Charge transfer between a metal-bound halide and a quinone through π-hole interactions leads to bulk conductivity

π-Hole interactions between a metal-bound halide and a quinoid ring are described in four novel isostructural co-crystals with the formula [Cu(terpy)ClX]·X'₄Q (terpy = 2,2':6',2''-terpyridine; Q = quinone; X = Br, I; X' = Cl, Br). An unusually strong π-hole interaction between Cu-X and the quinoid ring is noted. Periodic DFT computations estimate the energy of the X⋯quinone interaction to be -20.79 kcal mol^-1, indicating a very strong non-covalent interaction attributed to a higher degree of polarization along the bonding path. The black colour of the crystals originates from a cooperative intermolecular charge transfer between the [Cu(terpy)ClX] complex and the quinone π-system, with iodine playing a dominant role in this process by facilitating the π-hole interaction that enhances the charge transfer mechanism. All the compounds are considered to be weak semiconductors with the σ_DC magnitude ranging between 10^-11 and 10^-9 S cm^-1. It is anticipated that by a smart choice of electron donors and electron acceptors, one can substantially enhance the effect and engineer more efficient conductive materials.