Fourier Transform Infrared Studies of Molecular Ordering and Interactions in Langmuir-Blodgett Films Containing Nitrostilbene and Stearic Acid

Ultrathin Langmuir-Blodgett multilayer films containing a 1:1 mixture of an amidonitrostilbene dye and stearic acid have been characterized by Fourier transform infrared spectroscopy. Linear dichroism measurements on semiconductor and metal surfaces show that the molecules are packed (with the exception of the first monolayer) in well-ordered and strongly hydrogen-bonded arrays. We have developed a new method of calculating the average angle of a given transition dipole with respect to the surface normal. These data show that the alkyl chains are orientated at about 75 ± 4° to the substrate plane. Interactions between the two types of molecule appear to be of minor importance. The molecule-surface interactions, leading to disordering of the first monolayer, may easily be monitored through shifts of the v(NO₂) modes of the dye head group. The organic matrix “engineered” in this way is an ideal starting point for the construction of novel nonlinear optical device structures.