Thin films of discotic liquid crystals and their applications

ABSTRACT

Discotic liquid crystals (DLCs) are considered as fascinating systems due to their unique property of self-assembly to yield different columnar structures. DLCs are organic semiconductors and create pathways for the development of numerous optical and electrical devices. The thin films of DLCs can be considered as low dimensional system which can exhibit remarkable optical and physical properties. In this article, we present a review on ultrathin films of some interesting DLC molecules at air–water and air–solid interfaces. The Langmuir monolayer and Langmuir–Blodgett films of DLC molecules are extensively studied. The ultrathin films of DLC molecules can yield highly anisotropic layer wherein the molecular orientation and aggregation can have large impact on the physicochemical properties of the film. Different surface phases with different molecular orientations as function of surface density and temperature can be obtained by forming the Langmuir monolayer of the DLC molecules at the air–water interface. The Langmuir monolayer in a particular phase can be deposited onto the active area of a device layer-by-layer by employing a highly controlled Langmuir–Blodgett technique. Here, we report some interesting results related on molecular orientation of the DLC molecules at different interfaces. Such aggregation of DLC molecules in ultrathin films may find applications in thin film-based electro-optical devices.     

Triphenylene-based discotic liquid crystals: recent advances

Since the early work of Chandrasekhar and his co-workers on hexaesters of benzene published in 1977, discotic liquid crystals (DLCs), in particular, triphenylene-based DLC materials have been investigated intensively, especially over the last decade. The first successful commercialisation of triphenylene-based DLCs has been accomplished in Fuji ‘Wide-View’ optical compensation films. DLCs represent a broad well understood class of soft matter which possess the ability to self-organise into highly anisotropic and ordered structures such as columns that function not only as organic anisotropic semiconductors, but also contribute to the development of new smart materials in the field of organic electronics for many device applications such as photovoltaic devices, light-emitting diodes, field-effect transistors, memory elements, and sensors. Over the last 35 years, more than 1000 triphenylene derivatives have been synthesised and investigated starting from structure-properties to structure-device performance relationships. The very first review by Cammidge and Bushby followed by Kumar summarised the chemistry and physical properties of triphenylene-based discotics up to 2003. In this review, progress in the research of triphenylene DLC materials since 2004 is comprehensively outlined.     

Surface-assisted photoalignment of discotic liquid crystals by nonpolarized light irradiation of photo-cross-linkable polymer thin films

In this article, we describe the surface-assisted photoalignment of discotic liquid crystals (DLCs) on thin films of photo-cross-linkable polymers with cinnamoyl moieties as the side chains. Oblique irradiation of the polymer thin films with nonpolarized UV light at 313 nm brought about inclined orientation of the cinnamoyl residues as a result of their direction-selective photoisomerization and photodimerization. The DLC molecules on the photoirradiated polymer films were aligned in a tilted hybrid manner. This means that the DLC directors are continuously altered from the substrate to the DLC film surface so as to minimize the elastic free energy. Interestingly, we found that the tilted direction of aligned DLC molecules is clearly influenced by the chemical structures of the cinnamate-containing polymers. When a poly(vinyl cinnamate) thin film was obliquely exposed to nonpolarized UV light, the DLCs were inclined to the direction opposite to the UV light propagation. In a keen contrast, the thin film of poly(methacrylate)s tethering cinnamoyl groups, which was obliquely exposed to nonpolarized UV light in advance, provided the tilting DLC direction in parallel with the light propagation. The results were supported by tilted orientation of calamitic (rod-shaped) liquid crystal on the obliquely irradiated polymer films. Such photoalignment behavior of the DLCs can be rationalized by anchoring balance between intermolecular interaction of the DLC molecules with the photodimers of polymer films and those with the remaining E-isomers of cinnamoyl side chains at the film interface. The present technique of DLC photoalignment opens promising ways not only to understand anisotropic physical properties of DLCs, but also to design and fabricate novel nanodevices for photonics and electronics applications.     

