Semi-empirical PM3 calculation reveals the relationship between the fluorescence characteristics of 4,7-disubstituted benzofurazan compounds, the LUMO energy and the dipole moment directed from the 4- to the 7-position

Abstract

This article elucidates the relationships between the chemical structure and the fluorescence characteristics (fluorescence intensity, maximum excitation wavelength and maximum emission wavelength) of 4,7-disubstituted benzofurazan compounds with computer calculation. Our previous study using Hammett substituent constants of the substituent groups at the 4- and the 7-positions suggested that the total of electron densities on the benzofurazan skeleton and the dipole moment directed from the 4- to the 7-position might affect the fluorescence characteristics of these compounds. The sum of atomic charges on the benzofurazan skeleton, the HOMO energy and the LUMO energy were selected as parameters to reflect the total of electron densities on the benzofurazan skeleton and were obtained with the AM1 and PM3 calculations. The dipole moment directed from the 4- to the 7-position was also calculated with two hamiltonians. The LUMO energy and the dipole moment from the 4- to the 7-position obtained with the PM3 calculation is most closely related with the fluorescence characteristics. Using these parameters, the fluorescent 4,7-disubstituted benzofurazan compounds were classified into two groups, and the maximum excitation and emission wavelengths were different between these two groups. These relationships indicated that the fluorescence characteristics of 4,7-disubstituted benzofurazan compounds were determined by the total of electron densities on the benzofurazan skeleton and the dipole moment directed from the 4- to the 7-position. Furthermore, we predicted the fluorescence characteristics of four 4,7-disubstituted benzofurazan compounds based on the relationships obtained, and confirmed that the prediction agreed with the measured data.