Molecular rotational viscosity

Molecular Rotational Diffusion. Rotational diffusion is the dominant intrinsic cause of depolarization under conditions of low solution viscosity and low fluorophore concentration. Polarization measurements are accurate indicators of molecular size. Two types of measurements are used steady-state depolarization and time-dependent (dynamic) depolarization. 

Such is the newness of appreciation of near-IR fluorescence techniques that there is a dearth of examples in the literature of implementations of many of the classic fluorescence methods in the IR. Anisotropy is one striking example of this. However, in a comprehensive study of the anisotropy decay of dyes, including oxazine fluorescence at 720 nm, in mixed isotropic solvents Dutt et al.( T7 TS) have investigated the effects of viscosity on molecular rotation. 

The use of an electric field is not the only effective way to influence the LC polymer structure, magnetic fields displays a closely similar effect167 168). It is interesting as a method allowing to orient LC polymers, as well as from the viewpoint of determining some parameters, such as the order parameter, values of magnetic susceptibility, rotational viscosity and others. Some relationships established for LC polymer 1 (Table 15), its blends with low-molecular liquid crystals and partially deuterated polyacrylate (polymer 4, Table 15) specially synthesized for NMR studies can be summarized as follows ... 

To extract concrete predictions for experimental parameters from our calculations is a non-trivial task, because neither the energetic constant B nor the rotational viscosity yi are used for the hydrodynamic description of the smectic A phase (but play an important role in our model). Therefore, we rely here on measurements in the vicinity of the nematic-smectic A phase transition. Measurements on LMW liquid crystals made by Litster [33] in the vicinity of the nematic-smectic A transition indicate that B is approximately one order of magnitude less than Bo. As for j we could not find any measurements which would allow an estimate of its value in the smectic A phase. In the nematic phase y increases drastically towards the nematic-smectic A transition (see, e.g., [51]). Numerical simulations on a molecular scale are also a promising approach to determine these constants [52],... 

Fig. 9. Rotational relaxation time as a function of molecular weight (viscosity average) for fluorescein conjugates of polyacrylamide in dilute aqueous solutions. Filled circle represents the value of for free dye (recalculated from Fig. 5 of ref. 11, using the directly measured lifetime t)...

Table 3.14 Transition temperatures (°C), elastic constants fk/y, k22 kjj, 10 N), dielectric anisotropy ( e), dielectric constant measured perpendicular to the molecular long axis (e ), birefringence ( n), refractive index measured perpendicular to the director (noJ, rotational viscosity (y. Poise) and bulk viscosity (r, Poise) for tr ns-l-(4-cyanophe-nyl)-4-pentylcyclohexane (41), iTSins-l-(4-cyanophenyl)-4-[(E)-pent-l-enyl]cyclohexane (74) andtra.ns-l-(4-cyanophenyl)-4-[(E)-pent-3-enyI]cyclohexane (78) extrapolated to 100% at 22°...

An attempt to calculate the viscosity of a liquid on the basis of the classical kinetic theory led to the formula rj=Pal2c, where P=internal pressure, or=molecular diameter, and c=molecular velocity, and this was confirmed for liquid hydrogen (when molecular rotation is included), P being taken as afrom the van der Waals equation. 

However, viscometric measurements of dilute polymer solutions in a steady flow are inadequate for this purpose although, as already indicated, viscosity is related to molecular rotation. This has been demonstrated by Zimm s theory ). Zimm considered the kinetics of the motion and deformation of a kinetically flexible polymer chain in a weak mechanical field with harmonic velocity gradient g at frequency v. It has been found that under steady and weak flow conditions... 

Photoisomerization was studied from a purely photochemical point of view in which photo-orientation effects can be disregarded. While this feature can be true in low viscosity solutions where photo-induced molecular orientation can be overcome by molecular rotational diffusion, in polymeric environments, especially in thin solid film configurations, spontaneous molecular mobility can be strongly hindered and photo-orientation effects arc appreciable. The theory that coupled photoisomerization and photo-orientation processes was also recently developed, based on the formalism of Legendre Polynomials, and more recent further theoretical developments have helped quantify coupled photoisomerization and photo-orientation processes in films of polymer. 

