One-Dimensional π-Stacks of Pancake-Bonded 5,6-Dichloro-2,3-dicyanosemiquinone Radical Anions

A series of six novel salts of 5,6-dichloro-2,3-dicyanosemiquinone (DDQ) radical anions with aromatic cations is studied by structural characterization, magnetization, and conductivity measurements. Different types of stacking of DDQ radicals, which determine electrical and magnetic properties, are observed: regular stacks of equidistant radicals (resulting in semiconductivity along the direction of stacking), stacks of pancake-bonded dimers, isolated pancake-bonded dimers, and mixed stacks comprising DDQ radicals and aromatic cations. General features of pancake bonding of DDQ radicals are defined: parallel ring planes, offset along the carbonyl···carbonyl axis by 2.05–2.15 Å, and interplanar separations of 2.90–2.95 Å (for pancake-bonded dimers) or 3.08–3.24 Å (for regular stacks of equidistant radicals).