Transistor Properties of Charge-Transfer Complexes-Combined Requirements from Energy Levels and Orbital Symmetry

Tetracyanoquinodimethane (TCNQ) complexes of fluorene, dibenzofuran, dibenzothiophene, terphenyl, dithienothio-phene, and carbazole show n-channel transistor properties, whereas that of diaminoterphenyl and phenothiazine exhibits ambipolar characteristics. Ambipolar transport is observed when the highest occupied molecular orbital (HOMO) level of the donor-acceptor complex is located above the ordinary hole transport limit. Since the HOMO of the charge-transfer complex is deeper tha n that of the component donor owing to the charge-transfer interaction, not all hole-transporting materials show hole transport in the charge-transfer complexes. In addition, the effective (super-exchange) transfer integrals have to be sufficiently large; this is freque n t l y not satisfied when the donor HOMO is practically orthogonal to the acceptor's lowest unoccupied molecular orbital (LUMO). By appl y i n g these criteria to calculations starting from the crystal structures, transistor properties of mixed stack charge-transfer complexes are explained .