Rigid Aromatic Core Research Articles

Symmetrical liquid crystalline dimers containing azo and azomethine linkages: synthesis, characterisation, mesomorphic study and DFT studies

ABSTRACT In the current study, one new series of symmetrical liquid crystalline homologous series of dimers with a rigid aromatic core with azo and azomethine group were synthesized using an ether-linking group. Several spectroscopic methods, including FT-IR, 1H-NMR, 13C-NMR and elemental analysis, were used to describe the synthesised dimers. The differential scanning calorimetry (DSC) and polarised optical microscopy (POM) were used to assess their mesomorphic properties. The results showed that the number of methylene spacers significantly affected the thermal properties of the synthesized dimers. To ascertain photophysical behaviour, UV-visible spectroscopy was employed. All the dimers of the present series show excellent enantiotropic nematic mesophase. The relationships between structure and its mesogenic property, the effects of various flexible spacers, the mesophase ranges, and the photo-isomerization of liquid crystalline dimers were all investigated. DFT study was carried out to gain a better understanding of structural features.

Microporous, Crystalline, and Water‐Processable Framework Materials of Organic Amphiphile Salts

AbstractPorous framework materials are of major importance for a wide range of technologies. Nevertheless, many of these materials lack processibility as they are typically synthesized under rather harsh conditions and obtained as microcrystalline powders that cannot easily be coated or deposited from solution. Herein, a new approach to water‐processable metal–organic framework materials is presented. The materials are based on amphiphilic organic building blocks consisting of polar carboxylate groups and non‐polar alkyl chains connected to a rigid aromatic core. The amphiphilic building blocks assemble to porous framework structures via bonding to kinetically labile sodium ions from concentrated aqueous solution. The obtained crystalline materials, termed amphiphile salt frameworks , are thermally and mechanically stable (some derivatives up to 365 °C and up to at least 4000 bar hydrostatic pressure), exhibit persistent microporous channels accessible to several gases (N2, CO2, propane, propylene, n‐butane), and can be reversibly assembled/disassembled by crystallization from or dissolution in water. Systematic variation of the hydrophobic side chains of the amphiphile building blocks allows extracting structure‐property relationships and first design rules for this new class of water‐processable microporous framework materials.

Phosphorus core-shell tecto dendrimers for enhanced tumor imaging: the rigidity of the backbone matters.

Nanoplatforms with amplified passive tumor targeting and enhanced protein resistance can evade unnecessary uptake by the reticuloendothelial system and achieve high tumor retention for accurate tumor theranostics. To achieve this goal, we here constructed phosphorus core-shell tecto dendrimers (CSTDs) with a rigid aromatic backbone core as a nanoplatform for enhanced fluorescence and single-photon emission computed tomography (SPECT) dual-mode imaging of tumors. In this study, the phosphorus P-G2.5/G3 CSTDs (G denotes generation) were partially conjugated with tetraazacyclododecane tetraacetic acid (DOTA), cyanine5.5 (Cy5.5) and 1,3-propane sulfonate (1,3-PS) and then labeled with 99mTc. The formed P-G2.5/G3-DOTA-Cy5.5-PS CSTDs possess good monodispersity with a particle size of 10.1 nm and desired protein resistance and cytocompatibility. Strikingly, compared to the counterpart material G3/G3-DOTA-Cy5.5-PS with both the core and shell components being soft poly(amidoamine) dendrimers, the developed P-G2.5/G3-DOTA-Cy5.5-PS complexes allow for more efficient cellular uptake and more significant penetration in 3-dimensional tumor spheroids in vitro, as well as more significant tumor retention and accumulation for enhanced dual-mode fluorescence and SPECT (after labelling with 99mTc) tumor imaging in vivo. Our studies suggest that the rigidity of the core for the constructed CSTDs matters in the amplification of the tumor enhanced permeability retention (EPR) effect for improved cancer nanomedicine development.

The effect of amide bond orientation and symmetry on the self-assembly and gelation of discotic tripeptides.

