Effect Of Molecular Alignment Research Articles

O-6 Electronic and Optical Properties of Liquid Crystalline π-conjugated Polymer and their Molecular Alignment Effects

O-6 Electronic and Optical Properties of Liquid Crystalline π-conjugated Polymer and their Molecular Alignment Effects

Effect of Molecular Alignment in Smectic Mesophases on Carrier Transport of Self-Organizing Molecular Semiconductors

The general features of electrical properties of a new class of organic photoconductors, i.e., photoconductive liquid crystals, to which we prefer “Self-organizing Molecular Semiconductors” as a conceptual name, are reviewed briefly. And in two series of smectic mesophases, i.e., from SmA to SmE and from SmC to SmG in which the liquid crystalline molecules sit perpendicular to the smectic layer and tilted towards the layer, respectively, the effect of molecular alignments on the carrier transport was investigated by transient photocurrent measurements with 2-phenylnaphtalene and terthiophene derivatives corresponding to the two series. All the transient photocurrents measured were non-dispersive and ambipolar irrespective of the mesophases. The mobility increased stepwise in accordance with the upgrade of the molecular alignment and ranged from 10−4 cm2/Vs to 10−2 cm2/Vs, which was independent of electric field and temperature. The highest mobility was observed in SmE phase of 2-phenylnaphtalenes and SmG phase of terthiophenes. Judging from the analysis with two-dimensional hopping model in the smectic layers and the results from X-ray diffraction study, it was concluded that the upgrade of mobility at the phase transition from mesophase to mesophase is attributed primarily to the reduction of the molecular distance in the smectic layer irrespective of the type of the series. In addition, it was suggested that the hopping probability is enhanced possibly through a local overlap of molecular orbitals in highly ordered mesophases in which the molecular motion is limited.

Optical properties and electroluminescence characteristics of polyacetylene derivatives dependent on substituent and layer structure

Intense photoluminescence (PL) is observed in di-substituted polyacetylene derivatives even though solitonic mid-gap absorption is observed upon doping, contrary to non-substituted trans-polyacetylene and mono-substituted polyacetylene in which strong PL is not observed. Intense green and blue electroluminescence (EL) is realized utilizing poly(diphenylacetylene) derivatives and poly(1-alkyl-2-phenylacetylene) derivatives, respectively. Greenish-blue emission is also observed in poly (1-chloro-2-phenylacetylene) derivatives. The dependence of wavelength and intensity of PL and EL on the molecular structure of substituents is clarified in detail. The effects of molecular alignment and layer structure on the EL characteristics are also discussed. Upon intense light excitation, remarkable spectral narrowing due to stimulated emission is also observed in these di-substituted polyacetylene derivatives.

Quasiclassical Trajectory Study of Molecular Alignment Effects on the Dynamics of the Reactions of Cl, Br, and I with H2

The X + H2 (X = Cl, Br, and I) reactions may be taken as models for endoergic triatomic reactions with heavy−light−light kinematics and collinear saddle point. The dependence of scalar and two-vector properties, angular distributions (k, k‘), (k, j‘), (k‘, j‘), and (l‘, j‘), on (ET, v, j), as well as the effect of considering initial parallel (II), perpendicular (⊥), and random (null) k−j alignment has been studied using the quasiclassical trajectory (QCT) method. The threshold energy for II alignment is always higher than the ones for ⊥ and null alignments, but for high enough ET values σ(II) becomes larger than σ(⊥) and σ(null), and the same occurs for the j-dependence. In the v range of values explored σ(II) is in general equal or larger than σ(⊥) and σ(null). The expression 1/3σ(II) + 2/3σ(⊥) provides a very good estimate to σ(null) if the system is not in the vicinities of the threshold region, and some useful relations to simplify the QCT calculations for II alignment have also been given. For the two-vector properties considered, the results obtained for ⊥ alignment are in general closer to the ones for null alignment than the results obtained for II alignment. The angular correlations that result from the calculation are not a trivial result coming from a kinematic constraint; being particularly remarkable the role played by the rotation of the H2 molecule. These results may be explained taking into account the saddle point properties, the “effective molecular size” of the rovibrationally excited H2 molecule, and the geometrical implications of the alignments.

Novel intermolecular and intramolecular photochemical reactions initiated by excited state single electron transfer processes

Several examples of photoinduced electron transfer initiated cation radical carboncarbon bond fragmentation reactions are reported. 1,2-aminoalcohols and 1,2-diamines undergo analogous fragmentation reactions although they show quite different reactivities. In general, the cation radicals of 1,2-diamines fragment more efficiently than those of aminoalcohols and the reactions are not dependent on the basicity of the reduced acceptor counterion species. A systematic study of 1,2-diamines indicates that the quantum efficiencies of the reaction are highly dependent on the reaction conditions (solvent, type of sensitizer, cosensitization, etc.) and structural considerations of the molecules being studied. Higher quantum efficiencies can be obtained using a triplet sensitizer or cosensitization. The effects of molecular orbital alignment on the efficiency of fragmentation were analyzed by modifying the structures of the 1,2-diamines. Interestingly, higher quantum yields are observed when certain molecular orbital configurations are achieved. Similar conclusions are obtained from studies of the reactivity of “remote” aminoalcohols and diamines. In this report, we also demonstrate that the quantum efficiency of the cation radical fragmentation can be improved dramatically by utilizing an efficient fragmentable electron acceptor as sensitizer (such as carbon tetrachloride). A new concept of “double fragmentation” is proposed.

