Spectra of tetracyanoquinodimethane monovalent anion: Vibrational structure and polarization of electronic transitions

Abstract

The absorption spectra of tetracyanoquinodimethane (TCNQ) anion in the 960- to 350-nm range have been determined (a) in 2-methyltetrahydrofuran glass at 77 K, which allows resolution of vibronic structure, and (b) oriented in a liquid crystal solvent to obtain directions of the transition moments. The first electronic transition at 1.447 eV exhibits a vibronic structure indicative of two active upper-state vibrations, ω1′ = 1260 ± 3 and ω2′ = 331 ± 3 cm-1. Both this and the second electronic transition occurring at 2.84 eV are polarized along the molecular long axis, requiring the symmetry of the two lowest lying electronic excited states to be B1u. No transverse polarized transition was observed in the entire wavelength range; detection would have been possible if εtransverse/εlong-axis exceeded 0.13 at any wavelength. The results are in satisfactory agreement with predictions of previously published SCF-LCAO-MO calculations.