EVALUATION OF ION-ANNIHILATION REACTION-KINETICS USING HIGH-FREQUENCYGENERATION OF ELECTROCHEMILUMINESCENCE

The bimolecular rate constants for the annihilation reactions of the r adical ions of 9,10-diphenylanthracene (DPA), 9,10-dimethylanthracene (DMA), and ruthenium(II) tris(bipyridine) (Ru(bpy)3(2+)) in acetonitri le and DPA in propylene carbonate have been measured using electrogene rated chemiluminescence (ECL). In this work, a high-frequency multicyc le square wave was applied to a microelectrode and the resulting lumin escence curves were fit to an appropriate computer simulation. The ana lysis was complicated by the direct interaction of the emission with t he metallic electrode due to the close proximity of the ECL reaction l ayer to a reflecting surface. Significant deviations between theory an d experiment were apparent during the rising portion of the ECL curve and when high frequencies (short step times) were used. Under these co nditions, the ECL reaction layer is within a distance of 200 nm from t he electrode surface. These effects were least apparent with carbon-fi ber 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 pr opylene carbonate, a more viscous solvent. The unimolecular rate const ant for singlet formation for DPA in acetonitrile and propylene carbon ate was calculated to be ca. 3 x 10(9) and 5 x 10(8) s(-1), respective ly. The ca. 6-fold smaller unimolecular rate for DPA in propylene carb onate can be attributed to the longer solvent relaxation time for prop ylene 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-transfe r theory.