Organic/Inorganic Langmuir−Blodgett Films Based on Metal Phosphonates: Preparation and Characterization of Phenoxy- and Biphenoxy-Substituted Zirconium Phosphonate Films

Abstract

Zirconium phosphonate Langmuir−Blodgett (LB) films have been prepared where phenoxy and biphenoxy functional groups are placed in the organic tail at different positions relative to the phosphonate headgroup. The organophosphonic acids 4-octadecyloxyphenylphosphonic acid (P0), 4-(4‘-tetradecyloxyphenyl)butylphosphonic acid (P4), 4‘-octadecyloxybiphenylphosphonic acid (B0), and 4-(4‘-tetradecyloxybiphenyl)butylphosphonic acid (B4) were synthesized for this study. Pressure vs area isotherms of Langmuir monolayers are well-behaved except in the case of B0, where aggregation results in a very rigid monolayer. Using a previously developed three-step deposition procedure, both alternating layer and symmetric bilayer zirconium phosphonate films have been prepared and characterized. A combination of XPS, X-ray diffraction, and ATR-FTIR analyses indicate that organized zirconium phosphonate LB layers are formed in each case. Polarized ATR-FTIR was used to determine the orientation of the different functional groups leading to models for the molecular orientation within the films. In cases where the phenoxy or biphenoxy groups are separated from the phosphonate headgroup, deuteration of the terminal alkyl chain allows for the resolution of the different alkyl segments. Differences in molecular orientations are observed depending on whether the monolayers are deposited on the downstroke (template layer) or on the upstroke (capping layer) of the three-step deposition procedure, and these differences are attributed to the strong zirconium−phosphonate bonding. Subtle orientational differences are also seen for the same capping layers deposited onto different templates. The results show that the zirconium phosphonate LB deposition method may be extended beyond simple alkylphosphonates to organic groups containing aromatic moieties.