The Reflection Spectra of Simple Salts of the Tetracyanoquinodimethane Anion Radical

The reflection measurements were carried out for fourteen simple salts of the tetracyanoquinodimethane (TCNQ) anion radical over the range from 5000 to 42000 cm−1 by Avery’s method. The solid-state spectra show three bands (CT, YI, and YII) and exhibit distinct differences depending on the cation species. According to the differences in the transition energy (σi) and the oscillator strength (fi), the fourteen salts were classified into three groups (A, B, and C). The solid-state spectra of Group A resemble that of the dimer in an aqueous solution. Group B is characterized by the red shift of the CT band and the intensity increase in the YII band. The spectra of Group C exhibit a vibrational structure in the YI band which is similar to the monomer spectrum in an aqueous solution. Moreover, the large dispersion of the refractive index and the intensity increase in the bands of local excitation were observed for Group C. The electronic structures of these groups were studied by the use of the composite-system method. By taking account of the crystal structure of RbTCNQ-I, belonging to Group A, the stabilization energy (J*) of the ground state was calculated from σCT and fCT. The J* values for ten salts in Group A correspond well to the singlet-triplet separation in the ground state observed by ESR measurements.