Synthesis, Properties, and Light-Emitting Electrochemical Cell (LEEC) Device Fabrication of Cationic Ir(III) Complexes Bearing Electron-Withdrawing Groups on the Cyclometallating Ligands

The structure-property relationship study of a series of cationic Ir(III) complexes in the form of [Ir(C^N) 2 (dtBubpy)]PF 6 [where dtBubpy = 4,4'-di-tert-butyl-2,2'-bipyridine and C^N = cyclometallating ligand bearing an electron-withdrawing group (EWG) at C 4 of the phenyl substituent, i.e., -CF 3 (1), -OCF 3 (2), -SCF 3 (3), -SO 2 CF 3 (4)] has been investigated. The physical and optoelectronic properties of the four complexes were comprehensively characterized, including by X-ray diffraction analysis. All the complexes exhibit quasireversible dtBubpy-based reductions from -1.29 to -1.34 V (vs SCE). The oxidation processes are likewise quasireversible (metal + C^N ligand) and are between 1.54 and 1.72 V (vs SCE). The relative oxidation potentials follow a general trend associated with the Hammett parameter (σ) of the EWGs. Surprisingly, complex 4 bearing the strongest EWG does not adhere to the expected Hammett behavior and was found to exhibit red-shifted absorption and emission maxima. Nevertheless, the concept of introducing EWGs was found to be generally useful in blue-shifting the emission maxima of the complexes (λ em = 484-545 nm) compared to that of the prototype complex [Ir(ppy) 2 (dtBubpy)]PF 6 (where ppy = 2-phenylpyridinato) (λ em = 591 nm). The complexes were found to be bright emitters in solution at room temperature (Φ PL = 45-66%) with microsecond excited-state lifetimes (τ e = 1.14-4.28 μs). The photophysical properties along with density functional theory (DFT) calculations suggest that the emission of these complexes originates from mixed contributions from ligand-centered (LC) transitions and mixed metal-to-ligand and ligand-to-ligand charge transfer (LLCT/MLCT) transitions, depending on the EWG. In complexes 1, 3, and 4 the 3 LC character is prominent over the mixed 3 CT character, while in complex 2, the mixed 3 CT character is much more pronounced, as demonstrated by DFT calculations and the observed positive solvatochromism effect. Due to the quasireversible nature of the oxidation and reduction waves, fabrication of light-emitting electrochemical cells (LEECs) using these complexes as emitters was possible with the LEECs showing moderate efficiencies.