Solvent polarity influence on chemiexcitation efficiency of inter and intramolecular electron-transfer catalyzed chemiluminescence

Abstract

The parameters which determine the efficiency of chemiluminescence, and bioluminescence transformations are a continuous matter of experimental and theoretical studies. In this sense, we report here a study of the solvent polarity influence on four Chemically Initiated Electron Exchange Luminescence (CIEEL) systems. Two of them comprise intermolecular CIEEL systems with low efficiency – catalyzed decomposition of diphenoyl peroxide and spiroadamantyl-1,2-dioxetanone; the other two are highly efficient in producing excited states – the peroxyoxalate reaction through an intermolecular CIEEL, and the induced decomposition of a phenoxy-substituted 1,2-dioxetane derivative via intramolecular CIEEL. These four CIEEL systems are studied in binary solvent mixtures with similar viscosity but different polarity, as measured by the empirical solvent polarity parameter ET(30). Reaction rate constants for all the systems increase with increasing polarity, indicating the occurrence of electron or charge transfer in the rate-limiting step of these transformations. For the intermolecular systems, singlet quantum yields generally rise with medium polarity to a maximum value and decrease for higher polarities. This agrees with stabilization of the radical ion pair intermediate by solvation at an optimal medium polarity. In contrast, higher polarity promotes the separation of these charged species, thereby avoiding electron back-transfer between radical ion pairs, essential for chemiexcitation. The singlet quantum yields in the intramolecular CIEEL system show a steady increase with the medium polarity, reaching values close to the maximum possible quantum yield of 1.0 E mol−1, indicating the importance of biradical ion stabilization and the occurrence of the whole reaction sequence in an intramolecular manner.