Anisotropic Molecular Orientation Research Articles

Statement of Retraction: Anisotropic Molecular Orientation of Micellar Nematic Phase in a Binary Mixture of Two Nonmesogenic Compounds

This article refers to:Anisotropic Molecular Orientation of Micellar Nematic Phase in a Binary Mixture of Two Nonmesogenic Compounds

Light-driven molecular switching of atropisomeric polymers containing azo-binaphthyl groups in their side chains

Light-driven atropisomeric polymers containing azo and binaphthyl units with a conjugated structure were designed and synthesized. The polymers exhibit a glass transition temperature higher than 75 °C with thermal stability above 280 °C and form uniform and smooth thin films without grain boundaries. The trans-cis isomerization is efficiently reversible upon alternating photoirradiation with UV and visible light or with heat. The photoisomerization of the chiral polymer results in photomodulation of the ellipticity and optical rotation ( $$[\alpha ]_{\mathrm{D}}^{25}$$ ), owing to a chiroptical switching behavior. The maximum changes in the dihedral angle and $$[\alpha ]_{\mathrm{D}}^{25}$$ are 21% and 700°, respectively, from the initial state owing to a photoinduced molecular twisting motion of the binaphthyl moiety. The polymer also displays photoswitchable fluorescence with a maximum at 409 nm. A photoinduced change in the refractive index of the formed film is also observed after irradiation with linearly polarized light at 532 nm according to the anisotropic molecular orientation. These results suggest that atropisomeric polymers are potential candidates for light-driven chiroptical switches. Photoinduced molecular switches were achieved by atropisomeric polymers containing conjugated structures of azo and binaphthyl units. trans-cis Photoisomerization of the azo unit led to molecular twisting motion of the binaphthyl unit, yielding chiroptical switching in a neat film. The atropisomeric polymers also displayed photoswitchable fluorescence by repeated irradiation of UV and visible light. Irradiation of the neat film with linearly polarized light caused selective excitation of the atropisomeric polymer, leading to anisotropic molecular orientation.

In-Plane Anisotropic Molecular Orientation of Pentafluorene and Its Application to Linearly Polarized Electroluminescence.

By preparing parallelly aligned 1.9-μm-high SiO2 microfluidic channels on an indium tin oxide substrate surface, the solution flow direction during spin-coating was controlled to be parallel to the grating. Using this technique, a pentafluorene-4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP) binary solution in chloroform was spin-coated to embed a 40-50 nm-thick 10 wt %-pentafluorene:CBP thin film in the channels. In-plane polarized photoluminescence measurements revealed that the pentafluorene molecules tended to orient along the grating, demonstrating that one-dimensional fluid flow can control the in-plane molecular orientation. Furthermore, the dependences of the photoluminescence anisotropy on the spin speed and substrate material suggest that the velocity of the solution flow and/or its gradient in the vertical direction greatly affects the resulting orientation. This indicates that the mechanism behind the molecular orientation is related to stress such as the shear force. The effect of the solution flow on the molecular orientation was demonstrated even in organic light-emitting diodes (OLEDs). Linearly polarized electroluminescence was obtained by applying the in-plane orientation to OLEDs, and it was found that the dichroic ratio of the electroluminescence orthogonal (x) and parallel (y) to the grating is x/y = 0.75.

Influence of Molecular Shape on the Thermal Stability and Molecular Orientation of Vapor-Deposited Organic Semiconductors.

High thermal stability and anisotropic molecular orientation enhance the performance of vapor-deposited organic semiconductors, but controlling these properties is a challenge in amorphous materials. To understand the influence of molecular shape on these properties, vapor-deposited glasses of three disk-shaped molecules were prepared. For all three systems, enhanced thermal stability is observed for glasses prepared over a wide range of substrate temperatures and anisotropic molecular orientation is observed at lower substrate temperatures. For two of the disk-shaped molecules, atomistic simulations of thin films were also performed and anisotropic molecular orientation was observed at the equilibrium liquid surface. We find that the structure and thermal stability of these vapor-deposited glasses results from high surface mobility and partial equilibration toward the structure of the equilibrium liquid surface during the deposition process. For the three molecules studied, molecular shape is a dominant factor in determining the anisotropy of vapor-deposited glasses.

Studies on Anisotropic Molecular Orientation and Mesophase Stability in a Ternary Mixture of Liquid Crystalline Materials

ABSTRACTThe multicomponent system of cholesteryl linolenate (CL), sodium oleate (Naol), and orthophosphoric acid (H3PO4) exhibits liquid crystalline mesophases, like cholesteric and smectic phases, such as SmA, SmC, SmB, and SmG sequentially when the specimen is cooled from its isotropic phase. These phases have been observed by using microscopic technique. Helfrich potential and elastic moduli have also been estimated in the smectic phase using Helfrich model. Optical transmittance and electrical conductivity have also been discussed.

