Slow dynamics in a liquid crystal: 1H and 19F NMR relaxometry Slow dynamics in a liquid crystal: 1 H and 19 F NMR relaxometry (original) (raw)
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Slow dynamics in a liquid crystal: 1H and 19F NMR relaxometry
The Journal of Chemical Physics, 2011
Spin-lattice relaxation rates (R 1H and R 1F ) of two nuclear species ( 1 H and 19 F) are measured at different temperatures in the isotropic phase of a liquid crystal (4 -butoxy-3 -fluoro-4isothiocyanatotolane-4OFTOL), over a wide range of Larmor frequency (10 kHz-50 MHz). Their dispersion profiles are found to be qualitatively very different, and the R 1F in particular shows significant dispersion (varying over two orders of magnitude) in the entire isotropic range, unlike R 1H . The proton spin-lattice relaxation, as has been established, is mediated by time modulation of magnetic dipolar interactions with other protons (case of like spins), and the discernable dispersion in the mid-frequency range, observed as the isotropic to nematic transition is approached on cooling, is indicative of the critical slowing of the time fluctuations of the nematic order. Significant dispersion seen in the R 1F extending to very low frequencies suggests a distinctly different relaxation path which is exclusively sensitive to the ultra slow modes apparently present in the system. We find that under the conditions of our experiment at low Zeeman fields, spin-rotation coupling of the fluorine with the molecular angular momentum is the dominant mechanism, and the observed dispersion is thus attributed to the presence of slow torques experienced by the molecules, arising clearly from collective modes. Following the arguments advanced to explain similar slow processes inferred from earlier detailed ESR measurements in liquid crystals, we propose that slowly relaxing local structures representing such dynamic processes could be the likely underlying mechanism providing the necessary slow molecular angular momentum correlations to manifest as the observed low frequency dispersion. We also find that the effects of the onset of cross-relaxation between the two nuclear species when their resonance lines start overlapping at very low Larmor frequencies (below ∼ 400 kHz), provide an additional relaxation contribution.
Molecular motions in thermotropic liquid crystals studied by NMR spin-lattice relaxation
Brazilian Journal of Physics, 1998
Nuclear magnetic resonance relaxation experiments with eld cycling techniques proved to b e a v aluable tool for studying molecular motions in liquid crystals, allowing a very broad Larmor frequency variation, su cient to separate the cooperative motions from the liquidlike molecular di usion. In new experiments combining NMR eld cycling with the Jeener-Broekaert order-transfer pulse sequence, it is possible to measure the dipolar order relaxation time T 1D , in addition to the conventional Zeeman relaxation time T 1Z in a frequency range of several decades. When applying this technique to nematic thermotropic liquid crystals, T 1D showed to depend almost exclusively on the order uctuation of the director mechanism in the whole frequency range. This unique characteristic of T 1D makes dipolar order relaxation experiments specially useful for studying the frequency and temperature dependence of the spectral properties of the collective motions.
NMR proton spin dynamics in thermotropic liquid crystals subject to multipulse excitation
Physical Review E, 2003
Previous experiments of NMR spin-lattice relaxation times as a function of the Larmor frequency, as measured with the field-cycling technique ͑FC͒, were shown to be very useful to disentangle the various molecular motions, both local and collective, that dominate the relaxation in different time scales in liquid crystals. However, there are many examples where the known theoretical models that represent the molecular relaxation mechanisms cannot be fitted to the experimental trend in the region of low fields, making it difficult to obtain reliable values for the spectral densities involved, especially for the cooperative motions which dominate at low frequencies. In some cases, these anomalies are loosely ascribed to ''local-field'' effects but, to our knowledge, there is not a detailed explanation about the origin of these problems nor the range of frequencies where they should be expected. With the aim of isolating the dipolar effects from the influence of molecular dynamics, and taking into account the previous results in solids, in this work we investigate the response of the proton spin system of thermotropic liquid crystals 4-pentyl-4Ј-cyanobiphenyl ͑5CB͒ and 4-octyl-4Ј-cyanobiphenyl ͑8CB͒ in nematic and smectic A phases, due to the NMR multipulse sequence 90 y ؠ-(x-) N. The nuclear magnetization presents an early transient period characterized by strong oscillations, after which a quasistationary state is attained. Subsequently, this state relaxes towards internal equilibrium over a time much longer than the transverse relaxation time T 2. As occurs in solids, the decay time of the quasistationary state T 2e presents a minimum when the pulse width x and the offset of the radiofrequency are set to satisfy resonance conditions ͑spin-lock͒. When measured as a function of the pulse spacing in ''onresonance'' experiments, T 2e shows the behavior expected for cross relaxation between the effective Zeeman and dipolar reservoirs, in accordance with the thermodynamic theory previously developed for solids. Particularly, for values of comparable with T 2 , the relaxation rate follows a power law T 2e ϰ Ϫ2 , in all the observed cases, for the resonance conditions x ϭ/3 and equivalent frequency e ϭ/3. When is similar to or greater than typical dipolar periods, the relaxation rate becomes constant and for much shorter than T 2 , the thermodynamic reservoirs get decoupled. These experiments confirm that the thermodynamic picture is valid also in liquid crystals and the cross relaxation between the reservoirs can be detected without interference with spin-lattice relaxation effects. Accordingly, this technique can be used to estimate the frequency range, where cross-relaxation effects can be expected when Zeeman and dipolar reservoirs are put in thermal contact with each other and with the lattice, as in FC experiments. In particular, the present results allow us to associate the anomalies observed in low-field spin-lattice relaxation with nonadiabatic energy exchange between the reservoirs.
