Abstracts

Chemiluminescence (CL) and bioluminescence (BL) transformations are intimately linked to the chemistry of peroxides, specifically four-membered peroxidic rings, whose decomposition involves a symmetry-forbidden concerted [2+2] retro-cycloaddition, leading to the formation of two carbonyl fragments, in a highly exothermic reaction where the chemical energy can be transformed in electronical excitation energy. Studies on the stability and the chemiexcitation capacity of synthesized cyclic peroxides have led to the formulation of general chemiexcitation mechanisms which can be used to rationalize electronically excited state formation also in complex chemiluminescence and moreover bioluminescence transformations. In this contribution, we give a general introduction to these chemiexcitation mechanisms, outlining the main experimental observations which led to their formulation, and refer to recent experimental and theoretical results. (i) The unimolecular decomposition of 1,2-dioxetanes and 1,2-dioxetanones has been intensively studied und is believed to occur by a concerted biradial-like mechanisms, leading to the preferential formation of non-emissive triplet-excited species, with singlet quantum yields of lower than 0.1 %. Therefore, this system is not an adequate model for efficient CL and BL transformations, where singlet-excited states have to be formed in high yields. Several recent theoretical approaches have been performed with the objective to clarify this mechanism and to rationalize experimentally determined chemiexcitation quantum yields. (ii) The catalyzed decomposition of 1,2-dioxetanones and similar peroxides occurs with preferential formation of singlet-excited products, although with relatively low efficiency, involving the chemically initiated electron exchange luminescence (CIEEL) mechanism and recent experimental and theoretical studies indicate the importance of sterical hinderance on charge-transfer complex formation between an activator and the peroxide as reason for its low efficiency. (iii) The induced decomposition of 1,2-dioxetanes, which contain an electron rich phenolate group, proceeds with very high singlet-excitation quantum yields, in a process that has been shown to occur in an entirely intramolecular fashion. (iv) The peroxyoxalate reaction is one of the most efficient CL systems known with singlet-excitation quantum yields of up to 100% in favorable conditions, even so it proceeds by the, frequently low-efficient, intermolecular CIEEL mechanism. 1,2-Dioxetanedione appears to be the high-energy intermediate formed in this transformation, whose favorable interaction with the activator might be the key for its high efficiency. In summary, unimolecular peroxide decomposition leads mainly to non-emissive triplet-excited carbonyl compounds, whereas preferential formation of singlet-excited products is observed in catalyzed peroxide decomposition, although with low yields, except for the peroxyoxalate system which is highly efficient. Finally, intramolecularly induced decomposition of proper 1,2-dioxetanes leads also to extremely high singlet-excitation yields.