Supramolecular approaches to second-order nonlinear optical materials. Self-assembly and microstructural characterization of intrinsically acentric [(aminophenyl)azo]pyridinium superlattices

This paper describes the synthesis and characterization of self-assembled second-order nonlinear optical multilayer materials containing the high-hyperpolarizability [(aminophenyl)azo]pyridinium chromophore. The chromophoric multilayers are assembled on clean glass or single-crystal silicon substrates via the following iterative reaction sequence: (1) treatment with 4-ClCH2C6H4SiCl3, 3-BrC3H6SiCl3, or 3-IC3H6SiCl3 to afford a self-assembled coupling layer; (2) quaternization of 4-[[4-[N,N-bis(hydroxylethyl)amino]phenyl]azo]pyridine in a ''topotactic'' fashion to generate an [(aminophenyl)azo]pyridinium chromophore layer; (3) cross-linking with octachlorotrisiloxane to afford a capping layer (which also regenerates surface hydroxyl groups for subsequent layer deposition). The new chromophore precursor 4-[[4-[N,N-bis(hydroxylethyl)-amino]phenyl]azo]pyridine was synthesized by diazotization of LC-aminopyridine followed by coupling with N-phenyldiethanolamine and has been characterized by elemental analysis, mass spectrometry, and infrared, UV-vis, and NMR spectroscopies. The chromophoric multilayers have been characterized by X-ray photoelectron and transmission optical spectroscopies, spectroscopic ellipsometry, X-ray reflectivity, advancing contact angle measurements, and polarized second harmonic generation (SHG). The excellent structural regularity of the chromophoric multilayers is indicated by the linear dependence of the [(aminophenyl)azo]pyridinium chromophore longitudinal HOMO --> LUMO charge-transfer excitation absorbance at 572 nm and the ellipsometry- and X-ray reflectivity-derived multilayer thicknesses on the number of assembled trilayers, while the uniform polar order of the stacked chromophoric multilayers is evidenced by the quadratic dependence of the second harmonic generation intensities on the number of trilayers. The [(aminophenyl)azo]pyridinium multilayers are photochemically stable, have very high structural regularities, and exhibit a second-order nonlinear optical susceptibility (chi((2))) of similar to 3.6 x 10(-7) esu (similar to 150 pm/V) at a fundamental wavelength of 1064 nm. Finally, atomic force microscopy reveals that the surfaces of the self-assembled multilayers are very smooth (root mean square roughness = 12 Angstrom for a 10 trilayer sample), undoubtedly reflecting the high three-dimensional structural regularities of the individual layers.