π-stacking of semiquinone radicals determines magnetic and electrical properties of crystals

Semiquinones are a class of stable organic radicals with a great potential for design of novel organic electronics and multifunctional materials. Since the radicals are planar, they typically form π-stacks, which stabilise crystal packing and determine magnetic and electrical properties of the crystals. Two types of stacks π-have been described: 1) Peierls-distorted with alternating short ( 3.35 A) interplanar distances (i.e. comprising dimers of radicals with paired spins), which are diamagnetic and isolators and 2) stacks of equidistant radicals which are 1D antiferromagnetic and semiconductors. Despite its importance, π-stacking of planar organic radicals has been little studied from the fundamental point of view. Here we present a detailed study of a series of salts with different semiquinones: tetrachloro- (Cl4Q), tetrabromo (Br4Q) and 5, 6-dichloro-2, 3-dicyanosemiquinone (DDQ). We have correlated geometry of the π-interaction with electrical conductivity and magnetic properties, and used crystal engineering to optimise them. Electronegativity (i.e. electron-withdrawing) of the substituents plays a double role: i) it stabilises the radical by enhancing delocalisation of π electrons and ii) lowers the energy of the HOMO orbital, thus increasing band gap related to electronic transport. We have described a novel type of stacking with charge transfer between semiquinone radicals and neutral quinone molecules. Studies of X-ray charge density revealed fine details of electronic structure of the radical and the π-interactions of semiquinones.