Role of Structural Factors in the Nonlinear Optical Properties of Phthalocyanines and Related Compounds

1,1‘-Binaphthyl Dimers, Oligomers, and Polymers: Molecular Recognition, Asymmetric Catalysis, and New Materials†

Organophosphorus π-Conjugated Materials

The literature has shown numerous contributions on the synthesis and physicochemical properties of persistent organic radicals but there are a lesser number of reports about their use as building blocks for obtaining molecular magnetic materials exhibiting an additional and useful physical property or function. These materials show promise for applications in spintronics as well as bistable memory devices and sensing materials. This critical review provides an up-to-date survey to this new generation of multifunctional magnetic materials. For this, a detailed revision of the most common families of persistent organic radicals—nitroxide, triphenylmethyl, verdazyl, phenalenyl, and dithiadiazolyl—so far reported will be presented, classified into three different sections: materials with magnetic, conducting and optical properties. An additional section reporting switchable materials based on these radicals is presented (257 references).

Playing with organic radicals as building blocks for functional molecular materials

This contribution presents an overview of the principal classes of second-order nonlinear optical (NLO) transition metal (organometallic and coordination) complexes and illustrates how organometallic and coordination chemistry can offer a very large variety of NLO structures in relation to the metal nd configuration, oxidation state, spin state, etc. Moreover, metal complexes can satisfy the very different demands of second-order NLO materials such as switchable, tuneable, and multidimensional NLO properties, depending on the subtle interplay of structure–property relationships. Recent advances and perspectives in metal complexes as NLO materials are also presented.

Second-order nonlinear optical properties of transition metal complexes

OPTICALLY nonlinear organic materials show considerable potential for applications in optical signal processing and telecommunications1,2. Most materials are based on the p-nitro-aniline template, in which the optical nonlinearities are intimately associated with quasi-one-dimensional charge transfer. But there are problems associated with this conventional approach, arising from the strongly dipolar nature of the molecules2. It has recently been recognized3–5 that two- and three-dimensional stereochemistry offers new possibilities for the design and synthesis of optically nonlinear molecules, in which charge transfer is multidirectional rather than dipolar in character; octupolar nonlinearities have now been demonstrated in several molecular systems5–7. Tri-substituted ruthenium complexes6 appear particularly attractive because intense, multidirectional metal-to-ligand charge transfer leads to a significant enhancement of the optical nonlinearity, as quantified by the quadratic hyperpolarizability, β. Here we show that the choice of ligand can further increase β to values in excess of 10-27e.s.u., comparable to the best dipolar optically nonlinear molecules.

Chiral metal complexes with large octupolar optical nonlinearities

The scaling of the cubic nonlinearity γ with chain length in polyenic molecules has received considerable theoretical attention. Earlier experimental investigations have been restricted to oligomers with fewer than 20 double bonds because of problems associated with the synthesis and solubility of conjugated molecules. These synthetic difficulties have been overcome in the present study by the use of modern living polymerization techniques. Solution measurements of γ as a function of chain length in long-chain (up to 240 double bonds) model polyene oligomers are reported. A saturation of the increase of γ with chain length is observed, and the onset of this saturation occurs for chain lengths considerably longer than predicted from theory.

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Saturation of cubic optical nonlinearity in long-chain polyene oligomers

Quadratic nonlinear susceptibility of octupolar chiral ions

Experimental evidence is provided to supplement earlier molecular engineering propositions as to the high nonlinearities of octupolar origin. The absence of dipole moment in such structures precludes the utilization of electric-field-induced second-harmonic generation (EFISH) in solution: harmonic light scattering (HLS), which does not require dipolar electric field poling, is shown to provide quantitative access to the quadratic nonlinearity of octupolar nonlinear molecules. Crystal violet, a trigonal conjugated cationic dye with electronic charge transfer from peripheral dimethylamino donor groups to an electron deficient sp 2 hybridized central carbon atom, is chosen to illustrate the relevance of HLS in this context as well as provide an experimental estimate of the β magnitude of a prototypical octupolar system. A resonantly enhanced β value of 580 × 10 −30 esu is found at 1.064 μm, in acetone solution (λ max = 590 nm). Due care is taken of solvent calibration while a detailed account of symmetry considerations and tensorial averaging in solution are provided for various molecular point groups of relevance (D 3 , D 3h , C 2v and T d ). It is shown that besides the exploration of the new family of octupolar nonlinear systems, the HLS experiment should be also used to revisit the more traditional dipolar molecular systems: it complements EFISH experiments in providing a measure of the anisotropy of molecular quadratic hyperpolarizabilities.

Harmonic rayleigh scattering from nonlinear octupolar molecular media: the case of crystal violet

nant dielectric susceptibility of the crystal and T is the duration of the accelerating optical pulse. Using a 100-picosecond pulse with an energy of 100 wJ, we expect a 5 3900 for Sr,.,Ba&bO, as nonlinear crystal and electron pulses whose initial velocity is 4 x 105 m/s. When space charge is included the pulse spreading has to be compensated by dispersion obtainable from the transverse profile of the accelerating beam. Figure 2 shows the results of numerical calculation of the electron pulse duration including the space charge effects evaluated as a function of the relative delay of the excitation pulse and the initial 50-femtosecond pulse producing electrons. As can be seen from the inset, pulses shorter than femtosecond can be obtained. 1. H. Niu, V. l? Schelev, Proc. SPIE Intem. Soc. Opt. Eng, 79,1032 ~(1989). J. C. Williamson, A. H. Zewail, Chem. Phys. Letters, 209 (1,2), 10 (1993). 2.

Christophe Dhenaut

Papers

Chiral metal complexes with large octupolar optical nonlinearities

Saturation of cubic optical nonlinearity in long-chain polyene oligomers

Quadratic nonlinear susceptibility of octupolar chiral ions

Harmonic rayleigh scattering from nonlinear octupolar molecular media: the case of crystal violet

Chiral metal complexes with giant octupolar nonlinearities