Molecular Tilt Angle Research Articles

The dipole interaction model of the molecular aggregation in Y-type Langmuir-Blodgett film

Based on the classical electrostatic theory, the model of dipole-dipole interaction of rod-like amphiphilic molecules in Y-type Langmuir-Blodgett (LB) film is presented in the paper. The relations between the structure and spectral absorption peak of LB films were obtained by using this model. For Y-type films, the relationship of molecular aggregation state and the structure parameters, i.e. molecular separation distance a, separation d between two adjacent layers, molecular tilt angle θ, and layer number g, were studied using our model. Then we compared the similarities and differences of aggregation behavior between Y-type and Z-type films. The theoretical results agree well with experimental data.

Study of dependence of molecular orientation and optical properties of zinc phthalocyanine grown under two different pressure conditions

Zinc phthalocyanine (ZnPc) thin films were prepared by organic molecular beam deposition on Si (111) covered with a thin native oxide layer under two different pressure conditions. The samples are characterized by infrared spectroscopy and ellipsometry measurements aiming at the determination of dielectric functions and average molecular orientation in the ZnPc films. A two order of magnitude increase in the pressure during growth increases the average molecular tilt angle with respect to the substrate surface from ∼19° to ∼61° implying a change of crystal phase as well. Likewise sizeable differences are observed in the anisotropic dielectric functions of the ZnPc thin films in the infrared as well as in the visible and ultraviolet spectral ranges.

Dichroic Properties and the Molecular Tilt Angle of a Large-Tilt Angle Antiferroelectric Liquid Crystal Studied Using Polarized Infrared Spectroscopy

The voltage dependencies of the dichroic properties for a homogeneously aligned sample of a partly fluorinated chiral smectic liquid crystal (S)-4-(1-methylheptyloxy carbonyl)phenyl 4′-[6-(3,4,4,4-tetrafluoro-3-trifluoromethylbutylcarbonyloxy) hexyloxy] biphenyl-4-carboxylate at various temperatures have been studied using polarized Fourier transform infrared spectroscopy. The temperature dependence of the molecular apparent tilt angle has also been determined. For the antiferroelectric SmC A * phase, the molecular apparent tilt angle is found to be close to 45° for a temperature range of 65–80°C. In the SmA* phase, an application of a sufficiently high electric fields causes a substantial increase in the orientational order parameter. This results probably from a change in the azimuthal distribution of the molecular directors around the tilt cone. The material is found to exhibit an extraordinary large surface electroclinic effect and its helix is considerably distorted by the surfaces.

Ellipsometric study of an organic template effect: H2Pc/PTCDA

Abstract A key prerequisite for fabricating opto-electronic devices with organic molecules is to control the molecular growth mode. In this work we present an ellipsometric study of the templating effect of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) on the growth mode of metal free phthalocyanine (H 2 Pc). The difference in the anisotropic dielectric function between the H 2 Pc films grown on bare glass or silicon substrates compared to the anisotropic dielectric function of H 2 Pc films grown on PTCDA indicates a drastic change in molecular orientation. From the strong in-plane/out-of-plane anisotropy the average molecular tilt angle of H 2 Pc was found to be around 52° for the films grown on bare substrates and around 25° for H 2 Pc films grown on PTCDA. A splitting in the H 2 Pc Q-band was observed for the H 2 Pc films grown on PTCDA which indicates a deviation from the perfect H 2 Pc α-phase crystal.

Does the molecular orientation induce an electric dipole in Cu-phthalocyanine thin films?

The effect of the molecular orientation on the molecular electronic structure is studied on the Cu-phthalocyanine∕graphite system via film thickness dependences of metastable-atom electron spectra and ultraviolet photoelectron spectra. We observed a decrease in the vacuum-level position and a corresponding band-bending-like shift in the highest occupied state only for thick films where the molecular tilt angle increases gradually with the film thickness. These shifts are explained by electric dipoles produced in the film by a gradient of the intermolecular electronic interaction along the surface normal due to the continuous increase in the molecular tilt angle. The result indicates that the change in the molecular orientation is an important origin of the band-bending-like shift in the molecular electronic states even if the molecule has no permanent electric dipole.