Studies on relation between columnar order and electrical conductivity in HAT6 discotic liquid crystals using temperature-dependent Raman spectroscopy and DFT calculations

The vibrational property of 2,3,6,7,10,11-hexakis(hexyloxy)triphenylene (HAT6) discotic liquid crystal (DLC) material is investigated in this research by using temperature-dependent Raman spectroscopy technique. One-dimensional (1D) charge transport mechanism in the DLC molecules is enabled in the columnar liquid crystalline (D_h) phase. The result indicates a high core-to-core correlation in the liquid crystal columnar phase, which has a ‘memory’ like effect that extends into isotropic phase at femtosecond timescale. This correlation is also confirmed through electrical conductivity measurement of DLCs, in which the electrical conductivity is enhanced in the DLC phase. DFT simulation was also carried out in order to elucidate the basic properties of HAT6 such as the band gap in the light of Raman spectra. An interesting outcome is that a freely unspecified boundary model produces in a more flexible molecule, resulting in a reduced band gap. Thus, this work provides an understanding of relationship between columnar order and electrical conductivity of HAT6 molecule, and potential strategy for design of DLCs in electronics application.     

Efficient generation of Heisenberg Hamiltonian matrices for VB calculations of potential energy surfaces

The spin‐Hamiltonian valence bond theory relies upon covalent configurations formed by singly occupied orbitals differing by their spin counterparts. This theory has been proven to be successful in studying potential energy surfaces of the ground and lowest excited states in organic molecules when used as a part of the hybrid molecular mechanics—valence bond method. The method allows one to consider systems with large active spaces formed by n electrons in n orbitals and relies upon a specially proposed graphical unitary group approach. At the same time, the restriction of the equality of the numbers of electrons and orbitals in the active space is too severe: it excludes from the consideration a lot of interesting applications. We can mention here carbocations and systems with heteroatoms. Moreover, the structure of the method makes it difficult to study charge‐transfer excited states because they are formed by ionic configurations. In the present work we tackle these problems by significant extension of the spin‐Hamiltonian approach. We consider (i) more general active space formed by n ± m electrons in n orbitals and (ii) states with the charge transfer. The main problem addressed is the generation of Hamiltonian matrices for these general cases. We propose a scheme combining operators of electron exchange and hopping, generating all nonzero matrix elements step‐by‐step. This scheme provides a very efficient way to generate the Hamiltonians, thus extending the applicability of spin‐Hamiltonian valence bond theory. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Comprehensive study of threonine adsorption on carbon nanotube: A dispersion complemented density functional theory‐based treatment

The interaction mechanism of threonine (Thr) on the sidewall of (8, 8) single‐walled carbon nanotubes (CNTs) was investigated by density functional tight‐binding method. All the functional groups of Thr were used to interact with the surface of CNT. The structural parameters were analyzed to identify the noncovalent interactions, and the binding energy and strain energy were used to indicate the binding properties. We found that the CH/π interactions play more important roles than NH/π and OH/π interactions in stabilizing the complex structures. Furtherly, the charge transfer properties, density of states (DOS) and partial density of states, and highest occupied molecular orbitals and lowest unoccupied molecular orbitals were also studied to illustrate the adsorbed interactions. The results show that the DOS structure of CNT could be modified by the adsorption of Thr, and, therefore, the conductivity of CNT will be improved by introducing proper amino acids. Our data should be helpful for the design of biocompatible molecules for CNT modification. © 2015 Wiley Periodicals, Inc.     

Configuration-dependent interface charge transfer at a molecule-metal junction

The role of the molecule-metal interface is a key issue in molecular electronics. Interface charge transfer processes for 4-fluorobenzenethiol monolayers with different molecular orientations on Au(111) were studied by resonant photoemission spectroscopy. The electrons excited into the LUMO or LUMO+1 are strongly localized for the molecules standing up on Au(111). In contrast, an ultrafast charge transfer process was observed for the molecules lying down on Au(111). This configuration-dependent ultrafast electron transfer is dominated by an adiabatic mechanism and directly reflects the delocalization of the molecular orbitals for molecules lying down on Au(111). Theoretical calculations confirm that the molecular orbitals indeed experience a localization-delocalization transition resulting from hybridization between the molecular orbitals and metal surface. Such an orientation-dependent transition could be harnessed in molecular devices that switch via charge transfer when the molecular orientation is made to change.     

