Ionization effects on the sensitized photooxidation of 2,3-dihydroxypyridine and 2,4-dihydroxypyridine: a kinetic study

The kinetics of the Rose Bengal (RB)-sensitized photooxidation of 2,3-dihydroxypyridine (2,3-DHP) and 2,4-dihydroxypyridine (2,4-DHP), two compounds profusely employed in multiple fields such as pesticides design, pharmacology, and clinical therapeutics have been studied in water at pH 5, 11 and 14, and in the mixture MeCN–water 4:1 (v/v) with and without 0.01 M KOH. These compounds present different ionization states, depending on the pH of the medium. Rate constants values in the range of 8×105–6.80×108 M−1 s−1 for both overall (kt) and reactive (kr) singlet molecular oxygen, O2(1Δg), quenching processes were determined by time-resolved O2(1Δg) phosphorescence detection and by spectrophotometric, spectrofluorimetric and polarographic methods. The experimental evidence suggests a photooxidative process through a charge-transfer mediated mechanism involving an excited encounter complex. The ratios kr/kt indicate that the O2(1Δg) oxidation of both dihydroxypyridines is a relatively efficient pathway over an extended pH range, with respect to the same oxidation of the monohydroxypyridines. In a general sense, the increase of pH favours the overall interaction dihydroxypyridine–O2(1Δg), although this interaction does not correlate with the importance of the photooxidation reaction. In pH 5 medium or in the mixture MeCN–water, solvents where the respective un-ionized 2-pyridone forms predominate, only 2,3-DHP is photooxidized, whereas 2,4-DHP slightly deactivates O2(1Δg) only in a physical fashion. In pH 11 aqueous solution or in alkalinized MeCN–water, where the respective mono-ionized pyridone forms predominate, the highest kr/kt ratios are reached. In addition, in pH 14 water solution both di-ionized dihydroxypyridines show the highest kt values, but lower relative reactivity than in the pH 11 solutions. All these results indicate that in aquatic environments containing these dihydroxypyridines or related aquatic pollutants, a simple change in the pH conditions of the medium could allow the switch between reactive and non-reactive O2(1Δg)-mediated processes.