Molecular Orientation Analysis of a Single-Monolayer Langmuir−Blodgett Film on a Thin Glass Plate by Infrared Multiple-Angle Incidence Resolution Spectrometry

Molecular orientation analysis in a single monolayer deposited on a glass substrate has been a difficult matter, since the glass substrate absorbs infrared rays so strongly that the measurements of infrared spectra are difficult to perform, and the single monolayer is not suitable for X-ray diffraction analysis because no periodical structure is available. When a thin glass is used as the substrate, in particular, the infrared analysis becomes more difficult, since optical fringes appear strongly on the absorption spectra due to the multiple reflections in the glass. In the present study, infrared multiple-angle incidence resolution spectrometry (MAIRS) has been employed to remove the fringes from the spectra of single- and five-monolayer Langmuir-Blodgett (LB) films of cadmium stearate deposited on a thin glass plate. The MAIRS in-plane spectra gave quantitatively reliable infrared transmission spectra for both films with little fringes, which made it possible for the first time to analyze the molecular orientation in the single-monolayer LB film on glass. As a result, it has been revealed that the molecule in the single-monolayer LB film on thin glass exhibits a significantly larger molecular tilt angle than those prepared on other substrates such as gold and germanium.

Spectroscopic ellipsometry on a novel cyanine dyes in Langmuir–Blodgett multilayers

We have investigated on cyanine dye (HQ) in Langmuir–Blodgett (LB) films by spectroscopic ellipsometer (SE) and compared the results with that obtained by using small angle X-ray diffraction (SAXD). It is found from the isotherms that there is a critical point on Langmuir films near the area 0.8 nm 2/molecule for HQ LB films with and without Cd 2+ ions, respectively and suggested that the facts should result from the phase transition due to the change of molecular tilt angle on surface of sub-phase. The refractive indexes as a function of the wavelength λ are reported by the SE method using the three layers-model (air-LB film-silicon wafer) and Cauchy dispersion equation, for the first time presented in literature. The thickness of a single deposited layer is 2.45 nm on average. The thickness of per monolayer is agreement with those obtained by SAXD measurements.

Measurement of Dielectric Polarizations for Analyzing the Orientational Order of Langmuir Monolayers

The dielectric polarizations, i.e. spontaneous, linear, and second order non-linear polarizations, generated from Langmuir monolayers comprised of rod-shaped molecules were analyzed, and represented as functions of the orientational order parameters S 1 , S 2 , and S 3 ( S n = 〈 P n ( cos ⁡ θ ) 〉 , θ : molecular tilt angle, 〈 〉 : thermodynamic average, P n ( cos ⁡ θ ) : Legendre polynomial of n th order). It has been shown that the Maxwell displacement current (MDC), Brewster angle reflectometry (BAR), and optical second harmonic generation (SHG), which are attributed to the spontaneous, linear, and non-linear polarization, respectively, are capable of detecting the orientational order parameter S 1 , S 2 , and S 3 of Langmuir monolayers. An experiment using the MDC-BAR-SHG was conducted on Langmuir monolayers comprised of 4- octyl-4′- cyanobiphenyl rod-shaped molecules. It is shown that the MDC-BAR-SHG measurements are useful to investigate the orientational structure of Langmuir monolayers.

Self-assembly of biaxial ordering and molecular tilt angle of chiral smectic liquid crystals in homeotropically aligned cells investigated using infrared spectroscopy

Temperature dependences of the infrared absorbance have been measured for the four chiral liquid crystal samples in the homeotropic cell configuration. It is shown that the values of the orientational order parameter obtained using this method exhibit a remarkable similarity to the x-ray diffraction results of the smectic layer spacing and lead to accurate values of the molecular tilt angle. This has important consequences for the existing interpretation of the x-ray data. The proposed method, in many cases, may be considered as a valuable alternative to the x-ray diffraction, giving additional, important information about the orientations and the ordering of the molecular fragments. It is found that if the experimentally obtained order parameter is low, then the molecular biaxiality is exceptionally large. The average phenyl ring plane is found to lie close to the molecular tilt plane.

