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

Dependences of thermal behaviors on the number of layers in Langmuir-B lodgett (LB) films of 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (oc tadecyl-TCNQ) have been investigated for both morphological and molecu lar structural aspects by using ultraviolet-visible (UV-vis) and infra red (IR) spectroscopies. The temperature-dependent morphological chang es 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 s uggest that the domains in the one-layer film decompose progressively with temperature even just above room temperature. In contrast, the do mains in the seven- and 11-layer films are stable until 80 degrees C a nd undergo rather abrupt collapse between 80 and 90 degrees C. These d ifferences in the thermal behaviors of the domain structure between th e one-layer film and the seven- and 11-layer films may be attributed t o the differences in the film thickness, the effect of the interaction between the first layer and a CaF2 substrate, and the longitudinal in teraction between the domains. The IR spectral measurements for the LB films at elevated temperatures reveal that the order-disorder transit ion temperature of the one-layer LB film is lower by about 5 degrees C than that of the 11-layer film, which is close to the chain melting p oint of the bulk material. Of particular interest is that the domain s tructure of the one-layer film shows the gradual temperature change bu t that the molecular orientation and structure in the domains experien ce the clear order-disorder transition.