Isabelle Malfant

Azo-azulene derivatives as second-order nonlinear optical chromophores

The molecular and solid state nonlinear optical (NLO) properties of several (phenylazo)-azulenes are investigated. In particular, (4-nitrophenylazo)-azulene (2b) exhibits a quadratic hyperpolarizability (beta(vec)) of 80 x 10(-30) cm5esu recorded at 1.907 microm by the electric field-induced second-harmonic (EFISH) technique. This molecular material, which crystallizes in the monoclinic noncentrosymmetric space group Pc, exhibits an efficiency 420 times that of urea in second-harmonic generation. The origin of the optical nonlinearity in azo-azulene is discussed in relation with crystal structures and semiempirical calculations within the INDO/SOS formalism, and compared with that of the well known disperse red one (DR1) organic dye.

Infrared and Raman properties of [M(dmit)2] (M=Ni, Pd) based compounds

Abstract The Raman spectra of [M(dmit)2]−n (dmit = 1,3-dithiole-2-thione-4,5-dithiolato, M = Ni, Pd; 0 ≤ n ≤ 2) based complexes with different cations were studied. It was found that the anion Ag modes frequencies correlate linearly with its charge. In the polarized reflectance spectra of the 1:1, [NBu4][Ni(dmit)2] salt the intra- and intermolecular electronic transitions as well as the vibronic bands were assigned. Electron-molecular vibration coupling constants have been evaluated using the dimer model.

New Tetrathiafulvalene-π-Spacer-Acceptor Derivatives: Synthesis, Crystal Structure, Optical and Electrochemical Properties

4-[2-tetrathiafulvalenyl-ethenyl]pyridine (1) has been prepared from a Wittig reaction between formyltetrathiafulvalene and 4-picolyltriphenylphosphonium chloride hydrochloride. Conversion of the pyridine moiety of 1 by reaction with methyl iodide leads to 4-[2-tetrathiafulvalenyl-ethenyl]-1-methylpyridinium iodide (2a). Neutralization of 1 with a large excess of L-tartaric acid affords 4-[2-tetrathiafulvalenyl-ethenyl]-1-methylpyridinium hydrogen tartrate (3). These TTF-π-spacer-acceptor compounds have been characterized by elemental analysis, and IR and 1H NMR spectroscopy. The crystal structure of 2a has been determined by X-ray diffraction. The cation is essentially planar. Examination of the bond lengths in 2a, UV/Vis spectra and CV data, and calculations indicate that an intramolecular charge transfer occurs in the studied compounds, although it is rather limited, and larger in 2a and 3 than in 1.

A facile route for the preparation of nanoparticles of the spin-crossover complex [Fe(Htrz)2(trz)](BF4) in xerogel transparent composite films

Films and monoliths containing the spin crossover complex [Fe(Htrz)(2)(trz)](BF(4)) (trz = 1,2,4-triazole) as nanoparticles have been obtained. The dispersion and consecutive inclusion of the Fe complex in a silica matrix prepared from tetramethoxysilane or tetraethoxysilane afford monoliths or films with a violet colour at room temperature, which turns white above 380 K. This change of colour is reversible. This thermochromic behaviour has been characterized by measuring the magnetic properties together with thermogravimetric studies and Raman spectroscopy, the result of which all demonstrate that both films and monoliths undergo a spin crossover. Microscopy studies confirm the occurrence of the Fe complex as nanoparticles, in both the monoliths and the films. The facile synthesis of these materials as nanoparticles in transparent films should open the possibility of the synthesis of high quality films.

