Organizing radical species in the solid state with halogen bonding.

The electronic properties (conductivity, magnetism) of radical systems in the solid state essentially depend on (1) the extent of delocalization of the spin density in the molecule and (2) the intermolecular interactions between radicals. The halogen bond has proven very efficient in engineering such magnetic or conducting structures and recent advances along these lines are reviewed here. Three situations are considered: (1) halogenated radical species acting as halogen bond donors, as found in iodotetrathiafulvalene-based chiral conductors or bilayer systems, and in spin crossover (SCO) complexes with halogenated ligands, (2) radical species acting as halogen bond acceptors, such as neutral nitronyl species or anionic, mixed-valence dithiolene complexes, interacting with closed-shell halogen bond donors (iodoperfluoro alkanes and arenes, iodo- or bromo-pyridinium cations), and, finally, (3) charge transfer salts where both halogen bond donor and acceptor are radical species.