Nonlinear optical properties are a sensitive probe of the electronic and solid-state structure of organic compounds and as a consequence find various applications in many areas of optoelectronics including optical communications, laser scanning and control functions, and integrated optics technology. Because of their strongly delocalized π electronic systems, polymeric and non-polymeric aromatic compounds show highly nonlinear optical effects. Nowadays, polymer chemists are able to tailor specific materials properties for various applications. Some organic substances with π electronic systems exhibit the largest known nonlinear coefficients, often considerably larger than those of the more conventional inorganic dielectrics and semiconductors, and thus show promise for thin-film fabrication, allowing the enormous function and cost advantages of integrated electronic circuitry. The electronic origins of nonlinear optical effects in organic π electronic systems are reviewed, with special emphasis being given to second-order nonlinear optical effects. Methods for measuring nonlinear optical responses are outlined, and the critical relationships of the propagation characteristics of light to observed nonlinear optical effects and to solid-state structure are discussed. Finally, the synthesis and characterization of organic crystals and polymer films with large second-order optical nonlinearities are summarized.

Organic Polymeric and Non-Polymeric Materials with Large Optical Nonlinearities

Electric field poled organic electro-optic materials: state of the art and future prospects.

Tetrathiafulvalene (TTF) and its derivatives were originally prepared as strong electron-donor molecules for the development of electrically conducting materials. This Review emphasizes how TTF and its derivatives offer new and in some cases little-exploited possibilities at the molecular to the supramolecular levels, as well as in macromolecular aspects. TTF is a well-established molecule whose interest goes beyond the field of materials chemistry to be considered an important building block in supramolecular chemistry, crystal engineering, and in systems able to operate as machines. At the molecular level, TTF is a readily available molecule which displays a strong electron-donor ability. However, its use as a catalyst for radical-polar crossover reactions, thus mimicking samarium iodide chemistry, has only recently been addressed. Important goals have been achieved in the use of TTF at the macromolecular level where TTF-containing oligomers, polymers, and dendrimers have allowed the preparation of new materials that integrate the unique properties of TTF with the processability and stability that macromolecules display. The TTF molecule has also been successfully used in the construction of redox-active supramolecular systems. Thus, chemical sensors and redox-switchable ligands have been prepared from TTF while molecular shuttles and molecular switches have been prepared from TTF-containing rotaxanes and catenanes. A large synthetic effort has been devoted to the preparation of the so-called organic ferromagnets, many of which are derived from TTF. The main task in these systems is the introduction of ferromagnetic coupling between the conduction electrons and localized electrons. TTF has also played a prominent role in molecular electronics where TTF-containing D-sigma-A molecules have allowed the preparation of the first confirmed unimolecular rectifier. Recently, it has been confirmed that TTF can display efficient nonlinear optic (NLO) responses in the second and third harmonic generation as well as a good thermal stability. These findings can be combined with the redox ability of TTF as an external stimuli to provide a promising strategy for the molecular engineering of switchable NLO materials. Fullerenes endowed with TTF exhibit outstanding photophysical properties leading to charge-separated (CS) states that show remarkable lifetimes.

New Concepts in Tetrathiafulvalene Chemistry.

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

We report the observation of second harmonic generation in a new class of organic polymeric materials, namely, electric field poled polymer glasses. Both the observed second harmonic coefficient (d33=6.0±1.3×10−9 esu, at 1.58 μm wavelength) and the poling process are described by a thermodynamic model that we have developed. The ability to form thin films from these materials may qualify them for integrated optics applications.

Second harmonic generation in poled polymer films

We have investigated the influence of donor and acceptor substituents on the second and third order hyperpolarizabilities β and γ of large conjugated molecules such as stilbene and styrene derivatives. This was performed by two independent measurements of β and γ using the techniques of dc electric‐field induced second‐harmonic generation and tunable four‐wave mixing in liquids and solutions. For trans−stilbene derivatives, β and γ are typically 10 times larger than for the corresponding benzene compounds, and are strongly correlated with the mesomeric effect of the substituents. A series of disubstituted molecules with strong donor–acceptor intramolecular charge transfer exhibit very large β, and it is shown that this enhancement can be predicted from the basic properties of the first electronic excited state.

J. L. Oudar

Papers

Optical nonlinearities of conjugated molecules. Stilbene derivatives and highly polar aromatic compounds