Peroxyoxalate chemiluminescence efficiency in polar medium is moderately enhanced by solvent viscosity

• Peroxyoxalate efficiency in polar media is due to CT complex stabilization.
• Planarity of high-energy intermediate guarantees increased CT complex stability.
• High CT complex stability in polar medium supplants solvent-cavity effect.
• Viscosity increase is not sufficient to overcome inefficient CT complex formation.

The peroxyoxalate reaction is one of the most efficient chemiluminescence transformations and the only system which is believed to occur by an intermolecular Chemically Initiated Electron Exchange Luminescence (CIEEL) mechanism with proven high efficiency. Although this reaction system is utilized in a variety of analytical applications, its exact reaction mechanism is still unknown, specifically with respect to the structure of the high-energy intermediate, the mechanism of its interaction with a chemiluminescence activator, as well as the reason for the extraordinarily high chemiexcitation efficiency. The solvent viscosity influence on the singlet quantum yields of the peroxyoxalate reaction is studied in this work, using the polar binary solvent mixture of ethyl acetate and dimethyl phthalate spanning a viscosity range of 0.42–14.4 cP. The viscosity increase of about forty times leads to a quantum yield increase of a factor of around five. Although the viscosity effect on the quantum yields is significant, it is considerably lower than that obtained before in less polar media. The quantum yield dependency on the viscosity can be described with both the collisional and the frictional model, with good correlation coefficients (R2 > 0.95). However, the α values obtained from the frictional model would be more compatible with an intramolecular transformation than with the intermolecular interaction of a high-energy intermediate with an external activator. This apparent contradiction is thought to be due to stabilization of the charge-transfer complex between the high-energy intermediate and the activator by the polar medium, avoiding solvent cavity escape and increasing chemiexcitation efficiency. The high stability of the charge-transfer complex in the peroxyoxalate reaction might be due to the high planarity of the possible high-energy intermediate in this transformation, 1,2-dioxetanedione, the peroxidic carbon dioxide dimer.

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