Photoinduced Dynamics
of Oxyluciferin Analogues: Unusual
Enol “Super”photoacidity and Evidence for Keto–Enol
Isomerization
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Abstract
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 p<i>K</i><sub>a</sub>*
of 0.9 in dimethyl sulfoxide and an estimated p<i>K</i><sub>a</sub>* 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