An approximate method for calculating transfer integrals based on the ZINDO Hamiltonian

In this article, we discuss a method for calculating transfer integrals for pairs of molecules based on Zerner's Independent Neglect of Differential Overlap Hamiltonian which requires only a single self‐consistent field calculation on an isolated molecule to be performed to determine the transfer integral for a pair of molecules. This method is compared with results obtained by projection of the pair of molecules' molecular orbitals onto the vector space defined by the molecular orbitals of each isolated molecule. The two methods are found to be in good agreement using three compounds as model systems: pentacene, ethylene, and hexabenzocoronene. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Push-pull macrocycles: donor-acceptor compounds with paired linearly conjugated or cross-conjugated pathways

Two-dimensional π-systems are of current interest in the design of functional organic molecules, exhibiting unique behavior for applications in organic electronics, single-molecule devices, and sensing. Here we describe the synthesis and characterization of "push-pull macrocycles": electron-rich and electron-poor moieties linked by a pair of (matched) conjugated bridges. We have developed a two-component macrocyclization strategy that allows these structures to be synthesized with efficiencies comparable to acyclic donor-bridge-acceptor systems. Compounds with both cross-conjugated (m-phenylene) and linearly conjugated (2,5-thiophene) bridges have been prepared. As expected, the compounds undergo excitation to locally excited states followed by fluorescence from charge-transfer states. The m-phenylene-based systems exhibit slower charge-recombination rates presumably due to reduced electronic coupling through the cross-conjugated bridges. Interestingly, pairing the linearly conjugated 2,5-thiophene bridges also slows charge recombination. DFT calculations of frontier molecular orbitals show that the direct HOMO-LUMO transition is polarized orthogonal to the axis of charge transfer for these symmetrical macrocyclic architectures, reducing the electronic coupling. We believe the push-pull macrocycle design may be useful in engineering functional frontier molecular orbital symmetries.     

Theoretical insights into the 1D‐charge transport properties in a series of hexaazatrinaphthylene‐based discotic molecules

Discotic liquid crystal (DLC) materials have attracted considerable attention mainly due to their high charge carrier mobilities in quasi‐one‐dimensional columns. In this article, five hexaazatrinaphthylene‐based DLC molecules were investigated theoretically, and their frontier molecular orbital energy levels, crystal structures, and electron/hole drift mobilities were calculated by combination of density functional theory (DFT) and semiclassical Marcus charge transfer theory. The systems studied in this work include three experimentally reported molecules ( 1 , 2 , and 3 ) and two theoretically designed molecules ( 4 and 5 ). Compared with the 1 – 3 compounds, 4 and 5 have three more extended benzene rings in the π‐conjugated core. The present results show that the orders of the frontier molecular orbital energy levels and electron drift mobilities agree very well with the experiment. For 4 and 5 , the electron/hole reorganization energies are lower than those of compounds 1 – 3 . Furthermore, the calculated electron/hole transfer integral of 5 is the largest among all the five systems, leading to the highest electron and hole mobilities. In addition, the hydrophobicity and solubility were also evaluated by DFT, indicating that compound 5 has good hydrophobicity and good solubility in trichloromethane. As a result, it is expected that compound 5 can be a potential charge transport material in electronic and optoelectronic devices. © 2017 Wiley Periodicals, Inc.     

噻吩低聚物能级结构的量子化学研究

用量子化学方法计算了系列EDOT低聚物的紫外-可见吸收光谱和前线分子轨道,得到了相应的能级结构参数.与实验值进行对比发现,计算值与实验值具有相同的变化趋势.计算结果表明,通过选择性地改变共轭主链的长度、取代基的位置和取代基的类型,可以规律性地改变化合物光谱性质和能级结构.     

