Electric field-induced orientation

An electric field induces a dipole moment in non-polar solvent molecules and consequently these molecules become electrostatically aligned with the electric field according to the field polarity. Hence, the dielectric constant describes the capacity of a solvent to separate the electric charges of a solute through an appropriate orientation of its molecules. 

Electric field induced orientation of polar groups in these polymers occurs via the mechanism of large scale motion, common for rigid polymers. However, the incorporation of a spacer within a macromolecule (polymer 5, Table 16) changes sharply the mode of intramolecular aggregation and the mechanism of orientation in an electric field. 

Three synthetic approaches to donor-acceptor-substituted conjugated molecules with enhanced orientability in electric fields, potentially applicable to the preparation of electro-optic polymers via electric field poling, are summarized. The three approaches are parallel attachment of chromophores to a common framework, embedding the chromophore in a zwitterion, and head-to-tail oligomerization of chromophores. The oligomerization method as well as the use of dyes as curing agents are briefly discussed in relation to the stability of electric field-induced polar order in polymer matrices. 

Dopant orientation during and following electric field-induced poling can be studied continuously and in real time in order to examine the microenvironment surrounding the dopants in terms of the polymer relaxations and the applied corona field. In the results presented below, the SHG of 4-dimethylamino-4 -nitrostilbene (DANS) dispersed in polystyrene (PS) or poly(methyl methacrylate) (PMMA) matrices has been examined in corona poled films as a function of temperature in order to understand the effect of thermal conditions on the temporal stability of the dopant orientation. 

In the limit of the oriented gas model with a one-dimensional dipolar molecule and a two state model for the polarizability (30). the second order susceptibility X33(2) of a polymer film poled with field E is given by Equation 4 where N/V is the number density of dye molecules, the fs are the appropriate local field factors, i is the dipole moment, p is the molecular second order hyperpolarizability, and L3 is the third-order Langevin function describing the electric field induced polar order at poling temperature Tp - Tg. 

The high viscosity of the PMMA damps out orientational contributions so that the yqeo that is measured is thought to be =60-90% electronic. This has been ascertained by measuring the electric field induced second harmonic generation (EFISH) below the Tg of the polymer. From this can be obtained the microscopic elastic constant, which can in turn be used to estimate the magnitude of the two orientational contributions to yQEO- Details are provided else where (13,16). 

Boker A et al (2006) The influence of incompatibility and dielectric contrast on the electric field-induced orientation of lamellar block copolymers. Polymer 47(3) 849-857... 

The polyimide-base PR system [79,80] was designed on the premise that porphyrin-electron acceptor (quinones or imide moieties) systems are well-known model compounds for photosynthetic processes and exhibit very interesting charge transfer properties [81], A high quantum yield of charge separation can be achieved in these systems. Polyimides are found to be photoconductive and allow charge transport [82], Furthermore, polyimides possess high Tg and therefore, the electric field-induced dipole orientation can be fixed after imidization [83],... 

Note that the surface electric field, induced by the incident IR radiation characterizing the thin-film model catalysts, is mainly determined by the NiAl substrate. Consequently, because only the components of the dynamic dipole moment that are perpendicular to the metallic substrate contribute to the SFG signal, the effective dipole moment of tilted molecules is reduced. As a result, the intensity of the signal characterizing tilted molecules is smaller than that of CO molecules oriented perpendicular to the substrate (such as those on the particle top facet). 

At time t = 0, a dc field is applied it produces an electric field-induced Pockels effect (EFIPE), which is solely due to a third-order effect Eq) in the case of the copolymer because the molecules are not oriented by the dc field alone at room temperature, but which also contains a part due to the rotation, in a polar manner, of free chromophores in the guest-host system (induced The value of x is measured from the modulations of ATR... 

The first common method for molecular first hyperpolarizability determination is the electric field-induced second harmonic generation (EFISH) technique in solution [6-10]. This technique can be applied only to dipolar molecules. Under an applied external electric field, molecules in solution orient approximately in the direction of the field giving rise to second harmonic generation. The measured third-order nonlinear optical susceptibility is given by the following expression ... 

The Electric-Field-Induced Second-Harmonic Generation (EFISHG) technique makes it possible to measure the molecular hyperpolarizability, p, on liquids or molecular solutions. The centrosymmetry of tire solution is broken by applying a DC electric field to induce an average orientation of the molecules due to interactions of the permanent dipoles of the molecules and the electric field. The energy of a dipole with a permanent dipole fi in an electric field E is given by ... 

In addition to orienting dipoles, electric fields induce dipole moments in molecules, since electrons and nuclei experience forces in opposite directions in the same electric field and since electrons, being less massive, move much more easily than nuclei in a field. The quantity that measures the ease with which the electron cloud in a certain molecule can be distorted is the molecular polarizability ccq. The magnitude of an induced dipole is given as... 

Several alternative self-assembly approaches for producing thermally stable, acentric chromophoric multilayers have been reported [142-144]. The most prominent example is that developed by Katz et al. [145,146], which takes advantage of the zirconium phosphonate/phosphate coordinative bonding to fix layers of a dye to one another producing films with a good structural regularity and stability to orientational randomization of up to 150°C. Another example utilizes the electric field-induced LbL assembly technique of ionic species, followed by UV irradiation to convert the ionic bonds between layers into covalent bonds [147],... 

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