De Vries smectic A phase formed by a liquid crystal side chain copolymer? A 13C NMR study

We study the orientation and order parameter of a liquid crystalline random side chain copolymer by 13C NMR. Evidence has previously been presented that this material forms a de Vries smectic A phase. The NMR data show that the molecular tilt angle in the smectic A phase is very small or zero and the smectic A–smectic C* transition is attributed predominantly to a change of the molecular tilt rather than azimuthal ordering. We discuss the NMR results in the context of earlier X‐ray and elastic characterizations of structurally similar materials.

Monolayers of Diphenyldiacetylene Derivatives: Tuning Molecular Tilt Angles and Photopolymerization Efficiency via Electrodeposited Ag Interlayer on Au

An electrodeposited Ag adlayer (upd, underpotential deposition) is utilized to improve monolayer photopolymerization of diphenyldiacetylene derivatives (DPDAs) that would otherwise exhibit no polymerization in solid state. Topochemical reaction of diacetylene derivatives via solid-state 1,4-addition yields polydiacetylenes which are of great importance due to properties associated with their ene-yne conjugated backbones. The polymerization efficiency heavily depends on the molecular arrangement in the crystals. For example, crystals of most DPDA derivatives show no activity for topochemical reaction because the bulky phenyl end groups space out the triple bonds and thus DPDAs require relatively large translation and rotation angles for polymerization. In principle, topochemical reaction is viable if molecules are in optimal arrangement. The upd interlayer can be applied to tune the adsorbate-substrate interactions, intermolecular spacing, and the molecular tilt angle by controlling the coverage of the Ag adlayer. It is thus possible to manipulate the molecular arrangement of DPDAs for the subsequent polymerization. Successful photopolymerization of DPDA monolayers is realized from the decrease in nu(C[triple bond]C) intensity by infrared reflectance absorbance spectroscopy, growth of ene-yne pi-pi* transition by UV-vis measurements, and enhanced electrochemical stability by the cathodic desorption protocol. The optimal efficiency of polymerization takes place on upd-modified substrates that can generate approximately 45 degrees tilt angle for DPDA derivatives.

Electron Tunneling through Molecular Media: A Density Functional Study of Au/Dithiol/Au Systems

We report a density functional theory study of the electronic properties of n-alkanedithiols (CnS2, with n=4, 8 and 12) sandwiched between two Au(111) infinite slab electrodes. We investigate the influence of the distance between the two electrodes and of the molecular chain length, tilt angle, and coverage on the local density of states (LDOS) at the Fermi energy (E(f)). We find that the (small) value of the LDOS at Ef near the center of the molecular wires--a quantity that is related to the tunneling current--is mainly determined by the length n of the alkane chains: it originates from the tails of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) which are broadened by the interaction with the electrodes, and decays exponentially with the length of the molecular wire. This opens a nonresonance tunneling channel for charge transport at small bias voltages. While the length of the hydrocarbon chain appears to be the determining factor, the tilt angle of the molecular wires with respect to the electrode surfaces, and therefore the distance between these, has a small influence on the LDOS at the center of the molecule, while the effect of coverage can be ignored. The picture which emerges from these calculations is totally consistent with a through-bond tunneling mechanism.

Conformations and charge transport characteristics of biphenyldithiol self-assembled-monolayer molecular electronic devices: A multiscale computational study

We report a computational study of conformations and charge transport characteristics of biphenyldithiol (BPDT) monolayers in the (sqrt.3 x sqrt.3)R30 degrees packing ratio sandwiched between Au(111) electrodes. From force-field molecular-dynamics and annealing simulations of BPDT self-assembled monolayers (SAMs) with up to 100 molecules on a Au(111) substrate, we identify an energetically favorable herringbone-type SAM packing configuration and a less-stable parallel packing configuration. Both SAMs are described by the (2sqrt.3 x sqrt.3)R30 degrees unit cell including two molecules. With subsequent density-functional theory calculations of one unit cell of the (i) herringbone SAM with the molecular tilt angle theta approximately 15 degrees , (ii) herringbone SAM with theta approximately 30 degrees , and (iii) parallel SAM with theta approximately 30 degrees, we confirm that the herringbone packing configuration is more stable than the parallel one but find that the energy variation with respect to the molecule tilting within the herringbone packing is very small. Next, by capping these SAMs with the top Au(111) electrode, we prepare three molecular electronic device models and calculate their coherent charge transport properties within the matrix Green's function approach. Current-voltage (I-V) curves are then obtained via the Landauer-Buttiker formula. We find that at low-bias voltages (|V| < or = 0.2 V) the I-V characteristics of models (ii) and (iii) are similar and the current in model (i) is smaller than that in (ii) and (iii). On the other hand, at higher-bias voltages (|V| > or 0.5 V), the I-V characteristics of the three models show noticeable differences due to different phenyl band structures. We thus conclude that the BPDT SAM I-V characteristics in the low-bias voltage region are mainly determined by the -Au [corrected] interaction within the individual molecule-electrode contact, while both intramolecular conformation and intermolecular interaction can affect the BPDT SAM I-V characteristics in the high-bias voltage region.

