Photoinduced Dynamics of Oxyluciferin Analogues: Unusual Enol “Super”photoacidity and Evidence for Keto–Enol Isomerization

The first systematic pico-nanosecond time-resolved spectroscopic study of the firefly emitter oxyluciferin and two of its chemically modified analogues revealed that in the excited state the enol group is more acidic than the phenol group. The 6′-dehydroxylated derivative, in which only the 4-enolic hydroxyl proton is acidic, has an experimentally determined pKa* of 0.9 in dimethyl sulfoxide and an estimated pKa* of −0.3 in water. Moreover, this compound provided direct evidence that in a nonpolar, basic environment the keto form in the excited state can tautomerize into the enol, which subsequently undergoes excited-state proton transfer (ESPT) to produce enolate ion. This observation presents the first experimental evidence of excited-state keto–enol tautomerization of a firefly fluorophore, and it could be important in resolving the enol–keto conundrum related to the color-tuning mechanism of firefly bioluminescence. The 6′-dehydroxylated form of oxyluciferin adds a very rare case of a stable enol to the family of “super”photoacids.