Orientational order of difluorinated liquid crystals: a comparative 13C-NMR, Optical, and dielectric study in nematic and smectic B phases

Direct method to grasp molecular topology of mesogens through 13C-1H dipolar couplings

A prime factor in determining liquid crystalline phase formation is the overall molecular shape since molecules undergo rotational motion about the long axis. Molecular topology deals with the connectivity of atomic centers in a given molecular architecture, ultimately giving rise to the gross molecular shape. ¹³C NMR has emerged as the most important technique in establishing the molecular topology of mesogens in the liquid crystalline phase. In this work, we demonstrate the utility of ¹³C-¹H dipolar couplings determined from 2D separated local field NMR for finding the topology of three different mesogens in the liquid crystalline phase. The core unit of the investigated mesogens fundamentally differs, which may be categorized as rod-like, laterally substituted, and bent-core shapes. 1D and 2D ¹³C NMR measurements in the liquid crystalline phase revealed fascinating information. The ¹³C-¹H dipolar couplings extracted from 2D NMR are found to be sensitive to topologically variant core units. This permitted us to establish the molecular topology just by looking at the ¹³C-¹H dipolar couplings of the protonated carbons of the constituent phenyl rings. By considering the dipolar couplings of rod-like mesogens as a reference, the large variation in the magnitude of ¹³C-¹H dipolar couplings of the laterally substituted and bent-core mesogens is attributed to changes in the topology of their core units. The order parameters estimated from ¹³C-¹H dipolar couplings enabled visualization of the ordering array of phenyl rings of the mesogens. Interestingly large ¹³C-¹H dipolar couplings are observed for mesogens in which (a) laterally located phenyl ring and (b) central phenyl ring of bent-core mesogens exhibited different trends as revealed by the orientational order parameters.

Anisotropy and NMR spectroscopy

Abstract

In this paper, different aspects concerning anisotropy in Nuclear Magnetic Resonance (NMR) spectroscopy have been reviewed. In particular, the relevant theory has been presented, showing how anisotropy stems from the dependence of internal nuclear spin interactions on the molecular orientation with respect to the external magnetic field direction. The consequences of anisotropy in the use of NMR spectroscopy have been critically discussed: on one side, the availability of very detailed structural and dynamic information, and on the other side, the loss of spectral resolution. The experiments used to measure the anisotropic properties in solid and soft materials, where, in contrast to liquids, such properties are not averaged out by the molecular tumbling, have been described. Such experiments can be based either on static low-resolution techniques or on one- and two-dimensional pulse sequences exploiting Magic Angle Spinning (MAS). Examples of applications of NMR spectroscopy have been shown, which exploit anisotropy to obtain important physico-chemical information on several categories of systems, including pharmaceuticals, inorganic materials, polymers, liquid crystals, and self-assembling amphiphiles in water. Solid-state NMR spectroscopy can be considered, nowadays, one of the most powerful characterization techniques for all kinds of solid, either amorphous or crystalline, and semi-solid systems for the obtainment of both structural and dynamic properties on a molecular and supra-molecular scale.

Graphic abstract

Glass Transition Dynamics and Crystallization Kinetics in the Smectic Liquid Crystal 4-n-Butyloxybenzylidene-4'-n'-octylaniline (BBOA)

The molecular dynamics of 4-n-butyloxybenzylidene-4'-n'-octylaniline (BBOA, abbreviated also as 4O.8) was studied by broadband dielectric spectroscopy (BDS) for samples that were exposed to various thermal treatments. Phase transitions between liquid crystalline phases (N, SmA, SmB_hex, and SmB_Cr) were evidenced by abrupt changes in the temperature dependence of the dielectric permittivity spectra and dielectric relaxation times. A particularly complex dynamic behavior was revealed for the highly ordered SmB_Cr phase that showed clear evidence for cooperative dynamics of a glass-forming liquid as manifested by a Vogel-Fulcher-Tammann (VFT)-type temperature dependence of its structural relaxation time τ(T). At low temperatures, dependence τ(T) again changes from VFT to Arrhenius behavior, a phenomenon commonly observed for supercooled liquids confined to nanometer length scales and occasionally discussed as strong-fragile transition on heating. In this context, our observation supports the idea of partial orientation disorder in a quenched SmB_Cr phase, where the length scale of cooperative motions is restricted to the nanometer size of glassy domains causing the deviation from "bulk" VFT-type behavior. Finally, the isothermal crystallization kinetics of the metastable SmB_Cr phase was studied in detail. On the basis of structural information about the SmB_hex and SmB_Cr phases, determined by X-ray diffraction, the subtle relation between molecular order and relaxation behavior is discussed.

Self organization of main-chain/side-chain liquid crystalline polymer based on “jacketing” effect with different lengths of spacer: from smectic to hierarchically ordered structure

A series of combined main-chain/side-chain liquid crystalline polymers based on the “jacketing” effect, with different alkyl spacer lengths (n = 2–10), have been successfully synthesized and their self-organization behavior has been investigated.

Viscoelastic behavior of a binary system of strongly polar bent-core and rodlike nematic liquid crystals

We have investigated the permittivity and viscoelastic behavior of a binary system comprising bent-core and calamitic compounds, both of which are polar, the calamitic being more strongly so, and exhibiting only the nematic mesophase. The permittivity data in the nematic as well as the isotropic phase indicate strong polar interactions between the molecules, even for mixtures with a significant content of the bent-core compound. The thermal dependence of both the splay and bend elastic constants exhibit features different from the literature. The splay constant displays a large increase with increasing concentration of bent-core material, before undergoing a precipitous drop for small calamitic content materials. Upon lowering the temperature, certain mixtures exhibit a convex-shaped feature for the bend elastic constant; that is, the value of the elastic constant is maximum at a specific temperature in the nematic phase, diminishing when the temperature is either increased or decreased. Surprisingly, the pure compounds, especially the bent-core one, show only a monotonically increasing trend for the bend elastic constant. We present two arguments to explain these features: one of these is based on coupling between the molecular shape and director distortion presented in the literature. Then we put forth a new concept of frustration in the packing between the two types of molecules and the polar interactions as an alternative.

