Constantin Rosu

Mesomorphic behaviour and TSDC measurements of ortho-metallated palladium(II) and platinum(II) complexes with S,O-donor co-ligands

A series of ortho-metallated Pd and Pt complexes containing an imine ligand carrying three alkoxy chains and N-benzoylthiourea derivatives as co-ligands were prepared and their liquid crystalline properties investigated. Their structures were assigned based on elemental analysis, IR and 1H NMR spectroscopy, whereas thermal properties were investigated by differential scanning calotimetry and polarising optical microscopy. All the compounds exhibit monotropic transitions involving nematic and smectic A phases, with the mesomorphic behaviour strongly related to the type of N-benzoylthiourea as well as the metal centre used. The thermally stimulated depolarisation current technique was employed to determine the conduction mechanism, phase transition temperature and the activation energies for one of the ortho-metallated Pd complexes.

Thermally Stimulated Depolarization Currents and Optical Transmissionon Liquid Crystal/Cellulose Derivative Composite Devices

In this paper we examined the electrical and optical properties of a new composite device containing layers of a nematic liquid crystal and cellulose derivative. Using the thermally stimulated depolarization current (TSDC) method we obtained information regarding the conduction mechanism, the activation energy, and the pre-exponential frequency factor in the Arrhenius diagrams. The optical transmission is slightly dependent on the polarization field, and the thermal switch behavior was obtained at the nematic-isotrop transition. When a.c. voltage is applied to the sample, the optical transmission depends strongly on the field amplitude, and a very good contrast is obtained.

INFLUENCE OF POLARIZING ELECTRIC FIELDS ON THE ELECTRICAL AND OPTICAL PROPERTIES OF POLYMER–CLAY COMPOSITE SYSTEM

We examined the electrical and optical properties of a new material containing polymer–clay nano-composite dispersed in a nematic liquid. The clay (Cloisite-type) was modified by copolymerization of maleic anhydride and divinyl benzene, using azobisisobutytironitrile as the initiator. The final polymer–clay nano-composite has an intercalated structure according to XRD patterns. We measured the thermally stimulated depolarization currents and determined the activation energies. Simultaneously we measured the optical transmission and we studied the influence of the previously applied polarizing electric fields.

Thermal behaviour and electro-optical properties of a series of liquid crystals based on palladium complexes with mixed ligands: Schiff bases and N-benzoyl thioureas

A series of ortho-metallated palladium(II) complexes with two dimeric liquid crystals Schiff base (methoxy and decyloxy as terminal groups) as cyclometallated ligands and N-aryl-N′-benzoyl thiourea derivatives as co-ligands were prepared and investigated for their liquid crystalline properties. Their structures were assigned based on elemental analysis, FT-IR and 1H NMR spectroscopy while the thermal behaviour was investigated by differential scanning calorimetry and polarising optical microscopy. The complexes with Schiff base ligand bearing methoxy group as terminal group show extensive decomposition during melting while the complexes with Schiff base having decyloxy group as terminal group show monotropic nematic phases, with the mesophase stability strongly related to the type of N-aryl-N′-benzoyl thiourea derivative used. Their liquid crystalline properties are compared with their analogues having N,N-dialkyl-N′-benzoyl thiourea as co-ligands which were reported previously. One of the latter complexes was also investigated by thermally stimulated depolarisation currents method while the optical transmission was recorded simultaneously. The thermally stimulated depolarisation currents and optical transmission spectra confirmed the previous observation regarding the phase transition temperatures. The current intensity–applied voltage dependencies of this complex were investigated by specific electrical measurements.

Differential negative resistance in the current-voltage characteristics of a new palladium(II) metallomesogen

A new palladium(II) organometallic complex with liquid crystal properties was prepared and its electrical conduction was investigated in all specific mesophases. It was established that the electrical conduction is of ionic type. The negative differential electrical resistivity was evidenced. The effect is explained by the particular characteristics of the interactions between the charge carriers and the liquid crystal in electric field. A new method for determining the activation energy is proposed, based on the analysis of the ionic current dependency on voltage. The obtained results are of interest for the design of liquid crystal devices.

THERMALLY STIMULATED DEPOLARIZATION CURRENTS AND OPTICAL TRANSMISSION STUDIES ON A 3,4-DICYANOTHIOPHENE-BASED BENT-ROD LIQUID CRYSTAL

We study the phase transitions on LC 3,4-Dicyano-2,5-bis[4-(octyloxy)phenylethynyl]-thiophene by the thermally stimulated depolarization currents method and by differential scanning calorimetry. Analyzing the experimental data, information regarding the conduction mechanisms for each mesophase are obtained. The results are consistent with the existence of a space charge polarization where the trapped charges are characterized by a slow retrapping probability. We measure the value of the activation energy, specific to the conduction process. We analyze the influence of the polarizing field on the optical transmission measured simultaneously with the thermally stimulated depolarization currents.

Thermally Stimulated Depolarization Currents and Optical Transmission Studies on UV Cured Polymer Dispersed Liquid Crystal Films

Polymer-dispersed-liquid-crystal films were obtained using the photopolymerization-induced-phase-separation method. The thermally stimulated depolarization current measurements indicate a space-charge-limited current. Conduction of electronic type was proven and the activation energy, specific to the conduction process, was calculated. Optical measurements have been performed simultaneously with the measurements of thermally stimulated currents and the nematic-isotropic phase transition was detected by the switch of the optical transmission. The change of the optical transmission versus applied a.c. voltage was the indication for the on-off switch behavior, useful in electro-optic applications.

Influence of polarizing electric field on phase transition temperatures and optical transmission of PDLC films

Abstract We obtained polymer dispersed nematic liquid crystal films E7/PMMA. Using the thermally stimulated depolarization currents method and registering the optical signal simultaneously we studied the dependence of the phase transitions temperatures on the polarizing electric field. Thermal switch behavior and its dependence on polarizing electric field is obtained.

PHASE TRANSITION INVESTIGATIONS IN POLYMER/LIQUID CRYSTAL COMPOSITE MATERIALS

We prepared polymer dispersed liquid crystal (PDLC) composite films using polymethyl methacrylate (PMMA) and the ferroelectric liquid crystal (LC) Felix 015/000 (Hoechst) by the solvent-induced phase separation method. We studied the phase transitions by the thermally stimulated depolarization currents (TSDC) method and by differential scanning calorimetry (DSC), for the composite film and the corresponding liquid crystal. Polarized microscopy was also used to characterize the phase transitions. When the LC is mixed with the PMMA, its characteristic transition temperatures are shifted down a few degrees and the current peaks revealed by the TSDC method are broadened due to the dispersion of microdroplets and the consequential presence of a large interface between the LC and the polymer matrix.

Polymer-Dispersed Cholesteric Liquid Crystals Reflecting In the Infrared Region

Polymer-Dispersed Cholesteric Liquid Crystals(PDCLC)have been obtained using different cholesteric liquid crystals mixtures by the Photopolymerization-Induced Phase Separation method. PDCLC samples of different thickness and different LC concentrations have been studied. The selective reflection spectra for different voltages have shown maximum reflectivity in the infrared region.