Fluorescence characteristics of six 4,7-disubstituted benzofurazan compounds: an experimental and semi-empirical MO study
To elucidate the factors which determine the fluorescence characteristics (fluorescence quantum yield (Φ), maximum excitation/absorption and emission wavelengths) of 4,7-disubstituted benzofurazan compounds, we have studied the excitation and emission (or relaxation) processes of six typical 4,7-disubstituted benzofurazan compounds. The absorption and fluorescence spectra of these compounds were measured in twenty different solvents and the molecular orbitals relating to excitation and emission (or relaxation) were calculated with semi-empirical methods. The absorption/excitation and emission were ascribed to the electronic transition between HOMO and LUMO. The order of energy gaps between HOMO and LUMO of the 4,7-substituted benzofurazan compounds at the optimized geometry of the ground state agreed with the order of the maximum absorption/excitation energy in vapor phase obtained with the Taft–Kamlet treatment. The order of energy gaps at the optimized geometry of the excited state also agreed with the order of the maximum emission energy in the vapor phase obtained with the Taft–Kamlet treatment. The energy levels of the S1 state of the 4,7-disubstituted benzofurazan compounds were just under the energy level of the T2 state. The order of the energy gaps between the S1 state and the T2 state of the 4,7-disubstituted benzofurazan compounds agreed with the order of the fluorescence quantum yield (Φ) in cyclohexane. These results show that the maximum absorption/excitation, emission wavelengths and the fluorescence quantum yields (Φ) of the 4,7-disubstituted benzofurazan compounds are determined by the energy gaps between the HOMO and LUMO energy at the geometry of the ground state, of the excited state and the energy gap between the S1 and T2 states, respectively.