Studies on the Mechanism of the Excitation Step in Peroxyoxalate Chemiluminescence

Studies on the mechanism of excited state formation in the peroxyoxalate system have been performed, to corroborate the involvement of the well-known Chemically Initiated Electron Exchange Luminescence (CIEEL) mechanism in the chemi-excitation step of this complex sequence. The singlet quantum yields, extrapolated to infinite activator concentrations (ΦS∞), and relative rate constants (kCAT/kD) of the excitation step have been determined in the presence of several activators for two systems: (i) the complete peroxyoxalate reaction with bis(2,4,6-trichlorophenyl) oxalate; and (ii) the base-catalyzed reaction of 4-chlorophenyl O,O-hydrogen monoperoxyoxalate, an isolated key intermediate. For five activators commonly used in CIEEL studies (anthracene, 9,10-diphenylanthracene, 2,5-diphenyloxazole, perylene, and rubrene), a linear correlation of ln(kCAT/kD) with the voltammetric half-peak oxidation potential (Ep/2) of the activator was obtained for both systems. The values obtained with 9,10-dicyanoanthracene and 9,10-dimethoxyanthracene did not fit this correlation. A reasonable linear correlation between ln(ΦS∞) and Ep/2 was obtained for all activators. For the commonly used activators, this quantum yield (ΦS∞) dependence can be rationalized in terms of the free energy balance of the back electron transfer leading to the formation of the excited state of the activator. However, the ΦS∞ values obtained with 9,10-dimethoxyanthracene and 9,10-dicyanoanthracene cannot be explained on the basis of these considerations alone. Thus, although this work presents clear-cut evidence of the operation of the CIEEL mechanism in the peroxyoxalate reaction, the results obtained with less commonly used activators show that several mechanistic details of the CIEEL hypothesis remain to be elucidated.