Photoacids as singlet oxygen photosensitizers: Direct determination of the excited state acidity by time-resolved spectroscopy

Photophysical properties of some 1-naphthol and 2-naphthol sulfonate derivatives and their efficiency as singlet oxygen photosensitizers in aqueous media are reported. The fluorescence emission is mainly from the deprotonated form (conjugate base) and their maxima cover the range of 433-529 nm. The fluorescence decay of these derivatives was found to be fitted with bi-exponential function with χ2 of about 1.0 ± 0.1 in all cases. Rate constants for oxygen quenching of the excited singlet states of these compounds in H2O are found to be (1.2 ± 0.2) × 1010 M−1 s−1 for the neutral form and much lower for the quenching of their conjugate bases. pKa values were found to be in the range 7.4-9.5 while pKa* are close to zero and about 3.3 for 2-naphthol and 2-naphthol-6-sulfonate using Förster equation and in the range of 1.4-3.0 using the fluorescence titration method obtained from the relative integrated fluorescence intensity. Singlet oxygen quantum yields photosensitized by these photoacids were reported in D2O and found to be in the range 0.13-0.35 for the sulfonated derivatives.