Paramagnetism and semiconductivity in doped charge-transfer complexes

We have studied the epr and conductivity of the nonionic charge-transfer (D-A) complexes carbazole-tetracyanoquinodimethane and carbazole-2,3-dichloro-5,6-dicyanobenzoquinone. These complexes are intrinsically nonionic and therefore diamagnetic, but they exhibit weak, temperature-dependent paramagnetism due to the presence of paramagnetic impurities. Interactions among unpaired impurity spins result in a nonmagnetic ground state and paramagnetic excitations. The complexes behave as semiconductors with activation energy for conduction greater than that for the creation of the paramagnetic spins, but doping the complexes with A^-, analogous to n doping in inorganic semiconductors, suggests that both the conductivity and the paramagnetic properties are associated with the unpaired electrons on the A^- anions.