Azobenzene nonlinear optics

Katz, H. E., Schilling, M. L., and Washington, G. E., Solution-phase dielectric characterization of the 4-amino-4 -dicyanovinyl-azobenzene nonlinear-optical chromo-phore, J. Opt. Soc. Am. B, 7, 309-312 (1990). 

Table 2. Linear and nonlinear optical properties of azobenzene dendrimers...

The calculations predict that azobenzene derivatives have nearly identical dipole moments and molecular hyperpolarizabilities as the stilbenes. Selection of compounds for use in specific applications can therefore, be based on linear optical properties (absorption) and photochemical stability requirements without sacrifice of nonlinear optical response. 

The theoretical models discussed above indicate that the sulfonyl group, although slightly weaker in electron acceptor strength, is indeed a viable alternative to the nitro group. In particular, sulfonyl derivatives of stilbene and azobenzene display large molecular hyperpolarizabilities and can be used as bifunctional chromophores for the construction of materials with nonlinear optical properties. 

Alkyloxythiophene derivatives can be connected to an azobenzene moiety and the polymer showed nonlinear optical properties (03SM(138) 409 04MI2117 05MI2360). 

Donor/acceptor substituted azobenzenes, which have large second-and third-order nonlinear optical properties, show a fast thermal Z-F (cis-trans) conversion. 

This chapter is organized as follows. Section 4.2 addresses the study of photoisomerization and photoinduced orientation of azobenzene molecules at the molecular level in SAMs of azo-silane molecules. Section 4.3 discusses photoinduced effects in supramolecular assemblies, i.e., LBK multilayer structures containing azobenzene molecules, and compares the photoinduced movement of azobenzenes in these structures to that observed in spin-cast films. Section 4.4 focuses on the isomerization and sub-Tg photoinduced orientation in a series of very high Tg (up to 350°C) nonlinear optical polyimide and thermoplastic donor-embedded polyurethane polymers containing azo dye, especially focusing on polymer structure-Tg-photoinduced molecular movement relationships. Section 4.5 describes pressure effects on photoisomerization and photo-orientation in films of a PMMA polymer containing azo dye. Finally, we make some concluding remarks in Section 4.6. 

Similar conclusions concerning the relation between the relative molecular orientation and the first hyperpolarizability have been drawn by Hamada [13] in the case of the MNA dimer as well as by Okuno et al. [53] in a study on linear and parallel dimers of 4-dimethylamino-4 -carboxyazobenzene and 4-dimethyl-amino-2 -nitro-4 -carboxyazobenzene. The impetus was the desire for an explanation for the large second-order nonlinear optical responses of cone-shaped azobenzene dendrimers. Okono et al. also assessed the adequacy of three classical electrostatic models (Section IV). 

Zhang X, Wang C, Pan X et al (2009) Nonlinear optical properties and photoinduced anisotropy of an azobenzene ionic Uquid-crystallme polymer. Opt Commun 283 146-150... 

I. Novel network polyurethane with azobenzene dye in the main frame, Macromolecules, 1995, 28, 6437 Nonlinear optical polymers. 2. Novel NLO linear polyurethane with dipole moments aligned transverse to the main backbone, Macromolecules, 1996, 29, 592. 

Random main-chain nonlinear optical polymers Amino-sulfone azobenzene chromophores for second harmonic generation, CAe/n. Mater. 5 1439 (1993). 

Yesodha SK, Pillai CKS, Tsutsumi N. 2004. Stable polymeric materials for nonlinear optics a review based on azobenzene systems. Prog Polym Sci 29 45 74. 

Note that the sol-gel materials based on polysiloxane have been known for their durability and good optical quality. If functionalized with azobenzene moieties, they exhibit nonlinear optical properties and photoinduced anisotropy. There are a few examples of SRG inscribed onto materials (Kim et al., 2006b Chaput et al., 2000 Frey et al., 2000 Darracq et al., 1998). The gratings could be inscribed only in freshly prepared films, whereas in aged films, the cross-linking of the polymer completely hindered SRG formation. 

Azobenzene-containing polymers are reported in the literature at a rate of about 150 papers/year for the last five years. There are two main directions of interest the first relates to the donor-acceptor substituted azobenzenes as the structure generating second-order polarizability and - by noncentrosymmetrical alignment - second order nonlinear optical properties in poled films. The second direction of interest is related to the photoinduced isomerization between the trans (more stable) and cis configurations of the azobenzene. This photoisomerization has a wealth of unexpected and useful consequences which were noted only in the last decade. Both these directions appeared because of the availability of lasers, which on the one hand allowed the observation of the nonlinear optical properties and on the other hand... 

Tris(styiylbipyildine)zinc(II) complexes functionalized by dialkylamino-azobenzene groups have recently been designed by Le Bozec et al. to prepare photoisomerizable star-shaped nonlinear optical polymers 7 [25]. 

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