Structural characterization of Langmuir-Blodgett films derived from multisulfur heterocycles

A novel multisulfur heterocyclic compound with alkyl chains, C₃S₅(C₁₈H₃₇)₂ (4,5-bis(octadecylthio)-1,3-dithiole-2-thione), has been synthesized and fabricated into Langmuir-Blodgett (LB) films. The structure of this film on SiO₂ slides has been studied using X-ray photoelectron spectroscopy (XPS) and IR and UV transmission linear dichroism (LD) measurements. From the isotherm, it is known that the Langmuir film at the air-water interface is a bilayer. Angle-resolved XPS of films transferred to SiO₂ show that the alkyl chains of the molecule form the topmost layer of the film, with the C₃S₅ ring underneath the alkyl layer. Electronic structure calculations indicate that the transition moment associated with the observed UV band lies coincident with the C₂ axis of the C₃S₅ ring group. In order to properly interpret the polarized absorption data, a simple model addressing the local field effect in LD measurements was developed. Incorporating measured information on the polarizability of the chromophore, UV and IR LD measurements showed the ring plane and the chains to be stacked at 69° and 20°, respectively, relative to the surface normal. These data coupled with the XPS results permitted a structural model to be constructed which accounts for the multilayer structure of the transferred film.