Structure and Degree of Charge Transfer of Simple and Complex Cyanine-TCNQ Anion Radical Salts Studied by Resonance Raman and Infrared Spectroscopy

Abstract

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 = l-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 LPTCNQ-, suggesting the dominance of TCNQ-species in them. In contrast, the complex salts show bands attributed to TCNQO, TCNQ'l2-, 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 n-n* transition of cyanine dyes, are dominated by their contributions. Comparisons of the spectra with those of iodine salts of the cyanines (DMTzNCT, E M Q S e C T , DETOPI-) 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''2-species. The higher the conducting, the broader the Ag modes and the more remarkable the dips.

Pressure‐induced Fermi resonance between fundamental modes in 7,7,8,8‐tetracyanoquinodimethane

“…The Raman spectrum of TCNQ at ambient conditions is shown in Fig. , which is consistent with the previous reports . The strongest lines of the Raman spectrum are assigned to C═C stretching, C―H scissoring and lattice vibrational modes.…”

“…1, which is consistent with the previous reports. [30][31][32][33] The strongest lines of the Raman spectrum are assigned to C═C stretching, C-H scissoring and lattice vibrational modes. The band positions and assignments are given in Table 1.…”

A Semiconducting Cationic Square‐Grid Network with FeIII Centers Displaying Unusual Dynamic Behavior

“…Bands corresponding to CN vibrations were located at 2198, 2212 cm –1 (514 nm); 2161, 2198 cm –1 (633 nm) and 2198 cm –1 (785 nm); laser wavelengths are indicated in parentheses. These peaks closely match those of previously reported CT complexes containing TCNQ radical dimers . The Raman vibrational peak at 1388 cm –1 , which is independent of laser wavelength, is consistent with TCNQ in its monoanionic form …”

New chiral organosulfur donors related to bis(ethylenedithio)tetrathiafulvalene

“…22,28 The Raman spectra for these complexes each show a prominent signal at ca 1600 cm -1 and another one in the range 1434-1450 cm 1 which would be consistent with neutral TCNQ. 29,30 In contrast, the infra-red spectra of the TCNQ complexes of 13-15 show cyanide stretches in the range 2217-2213 cm 1 , which from the correlation of anionic charge with stretching frequency corresponds to a net negative charge of ca 0.35e. 28 However, the range in stretching frequencies between neutral and monoanionic TCNQ is just 44 cm 1 , so other effects from the crystal environment on the CN stretching frequency may easily upset the correlation between charge and stretching frequency.…”