Excited state dipole moment of PRODAN as determined from transient dielectric loss measurements

Time-resolved changes in microwave dielectric absorption have been studied to determine the excited state dipole moment of 6-propanoyl(N,N-dimethylami no) naphthalene (PRODAN), a molecule extensively used as a fluorescence pro be for the study of microheterogeneous environments of a wide variety of ch emical and biological systems. This study, which provides by far the most r eliable value of the excited-state dipole moment of PRODAN, shows that the change in the dipole moment of the molecule upon excitation (4.4-5.0 D) is far less than the commonly mentioned value (similar to 20 D). The results i mply that solvatochromism of the fluorescence band of PRODAN, the property that makes this system an ideal probe, cannot be attributed to an extraordi narily large magnitude of Delta mu. Other forces, such as a hydrogen-bondin g interaction of the molecule with the solvent, must play a crucial role in determining the location of the fluorescent state of PRODAN. Furthermore, the results imply that internal rotation or twisting of the dimethylamino g roup in the excited state, if any occurs, does not influence the fluorescen ce properties of PRODAN and that the fluorescence originates from a locally excited state.