Polarized near-edge x-ray-absorption fine structure spectroscopy of C60-functionalized 11-amino-1-undecane thiol self-assembled monolayer: Molecular orientation and Evidence for C60 aggregation

Near-edge x-ray-absorption fine structure (NEXAFS) spectroscopy was adopted to probe the unoccupied electronic states of C60 anchored onto an organized assembly of 11-amino-1-undecane thiol on Au(111). The polarization dependence of the intensity of pi* resonance associated with C60 pi network revealed the self-assembled monolayer (SAM) system to be oriented with an average molecular tilt angle of 57 degrees with respect to the surface normal. Invoking the absence of solid-state band dispersion effects and in comparison to solid C60 and /or 1-ML C60/Au(111), the electronic structure of the resulting assembly was found dominated by spectral position shift and linewidth and intensity changes of the lowest unoccupied molecular orbital (LUMO), LUMO+1, and LUMO+2 orbitals. The latter implied hybridization between N Pz of -NH2 group of thiolate SAM and pi levels of C60, resulting in a nucleophilic addition with a change in the symmetry of C60 from Ih to C1 in the SAM. Occurrence of a new feature at 285.3 eV in the NEXAFS spectrum, assigned previously to pi* graphitic LUMO, signified the formation of aggregated clusters, (C60)n of C60 monomer. Low tunneling current scanning tunneling microscopy confirmed them to be spherical and stable aggregates with n approximately 5.

Infrared Spectroscopy Analysis of the Structure of Multilayer Langmuir−Blodgett Films: Effect of Deposition Velocity and pH

Multiple Langmuir-Blodgett (LB) films of arachidic acid were deposited on germanium (Ge) substrates from subphase solutions of 10(-4) M CdCl2 at different pH values and at different deposition speeds. Attenuated total reflectance infrared (ATR-IR) spectroscopy was used to obtain information on the molecular order and structure of these multilayer LB films. At pHs higher than the pKa of the fatty acid/cation system, transfers took place only during the downstroke, indicating X-type deposition. At pH = pKa and large deposition speeds, deposition partially failed during the downstroke, resulting in Z-type depositions. Analysis of the infrared spectra indicates that multiple LB films deposited only during the upstroke (Z-type) or during downstrokes (X-type) have a centrosymmetric structure typical of films deposited during the upstroke and downstroke, except for a slight decrease in molecular order and tilt angle as the pH increases (X-type). The centrosymmetric structure indicates that rearrangement of layers takes place between cycles. Experimental evidence of such rearrangement occurring in a fatty acid/divalent cation salt subphase is shown here, and rearrangement alternatives are discussed.

Siloxane antiferroelectric liquid crystalline dimer exhibiting two field-free zero birefringent states

We studied the optical and electro-optical properties of a siloxane antiferroelectric liquid crystalline dimer exhibiting a large molecular tilt that reached a maximum value of 53°. When the molecular tilt angle in the antiferroelectric phase exceeded 45°, it was found that the field-free position of the slow vibration direction, which corresponds to the largest refractive index, flipped from a position along the smectic layer normal, to a position perpendicular to it. When the tilt angle became 45°, a zero birefringent state, which was theoretically predicted by de Meyere in 1996, was also found to occur twice in the antiferroelectric phase of the siloxane dimer.