New Molecular Donors with Dithienopyrrole as the Electron-Donating Group for Efficient Small-Molecule Organic Solar Cells

Six new molecules with donor–acceptor–acceptor (D–A–A) configuration bearing coplanar electron-donating dithieno[3,2-b:2′,3′-d]pyrrole (DTP) or aryl-substituted DTP as the D unit and the electron-withdrawing pyrimidine–dicyanovinylene or benzothiadiazole–dicyanovinylene as the A–A block are synthesized. The introduction of aryl groups (p-tolyl or p-anisolyl) onto the α position of DTP is found to greatly benefit the chemical stability and extend the molecular conjugation of the DTP-based D–A–A molecules. The observed broad absorption spectra and anisotropic molecular orientation in the thin films allow for these new D–A–A molecules to perform good capability of light harvesting. These new D–A–A donors are subject to the fabrication of vacuum-processed small-molecule organic solar cells (SMOSCs). The results showed that the structural modulation on the central A block (pyrimidine versus benzothiadiazole) as well as the terminal substitution (p-tolyl versus p-anisolyl) give an evident trade-off between the open circuit voltage (Voc) and short circuit current density (Jsc) values. Among them, SMOSC-adopted D–A–A molecule TDPM composed of a p-tolyl terminal group and a D–A–A configuration of DTP–pyrimidine–dicyanovinylene as the electron donor combined with C70 as the electron acceptor shows a Jsc of 11.34 mA/cm2, a Voc of 0.94 V, and a fill factor (FF) of 0.52, giving a best power conversion efficiency (PCE) as high as 5.6%.

Anisotropic Molecular Orientation of Micellar Nematic Phase in a Binary Mixture of Two Nonmesogenic Compounds

We report the results of our studies on the optical and thermal properties of two nonmesogenic compounds, namely, Cetyl-dimethyl-ethylammonium bromide (CDEAB) and Glacial acetic acid. The mixture exhibits very interesting schilieren texture of micellar nematic phase, SmA, SmC*, and SmE phases, respectively, at different concentrations of CDEAB sequentially when the specimen is cooled from its isotropic phase. The order parameter (S) of the micellar nematic phase is estimated with the help of temperature dependence of optical anisotropy from the measured values refractive index and density data. The temperature variation of order parameter of the experimental curve is very well fitted with the Mayer–Saupe theoretical curve. X-ray studies have been discussed. The formation of above phases has been confirmed by optical and DSC studies.

Anisotropy in amorphous films of cross-shaped molecules with an accompanying effect on carrier mobility: Ellipsometric and sum-frequency vibrational spectroscopic studies

Variable angle spectroscopic ellipsometry and sum-frequency vibrational spectroscopy have been used to study molecular orientations in thin films used in an organic light-emitting-diode. The films consist of sterically bulky and cross-shaped molecules that have small anisotropy in shape, 2-methyl-9,10-di(2-naphthyl)anthracene (MADN). As a result, anisotropic molecular orientation in the amorphous films has been observed with respect to the surface normal. The short axis of anthracene in MADN molecules, more or less, slightly tilts from the surface plane but preferentially close to the surface with a certain orientational distribution, while the long axis of anthracene is, on average, oriented close to the magic angle from the surface normal. This anisotropic molecular orientation gives rise to better carrier transportation properties than the isotropic orientation.

Quantitative compositional analysis of organic thin films using transmission NEXAFS spectroscopy in an X-ray microscope

Near edge X-ray absorption fine structure (NEXAFS) spectroscopy is well suited for the quantitative determination of the composition of soft matter thin films. Combined with the high spatial resolution of a scanning transmission X-ray microscope, compositional maps of submicron morphologies can be derived and have been used successfully to characterize a number of materials systems. However, multiple sources of known systematic errors limit the accuracy and are frequently not taken into account. We show that these errors can be significant (more than 10%) and demonstrate simple methods to eliminate them. With suitable precautions, a compositional measurement can be made on a thin film sample in a matter of minutes with sub-micron spatial resolution and sub-percent compositional precision. NEXAFS measurements are furthermore known to be sensitive to anisotropic molecular orientation and a strategy to account for that and extract preferential molecular orientation relative to a reference is presented. The spatial resolution of the measurement can be increased to below 100nm at the expense of compositional precision, depending on the point spread function of the zone plate focusing optics of the microscope.

