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

Abstract

Time-resolved changes in microwave dielectric absorption have been studied to determine the excited state dipole moment of 6-propanoyl(N,N-dimethylamino) naphthalene (PRODAN), a molecule extensively used as a fluorescence probe for the study of microheterogeneous environments of a wide variety of chemical and biological systems. This study, which provides by far the most reliable 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 (~20 D). The results imply that solvatochromism of the fluorescence band of PRODAN, the property that makes this system an ideal probe, cannot be attributed to an extraordinarily large magnitude of Δμ. Other forces, such as a hydrogen-bonding 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 group in the excited state, if any occurs, does not influence the fluorescence properties of PRODAN and that the fluorescence originates from a locally excited state.