Advances in Science and Technology Vol. 55

Abstract: The peculiar property of reversible change of the refractive index (Δn) of photochromic systems, is suitably exploited to realize volume phase holographic gratings (VPHGs), which are finding applications in optics and in spectroscopic instrumentations. A large modulation of the refractive index between the two photochromic states and a relatively thick layer of good optical quality are the fundamental requirements for the materials to achieve a good diffraction efficiency. A backbone diarylethene polymer which combines good processing properties with a proper photochromic sensitivity is synthesised and studied. The large Δn between the two isomer states (i.e. Δn=0.03 measured at 1500 nm and the value is even higher at shorter wavelengths, approaching 0.1 near the visible) and the high transparency in the near infrared range make it a promising substrate for the realization of efficient VPHGs. First prototype of VPHGs based on photochromic diarylethene polymer films has been written by transferring the pattern of lines from a master and its functionality is shown.

Abstract: Nd0.3Sm0.7NiO3 ceramic has been synthesised using a sol-gel method and annealing at oxygen pressure. Nd0.3Sm0.7NiO3 exhibits a reversible metal insulator phase transition which is, in the infrared range, responsible of a thermal-optical contrast. Samples are structurally characterized by X-ray diffraction and thermogravimetric analysis. The Nd0.3Sm0.7NiO3 thermal – optical contrast is characterized in the mid-infrared range, using reflectance and thermal-optical measurements.

Abstract: The reversible charge ordering transition of (Sm1-x,Cax)MnO3 is associated with strong changes in electrical, magnetic and infrared properties. In this work, normal spectral emissivity is calculated from infrared reflectance of bulk polished ceramics, measured by Fourier Transform Infrared Spectroscopy (FTIR). Emissivity versus temperature shows a sharp transition at the charge ordering transition. By comparing the apparent temperature of the sample with the absolute temperature, we show that (Sm1-x,Cax)MnO3 is a good candidate for infrared furtivity around room temperature.

Abstract: This paper describes the elaboration and optical characterizations of electrochromic devices (ECDs) based on a semi-Interpenetrating Polymer Network architecture. These ECD exhibit contrast ration of 36% in the full visible range and are able to switch 10,000 times at low voltage between +1.2V and -1.2V with only 0.2% of electroactivity lost. By the way, these IPN-based materials present interesting properties in the NIR region.

Abstract: Nickel oxide-based thin films were deposited onto indium-tin oxide (ITO) coated glass substrates by the chemical bath method. The synthesis, based on the decomposition of urea in an aqueous nickel nitrate solution, promotes the deposition of the turbostratic
-Ni(OH)2 phase. The electrochromic behavior of films obtained from low and high urea concentrations was tested after air annealing at temperatures of 250 and 300 °C. Using cyclic voltammetry, chronoamperometry and in-situ single wavelength transmittance it is shown that the films exhibit good reversibility and coloration efficiency. Transmittance spectra in the visible range show the high optical contrast of the films. It was found that in films annealed at 250 °C the bleached-colored switching is between the Ni(OH)2 and -NiOOH phases. Films annealed at 300 °C are comprised by a Ni(OH)2-NiO mixture and Ni2O3 units or the -NiOOH phase are responsible for coloring depending on urea concentration in solution. The -NiOOH phase was clearly identified in over-colored films. X-ray diffraction, Raman scattering, and infrared reflectance were used for phase identification.

Abstract: This paper reports the enhancement of the electrochromic properties in the infrared, IR, region of radio frequency sputtered WO3 thin films thanks to a careful optimization of the deposition conditions. From the combination of various techniques of characterization, the optical performances, in particular, the contrast in reflectance between the inserted state and the deinserted one, upon cycling in H3PO4 liquid electrolyte, are correlated to the film morphology and composition. Films, grown in higher chamber pressure, exhibiting a lower density and a O/W ratio higher than 3, show the most promising behavior. Finally, first trends on the incorporation of the WO3 single layer in full electrochromic devices are discussed.

Abstract: Fluorescent dyes with a high sensitivity of their optical spectra towards changes of the environment were prepared via aldol condensation or Horner olefinations. The main chromophore is a quadrupolar N-substituted 1,4-distyrylbenzene which allows protonation and complexation at various positions resulting in a series of different and significant changes of the optical spectra. The sensitivity of the absorption and emission spectra on solvent polarity, acid, and cations is reported.

Abstract: We have synthesized two types of all-organic radical liquid crystalline (LC) compounds, trans-2-alkoxyphenyl-5-[4-(4-alkoxybenzenecarbonyloxy)phenyl]-2,5-dimethylpyrrolidine-1-oxy (1) and 4-alkoxyphenyl trans-4-[5-(4-alkoxyphenyl)-2,5-dimethylpyrrolidine-1-oxy-2-yl]benzoate (2) and have fully characterized their LC properties. Although the only difference in the molecular structure between 1 and 2 is the orientation of a binding group connecting the core portion and one side-chain (-OCO- and -COO- for 1 and 2), the racemic or enantiomerically enriched 2 showed an SmA phase, or SmA* and TGBA* phases, which were not observed for 1, besides N and SmC, or N* and SmC* phases, respectively. Here we discuss the origin of these differences on the basis of their crystal structures determined by X-ray crystallographic analysis.