Fig. 2: Evaluation of the intramolecular spirocyclization of fluorescein derivatives.
From: Halo-fluorescein for photodynamic bacteria inactivation in extremely acidic conditions
a Thermal equilibrium of intramolecular spirocyclization between the fluorescent open form and the nonfluorescent closed form, here χ and Z represents electronegativity and atomic number of chlorine, bromine, and iodine, respectively; b the electrostatic potential (ESP) map of FL (no-substitution), DCF (di-substitution in xanthene), TCF-1 (tetra-substitution in xanthene), and TCF-2 (tetra-substitution in benzene); c the relationship between electronegativity and ESP of deprotonated hydroxyl group on the xanthene core (site A) and the carboxyl group on the benzoic acid (site B), including non-substituted group (gray), chloro-substituted in benzene moiety (cyan), di-substituted in xanthene moiety (blue), and tetra-substituted in xanthene moiety (red ball); d pH titration results of the fluorescein derivatives with different substituting position; e pH titration results of the fluorescein derivatives with different kinds of halogen substitutions; and f pH titration results of the fluorescein derivatives with different number of halogen substitutions.
