Effects of protolytic interactions on the photophysics of phenyl pyridyl ketones

Abstract

The photophysics of phenyl 3-pyridyl ketone (3-PPK) and phenyl 4-pyridyl ketone (4-PPK) were investigated in aqueous solution as a function of pH (0–10) by steady-state and pulsed emission spectroscopy and by nanosecond laser flash photolysis.

From phosphorimetric and triplet–triplet absorption acid–base titrations of the excited state, two protonation steps were evidenced for 3-PPK, which involved both the carbonyl oxygen and nitrogen atoms. The triplets of the neutral molecule and of the mono- and di-cation were identified from the shape of the transient absorption spectra, analysis of the kinetic profiles and effect of charged quenchers.

For 4-PPK, the titration curves showed only one inflection point. Phosphorescence emission and T₁→ T_n absorption spectra, over the whole pH range explored, showed only the triplet of the neutral molecule. Indirect evidence of protonation was obtained from the weaker emission and absorption intensities and shorter lifetimes with increasing acidity.

The comparison with ground-state pKs showed that the basicity of these molecules increases greatly upon electronic excitation.