DIRECT OBSERVATION OF A RADICAL-ION INTERMEDIATE IN THE CHEMIEXCITATION STEP OF PEROXYOXALATE CHEMILUMINESCENCE

Abstract

INTRODUCTION The chemically initiated electron exchange luminescence (CIEEL) mechanism was proposed by Schuster in the late 1970's and represents the most general explanation for efficient bioand chemiluminescence phenomena. It consists in a combination of the triplet chemiexcitation, observed in unimolecular decomposition or rearrangement of high-energy compounds (e.g. organic peroxides and hydroperoxides), and electrochemiluminescence, the light emission observed by annihilation of radical-ions generated electrochemically.' The intermolecular variation of the CIEEL mechanism is widely associated with the chemiluminescent reaction of cyclic and linear peroxides with highly fluorescent polycondensate aromatic hydrocarbons, the so-called activators (ACTS). As light emission intensities and peroxide decomposition rate constants are both dependent on the oxidation potential of the activator used, an electron transfer process should take place in the rate-limiting step. This mechanism proposes the formation of a charge transfer complex between a low oxidation potential activator and the peroxide, followed by an irreversible electron transfer (ET) and loss of a neutral fragment or rearrangement. The resulting radical ion pair is annihilated, and the back electron transfer (BET) is able to release enough energy to generate the ACT in its first singlet-excited state. Despite the fact that this proposition is based on activated diphenoylperoxide decomposition, which is known to be an inefficient system: it was also used to interpret the results obtained with the peroxyoxalate system, which shows chemiexcitation quantum yields (QS) of ca. 0.3. In the latter reaction, a peroxidic high energy intermediate (HEI) is produced by the reaction between an oxalic ester and H202 in the presence of a base. The interaction between the HE1 and the ACT, which is supposed to involve the CIEEL mechanism, results in chemiexcitation of the ACT. Our group has pursued experimental evidence on the occurrence of the CIEEL mechanism, especially concerning the chemiexcitation step, and on the nature of the HE1 in the peroxyoxalate ~ y s t e m . ~ ' ~ This work shows direct experimental evidence for the occurrence of the CIEEL in the reaction of bis(2,4,6-trichlorophenyl)oxalate (TCPO) with hydrogen peroxide in the presence of imidazole (IMI-H) and an ACT, as it presents direct experimental evidence on the formation of the carbon dioxide radical anion in this system. A radical adduct could be detected by electron