Surface-mediated photoalignment of discotic liquid crystals on azobenzene polymer films

This paper describes a simple strategy for the formation of photoaligned and micropatterned discotic liquid crystal (DLC) film on the surface of photoirradiated azobenzene-containing polymer thin film. The key material for the surface-mediated photoalignment of the DLCs was poly[4-(4-cyanophenylazo)phenyl methacrylate] (pMAzCN). Optical anisotropy was generated in a pMAzCN film by oblique exposure to nonpolarized light which resulted in angle-selective photoisomerization and reorientation of the azobenzenes. Subsequent annealing of the film at 240 degrees C enhanced the photoaligned state of the p-cyanoazobenzenes due to strong intermolecular dipole-dipole interaction and semicrystalline nature of the pMAzCN. This combination of photoirradiation and subsequent annealing of the pMAzCN film made it possible to realize the surface-assisted orientation control of a DLC molecule, which displays both columnar (Col) and discotic nematic (N(D)) phases over 152 degrees C. When the pMAzCN film was exposed to linearly polarized light from the surface normal, the DLC molecules showed homeotropic orientation with the director perpendicular to the substrate surface. In the contrast, oblique irradiation of the pMAzCN film with nonpolarized light gave rise to tilted DLC orientation with well-ordered optical birefringence at the N(D) phase. Rapid cooling from the N(D) phase produced a well-aligned glassy N(D) state at room temperature, which was adequately stable for 10 months even though no covalent cross-linking among the DLCs was performed. The spatial orientation of photoaligned DLCs in both their bulk film and in their interface region was characterized by means of optical birefringence, X-ray diffraction, and fluorescence measurements. At the N(D) phase, the DLC molecules were aligned in a hybrid manner such that their tilt angles varied throughout the thickness of DLC film. The direction of tilted DLCs was opposite to the propagation of the actinic nonpolarized light. The photoaligned DLC films exhibited polarized fluorescence emission with an s-polarized/p-polarized intensity ratio of 4.1, despite the nonpolarized excitation of only DLC at outmost surface. These results indicate that the three-dimensionally aligned azobenzene moieties of the pMAzCN thin film were transferred to the tilted DLC molecules at air/DLC interface. Finally, we demonstrated micrometer-scale photopatterned orientation of DLC molecules on the pMAzCN surface by oblique nonpolarized irradiation of the film through a photomask.     

Adsorption of organic molecules on the TiO2(011) surface: STM study

High resolution scanning tunneling microscopy has been applied to investigate adsorption and self-assembly of large organic molecules on the TiO(2)(011) surface. The (011) face of the rutile titania has been rarely examined in this context. With respect to possible industrial applications of rutile, quite often in a powder form, knowledge on behavior of organic molecules on that face is required. In the presented study we fill in the gap and report on experiments focused on the self-assembly of organic nanostructures on the TiO(2)(011) surface. We use three different kinds of organic molecules of potential interest in various applications, namely, PTCDA and CuPc representing flat, planar stacking species, and Violet Landers specially designed for new applications in molecular electronics. In order to reach a complete picture of molecular behavior, extended studies with different surface coverage ranging from single molecule up to 2 monolayer (ML) thick films are performed. Our results show that the adsorption behavior is significantly different from previously observed for widely used metallic templates. Creation of highly ordered molecular lines, quasi-ordered wetting layers, controlled geometrical reorientation upon thermal treatment, existence of specific adsorption geometries, and prospects for tip-induced molecule ordering and manipulation provide better understanding and add new phenomena to the knowledge on the (011) face of rutile titania.     

Theoretical Characterization of Charge Transport in One‐Dimensional Collinear Arrays of Organic Conjugated Molecules

A great deal of interest has recently focused on host–guest systems consisting of one‐dimensional collinear arrays of conjugated molecules encapsulated in the channels of organic or inorganic matrices. Such architectures allow for controlled charge and energy migration processes between the interacting guest molecules and are thus attractive in the field of organic electronics. In this context, we characterize here at a quantum‐chemical level the molecular parameters governing charge transport in the hopping regime in 1D arrays built with different types of molecules. We investigate the influence of several parameters (such as the symmetry of the molecule, the presence of terminal substituents, and the molecular size) and define on that basis the molecular features required to maximize the charge carrier mobility within the channels. In particular, we demonstrate that a strong localization of the molecular orbitals in push–pull compounds is generally detrimental to the charge transport properties.     

