Paramagnetism and semiconductivity in organic π—π* molecular complexes

Abstract

The semiconduction and electron spin resonance of hexamethylbenzene, pyrene and perylene complexes of tetracyanoquinodimethane (TCNQ), and perylene complexes of TCNQ, chloranil and fluoranil have been studied as a function of temperature. The e.s.r. data are interpreted in terms of isolated and clustered M⁺(acceptor)^– impurities (with M⁺ probably Na⁺). Space charge limited current studies suggest that lattice perturbations surrounding large M⁺(acceptor)^– clusters are responsible for carrier trapping. Impurity concentrations and trap densities are approximately constant for complexes of a given electron acceptor. The magnitude of semiconduction is too large to be due to the measured concentrations of M⁺(acceptor)^–, but of the correct order for intrinsic conduction. Similar studies on TCNQ crystals indicate extrinsic conduction, with activation energy 0.49 eV, at low temperatures and intrinsic conduction, with activation energy 0.88 eV, at higher temperatures.