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

Abstract

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.

Nanoarchitectonics for Analytical Science at Interfaces and with Supramolecular Nanostructures

“…Hasegawa and co-workers have demonstrated several pioneering research efforts for the evaluation of thin films with perfluoroalkyl chains. 50 In molecules having chemical bonding with large permanent dipoles such as C=O and C-F, vibrational couplings across the molecules are induced by dipole-dipole interaction, resulting in extra bands of surface phonon or polariton band. From the infrared spectral shape shifts upon a decrease in the thickness of the films, changes of molecular packings of perfluoroalkanes on a surface can be estimated with verification of stratified dipole-array theory for perfluoroalkyl compounds.…”

Structural and Functional Properties of Fluorinated Silica Hybrid Barrier Layers on Flexible Polymeric Foil

“…The rocking bands were stronger in the case of (CF)10, which had a higher content of fluorinated groups. Moreover, the longer fluorinated chain in this sample caused a characteristic shift of the -CF 2 symmetric stretching vibrations' bands toward a higher wavenumber (from 1142 cm −1 as in (CF)6 to 1149 cm −1 ) [50]. Eventually, the spectrum of (CF)10 was distinguished by two strong bands related to the stretching vibrations observed at 1149 and 1204 cm −1 .…”

“…However, in the other two samples, when the number of fluorinated carbons in the alkyl chain increased, noticeable changes in the FTIR characteristics were observed. In comparison to (CF)0, the presence of the fluorinated groups can be confirmed by the following absorption bands located in the ranges: 520-730 cm −1 (bands assigned to the -CF 3 rocking and the -CF 2 wagging vibrations) and 1120-1250 cm −1 (the C-F stretching vibrations) [41,49,50]. The rocking bands were stronger in the case of (CF)10, which had a higher content of fluorinated groups.…”

Quantitative Anisotropic Analysis of Molecular Orientation in Amorphous N2O at 6 K by Infrared Multiple-Angle Incidence Resolution Spectrometry

“…Ovchinnikov and Wight also suggested that the line shape of the ν3 band depends on the macroscopic shape and orientation of the sample owing to the long-range nature of the dipole–dipole interactions. This implies that both the band shape and intensity can differ between the OP and IP spectra in IR-MAIRS, irrespective of molecular orientation. , We therefore speculate that some morphological changes occur in amorphous N 2 O as the film thickness increases during gas deposition over 1–128 min, leading to the change of the band shape and intensity of the OP spectra (Figure C,D). , …”

“…43,47 We therefore speculate that some morphological changes occur in amorphous N 2 O as the film thickness increases during gas deposition over 1−128 min, leading to the change of the band shape and intensity of the OP spectra (Figure 2C,D). 29,48 In addition, the vibrational Stark effect can induce a frequency shift in the ν3 band of N 2 O, because an electric field can be applied across amorphous N 2 O film owing to anisotropic molecular orientation (dipole alignment). 22,25 The vibrational Stark effect on molecular-solid thin films at low temperatures has been studied for polycrystalline N 2 O.…”