A series of discotic tripeptides containing a rigid aromatic core and l-phenylalanine have been developed. The orientation of the amide bonds yielded variations of the structure and self-assembly properties of the compounds. The aggregation behavior of the discotic tripeptides was studied by various spectroscopic techniques. The morphology of the resulting aggregates was studied by field emission electron microscopy and atomic force microscopy. These studies showed that the orientation of the amide bonds has a strong influence on the intermolecular interactions, resulting in huge differences in the aggregation properties, and morphology of the discotic tripeptides. Only the C3-symmetric discotic tripeptides formed organogels. The supramolecular aggregation mechanism of N-centered and C[double bond, length as m-dash]O-centered discotic tripeptides for forming 3-fold intermolecular H-bonded helical column were the same, there was only a smaller enthalpy change due to the occurrence of longer distances for the N-HO[double bond, length as m-dash]C bonds of the N-centered discotic tripeptide. Whereas, the C2-symmetric discotic tripeptides 2 and 3 adopted a 6-fold intermolecular H-bonded dimer structure. Thus, this report presents a valuable approach for the fine-tuning of the discotic tripeptide based functional material.

Influence of the Growing Flexible Shell on the Molecular Behavior of Hybrid Dendrimers

A versatile approach to the synthesis of hybrid dendrimers composed of a rigid aromatic core and a flexible carbosilane shell is proposed. For this, carbosilane monodendrons of the first, second, a...

(Invited) Design and Fabrication of Organic Thin Film Transistors Using Solution-Processable Liquid Crystalline Phthalocyanine Derivatives

Phthalocyanines (Pcs) are an extremely non-toxic organic chromophores with a range of versatile range applications from industrial pigments to photodynamic agents in cancer therapy and photosensitizers in photocopiers and compact discs. The central atom is coordinated with four pyrrolic nitrogen atoms. The conjugated π-systems of eighteen electrons in the macrocyclic ring gives rise to unique semiconducting properties which can be tuned by substituents of alkyl, alkoxy, or alkoxymethyl chains at sixteen sites on the benzenoid rings and axial ligation sites at the central metal or metalloid. Unsubstituted Pcs are generally both thermally and chemically stable but ring substituents such as SO3H and aliphatic chains confer solubility in water and organic solvents, respectively. They can form liquid crystalline phases. The present talk will highlight the achievements in application of phthalocyanine layers in different electronic devices such as field effect transistors (OFET) and memory devices taking into account literature data and our experimental results. Special attention will be focused to liquid crystalline phthalocyanines. Disk-like molecules, typically comprising a flat rigid aromatic core and flexible peripheral substituents, self-organizes into one-dimensional "supramolecular wires". Due to high charge carrier mobility along the columns, discotic mesogens are considered as realistic candidates for application in organic electronic devices. The well-ordered films with controllable molecule alignment can be obtained by slow cooling from isotropic liquid temperature.

Aldehyde group driven aggregation-induced enhanced emission in naphthalimides and its application for ultradetection of hydrazine on multiple platforms.

Restriction of intramolecular motion (RIM) of rigid aromatic cores is the most universal mechanism so far that can successfully explain almost all AIE/AIEEgenic systems. By designing two novel naphthalimide derivatives (NIB and NIBD), we experimentally demonstrated the effect of a single formyl group that can efficiently transform an aggregation caused quenching (ACQ) system (NIB) into an AIEEgenic system (NIBD) by strengthening the RIM process. Besides, the newly designed naphthalimide AIEEgen (NIBD) accomplished ultrasensitive detection of hydrazine at the parts per trillion level (LOD/81 ppt) in aqueous media with high selectivity and enormous improvement over the existing state of the art. An exceptional sensitivity is also achieved in the vapor phase (LOD/0.003%) using a Whatman paper strip based portable device for simple and cost-effective on-site detection. The detection mechanism involved a reaction-based spontaneous formation of a non-fluorescent hydrazone Schiff base derivative (NIBDH). The in vitro potentiality of the AIEEgenic probe was also demonstrated in two mammalian cell lines i.e. HeLa (human cervical cancer cell line) and HEK293T (Human embryonic kidney cell line that expresses a mutant version of the SV40 large T antigen). Owing to the highly selective formation of the hydrazone Schiff base complex with hydrazine, NIBD responds to the existence of hydrazine in both these cell lines without any interference from other biologically rich metal ions and amino acids. These outcomes could initiate a much wider use of formyl group induced condensed state emission and a key hypothesis that could generate newer avenues for ACQ to AIEE transformations for several practical applications including hydrazone Schiff base complexation for probing and manipulating hydrazine biology associated with several metabolic activities.

