An Infrared Study on Order-Disorder Transitions in Evaporated Films of 2-Dodecyl-, 2-Pentadecyl-, and 2-Octadecyl-7,7,8,8-tetracyanoquinodimethane. A Comparison of the Transitions with Those in the Corresponding Langmuir-Blodgett Films

Abstract

Order-disorder transitions of evaporated 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. Four kinds of evaporated films have been prepared for octadecyl-TCNQ ; two of them have been deposited on CaF 2 plates at 5 and 15 °C and the other two on Au-evaporated glass slides at the same temperatures. For all the evaporated films of octadecyl-TCNQ, a CH 2 symmetric stretching band of the hydrocarbon chain shows an abrupt upward shift near 125 °C, revealing that the order-disorder transition occurs near 125 °C irrespective of the substrates used and temperatures at which the sample has been deposited. However, the molecular orientation of the hydrocarbon chain in the evaporated films depends upon the substrates and temperatures. The molecular axis of the hydrocarbon chain is nearly parallel with the substrate surface with its molecular plane nearly perpendicular to the surface in the evaporated films of octadecyl-TCNQ deposited on the CaF 2 plate and Au-evaporated glass slide at 5 °C. On the other hand, it is tilted with respect to the normal in the films prepared on the Au-evaporated glass slide and CaF 2 plate at 15°C, respectively. There is no or little pretransitional change in the frequency of the CH 2 stretching symmetric band, but its bandwidth undergoes pretransitional alterations for all the octadecyl-TCNQ evaporated films. These observations lead us to the conclusion that the alkyl chain mobility begins to increase before their conformational disorder takes place. As in the case of corresponding Langmuir-Blodgett (LB) films, the order-disorder transition of the octadecyl-TCNQ evaporated film deposited on the CaF 2 plate at 15 °C occurs at a higher temperature (125 °C) than that (121 °C) of the dodecyl-TCNQ evaporated film prepared under the same conditions, but the evaporated film of pentadecylTCNQ with the odd-numbered hydrocarbon chain exhibits a transition temperature (121 °C) similar to that of the dodecyl-TCNQ evaporated film with the shorter even-numbered hydrocarbon chain. Cyclic temperature treatment up to 120 °C performed for the octadecyl-TCNQ evaporated film deposited on the CaF 2 plate at 5 °C changes the molecular orientation largely ; the hydrocarbon chain becomes nearly perpendicular to the substrate surface after the cycle to 120 °C.