Abstract
The structure–property relationship study of a series of cationic Ir(III) complexes in the form of [Ir(C^N)2(dtBubpy)]PF6 [where dtBubpy = 4,4′-di-tert-butyl-2,2′-bipyridine and C^N = cyclometallating ligand bearing an electron-withdrawing group (EWG) at C4 of the phenyl substituent, i.e., −CF3 (1), −OCF3 (2), −SCF3 (3), −SO2CF3 (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 (λem = 484–545 nm) compared to that of the prototype complex [Ir(ppy)2(dtBubpy)]PF6 (where ppy = 2-phenylpyridinato) (λem = 591 nm). The complexes were found to be bright emitters in solution at room temperature (ΦPL = 45–66%) with microsecond excited-state lifetimes (τe = 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 3LC character is prominent over the mixed 3CT character, while in complex 2, 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.
Highlights
Over the past few decades, heteroleptic cationic Ir(III) complexes have garnered widespread interest due to their frequently bright phosphorescence that can be tuned across the visible spectrum through simple ligand modification.[1−4] Due to facile color tuning, high photoluminescence quantum yield (ΦPL) and short emission lifetimes, iridium complexes are ideal emissive materials for electroluminescent (EL) devices and remain the most popular materials for use in organic lightemitting diodes (OLEDs) and in light-emitting electrochemical cells (LEECs)
Complexes [1−4] are air- and moisture-stable solids that are soluble in polar organic solvents including acetonitrile and dichloromethane
Contrary to the density functional theory (DFT) calculated HOMO−LUMO gap for [1−4] (Figure 3), the lowest-energy absorption maxima of these complexes are red-shifted compared to that of R1, which may suggest an additional stabilization of the LUMOs of the complexes [1−4] compared to the energy of the LUMO of complex R1 due to the strong electron-withdrawing nature of the electron-withdrawing group (EWG)
Summary
Over the past few decades, heteroleptic cationic Ir(III) complexes have garnered widespread interest due to their frequently bright phosphorescence that can be tuned across the visible spectrum through simple ligand modification.[1−4] Due to facile color tuning, high photoluminescence quantum yield (ΦPL) and short emission lifetimes (τe), iridium complexes are ideal emissive materials for electroluminescent (EL) devices and remain the most popular materials for use in organic lightemitting diodes (OLEDs) and in light-emitting electrochemical cells (LEECs). Contrary to the DFT calculated HOMO−LUMO gap for [1−4] (Figure 3), the lowest-energy absorption maxima of these complexes are red-shifted compared to that of R1, which may suggest an additional stabilization of the LUMOs of the complexes [1−4] compared to the energy of the LUMO of complex R1 due to the strong electron-withdrawing nature of the EWGs. Figure 6 illustrates the normalized room temperature emission spectra of [1−4] upon photoexcitation into the CT band (at 360 nm) in degassed acetonitrile. Incorporation of electron-withdrawing substituents on the C^N ligands promotes the expected stabilization of the frontier molecular orbitals (Figure 3) and the blue-shift in the emission observed for complexes [1−4] compared to the reference. For LEEC 2, (0.44, 0,53) for LEEC 3, and (0.45, 0.53) for LEEC 4
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