Self-Assembly of X-Shaped Bolapolyphiles in Lipid Membranes: Solid-State NMR Investigations.

A novel class of rigid-rod bolapolyphilic molecules with three philicities (rigid aromatic core, mobile aliphatic side chains, polar end groups) has recently been demonstrated to incorporate into and span lipid membranes, and to exhibit a rich variety of self-organization modes, including macroscopically ordered snowflake structures with 6-fold symmetry. In order to support a structural model and to better understand the self-organization on a molecular scale, we here report on proton and carbon-13 high-resolution magic-angle spinning solid-state NMR investigations of two different bolapolyphiles (BPs) in model membranes of two different phospholipids (DPPC, DOPC). We elucidate the changes in molecular dynamics associated with three new phase transitions detected by calorimetry in composite membranes of different composition, namely, a change in π-π-packing, the melting of lipid tails associated with the superstructure, and the dissolution and onset of free rotation of the BPs. We derive dynamic order parameters associated with different H-H and C-H bond directions of the BPs, demonstrating that the aromatic cores are well packed below the final phase transition, showing only 180° flips of the phenyl ring, and that they perform free rotations with additional oscillations of the long axis when dissolved in the fluid membrane. Our data suggests that BPs not only form ordered superstructures, but also rather homogeneously dispersed π-packed filaments within the lipid gel phase, thus reducing the corrugation of large vesicles.

Self-Assembly of Four-Claw Discotic Mesogenic Molecules: Influence of Core on Chirality

The morphology of self-assembled monolayers determined by molecular structure and involved interactions plays a crucial role in their properties. Herein, we report a study on 2D self-assembly of two kinds of triphenylene-submitted discotic mesogenic species, which have similar four TP moieties with alkyne spacer but different rigid aromatic cores pyrene and carbazole. Two types of stable periodic long-ranged supramolecular networks on the highly oriented pyrolytic graphite surface have been visualized via high-resolution scanning tunneling microscopy technique. A comparative study reveals how the aromatic core of molecular building blocks affects achiral and chiral arrangement of discotic molecule on surface. The asymmetry backbone results in molecular orientation and varied van der Waals forces between molecule–molecule and molecule–substrate. Our results demonstrate that design of functional molecules plays an important role in the construction of 2D supramolecular assembly possessing desirable structur...

Conformational Properties and Orientational Order of a de Vries Liquid Crystal Investigated through NMR Spectroscopy

Solid-state and liquid-state NMR spectroscopic techniques are used to describe at molecular level the behaviour of a de Vries liquid crystal (namely the mesogen 9HL) at the SmA-SmC* transition, which is characterized by the absence of the layer shrinkage, typical of non-de Vries smectogens. Previous (2)H NMR studies on the same smectogen, performed at a different magnetic field (from 4.70 to 18.80 T), provided evidence of the occurrence of a tilt of one of the three phenyl rings, constituting the aromatic core of 9HL, at the SmA-SmC* phase transition. In this work, the study is extended to the whole rigid aromatic core of the 9HL. In particular, the variable temperature behavior of the mesogen studied by 1D (13)C NMR cross-polarization (CP) and 2D (1)H-(13)C PDLF (proton-encoded (13)C-detected, local field) NMR experiments made possible the characterization of the conformational and orientational properties in the two smectic phases. These results are compared with various proposed models invoked to describe the SmA-SmC* transition in de Vries smectogens at a molecular level.

Order Induced Charge Carrier Mobility Enhancement in Columnar Liquid Crystal Diodes

Discotic molecules comprising a rigid aromatic core and flexible side chains have been promisingly applied in OLEDs as self-organizing organic semiconductors. Due to their potentially high charge carrier mobility along the columns, device performance can be readily improved by proper alignment of columns throughout the bulk. In the present work, the charge mobility was increased by 5 orders of magnitude due to homeotropic columnar ordering induced by the boundary interfaces during thermal annealing in the mesophase. State-of-the-art diodes were fabricated using spin-coated films whose homeotropic alignment with formation of hexagonal germs was observed by polarizing optical microscopy. The photophysical properties showed drastic changes at the mesophase-isotropic transition, which is supported by the gain of order observed by X-ray diffraction. The electrical properties were investigated by modeling the current-voltage characteristics by a space-charge-limited current transport with a field dependent mobility.

