Determining a fluorophore's transition dipole moment from fluorescence lifetime measurements in solvents of varying refractive index

The transition dipole moment of organic dyes PM546 and rhodamine 123 is calculated from fluorescence lifetime measurements in solutions of different refractive index. A model proposed by Toptygin et al (2002 J. Phys. Chem. B 106 3724-34) provides a relationship between the radiative rate constant and refractive index of the solvent, and allows the electronic transition dipole moments to be found: it is (7.1 +/- 1.1) D for PM546 which matches that found in the literature, and (8.1 +/- 0.1) D for rhodamine 123. Toptygin's model goes further in predicting the shape of the fluorescent dye and here we predict the shape of PM546 and rhodamine 123 to be ellipsoidal.