Attenuated total reflection/FT-IR studies on Langmuir-Blodgett films of 3-octadecyl-2-[3-(3-octadecyl-2-benzothiazolinylidene)-1-propenyl] benzothiazolium iodide

Abstract

Attenuated total reflection (ATR)/Fourier transform infrared (FT-IR) spectra have been measured for Langmuir-Blodgett (LB) films of 3-octadecyl-2-[3-(3-octadecyl-2-benzothiazolinylidene)1-propenyl] benzothiazolium iodide (NK-2560) deposited on a germanium plate with various monolayers. The spectra obtained are very close to the IR transmission spectrum of 3-ethyl-2-[3-(3-ethyl-2-benzothiazolinylidene)-1-propenyl]benzothiazolium iodide (NK-76) in the powdered microcrystalline state which is thought to take conformation A. This observation implies that NK-2560 assumes conformation A in the LB assemblies. The LB film spectra are, however, slightly different from IR transmission spectra of NK-2560 in the solid states (powdered microcrystalline and KBr disc) in that IR bands due to CC and CC stretching modes of the central conjugated system show an upward shift by 7–99 cm−1 in the former compared with the latter. This result can be interpreted by assuming that the electronic structure of the central conjugated system of NK-2560 is significantly different between the LB films and the solid states. A comparison of the ATR/IR spectra of the 1-monolayer LB film with “head-on” configuration and that with “tail-on” configuration shows that NK-2560 molecules have highly ordered (trans-zigzag) hydrocarbon chains in the former film, but in the latter film they have much less ordered hydrocarbon chains.