Dependences of Thermal Behaviors on the Number of Layers in Langmuir−Blodgett Films of 2-Octadecyl-7,7,8,8-tetracyanoquinodimethane. 1. Order−Disorder Transitions in the Films Investigated by Ultraviolet−Visible and Infrared Spectroscopies

Abstract

Dependences of thermal behaviors on the number of layers in Langmuir−Blodgett (LB) films of 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (octadecyl-TCNQ) have been investigated for both morphological and molecular structural aspects by using ultraviolet−visible (UV−vis) and infrared (IR) spectroscopies. The temperature-dependent morphological changes have been monitored by the base-line shift in the UV−vis spectra of the LB films. The changes in the base-line in the 450−600 nm region suggest that the domains in the one-layer film decompose progressively with temperature even just above room temperature. In contrast, the domains in the seven- and 11-layer films are stable until 80 °C and undergo rather abrupt collapse between 80 and 90 °C. These differences in the thermal behaviors of the domain structure between the one-layer film and the seven- and 11-layer films may be attributed to the differences in the film thickness, the effect of the interaction between the first layer and a CaF2 substrate, and the longitudinal interaction between the domains. The IR spectral measurements for the LB films at elevated temperatures reveal that the order−disorder transition temperature of the one-layer LB film is lower by about 5 °C than that of the 11-layer film, which is close to the chain melting point of the bulk material. Of particular interest is that the domain structure of the one-layer film shows the gradual temperature change but that the molecular orientation and structure in the domains experience the clear order−disorder transition.