Two-piece Imaging Probe

For a molecule to be a good near-infrared fluorescence-imaging probe, it must fluoresce brightly and it has to be long-lived enough to shine through the duration of an imaging experiment. Squaraine dyes possess an intense fluorescence emission, but their electron-deficient squaric acid core makes them prone to nucleophilic attack, which can prematurely quench fluorescence. Bradley D. Smith, Jeremiah J. Gassensmith, Easwaran Arunkumar, and coworkers at the University of Notre Dame, in Indiana, found that by threading a squaraine dye molecule through an anthracene-containing tetralactam macrocycle (shown, hydrogen bonds are dashed lines), they can stave off nucleophilic attack to extend the molecule's fluorescence lifetime (J. Am. Chem. Soc. 2007, 129, 15054). The inclusion complex's emission is red-shifted from that of the lone squaraine, making it easy to differentiate the two-piece probe from its individual components. The chemists also found that the complex readily assembles within the complicated environment of a living cell. They plan to modify the system so it can be used for cell and whole-animal imaging.