Improved Deep-Red Phosphorescence in Cyclometalated Iridium Complexes via Ancillary Ligand Modification

Abstract

In this work, we describe bis-cyclometalated iridium complexes with efficient deep-red luminescence. Two different cyclometalating (C^N) ligands‒1-phenylisoquinoline (piq) and 2-(2- pyridyl)benzothiophene (btp)‒are used with six strong π-donating ancillary ligands (L^X) to furnish a suite of 10 new complexes with the general formula Ir(C^N)2(L^X). Improvements in deep-red photoluminescence quantum yields were accomplished by the incorporation of sterically encumbering substituents onto the ancillary ligand, which can enhance the radiative rate constant (kr) and/or reduce the non-radiative rate constant (knr). Five of the complexes were characterized by X-ray crystallography, and all of them were investigated by in-depth spectroscopic and electrochemical measurements.<br>

Enhanced deep red to near-infrared (DR-NIR) phosphorescence in cyclometalated iridium(iii) complexes

“…The cyclometalating ligands are chosen to engender the complexes with emission in the DR-NIR region of the spectrum 34,43 by adding additional π conjugation ( pphen) or a cyano group ( piqCN) to 1-phenylisoquinoline ( piq), a ubiquitous C^N ligand in red-phosphorescent iridium complexes. 31,32,39,41 These C^N ligands are paired with five ancillary ligands (L^X) previously used in studies of red and deep-red emitters. Besides β-ketoiminate (acNac) and β-diketiminate (NacNac) ancillary ligands that we have extensively worked with, 39,44,45 the more sterically encumbered NacNac ((dmp) 2 NacNac) and amidinate (dipba mes ), and more electron-rich acNac ((Cy)acNac) are included in this study.…”

“…Besides β-ketoiminate (acNac) and β-diketiminate (NacNac) ancillary ligands that we have extensively worked with, 39,44,45 the more sterically encumbered NacNac ((dmp) 2 NacNac) and amidinate (dipba mes ), and more electron-rich acNac ((Cy)acNac) are included in this study. 41 To prepare the complexes, [Ir(C^N) 2 (µ-Cl)] 2 was treated with the potassium or lithium salts of the appropriate ancillary ligand in tetrahydrofuran at room temperature or 85 °C. 41,46 Some Scheme 1 Synthesis of the iridium(III) complexes.…”

“…38 Recent notable advances in DR-NIR-phosphorescent iridium complexes have focused on modifications to the cyclometalating ligand, including complexes supported by substituted 1-phenylisoquinline ( piq) analogues with emission beyond 700 nm, 34 and complexes with a "core-shell" cyclometalating ligand design that combines a rigid, conjugated core ligand with a flexible, electron-rich thienyl or diphenylamino shell. 35 Our group has tackled the above-mentioned challenges in the red (λ em = 600-650 nm) 39,40 and deep-red regions (λ em = 650-700 nm) 41,42 with a new class of bis-cyclometalated iridium phosphors. We have demonstrated that electron-rich, π-donating ancillary ligands strongly perturb the metal-centered HOMO and increase the metal d-orbital participation in the excited state, resulting in augmented k r and in some cases Φ PL values.…”

“…With this strategy, we have achieved solution Φ PL values as high as 0.8 in the red region of the spectrum, 39 and eclipsed Φ PL = 0.5 in the deep red region. 41 Another insight to emerge from these previous works is that the nature of the C^N ligand plays a major role in the effectiveness of this approach. In general, the phosphorescence in complexes where the C^N ligands have electron-rich thiophene-based cyclometalated aryl groups is minimally influenced by the ancillary ligand, whereas with phenyl-based C^N ligands the phosphorescence wavelength and k r values are both strongly responsive to how electron-rich the ancillary ligand is.…”

“…34,43 The electron-rich ancillary ligands are chosen among the ones previously used in our group that resulted in increased quantum yields in the red and deep-red regions. 39,41 A total of six new compounds are described in this paper. Cyclic voltammetry reveals the effects of the electron-rich ancillary ligands on the HOMO energies, and detailed photophysical analysis in three solvents (MeCN, THF and toluene) and in transparent polymer film catalogues the effects on the excited-state dynamics and Φ PL .…”