The vibrational and electronic spectra of the mono-, di-, and trianon salts of TCNQ

Abstract

The vibrational and electronic spectra for the sodium mono‐, di‐, and trianion salts of TCNQ prepared in vacuo by solvent‐free vapor codeposition methods, have been measured. These data, combined with published vibrational assignments for neutral TCNQ and the monoanion salts of TCNQ, have permitted an empirical analysis of the effect of increasing electronic charge on the π‐ bonding of this conjugated system. Thus, empirical molecular force constants determined from new vibrational assignments, which are somewhat tentative in the case of the di‐ and trianion, indicate a regular transition of the p‐quinoid ring system of TCNQ⁰ to a benzenoid system for TCNQ³⁻. Simultaneously, the C≡N and C≡C bonds of the TCNQ wings are considerably reduced in strength while the wing C––C bonds gain double bond character eventually surpassing the original C=C bonds in strength in the case of TCNQ³⁻. The dianion salt, Na₂TCNQ, is colorless while the trianion salt, Na₃TCNQ, is golden yellow but there is little evidence in either the visible or infrared spectra to suggest that these salts are other than insulators. The vibrational data are indicative of a severe distortion of the anion in Na₂TCNQ and, especially, Na₃TCNQ.