Solvent cage effects: basis of a general mechanism for efficient chemiluminescence

The induced decomposition of 1,2-dioxetanes results in the efficient formation of singlet-excited carbonyl compounds. This transformation has been assumed to involve two sequential electron-transfer steps, and the viscosity dependence of the chemiexcitation efficiency (solvent cage effect) has been considered as evidence for the occurrence of an intermolecular electron back-transfer, despite the very high chemiexcitation quantum yields observed. However, all other chemiluminescent reactions assumed to occur according to the entirely intermolecular mechanism, referred to as CIEEL, are inefficient, except for the peroxyoxalate system. Therefore, we have investigated the solvent cage effect on the singlet quantum yields in both the induced decomposition of 1,2-dioxetanes and the peroxyoxalate reaction. Analysis of the viscosity effect observed for both systems, using a collisional as well as a free-volume model, indicates a very distinct behavior, which was interpreted as the occurrence of intramolecular chemiexcitation in the induced 1,2-dioxetane decomposition. We propose a general mechanism for efficient chemiluminescence in which the required electron back-transfer and C-C bond cleavage are concerted and compete with conformational changes that compromise the chemiexcitation. This mechanism is in agreement with both experimental and theoretical data available on the induced 1,2-dioxetane decomposition as well as with the high quantum efficiency of this transformation.