Multinuclear NMR relaxometry studies in singly fluorinated liquid crystal
Nuclear spin-lattice relaxation rate dispersion study of 1 H and 19 F in the isotropic phase of a singly fluorinated liquid crystal 4 0 -butoxy-3-fluoro-4-isothiocyantotolane (4OTOLFo) points to their differing relaxation pathways and hence sensitivity to qualitatively different time modulations. In particular fluorine nuclear spins, with strong lattice coupling (larger by two orders) extending to very low frequencies, detect slowly relaxing local structures via the spin-rotation interaction. The field-cycling technique used to carry out these very low frequency measurements, provides for level crossing of the two nuclear species at low enough jump fields, facilitating an additional mechanism of cross-relaxation in the strong coupling limit.
Journal of Physical Chemistry B, 2001
Transverse nuclear spin relaxation measurements, employing the Carr-Purcell-Meiboom-Gill sequence, can provide detailed information on the dynamics of director fluctuations in liquid crystals. In principle, a full characterization of the rate dispersion of the director modes can be derived from such measurements. However, the rigorous analysis of these experiments is generally hampered by the lack of a timescale separation between the slow director fluctuations and the transverse magnetization decay. Under these conditions, the use of a fast-motion theory (i.e., Redfield theory) is no longer justified and one should resort to a slowmotional approach based on the stochastic Liouville equation for the simultaneous evolution of the stochastic variables (i.e., the director field) and the spin degrees of freedom. In this paper, explicit expressions for transverse deuteron spin relaxation times are derived without invoking any timescale separation, on condition that (i) the stochastic variables are described as a multidimensional Gaussian process and that (ii) the spin Hamiltonian linearly depends on the stochastic variables. These are precisely the conditions usually adopted for the modeling of director fluctuations and their relaxation effects in the harmonic approximation. Thus, the present theory allows for a rather general analysis of the transverse magnetization decay in different kinds of experiments. In particular, analytical expressions are derived for the transverse relaxation times in Carr-Purcell-Meiboom-Gill sequences and evaluated as a function of the pulse spacing and the number of cycles in the sequence. It is shown that in the limit of an infinite number of pulses, one can interpret the measured asymptotic relaxation time as a superposition of independent contributions evaluated according to the Luz-Meiboom equation (Luz, Z.; Meiboom, S. J. Chem. Phys. 1963, 39, 366), originally derived within the Redfield limit.
Journal of Chemical Physics, 2003
Transverse nuclear spin relaxation measurements, employing the Carr-Purcell-Meiboom-Gill sequence, can provide detailed information on the dynamics of director fluctuations in liquid crystals. In principle, a full characterization of the rate dispersion of the director modes can be derived from such measurements. However, the rigorous analysis of these experiments is generally hampered by the lack of a timescale separation between the slow director fluctuations and the transverse magnetization decay. Under these conditions, the use of a fast-motion theory (i.e., Redfield theory) is no longer justified and one should resort to a slowmotional approach based on the stochastic Liouville equation for the simultaneous evolution of the stochastic variables (i.e., the director field) and the spin degrees of freedom. In this paper, explicit expressions for transverse deuteron spin relaxation times are derived without invoking any timescale separation, on condition that (i) the stochastic variables are described as a multidimensional Gaussian process and that (ii) the spin Hamiltonian linearly depends on the stochastic variables. These are precisely the conditions usually adopted for the modeling of director fluctuations and their relaxation effects in the harmonic approximation. Thus, the present theory allows for a rather general analysis of the transverse magnetization decay in different kinds of experiments. In particular, analytical expressions are derived for the transverse relaxation times in Carr-Purcell-Meiboom-Gill sequences and evaluated as a function of the pulse spacing and the number of cycles in the sequence. It is shown that in the limit of an infinite number of pulses, one can interpret the measured asymptotic relaxation time as a superposition of independent contributions evaluated according to the Luz-Meiboom equation (Luz, Z.; Meiboom, S. J. Chem. Phys. 1963, 39, 366), originally derived within the Redfield limit.