Tetrathiafulvalene-based conducting deposits on silicon substrates

Tetrathiafulvalene-based molecular conductors, namely [TTF][TCNQ], (TTF)6[N(C2H5)4](HPM12O40) (M = W, Mo), and TTF[Ni(dmit)2]2 have been processed as thin films on microrough (001)-oriented silicon substrates using three different techniques. CVD-grown [TTF][TCNQ] deposits consist of platelets (10 × 5 µm) and filaments (diameter ∼1 µm). CN stretching modes in the infrared spectrum and CC stretching modes in the Raman spectrum are in agreement with a charge transfer of ∼0.6 from the TTF donor to the TCNQ acceptor. The N(1s) photoelectron spectrum confirms the presence of a reduced tetracyanoquinodimethane moiety. The films exhibit a semiconducting behaviour with a room-temperature conductivity of ∼0.4 S cm−1. Deposits of (TTF)6[N(C2H5)4](HPW12O40) are electrodeposited on microrough Si(001) at constant current from TTF and [N(C2H5)4]3(PW12O40) in acetonitrile solution. The films are made of stacked sheets (5 < thickness < 25 µm). Vibrational spectra, conductivity measurements and magnetic susceptibility data are similar to those obtained on single crystals of (TTF)6[N(C2H5)4](HPW12O40) grown on a platinum electrode. Thin films of TTF[Ni(dmit)2]2 are grown on microrough Si(001) by an adsorption process in organic solution. The deposits are characterized by Raman micro-probe and exhibit a pseudo-metallic behaviour with a room-temperature conductivity of about 2 S cm−1.

Synthesis, structure and characterization of mixed cyclopentadienyl–dithiolene metal complexes, [Co(η-C5H5)2][Ni(C3S5)2]3·2MeCN and [Ni(η-C5H5)(C3S5)]

Two new compounds [Co(η-C5H5)2][Ni(C3S5)2]3·2MeCN and [Ni(η-C5H5)(C3S5)], both obtained by combination of the nickel complex anion [Ni(C3S5)2]–(C3S5= 4,5-disulfanyl-1,3-dithiole-2-thionate) and the metallocenium cation [M(η-C5H5)2]+(M = Co or Ni), respectively, have been prepared and characterized crystallographically. The non-integral oxidation-state cobalt complex with the unusual 1 : 3 stoichiometry was obtained by electrochemical oxidation of [NBu4][Ni(C3S5)2] in the presence of [Co(η-C5H5)2]PF6. It is semiconductive (σ= 0.06 S cm–1 at room temperature, Ea= 0.18 eV between 300 and 250 K). The crystal structure shows that the asymmetric unit contains one Ni(C3S5)2 unit, one Ni(C3S5) entity, one Co (η-C5H5) unit and one molecule of acetonitrile. The Ni(C3S5)2 units are associated in triads stacked along the [101] direction. No significant differences are observed in the distances and angles within the two symmetry-unrelated Ni(C3S5)2 units, indicating that the charges of both units are very similar. The cyclopentadienyl rings are staggered due to the location of the Co atoms on inversion centres. The average Co–C distance is comparable to those found in several cobaltocenium salts. The compound [Ni(η-C5H5)(C3S5)] was obtained unexpectedly from the reaction of [Ni(η-C5H5)2]BF4 with Na[Ni(C3S5)2] and characterized by mass spectrometry, conductimetry and X-ray analysis. The crystal structure shows that the asymmetric unit consists of one [Ni(η-C5H5)(C3S5)] entity. The C5H5 ring is disordered over two positions. The Ni(C3S5) unit is planar and almost perpendicular to the C5H5 rings.

Molecular materials for switchable nonlinear optics in the solid state, based on ruthenium-nitrosyl complexes

The promising class of (polypyridine-ruthenium)-nitrosyl complexes capable of high yield Ru–NO/Ru–ON isomerization is targeted as a potential molecular device for the achievement of complete NLO switches in the solid state. A computational investigation conducted at the PBE0/6-31+G** DFT level for benchmark systems of general formula [R-terpyridine-RuIICl2(NO)](PF6) (R being a substituent with various donating or withdrawing capabilities) leads to the suggestion that an isomerization could produce a convincing NLO switch (large value of the βON/βOFF ratio) for R substituents of weak donating capabilities. Four new molecules were obtained in order to test the synthetic feasibility of this class of materials with R = 4′-p-bromophenyl, 4′-p-methoxyphenyl, 4′-p-diethylaminophenyl, and 4′-p-nitrophenyl. The different cis-(Cl,Cl) and trans-(Cl,Cl) isomers can be separated by HPLC, and identified by NMR and X-ray crystallographic studies.

