Electrochemical processes leading to light emission are reviewed, with emphasis on aspects of this subject relevant to the understanding and optimization of electrogenerated luminescence (EL) in organic thin-film materials. The basic energetic requirements of light emission from electrochemically initiated solution redox reactions [electrogenerated chemiluminescence (ECL)] are reviewed first. This review is followed by a discussion of light-emitting electrochemical processes that have been observed in hybrids of ionically conducting polymers and electronically conducting polymers. Finally, the features of EL in insulating polymers and molecular thin films are reviewed, along with recent electrochemical and ECL studies of the small-molecule components of certain organic light-emitting diodes. These studies provide a conceptual framework for understanding and optimizing these materials and the EL process.

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The electrochemiluminescent (ECL) reaction mechanism between tris(2,2‘-bipyridyl)ruthenium(II) (Ru(bpy)3 2+) and tripropylamine (TPrA) in aqueous solution at pH 7.4 was examined using fast potential pulses at carbon fiber microelectrodes. High-stability ECL emission was found with 0.5 ms pulses and a 25% duty cycle. In addition, stability was increased with negative rest potentials. Direct evidence for the strongly reducing free radical intermediate was obtained when the light was quenched upon addition of nitrobenzene derivatives. The formation of this free radical becomes rate-limiting at high concentrations of Ru(bpy)3 2+ and TPrA, as its production can be limited when there is an insufficient supply of one or both of its precursors (TPrA and TPrA•+) relative to Ru(bpy)3 2+. When TPrA is in sufficient excess of Ru(bpy)3 2+, the ECL efficiency (photons emitted/Ru(bpy)3 3+ generated) was determined to be very high (∼90%) by comparison to a Ru(bpy)3 3+/ Ru(bpy)3 + standard in acetonitrile. Rapid potential pulses also generated ECL from Ru(bpy)3 2+ when other tertiary amines, trimethylamine, diisopropylethylamine, and histamine, were used as co-reactants. The secondary amine epinephrine also produced light, but not norepinephrine, a primary amine.

Electrogenerated chemiluminescence (ECL) is the production of light via electron transfer reactions between electrochemically produced reagents. ECL-based biosensors use specific biological interactions to recognize an analyte and produce a luminescent signal. Biosensors fabricated with novel biorecognition species have increased the number of analytes detected. Some of these analytes include peptides, cells, enzymes and nucleic acids. ECL biosensors are selective, simple, sensitive and have low detection limits. Traditional methods use ruthenium complexes or luminol to generate ECL. Nanomaterials can be incorporated into ECL biosensors to improve efficiency, but also represent a new class of ECL emitters. This article reviews the application of ruthenium complex, luminol and nanomaterial-based ECL biosensors to making measurements in biological matrices over the past 4 years.

Solution electrogenerated chemiluminescence (ECL) was evaluated for molecules of interest for organic light-emitting diodes (OLEDs), using high-frequency voltage pulses at a microelectrode. Radical cations of different energies were electrogenerated from a series of triarylamine hole-transport materials (x-TPD), in the presence of radical anions of a high electron affinity sulfonamide derivative of tris(8-hydroxyquinoline)aluminum (Al(qs) 3 ), or a bis(isoamyl) derivative of quinacridone (DIQA). The resultant emission was from the excited singlet states 1 Al(qs) 3 * or 1 DIQA*, the same excited state produced in OLEDs based on these molecules. In solution, the majority of the reaction pairs had insufficient energy to populate 1 Al(qs) 3 * or 1 DIQA* directly, but could form the triplet states 3 Al(qs) 3 * or 3 DIQA*. The reaction order and the temporal response of the emission were consistent with subsequent formation of the excited singlet states via triplet-triplet annihilation (TTA). For reactions with a low excess Gibbs free energy to form the triplet state (∆ T G), the efficiency increased exponentially with an increase in driving force (increase in oxidation potential of x-TPD), then reached a plateau. At the maximum, the efficiencies for formation of 1 Al(qs) 3 * or 1 DIQA* via the TTA route reach as high as a few percent. The computed energetics of these reactions suggest that similar lightproducing electroluminescent reactions, proceeding via triplet formation, could also occur in condensed phase organic thin films.