Molecular dynamics in a blue phase liquid crystal: a 1H fast field-cycling NMR relaxometry study
Soft Matter, 2013
Liquid crystals exhibiting Blue Phases (BPs) have been the focus of academic and commercial research interest in the last few years due to their highly interesting properties in the fields of optics and photonics. In order to better understand the properties of the BPs, it is important to study molecular dynamics in these phases, including molecular rotations/reorientations, diffusion, and collective motions. Here, we present the first study of molecular dynamics in a BP system by means of 1 H fast field-cycling relaxometry. The investigated sample, called 10BBL, was a lactate derivative, containing two chiral centers, exhibiting BP, TGBA*, TGBC*, and SmC* phases stable in rather large temperature ranges. Molecular dynamics was investigated by analyzing the temperature-and frequency-dependencies of spin-lattice relaxation. We compare the dynamics in the TGB phases with the one in the BP and with the TGB phases investigated in previous studies.
Quasiequilibrium states in thermotropic liquid crystals studied by multiple-quantum NMR
The Journal of Chemical Physics, 2009
Previous work showed that by means of the Jeener-Broekaert ͑JB͒ experiment, two quasiequilibrium states can be selectively prepared in the proton spin system of thermotropic nematic liquid crystals ͑LCs͒ in a strong magnetic field. The similarity of the experimental results obtained in a variety of LC in a broad Larmor frequency range, with crystal hydrates, supports the assumption that also in LC the two spin reservoirs, into which the Zeeman order is transferred, originate in the dipolar energy and that they are associated with a separation in energy scales: A constant of motion related to the stronger dipolar interactions ͑S͒, and a second one ͑W͒ corresponding to the secular part of the weaker dipolar interactions with regard to the Zeeman and the strong dipolar part. We study the nature of these quasi-invariants in nematic 5CB ͑4Ј-pentyl-4-biphenyl-carbonitrile͒ and measure their relaxation times by encoding the multiple-quantum coherences of the states following the JB pulse pair on two orthogonal bases, Z and X. The experiments were also performed in powder adamantane at 301 K which is used as a reference compound having only one dipolar quasi-invariant. We show that the evolution of the quantum states during the buildup of the quasiequilibrium state in 5CB prepared under the S condition is similar to the case of powder adamantane and that their quasiequilibrium density operators have the same tensor structure. In contrast, the second constant of motion, whose explicit operator form is not known, involves a richer composition of multiple-quantum coherences of even order on the X basis, in consistency with the truncation inherent in its definition. We exploited the exclusive presence of coherences of Ϯ4, Ϯ 6, Ϯ 8, besides 0 and Ϯ2 under the W condition to measure the spin-lattice relaxation time T W accurately, so avoiding experimental difficulties that usually impair dipolar order relaxation measurement such as Zeeman contamination at high fields and also superposition of the different quasi-invariants. This procedure opens the possibility of measuring the spin-lattice relaxation of a quasi-invariant independent of the Zeeman and S reservoirs, so incorporating a new relaxation parameter useful for studying the complex molecular dynamics in mesophases. In fact, we report the first measurement of T W in a LC at high magnetic fields. Comparison of the obtained value with the one corresponding to a lower field ͑16 MHz͒ points out that the relaxation of the W-order strongly depends on the intensity of the external magnetic field, similarly to the case of the S reservoir, indicating that the relaxation of the W-quasi-invariant is also governed by the cooperative molecular motions.
Chemical Physics Letters, 2012
F Fast Field-Cycling (FFC) NMR relaxometry was applied for the first time for the investigation of the dynamics of a liquid crystal. Longitudinal 19 F relaxation rates were measured at different temperatures in the nematic phase of 4DBF2 in the frequency range between about 9.5 kHz and 33 MHz. The observed relaxation dispersions could be well reproduced using a simple model in which contributions due to order director fluctuations and molecular rotational diffusion motions were considered. The obtained results indicate 19 F FFC NMR relaxometry as a valuable method for the investigation of molecular and collective motions of fluorinated liquid crystals.