Jacques Muller

Intramolecular charge transfer and enhanced quadratic optical non-linearities in push-pull polyenes

Abstract Push-pull polyenes, which have an electron-donating group (D) and an electron-withdrawing group (A) grafted on opposite ends of a conjugated polyenic chain, are of particular interest as model compounds for long-distance intramolecular charge transfer (ICT), as well as potent non-linear optical chromophores. Several series of push-pull polyenes of increasing length, combining aromatic donor moieties and various acceptor groups, have been prepared and studied. Their linear and non-linear optical properties have been investigated by performing electro-optical absorption measurements (FOAM) and electric-field-induced second-harmonic generation (EFISH) experiments in solution. Each molecule shows a broad and intense ICT absorption band in the visible associated with an increase in the dipole moment (Δμ). Lengthening the polyenic chain linking the D and A groups results in a bathochromic shift of the ICT absorption band and induces a linear increase in the excited state dipole. In contrast, the ground state dipole remains roughly constant. As a result, the longest molecules exhibit huge Δμ values (up to 42 D) as well as markedly enhanced quadratic hyperpolarizabilities (β). In addition, the nature of the end groups has been found to influence strongly both the ICT and optical non-linearities: larger β and Δ μ values, as well as steeper length dependences, are obtained with push-pull phenylpolyenes bearing strong acceptors.

A general method to determine twinning elements

Based on the minimum shear criterion, a direct and simple method is proposed to calculate twinning elements from the experimentally determined twinning plane for Type I twins or the twinning direction for Type II twins. It is generic and applicable to any crystal structure.

Determination of surface crystallography of faceted nanoparticles using transmission electron microscopy imaging and diffraction modes

A general calculation method is proposed to characterize the crystalline planes and directions of a faceted nanoparticle using transmission electron microscopy (TEM) imaging and diffraction modes. With the determination of the edge vectors and then the plane normal vectors in the screen coordinate system of TEM, their Miller indices in the crystal coordinate system can be calculated through coordinate transformation. The method is helpful for related studies of the determination of the surface structure of nanoparticles.

Chain-length dependence of the optical properties of a series of push–pull polyenes: application to doped photorefractive polymers

Abstract Push–pull molecules are of considerable interest for their large quadratic hyperpolarizabilities. However, incorporated as doping chromophores in low glass-transition temperature photorefractive polymers, their Pockels contribution to the modulation of the refractive index is generally weak or even negligible. We present an experimental and analytical study of a series of push–pull polyenes with a number of double bonds in the chain increasing from 1 to 5. The molecules were dissolved in chloroform and the two contributions to the total birefringence, i.e. the orientational birefringence and the Pockels effect, were measured using a specific ellipsometric technique and analyzed in a two-form two-state formalism. For the longest polyene studied, a giant efficiency of the refractive index modulation, arising equally from the orientational contribution and from the Pockels effect, can be expected.

Structure/quadratic hyperpolarizability correlation: solvent effect

In this work, we have investigated theoretically the structure/hyperpolarizability correlations of push-pull molecules sing a two-form two-state model. For this, we have defined a parameter MIX characterizing the mixing between the two-limiting resonance forms and thus ruling the molecular structure and polarization. Also, we have analyzed the solvent effect on the structure and on the polarizabilities of push-pull molecules using the Onsager reaction field theory. We have shown that the dependences of the linear and nonlinear polarizabilities on the solvent dielectric constant look similar to the structure/(hyper)polarizability correlations. Finally, we have investigated experimentally the solvent effect on a series of push-pull polyenes of increasing length. In particular, we have determined the scalar (mu) g(Beta) (0) product (where (mu) g is the ground-state dipole and (Beta) (0) the vector part of static quadratic hyperpolarizability tensor) in different solvents using the electric-field-induced-second-harmonic generation technique. For the different chain lengths, we have obtained a mapping of the positive (mu) g(Beta) (0) peak. This leads for the longest compound of the series to a record high value of (mu) g(Beta) (0) product in chloroform.

Volume fractions of texture components in polycrystalline materials

The orientation distribution of crystallites has until now been interpreted by a small number of texture components. In the frame of the harmonic formalism, an analytical method for the accurate determination of the volume fractions of peak-type components in restricted regions is developed without recourse to additional assumptions. Its validity and feasibility are demonstrated through two numerical examples.

New results on low-Tg photorefractive materials

Push-pull molecules are key components for doped low glass transition temperature photorefractive polymers. In this paper we report on the optimization of the factor of merit of photorefractive polymers doped by molecules belonging two different classes of push-pull molecules, on one hand molecules with a dominant neutral resonance form and, on the other hand, zwitterionic or charge separated molecules. The theoretical analysis is illustrated by experimental result obtained both on a molecular level and on polymeric materials of the interest of the analysis developed.

Calculation Methods to Determine Crystallographic Elements: Interface Plane, Surfaces Plane and Twinning Elements, Based on Electron Diffraction Orientation Measurements by SEM and TEM

In the present work, we summarize three calculation methods to determine some specific crystallographic elements based on electron diffraction orientation measurements by SEM and TEM. The first one is to determine the plane indices of the faceted interfaces where the orientation relation¬ships (ORs) between the adjacent crystals are reproducible. To acquire the orientation data, we need to prepare only one sample surface but not two perpendicular sample surfaces as usually required in the standard double trace method. The second is to characterize the surface crystalline planes and directions of a faceted nano-particle under TEM imaging and diffraction mode. With the determination of the edge trace vectors and then the plane normal vectors in the screen coordinate system of TEM, their Miller indices in the crystal coordinate system can be calculated through coordinate trans¬formation. The third method is to determine the twin type and the twinning elements based on the orientation information acquired by SEM EBSD measurements from the two twinned crystals through misorientation calculations. These methods will facilitate related studies.