Light-Emitting Metallacycles Put Themselves Together

Chemists are often described as being molecular architects. Nowhere is that moniker more appropriate than for the researchers who recently created a collection of light-emitting platinum-organic metallacycles and metallacages. Peter J. Stang of the University of Utah described the self-assembling supramolecular coordination complexes this week in lectures in the Divisions of Fluorine and Inorganic Chemistry at the American Chemical Society national meeting in Denver.

By selecting combinations of linear and angular metal complexes and organic linking groups, researchers such as Stang and his colleagues can build two- and three-dimensional molecules with fantastic geometries and versatile properties. The scientists are exploring the molecules as optoelectronic chemical sensors, as building blocks for polymeric hydrogels and fibers, as bioprobes for visualizing and monitoring physiological processes, and as therapeutics.

Stang described one study, led by J. Bryant Pollock in his group, in which researchers coupled bis(ethynylpyridine)aniline ligands with a platinum phenanthrene complex to form rhombus-shaped rings. The color of light the molecules emit when they fluoresce can be tuned to span the visible spectrum by changing substituents on the aniline ring. Working with Bogdan Z. Olenyuk’s group at the University of Southern California, Stang’s team showed that the rhomboidal metallacycles get slurped up by tumor cells and have anticancer activity—a first for supramolecular coordination complexes, Stang believes. The molecules potentially could be developed for image-guided drug delivery.

Stang also reported a collaboration with Xuzhou Yan and Feihe Huang of China’s Zhejiang University to link a pyridyl-functionalized tetraphenylethylene, a benzene dicarboxylate, and a platinum phosphine complex to create metallacages that can be decorated with functional groups such as sugars on the outside and house molecules up to the size of enzymes on the inside.

These 3-D metallacages maintain their light emission when they are discrete molecules in dilute solution or when they aggregate together in high concentrations. Other such “luminophores” tend to self-quench and extinguish themselves at high concentrations or fall dark when aggregates come apart at low concentrations, Stang explained. The metallacage light emission does, however, change with solvent polarity and solvation power, which offers an easy way to tune the color and intensity of light emitted by the complexes—including white light—simply by changing solvent.

“These metallacycles are a great contribution to the search for luminescent materials with macroscopic processability and multifaceted functionality,” said Ben Zhong Tang of Hong Kong University of Science & Technology, who discovered the aggregation-induced-emission phenomenon.

Metal-organic frameworks have attracted attention as functional hybrid materials, Tang added, but they are insoluble in common solvents and difficult to process. Stang’s supramolecular coordination complexes overcome that limitation, he said. The fact that they can emit white light, Tang observed, “is remarkable because conventional white-light emitters are usually blends of multiple components in mixtures of two or more solvents.”