Abstract
The phosphorescence quantum yield, zero-time anisotropy, and lifetime are the three factors determining the suitability of a phosphorescent dye for rotational diffusion experiments. Furthermore, knowledge of the absolute orientation of the phosphorescence dipole moment in the molecular frame is a prerequisite for a quantitative analysis of time-resolved phosphorescence anisotropy (TPA) measurements in orientationally anisotropic systems. These factors have been characterized for four dye molecules, consisting of a fluorescein core with two or four substituted heavy-atoms (iodine and bromine). The characterization was performed by a combination of spectral and time-resolved phosphorescence and fluorescence measurements on dye molecules embedded in immobilizing polymer matrices. A quantitative criterion was found for the time-region where to use which dye, as well as for the dye concentrations to be used. For the latter criterion the effect of Förster radiationless energy transfer on the phosphorescence anisotropy was treated explicitly. The phosphorescence dipole moment was found to be tilted somewhat out of the molecular plane, and aligned more towards the in-plane fluorescence dipole moment than to the in-plane S
1
←S
0
absorption dipole moment. The results for the four fluorescein derivatives indicate that the effect of the spin-orbital coupling is to pull the in-plane component of the phosphorescence dipole moment towards the fluorescence dipole moment, causing an increase of the phosphorescence quantum yield. This effect is primarily influenced by the Z-number of the substituted heavy atoms. The number of substituted heavy atoms plays a less important role.