Solid State Assemblies and Photophysical Characteristics of Linear and Bent-Core π-Conjugated Oligophenylenevinylenes

New classes of luminescent linear, bent-core, and star-shaped oligophenylenevinylenes (OPVs) having 1,4-para and 1,3-meta rigid aromatic cores were designed and developed. 3-Pentadecylphenol, a renewable resource molecule, was chosen as the flexible unit at the longitudinal or middle position of the OPV aromatic core for solid state ordering. Depending upon the nature of the π-core, the OPVs exhibited either mosaic-type liquid crystalline textures or spherulitic crystalline solids. The enthalpies of melting transitions revealed that the bent-core OPV structure showed enhanced solid state packing compared to linear or star-shaped OPVs. Small and wide-angle X-ray diffraction analysis confirmed layered-like assemblies in OPV molecules. Photophysical experiments such as excitation, emission, and time-resolved fluorescence decay dynamics were carried out to trace the molecular self-organization of OPV chromophores. Time correlated single photon counting technique (TCSPC) luminescent decay profiles and decay lifetimes (τ1 and τ2 values) revealed that the OPV chromophores showed faster exciton decay in the tightly packed bent-core structure. The weakly packed star-shaped OPV showed enhanced excited state luminescence stability up to 10 ns. A direct correlation between the OPV chemical structure, solid state ordering, and photophysical characteristics was established.

Amphiphilic π-Conjugated Poly(m-phenylene) Photosensitizer for the Eu3+ Ion: The Role of Macromolecular Chain Aggregation on the Color Tunability of Lanthanides

Here, we report new carboxylic functionalized poly(phenylene)s and their oligomers as selective and efficient photosensitizers for Eu(3+) ions. Palladium-catalyzed Suzuki polycondensation was developed for the synthesis of carboxylic functionalized π-conjugated materials. The chemical structures of the polymer skeleton were varied using two anchoring groups consisting of ethylhexyloxy and methoxy substitution in the chain backbone. The molecular weights of the polymer samples were obtained in the range of 4000-8000 containing 20 aromatic units in the chain. Photoexcitation of the oligomer-Eu(3+) complexes resulted in magenta color emission as a result of the combination of partial blue self-emission from the chromophores along with the red color from the metal center. The ethylhexyl substituted polymer-Eu(3+) complex showed complete excitation energy transfer from the macromolecular chains to the metal center and produced bright and sharp red emission. The polymer-containing methoxy unit was found to show largely self-emission rather than photoexcitation to the metal center. Singlet and triplet energy levels of the complexes and chromophores revealed that both oligomers and polymers have almost identical energy levels for photosensitizing Eu(3+) ions. The polymers possessed typical amphiphilic structures via a rigid aromatic hydrophobic core and hydrophilic anionic periphery for self-organization in water. Both dynamic light scattering and atomic force microscope analysis confirmed the existence of the spherical shape nanometer size aggregates of the polymer chains in water. The branched ethylhexyl polymer showed the formation of loosely packed 500 nm aggregates whereas tightly packed 200 nm particles are produced by the methoxy substituted rigid polymer. These molecular aggregates behaved as templates for complexation as well as photosensitizing of the Eu(3+) ions. The loosely packed nanoaggregates (ethylhexyl polymer) contain Eu(3+) ions in the entire scaffold and showed efficient and complete energy transfer from the conjugated chain to metal ions. The tightly packed rigid-chains in methoxy polymer restricted the complete energy transfer to metal center. The molecular self-organization of the polymers played a crucial role on the efficient energy transfer from the polymer chain to metal center, more specifically Eu(3+) ion-based red emission.

Synthesis and characterisation of photopolymerisable liquid crystals based on the π-extended fluorene core and their corresponding non-reactive analogues

We have synthesised a series of new reactive mesogens with photopolymerisable di-acrylates and their corresponding non-reactive analogues based on the π-conjugated aromatic core, fluorene (F)-di-[thiophene (T)-benzene (B)], using the Stille and Suzuki coupling reaction. The effect of lateral alkyl chains on the 9-position of the central fluorene moiety as well as α, ω- side alkyl chains attached to the π-conjugated aromatic core on the mesomorphism was investigated by utilising differential scanning calorimetry (DSC) and polarising optical microscopy (POM). A wide angle X-ray scattering (WAXS) study at the various temperatures was also carried out to reveal phase structures. Photopolymerisable di-acrylates connected directly to the rigid aromatic core showed higher phase transition temperatures, probably due to the induced dipole moment in comparison with those of a non-reactive methyl–ether counterpart.

