Journal of Physical Chemistry, Vol.99, No.11, 3618-3628, 1995
Structure and Degree of Charge-Transfer of Simple and Complex Cyanine TCNQ Anion-Radical Salts Studied by Resonance Raman and Infrared-Spectroscopy
The visible and near-infrared excited resonance Raman, infrared, and visible-near-infrared spectra have been measured for six cyanine-7,7,8,8-tetracyanoquinodimethane (TCNQ) anion radical salts. Among them three are insulating simple salts, [DMTzNC][TCNQ], [EMQSeC][TCNQ], and [DETOC][TCNQ] (DMTzNC = 3,3’-dimethyl-2,2’-thiazolinocyanine, EMQSeC = 1-ethyl-3’-methyl-2,2’-quinoselenacyanine, DETOC = 3,3’-diethyl-2,2’-thiaoxacyanine), and another three are moderately conducting complex salts, [DMTzNC](2)[TCNQ](3), [EMQSeC][TCNQ](2), and [DETOC][TCNQ](2). The 1064-nm excited Raman spectra of both the simple and complex cyanine-TCNQ salts are resonance enhanced with a charge transfer transition from TCNQ(-) to TCNQ(-). The simple salts give a spectrum almost identical to that of Li(+)TCNQ(-), suggesting the dominance of TCNQ(-) species in them. In contrast, the complex salts show bands attributed to TCNQ(0), TCNQ(1/2-), and TCNQ(-), indicating the coexistence of the species in them. Of particular note is that the spectral patterns of the complex salts are markedly different from each other. Therefore, it seems that the mechanism of the charge transfer differs from one compound to another. The 457.9-, 488.0-, and 514.5-nm excited Raman spectra of the cyanine-TCNQ salts, which are resonance enhanced with a pi-pi* transition of cyanine dyes, are dominated by their contributions. Comparisons of the spectra with those of iodine salts of the cyanines (DMTzNC(+)I(-), EMQSeC(+)I(-), DETOC(+)I(-)) suggest that the geometric and electronic structure of the dyes are not perturbed upon complex formation with TCNQ. Although all the infrared spectra of the cyanine-TCNQ salts are characterized by appearances of Ag modes, the spectral patterns of the simple and complex salts can be differentiated by a glance. As in the case of the 1064-nm excited Raman spectra, the infrared spectra of the simple salts exhibit bands due to TCNQ(-) but do not show those assignable to other species of TCNQ. On the other hand, the infrared spectra of the complex salts are more complicated, showing the coexistence of TCNQ(-) and TCNQ(1/2-) species. The higher the conducting, the broader the Ag modes and the more remarkable the dips.
Keywords:MAGNETIC-PROPERTIES;ORGANIC CONDUCTORS;TETRACYANOQUINODIMETHANE TCNQ;CRYSTAL-STRUCTURE;PHASE-TRANSITION;SPECTRA;SYSTEMS;METAL