J. Am. Chem. Soc. 134, 1553–1559 (2012)

One particularly attractive feature of coordination networks — formed by metal centres, usually acting as network nodes, linked through organic ligands that serve as vertices — is their intrinsic ability to be tuned. Modification strategies typically aim to functionalize the organic vertices without affecting the coordination situation at the metal nodes, thus retaining the framework's structure. Rather than exclusively functionalizing the ligands, Zhengtao Xu and co-workers have now investigated side chains from the ligands that can also coordinate to the metal centres yet still preserve the overall network topology — and have observed a dramatic change in properties.

Two materials were constructed from the same lead–carboxylate chains, but with organic linkers featuring different pendant groups: either a thio or a hydroxyl moiety. Only the hydroxyl groups coordinate to the lead, and because it is able to accommodate different numbers of ligands this 'secondary group participation' only disrupted the lead–carboxylate chains in a subtle manner. This weaker coordination interaction, however, induced dramatic changes in properties: the thio-based material exhibited a yellow–green luminescence, whereas a bright white emission was observed with the hydroxyl-based one. This may partly arise from the weakening of the lead–carboxylate bonds, and its effect on electronic transfers.

A commercial ultraviolet light-emitting diode with a blue tinge was converted to a bright white-light one on coating with the hydroxyl-based material. Furthermore, the specific arrangement of the chiral hydroxyl-based side chain also conferred the network significant nonlinear optical properties. This functionalization is reminiscent of hemilabile ligands in catalysis, and points to the potential of using secondary group participation to block or free a metal site for further applications.