2-Azafluorenone derivatives: Photocatalyst for oxygenation of toluene via electron transfer and hydrogen-atom transfer

2-Aza-9-fluorenone derivatives were synthesized and their optical properties were investigated. The UV–visible absrption spectra revealed λmax values in the range of 350–380 nm, with a shift to the longer wavelength when an electron-donating group was substituted on the aryl group at the 3-position adjacent to the nitrogen in the azafluorenone. N-Methyl-2-azafluorenone was synthesized by methylating the nitrogen of azafluorenone. The N-methyl derivative was found to have a maximum absorption at 426 nm from UV-Vis spectral measurements, and electrochemical measurements revealed that it has a lower first reduction potential and higher electron-accepting ability than azafluorenone. In addition, the application of azafluorenone as a photocatalyst was investigated, focusing on its structural and electronic features. Oxidation of toluene with azafluorenone as a photocatalyst yielded benzoic acid in high yield. Electron-transfer dynamics analysis using nanosecond time-resolved laser flash photolysis suggests that azafluorenone and N-methylazafluorenone act as hydrogen transfer and electron transfer catalysts, respectively.