Daniela Imbardelli

Conformation of ethylene glycol dissolved in a nematic-lyotropic solution: An N.M.R. analysis

The conformation of ethylene glycol (EG) has been investigated by means of liquid crystal nuclear magnetic resonance spectroscopy (LICRY-N.M.R.). The proton and 13C N.M.R. spectra were obtained from a quarternary solution of potassium laurate (KL), decanol, water and EG. These spectra were analysed to get the proton-proton and proton-carbon direct dipolar couplings. Spectra obtained from an aqueous solution have also been analysed. The experimental results were then used to investigate the conformational equilibrium of the molecule. It has been found that in the lyotropic liquid crystalline solution EG exists in a gauche conformation only, and that the value of the OC-CO dihedral angle is 72°.

Measurement of the Polarization Field in Pdlc with Memory State

Abstract A memory state can be induced in PDLC films through the application of a strong D.C. voltage. This memory state is due to the onset of an internal polarization field, which keeps the film in a high transparent state. This polarhation field remains in the film from hours to weeks, depending on the temperature at which the external polarizing field is applied. We have found the way to measure such an internal field by observing the electrooptical response of the film, under a low frequency driving voltage. Due to the presence of the polarization field the light transmission obtained during the positive half period of the driving voltage is not equal to that observed in the negative half period. The polarization field corresponds to the D.C. bias voltage which is needed to equalize the light transmission in the two half periods. The time dependence of the polarization field has been determined.

Photoelectrochromic switchable nematic emulsions

Switchable nematic emulsions are composite systems formed by liquid-crystal droplets dispersed in a fluid, homogeneous, monomer matrix. These systems can be switched from an opaque to a transparent state by application of a suitable alternating electric field. An electrochromic device provides a reversible and visible change in its transmittance as a result of either oxidizing or reducing electrochemical processes. A photochromic device provides a reversible and visible change in its transmittance as the result of a single chemical transformation of the species induced by suitable electromagnetic radiation. These devices have been proven to be useful for a variety of electro-optical applications as switchable windows, electromagnetic shutters, and displays. Our article reports preliminary results on trifunctional devices based on nematic emulsions that host electrochromic and photochromic guest molecules. These trifunctional films are able to modulate the light transmission (with an external alternating electric field) and to change their color (with static field or UV light) by means of suitable electric fields or electromagnetic radiation.

Nuclear relaxation of partially oriented pyridine determined by line width analysis of 1H NMR spectra

An NMR lineshape study of pyridine dissolved in nematic PCH mesophase has been carried out. Measurements of spectral frequencies and intensities enabled us to determine the order parameters defining the molecular orientation. Lineshape analysis indicates that the highly selective broadening of lines is mainly due to the coupling of protons with the rapidly relaxing 14N quadrupolar nucleus. A further contribution to the line width comes from the dipolar intermolecular relaxation mechanism. The quadrupolar relaxation time of the 14N nucleus and the correlation time of the intermolecular interaction were calculated.

n-Type Columnar Liquid Crystal Combining Ionic and Electronic Functions

The research in the field of electronic and photonic devices based on ordered organic semiconducting materials has been boosted by the development of new multifunctional materials. The order characterizing liquid crystals has been exploited for improving charge mobility performances. However, while in the last decade much effort has been addressed to the synthesis of hole-conducting materials (p-type), less attention has been paid on the development of electron-conducting ones (n-type). Here we show the bulk multifunctional properties of a π-conjugated thienoviologen ionic liquid crystal which exhibits strong electron acceptor character. Notably, the combination of its optical properties with high ionic conductivities and fast intermolecular electron transport, leads to unique bulk electrochromic functions.

2H-NMR study of molecular order and dynamics in mixtures of 8OBCAB and nTPCHB

2H-NMR measurements have been performed on pure 8OBCAB-d 2 and on its mixture with compounds of the nTPCHB series. The aim of this investigation was to achieve an insight into the unusual mixing properties of these compounds, which have been associated to the formation of molecular pairs in the smectic A phase. By quadrupolar splittings measurements, performed at various temperatures, ordering information on the various mixtures was obtained. The Zeeman (T 1Z) and the quadrupolar (T 1Q) relaxation times were simultaneously measured by the Jeener-Broekaert experiment; then the temperature dependence of the individual spectral densities J(ω) and J(2ω) was analysed. The fitting of these parameters has been obtained using Nordios rotational diffusion model. Comparison of all the mixtures at the same reduced temperature (T/T NI) showed a connection between the presence of the nematic re-entrant behaviour and the slowing down of both D ‖ and D ⊥ elements of the rotational diffusion tensor.

2H-NMR Investigation of Aerosol OT/Water/n-Hexane Liquid Crystalline Mixture

Abstract Lyotropic lamellar mesophases formed by mixtures of Aerosol OT (AOT), n-hexane and water have been investigated by 2H-NMR. The spectral profiles obtained from perdeuterated n-hexane depend upon the mechanical treatment of the samples. Spectra recorded immediately after centrifugation of the specimens have been compared to the ones obtained after an interval of about 15 days. Analysis of spectral profiles indicates that mechanical stress induces two effects: a reduction of the quadrupolar splitting and an asymmetry of the electric field gradient tensor acting on the deuterium nuclei. The magnitude of these effects is constant along the hexane chain, indicating a modification of the lyotropic aggregates. Data are interpreted in terms of a model in which ripples are induced on the lamellar bilayers by the mechanical treatment. Results are in agreement with the behaviour observed in the lamellar mesophase of the two component system AOT/Water.

THE THERMODYNAMICS OF RIBBON LYOTROPIC MESOPHASES

Abstract A thermodynamic model for ribbon mesophases obtained by mixing lipids of different chain lengths with water has been developed. The model permits the deviation of the shape of aggregates on the basis of factors such as the composition of the mixtures, the temperatures and other molecular physical parameters. Simple analytical solutions have been obtained in some limiting cases. The prediction of the model has been tested by performing deuterium nuclear magnetic resonance analysis of lyotropic mixtures in appropriate ranges of temperature and composition.

An investigation of the dynamics of CH3I in a nematic mesophase by NMR linewidth analysis

Abstract Relaxation mechanism acting on CH 3 I dissolved in a nematic mesophase were studied by linewidth analysis of 1 H and 13 C NMR spectra. Intramolecular and intermolecular dipolar interaction fluctuations were found to be the most important relaxation phenomena responsible for the line broadening.

Conformation of succinic acid: its pH dependence by Licry-NMR analysis

The conformations of fully protonated (H2SA), fully deprotonated (SA=) and monoprotonated (HSA-) succinic acid have been investigated by means of nuclear magnetic resonance spectroscopy in liquid crystal mesophases (Licry-NMR). H-H and 13C-H direct dipolar couplings have been determined by measuring 1H-NMR and 13C-NMR spectra from quaternary nematic-lyotropic solutions of myristyltrimethylammonium bromide (MTAB), decanol, deuterated water and succinic acid (in each of its three different protonated forms). Direct dipolar couplings have been used to investigate the conformational equilibrium of the molecule in its three different protonation forms. Data could be interpreted in terms of a single conformation for each of the investigated forms. The dihedral angle between the H3-C5′-C5 and the C5′-C5-H1 planes gradually increases when going from the fully protonated H2SA species to the SA= ions. Our findings are different from those obtained by other authors by analysis of Jij couplings. In that case an equil...