3-Carboxy-6-chloro-7-hydroxycoumarin: a highly fluorescent, water-soluble violet-excitable dye for cell analysis.

In our search for new violet-excitable dyes with improved photophysical and photochemical properties, we examined several halogen-substituted hydroxycoumarins and found that chlorinated derivatives are at least as bright as their fluorinated analogs. A monochlorinated hydroxycoumarin was found to have a high quantum yield (approximately 0.98), and human leucocyte-specific monoclonal antibodies (CD3, CD4, and CD45) conjugated with this dye exhibited reliable performance in flow cytometry assays.