Quantum Chemical Investigations on Electron Transport Characteristics of Porphyrin and Metal-porphyrin

IntroductionRecently, great advances have been made in theresearch of molecular electronics[1,2]. Molecular de-vices based on single molecule[3—5]or molecular clus-ters[6—8], negative differential resistance[9], electro-static current switching[10—12],…     

Quantum molecular mechanics—a noniterative procedure for the fast Ab Initio calculation of closed shell systems

In this article, we advance the foundations of a strategy to develop a molecular mechanics method based not on classical mechanics and force fields but entirely on quantum mechanics and localized electron‐pair orbitals, which we call quantum molecular mechanics (QMM). Accordingly, we introduce a new manner of calculating Hartree–Fock ab initio wavefunctions of closed shell systems based on variationally preoptimized nonorthogonal electron pair orbitals constructed by linear combinations of basis functions centered on the atoms. QMM is noniterative and requires only one extremely fast inversion of a single sparse matrix to arrive to the one‐particle density matrix, to the electron density, and consequently, to the ab initio electrostatic potential around the molecular system, or cluster of molecules. Although QMM neglects the smaller polarization effects due to intermolecular interactions, it fully takes into consideration polarization effects due to the much stronger intramolecular geometry distortions. For the case of methane, we show that QMM was able to reproduce satisfactorily the energetics and polarization effects of all distortions of the molecule along the nine normal modes of vibration, well beyond the harmonic region. We present the first practical applications of the QMM method by examining, in detail, the cases of clusters of helium atoms, hydrogen molecules, methane molecules, as well as one molecule of HeH⁺ surrounded by several methane molecules. We finally advance and discuss the potentialities of an exact formula to compute the QMM total energy, in which only two center integrals are involved, provided that the fully optimized electron‐pair orbitals are known. © 2012 Wiley Periodicals, Inc.     

Functional Organic Semiconductors Based on Bay‐Annulated Indigo (BAI)

The advancement of organic electronics has been continually pushed by the need for stable and high performance acceptor materials. By utilizing inexpensive and stable indigo dye as a starting material, Bay‐Annulated Indigo (BAI) provides a new motif for the development of semiconducting materials. Modular and straightforward synthesis makes BAI an outstanding platform for molecular design, while excellent stability, strong absorption, and high ambipolar mobility render BAI‐based materials excellent candidates for organic electronics. BAI‐based polymers and small molecules have taken advantage of these properties to show promising results in a variety of applications.     

Approaching the Integer‐Charge Transfer Regime in Molecularly Doped Oligothiophenes by Efficient Decarboxylative Cross‐Coupling

A library of symmetrical linear oligothiophene was prepared employing decarboxylative cross‐coupling reaction as the key transformation. Thiophene potassium carboxylate salts were used as cross‐coupling partners without the need of co‐catalyst, base, or additives. This method demonstrates complete chemoselectivity and is a comprehensive greener approach compared to the existing methods. The modularity of this approach is demonstrated with the preparation of discreet oligothiophenes with up to 10 thiophene repeat units. Symmetrical oligothiophenes are prototypical organic semiconductors where their molecular electrical doping as a function of the chain length can be assessed spectroscopically. An oligothiophene critical length for integer charge transfer was observed to be 10 thiophene units, highlighting the potential use of discrete oligothiophenes as doped conduction or injection layers in organic electronics applications.     

Control of electron transport using redox‐active core dendrimers

We are constructing a model system to elucidate the molecular structure‐property relationships for attenuation of electron transfer (e.g. electron encapsulation). This information is relevant in bio‐electron transfer schemes and in emerging molecular electronics schemes such as storage of information using individual molecules. Our system consists of an inorganic cluster surrounded by dendritic ligands which act as an organic coating. Although the electrochemical and photophysical properties of a variety of metal clusters have been established, very little has been described on the chemistry on metal clusters.     

Discotic liquid crystals derived from polycyclic aromatic cores: from the smallest benzene to the utmost graphene cores

ABSTRACT

Past decades we have witnessed many breakthroughs in research on liquid crystals (LCs) as well as significant amplification in the application of LCs. LCs are currently attracting great attention of scientists from all over the world where various researches have been implemented on the varied facets of LCs. In this review we present some recent developments in the field of discotic liquid crystals (DLCs). A large number of DLCs from various aromatic cores have been realised. However, due to paucity of space only DLCs derived from four main aromatic cores, benzene, triphenylene, hexabenzocoronene and graphene, are covered here. An outlook on these emerging two-dimensional organic semiconductor materials with relevant scientific application background has been presented.