Layer-by-layer growth of acentric multilayers of Zr and azobenzene bis(phosphonate): Structure, composition, and second-order nonlinear optical properties

This paper describes the use of layer-by-layer growth of metal-bis(phosphon ate! multilayers to produce acentric thin films with second-order nonlinear optical properties. To incorporate such properties, organic "chromophore" molecules containing conjugated pi systems situated between electron donor and electron acceptor groups are oriented uniformly within the film such th at the bulk structure is noncentrosymmetric. This is accomplished using chr omophoric alpha,omega-bis(phosphonate) molecules that have one terminal pho sphonate group "protected" in ester form, whereas the other is a free phosp honic acid that will bind to a metal-primed silicon or glass surface. After deposition of the acid moiety onto metal-primed silicon, the ester groups are hydrolyzed to enable deposition of additional metal and chromophore lay ers. We report here the results of this approach using the chromophore bis( 1-ethyl)3-{N-methyl[(4-[(4-phenylphosphonic acid)azo] phenyl)amino]decyl}ph osphonate, or azobenzene molecule I. Results from multilayer studies and. m onolayer and solution studies are discussed, with emphasis on results from UV-vis spectroscopy, grazing angle X-ray diffraction, and second harmonic g eneration.