The mechanism of denitrogenation for 2,3-diazabicyclo[2.2.1]hept-2-ene (DBH) derivatives was investigated using a visible 5-fs pulse laser. The time-dependent wavenumber changes observed in this study revealed that the concerted denitrogenation occurs in the parent DBH. However, a stepwise denitrogenation mechanism was found for the 7,7-diethoxy-substituted DBH derivative. (C) 2012 Elsevier B. V. All rights reserved.