Efficient electrogenerated chemiluminescence from bis-cyclometalated iridium(III) complexes with substituted 2-phenylquinoline ligands

The ECL behavior of bis-cyclometalated (pq)(2)Ir(LX) complexes, in which pq is a 2-phenylquinoline anion and LX is a monoanionic bidentate ligand (e.g., acetylacetonate, picolinate, etc.), and the specific influences of the electrochemical stability and photoluminescence quantum yield (PL QY) of luminophores on ECL generation have been investigated. In this study, efficient ECLs, some of which even approached phi(ECL) = 0.88 (18 x higher than that of Ru(bPY)(3)(2+)), have been observed in the annihilation process. The simultaneous accumulation to excited singlet S, and triplet T, states and the spin-orbit coupling characteristics of transition metal complexes are expected to produce efficient annihilation ECL, which permits the high efficiency to exceed PL QY. A cyclic voltammetric study has revealed that the ECL intensity depends primarily on the electrochemical stability of the redox precursors of (pq)(2)Ir(LX)s. For example, (pq)(2)Ir(acac) (acac = acetylacetonate anion), which shows irreversible reduction, has produced efficient ECL during the oxidative-reductive process but less intense ECL during both the annihilation and reductive-oxidative processes. In the oxidative-reductive process, (pq)(2)Ir(LX)s also produces efficient ECL reacting with tri-n-propylamine radical precursors (TPA'(.)). During the oxidative-reductive process, (pq)(2)Ir(LX)/FPA couples undergo many competitive pathways involving both heterogeneous and homogeneous reactions. The tendency of ECL intensities with respect to their PL QYs is more complicated than that in the case of the annihilation process. These findings provide useful information on the fundamental ECL studies and the search for new ECL luminophores or practical ECL applications, such as analysis based on ECL and electroluminescent devices.