Growth and characterization of photoactive and electroactive zirconium bisphosphonate multilayer films

Zirconium alkylbisphosphonates are prepared by treating a derivatized substrate alternately with solutions of Zr4+ and a bisphosphonic acid. We report here the preparation of electroactive and photoactive metal phosphonate thin films, containing neutral and cationic organic groups. Electronic spectroscopy, ellipsometry, atomic force microscopy, and electron probe microanalysis were used to characterize the structure and composition of these thin films. An N,N'-dialkylphenylenediamine bisphosphonate (1,4-bis(4-phosphonobutylamino)-benzene) gives uniform lamellar thin films, with low root-mean-square roughness. The stoichiometry of this film is low in Zr relative to other zirconium bisphosphonate films [i.e., Zr-0.75(bisphosphonate)]. Bisphosphonic acids with cationic organic groups (e.g., H2O3PCH2CH2-(4,4'-bipyridinium)-CH2CH2PO3H2) do not follow the typical layered growth motif but instead form crystallites on the substrate surface. Lamellar growth in films prepared with neutral bisphosphonic acids can be perturbed by adding ammonium ions to the bisphosphonic acid growth solutions, leading to the growth of large crystallites. Electrochemical studies were carried out on crystallite films of viologen bisphosphonate based materials, grown on both gold foil and Sn[Sb]O-x-coated glass substrates. The estimated reduction potential for viologen in these films (-0.83 V) is independent of film thickness but shows kinetic limitations that are directly related to film thickness. The E degrees of the viologen moiety in ZrPV(X) films is 150 mV more negative than that of dimethylviologen in solution or in other thin films.