Organic/inorganic Langmuir-Blodgett films based on metal phosphonates. 3. An azobenzene-derivatized phosphonic acid forms continuous lattice layers with divalent, trivalent, and tetravalent metal ions

Metal phosphonate Langmuir-Blodgett (LB) films of an azobenzene-functionalized phosphonic acid amphiphile, (4-(4'-tetradecyloxyphenyldiazenyl)phenyl)butylphosphonic acid (A4), have been prepared with Zr4+, La3+, Gd3+, Ba2+, Mn2+, and Cd2+. Film formation and quality are characterized with optical spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. The azobenzene chromophores are strongly H-aggregated in all of the metal phosphonate LB films. With divalent and trivalent ions, the in-plane metal phosphonate structure observed in the LB films is the same as that found in analogous solid-state layered phosphonates and depends on the identity of the metal ion. Films of A4 with the lanthanide ions La3+ and Gd3+ form with stoichiometry MH(O3PR)(2). When Ba2+ is incorporated into the films, the M(HO3PR)(2) structure-type is adopted, while Mn2+ and Cd2+ give rise to films of A4 with the stoichiometry M(O3PR). H2O. In each of these cases, the inorganic continuous lattice crystallizes in the hydrophilic portion of the film formed by the A4 amphiphiles during deposition. Films incorporating Zr4+ were prepared with a previously described three-step procedure which exploits the strong oxophilicity of the Zr4+ ion. The films form with stoichiometry Zr(O3PR)(2), the same as that observed in solid-state zirconium phosphonate materials. Film transfer is shown to be facile when the spacing of the inorganic network and the packing of the organic chromophores is commensurate, while a mismatch in these preferred spacings affects the deposition process. The results demonstrate that the metal phosphonate continuous inorganic network can be formed in LB films of amphiphiles other than alkylphosphonic acids, including those with larger organic groups.