Conducting oriented-[(n-C4H9)4N]2[Ni(dcbdt)2]5 and new (BEDT-TTF)[Ni(dcbdt)2] phases as microcrystalline films, electrodeposited on silicon substrates

Ni(dcbdt)2-based molecular materials, namely [(n-C4H9)4N]2[Ni(dcbdt)2]5, and (BEDT-TTF)[Ni(dcbdt)2] have been processed as microcrystalline films on (001)-oriented silicon substrates using the electrodeposition technique [dcbdt2−: 4,5-dicyanobenzene-1,2-dithiolato; BEDT-TTF: bis(ethylenedithio)tetrathiafulvalene]. Electrodeposited [(n-C4H9)4N]2[Ni(dcbdt)2]5 is made of thin platelets. Elemental analysis and X-ray photoelectron spectroscopy data are in agreement with a 2 : 5 stoichiometry. X-Ray powder diffraction measurements indicate that the growth is highly anisotropic, the ab-plane being parallel to the silicon surface. The films exhibit a semiconducting behaviour with a room-temperature conductivity of about 1.2 × 10−2 S cm−1. Electrodeposition run in the presence of the BEDT-TTF donor molecule leads to faceted microcrystals (size: 5–100 μm) of (BEDT-TTF)[Ni(dcbdt)2] as evidenced by scanning electron microscopy. Single crystal data show that this new (BEDT-TTF)[Ni(dcbdt)2] phase is isostructural to the previously described (BEDT-TTF)[Au(dcbdt)2]. Again, elemental analysis and X-ray photoelectron spectroscopy data are in agreement with a 1 : 1 stoichiometry. The room-temperature conductivity of the film is about 3 × 10−6 S cm−1. This low value is comparable to that measured on (BEDT-TTF)[Au(dcbdt)2] single crystals. For both materials the charge transfer is similar, as consistently evaluated from Raman, infrared and photoemission measurements.

Establishing the Two-Photon Linkage Isomerization Mechanism in the Nitrosyl Complex trans-[RuCl(NO)(py)4](2+) by DFT and TDDFT.

The density functional theory calculations presented in this work allow the first rationalization of the full linkage photoisomerization mechanism of trans-[RuCl(NO)(py)4](2+), in both the forward and reverse directions. These mechanisms are consistent with the experimental data establishing that blue-light irradiation triggers the forward process, while red or IR photons trigger the reverse process. Characterization of the singlet and lowest triplet potential energy surfaces shows that, despite the unfavorable thermodynamic character of the forward process, the topologies of the surfaces and particularly some crucial surface crossings enable the isomerization. In the forward Ru-NO → Ru-ON direction, a sequential two-photon absorption mechanism is unraveled that involves a sideways-bonded metastable state. In contrast, in the reverse reaction, two mechanisms are proposed involving either one or two photons.

Conductive thin films of θ-(BETS)4[Fe(CN)5NO] on silicon electrodes – new perspectives on charge transfer salts

An alternate electrodeposition method using large surface silicon electrodes as anodes to prepare thin films of radical cation salts has been developed. We report on its use for the preparation of the radical cation salt containing the BETS molecule [BETS=bis(ethylenedithio)tetraselenafulvalene] and the photochromic nitroprusside [Fe(CN)5NO]2− anion, i.e. θ-(BETS)4[Fe(CN)5NO]. The use of various techniques, such as SEM/EDX, XPS, X-ray, IR and Raman, unambiguously confirmed that the film is made of the θ-(BETS)4[Fe(CN)5NO] phase previously obtained as single crystals on platinum electrodes. These results show that it is possible to electrodeposit cation radical salts on silicon wafers. In addition, this new method of electrodeposition proves to be extremely useful for preparing the large amounts of product (several hundreds of milligrams) often required for physical measurements.