Semi-empirical PM3 calculations predict the fluorescence quantum yields (Φ) of 4-monosubstituted benzofurazan compounds

Abstract

We have elucidated the relationship between the fluorescence quantum yield (Φ) of 4-monosubstituted benzofurazan (2,1,3-benzoxadiazole) compounds and the substituent group at the 4-position using a group of eleven such compounds. The absorption and fluorescence spectra of these compounds were measured in fifteen different solvents. Semi-empirical PM3 calculations were carried out to obtain the optimized geometry, energies, charges and dipole moments of this group in the ground and excited states. The S₁–T₂ energy gaps obtained from the PM3-CAS/CI calculations, which reflected the probability of S₁→T₂ intersystem crossing, related well to the fluorescence quantum yield (Φ) of the 4-monosubstituted benzofurazan compounds in nonpolar solvents. It also appeared that the decrease in the S₁–T₁ energy gaps was related to the change in the fluorescence quantum yield (Φ) due to solvent effects. In highly polar solvents, non-radiative S₁→T₁ intersystem crossing occurs, in addition to non-radiative S₁→T₂ intersystem crossing, in 4-monosubstituted benzofurazan compounds, as they have a larger dipole moment in the S₁ state than in the T₂ state. These studies have enabled us to predict the fluorescence quantum yield (Φ) for a series of 4-monosubstituted benzofurazan compounds.