ChemInform Abstract: Ferroelectric, Ferrielectric, and Antiferroelectric Properties in the ( R)- and (S)-1-Methylalkyl 4′-(4′′-n-Alkoxybenzoyloxy)biphenyl-4- carboxylate Liquid Crystals.

A series of chiral esters has been prepared that exhibits unusual liquid-crystalline properties. In this family of materials the general molecular structure is one where a rigid aromatic core has two peripheral aliphatic chains attached to it, one of which carries a chiral carbon atom adjacent to the core itself. The liquid-crystalline properties were investigated as a function of terminal-aliphatic-chain length for changes to both peripheral groups. It was found that when the terminal chain that carries the asymmetric centre was increased in length, thereby increasing the chirality of the system, ferroelectric, ferrielectric, and antiferroelectric properties were observed.

Molecular Reorganization of Paired Assemblies of T-Shaped Rod−Coil Amphiphilic Molecule at the Air−Water Interface

A T-shaped aromatic amphiphilic molecule based on linear oligo(ethylene oxide) was synthesized. We suggest that its peculiar interfacial behavior at the air-water interface and the structure of the Langmuir-Blodgett monolayer are associated with its peculiar T-shape and competing steric and amphiphilic interactions at different surface pressures. At low surface pressure, uniform and smooth monolayers were formed. Upon compression, the molecular reorganization from spherical to cylindrical transformation occurred, as caused by the submerging of the oligo(ethylene oxide) chains, providing for efficient pi-pi interactions of the central core. At the highest surface pressure, the monolayer collapses into bilayer domains, following a bicontinuous network formation which tends to transform into a perforated film. The unique shape of T-like rigid aromatic cores makes their structural reorganization very peculiar with paired, dimerlike molecular packing dominating in gas and solid states. This paired aggregation is so strong that it is preserved in the course of flipping and formation of vertically oriented backbones.

Effect of Chemical Structure and Composition of the Resin Phase on Vinyl Conversion of Amorphous Calcium Phosphate-filled Composites.

The objective of this study was to elucidate the effect of chemical structure and composition of the polymer matrix on the degree of vinyl conversion (DC) of copolymers (unfilled resins) and their amorphous calcium phosphate (ACP) composites attained upon photo-polymerization. The DC can also be an indicator of the relative potential of these polymeric materials to leach out into the oral environment un-reacted monomers that could adversely affect their biocompatibility. The following resins were examined: 1) 2,2-bis[p-(2'-hydroxy-3'-methacryloxypropoxy)phenyl]propane (Bis-GMA)/triethylene glycol dimethacrylate (TEGDMA) (1:1 mass ratio; BT resin) combined with hydroxyethyl methacrylate (HEMA; BTH resin) and with HEMA and zirconyl dimethacrylate (BTHZ resin), 2) urethane dimethacrylate (UDMA)/HEMA resins, and 3) pyromellitic glycerol dimethacrylate (PMGDMA)/TEGDMA (PT resin). To make composite specimens, resins were mixed with a mass fraction of 40 % zirconia-hybridized ACP. Copolymers and their composites were evaluated by near infra-red spectroscopy for DC after 1 d and 28 d post-cure at 23 °C. Inclusion of HEMA into the BT and UDMA resins yielded copolymers and composites with the highest DCs. The significantly lower DCs of PT copolymers and their composites are attributed to the rigid aromatic core structure, tetra-vinyl functionality and limited methacrylate side-chain flexibility of the surface-active PMGDMA monomer. There was, however, an increase in the 28 d DC for the PT materials as there was for the BTHZ system. Surprisingly, the usual decrease observed in DC in going from unfilled polymer to composite was reversed for the PT system.

