Valentina Domenici

Order and dynamics of rod-like and banana-shaped liquid crystals by 2H NMR

Deuterium NMR spectroscopy is a very powerful technique for studying partially or totally ordered systems, such as liquid crystals (LCs) and polymers. LCs represent a branch of the most general class of soft materials, with peculiar physical and chemical properties which attracted scientific attention for their potentiality for technological applications. From a chemical point of view, there are three aspects in which 2H NMR could provide significant insights: (i) the conformational and structural properties; (ii) the molecular dynamics and mobility; and (iii) the orientational order and aggregation/distribution of molecules in the different liquid-crystalline phases. In this work, some of the recent developments in this field are discussed, focusing on two main topics: (1) the molecular dynamics of the smectic liquid-crystalline phases formed by rod-like molecules and (2) the unusual orientational and dynamic properties of the new liquid-crystalline mesophases formed by banana-shaped molecules (BLCs).

How do banana-shaped molecules get oriented (if they do) in the magnetic field?

In this work the orientation of banana-shaped molecules in the magnetic field is investigated. A new and original hypothesis of the peculiar aggregation of banana-shaped molecules in the nematic phase in the presence of a magnetic field is here reported. Deuterium NMR measurements on two selectively deuterium labelled mesogens and preliminary calculations of the magnetic susceptibility anisotropy of the aromatic core of these bent molecules are reported and discussed to support our hypothesis.

The role of NMR in the study of partially ordered materials: Perspectives and challenges

The development of NMR techniques applied in the last 10 years to partially oriented systems, and in particular to liquid crystals (LCs), is the object of this brief perspective. The evolution of NMR methods (i.e., new NMR pulse sequences) and the improvement of both theoretical models and mathematic tools for the analysis of NMR data (specifically, for partially ordered systems) allowed scientists to extend their research to increasingly complex materials, such as dendrimers, polymers, and membranes, and to investigate unique phenomena, such as field-induced alignment and confining effects. Furthermore, the fast development of nanoscience and biomedicine is offering a rich variety of new “physical chemical” problems related to partially ordered materials. Starting from a brief perspective of recent works on thermotropic and lyotropic LCs based on different NMR methods, new challenges in this field will be drawn. Moreover, recent selected research works will be discussed in detail with particular emphasis on: (i) the effect of high magnetic fields on the supramolecular structure of chiral liquid-crystalline phases, such as the SmC*, TGBA*, and “de Vries”-type SmA* phases, by means of solid-state 2H NMR; (ii) the slow dynamics in the isotropic phase of bent-core LCs (BLCs) and of liquid single-crystal elastomers evidenced by 2H NMR relaxation studies; and (iii) the influence of the LC environment on the conformational properties of rod-like mesogens studied by high-resolution solid-state 13C NMR methods. This work aims to offer an occasion of reflection on this field of physical chemistry with a glance at future trends and challenges in view of the celebration of the International Year of Chemistry, 2011.

Dynamics in the isotropic and nematic phases of bent-core liquid crystals: NMR perspectives

The dynamic properties of low-molecular-weight bent-core liquid crystals were revealed to be unusual with respect to the analogous rod-like mesogens. First pioneering investigations based on dynamic light scattering measurements were followed by intense and deep studies based on nuclear magnetic resonance (NMR) spectroscopy, mainly 2H NMR relaxation and line width analyses, 1H NMR relaxometry and diffusometry. These studies were of help in identifying the molecular motions responsible of the observed slow relaxation and the large 2H NMR spectral width. Viscosity and rheology measurements provided further elements confirming the slow dynamics of bent-core liquid crystals at a macroscopic level. All these experimental techniques indicate the presence of restricted dynamic motions as a distinctive feature of bent-core liquid crystals both in the isotropic and nematic phases. In this review, the case of several bent-core mesogens studied by different NMR methods is reported and the main results are discussed in view of the recent achievements concerning the tendency of bent-core molecules to form aggregates or temporary clusters both in the isotropic and nematic phases.

2H-NMR and SAXS of a ferroelectric liquid crystal : unwinding of the ferroelectric chiral helix by high magnetic fields

Abstract 2 H-NMR in three different magnetic fields and small angle X-ray scattering (SAXS) are employed to investigate the orientational order, molecular organisation and phase transitions of the chiral smectic liquid crystal (−)-(S)-[4-(2-methylbutyloxycarbonyl)phenyl] 4- n -heptyl-biphenyl-carboxylate (MBHB) suitably deuterated for 2 H-NMR measurements. Unwinding of the ferroelectric chiral helix by high magnetic fields is observed and discussed. Tilt angle values are obtained with the two different techniques and compared. The results show the importance of molecular conformation for the structural properties of this mesogen.