Orientational ordering studies of fluorinated thermotropic liquid crystals by NMR spectroscopy

Fluorinated calamitic thermotropic liquid crystals represent an important class of materials for high-tech applications, especially in the field of liquid crystal displays. The investigation of orientational ordering in these systems is fundamental owing to the dependence of their applications on the anisotropic nature of macroscopic optical, dielectric, and visco-elastic properties. NMR spectroscopy is the most powerful technique for studying orientational order in liquid crystalline systems at a molecular level thanks to the possibility of exploiting different anisotropic observables (chemical shift, dipolar couplings, and quadrupolar coupling) and nuclei ((2)H, (13)C, and (19)F). In this paper, the basic theory and NMR experiments useful for the investigation of orientational order on fluorinated calamitic liquid crystals are reported, and a review of the literature published on this subject is given. Finally, orientational order parameters determined by NMR data are discussed in comparison to those obtained by optical and dielectric anisotropy measurements.

Orientational order of two fluoro- and isothiocyanate-substituted nematogens by combination of 13C NMR spectroscopy and DFT calculations

Orientational order properties of two nematogens containing a fluoro- and isothiocyanate-substituted biphenyl moiety have been investigated by means of (13)C NMR spectroscopy. (13)C NMR spectra acquired on static samples under high-power (1)H-decoupling allowed both (13)C chemical shift anisotropies and (13)C-(19)F couplings to be measured. These data were used to determine the local principal order parameter and biaxiality for the different rigid fragments of the molecules. To this aim, advanced DFT methods for the calculation of geometrical parameters and chemical shift tensors were used. The orientational order parameters obtained by NMR have been critically compared with those obtained by dielectric spectroscopy. Trends of order parameters with temperature have been analyzed in terms of both mean field theory and the empirical Haller equation.

Investigations on ferroelectric liquid crystal by high resolution TEM and solid state 13C NMR

In order to investigate the structural and dynamical properties of ferroelectric liquid crystal (FLC) in different phases a model compound [4-(3)-(S)-methyl-2-(S)-chloropentanoyloxy)]-4'-nonyloxy-biphenyl (3M2CPNOB) is synthesized. High resolution transmission electron microscopy (HR-TEM) is applied to observe the morphology of 3M2CPNOB and temperature-dependent solid state (13)C NMR to record (13)C chemical shifts at different phases. A liquid nitrogen quenching method is used to maintain the conformation of the mesophases for HR-TEM experiments. TEM images show that all the smectic A (SmA), smectic C* (SmC*) and crystalline phases have lamellar morphology. The interplanar distances in the crystalline phase are smaller than those in SmA and SmC* phases because of denser arrangement of the molecules. Both (13)C chemical shifts and line shape vary with different phases. The experimental results suggest that SmC* phase as an intermediate occurs in the anisotropy transition process from SmA to crystalline phase, the helical structure of the SmC* phase unwinds in the magnetic field and the conformations of the SmA and isotropic phase are very similar.

Density-functional-theory investigation of conformations, 13C shielding, and magnetic field interactions in a V-shaped phenylene bis carboxylate homologous series

Density functional theory (DFT) has been employed in a conformational study of a bent-core PBBC homologous series. IR spectra, GIAO-DFT chemical shielding tensors (CSTs), and molecular susceptibility tensors (MSTs) are theoretically calculated for various optimized conformations established by searching the potential energy surface of each PBBC V-shaped molecule. The IR results could aid the understanding of vibration normal modes, while the MST results can illuminate the alignment properties of the V-shaped molecules in an external magnetic field. CSTs of the aromatic carbons, and those previously measured by the 2D-NMR SUPER technique (with some (13)C NMR peaks reassigned for correctness based on new DFT calculations), were found to be in good agreement. These verified experimental CSTs are then used to revisit the (13)C NMR data to yield structural and local orientational order parameters for two members of the PBBC series. The PBBC series studied using the combined DFT-(13)C NMR approach strongly supports the notion that lesser populated conformational states found by DFT could be reached in the studied mesogens 10DClPBBC and 11ClPBBC upon decreasing temperature, as revealed by the change in the bend angle determined by NMR and identified with those of the DFT molecular structures optimized for various conformers.

Orientational order in liquid crystals by combining 2H and 13C nuclear magnetic resonance spectroscopy and density functional theory calculations

Structural and orientational order properties of the liquid crystal 4,4'-bis-heptyl-azoxybenzene (HAB) have been obtained in its nematic and smectic- A phases by simultaneously analyzing several observables extracted from 2H and 13C nuclear magnetic resonance (NMR) spectra, i.e., 2H quadrupolar, 2H-1H and 13C-2H dipolar couplings, as well as 13C chemical shift anisotropy. 13C experiments required the application of high-resolution solid-state NMR techniques like 1H high-power decoupling and cross polarization, as well as the independent determination of chemical shift tensors by means of density functional theory (DFT) calculations, here performed taking into account the effect of the anisotropic medium by the polarizable continuum model method. The approach, consisting in the simultaneous analysis of all the 2H and 13C experimental data to derive orientational order parameters, and in the use of geometrical parameters determined by DFT methods, allows more detailed and reliable results to be obtained with respect to the traditional approach based on the sole analysis of 2H experiments.