Direct observation of denitrogenation process of 2,3-diazabicyclo[2.2.1]hept-2-ene (DBH) derivatives, using a visible 5-fs pulse laser

► The denitrogenation process was directly observed using 5-fs ultrafast spectroscopy. ► The substituent effect on the denitrogenation of 2,3-diazabicyclo[2.2.1]hept-2-ene (DBH) was revealed. ► The stepwise denitrogenation was observed for the diethoxy-substituted azoalkane. ► The concerted denitrogenation was observed for the parent DBH.