Fluorescent tags to see catalysts in action

German researchers have used a fluorescent tag to monitor the state of a catalyst during a chemical reaction.

Herbert Plenio and colleagues from the Darmstadt University of Technology tagged an N-heterocyclic carbene ligand in a palladium catalyst with a fluorescent dye. They followed the catalyst's progress in a Suzuki cross-coupling reaction using fluorescence spectroscopy. The team found that the fluorescence signal changed at each stage of the reaction. When the catalyst was activated by a base, the signal decreased within a few seconds. It then remained stable until the substrate was added, then decreased gradually until the end of the reaction. The tag also allowed Plenio to see any catalyst impurities left in the product.

'Our work provides a highly sensitive tool to monitor catalysts in action in low concentrations,' says Plenio. 'Little is known about the nature of catalyst complexes involved in catalytic transformations. By definition, the amount of a catalyst is small compared to the substrates and, to make things worse, this often acts as only a reservoir for the even smaller amount of active species that is actually doing the work.'

Fluorescence has the high sensitivity required to detect these very small quantities, even with just a few nanograms of palladium. 'This is a nice application of fluorescence that allows a deeper insight into catalytic processes,' says Jay Winkler, an expert in the photochemistry of transition metal complexes at the California Institute of Technology, Pasadena, US. 

Plenio says he hopes that in the future, he will have tags that are sensitive enough to reveal the concentration of the active species in the catalytic cycle and the nature of the metal complexes involved. 'We still need to understand so much more about fluorescence dyes and the photophysics behind them,' he says, 'but I am optimistic - Shimomura, Chalfie and Tsien's 2008 Nobel Prize award for the discovery and development of the green fluorescent protein is providing a strong stimulus.' 

Michael Spencelayh, RSC Publishing.