Photo-control of bimolecular reactions: reactivity of the long-lived Rhodamine 6G triplet excited state with ˙NO.

Photo-chemistry provides a non-intuitive but very powerful way to probe kinetically limited, sometimes thermodynamically non-favored reactions and, thus, access highly specific products. However, reactivity in the excited state is difficult to characterize directly, due to short lifetimes and challenges in controlling the reaction medium. Among photo-activatable reagents, rhodamine dyes find widespread uses due to a number of favorable properties including their high absorption coefficient. Their readily adaptable synthesis allows development of tailor-made dyes for specific applications. Remarkably, few studies have directly probed the chemical reactivity of their triplet excited state. Here we present a new conceptual approach to examine the specific chemistry of the triplet excited state. We have developed a pump (488nm) – probe (600nm) strategy to examine the gas-phase lifetime and reactivity of the triplet cation of Rhodamine 6G (3Rh6G+) in an ion trap mass spectrometer. The confounding effects of solvent, aggregation and formation of other reactive intermediates is thus avoided allowing fundamental reactivity to be explored. In the presence, in the ion trap, of helium seeded with 1 % of nitric oxide (•NO) (~ 60 ion/•NO collisions per second), the triplet lifetime is shortened from from 1.9 s to 0.7 s. Simultaneously, the reaction products [Rh6G -H]•+ and [Rh6G -H +NO]+ are observed. Reaction of 3Rh6G+ with •NO2 yields [Rh6G -H]•+, [Rh6G -H +NO2]+ and [Rh6G -2H]+. None of these products are observed for the singlet, 1Rh6G+. DFT calculations suggest a stepwise mechanism only allowed from 3Rh6G+, in which H atom abstraction by •NOx (x = 1 or 2) yields [Rh6G -H]•+ which, then, reacts with another •NOx molecule. This illustrates the power of light to initiate specific chemical reactions, and the relevance of gas-phase ion-molecule reaction approaches to understand stepwise reaction mechanism from specific excited states.