A Complete Description of the Ultrafast Proton Transfer Dynamics in the Excited State of D-Luciferin.

Excited-state proton transfer (ESPT) in organic photoacids is a widely studied phenomenon in which D-luciferin is of special mention, considering the fact that apart from its phenolic OH group, the nitrogen atoms at either of the two thiazole moieties could also participate in hydrogen bonding interactions with a proton-donating solvent during ESPT. As a result, several transient species could appear during the ESPT process. We hereby deploy subpicosecond time-resolved fluorescence upconversion (FLUP) and transient absorption (TA) spectroscopic techniques to understand the detailed photophysics of D-luciferin in water as well as in dimethyl sulfoxide (DMSO) and ethanol. These transient-state spectroscopic studies reveal the population of two kinds of hydrogen-bonded (HB) complexes (HBCs) in the excited singlet (S₁) state─HBC-I, which is formed at the OH site of the hydroxy benzothiazole moiety with a proton-accepting solvent, and the other one is HBC-II, which is formed at the N atom site of the thiazoline moiety with a proton-donating solvent. This study provides a complete description of the mechanism of the deprotonation process in HBC-I through distinct identification and characterization of the spectroscopic properties and temporal dynamics of those transient species associated with the four stages of the ESPT process as proposed by the Eigen-Weller model. This study also identifies and characterizes HBC-II, which, however, does not participate in the deprotonation process but provides an efficient nonradiative relaxation mechanism via geminate proton recombination.