Semiconductive 2D arrays of pancake-bonded oligomers of partially charged TCNQ radicals

Multicentre two-electron (mc/2e or `pancake bonding') bonding between 7,7,8,8-tetracyanoquinodimethane (TCNQ) radical anions was studied on its 14 novel salts with planar organic cations. The formal charges of the TCNQ ^δ− moieties are −1/2 and −2/3, and they form mc/2e bonded dimers, trimers and tetramers which are further stacked into extended arrays. Multicentre bonding within these oligomers is characterized by short interplanar separations of 2.9–3.2 Å; distances between the oligomers are larger, typically >3.3 Å. The stacks are laterally connected by C—H...N hydrogen bonding, forming 2D arrays. The nature of mc/2e bonding is characterized by structural, magnetic and electrical data. The compounds are found to be semiconductors, and high conductivity [10 ⁻² (Ω cm) ⁻¹ ] correlates with short interplanar distances between pancake-bonded oligomers.