Nature of the fluorescent state of N-arylcarbazole derivatives as derived from directly measured values of the excited state dipole moment

Laser-induced changes in microwave dielectric loss of benzene solutions of two electron donor-acceptor systems, N-(4-cyanophenyl)carbazole and N-(1-na phthyl)carbazole, have been quantitatively measured with a view to determin ing the dipole moments and hence the nature of the fluorescent states of th ese systems. The dipole moments so determined are found to be much lower th an that expected for a twisted intramolecular charge transfer state. On the basis of the measured values and the results of AMl calculations, it is co ncluded that, contrary to what is commonly believed, these donor-accepter s ystems emit from a locally excited state.