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.

Solid state static and magic angle spinning (MAS) 31 P NMR have been used to assess the efficiency of hydrolysis of surface-bound phosphonate ester moieties using variations of two hydrolysis reactions. Inefficient phosphonate ester hydrolysis has limited the quality of polar hafnium R,ω-bis(phosphonate) multilayer films with nonlinear optical properties prepared in our laboratory. To incorporate second-order nonlinear optical (NLO) activity into self-assembled films, oriented monolayers are prepared using NLO chromophores with a phosphonic acid moiety on one end of the molecule and a phosphonate ester group on the other. After the phosphonic acid end is bound to a surface metal layer, the terminal ester must be converted to a phosphonic acid group via hydrolysis in order to bind additional metal and bis(phosphonate) layers. Such hydrolysis reactions are well-known in solution but are not necessarily efficient when one of the reactants is confined to a surface. To determine the best method of hydrolysis for surface-bound phosphonate esters, 10-(diethylphosphonate)decylphosphonic acid was self-assembled onto Hf-functionalized Cab-O-Sil to give a high-surface-area silica with surface phosphonate ester groups. Samples were hydrolyzed via two different chemical methods under varying conditions. Phosphonic acid and phosphonate ester surface groups have distinct signatures in their solid state static and MAS 31 P NMR spectra, and the latter technique provides an unambiguous assessment of the efficiency of different hydrolysis procedures. This type of study is of general utility for evaluating a wide variety of surface reactions.

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The wavelength dependence of the second harmonic response from native single crystal silver electrodes is examined for Ag(111) and Ag(110). Both the rotational anisotropy and potential dependence of the second harmonic response are significantly different at the two wavelengths examined, 1064 nm and 532 nm excitation. These differences are attributed to optical resonances between the incident or outgoing radiation and surface electronic or bulk electronic transitions of the native metal. The anisotropy at 532 nm reflects the increased contribution of surface interband transitions as this higher excitation energy. It is proposed that the potential dependence at 532 nm excitation involves resonances with surface electronic states on the metal.