Photophysical properties of BODIPY-derived hydroxyaryl fluorescent pH probes in solution

The photophysical properties of five fluorescent pH probes derived from 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene with phenolic or naphtholic subunits at position 8 and with substituents having different electron driving forces at positions 3 and 5 have been investigated in several organic solvents, by means of absorption, steady-state, and time-resolved fluorimetry. For each compound, the fluorescence quantum yield and lifetime are lower in solvents with higher polarity, owing to an increase in the rate of nonradiative deactivation. The rate constants for radiative deactivation, k(f), are nearly constant for all dyes in all solvents studied [k(f)=(1.7+/-0.2)x10(8) s(-1)]. In aqueous solution, these probes undergo a reversible protonation-deprotonation in the near-neutral to basic pH range, producing intensity increases with lower pH. The pK(a) values of the indicators are between 7.5 and 9.3, depending on the substitution pattern on positions 3, 5, and 8. The difference between the absorption and excitation spectra as a function of pH is indicative of the presence of two species in aqueous solution: the phenol- or naphthol-based indicator and its conjugate base.