The dependence of thermal behaviors on the number of layers in Langmuir-Blodgett (LB) films of the mixedstack charge transfer (CT) complex of 7,8, 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 CtN 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°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 CaF 2 substrate.

Morphology and annealing effects of Langmuir-Blodgett (LB) films of a mixed-stack charge transfer (CT) complex of 7,8, doped by 3,3′,5,5′-tetramethylbenzidine (TMB) have been investigated by atomic force microscopy (AFM) and ultraviolet-visible-near infrared (UV-vis-NIR) and infrared (IR) spectroscopies. AFM images of the LB films of the CT complex scanned at room temperature show that the films consist of numerous one-dimensional needle-like microcrystals and that a periodic structure exists inside these needle-like microcrystals. A cyclic thermal treatment on one-, three-, and 11-layer films of the CT complex up to 110°C, which is below their dedope temperatures, makes the needle-like microcrystals thinner and flatter while the molecular arrangement and structure in the crystals are nearly unchanged. However, when the temperature of the CT complex films is elevated above their respective dedope temperatures, the needle-like microcrystals become much thinner, resulting in the formation of plate-like microcrystal domains which are characteristics of LB films of octadecyl-TCNQ. The molecular arrangement and structure in the CT complex films after the annealing above the dedope temperature differ from those before the annealing and seem to exist in two different forms. One is just like that of LB films of octadecyl-TCNQ, while the other is neither like that of the CT complex films before the annealing nor like that of the LB films of octadecyl-TCNQ.

Molecular orientation and structure in mixed-stack charge transfer (CT) films of 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (octadecyl-TCNQ) doped by 5,10-dimethyl-5,10-dihydrophenazine ((Me)2P) have been investigated by using ultraviolet−visible−near-infrared (UV−vis−NIR) and infrared (IR) spectroscopies and X-ray diffraction. The degree of charge transfer (ρ) was determined to be 0.5 by a shift of a b1u C⋮N stretching band of the TCNQ chromophore, suggesting that the complex is close to the neutral−ionic boundary. An X-ray diffraction pattern indicates that the CT films consist of a highly ordered multilayered structure in which donor ((Me)2P) and acceptor (octadecyl-TCNQ) molecules are periodically arranged. The d value was calculated to be 3.3 nm, implying that each layer of the CT films comprises bimolecular layers. Comparison of IR intensities of b1u, b2u, and ag C⋮N stretching bands of the acceptor (A) and a totally symmetric band at 1255 cm-1 of the donor (D) between the transmission and reflectance absorption (RA) IR spectra indicates that the chromophores of D and A are almost perpendicular to the substrate surface with their long molecular axes being nearly parallel to the surface normal in the one-layer mixed-stack CT films. The hydrocarbon chain of octadecyl-TCNQ was evaluated to lie on the substrate. In the multilayer CT films the D and A chromophoric planes seem to be slightly tilted with respect to the substrate surface.