ternary sulfides. Neutron diffraction data from powders was collected on the 3T2 diffractometer at the Laboratoire LØon Brillouin, Saclay, France, to follow the crystal and magnetic structures of the title compound as a function of temperature. Rietveld refinements made use of the FullProf Rietveld program [18].Variable-temperature Mössbauer spectra with good statistics were measured from 300 K down to 4.2 K using a source of 57 Co in rhodium. The data were collected with the sample in a helium cryostat. The program MOSFUN [19] was used to analyze the data.First principles calculations of the electronic structures were performed using the LMTO-ASA method. A detailed description of the LMTO-ASA method, including its applications, can be found elsewhere [20]. The scalar± relativistic Kohn Sham equations were solved, taking all relativistic effects into account except for spin±orbit coupling. The basis set included the s, p, and d wavefunctions for Fe, Cu, Cr, and S. The S d wavefunctions were treated using the down-folding procedure. The k integrated functions were evaluated by the tetrahedron method on a grid of 735 k points in the irreducible part of the Brillouin zone (1/48 of the BZ). We used lattice constants from the neutron diffraction refinement and constructed a supercell with an ordered arrangement of Fe and Cu on the tetrahedral sites in the space group F43m. For comparison, we have also performed calculations on the end members in the series, that is FeCr 2 S 4 and CuCr 2 S 4 .Because of magnetic ordering, the calculations were performed (using the von Barth±Hedin [21] form of the exchange correlation within the local spin density approximation) with the assumption of a magnetic ground state (spin-polarized calculation) as these, in agreement with experiment, yielded more stable total energies.

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In this article, we investigate the linear and nonlinear optical properties of the thiophene/phenylene-based oligomer (SL128G) and polymer (FSE59) chemically modified with alquilic chains, which allow greater solubility and provide new optical properties. These compounds present a strong absorption in the UV-visible region, providing a wide transparence window in visible-IR, ideal for applications in nonlinear optics. Employing the Z-scan technique with femtosecond pulses, we show that these compounds exhibit considerable two-photon absorption (2PA), with two 2PA allowed states located at 650 and 800 nm for SL128G and 780 and 920 nm for FSE59. Moreover, we observe the resonance enhancement effect as the excitation wavelength approaches the lowest one-photon-allowed state. By modeling the 2PA spectra considering a four-energy-level diagram within of the sum-over-essential states approach, we obtained the spectroscopic parameters of the electronic transitions to low-energy singlet excited states. Additionally, photoluminescence excited by femtosecond and picosecond pulses were performed to confirm the order of the multiphoton process and estimate the fluorescence lifetime, respectively.

Langmuir-Blodgett (LB) films of a poly(p-phenylenevinylene) (PPV) precursor were prepared using an amphiphilic precursor, poly(xylylidenetetrahydrothiophenium chloride) (PTHT), where the counterion chloride was replaced by a long chain dodecylbenzenesulfonate (DBS) ion. Stable PTHT-DBS monolayers were transferred onto quartz substrates in the form of LB films. Using DBS allowed PPV films to be converted under atmospheric pressure at temperatures as low as 80 °C, with conjugation length and optical properties better than for standard films converted at temperatures above 200 °C under controlled atmospheres. These LB-PPV films are highly anisotropic as demonstrated by linear dichroism experiments using linearly polarized optical absorption and emission. The dichroic ratio between the intensity of the emitted light parallel and perpendicular to the dipping direction was ca. 17, which is much higher than the dichroic ratio of 3.6 found in the polarized absorption experiment. This difference indicates an efficient excitation transfer between the low conjugated segments (disordered) and the highly conjugated (ordered) ones after excitation.

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Layer-by-layer (LBL) thin films of a poly(p-phenylenevinylene) (PPV) precursor have been processed using the long chain dodecylbenzenesulfonate anion (DBS). We adopted the alternative approach with controlled substitution of the chloride counterion of the water-soluble PPV precursor by DBS. The internal layer structure of these films is likely to be different from conventional LBL films, since DBS is incorporated into the PPV precursor chain. In addition to the fast adsorption of the sulfonic salt, introducing DBS allows the films to be converted at 110 °C within 12 min with a high conjugation length. By varying the amount of DBS in the solution, the conjugation length of the PPV segments can be controlled. The luminescence and absorption spectra show intense vibronic structures that are better resolved than for cast and Langmuir−Blodgett films of PPV. LBL films converted at lower temperatures display higher quantum efficiency, owing to the absence of carbonyl groups arising from high-temperature conversion procedures.

The control of emission properties in luminescent polymers such as poly(p-phenylene vinylene) (PPV) is important for various applications, and may be achieved with suitable molecular architectures in nanostructured films. This paper reports on optical properties of PPV films, using ellipsometry measurements for emitted light in the scope of the Stokes' theory. Organized PPV films obtained with the Langmuir-Blodgett (LB) method exhibited high degree of polarization for the emitted light, while cast films emitted mainly non-polarized light. From ellipsometry data, a secondary structure was inferred for poly(xylylidene tetrahydrothiophenium) chloride (PTHT), a PPV precursor, in solution, which is retained only to a small extent in the PPV cast film as thermal conversion was performed close to the glass transition temperature of PPV. On the other hand, a higher intensity of emitted light with circular polarization was observed for the LB film, which is attributed to PPV molecular secondary structure that was enhanced during the LB film deposition. Circular dichroism experiments were performed to corroborate this hypothesis. It is suggested that such a secondary structure has not been predicted in theoretical models for PPV because possible conformational changes induced in the processing steps are not taken into account.