Neutral Cyclometalated Iridium(III) Complexes Bearing Substituted N-Heterocyclic Carbene (NHC) Ligands for High-Performance Yellow OLED Application

Abstract

The synthesis, crystal structure, and photophysics of a series of neutral cyclometalated iridium(III) complexes bearing substituted N-heterocyclic carbene (NHC) ancillary ligands ((C ∧ N) 2 Ir(R-NHC), where C ∧ N and NHC refer to the cyclometalating ligand benzo[h]quinoline and 1-phenylbenzimidazole, respectively) are reported. The NHC ligands were substituted with electron-withdrawing or -donating groups on C4′ of the phenyl ring (R = NO 2 (Ir1), CN (Ir2), H (Ir3), OCH 3 (Ir4), N(CH 3 ) 2 (Ir5)) or C5 of the benzimidazole ring (R = NO 2 (Ir6), N(CH 3 ) 2 (Ir7)). The configuration of Ir1 was confirmed by a single-crystal X-ray diffraction analysis. The ground-and excited-state properties of Ir1− Ir7 were investigated by both spectroscopic methods and time-dependent density functional theory (TDDFT) calculations. All complexes possessed moderately strong structureless absorption bands at ca. 440 nm that originated from the C ∧ N ligand based 1 π,π*/ 1 CT (charge transfer)/ 1 d,d transitions and very weak spin−forbidden 3 MLCT (metal-to-ligand charge transfer)/ 3 LLCT (ligand-to-ligand charge transfer) transitions beyond 500 nm. Electron-withdrawing substituents caused a slight blue shift of the 1 π,π*/ 1 CT/ 1 d,d band, while electron-donating substituents induced a red shift of this band in comparison to the unsubstituted complex Ir3. Except for the weakly emissive nitro-substituted complexes Ir1 and Ir6 that had much shorter lifetimes (≤160 ns), the other complexes are highly emissive in organic solutions with microsecond lifetimes at ca. 540−550 nm at room temperature, with the emitting states being predominantly assigned to 3 π,π*/ 3 MLCT states. Although the effect of the substituents on the emission energy was insignificant, the effects on the emission quantum yields and lifetimes were drastic. All complexes also exhibited broad triplet excited-state absorption at 460−700 nm with similar spectral features, indicating the similar parentage of the lowest triplet excited states. The highly emissive Ir2 was used as a dopant for organic light-emitting diode (OLED) fabrication. The device displayed a yellow emission with a maximum current efficiency (η c ) of 71.29 cd A −1 , a maximum luminance (L max ) of 32747 cd m −2 , and a maximum external quantum efficiency (EQE) of 20.6%. These results suggest the potential of utilizing this type of neutral Ir(III) complex as an efficient yellow phosphorescent emitter.

Highly Efficient Far‐Red/NIR‐Absorbing Neutral Ir(III) Complex Micelles for Potent Photodynamic/Photothermal Therapy

“…In water, the vibronic structures of this absorption band were less pronounced and the band became broader due to the aggregation of BODIPY‐Ir. The Ir(NHC)(bqu) 2 (bqu represents benzo[ h ]quinoline) motif mainly contributes to the absorption in the regions of <500 nm referring to the absorption spectrum of Ir(NHC)(bqu) 2 reported in our previous studies [ 19k,22 ] and confirmed by the TDDFT calculation results shown in Table S1, Supporting Information.…”

Unexpected ortho C–H bond activation in coordinated 7,8-benzoquinoline: synthesis and characterisation of heteroleptic Ir(iii)-7,8-benzoquinoline complexes

“…Since then numerous heteroleptic Ir(III)-7,8-benzoquinoline complexes were reported with very same coordination mode. [12][13][14][15][16][17][18] The Ir-benzoquinoline complexation could be viewed as the formation of initial Ir-N(quinoline) bond, which selectively induces the C10-H bond activation and leads to Ir-C10 coordination. This results in a stable vemembered chelation by 7,8-benzoquinoline.…”

“…The Ir-C, Ir-N and Ir-Cl [2.3683( 12) A] bond lengths in complex (2) were in good agreement with the values reported for several similar Ir(III) complexes. 13,32 The structure of complex (3) displayed two benzoquinoline ligands in different coordination modes. One of them had the ortho C-H bond activation and coordinated in a monodentate way through C14 to the iridium centre, whereas the other benzoquinoline had acted as a bidentate ligand and chelated the Ir(III) centre through N1 and C11.…”

“…The centroid to centroid (Cg-Cg) and C-H/p distances are tabulated in Table S4. † Till date the available structural reports of Ir(III)-7,8-benzoquinoline complexes [12][13][14][15][16][17][18] displayed similarity with complex (2), where the benzoquinolines chelated iridium centre through N1 and C10 in the cyclometallated form. The formation of the Ir-C10 bond resulted with C-H bond activation during the preparation of the starting material (1) from IrCl 3 .…”

“…A range of different bidentate ancillary ligands have been employed in the synthesis of arrays of Ir(III) complexes. [12][13][14][15][16][17][18] We have employed pyridine-2-aldoxime as a bidentate ancillary ligand and expected the formation of a heteroleptic complex having the formula [Ir(benzq-kN, kC 10 ) 2 (pyrald)] (2). We have already reported a phenyl pyridine-Ir(III) complex with pyridine-2-aldoxime having the formula [Ir(ppy) 2 (pyrald)] ( 4) 31 (Fig.…”