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.