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(d<i>t</i>Bubpy)]­PF₆ [where d<i>t</i>Bubpy = 4,4′-di-<i>tert</i>-butyl-2,2′-bipyridine and C^N = cyclometallating ligand bearing an electron-withdrawing group (EWG) at C₄ of the phenyl substituent, i.e., −CF₃ (<b>1</b>), −OCF₃ (<b>2</b>), −SCF₃ (<b>3</b>), −SO₂CF₃ (<b>4</b>)] 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 d<i>t</i>Bubpy-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 <b>4</b> 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(d<i>t</i>Bubpy)]­PF₆ (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 <b>1</b>, <b>3</b>, and <b>4</b> the ³LC character is prominent over the mixed ³CT character, while in complex <b>2</b>, the mixed ³CT 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