Electron-spin relaxation and ordering in smectic and supercooled nematic liquid crystals

We report on careful line shape studies of slow motional and orientation dependent ESR spectra of a deuterated liquid\u2010crystal\u2010like spin probe dissolved in a benzilidene\u2010derivative (40,6) and in cyanobiphenyl derivative (S2 and 5CB) liquid crystals. The simulation of the ESR spectra is based on the Lanczos algorithm recently applied by Moro and Freed in a general and efficient formulation of slow motional and ordering effects on ESR line shapes. With 40,6 which exhibits monolayer smectic phases, we find that the main change in the spin relaxation upon passing from the nematic to the smectic A phase consists of changes occuring in ordering attributable to packing forces on functional groups. Such ordering effects appear to be further enhanced in the SB phase with consequent alterations in dynamics. With S2, which exhibits an interpenetrating bilayer smectic A phase, we find unusual ESR spectra in that phase which may be simulated on the basis of a model of cooperative distortions static on the ESR time scale, and superimposed on individual molecular reorientation. This mode is interpreted as a collective chain distortion which affects the orientational distribution of the piperidine ring of the spin probe. A similar phenomenon is observed in the supercooled nematic phase of 5CB, which is aligned by an electric field, and evidence is also found that the reorientational dynamics of this ring are affected by interaction with local cooperative modes in the liquid crystal (i.e., a SRLS mechanism previously proposed by Freed and co\u2010workers). Some microscopic characteristics of liquid crystals revealed by this and previous ESR spin probe studies are discussed

Electron spin resonance and electron-spin-echo study of oriented multilayers of L alpha-dipalmitoylphosphatidylcholine water systems.

A detailed electron spin resonance (ESR) study of spin-labeled-oriented multilayers of L alpha-dipalmitoylphosphatidylcholine (DPPC) water systems for low water content (2-10% by weight) is reported with the purpose of characterizing the dynamical and structural properties of model membrane systems. Emphasis is placed on the value of combining such experiments with detailed simulations based on current slow-motional theories. Information is obtained regarding ordering and anisotropic rotational diffusion rates via ESR lineshape analysis over the entire motional range, from the fast motional region through the moderately slow and slow to the rigid limit. This includes the low-temperature gel phase, the liquid crystalline L alpha (1) phase and what appears to be a third high-temperature phase above the L alpha phase. Cholestane (CSL) and spin-labeled DPPC (5-PC, 8-PC, and 16-PC) have been used to probe different depths of the bilayer. While CSL and 5-PC both reflect the high ordering of the bilayer close to the lipid-water interface, CSL appears to be located close enough to the water for the nitroxide to be involved in hydrogen bonding with water molecules. 16-PC reflects the relatively low ordering near the tail of the hydrocarbon chain in the bilayer. Quantitative estimates of ordering and motion are obtained for these cases. The results from CSL indicate that close to the lipid-water interface the DPPC molecule is oriented approximately perpendicular to the bilayer in these low water-content systems. However, all three labeled lipid probes indicate that the hydrocarbon chain of DPPC may be bent away from the bilayer normal by as much as 30 degrees and this evidence is stronger at low temperatures. When cholesterol is added to the DPPC-water system at a concentration greater than or equal to 2.5 mol %, the ordering is greatly increased although the rotational diffusion rate remains almost unaffected in the gel phase. Electron spin echoes (ESE) are observed for the first time from oriented lipid-water multilayers. Results obtained from cw ESR lineshape analysis are correlated with data from ESE experiments, which give a more direct measurement of relaxation times. These results indicate that for detection of very slow motions (close to the rigid limit) ESE experiments are more sensitive to dynamics than continuous wave ESR for which inhomogeneous broadening becomes a major problem

Deuterium NMR studies of n-alkyl-β-D-glucopyranosides liquid-crystalline systems

Liquid-crystalline systems consisting of n-alkyl-β-D-glucopyranosides (heptyl to dodecyl) and solutes (water, n-octanol, benzene and methylene dichloride) have been studied by measurement of the deuterium NMR quadrupolar splittings. Particular attention was given to the system of n-octyl-β-D-glucopyranoside (OG)-solute for which the OG-water and the OG-benzene phase diagrams were determined. Information about the orientational order of the aliphatic chain was obtained from deuterium NMR measurements of chain deuteriated OG. The order parameter profiles for the chain in this molecule were calculated using a quantitative theoretical model. A qualitative model served to interpret the dependence of the splittings in different solutes upon their concentration. DSC and preliminary X-ray data supplement the NMR measurement