In order to elucidate the fluorescence â onâ offâ switching mechanism of benzofurazan compounds bearing an aromatic substituent group, we first studied the intermolecular electron transfer between fluorescent benzofurazan compounds and other aromatic compounds. The fluorescence was quenched by the added aromatic compounds without changing the absorption and excitation wavelengths, indicating that quenching was caused by intermolecular electron transfer. The quenching abilities of the added aromatic compounds reflected the K values of the Sternâ Volmer plot. The K values were also co-related with the molecular orbital energies of the benzofurazan and the added aromatic compounds calculated with the semi-empirical PM3/COSMO method. Next, 11 benzofurazan compounds bearing an aromatic substituent group were synthesized and the effects of the intramolecular electron transfer between the benzofurazan skeleton and the aromatic substituent group were studied. A benzofurazan compound bearing an aromatic substituent group which has a high quenching ability (large K value) in the study of intermolecular electron transfer did not fluoresce (fluorescence â offâ ) because of the intramolecular electron transfer, whereas a compound bearing an aromatic substituent group which has a low quenching ability (small K value) fluoresced (fluorescence â onâ ). From the results, the fluorescence â onâ offâ properties of benzofurazan compounds bearing an aromatic substituent group were predictable from the estimation of the quenching ability of the aromatic substituent group using the Sternâ Volmer plot. Finally, the prediction of the fluorescence â onâ offâ properties of benzofurazan compounds was applied to the development of a new fluorescent â onâ offâ reagent for oxidation. The results are useful in the context of the development of new fluorescent â onâ offâ reagents for use in analytical chemistry.