Photo-Induced Photonic Band Gap Shift of SiO2 Inverse Opal Films Infiltrated by Azo-Tolane Copolymer

Photochemically tunable photonic band gap materials were prepared by the infiltration of liquid crystal polymers with azobenzene side chains and tolane side chains indicating larger birefringence into voids of SiO2 inverse opal films. Linearly polarized light irradiation resulted in transformation from random to anisotropic molecular orientation of azobenzene and tolane side chains, then the azobenzene-tolane copolymer film showed larger birefringence. Thus, the reflection band of the SiO2 inverse opal films infiltrated with the azobenzene-tolane copolymers was shifted to longer wavelength region by the irradiation of linearly polarized light.

Photoswitching Behavior of SiO2 Inverse Opal Films Infiltrated with Azo-Tolane Copolymer: Effect of Polymer Main Chain Structure

Phototunable photonic band gap materials were prepared by infiltration of a liquid crystalline azopolymer into voids of a SiO2 inverse opal film. Linearly polarized light irradiation resulted in a reversible and stable shift of the reflection band to longer wavelength region due to the transformation of azobenzene groups from random to anisotropic molecular orientation. Acrylate and methacrylate copolymers having both azobenzene and tolane groups in side chain have been synthesized in order to improve switching property. Upon irradiation of linearly polarized light SiO2 inverse opal film infiltrated with methacrylate copolymer showed a larger wavelength shift of the reflection band compared to that of the composite film infiltrated with acrylate copolymer.

Enhanced photochemical‐shift of reflection band from an inverse opal film based on larger birefringent polymer liquid crystals: Effect of tolane group on the photochemical shift behavior

AbstractPhoto‐chemically tunable photonic band gap materials are prepared by infiltration of liquid crystal polymers having azobenzene groups into voids of SiO2 inverse opal films. Linearly polarized (LP) light irradiation results in transformation from a random to an anisotropic molecular orientation of azobenzene side chains in the voids of the SiO2 inverse opal film, leading to the reversible and stable shift of the reflection peak to longer wavelength more than 15 nm. To improve switching properties, we use copolymers of azobenzene monomer and tolane monomer, which have higher birefringence, as infiltration materials into the voids. The azobenzene‐tolane copolymers are found to show higher birefringence than azobenzene homopolymers by the LP light irradiation at higher temperature. Consequently, the reflection band of the SiO2 inverse opal film infiltrated with the azobenzene‐tolane copolymer can be shifted to longer wavelength region more than 55 nm by the irradiation of LP light. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1981–1990, 2009

Photoswitching properties of photonic band gap material containing azo-polymer liquid crystal

Abstract A photochemically tunable photonic crystal was prepared by infiltration of the polymer liquid crystal (LC) having azo-chromophores in a SiO 2 inverse opal structure. The SiO 2 inverse opal film infiltrated with the polymer LC reflected a light corresponding to the periodicity as well as the refractive indices of the inverse opal structure. Linearly polarized light irradiation caused the shift of the reflection band to longer wavelength more than 15 nm. This is caused by the formation of uniaxially anisotropic molecular orientation of the polymer LC. The switched state was stable under interior light, and reversible switching of the reflection band can be achieved by the linearly and circularly polarized light irradiation. This photoswitching property will be suitable not only for various optical materials, but also for introduction of defects in the photonic crystals.

Photoorientation of Liquid Crystalline Azo-Dendrimer by Nanosecond Pulsed Laser for Liquid Crystal Alignment

Upon irradiation with nanosecond pulsed UV laser, surface periodic microstructure and photoorientation occurred simultaneously on the azobenzene functionalized liquid crystalline dendrimer films. The orientation of azo groups was perpendicular to the polarization of pulsed laser and dependent on the laser fluence. The in-plane orientation of azo groups presented parabola-shaped behavior with the increase of laser fluences. The out-of-plane reorientation of azo groups was closely related to thermal effect induced in the procedure of nanosecond pulsed irradiation, which led to the transformation of phase state of azo groups from smectic liquid crystal phase to isotropic phase. Annealing of irradiated film at its liquid crystalline temperature resulted in dramatic enhancement of in-plane orientation and out-of-plane redistribution of azo groups. Liquid crystal alignment on the irradiated film was controlled by a cooperative effect of surface topography and anisotropic molecular orientation simultaneously. Bo...

