Interpreting the different emissive properties of cyclic triimidazole-based CuI and AgI coordination polymers: a QTAIM and IQA study

A QTAIM and IQA investigation on model compounds of two isostructural AgI and CuI coordination polymers (CPs) based on cyclic triimidazole (L), i.e. the [MIL]n 1D double-stranded stair chain and the [MClL]n 3D network (M = Cu, Ag), has allowed light to be shed on the different emissive behaviour associated with the two metal ions. According to a previously reported investigation [Malpicci et al. (2021). Inorg. Chem. Front. 8, 1312-1323], AgI CPs showed both fluorescence and multiple ligand-centred room-temperature phosphorescences, whereas CuI CPs displayed non-thermally equilibrated halogen and metal-toligand charge transfer and two ligand-centred phosphorescences, the latter observed only by their selective activation. Analysis of both local and integral QTAIM descriptors, including delocalization indices and source function, of the Ag--N and Cu--N bonds reveals a higher covalent and local character for the latter, explaining the greater metal-ligand electronic communication observed for the Cu compounds. Moreover, IQA investigation shows that the Cu--N bond is characterized by higher interaction energy, due to both higher electrostatic and exchange-correlation contributions. Analysis on the M--X (M = Ag, Cu; X = I, Cl) bonds, also present in these structures, highlights a much higher covalent and local character with respect to the M--N bonds.