Layer-by-Layer Growth of Acentric Multilayers of Zr and Azobenzene Bis(phosphonate): Structure, Composition, and Second-Order Nonlinear Optical Properties

Abstract

This paper describes the use of layer-by-layer growth of metal−bis(phosphonate) multilayers to produce acentric thin films with second-order nonlinear optical properties. To incorporate such properties, organic “chromophore” molecules containing conjugated π systems situated between electron donor and electron acceptor groups are oriented uniformly within the film such that the bulk structure is noncentrosymmetric. This is accomplished using chromophoric α,ω-bis(phosphonate) molecules that have one terminal phosphonate group “protected” in ester form, whereas the other is a free phosphonic 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 layers. 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}phosphonate, or azobenzene molecule I. Results from multilayer studies and monolayer and solution studies are discussed, with emphasis on results from UV−vis spectroscopy, grazing angle X-ray diffraction, and second harmonic generation.