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 (¹ O₂ ,¹ ∆_g ) in aqueous media. It is based on the disruption, by a water-soluble substrate S, of the ¹ O₂ chemiluminescence (CL) generated by the H₂ O₂ /Na₂ MoO₄ 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 (H₂ O), ¹ O₂ lifetime is short and the CL intensity is weak allowing solely the investigation of very reactive substrates for which (k_r + k_q ) > 3 × 10⁶ m^-1 s^-1 while, in D₂ O, ¹ O₂ 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 × 10⁵ 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 ¹ O₂ 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.