Thermal behavior of mixed-stack charge transfer films of 2-octadecyl-7,7,8,8-tetracyanoquinodimethane and 3,3',5,5'-tetramethylbenzidine prepared by the Langmuir-Blodgett technique and donor doping .1. Dependence of thermal behavior on the number of layers studied by ultraviolet visible near infrared and infrared spectroscopies

The dependence of thermal behaviors on the number of layers in Langmuir-Blodgett (LB) films of the mixed-stack charge transfer (CT) complex of 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (octadecyl-TCNQ) doped by 3,3',5,5'-tetramethylbenzidine (TMB) has been investigated by using ultraviolet-visible-near infrared (UV-vis-NIR) and infrared (IR) spectroscopies. The appearance of a broad CT absorption band in the NIR region reveals that the mixed-stack CT films are formed by donor doping. The degree of the charge transfer determined by a shift of the C=N stretching band of the TCNQ chromophore verifies that the CT complex films are in a quasi-neutral state. Their temperature-dependent UV-vis-NIR and IR spectra show that donor molecules (TMB) dedope from the CT complexes in the LB films, resulting in the restoration of acceptor molecules (octadecyl-TCNQ) to their neutral states at their respective dedope temperatures. The dedope temperature of the CT complex films increases with the number of layers up to a seven-layer film. Both the UV-vis-NIR and IR spectra show that the one-layer CT complex film undergoes a progressive thermal process. In contrast, the seven- and 11-layer films are stable up to 120 degrees C and show rather abrupt changes near their dedope temperatures. Furthermore, pre-dedope phenomena are observed for the multilayer CT complex films but not for the one-layer CT film. This dependence of the thermal behaviors of the CT films on the number of layers may be attributed to the differences in the film thickness, the longitudinal interactions between the one-dimensional needle-like microcrystals of the CT complex, and the effects of the interaction between the first layer and a CaF2 substrate.