Role of Hydrogen Bonding in Azobenzene-Urea Assemblies. Structural Evaluations of Multilayers on Solid Substrates

The multilayers of three homologous azobenzene (Az)-containing urea derivatives, having two, one and no hydrogen atom(s) at the terminal nitrogen atom in the urea moiety, and a simple long chain urea (hexadecylurea) were prepared by the Langmuir-Schaefer method. X-ray reflectometry, contact angle measurements of water, and Fourier transform infrared (FTIR) spectroscopy were undertaken to evaluate the layer structure and properties of these multilayered films. Of the four compounds, only the Az amphiphile, having no hydrogen atom at the terminus nitrogen atom, provided a different type of multilayers in terms of the molecular orientation, flexibility of the packing state, and characteristics of the intermolecular hydrogen bonding. The obtained data strongly support the formation of bifurcated hydrogen bonds for compounds having one or two hydrogen atom(s) at the terminus nitrogen of urea. The aspects obtained in this structural study can be exactly correlated with the properties of the Langmuir monolayers floating on water (Seki et al., Langmuir, 18, 5462 (2002).). Based on the X-ray and IR spectroscopic data, plausible models of the packed structure of the urea compounds in the multilayers are proposed.