Phase dependence and mechanical and thermal ductility of the luminescence properties of tetranuclear Cu(I) metallacycle assemblies stabilized by ditopic organo-pnictogen (P,As) ligands

A new highly solid-state luminescent dicationic Cu(i)(4) metallacycle A based on a mixed P,As ligand assembly is reported for the first time. Surprisingly, as compared to a previously described related Cu(i)4 metallacycle B based on a similar P,P ligand, a moderate effect of the presence of arsenic atoms in the photophysical processes is observed. A thorough combined experimental and theoretical study is conducted revealing that, in Cu(i) polymetallic assemblies bearing pnictogen ligands, the substitution of a phosphorus atom by an arsenic atom does not cause systematically an alteration of the SOC values and therefore an improvement of the radiative relaxation rates. Intermolecular constraints applied on the gross molecular backbones can play a major role, inducing subtle but noticeable structural alteration impacting optical changes that may differentiate the photophysics of the assemblies regarding the pnictogen atom present. In line with this point, luminescence thermochromism and mechanochromism are observed and analyzed for both A and B assemblies, which moreover also display in the solid state at high temperature an original irreversible thermal crystalline transition that impacts photophysical properties. All in all, this study highlights the subtle and competitive effects that can rule out the photophysics of Cu(i) polymetallic assemblies based on organo-pnictogen ligands and reveals their stimuli-sensitive behaviours, allowing the facile preparation of different phases bearing contrast efficient luminescence properties.