Fluorescence Characteristics of Methoxycoumarins as Novel Fluorophores

The fluorescence characteristics of various methoxycoumarin fluorophores for the development of fluorescence reagents were examined in relation to their structures. The fluorescence emission mechanisms were also considered from the viewpoint of the intramolecular charge-transfer (CT) between methoxyl substituents and the coumarin ring. The arrangement of 6-methoxyl and 3-acetyl group pairs on the coumarin ring significantly contributed to the fluorescence enhancement through the intramolecular CT. We found that the structural features of methoxycoumarins required for intense fluorescence are to hold both diether bonds at the 6- and 7-positions and an electron-withdrawing group at the 3-position, as shown in 3-acetyl-6, 7-dimethoxycoumarin with 0.52 in quantum yield.