Infrared Band Assignments and Structure of Even-Numbered 2-Alkyl-7,7,8,8-Tetracyanoquinodimethane in Cast Films-Two Components of the CH2 Scissoring Vibrations not Related to Crystal Field Splitting
Infrared (IR) spectra were measured for 2-octyl-, 2-dodecyl-, and 2-octadecyl-7, 7, 8, 8-tetracyanoquinodimethane (C8TCNQ, C12 TCNQ and C18 TCNQ) in cast films, and it was found that each spectrum shows two components for the CH2 scissoring band at 1 471 and 1 462 cm(-1). Polarized IR measurements showed that the micro-crystallites in the cast films take a random orientation in the plane of the plate. The intensity ratio of the two bands at 1 471 and 1 462 cm(-1) (I1 471/I1 462) decreases observably with the increase in the length of the alkyl chain. Moreover, the relative intensity of the 1 471 cm(-1) CH2 band to a band at 1 529 cm(-1) (C=C stretching mode of the TCNQ chromophore ring) does not change significantly for the three kinds of C(n) TCNQ while the relative intensity of the 1 462 cm(-1) CH2 band to the band at 1 529 cm(-1) increases markedly with the length of the alkyl chain. The above variations of the CH2 scissoring doublet of C(n) TCNQ are quite different from those of long-chain fatty acids (stearic acid and lignoceric acid) where the splitting of the CH2 scissoring vibration occurs due to a crystal field splitting. Considering the crystal structure of C12 TCNQ and the above spectral variations, the authors assign the two components of the CH2 scissoring bands at 1 462 and 1 471 cm(-1) of the C(n) TCNQ cast films to the interdigitated and non-interdigitated parts of the alkyl chains, respectively. Furthermore, the conclusion that the length of the non-interdigitated part of the alkyl chain is almost unchanged in the three kinds of even-numbered C(n) TCNQ could also be reached.