Nuclear spin-lattice relaxation rates in liquid crystals

Comparison of the dielectric and NMR results for liquid crystals: dynamic aspects

Critical analysis of the results of studies of molecular rotational dynamics in liquid crystalline substances with the aid of the dielectric spectroscopy (DS) and nuclear magnetic resonance (NMR) is given. Both methods are known to be sensitive to different aspects of molecular rotations: the polarization vector and the relaxation time τ(DS) in the case of DS, a tensor describing a nuclear interaction and the correlation time τ(NMR) for NMR method. Furthermore, both methods provide correlation functions with different rank values. A common basis for the comparison between τ(DS) and τ(NMR) is postulated. Several examples of the temperature dependence of the correlation times coming from the two spectroscopic methods are presented. Qualitative agreements of the correlation times were achieved in most cases.

A molecular and thermodynamic view of the assembly of gold nanoparticles in nematic liquid crystal

The molecular interactions driving the assembly of gold nanoparticles (AuNPs) in a nematic liquid crystal (LC) are directly detected by nuclear magnetic resonance (NMR) spectroscopy and thermodynamically analyzed. The orientational orders of the selectively deuterated LC matrix and AuNP ligands, each separately followed by variable temperature (2)H NMR as a function of particle concentration, were observed to be strongly correlated. The mechanism of the reversible formation of long-range, quasi-periodic nanoparticle structures is attributed to the coupling of the AuNP ligands to the LC matrix, inducing an isotropic-nematic biphasic state. Experimentally validated thermodynamic modeling shows that, in contrast to colloidal nematics that are dominated by elastic forces, nematic dispersions of nanoparticles self-organize through a subtle balance of entropic forces and excluded volume, interface-mediated mesogen and nanoparticle molecular interactions, and couplings between conserved and nonconserved order parameters. Fine-tuning of these interactions through ligand and mesogen chemistry, together with mesoscale modeling, provides a route for materials innovations by merging structured fluid physics and nanoscience.

Orientational order, molecular organization, and dynamics in mixtures of bent-core and rod-shaped mesogens: a 2H NMR study

Mixtures of a bent-core mesogen (ClPbis10BB) and a calamitic mesogen (6OO8), showing a nematic phase over the entire compositional range and one or two smectic phases (namely, SmA, SmC, or SmC(A)) below the nematic one over a wide concentration range, were investigated by means of (2)H NMR spectroscopy, exploiting selectively deuterated isotopomers of both mesogens. The analysis of (2)H NMR spectra recorded in the liquid crystalline phases on several representative mixtures gave information on the orientational order properties and the molecular organization within the phases as well as on the alignment properties upon application of a magnetic field. On the other hand, the analysis of (2)H longitudinal relaxation times (T(1Z) and T(1Q)) in 6OO8-d(2)/ClPbis10BB mixtures and pure 6OO8-d(2) allowed the influence of the bent-core mesogen on the dynamics of the calamitic one to be highlighted.

Dynamics of partially oriented L-phenylalanine-d(8) in the CsPFO/H(2)O lyotropic system via (2)H NMR relaxation studies

Dynamics of the l-phenylalanine-d(8) has been here investigated by analyzing the (2)H NMR spin-lattice relaxation times of this selectively deuterium enriched amino acid diluted in the cesium pentadecafluorooctanoate/water (CsPFO/H(2)O) lyotropic system both in the nematic (N(+)(D)) and in the lamellar (L(D)) phases. Information on the internal and overall molecular motions as well as on collective motions has been achieved by a global fitting procedure. The dynamic processes affecting this probe molecule reflect its particular conformational and interaction properties with respect to the lyotropic environment. The best reproduction of the experimental data is obtained by assuming free internal reorientations of the benzylic moiety, which results in diffusion constants of the same order of magnitude of the overall molecular spinning motion. Moreover, the contribution of collective motions (order director fluctuations and layer undulations) is estimated to be greater than that commonly observed by other techniques in lyotropic systems.