Photochemical switching behavior of azofunctionalized polymer liquid crystal/SiO2 composite photonic crystal

A photochemically tunable photonic crystal was prepared by infiltrating azopolymer liquid crystal in a SiO2 inverse opal structure. The SiO2 inverse opal film obtained reflected a light corresponding to the periodicity as well as the refractive indices of the inverse opal structure. Linearly polarized light irradiation shifted the reflection band to longer wavelength more than 15nm. This is caused by the formation of anisotropic molecular orientation of the azopolymer. The switched state was stable in the dark, and the reversible switching of the reflection band can be achieved by the linearly and circularly polarized light irradiations.

Stability of Azobenzene-Containing Polyimide Film to UV Light

We have investigated the stability of an azobenzene-containing polyimide (Azo-PI) film with anisotropic molecular orientation to linearly polarized light (LP-L) of wavelength 340–500 nm. The anisotropic molecular orientation was induced by exposing the corresponding polyamic acid film to LP-L of 72 J/cm2, and then the film was thermally imidized. No significant change was observed in the in-plane molecular order of the Azo-PI film over prolonged light exposure of 468 J/cm2, but a 10% decrease of the number of the Azo-PI backbone structure was observed. From these results we found that: (i) the reorientation of the backbone structure through photo-isomerization cycles of azobenzene is inhibited in the Azo-PI film; (ii) the photo-decomposition of the Azo-PI film proceeds with about 2 times the rate for a poly[4,4'-oxydiphenylene-pyromellitimide] film.

Anisotropic Molecular Orientation of Liquid Crystal Monolayer in Contact with UV-irradiated Polyimide Film

By polarized infrared (IR) absorption spectroscopy we have determined the in-plane anisotropy in the molecular orientation of a liquid crystal (LC) monolayer in contact with a polyimide film exposed to linearly polarized ultraviolet light (LPUVL). The in-plane anisotropy of the underlying polyimide film surface was also determined. The average orientational directions of the LC molecules and the polyimide molecules were the same, both being perpendicular to the polarization direction of LPUVL. The in-plane molecular order of the LC molecules was smaller than that of the polyimide molecules. This result indicates that the anisotropic orientational distribution of the LC molecules in contact with the LPUVL-exposed polyimide film is induced through a short-range interaction between the LC molecules and the polyimide molecules.

Alignment mechanism of a nematic liquid crystal on a pre-rubbed polyimide film with laser-induced periodic surface structure

A periodic surface structure was prepared on a pre-rubbed polyimide (PI) film surface with a pulsed UV laser polarized perpendicular to the rubbing direction. The experimental results demonstrate that the rubbing-induced molecular anisotropic orientation was relaxed by the pulsed laser irradiation, and the laser induced molecular orientation was perpendicular to the line of the laser-induced periodic structure. The dichroism of the anisotropy of molecular orientation increased with the increase of laser energy. Since the direction of the laser-induced molecular anisotropy was perpendicular to the surface groove direction of the pre-rubbed PI surface, the effects of surface microgroove and anisotropic molecular orientation of the PI chain on liquid crystal (LC) alignment can be distinguished from each other. LC alignment was investigated by evaluating the anchoring energy of the PI surface, which was calculated according to Berreman's theory using the twist angle of the LC in the cells. The experimental results demonstrate that the exact alignment direction of the LC molecules is determined by the relative strength of both factors.

Alignment of polyamic acid molecules containing azobenzene in the backbone structure: Effects of polarized ultraviolet light irradiation and subsequent thermal imidization

We have investigated the molecular orientation in the films of polyamic acid (PAA) with azobenzene units in the backbone structure. Anisotropic molecular orientation was induced by irradiation of linearly polarized ultraviolet light (LPUVL). The change in the molecular orientation caused by subsequent thermal imidization was also investigated. The orientation of the PAA and polyimide backbone structures was determined by measuring the polarized infrared absorption spectra of the films. When the PAA film was exposed to LPUVL of wavelength 365 to 400 nm at normal incidence, permanent orientational change of the PAA backbone structure occurred through repeated photoisomerization reactions of the azobenzene unit. The PAA backbone structure rotated toward the plane perpendicular to the polarization direction of LPUVL. In subsequent thermal imidization the molecular order increased significantly around the direction perpendicular to both the polarization direction of LPUVL and the surface normal. This enhancement of the molecular order was tentatively attributed to the crystallization of the film caused by thermal imidization.

Lasing in a Microcavity with an Oriented Liquid-Crystalline Polyfluorene Copolymer as Active Layer

The use of layers with anisotropic molecular orientation in microcavities provides a means for the fabrication of lasing devices based on polyfluorene copolymers. The Figure shows the structure of the polymer film formed by the spin-coating of two mirrors with chloroform solutions of the polymers and pressing them together. The photopumped laser thus achieved shows a low lasing threshold.