Limiting fluorescence anisotropies of perylene and xanthene derivatives

The limiting fluorescence anisotropies (r0) of perylene, perylenyl, perylenoyl and xanthene derivatives in different solvents, micelles and lipid vesicles have been determined. It is found that r0= 0.369 ± 0.002 for the perylene and the perylenyl compounds, r0= 0.377 ± 0.002 for the perylenoyl compounds and r0= 0.373 ± 0.002 for the xanthene derivatives. Among the xanthenes, fluorescein monoglucoronide (FG) is an exception with r0= 0.392. It appears, however, that the absorption and fluorescence spectra of FG also differ markedly in shape from those of fluorescein and different rhodamines. The reason for r0 < ⅖ is not compatible with Jablonski′s theory of torsional vibrations of a rigid molecule, or a free rotational motion in the solvent cavity, but seems to be an inherent property of the chromophoric part of a molecule.