Hydrolysis of Surface-Bound Phosphonate Esters for the Self-Assembly of Multilayer Films: Use of Solid State Magic Angle Spinning 31P NMR as a Probe of Reactions on Surfaces

Abstract

Solid state static and magic angle spinning (MAS) 31P 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 α,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 31P 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.