Study of excited states of fluorinated copper phthalocyanine by inner shell excitation

Near edge X-ray absorption fine structure (NEXAFS) spectra of hexadecafluoro copper phthalocyanine (FCuPc) films (thickness of 50 Å) on MoS 2 substrates were observed near the carbon (C) and fluorine (F) K-edges. From the analysis of the dependence of C and F K-edge NEXAFS spectra on the photon incidence angle ( α), the average molecular tilt angle was determined to be 30°. The lowest and second lowest peaks in the F K-edge NEXAFS were assigned to the transition to σ ∗. In the ion time-of-flight mass spectra of FCuPc excited by photons near the F K-edge, F +, CF +, and CF 3 + ions were mainly observed. These results indicate that CC bonds as well as CF bonds are broken by the photon irradiation. From the analysis of the partial ion yield spectra of F + and CF + near the F K-edge, the lowest and second lowest peaks in the F K-edge NEXAFS spectra could be assigned to transitions to σ(CF) ∗ and σ(CC) ∗, respectively.

X-ray diffraction analysis of the layer structure in a ferroelectric liquid crystal with a chiral nematic–chiral smectic C phase transition

A half-V-shaped switching ferroelectric liquid crystal (FLC) is a promising candidate for fast response displays. In the half-V FLC display, a liquid crystal with a chiral nematic–chiral smectic C phase transition is used, and the smectic layer is formed by cooling from N* to SmC* with an applied d.c. field. We studied the layer structure by means of X-ray measurements for two axes (ω and χ). By using a point-focused X-ray tube and optimizing the slit width, we succeeded in the two-axis measurement with a commercial X-ray system. The ω–χ profile of the half-V FLC showed two broad peaks in an arc-shaped high-intensity area. Our interpretation of this result is that the major part of the layer consists of a tilted-bookshelf structure and that the minor part consists of a near-bookshelf structure. Since optical microscopy observations on the half-V FLC cells showed a stripe-shaped texture, we consider that the coexistence of the tilted-bookshelf and the near-bookshelf structures forms the stripe-shaped patterns. The radius of the arc-shaped high-intensity area was nearly equal to the molecular tilt angle. This result can explain why the half-V FLC showed a desirable black appearance in spite of the stripe-shaped texture.

Spectroscopic Study of Surface Recovery of Germanium Substrate for Langmuir-Blodgett Films by Infrared Multiple-Angle Incidence Resolution Spectrometry

Surface recovery of germanium substrate for infrared spectroscopic studies of Langmuir-Blodgett (LB) films has been investigated by infrared multiple-angle incidence resolution spectroscopy (MAIRS). A Gesubstrate is, in general, repeatedly used for fabrication of LB films for infrared spectroscopic studies. For the recycling process, the substrate covered by LB films is washed by sonication in organic solvents and pure water. This cleaning generally works well for ordinary analytical use, especially for thick samples, such as polymer and liquid materials. Nonetheless, in the present study, the cleaning by sonication only has been found to be inappropriate for the measurements of monolayer-level samples. The infrared MAIRS analysis has revealed that the 5-monolayer cadmium stearate LB film deposited on a fresh Ge substrate has a molecular tilt angle of ca. 13° from the surface normal, while that deposited on a recycled substrate by sonication has an angle of ca. 30°. Infrared MAIRS analysis has experimentally proved that the ideal recovery of the Ge surface can be achieved by cleaning the substrate with chemical rinse with the use of acid solution.

DIRECTOR ORIENTATION IN NEMATIC LIQUID CRYSTALS UNDER ORTHOGONAL MAGNETIC AND ELECTRIC FIELDS

The orientation of the director in nematic liquid crystals can be driven by an electric or a magnetic field or a crossed combination of both. The dynamic response of nematics to crossed fields has been studied experimentally by Faetti et al. [1] and others in an attempt to measure quantities such as the rotational viscosity coefficient. We investigate the director orientation of an incompressible nematic liquid crystal under the influence of tilted, mutually perpendicular, constant magnetic and electric fields. We assume that flow effects are negligible and employ an averaging method to derive an equation describing the dynamic response of the average molecular tilt angle. Travelling wave solutions are calculated and we discuss criteria for their stability. Finally, we investigate director response when both or either of the applied fields are turned off with a view to gaining information about the characteristic times of relaxation. These analytical characteristic times are shown to depend on the various material properties.