Infrared external reflection spectroscopic study on the structures of a conducting Langmuir-Blodgett film of a tetrathiafulvalene derivative

Infrared (IR) external reflection absorption spectroscopy was applied to study the structure and its change associated with iodine doping of a Langmuir-Blodgett (LB) film of a tetrathiafulvalene (TTF) derivative, 1,3-dithiole-4-carboxylic acid-2-(1,3-dithiol-2-ylidene)hexadecyl ester (HDTTF, Figure 1), prepared on a silicon wafer. The incidence angle (phi) dependence of the p- and s-polarized IR external reflection spectra were measured and analyzed by a theoretical calculation of reflection absorbance (A(p) and A(s)) as a function of the tilt angle of the transition moment of each IR band with an assumed complex refractive index. The results indicated that the LB film before doping consists of cis and trans structures, which have the C=O bonds with cis and trans configurations, respectively, with respect to the neighboring C=C bond of the TTF ring. The TTF ring in both structures are almost perpendicular to the substrate surface with the central C=C bond parallel to the surface. After iodine doping the LB film consists of the cis structure and gives rise to charge-transfer-induced vibronic bands at 1352 and 1252 cm(-1). The analyses of the phi dependence of A(p) of these bands proved that the transition moments of both bands make a tilt angle of about 70 degrees with respect to the surface normal of the substrate, which is consistent with a face-to-face stacking of the TTF rings nearly perpendicular to the surface and the high conduction parallel to the substrate.