Discotic liquid crystals: a new generation of organic semiconductors

Discotic (disc-like) molecules typically comprising a rigid aromatic core and flexible peripheral chains have been attracting growing interest because of their fundamental importance as model systems for the study of charge and energy transport and due to the possibilities of their application in organic electronic devices. This critical review covers various aspects of recent research on discotic liquid crystals, in particular, molecular design concepts, supramolecular structure, processing into ordered thin films and fabrication of electronic devices. The chemical structure of the conjugated core of discotic molecules governs, to a large extent, their intramolecular electronic properties. Variation of the peripheral flexible chains and of the aromatic core is decisive for the tuning of self-assembly in solution and in bulk. Supramolecular organization of discotic molecules can be effectively controlled by the choice of the processing methods. In particular, approaches to obtain suitable macroscopic orientations of columnar superstructures on surfaces, that is, planar uniaxial or homeotropic alignment, are discussed together with appropriate processing techniques. Finally, an overview of charge transport in discotic materials and their application in optoelectronic devices is given.

Electronic Charge Transport in Extended Nematic Liquid Crystals

We report a systematic study of charge transport in a range of low-molar-mass and extended (having at least six aromatic rings) nematic liquid crystals, some of which are reactive mesogens, with a high degree of shape anisotropy, i.e., the length-to-width (aspect) ratio is exceptionally high. We demonstrate that the hole mobility is independent of the macroscopic, but not microscopic, ordering of the nematic and isotropic phases of these nematic liquid crystals with a long, rigid, and extended aromatic molecular core, because no discontinuity is observed at the transition between these phases. A room-temperature mobility of up to 1.0 × 10-3 cm2 V-1 s-1 is obtained in the nematic phase, which is attributed to the short intermolecular distances between the highly polarizable but rigid long aromatic cores. We show that the intermolecular separation can be easily fine-tuned by changing the lateral and terminal aliphatic groups of these nematic liquid crystals. Hence, the charge mobility can be varied by up to 2 orders of magnitude without altering the core structure of the molecules, and this chemical fine control could be used to limit hole transport and so provide better charge balance in organic light-emitting diodes. X-ray diffraction is used to obtain the intermolecular separation and shows local lamellar order in the nematic phase.

Alkyl Tail Length Dependence of Structures in a Series of Symmetrically Tapered Bisamides Exhibiting Self‐Assembled Supramolecular Columnar Phases

Previously, we synthesized a series of symmetrically tapered 1,4‐bis[3,4,5‐tris(alkan‐1‐yloxy)benzamido] benzene bisamides (CnPhBA, where n is the number of carbon atoms in the alkyl chains, n=10, 12, 14, and 16). These bisamides consist of a rigid aromatic core with three flexible alkyl chains at each end of the core. It was found that all of these CnPhBA bisamides formed a highly ordered oblique columnar (ΦOK) phase and a low‐ordered oblique columnar (ΦOB) phase.[1,2] To study the dependence of structure on alkyl chain length in this series of symmetrically tapered bisamides, we focus on the structural and phase transitions of CnPhBA (n=6 and 8) via differential scanning calorimetry, one‐dimensional (1D) and 2D wide angle X‐ray diffraction (WAXD) and Fourier transform infrared spectroscopy. Structural evolutions and phase identifications were also supported by texture changes observed by polarized optical microscopy. It was interesting that there was only one ordered structure in C6PhBA which contains the shortest alkyl tails in this series of bisamide. Its 2D lattice (a=1.94 nm, b=3.43 nm, and γ=63.5°) was different from the samples with longer alkyl chains (n=10, 12, 14, and 16, n≥10, even).[1,2] However, the alkyl tail length did not have a significant influence on the packing along the column (c=0.96 nm). No liquid crystalline (LC) columnar phase was observed. When the length of alkyl tails contains eight carbons (C8PhBA), there was a sudden jump in the isotropization temperature of its LC phase compared with that of molecules with longer alkyl tails (n≥10, even).[1,2] Dramatic structure and phase behavior differences between C8PhBA and other CnPhBA bisamides were observed. This finding indicated that the C8PhBA having eight carbons in its alkyl tails represents a transition from a weak nano‐phase separation effect of the alkyl chains with respect to the rigid cores in CnPhBA with n≤6 (even) to a strong nano‐phase separation effect in CnPhBA (n≥10, even). On the occasion of Prof. Phil Geil's 75th birthday for his pioneering contributions in polymer crystal physics.