Pascal G. Lacroix

Second‐Order Optical Nonlinearities in Coordination Chemistry: The Case of Bis(salicylaldiminato)metal Schiff Base Complexes

Since the mid-1980s, there has been a growth of interest in the search and development of molecular second-order nonlinear optical (NLO) materials that possess various device applications. Beside the most traditional donor-acceptor (“push-pull”) organic chromophores (e.g. the stilbene family), chemists have increasingly extended their field of investigations to newer generations of organic molecules of greater complexity. In recent years, coordination complexes through their unique characteristics such as various redox and magnetic properties, in addition to their great diversity of geometries, have introduced a new dimension to the area. This review attempts to summarize the NLO capabilities of bis(salicylaldiminato)metal Schiff base complexes: (i) chromophores with enhanced hyperpolarizabilities (β) can be obtained by virtue of the coordinated metal center; (ii) the use of chiral substituents allows the engineering of the chromophores into various non-centrosymmetric environments and efficiencies of about 10 times that of urea in second-harmonic generation; (iii) the general trend observed for the high temperature stability in this family of chromophores, while combined with the large β×μ products, makes these systems interesting candidates for their incorporation into poled polymers with high glass transition temperatures (Tg); (iv) the possibility for β modulation induced by magnetic transition is also seen with respect to the concept of molecular switching.

Stilbazolium-MPS3 Nanocomposites with Large Second-Order Optical Nonlinearity and Permanent Magnetization

Intercalated layered materials comprising an organic dye and inorganic MPS3 [where M is either the manganese ion (Mn2+) or the cadmium ion (Cd2+)] phases have been prepared. The intercalation process induces a spontaneous poling, giving rise to an efficiency of 750 times that of urea in second-harmonic generation for the cadmium derivative. In addition, the manganese derivative displays a permanent magnetization below 40 kelvin. Thus, these materials exhibit both a large optical nonlinearity and magnetic ordering.

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.

Synthesis, characterization and nonlinear optical (NLO) properties of a push–pull bisboronate chromophore with a potential electric field induced NLO switch

A new bisboronate with five six-membered ring heterobicycles was prepared by reaction of [2-(2-hydroxy-4-diethylaminobenzylidene)amino]-5-nitroaminophenol and 1,4-phenyldiboronic acid, and its quadratic nonlinear optical (NLO) response was compared to that of a related monoboronate species. A computational investigation conducted within the framework of the DFT theory, at the B3PW91/6-31G* level, indicates that, while the diboronate derivative exhibits a centrosymmetric conformation in the ground state, and hence a vanishing quadratic hyperpolarizability (β), the application of an external electric field provides a gradual intramolecular rotation of the two “push–pull” sub-units which reaches 128° for a field intensity of 10−3 a.u. and leads to a β value equal to 49.2 × 10−30 cm5 esu−1. This result suggests a possibility for a molecular NLO switch induced by an electric field in such systems. Experimentally, the NLO response measured by the electric field induced second harmonic (EFISH) technique indicates an NLO response (expressed as the μ × β product) 1.95 times larger for the bisboronate than for the monoboronate analogue, thus suggesting that, although the ground state conformation is centrosymmetric, the “push–pull” sub-units are in quasi-free rotation at room temperature.

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.

Dithiazolylethene-based molecular switches for nonlinear optical properties and fluorescence: synthesis, crystal structure and ligating properties

Two photochromic dithiazolylethene-based derivatives (1a and 2a) incorporating a push–pull structural motif D–π–A are synthesized and structurally characterized. The photochemical reactions, which reversibly modify the connection between the electron donor and acceptor, result in simultaneous and effective photo-modulations of their NLO and fluorescent properties. The photo-switchable NLO behaviors are investigated theoretically by performing INDO calculations. The presence of chelating sites in 2a allows the synthesis of various metal complexes: Cu(hfac)2·2a, Mn(hfac)2·2a and Ag(2a)2CF3SO3. Their crystal structures are determined.

A Novel and Perfectly Aligned Crystal of a Ferrocenyl Chromophore Displaying High Quadratic Nonlinear Optical Bulk Efficiency

The highest nonlinear optical bulk efficiency for a 2-(4-nitrophenyl)ethenylferrocene (140 times that of urea) has been achieved for E-4 owing to a favourable noncentrosymmetrical packing in which all molecules are perfectly aligned (P1 space group).

PUSH-PULL AZULENE-BASED CHROMOPHORES WITH NONLINEAR OPTICAL PROPERTIES

Abstract A new chromophore 6-methoxy-1-[2-(4-nitrophenyl)ethenyl]-azulene was synthesized and tested for the Second Harmonic Generation (SHG) in solid state by the Kurtz powder technique. This compound was six times more efficient (58 times than the urea standard) than the unsubstituted compound 1-[2-(4-nitrophenyl)ethenyl]-azulene (10 times than the urea).

Synthesis and non-linear optical characterization of novel borinate derivatives of cinnamaldehyde

Herein is reported the synthesis of three new borinates (2a–2c) derived from bidentate imine ligands (1a–1c), which were prepared from the corresponding cinnamaldehyde derivatives. All compounds were characterized by MS, IR and NMR spectroscopy. The nonlinear optical (NLO) characterization of these molecules having push–pull properties is included. Second order nonlinearities were evaluated at 1.907 μm by the electric-field-induced second harmonic (EFISH) technique through the β×μ product, showing a trend for a general increase of the NLO response after boron complexation. For borinate 2c, which has strong electron-donating and electron-withdrawing groups at opposite ends of the π-backbone, βμ = 1560 × 10−30 D cm5 esu−1, showing a three-fold increase with respect to the ligand 1c (βμ = 542 × 10−30 D cm5 esu−1). Third-order nonlinearities, in solid films, were evaluated employing the third harmonic generation (THG) Maker Fringe technique showing χ(3) susceptibilities of the order of χ(3)∼10−12 esu at the infrared wavelength of 1.2 μm. Compounds with the highest χ(3) values correspond to those having the largest βμ product: for 1cχ(3) = 1.1 × 10−11 esu and for 2cχ(3) = 8.4 × 10−12 esu.

Synthesis and non-linear optical properties of substituted styrylpyridinium–MPS3 intercalation compounds (M = Mn, Cd)

New intercalation compounds, M1–xPS3(G)2x(H2O)y(M = Mn, Cd) have been synthesized, where G denotes the 4-[4-(dimethylamino)-α-styryl]-1-methylpyridinium cationic chromophore. The non-linear optical properties of these materials were studied by the Kurtz powder technique. Second-harmonic generation has been evidenced, with an efficiency comparable to that of urea. This preliminary result shows that MPS3 layered host lattices can be envisaged for achieving spontaneous poling of cationic chromophores.