Molecular-orientation effects in the dissociative ionization of CH4 in intense laser fields.

Differences in the dissociative ionization pattern of ${\mathrm{CH}}_{4}$ obtained in interactions with intense laser fields, and by fast-ion impact, are discussed in terms of molecular-alignment effects.

Molecular-alignment dependence in the transfer excitation ofH2

Molecular-alignment effects in the transfer excitation of ${\mathrm{H}}_{2}$ by high-velocity heavy ions are studied using a two-step mechanism with amplitudes evaluated from first-order perturbation theory. Two-electron transfer excitation is treated as a result of two independent collision processes (excitation and electron transfer). Cross sections for each one-electron subprocess as well as the combined two-electron process are calculated as functions of the molecular-alignment angle. Within the independent-electron approximation, the dynamic roles of electron excitation and transfer in conjunction with molecular alignment are explored. While both excitation and transfer cross sections may strongly depend on molecular alignment, it is electron transfer that is largely responsible for the molecular-alignment dependence in the transfer excitation process. Interpretation of some experimental observations based on this model will also be discussed.

Studies of turbulent gas flows using electric and magnetic probes

The possibility that regions of shear in turbulent gas flows can align molecules and give rise to detectable electromagnetic radiation has been suggested by Clark (1958, 1975). This paper describes further research in this area and in particular considers the effect of acoustic interaction and vibration on the sensitive magnetic and electric probes used to detect the signals. The conclusion is that vibration effects are likely to dominate in most practical configurations in the laboratory. Calculations suggest an upper limit of approximately 1017 T for the molecular alignment effect, which is a factor of 106 below the background noise level when vibration effects are excluded. The probes can, however, be adapted for studies of flow-induced vibration.

Fluctuations and light scattering from anisotropic interfaces

The normal modes of fluctuations in an interface, separating two fluid phases, which possess both viscoelasticity and optical anisotropy, are determined. The modes separate into the following four groups if the two surrounding fluids are the same: shear; twist; dilatation and ordering; and transverse vibration and splay modes. In the limit of strong coupling between orientation and transverse vibration, the main effect of molecular alignment is to increase the apparent surface tension by a term proportional to q 2, where q is the momentum transfer in the surface fluctuations. The intensity and frequency spectrum of coherent light scattered from the thermal fluctuations are calculated. By measuring the depolarization in a suitable geometry and the frequency spectrum, contribution from both the orientation effect and the various viscosity coefficients can be determined. Applications to artificial and biological membranes are briefly discussed.

NMR study of induced dielectric alignment in pure and binary liquids

The nmr spectra of polar liquids may be affected by application of a strong external electric field. Dipolar molecules are slightly aligned by this field and the nuclear quadrupole couplings and the direct spin—spin dipolar interactions may no longer be-averaged to zero. The proton spectrum of p-nitrotoluene, modified by dipolar induced couplings, is given as an example. Quadrupolar electric field effects have been measured in the '4N spectra of a series of pure nitriles and nitrocompounds. From these spectra the molecular alignment, induced by the electric field, has been obtained. Alignments of both polar and nonpolar molecules have been detected in the nmr spectra of nitrobenzene—deuterobenzene mixtures, if the polar molecules are aligned by the electric field, the orientation is transferred to the nonpolar particles. Theoretical expressions for the alignments in pure and binary liquids are discussed. Short range interactions between the molecules, giving rise to angular correlations, turn out to be important. This is particularly apparent in the case of the alignment of nonpolar molecules in mixtures. INTRODUCTION When an external electric field is applied to a polar liquid the molecular dipoles will tend to align themselves along this field. In competition with this effect the thermal motion disturbs the ordering but on the average the molecules will preserve a preferential orientation. As a result the nmr spectrum of the liquid may be modified because molecular tensor properties; which have no influence on the spectrum of an isotropic liquid, become manifest. Well known examples are the direct dipole—dipole interactions between nuclear spins and the quadrupole interaction between a nucleus and the inhomogeneous electric field originating from the surrounding electrons and nuclei. The possibility of observing the effect of molecular alignment, induced by an electric field, in nuclear magnetic resonance phenomena was mentioned as early as 1954 by Hahn1. Since then several attempts2 have been made to detect dipolar-induced electric field effects in nmr spectra. Most Present address: Department of Physical Chemistry, University of Nymegen, Nymegen, The Netherlands.

On the transient scattering of light by pulsed liquid crystal cells

Results obtained from several experiments with liquid crystal cells indicate that a light scattering transient which has been attributed to molecular alignment effects in the nematic liquid crystal material may actually be caused by dynamic scattering.