Highly efficient and thermally stable nonlinear optical dendrimer for electrooptics.

For the fabrication of practical E-O devices, critical material requirements, such as large E-O coefficients, high stability (thermal, chemical, photochemical, and mechanical), and low optical loss, need to be simultaneously optimized.1 In the past decade, a large number of highly active nonlinear optical (NLO) chromophores have been synthesized, and some of these exhibit very large macroscopic optical nonlinearities in high electric field poled guest/host polymers.2 To maintain a stable dipole alignment, it is a common practice to utilize either high glass-transition temperature (Tg) polymers with NLO chromophores as side chains or cross-linkable polymers with NLO chromophores that could be locked in the polymer network.3 However, it is difficult to achieve both large macroscopic nonlinearities and good dipole alignment stability in the same system. This is due to strong intermolecular electrostatic interactions among high dipole moment chromophores and high-temperature aromatic-containing polymers, such as polyimides and polyquinolines that tend to form aggregates. The large void-containing dendritic structures4-8 may provide an attractive solution to this critical issue because the dendrons can effectively decrease the interactions among chromophores due to the steric effect. Furthermore, these materials are monodisperse, well-defined, and easily purifiable compared to polymers that are made by the conventional synthetic approaches. In this paper, we report the synthesis and characterization of a cross-linkable NLO dendrimer 3 exhibiting very large optical nonlinearity and excellent thermal stability. This NLO dendrimer was constructed through a double-end functionalization of a 3-D shape phenyl-tetracyanobutadienyl (Ph-TCBD) thiophene-stilbene-based NLO chromophore9 as the center core and the crosslinkable trifluorovinyl ether-containing dendrons10,11 as the exterior moieties (Scheme 1). Spatial isolation from the dendrimer shell decreases chromophore-chromophore electrostatic interactions, and thus enhances macroscopic optical nonlinearity because electrostatic interactions among chromophores play a critical role in defining the maximum macroscopic optical nonlinearity that can be achieved for a given chromophore.12 In addition, the NLO dendrimer can be directly spin-coated without the usual prepolymerization process needed to build up viscosity, since it already possesses a fairly high molecular weight (4664 Da). The chromophore loading density of the dendrimer is 33 w/w %, which is confirmed by elemental analysis. There are also several other advantages derived from this approach, such as excellent alignment stability and mechanical properties, which are obtained through the sequential hardening/cross-linking reactions during the high-temperature electric-field poling process. Very large E-O coefficient (r33 ) 60 pm/V at 1.55 μm), and long-term alignment stability (retaining >90% of its original r33 value at 85 °C for more than 1000 h) were achieved for the poled dendrimer. The dendrimer 3 was synthesized by the Mitsunobu condensation between the carboxyl groups on the three branches of the desirable core molecule 2 and the hydroxy-containing chromophore precursor 1 that has cross-linkable trifluorovinyl ether on the dendrons. Then, the intermediate was reacted with tetracyanoethylene (TCNE) to activate the Ph-TCBD electron acceptor (Scheme 1). 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