The importance of nonlinear optical phenomena has been known for some time, however, since the mid-1980s, there has been an explosion of interest in the search for and development of non-linear optical materials that possess commercial device applications. To date, the systems have been utilized in information processing, optical switching, optical frequency conversion, and telecommunications, and with the advancing development of optotechnology, burgeoning demands for suitable materials are becoming apparent. Of the many systems studied for example inorganic crystals and semiconductors, organic crystalline monomers, and long-chain polymers with delocalized π electrons, no one species has proved to be all-encompassing, with advantages for one application being negated by disadvantages for another. This is likely to remain the case for the foreseeable future with the criteria of the application governing the type of material used. However, chemists are gradually elucidating what actually governs second- and third-order nonlinear activity and thereby, tailoring species to show greater effects. In recent years, organometallic compounds, through their unique characteristics such as diversity of metals, oxidation states, ligands, and geometries, have found success and brought a new dimension to the area. This review attempts, with critical appraisal, to bring the discussion of organometallic nonlinear optical systems up to date, as well as giving a brief but general introduction to the field of nonlinear optics.

For corrigendum see DOI:10.1002/anie.199508261

Organometallic Compounds for Nonlinear Optics—The Search for En‐light‐enment!

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

The current state of materials development in nonlinear optics is summarized, and the promise of these materials is critically evaluated. Properties and important materials constants of current commercial materials and of new, promising, inorganic and organic molecular and polymeric materials with potential in second- and third-order nonlinear optical applications are presented.

https://science.sciencemag.org/content/sci/253/5017/281.full.pdf

Nonlinear optical materials.

Efficiency of three-wave interactions in molecular crystals depends on the conjugation of the molecular unit, which in turn is a one- or two-dimensional property. This strong anisotropy reduces the number of non-negligible molecular lowest-order hyperpolarizability coefficients to four. The lowest-order macroscopic optical nonlinearity can be expressed, in the absence of significant intermolecular effects, as the tensorial sum of molecular hyperpolarizabilities. This analysis is applied to the 17 relevant noncentrosymmetric crystal point groups, generalizing a previous analysis of nonlinear-optical properties of methyl-(2,4-dinitrophenyl)-aminopropanoate crystals. In several cases, the molecular unit anisotropy is shown to impose structural relations between coefficients of macroscopic nonlinearities, in addition to the usual relations resulting from the crystal point symmetry only. In such cases, nonlinear-optics experiments can be used for testing molecular anisotropy and molecular orientations within the unit cell in the absence of significant nonlinearity arising from intermolecular coupling. Similar relations can be derived between electro-optic coefficients, but limited to the case of weak contributions of intermolecular vibration to the electro-optic effect. We investigate for each point group the possibility of inferring hyperpolarizability coefficients from macroscopic nonlinear measurements, a complementary approach to that based on theoretical molecular calculations or electric-field-induced second-harmonic generation in solution. In the case of highly anisotropic one-dimensional charge-transfer systems (exemplified by $p$-nitroaniline), for each point group and a given molecular hyperpolarizability, the optimal orientation of the charge transfer axis, leading to the highest phase-matchable coefficient, is given. It is shown that crystal point groups 1,2,$m$, and $\mathrm{mm}2$ correspond to the highest possible value of this coefficient, while other crystal symmetry is less favorable. These considerations are applied to four available efficient molecular crystals and used either as a check of molecular orientations in a case of low crystalline symmetry or to estimate otherwise unavailable molecular nonlinear coefficients.

Relations between microscopic and macroscopic lowest-order optical nonlinearities of molecular crystals with one- or two-dimensional units

The higher second‐harmonic efficiency, in powder, of a new organic molecular crystal N‐(4‐nitrophenyl)‐(L)‐prolinol (NPP) is reported. Electronic polarizability of NPP molecules relates to that of other para‐nitroaniline‐like structures such as that of previously reported N‐(2,4‐dinitrophenyl) methyl alaninate (MAP), and the increase in crystalline nonlinear efficiency by one order of magnitude above the latter is ascribed to an ‘‘optimized’’ crystalline structure rather than to minor changes of the molecular hyperpolarizability. The simultaneous chiral and hydrogen‐bonding character of the prolinol electron‐donating group leads to a quasioptimal angle, with respect to quadratic phase‐matched nonlinear interactions, between the molecular transition dipole moments and the twofold axis of the monoclinic P21 crystal structure. Based on a simple oriented gas description of the quasiplanar structure of the crystal, two possible mutually exclusive second‐harmonic phase‐matched configurations are evidenced and s...

Chirality and hydrogen bonding in molecular crystals for phase‐matched second‐harmonic generation: N‐(4‐nitrophenyl)‐(L)‐prolinol (NPP)

The dc linear electro‐optic effect and the x‐ray crystal structure of the organic molecular solid 2‐methyl‐4‐nitroaniline (MNA) have been studied. Of two crystal orientations measured, one polar orientation was found to have an exceptionally large figure of merit for electro‐optic phase retardation, comparable to that of LiNbO3. Comparison with second harmonic generation results demonstrates that the linear electro‐optic effect in MNA is primarily electronic in nature. The origin of the large macroscopic second order nonlinear optical susceptibility is traced to a single large component of the corresponding microscopic molecular susceptibility.

An exceptionally large linear electro‐optic effect in the organic solid MNA

The dc linear electro‐optic effect and the x‐ray crystal structure of the organic molecular solid 2‐methyl‐4‐nitroaniline (MNA) have been studied. One polar orientation was found to have an exceptionally large figure of merit for electro‐optic phase retardation ( 1/2 (n13r11−n33r31) = 270±50×10−12 m/V), which is traced to a single, large component of the corresponding microscopic molecular susceptibility. Comparison with second‐harmonic generation data demonstrates that the liner electro‐optic effect in MNA is primarily electronic in nature.

A large linear electro‐optic effect in a polar organic crystal 2‐methyl‐4‐nitroaniline

An exceptionally large linear electrooptic effect in the organic solid 2-methyl-4-nitroaniline

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An exceptionally large linear electro‐optic effect in the organic solid MNA

A large linear electro‐optic effect in a polar organic crystal 2‐methyl‐4‐nitroaniline

An exceptionally large linear electrooptic effect in the organic solid 2-methyl-4-nitroaniline