Conformational Properties and Orientational Order of a de Vries Liquid Crystal Investigated through NMR Spectroscopy

Solid-state and liquid-state NMR spectroscopic techniques are used to describe at molecular level the behaviour of a de Vries liquid crystal (namely the mesogen 9HL) at the SmA-SmC* transition, which is characterized by the absence of the layer shrinkage, typical of non-de Vries smectogens. Previous (2)H NMR studies on the same smectogen, performed at a different magnetic field (from 4.70 to 18.80 T), provided evidence of the occurrence of a tilt of one of the three phenyl rings, constituting the aromatic core of 9HL, at the SmA-SmC* phase transition. In this work, the study is extended to the whole rigid aromatic core of the 9HL. In particular, the variable temperature behavior of the mesogen studied by 1D (13)C NMR cross-polarization (CP) and 2D (1)H-(13)C PDLF (proton-encoded (13)C-detected, local field) NMR experiments made possible the characterization of the conformational and orientational properties in the two smectic phases. These results are compared with various proposed models invoked to describe the SmA-SmC* transition in de Vries smectogens at a molecular level.

NMR studies of the ferroelectric SmC* phase

In this work an attempt is made to compare and rationalise the structural and dynamic behaviour of some ferroelectric rod-like mesogens studied by our group in recent years, mainly by means of 2H- and 13C-nuclear magnetic resonance (NMR) spectroscopy. This comparison concerning the local orientational order, the average molecular conformation and the chiral smectic C (SmC*) phase structure, with a brief overview on the reorientational dynamic properties, allowed us to identify some common behaviour of these ferroelectric rod-like mesogens when decreasing the temperature from lower ordered phases to and within the SmC* phase. The main results obtained by means of NMR studies on these mesogens are rationalised and discussed in comparison with detailed discrete Fourier transform computations of their molecular conformations (in the limit of isolated molecules) with the purpose of enlightening the role of intermolecular and packing interactions in the ferroelectric phase with respect to the smectic A and nematic phases.

Synthesis and mesomorphic properties of new compounds exhibiting TGBA and TGBC liquid crystalline phases

We synthesised a series of rod‐like mesogens with a (S)‐2‐methylbutyl‐(S)‐lactate unit in the chiral chain that exhibited extremely wide temperature ranges in the TGBA and TGBC* phases. TGB phases were identified, based on typical textures in confined samples and in free‐standing films, by Grandjean‐Cano texture and by NMR studies on a deuterium‐labelled isotopomer. A sufficiently high electric or magnetic field transformed the TGBA and TGBC* phases into their respective SmA and SmC* phases, the TGB structures being restored within some 20–30 minutes. Therefore values of the spontaneous polarisation and spontaneous tilt angle, when measured under a sufficiently high field, gave evidence of the properties of the SmC* phase. Temperature dependencies of relaxation frequency, dielectric strength, selective reflection and layer spacing showed anomalies at a certain temperature within the TGBC* phase range. Also, changes in textures, as well as in 2H‐NMR spectra, occurred at this same temperature. These results suggest the existence of two TGBC* phases.

Twist grain boundary liquid-crystalline phases under the effect of the magnetic field: a complete 2H and 13C NMR study.

A liquid crystal (HZL 7/*) containing an (S)-2-methylbutyl-(S)-lactate unit in the chiral chain, is investigated by means of (2)H and (13)C NMR spectroscopy in order to obtain information on its orientational order, its molecular structure and the effect of external magnetic fields on the supramolecular structure of its phases. This mesogen presents very peculiar mesomorphic properties and exhibits frustrated TGBA* and TGBC* phases in a wide temperature range up to 60 degrees C, as well as an additional phase transition from TGBC(1)* to TGBC(2)*. (2)H NMR measurements show, for the first time, a peculiar magnetic field effect in unwinding the supramolecular structure of both the TGBA* and TGBC* phases. This effect is particularly evident at higher magnetic fields, while different behaviour is observed at lower magnetic fields. This indicates that the supramolecular structure is very sensitive to magnetic fields of the order of 1 Tesla. Moreover, the analysis of the (2)H and (13)C NMR spectra of HZL 7/* allow us to obtain several structural properties, such as the tilt angle of the TGBC* phases and the local orientational order parameters referred to the phenyl and biphenyl fragments. This is the first structural characterization of the frustrated phases of these complexes by means of NMR.

Polymer-dispersed liquid crystal elastomers.

The need for mechanical manipulation during the curing of conventional liquid crystal elastomers diminishes their applicability in the field of shape-programmable soft materials and future applications in additive manufacturing. Here we report on polymer-dispersed liquid crystal elastomers, novel composite materials that eliminate this difficulty. Their thermal shape memory anisotropy is imprinted by curing in external magnetic field, providing for conventional moulding of macroscopically sized soft, thermomechanically active elastic objects of general shapes. The binary soft-soft composition of isotropic elastomer matrix, filled with freeze-fracture-fabricated, oriented liquid crystal elastomer microparticles as colloidal inclusions, allows for fine-tuning of thermal morphing behaviour. This is accomplished by adjusting the concentration, spatial distribution and orientation of microparticles or using blends of microparticles with different thermomechanical characteristics. We demonstrate that any Gaussian thermomechanical deformation mode (bend, cup, saddle, left and right twist) of a planar sample, as well as beat-like actuation, is attainable with bilayer microparticle configurations.