An anti-quinoid structure in dual fluorescence of fluozazene molecule and solvent effect of intramolecular charge transfer

In this paper, the dual fluorescence of fluozazene is investigated using quantum chemical methods. Theoretical analyses for the properties of the geometries and the ground state, the locally excited states and the charge transfer states are made. The emission energy of the intramolecular charge transfer state is corrected using nonequilibrium solvation theory developed by our laboratory. Calculations predict two stable geometries for the first excited state. One is resulted from local excitation of pyrrole moiety, which has a structure resembling quinoid and exhibits the feature of normal fluorescence. The other displays an efficient character of intramolecular charge transfer with anti-quinoid geometry, and its emission is the anomalous fluorescence. The anti-quinoid structure has a sufficiently larger dipole moment than the quinoid-like one because of the electron transfer from pirrolo group to the phenyl moiety. The change of charge distribution in the molecule leads to the quinoid-like or anti-quinoid structure. It is found that the anti-quinoid structure is lower in energy than the quinoid-like one by ∼0.330 eV, indicating a state-crossing mechanism for the emission of dual fluorescence. The calculated absorption and emission spectra are in good agreement with the experimental observations.