Chromophores for nonlinear optics

Figure 9. Tetraethynylethene (TEE) and diethynylethene (DEE) molecular scaffoldings, a) Donor-acceptor substituted chromophores for nonlinear optical studies, b) A three-way...

Table 6.1. Some Examples of Chromophores for Nonlinear Optics ...

F. Steybe, F. Effenberger, U. Gubler, C. Bosshard and P. Gunter, Highly polarizable chromophores for nonlinear optics syntheses, structures and properties of donor-acceptor substituted thiophenes and oligothiophenes. 

In guest/host PR polymers the macroscopic nonlinear optical properties result from the molecular constants of the dopant molecule (also called dinmiophore). Chromophores for nonlinear optical properties are usually based upon aromatic ra-electron systems unsymmetrically eiid-capped with electncm donat and electron accepting groups, as shown in Fig. 3. The molecular polarization of such molecules is a nonlinear fonction of the electric field and can be simplified to (4,5) ... 

Zhang Y, Wang L, Wada T, Sasabe H (1996) One-pot synthesis of a new hyperbranched polyester containing 3,6-di-acceptor-substituted carbazole chromophores for nonlinear optics. Macromol Chem Phys 197 667-676... 

Polymers and supermolecules modified using electron push-pull chro-mophores are also of particular interest for nonlinear optics (NLO) [10-15]. NLO material has attracted much interest over the past 20 years and has been widely applied in various field (telecommunications, optical data storage, information processing, microfabrication, etc.). Chemists have developed ways to introduce NLO chromophores into many type of polymers, such as Hnear polymers, cross-linked polymers, and branched polymers, and have demonstrated their performance in NLO appHcations. 

Chromophore—Polymer Assemblies for Nonlinear Optical Materials... 

Bosch M, Fischer C, Cai C, Liakatas I, Jager M, Bosshard Ch, Gunter P (1999) Photochemical stability of highly nonlinear optical chromophores for electro-optic applications. In Organic thin films for photonics applications. Optical Society of America, Washington DC, p 75... 

There have also been reports on the preparation of polar materials by a photo-electro-poling technique that combines the optically induced quadrupolar depletion of chromophores in the direction of the light electric vector with an additional field-induced orientation of dipolar chro-mophores. The latter allows the preparation of cold electrets, which are interesting for nonlinear optical applications, such as optical harmonic generation, wave mixing, etc. ... 

Polyphosphazenes are suitable materials to be used as carriers for nonlinear optical (NLO) chromophores. Second order NLO properties have been studied for the polymer (128) and blends of (129) with the free chromophore (130) or the cyclophosphazene (131). All systems have glass transition temperatures higher than 135°C and a wide transparency window. The system (129)-(130) appears to exhibit the highest second-harmonic generation (SHG) response. For possible applications the SHG capability has to be enhanced. ... 

Azo based chromophores are also used as side chains in PAl for nonlinear optical applications. The synthesis of such compounds is shown in Figure 14.8. 

LB multilayers of DCANP are highly anisotropic with respect to the dipping direction. Preferential orientation of the aminonitropyridine chromophore was established both by SHG measurements and by UV/Vis spectroscopy of transferred multilayers. Since this orientational anisotropy is essential for nonlinear optical purposes, its origin must be assessed, in order to achieve the necessary structural control. To date, there is only little understanding of the processes leading to the in-plane anisotropy of LB films. 

Samyn et al. reported preparation of chiral chromophore-functionalized polybinaphthalenes for nonlinear optics. These polymers were... 

Walba, D. M. Dyer, D. J. Sierra, T. Cobb, P. L. Shao, R. Clark, N. A. Ferroelectric liquid crystals for nonlinear optics orientation of the disperse red 1 chromophore along the ferroelectric liquid crystal polar axis. /. Am. Chem. Soc. 1996,118, 1211-1212. 

Of course, for chromophores to be of utility for nonlinear optical applications, they must be assembled into a noncentrosymmetric lattice. This assembly process can place an additional number of requirements on chromophore design. Let us now turn our attention to the many aspects of the assembly process. No attempt will be made to provide a comprehensive historical review, but rather an introduction is provided to the various methods that have been pursued in the attempt to realize device quality organic nonlinear optical lattices. 

For nonlinear optical molecules which do not possess the desired centrosymmetric crystallization, there are very potent methods to incorporate these chromophores into polymer hosts ( electric field poling ) (Singer et al., 1986,1987 Hubbard et al., 1989 Bjorklund et al., 1991), self-assembly of molecular layers (Li et al., 1990 Katz et al., 1991), and Langmuir-Blodgett assembly of films (Roberts et al 1990 Kajikawa et al., 1992 Marder et al., 1994). 

Organic compounds with delocalized 7r-electron systems are leading candidates for nonlinear optical (NLO) materials, and interest in these materials has grown tremendously in the past decade [108-118]. Reliable structure-property relationships—where property here refers to first-order (linear) polarizability a, second-order polarizability and third-order polarizability y—are required for the rational design of optimized materials for photonic devices such as electro-optic modulators and all-optical switches. Here also, quantum-chemical calculations can contribute a great deal to the establishment of such relationships. In this section, we illustrate their usefulness in the description of the NLO response of donor-acceptor substituted polymethines, which are representative of an important class of organic NLO chromophores. We also show how much the nonlinear optical response depends on the interconnection between the geometric and electronic structures, as was the case of the properties discussed in the previous sections [ 119]. 

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