Excited-state dipole moment of 7-aminocoumarins as determined from time-resolved microwave dielectric absorption measurements

The excited-state dipole moments of several structurally related 7-aminocoumarin derivatives, used as laser dyes and in nonlinear applications, have been measured directly by following time-resolved changes in photoinduced microwave dielectric absorption. The dipole moments of the systems in the fluorescent state lie between 8.1 and 11.9 D, and the changes in the dipole moment on electronic excitation range between 1.7 and 6.1 D. Further, the excited-state dipole moment of any given system is found to be almost the same in benzene and in 1,4-dioxane. The results unambiguously suggest that these aminocoumarins fluoresce from the locally excited state and not from a zwitterionic or twisted intramolecular charge-transfer state, as often indicated in the literature.