The Lowest Triplet of Tetracyanoquinodimethane via UV–vis Absorption Spectroscopy with Br-Containing Solvents

This study was undertaken to find the previously unknown lowest triplet of the isolated molecule of tetracyanoquinodimethane (TCNQ), which is a widely used organic semiconductor. The problem is topical because the triplet excitation of this compound is involved in some processes which occur in electronic devices incorporating TCNQ and its derivatives, and information on the TCNQ triplet is needed for better understanding of these processes. The lowest triplet of TCNQ was obtained at 1.96 eV using UV–vis absorption spectroscopy with Br-containing solvents. Production of the triplet band with sufficient intensity in the spectra was provided by the capacity of the Br atom to augment the triplet excitation and through using a 100 mm cuvette. The assignment of the corresponding spectral band to the triplet transition was made by observation that this band appeared only in the spectra recorded in Br-containing solvents but not in spectra recorded in other solvents. Additional support for the triplet assignment came from the overall UV–vis absorption spectra of TCNQ recorded in various solvents, using a 10 mm cuvette, in the 1.38–6.5 eV energy range. Singlet transitions of the neutral TCNQ^o molecule and doublet transitions of the TCNQ^¯ negative ion were identified in these overall spectra and were assigned with TD B3LYP/6-31G calculations. Determination of the lowest triplet of TCNQ attained in this work may be useful for theoretical studies and practical applications of this important compound