Irreversible photo-oxidation of propranolol triggered by self-photogenerated singlet molecular oxygen

Abstract

The photochemistry of propranolol (PR), 1-isopropylamino-2-(1-naphthyloxy)propan-2-ol, has been studied by combining laser-flash photolysis and steady-state measurements. Like other compounds bearing the naphthalene chromophore, the triplet state is produced quite efficiently (Φ_isc = 0.35) and it is able to transfer its energy to oxygen, sensitizing singlet oxygen formation with Φ_Δ = 0.24. PR photodecomposition takes place in an aerated aqueous medium and leads to 6-hydroxy-1,4-naphthoquinone as the sole stable photoproduct. This seems to be consistent with the intermediary formation of the highly photolabile 1,4-naphthoquinone. It is demonstrated that photodegradation occurs via a type II mechanism involving irreversible trapping of self-photogenerated singlet molecular oxygen.