Simultaneous Determination of the Chemical (kr) and the Physical (kq) Quenching Rate Constants of Singlet Oxygen in Aqueous Solution by the Chemiluminescence-quenching Method†

This work reports a novel and visual method for the simultaneous determination of the chemical (k(r)) and the physical (k(q)) quenching rate constants of singlet oxygen (O-1(2),(1) increment (g)) in aqueous media. It is based on the disruption, by a water-soluble substrate S, of the O-1(2) chemiluminescence (CL) generated by the H2O2/Na2MoO4 catalytic system. A mathematical analysis of the CL signal at 1270 nm vs time provides separately the overall (k(r) + k(q)) and the chemical (k(r)) quenching rate constants. In ordinary water (H2O), O-1(2) lifetime is short and the CL intensity is weak allowing solely the investigation of very reactive substrates for which (k(r) + k(q)) > 3 x 10(6) m(-1) s(-1) while, in D2O, O-1(2) lifetime is significantly longer lifetime and the CL signal is much stronger allowing the study of poorly reactive substrates for which (k(r) + k(q)) > 4 x 10(5) m(-1) s(-1). The method has been successfully tested on a series of anionic and nonionic water-soluble naphthalene derivatives commonly used as bio-compatible O-1(2) carriers. The obtained k(r) and k(q) values are in good agreement with the values determined by conventional techniques, namely, flash photolysis and competitive kinetics with a reference quencher.