Orientation, in electric fields

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. 

Figure 2-3. An illustration of how well molecules can be oriented in electric fields of moderate strength. The wavefunction shown in the figure is for the = 0 pendular state for N2-HF at 16.6 kV cm 1.

M. F. Schiekel and K. Fahrenschon, Deformation of nematic liquid crystals with vertical orientation in electric fields, App/. Phys. Lett. 19, 391 (1971). 

Hilbers, C.W., MacLean, C. NMR of Molecules Oriented in Electric Fields. 7, 1-52... 

Equation (21.18) will be useful for modelling polar molecules orienting in electric fields and gating in ion channels (see page 425). 

M. F. Shiekel and K. Fahrenschon, "Deformation of Nematic Liquid Crystals with Vertical Orientation in Electrical Fields, Appi. Phys. Lett., 19, p. 393 (1971). 

Liquid crystals (LCs) are organic liquids with long-range ordered structures. They have anisotropic optical and physical behaviors and are similar to crystal in electric field. They can be characterized by the long-range order of their molecular orientation. According to the shape and molecular direction, LCs can be sorted as four types nematic LC, smectic LC, cholesteric LC, and discotic LC, and their ideal models are shown in Fig. 23 [52,55]. 

Orientation polarization can occur in materials composed of molecules that have permanent electric dipole moments. The permanent dipoles tend to become aligned with the apphed electric field, but entropy and thermal effects tend to counter this alignment. Thus, orientation polarization is highly temperature-dependent, unlike the forms of induced polarization which are nearly temperature-independent. In electric fields of moderate intensity, the orientation polarization is proportional to the local electric field, as for the other forms of polarization... 

Structure, then for every tetrahedron there is another tetrahedron which has the exact opposite orientation the electric fields of the dipoles compensate each other. If, however, all tetrahedra have the same orientation or some other mutual orientation that does not allow for a compensation, then the action of all dipoles adds up and the whole crystal becomes a dipole. Two opposite faces of the crystal develop opposite electric charges. Depending on the direction of the acting force, the faces being charged are either the two faces experiencing the pressure (longitudinal effect) or two other faces in a perpendicular or an inclined direction (transversal effect). 

One of the most specific and unique features of low-molecular liquid crystals is their ability for orientation in external fields — mechanical, electric and magnetic. It is this property that establishes wide capabilities for technical application of liquid crystals. Today electric and magnetic optics of liquid crystals are an independent and useful for practics branch of the physics of the condensed state of matter 42 43 ... 

Thus the ability of LC polymers to orient in electric and magnetic fields reveals promise for the investigation of the specific features of LC polymer structure, as well as for the study of the mechanism of orientation and structural rearrangement processes in low-molecular liquid crystals, where they are very fast and in some cases are even hard to measure. On the other hand, this provides a method to control the structure of a polymer and thus create new materials with interesting optical properties. 

Here directions z and x correspond to the orientation of electric field across and along the layers correspondingly, Exz is the DC components of electric field, index (0 is related to the AC electric field components associated with the motion of JVL. In earlier calculations of dissipation in JFF regime [19, 24] only two first terms of Eq. (6) have been taken into account. However, as that has been pointed out recently [23], the third term can have the leading contribution for the highly anisotropic layered superconductors, satisfying the condition T = ( 1. For materials like BSCCO at low... 

To examine the role of the LDOS modification near a metal nanobody and to look for a rationale for single molecule detection by means of SERS, Raman scattering cross-sections have been calculated for a hypothetical molecule with polarizability 10 placed in a close vicinity near a silver prolate spheroid with the length of 80 nm and diameter of 50 nm and near a silver spherical particle with the same volume. Polarization of incident light has been chosen so as the electric field vector is parallel to the axis connecting a molecule and the center of the silver particle. Maximal enhancement has been found to occur for molecule dipole moment oriented along electric field vector of Incident light. The position of maximal values of Raman cross-section is approximately by the position of maximal absolute value of nanoparticle s polarizability. For selected silver nanoparticles it corresponds to 83.5 nm and 347.8 nm for spheroid, and 354.9 nm for sphere. To account for local incident field enhancement factor the approach described by M. Stockman in [4] has been applied. To account for the local density of states enhancement factor, the approach used for calculation of a radiative decay rate of an excited atom near a metal body [9] was used. We... 

There are sub-effects of these changes in electric field strengths too. The magnitude of such fields distort the shape of molecules in it and clearly this changes their reactivity or at least modifies the effect these squeezed and oriented molecules have on nearby reactants and products. 

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... 

Recently liquid crystals have received much attention because of their optical properties that enable one to monitor minute changes in temperature and mechanical stress, for their ability to orient in electric and magnetic fields, and because of their implications on structures observed in biological systems. 

The dynamic shape factor is almost always greater than 1.0 for irregular particles and flows at small Reynolds numbers and is equal to 1.0 for spheres. For a nonspherical particle of a given shape % is not a constant but changes with pressure, particle size, and as a result of particle orientation in electric or aerodynamic flow fields. 

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