ESTIMATION OF EXCITED-STATE DIPOLE-MOMENT OF SUBSTITUTED COUMARINS

Coumarins are important constituents of natural products. Many of them exhibit fluorescence. We report UV-absorption and fluorescence emission spectra of 4-methyl-5,7-diethoxycoumarin, 4-methyl-5-ethoxy-7-methoxycoumarin and 4-methyl-7,8-diethoxycoumarin in different polar and nonpolar organic solvents. We estimated the change in dipole moment upon excitation using solvatochromic data, and interpret the increase in excited state dipole moment in terms of the possible resonance structure. This method cannot be used when substituent groups are present at two consecutive positions because of the specific interaction and hindrance occurring in such cases.