Fluorescence spectroscopy is an extremely sensitive technique for the detection of fluorescent molecules. This detection sensitivity perfectly matches the needs of organometallic complexes in homogeneous catalysis. Catalysis – almost by definition means – means a small amount of catalyst relative to a large amount of substrate molecules. To make it even more difficult, typically the amount of active species formed from a precatalyst tends to be minute, i.e. the vast majority of transition metal species is sitting around doing nothing – other than populating unfavorable equilibria. In order to obtain information on the active species a highly sensitive spectroscopic technique is thus essential.
We have decided to label transition metal complexes with Bodipy fluorophores. This is especially useful since we were able to show, that changes in the electron density at the metal center, translate into changes in the fluorescence properties of the attached fluorophore. Based on this, it is possible to “measure” the electron density at the metal center via fluorescence spectroscopy during catalytic transformations. In the scheme the changes in the fluorescence during the reaction of an alkyne with a gold complex are shown – depending on the stoichiometry of the reactants different fluorescence level indicate the existence of different gold species.
Using complementary fluorescence labels it is even possible to obtain information on the spatial vicinity of gold complexes during a catalytic reaction. Specifically the observation of a red FRET signal provides unequivocal evidence on the formation of a dinuclear gold complexes (from two mononuclear complexes) during a catalytic reaction.
