cluster which is held together by eight phosphate ligands.

Azoarenes with Opposite Chiral Configurations: Light‐Driven Reversible Handedness Inversion in Self‐Organized Helical Superstructures

Azoarenes with Opposite Chiral Configurations: Light‐Driven Reversible Handedness Inversion in Self‐Organized Helical Superstructures

Azoarenes with Opposite Chiral Configurations: Light‐Driven Reversible Handedness Inversion in Self‐Organized Helical Superstructures

On the other hand: Azoarene compounds with axially chiral binaphthyl units of the same and opposite chiral configurations were doped into achiral liquid crystals (LCs). They were found to efficiently induce self-organized helical superstructures, which could be reversibly tuned by light irradiation using trans–cis photoisomerization to change the handedness of the helix (see scheme) in LC hosts.

Synthesis and mesomorphism of new chiral imines and copper(II) complexes

The synthesis, characterization and mesomorphic properties of a series of N-(2-hydroxy-4-n-alkyloxybenzylidene) 4′-amino-2-methylbutylcinnamate and their copper(II) complexes are reported. The imines exhibited smectic A (SmA) and chiral smectic C (SmC*) phases while their copper(II) complexes displayed SmA–SmC phase variant. The compounds have been characterized by elemental analysis and spectroscopic studies. The phase transition temperatures were detected by differential scanning calorimetry (DSC) analysis and the phases are confirmed by polarizing optical microscopy (POM). The chiral phase (SmC*) of the ligand disappeared on complexation. The unwinding of the helical superstructure of SmC* phase under the influence of electric field and the observed optical textures of SmC* phase are also discussed.

Oscillatory polarons generation by near IR and spin induced chirality studies in optically nonlinear 1,3-dinitrobenzene crystal

Paramagnetic species in the 1,3-dinitrobenzene (mDNB) crystal have been detected by EPR spectroscopy and the intervalence (conductivity) band at ∼10,400cm−1 along the polar c axis, and the band at ∼7400cm−1 along the a axis, of unknown origin, have been found in its polarized NIR spectra. The radical anions occurrence and supposed helical superstructure formed due to nitro groups interactions have induced the chirality of solid mDNB confirmed by the CD spectra record and ab initio quantum chemical calculations. The several times irradiation of mDNB crystal samples by the 980nm and 1550nm laser diode beams have revealed temporal fluctuations of radical anions concentration (EPR) and of NIR band intensities, as well as CD spectra changes. An autocatalytic reduction by NIR is supposed to be responsible for the observed fluctuations. The proposed conclusions were reinforced by the density functional (DFT) calculations of electronic properties (ionization energy, electron affinity, dipole moment, hyperpolarizability) of mDNB neutral and charged clusters formed along the hypothetical helix and by the calculation of crystal band structure. The UV–Vis–NIR absorption and CD spectra of mDNB molecular systems were computed at non-empirical all-electron level by DFT method within the correlation consistent 6-311G**(2d,2p) basis set and hybrid B3LYP density functional.

Self-assembly Behavior of Interlocked Helical Supramolecule of Cyclodextrin Modified by 2-Furanmethanethiol

The mono-modified β-cyclodextrin, mono-[6-S-6-(2-methylfuran)]-β-cyclodextrin, was synthesized by the substituting 2-furanmethanethiol for toluenesulfonyl group at the primary rim of β-cyclodextrin. Its self-assembly behavior was measured in both solution and the solid state by X-ray crystallography and H NMR spectroscopy. In the crystal structure, the complex, C47H106O51S, crystallizes in the orthorhombic space group P212121. The furan group is located above the primary rim of β-cyclodextrin and stretches slantwise along the side wall of β-cyclodextrin with the dihedral angle of 104.4° between furan ring and the plane of glycosidic oxygen atoms O(4) of β-cyclodextrin. The furan group is inserted deeply into the hydrophobic cavity of the adjacent β-cyclodextrin from the second hydroxyl rim and makes an angle of 67.6° with the O(4) atoms plane of the adjacent β-cyclodextrin. The dihedral angle of the O(4) atoms plane between the adjacent β-cyclodextrin is 38.4°. The consequence is the formation of an interlocked helical columnar superstructure formed by the self-assembly of the complex molecules in which the modified cyclodextrins are stacked along a two-fold screw axis parallel to the a crystal axis. Thus each molecule behaves both a host and a guest molecule. And the position and orientation of the furan ring within the cyclodextrin cavity is determined by host guest interactions which include the van der Waals contacts and hydrogen bonds between the furan ring and cyclodextrin. The interlocked helical column is stabilized by the hydrogen bonds formed between the primary and secondary hydroxyl groups of the adjacent cyclodextrin or through intervening water molecules. Furthermore, ROESY data indicate that the furan ring is included in the cyclodextrin cavity, which is in accordance with the conformation of the solid state structure. H NMR concentration dependent studies show that the complex molecule forms a dimer at concentrations of >10 mol•L. And the effective binding constant K and the aggregation number n were calculated to be 450 mol•L and 1.9, respectively.

Responsive biomimetic networks from polyisocyanopeptide hydrogels

Mechanical responsiveness is essential to all biological systems down to the level of tissues and cells. The intra- and extracellular mechanics of such systems are governed by a series of proteins, such as microtubules, actin, intermediate filaments and collagen. As a general design motif, these proteins self-assemble into helical structures and superstructures that differ in diameter and persistence length to cover the full mechanical spectrum. Gels of cytoskeletal proteins display particular mechanical responses (stress stiffening) that until now have been absent in synthetic polymeric and low-molar-mass gels. Here we present synthetic gels that mimic in nearly all aspects gels prepared from intermediate filaments. They are prepared from polyisocyanopeptides grafted with oligo(ethylene glycol) side chains. These responsive polymers possess a stiff and helical architecture, and show a tunable thermal transition where the chains bundle together to generate transparent gels at extremely low concentrations. Using characterization techniques operating at different length scales (for example, macroscopic rheology, atomic force microscopy and molecular force spectroscopy) combined with an appropriate theoretical network model, we establish the hierarchical relationship between the bulk mechanical properties and the single-molecule parameters. Our results show that to develop artificial cytoskeletal or extracellular matrix mimics, the essential design parameters are not only the molecular stiffness, but also the extent of bundling. In contrast to the peptidic materials, our polyisocyanide polymers are readily modified, giving a starting point for functional biomimetic hydrogels with potentially a wide variety of applications, in particular in the biomedical field.