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

Abstract

π-hole interactions between a metal-bound halide and a quinoid ring are described in four novel isostructural co-crystals of 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 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 σDC 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 could substantially enhance the effect and engineer more efficient conductive materials.