HIGH-FREQUENCY GENERATION OF ELECTROCHEMILUMINESCENCE AT MICROELECTRODES

High-frequency generation of electrochemiluminescence (ECL) from ion-annihilation reactions has been investigated at microelectrodes. Well-defined, stable, reproducible luminescence curves were obtained when a multicycle square wave was applied to a microelectrode in the presence of either 9,10-diphenylanthracene, 9,10-dimethylanthracene, or 9-phenylanthracene. ECL efficiencies of ca. 5 %, 2 %, and 2 % were obtained for these compounds, respectively, using ruthenium(II) tris(bipyridine) as a standard. Due to the reduced effects of double-layer capacitance and ohmic drop, the microelectrode can be pulsed at a more rapid rate than electrodes of conventional size. The electrochemical-time constant, which was directly evaluated from the luminescence data, determined the upper frequency limit. For a 5 mum radius electrode in acetonitrile containing 0.1 M electrolyte, this corresponded to 20-30 kHz. The rapidity of the experiment improved the stability of the luminescence and enabled ECL to be investigated from dilute solutions as well as from solutions containing 10% added water. Through variation in both the time scale of the experiment and the precursor concentration, a lower limit on the ion-annihilation rate constant for DPA of 2 x 10(9) M-1 s-1 was determined. This is close to the diffusion-controlled value of 10(10) M-1 s-1 theoretically expected.