Eosin-5-maleimide is a good photosensitizer and can be used to selectively label thiols. It has a quantum yield of 0.57 for singlet oxygen generation, which makes it useful for photoconversion of electron-rich materials such as diaminobenzidine (DAB) or fluorescent dyes into highly electron-dense materials for high resolution electron microscopy studies. Additionally, eosin is a good fluorescence acceptor for fluorescein (FAM), dansyl and coumarin dyes in fluorescence resonance energy transfer (FRET).    &lamba;Ex/&lamba;Em (MeOH) = 524/545 nm  ε = 100,000  Orange solid soluble in DMF or DMSO  Store at -20°C and protect from light  C24H9Br4NO7  MW: 742.95