Evaluation of Ion-Annihilation Reaction Kinetics Using High-Frequency Generation of Electrochemiluminescence

The bimolecular rate constants for the annihilation reactions of the radical ions of 9,10-diphenylanthracene (DPA), 9,10-dimethylanthracene (DMA), and ruthenium(II) tris(bipyridine) (Ru(bpy)3(2+)) in acetonitrile and DPA in propylene carbonate have been measured using electrogenerated chemiluminescence (ECL). In this work, a high-frequency multicycle square wave was applied to a microelectrode and the resulting luminescence curves were fit to an appropriate computer simulation. The analysis was complicated by the direct interaction of the emission with the metallic electrode due to the close proximity of the ECL reaction layer to a reflecting surface. Significant deviations between theory and experiment were apparent during the rising portion of the ECL curve and when high frequencies (short step times) were used. Under these conditions, the ECL reaction layer is within a distance of 200 nm from the electrode surface. These effects were least apparent with carbon-fiber microelectrodes consistent with their lower electrode reflectivity and density-of-states. Diffusion-controlled ion-annihilation rates of (2 +/- 1) x 10(10) M(-1) s(-1) were measured far DPA, DMA, and Ru(bpy)3(2+) in acetonitrile and (4 +/- 1) x 10(9) M(-1) s(-1) for DPA in propylene carbonate, a more viscous solvent. The unimolecular rate constant for singlet formation for DPA in acetonitrile and propylene carbonate was calculated to be ca. 3 x 10(9) and 5 x 10(8) s(-1), respectively. The ca. 6-fold smaller unimolecular rate for DPA in propylene carbonate can be attributed to the longer solvent relaxation time for propylene carbonate compared to acetonitrile. The rate to form the tripler state proceeds at the diffusion-controlled limit for DMA, DPA, and Ru(bpy)(3)(2+) consistent with the predictions based on electron-transfer theory.