Coumarin Dyes

　Coumarins are aromatic lactone compounds and many of them exist in natural plants. A coumarin compound with an electron-donating group on the 7-position shows strong light absorption and luminescence, whereas unsubstituted coumarin compounds are hardly luminescent. This is because intramolecular charge transfer occurs when electron-donating and -withdrawing groups coexist in the coumarin molecule. An introduction of a group on the 3- or 4-position largely controls the wavelength of light absorption and luminescence. Furthermore, an introduction of an electron-withdrawing group on these positions can enhance the intensity of luminescence. Such coumarin dyes are useful for laser dyes emitting blue-green light.