Medium effects of fluorescence quantum yields and lifetimes for coumarin laser dyes

Fluorescence quantum yields and lifetimes for a series of aminocoumarin laser dyes have been measured. The characteristics red shift of fluorescence in polar solvents is accompanied by a marked decrease in emission yield and lifetime for coumarins in which the substituted amine group at the 7 position is free to rotate. Strong emission is maintained in structurally rigid dyes. The non-radiative decay rates for non-rigid dyes which increase in polar solvents are associated with relaxation of emissive planar dye conformations to twisted zwitterionic species.