Liquid crystal 8CB in random porous glass: NMR relaxometry study of molecular diffusion and director fluctuations

We present the measurements of the proton spin-lattice relaxation time T1 of liquid crystal 4-n-octyl-4'-cyanobiphenyl (8CB) confined into randomly oriented approximately 15 nm pores of untreated porous glass. In the low kilohertz range the spin-lattice relaxation rate in the nanoconfined 8CB is about ten times larger than in the bulk. We show that the increase is mainly due to molecular reorientations mediated by translational displacements (RMTD). In the paranematic phase the power law describing the RMTD dispersion, (T1(-1))RMTD proportional, omega(-p), is well characterized by the exponent p=0.5+/-0.06 and suggests an equipartition of diffusion modes with different wavelengths. The largest distance related to the decay of the orientational correlation function is about twice the diameter of the cavity. The situation is different in the nematic phase, where the orientational correlation is eventually lost at approximately 60 nm in the direction along the pore, a distance corresponding roughly to the length of a pore segment in the glassy matrix. The exponent p is between 0.65 and 0.9, depending on the temperature, which implies that in the nematic phase long wavelength modes are relatively more important--a consequence of the uniform director field along the pore. These observations are in agreement with the model of mutually independent pores with nematic director parallel to the pore axis in each segment. We point out that in strongly confined liquid crystals the proton NMR relaxometry does not provide the evidence of director fluctuations correlated over micrometer distances as was suggested earlier. The local translational diffusion of molecules within the cavities is found about as fast as in bulk.

Expected and Unexpected Behavior of the Orientational Order and Dynamics Induced by Azobenzene Solutes in a Nematic

We have explored the changes in the phase stability, orientational order, and dynamics of the nematic 4-cyano-4'-n-pentylbiphenyl (5CB) doped with either the trans or the cis form of different p-azobenzene derivatives using the ESR spin-probe technique. In particular, we have studied the effects induced by each of the seven nonmesogenic 4-R-phenylazobenzenes (R = H, F, Br, CH3, CF3, On-Bu, Ot-Bu) at 1% and 7% mole fraction on the order parameter <P2> and on the shift of the nematic-isotropic transition temperature (TNI), as reported by a nitroxide spin probe, and we have tried to relate them to the solute shape and charge distribution. In all the cases the presence of the azo-derivative causes a depression of T(NI), more pronounced for the cis isomers. The dependence of <P2> on the reduced temperature T* = T/T(NI) remains the same as that of pure 5CB in all trans-doped samples at 1% and 7% and decreases only slightly in the cis at 1%. However, we observe different and in some cases large variations (up to 25%) in <P2> for the cis at 7%, showing solute effects that go beyond the shift in T(NI). Surprisingly enough, even at the highest concentration, the probe dynamics appears to be essentially independent of the nature, the configuration, and the concentration of the different solutes and very similar to that observed in the pure 5CB.

Headgroup Motion in a Lyotropic Lamellar Phase by Deuterium and Nitrogen-14 Relaxation Studies

Deuterium (2H) and nitrogen-14 (14N) NMR spectroscopy were used to investigate the molecular dynamics of a lyotropic liquid crystal. Deuterium spectral densities of motion for the C1 deuterated site on the chain of the molecule decylammonium chloride (DACl) at the Larmor frequency 61.4 MHz and those for the (14)N at the headgroup (NH(3)(+)) at 28.9 MHz are analyzed quantitatively in the lamellar phase of the DACl-d(11)/water binary system to shed light on the headgroup dynamics. The motional model used is the small step rotational diffusion for reorientations plus internal rotations of the methylene group in the strong collision limit. The tumbling motion of the long axis of the DACl molecule in the aggregates seems to be as rigorous as the molecular spinning motion, likely due to the proposed motional model. The similarity of deuterium spectral densities from the C1 and C2/C3 sites may indicate a relatively rigid unit of C1-C2-C3 in the backbone.

Dielectric and elastic properties of liquid crystals

The structural properties, the static and relaxation dielectric coefficients [epsilon(j) and epsilon(j)(omega) (j= ||, perpendicular)], the rotational diffusion constants D( perpendicular) and D( ||), the orientational correlation times tau(1)(i0) (i=0,1), and the bulk elastic constants K(i) (i=1,2,3) are investigated for polar liquid crystals, such as 4-n-pentyl-4(')-cyanobiphenyl (5CB). epsilon(j) are calculated by a combination of the existing molecular theory and statistical-mechanical approach (SMA) that takes into account translational and orientational correlations as well as their coupling, whereas epsilon(j)(omega) are calculated by combining SMA and nuclear magnetic resonance relaxation theory, both based on a rotational diffusion model in which the reorientation of an individual molecule is assumed as stochastic Brownian motion in a potential of mean torque. Reasonable agreement between the calculated and experimental values of epsilon(j) and epsilon(j)(omega) for 5CB is obtained. The bulk Frank elastic constants K(i) (i=1,2,3), for splay, twist, and bend distortion modes, as well as their ratios K(3)/K(1) and K(2)/K(1) are also obtained.

