Energy band structure and metal–organic interactions in tetracyanoquinodimethane (TCNQ) and N,N′-dicyanoquinonediimine (DCNQI) materials

Abstract

The interplay of metal orbitals in metal TCNQ (7,7,8,8-tetracyanoquinodimethane) and DCNQI (N,N′-dicyanoquinonediimine) complexes is investigated on the basis of molecular orbital calculations. For the M(TCNQ) materials, where M = Na, K, Rb, Cs, Tl, Cu, and Ag, the conductivity decreases with the increase in dimerization. Some Cu compounds, however, exhibit uniform columns and higher conducting properties, for which hybridization of the Cu 3d orbitals with the organic radicals is expected. The energy level of Tl 6s is slightly more separated from the TCNQ LUMO, but could potentially mediate similar metal–organic interactions. A simple analytical energy band model is proposed to evaluate the hybridization of the metal orbitals, providing the criteria to form a closed Fermi surface. Accordingly, semiconducting and comparatively high-conducting states of many Cu and Tl TCNQ complexes with uniform columns are to be regarded as a Mott insulating state rather than a Peierls insulating state.