Infrared and Ultraviolet−Visible Spectroscopic Studies on Premelting Behavior of One- and Eleven-Layer Langmuir−Blodgett Films of 2-Dodecyl-, 2-Pentadecyl-, and 2-Octadecyl-7,7,8,8-tetracyanoquinodimethane

Abstract

Premelting behavior of one- and eleven-layer Langmuir-Blodgett (LB) films of 2-dodecyl-, 2-pentadecyl-, and 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (dodecyl-TCNQ, pentadecyl-TCNQ, and octadecyl-TCNQ) has been investigated by infrared (IR) and ultraviolet-visible (UV-vis) spectroscopies. It has been suggested from the temperature-dependent variations in the CH 2 scissoring band region of the one-layer LB film of pentadecyl-TCNQ that the packing of the alkyl chains is changed to the hexagonal state from the highly packed state around 60 °C. A CH 2 symmetric stretching band near 2850 cm - 1 of the pentadecyl-TCNQ LB film shows an apparent shift with temperature, indicating that the alkyl chain undergoes premelting phase transitions near 60 and 90 °C. It seems likely that the thermal movement by the alkyl chains in the hexagonal state plays the role of the trigger for the melting phenomenon. The IR and UV-vis studies of the premelting behavior of the pentadecyl-TCNQ LB film suggest that there are one intermediate state between 60 and 90 °C and another intermediate state between 90 and 120 °C. The comparison among the temperature-dependent profiles of the peak maximum position of the CH 2 symmetric stretching band of one-layer LB films of dodecyl-, pentadecyl-, and octadecyl-TCNQ suggests that all the one-layer LB films have two intermediate states between the solid and liquid states and that the longer alkyl chain assumes the more ordered conformation due to van der Waals attraction force among the alkyl chains. The eleven-layer LB films of dodecyl-, pentadecyl-, and octadecyl-TCNQ show the temperature-dependent profiles different from those of the one-layer LB films of dodecyl-, pentadecyl-, and octadecyl-TCNQ. The temperatures for the formation of the second intermediate state of the eleven-layer LB films of dodecyl-, pentadecyl-, and octadecyl-TNCQ are lower than those of the corresponding one-layer LB films, probably because of the layer-layer interaction in the eleven-layer LB films.