The two kq values calculated for the 3 PC5-N4 and the 3 N4-1a EnTs are close to diffusion (10 9 -10 10 L mol -1 sec -1 ), which is most often the ceiling rate of homogeneous chemical events. These results strongly suggest that the reaction involves a first EnT between PC5 and N4, followed by a second one between the latter and 1a (Figure ).