Ligands containing 7-azaindole functionality for inner-sphere hydrogen bonding: structural and photophysical investigations

The synthesis, structural analysis and spectroscopic characterisation of three new 7-azaindole ligands is reported, alongside a novel 7-azaindole derived coordination polymer, with the aim of identifying new bridging ligands containing inner-sphere hydrogen bond donor functionality. Structural characterisation shows that the 7-azaindole hydrogen bond donor ability is significantly stronger in the hydrazone and imine species 1 and 2 compared to the amine 3, with the opposite trend evident in their hydrogen bond acceptor character. These findings are mirrored by the fluorescence spectroscopy results which show bimodal emission, characteristic of multiple emissive species related by proton transfer, is only evident in the amine species and not the more acidic imines. The polymeric copper(II) complex of the hydrazone ligand 1 shows the anticipated inner-sphere hydrogen bonding with a similar donor strength to that observed in the free ligand, which leads to deformation in the remainder of the coordination sphere. These results show the untapped versatility of the 7-azaindole functional group as a building block for ligands in coordination polymers and other multinuclear assemblies, with the potential for both stabilisation through hydrogen bonding and interesting photophysical properties.