TAUTOMERISM AND FLUORESCENCE OF LUMAZINE

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Abstract
Abstract— The fluorescence properties of lumazine, 1‐methyl‐lumazine (1‐ML) and 3‐methyl‐lumazine (3‐ML) in aqueous solution are pH dependent. Emissions with the following maxima were attributed to the four ionic species of lumazine: dianion (483 nm), monoanion (467 nm), neutral (380 nm) and monocation (505 nm). Neutral lumazine emitted, besides, a fluorescence at 481 nm with a large Stokes shift (10 000 cm‐1). As a similar emission is observed with 3‐ML but not with 1‐ML, we suggest as emitting species the N(8)‐H‐phototautomer resulting from the N(l) to N(8) proton transfer in the excited state. At pH 10, the fluorescence quantum yield of 3‐ML, Qf= 0.24 ± 0.02 was higher than that of 1‐ML, Qf= 0.015 ± 0.002. At this pH, lumazine is a mixture of N(l)‐deprotonated and N(3)‐deprotonated monoanions, the emitting properties being principally due to the N(l)‐deprotonated species, Qf= 0.24 ± 0.02. The evaluation of the ionization constants in the excited state are discussed in relation to the tautomeric nature of the emitting species.