Rigid dipoles response

We shall remove an important drawback of the polarization model described in Section VI by considering another variant of a composite model than that described in previous Section VILA. We use again a linear-response theory to find the contribution of a vibrating dipole to the total permittivity . We split the total concentration N of polar molecules into the sum Nm and Nv b, where each term refers to rotation of a like rigid dipole (viz. with the same electric moment p) but characterized by different law of motion ... 

It has been shown that the main medianism responsible for EB in solutions of rigid-chain polymers is the rotation of their polar molecules as a whole whereas the anisotropy of the dielectric polarizability of the macromolecules only provides a small contribution to the Kerr effect. Hence, the general theory of the Kerr effect for rigid dipole particles with axial symmetry of the optical polarizability can be... 

The derived system of equations of motion describes simultaneously a periodic variation of the H-bond length and rotation of this bond. We obtain dielectric response both to small translational oscillations of charges and to rigid-dipole reorientations. Each response (for charges and for dipoles) is characterized by two Lorentz lines. 

Mechanism c, responsible for the V-peak shown in the same Figures, concerns elastic reorientation of a rigid dipole about the H-bond this motion is governed by the dimensionless angular force constant crot. 

In Section VI we study in detail two fast short-lived vibration mechanisms b and c, which concern item 2. The dielectric response to the elastic rotational vibrations of hydrogen-bonded (HB) polar molecules and to translational vibrations of charges, formed on these molecules, is revealed in terms of two interrelated Lorentz lines. A proper force constant corresponds to each line. The effect of these constants on the spectra of the complex susceptibility is considered. The dielectric response of the H-bonded molecules to elastic vibrations is shown to arise in the far IR region. Namely, the translational band (T-band) at the frequency v about 200 cm-1 is caused by vibration of charges, while the neighboring V-band at v about 150 cm-1 arises due to elastic rigid-dipole reorientations. In the case of water these bands overlap, and in the case of ice they are resolved due to longer vibration lifetime. 

In the copolyamides under consideration, the dipoles that are responsible for the dielectric relaxations are associated with the C = 0 groups of the amide functions. Due to the quasi-conjugated character of the CO - NH bond, the amide group takes on a rigid plane conformation in such a way that the dielectric relaxations of copolyamides should correspond to motional modes that involve amide groups and not only the carbonyls, in contrast to what happens with the ester groups encountered in polyethylene fere-phthalalc (Sect. 4.1.2). 

The response range of the local environment to the excited Trp-probe is mainly within 10 A because the dipole-dipole interaction at 10 A to that at —3.5 A of the first solvent shell drops to 4.3%. This interaction distance is also confirmed by recent calculations [151]. Thus, the hydration dynamics we obtained from each Trp-probe reflects water motion in the approximately three neighboring solvent shells. About seven layers of water molecules exist in the 50-A channel, and we observed three discrete dynamic structures. We estimated about four layers of bulk-like free water near the channel center, about two layers of quasi-bound water networks in the middle, and one layer of well-ordered rigid water at the lipid interface. Because of lipid fluctuation, water can penetrate into the lipid headgroups, and one more trapped water layer is probably buried in the headgroups. As a result, about two bound-water layers exist around the lipid interface. The obtained distribution of distinct water structures is also consistent with —15 A of hydration layers observed by X-ray diffraction studies from White and colleagues [152, 153], These discrete water stmctures in the nanochannel are schematically shown in Figure 21, and these water molecules are all in dynamical equilibrium. 

The attachment of several chromophores to a rigid polymeric backbone can also lead to a significant enhancement of the overall hyperpolarizability and dipole moment of the polymer. The three polymeric conformations that have been shown to affect the overall nonlinear response are schematically depicted in Figure 6. They include I) polymers with a helical backbone and pendant groups that have a fixed orientation relative to the axis of the central helix, II) dendrimers with pendant chromophores attached in the outer shell, and III) main-chain polymers with a rigid backbone. 

Endo et al. (1992) measured the optical transmission and the polarity-reverse current during the polarity reversion of a side chain ferroelectric liquid crystalline polymer. It was found that both parameters reached peak values at the same time. It was concluded that the rigid core of the side groups responsible for birefringence moves simultaneously with the dipole moment reversion and the latter contributes to the polarity reversion current. The FTIR experiment suggested that the backbone moves when the polarity is reversed. 

The loss and absorption peaks at v 700 cm-1, located near the border of the IR region, arise due to mechanism a—that is, due to reorientation of a rigid (permanent) dipole in the hat well. This mechanism is also responsible for the microwave loss peak located between the frequencies 0.1 and 1cm-1. The complex permittivity s of the corresponding relaxation band is actually governed by Debye theory, which is involved formally in our calculation scheme. 

The most intriguing aspect of the emission response is the blue shift that occurs upon the transition from fluid to rigid media, as in the transformation of a sol to a xerogel (115, 136-140). Formerly attributed to rigidochromism this phenomenon is actually a result of inhibition of solvent reordering around the dipole, which is created by an asymmetric excited-state charge distribution (136, 140). The solvent molecules that reorient themselves around the dipole lower... 

A detailed evaluation of the structural parameters affecting the photophysical properties was performed for the didodecyloxy-substituted quinquephenyl rigid-flexible polyethers 60. More particularly, the odd-even effect was observed for dilute polymeric solutions by means of steady-state and time-resolved fluorescence anisotropy. The different orientations of the quinquephenyl chromophores, and subsequently of their luminescent dipoles, concerning the polymers with an odd number of methylene units x=7, 9, 11) and those with an even (x = 8, 10, 12) one, were found responsible for the observed strong deviations in frozen and dilute solutions, where the flexible aliphatic chains are forced to adopt a nearly sta ered, lower energy conformation. 

---