Nonlinear optical molecular properties

One purpose of this tutorial paper on optical characterization is to provide a brief introduction for chemists to the concepts and methods involved in studies of the nonlinear optical properties of molecules and materials. The intent is to familiarize chemists with the range of commonly used techniques and their physical basis. An attempt is made to provide some background on macroscopic nonlinear optics, relating to what is actually measured, and the connection to molecular nonlinear optical properties. This paper is not intended to be a detailed or comprehensive review. The reader is referred to introductory (1, 2) and advanced (3-6) texts on nonlinear optics for more detailed or complete coverage of the subject. 

Bulk and molecular nonlinear optical properties have been measured by laser optical techniques such as second and third harmonic generation (SHG, THG), electric field-induced second harmonic generation (EFISH), and degenerate four-wave mixing (DFWM). Molecular NLO responses can also be calculated by quantum-mechanical (ab initio and semiempirical) methods, and suitable computing programs are being developed. 

This section will outline the simplest models for the spectra of both metal and semiconductor nanocrystals. The work described here has illustrated that, in order to achieve quantitative agreement between theory and experiment, a more detailed view of the molecular character of clusters must be incoriDorated. The nature and bonding of the surface, in particular, is often of crucial importance in modelling nanocrystal optical properties. Wlrile this section addresses the linear optical properties of nanocrystals, both nonlinear optical properties and the photophysics of these systems are also of great interest. The reader is referred to the many excellent review articles for more in-depth discussions of these and other aspects of nanocrystal optical properties [147, 148, 149, 150, 151, 152, 153 and 1541. 

Liquid crystal polymers are also used in electrooptic displays. Side-chain polymers are quite suitable for this purpose, but usually involve much larger elastic and viscous constants, which slow the response of the device (33). The chiral smectic C phase is perhaps best suited for a polymer field effect device. The abiHty to attach dichroic or fluorescent dyes as a proportion of the side groups opens the door to appHcations not easily achieved with low molecular weight Hquid crystals. Polymers with smectic phases have also been used to create laser writable devices (30). The laser can address areas a few micrometers wide, changing a clear state to a strong scattering state or vice versa. Future uses of Hquid crystal polymers may include data storage devices. Polymers with nonlinear optical properties may also become important for device appHcations. 

Hedstrom, M., Benachenhou, N.S. and Calais, J.L. (1994) Nonlinear optical properties of some substituted biphenyls. Molecular Engineering, 3, 329-342. 

Meyers F, Marder SR, Pierce BM, Bredas JL (1994) Electric field modulated nonlinear optical properties of donor-acceptor polyenes sum-over-states investigation of the relationship between molecular polarizabilities (a, p, and y ) and bond length alternation. J Am Chem Soc 116 10703-10714... 

A synthesis and physicochemical characterization, including molecular second-order nonlinear optical properties, of new push-pull-based chromophores 170 properly functionalized for polymerization and containing oxadiazole rings were reported <2002J(P2)1791>. 

Molecular Optics Nonlinear Optical Properties of Organic and Polymeric Crystals... 

Except through the study of linear and nonlinear optical properties of molecular crystals, methods to determine the nature of / require evaluation of appropriate characteristics of... 

The large molecular hyperpolarizability of the merocyanine chromophore (4,5) and the highly polar environment of the quasicrystals has prompted studies of the second order nonlinear optical properties of these materials (6). 

The study of chiral materials with nonlinear optical properties might lead to new insights to design completely new materials for applications in the field of nonlinear optics and photonics. For example, we showed that chiral supramolecular organization can significantly enhance the second-order nonlinear optical response of materials and that magnetic contributions to the nonlinearity can further optimize the second-order nonlinearity. Again, a clear relationship between molecular structure, chirality, and nonlinearity is needed to fully exploit the properties of chiral materials in nonlinear optics. 

The Suzuki coupling has been utilized to craft (Toctasubstituted tetramesitylporphyrins using various arylboronic acids [62], and Schluter has adopted this reaction to prepare phenyl-pyrrole mixed polymers 75 [63]. The BOC group is easily removed by heating [64] and polymers with molecular weights of up to 23,000 were synthesized. These polymers are potentiaEy interesting for their electrical and nonlinear optical properties [65]. 

Recently, Romero and Andrews [1], and Lipinski [2] have shown that the calculated sum over states of a one-electron nonlinear optical property of a molecular system must vanish provided that the wave function employed satisfies the Hellmann-Feynman theorem. This statement applies, in particular, to the electric dipole polarizahility and, as a consequence, there must exist systems which exhibit, most prohahly in excited states, a negative polarizability. Several examples of atomic and molecular systems with negative polarizability can be found in Refs. [3-8]. In search for such systems we study the state of... 

Oligomerization and polymerization of terminal alkynes may provide materials with interesting conductivity and (nonlinear) optical properties. Phenylacetylene and 4-ethynyltoluene were polymerized in water/methanol homogeneous solutions and in water/chloroform biphasic systems using [RhCl(CO)(TPPTS)2] and [IrCl(CO)(TPPTS)2] as catalysts [37], The complexes themselves were rather inefficient, however, the catalytic activity could be substantially increased by addition of MesNO in order to remove the carbonyl ligand from the coordination sphere of the metals. The polymers obtained had an average molecular mass of = 3150-16300. The rhodium catalyst worked at room temperature providing polymers with cis-transoid structure, while [IrCl(CO)(TPPTS)2] required 80 °C and led to the formation of frani -polymers. 

Shcheslavskiy, V., Saltiel, S., Ivanov, D. A., Ivanov, A. A., Petrussevich, V. Y., Petrov, G. I., and Yakovlev, V. V. 2006b. Nonlinear optics of molecular nanostructures in solution Assessment of the size and nonlinear optical properties. Chem. Phys. Lett. 429 294-98. 

A more comprehensive discussion of the theoretical background can be found in the first part of this review.1 This necessarily more abbreviated account focuses on those aspects relevant to third-order properties. As discussed in the first part,1 a convenient way to describe the nonlinear optical properties of organic molecules is to consider the effect on the molecular dipole moment p of an external electric field ... 

The D-PS-A systems investigated for their ability to yield charge-separated states possess suitable features (see Section 8.2.3). Ion-dependent optical changes produced by indicator ligands [8.34-8.40] could lead to ion-selective control of nonlinear optical properties. Molecular electron donor-acceptor complexes may present NLO effects since they possess polarized ground states and undergo inter-... 

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