The molecular structure of Langmuir-Blodgett films of 14,15-bis(hydroxyimino)-13,16-dithiaoctacosane using Fourier Transform Infrared spectroscopy

Fourier Transform Infrared (FTIR), p-polarized grazing angle (GAIR) and horizontal attenuated total reflectance (HATR) spectra have been recorded of Langmuir-Blodgett (LB) films of 14,15-bis(hydroxyimino)-13,16-dithiaoctacosane (DOC) deposited on an aluminium plate with 5, 11, 15, 21, 25 and 31 monolayers. Changes in frequency and intensity of the νa(CH)2 vibration can be used to characterize film ordering and preferential molecular packing. The obtained results show that the spectra of the LB films for 5-15 monolayers feature relatively strong CH3 contributions. However, the CH2 contributions slightly increase in the spectra of the LB films for 21-31 monolayers. The changes in frequency and intensity of the νa(CH)2 vibration band can be explained that all LB films of DOC exist as a mixture of gauche and trans conformers. Some portion of the films for 5-15 monolayers may exist high population of gauge conformers and slightly high population of trans conformers for 21-31 monolayers. The absence of splitting of the δ(CH2) band indicate that the LB film of DOC is amorphous or has a hexagonal or triclinic packing of the alkyl chains. Changes in intensity between the p-polarized GAIR and HTAR spectra leads to information about the degree of ordering. Especially, the methylene vibrations (νa(CH)2 in the 2915-2925 cm−1 region and νs(CH)2 at ∼2849 cm−1) show strong dichroism for 31 monolayers. In the HATR spectrum these vibrations are greatly diminished in relative intensity while they are intense in the p-polarized GAIR spectrum. This result indicates that the alkyl chains are almost parallel to the substrate.