Infrared active surface modes found in thin films of perfluoroalkanes reveal the dipole–dipole interaction and surface morphology

Infrared (IR) spectra of an organic thin film are mostly understood by considering the normal modes of a single molecule, if the dipole–dipole (D–D) interaction is ignorable in the film. When the molecules have a chemical group having a large permanent dipole moment such as the C=O and C—F groups, the D–D interaction induces vibrational couplings across the molecules, which produces an extra band as a surface phonon or polariton band because of the small thickness. Since the dipole moment of an organic compound is much less than that of an inorganic ionic crystal, we have a problem that the extra band looks like a normal-mode band, which are difficult to be discriminated from each other. In fact, this visual similarity sometimes leads us to a wrong direction in chemical discussion because the direction of the transition moment of the extra band is totally different from those of the normal modes. Here, we show useful selection rules for discussing IR spectra of a thin film without performing the permittivity analysis. The apparent change in the spectral shape on decrease in the thickness of the sample can be correlated with the morphological change in the film surface, which can also be discussed with changes in the molecular packing. This analytical technique has effectively been applied for studying the chemical properties of perfluoroalkanes as a chemical demonstration, which readily supports the stratified dipole-array theory for perfluoroalkyl compounds.