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) film s of 2-dodecyl-, 2-pentadecyl-, and 2-octadecyl-7,7,8,8-tetracyanoquin odimethane (dodecyl-TCNQ, pentadecyl-TCNQ, and octadecyl-TCNQ) have be en investigated by using infrared spectroscopy. The CH2 symmetric stre tching bands of the hydrocarbon chains in the dodecyl-, pentadecyl-, a nd octadecyl-TCNQ LB films show a marked upward shift in the temperatu re ranges 110-120, and 120-130-degrees-C, respectively. These results indicate that the order-disorder transition of the LB film of octadecy l-TCNQ with the longer even-numbered hydrocarbon chain occurs at a hig her temperature than that of the LB film of dodecyl-TCNQ with the shor ter even-numbered hydrocarbon chain, but the LB film of pentadecyl-TCN Q with the odd-numbered hydrocarbon chain shows a transition temperatu re similar to the LB film of dodecyl-TCNQ. Below the transition temper ature a pretransitional disordering of the hydrocarbon chain was obser ved for all the LB films: the shorter the hydrocarbon chain is, the la rger the disordering is. The bandwidths of the CH2 symmetric stretchin g modes of the three kinds of LB films change almost in parallel with their frequencies, suggesting that the mobility of the hydrocarbon cha ins increases in concert with the progress of disordering. The cyclic temperature treatment experiments performed using the octadecyl-TCNQ L B film elucidate that its order-disorder transition takes place nearly reversibly if the temperature of the sample is kept below 140-degrees -C.