The viscosity of a sample reflects its ability to absorb microwave energy because it affects molecular rotation. The effect of viscosity is best illustrated by considering ice water. When water is frozen, the water molecules become locked in a crystal lattice. This greatly restricts molecular mobility and makes it difficult for the molecules to align with the microwave field. Thus, the dielectric dissipation factor of ice is low (2.7 X 10 at 2450 MHz). When the temperature of the water is increased to 27°C, the viscosity decreases and the dissipation factor rises to a much higher value (12.2). 

The switching time r of a TN cell depends mostly on the rotational viscosity which can be influenced by molecular design, and on the elastic splay constant Kp, the correlation of with molecular structure remains quite elusive. 

Figure 6.42. The switch time r (a) and the rotational viscosity q (b) vs. temperature for small molecular mass (L1-L3) and polymeric (P1-P3) ferroelectric liquid crystals. (Modified from Sekiya et al., 1993. Reproduced by permission of Taylor Francis, )...

Figure 6.44. The relation of rotational viscosity r) and molecular weight Mw at 600 °C. (Modified from Endo et al., 1992.)...

The molecule contains Si-0 bonds. FTIR suggested that the Si-0 bonds move when the side groups move. Hence, ferroelectric liquid crystalline polymers have higher rotational viscosities than small molecular mass ferroelectric liquid crystals. In Figure 6.44 the relation of rotational viscosity r/ and molecular weight Mw at 600 °C is plotted, rj increases as Mw increases and the quadratic law is observed. 

As polarization arises from the mode of vibration of an atom or molecule, both bands and lines in luminescence spectra suffer polarization. Since polarization of fluorescent light is closely associated with the life time of the excited state, a lower limit for the life time of excited state of many organic compounds was obtained37 by consideration of the linear polarization of fluorescence in media of high and low viscosity by using Perrin s law of depolarization41, which connects polarization with the life time of the excited state and the relaxation time of molecular rotation. 

The conventional NOEs in the laboratory frame can be positive or negative and pass through zero when o)otc, the product of spectrometer angular frequency and molecular rotational correlation time (which depends on the size and shape of the molecules and on the viscosity of the rotating medium) is approximately equal to unity. Such a problem, which is typical of middle-sized molecules like saponins, can be solved by performing the experiment in the rotating frame, the so-called ROESY [26, 27]. 

The effect of nitrile geometry (linear or bent) on the singlet-state properties of benzonitrile and p-dimethylaminobenzonitrile has been investigated by INDO/S calculations. A previously low-lying hidden state is the bent form. Solvent viscosity has a marked effect on the fluorescence yield of / -A,A-dialkyl-aminobenzylidenemalononitrile. The yield is increased by decreasing molecular rotation of surrounding molecules and so the molecule can be used as probe of microenvironments. 

Supercritical media have very promising features for NMR spectroscopy. Because of their low bulk viscosity and the associated small molecular rotational correlation times of solutes in these media [68], relaxation times of quadrupolar nuclei increase significantly and, hence, their line widths (HHW) will be considerably reduced compared to those in common solvents [30,69,70]. This feature may be advantageously employed for the detection of NMR resonances of quadrupolar nuclei, because measurements in the liquid state are often troubled due to extremely broad line widths. An illustrative example involving Co was described in Section 3.2-3. 

The lack of fluorescence of TAM dyes in solution is attributed to an extremely rapid, nomadiative deactivation process brought about by intramolecular rotation of the flexible aryl groups. Suppression of molecular rotation, by increasing the viscosity of the medium, by binding of the dye to a polymer or protein, or by selfassociation, diminishes this radiationless process. Coincident with this change in photophysical properties is often a dramatic increase in sensitivity toward photofading due to an increase in the quantum yield of the relatively long-lived and photochemically active triplet state [139-143]. The triplet state of the dye cation... 

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