INFRARED STUDY ON ORDER-DISORDER TRANSITIONS IN LANGMUIR-BLODGETT-FILMS OF 2-DODECYL-7,7,8,8-TETRACYANOQUINODIMETHANE, 2-PENTACECYL-7,7,8,8-TETRACYANOQUINODIMETHANE, AND 2-OCTADECYL-7,7,8,8-TETRACYANOQUINODIMETHANE

Order-disorder transitions of 11-monolayer Langmuir-Blodgett (LB) films of 2-dodecyl-, 2-pentadecyl-, and 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (dodecyl-TCNQ, pentadecyl-TCNQ, and octadecyl-TCNQ) have been investigated by using infrared spectroscopy. The CH2 symmetric stretching bands of the hydrocarbon chains in the dodecyl-, pentadecyl-, and octadecyl-TCNQ LB films show a marked upward shift in the temperature ranges 110-120, and 120-130-degrees-C, respectively. These results indicate that the order-disorder transition of the LB film of octadecyl-TCNQ with the longer even-numbered hydrocarbon chain occurs at a higher temperature than that of the LB film of dodecyl-TCNQ with the shorter even-numbered hydrocarbon chain, but the LB film of pentadecyl-TCNQ with the odd-numbered hydrocarbon chain shows a transition temperature similar to the LB film of dodecyl-TCNQ. Below the transition temperature a pretransitional disordering of the hydrocarbon chain was observed for all the LB films: the shorter the hydrocarbon chain is, the larger the disordering is. The bandwidths of the CH2 symmetric stretching modes of the three kinds of LB films change almost in parallel with their frequencies, suggesting that the mobility of the hydrocarbon chains increases in concert with the progress of disordering. The cyclic temperature treatment experiments performed using the octadecyl-TCNQ LB film elucidate that its order-disorder transition takes place nearly reversibly if the temperature of the sample is kept below 140-degrees-C.