Rotational viscosity, dynamic phenomena, and dielectric properties in a long-chain liquid crystal: NMR study and theoretical treatment

The rotational diffusion constants D(perpendicular) and D(parallel), rotational viscosity coefficients gamma(i) (i=1,2), the orientational correlation times tau(L)mn, and the dielectric permittivities for nematic liquid crystals (NLCs) are investigated. gamma(i) are calculated by a combination of existing statistical-mechanical approach (SMA) and NMR relaxation theory, both based on a rotational diffusion model. In the rotational diffusion model, it is assumed that the reorientation of an individual molecule is a stochastic Brownian motion in a certain potential of mean torque. According to the SMA, gamma(i) are found to be a function of temperature, density, rotational diffusion constant for tumbling motions, and the orientational order parameters. The order parameters and rotational diffusion constant are obtained from an analysis of NMR measurements. Reasonable agreement between the calculated and experimental values of gamma(i) for 4-n-octyloxy-4'-cyanobiphenyl (8OCB) is obtained. The orientational correlation times, and the longitudinal and transverse components of the real chi'(j)(omega) and imaginary chi"(j)(omega) (j= parallel, perpendicular) parts of the complex susceptibility tensor for 8OCB molecules in the nematic phase are also obtained.

Conformational dynamics of a metallomesogen studied by 2H-NMR spectroscopy

In this work we present a quantitative analysis of both quadrupolar splittings and deuterium Zeeman and quadrupolar spin-lattice relaxation times reported in the literature for two isotopomers of Azpac, an acetylacetonate derivative of the cyclopalladated 4, 4'-bis(hexyloxy) azoxybenzene. Azpac-d(4) is deuterated at the aromatic rings and Azpac-d(26) is deuterated on the alkoxy chains. The additive potential method is used to model the splittings, while the derived spectral densities are interpreted using the decoupled model in conjunction with the Nordio model. The two side chains are assumed to be noninteracting and identical in their conformations in order to limit the size of the transition rate matrix needed to describe correlated internal bond rotations in the chains. Rotational diffusion constants and internal jump rate constants are derived for this metallomesogen.

Deuteron NMR study of a long-chain smectic liquid crystal: molecular order and dynamics

Deuteron Zeeman (T(1Z)) and quadrupolar (T(1Q)) spin-lattice relaxation times and quadrupolar splittings were measured in the nematic and smectic A phases of a chain-deuterated 4-n-octyloxy-4'-cyanobiphenyl (8OCB-d17) at 15.1 and 46 MHz. To model the NMR observables, the so-called pentane effect is used to limit the number of possible conformations in the chain, and is found to be a good approximation. The additive potential method is employed to construct the potential of mean torque using the quadrupolar splittings. A decoupled model is used to describe correlated internal motions in the chain, which are independent of the molecular reorientation. The latter motion is treated using the small-step rotational diffusion model of Nordio (Tarroni and Zannoni), while the former motion is described using a master rate equation. In the nematic phase, order director fluctuations are found to be necessary. The relaxation data in both mesophases were treated using a global target method, and the derived motional parameters are acceptable in comparison with those found for 6OCB.

Rotational viscosity in a nematic liquid crystal: a theoretical treatment and molecular dynamics simulation

The rotational viscosity coefficient gamma(1) of 4-n-pentyl-4(')-cyanobiphenyl in the nematic phase is investigated by combination of existing statistical-mechanical approaches (SMAs), based on a rotational diffusion model and computer simulation technique. The SMAs rest on a model in which it is assumed that the reorientation of an individual molecule is a stochastic Brownian motion in a certain potential of mean torque. According to the SMAs, gamma(1) is found to be a function of temperature, density, rotational diffusion coefficient, and a number of order parameters (OPs). The diffusion coefficient and the OPs were obtained from an analysis of a trajectory generated in a molecular dynamics simulation using realistic atom-atom interactions. In addition, a set of experimentally determined diffusion coefficients and OPs was used for evaluation of gamma(1). Reasonable agreement between calculated and experimental values of gamma(1) is obtained. It is shown that near the clearing point gamma(1) is proportional to (-)P22, where (-)P2 is the second-rank OP. This limiting value of gamma(1) is in agreement with mean-field theory.