LASER PHOTOLYSIS STUDY OF THE KINETICS AND MECHANISM OF PHOTOINITIATED PEROXYOXALATE CHEMILUMINESCENCE

Kinetics of photoinitiated peroxyoxalate chemiluminescence have been studied in order to further elucidate the mechanism of the reaction. Intensity versus time profiles obtained from photolysis by a single laser pulse displayed three prominent features: (1) prompt emission rising to a maximum in less than 0.3 s and decaying by a first-order process with a time constant of almost-equal-to 35 s when no imidazole catalyst is present, (2) a sharp burst of light with rise and fall constants of almost-equal-to 2 s when imidazole is added postphotolysis, and (3) a broad, symmetrical emission profile having nearly identical rise and fall rate constants when imidazole is added either before or after photolysis. The broad emission profile was identified as being due to photosensitized production of H2O2.In this case, pseudo-first-order rate constants for the rise (r) and fall (f) of the chemiluminescence intensity versus time profile were measured as a function of initial reactant concentrations and number of laser shots used to initiate the reaction. Both the rise and fall rate constants were found to be independent of the fluorophore concentration and to vary only slightly with the number of laser shots. Imidazole has a complex and nearly identical effect on the rise and fall, the dependence on imidazole concentration increasing from first to second order as the imidazole concentration increases. A finding that both the rise and fall pseudo-first-order rate constants vary linearly with oxalate ester concentration implies that two molecules of oxalate ester are involved in the reaction. These results were confirmed by additional kinetics experiments in which photochemical generation of H2O2 was replaced by direct addition. The kinetics of the prompt emission and of the sharp burst resulting from postphotolysis addition of imidazole is consistent with the formation in the initial photolysis reaction of the peroxydioxalate, ArO(C = O)2OO(C = O)2OAr, where Ar represents 2,4,6-trichlorophenyl. Based on kinetics results, it is proposed that the H2O2-initiated reaction also produces this species, which subsequently reacts to form the high-energy intermediate capable of transferring energy to the fluorophore. New high-energy intermediates derived from the peroxydioxalate are proposed. Rate constants for the rate-limiting steps of the mechanism are derived, and a computer simulation of the mechanism is found to be consistent with the experimental results.