Elsewhere, 1 DCA* is known as an efficient singlet oxygen sensitizer (k q = 4.3 x 10 9 M -1 s -1 ) via a photosensitized energy transfer (E n T) mechanism. The high quantum yield (reaching almost 2.0) supports the generation of two molecules of 1 O 2 per 1 DCA*. Yet, comparison of quenching rate constants revealed that 1a and 1x (N-methyl tetrahydroisoquinoline) as surrogate trialkylamines quench 1 DCA* more efficiently than O 2 does (Figure ). Furthermore, our enhanced yields with increasing [O 2 ] in solution are inconsistent with an initial SET between 1 DCAS* and amine, as O 2 should decrease the population of 1 DCAS* available for "initial SET." This was reflected in the relative intensity change of light transmitted through the coil of the aforementioned tubular flow reactor as detected by an online fibre-optic transmission spectrometer probe. When an aerated reaction mixture of DCA (5 mol%) + 1a (12 mM) was compared with an aerated solution of DCA only, the latter led to strong absorption (Figure , A), indicating a larger steady-state concentration of DCA directly afforded via the rapid E n T quenching of 1 DCA* by O 2 . On the other hand, the reaction mixture (Table , entry 6) gave minimal absorption of light (Figure ). Reductive quenching of 1 DCA* by 1a, even faster than quenching by O 2 , does not directly afford DCA but affords DCA •-whose absorption is red-shifted far into the visible (≥580 nm). In contrast, k q for quenching of 1 DCAS* by O 2 was comparable, if slightly higher than that of 1 DCA*, and was markedly (100x) higher than k q for quenching of 1 DCAS* by 1a.