Donald L. Maricle

Pre-annihilation electrochemiluminescence☆

Abstract Electrochemiluminescence generated by alternating electrolysis of solutions of fluorescent aromatic hydrocarbons is attributed primarily to radical-anion—cation annihilation. However, light can also be detected while oxidizing the anion or reducing the cation of certain molecules at potentials insufficient to generate the oppositely charged radical. The mechanism of the pre-annihilation ecl has been studied for two aromatic hydrocarbons. The light pulse produced while oxidizing the rubrene anion in a double-potential-step experiment was found to be singlet emission (spectrum identical to fluorescence) even though the overpotentials applied were considerably less than the 2·3 eV singlet energy of this molecule. Possible energy doubling mechanisms are discussed. Most of the pre-annihilation light generated from the rubrene anion is shown to result from the oxidation of an anion decomposition product, followed by a homogeneous chemiluminescent reaction. Phenanthrene, unlike most molecules, exhibits pre-annihilation luminescence which is not the same colour as its fluorescence (violet). Oxidation of the anion at overvoltages of about 2·6 V produces a green emission with a maximum between 4900 and 5000 A. This corresponds closely to the published room temperature solution phosphorescence. The emission is quenched by appropriate triplet quenchers. This is believed to be the first positive evidence for electrochemically generated triplets.