Development of firefly oxyluciferin derivatives as pH sensitive fluorescent Probes: A DFT/TDDFT study.

Because of its high fluorescence quantum yield, high sensitivity and high signal-to-noise ratio, firefly oxyluciferin is widely used in the field of fluorescent probe. However, there are six chemical forms which can possibly co-existence in solution. Reliable relative stability of the different forms in solution made the analysis of their photophysical properties difficult. In order to address this issue and gain insights into the nature of spectra feature observed in the experiments, we designed six oxyluciferin derivatives. The distribution of the species at the ground and excited states demonstrate that only keto(0), keto(−1) and enol(−2) are present for all oxyluciferin derivatives. The majority of molecules are surprisingly more acidic at the ground state than at the excited state, which indicate that they are photobases. The distribution of the species at the excited state indicate that the most pH-sensitive molecules are 2-(6′-hydroxybenzothiazol-2-yl)-2,3-dihydro-4-hydroxypyrrole (Oxyan-1) and 1-(5′-brominated-6′-hydroxybenzothiazol-2-yl)-2-(4-hydroxythiazole) ethylene (Oxyan-4), as their chemical equilibria consist on the same three species: keto(0), keto(−1) and enol(−2). While the first deprotonation occurs at acidic pH, the second occurs at basic pH, with a single deprotonated species existing at more neutral pH. This indicates that both Oxyan-1 and Oxyan-4 can be good candidates for fluorescent pH probing. Exhaustive analyses of the emission spectra in water revealed that the firefly OxyLH 2 analogues can emit in the UV–visible and near-infrared wavelength range. [ABSTRACT FROM AUTHOR]