The 3 N4 and 1 can undergo a second EnT, delivering a sensitized substrate. The phosphorence of 3 N4 was detected in frozen 2-MeTHF at 77 K, and we measured an ET (0-0 band) of 56.1 kcal/mol, which is close to the calculated value of 57.0 kcal/mol. The result indicates the feasibility of the 3 N4-1 EnT, which is exergonic. At 77 K, 3 N4 has a very long lifetime (t = 1.5 sec), suggesting that it is a powerful triplet-state reservoir. 28 It was not possible to directly measure the were 6.7 and 1.8 nsec, respectively. The former luminescence band is typical of isolated 1-naphthyl groups while the latter band has been ascribed to the emission of an excimer of N4 in which the two aryl groups are interacting. Gratifyingly, the intensity of this band was linearly affected by the addition of 1a (KSV = 106 mol -1 ). Static luminescence quenching between the chromophore and the substrate was ruled out because we did not observe shifts in the corresponding absorption and emission UV-vis spectra (spectra in SI). Therefore, the linear correlation between the intensity of the excimer luminescence and the concentration of 1a indicates that a dynamic quenching occurs. This, in turn, implies that the rate of decay of the excimer is slower than the 3 N4-1a EnT. By taking into account the measured lifetime of the N4 excimer (t = 1.8 nsec), the bimolecular 3 N4-1a quenching constant kq results higher than 1.6 x 10 10 L mol -1 sec -1 .