Using Substituted Cyclometalated Quinoxaline Ligands To Finely Tune the Luminescence Properties of Iridium(III) Complexes

Abstract

The syntheses of five new heteroleptic iridium complexes [Ir(L(1-4))(2)(Diobpy)]PF(6) (where Diobpy = 4,4'-dioctylamido-2,2'-bipyridine) and [Ir(L(3))(2)(bpy)]PF(6) (where L = para-substituted 2,3-diphenylquinoxaline cyclometalating ligands; bpy = 2,2'-bipyridine) are described. The structures of [Ir(L(3))(2)(Diobpy)]PF(6) and [Ir(L(3))(2)(bpy)]PF(6) show that the complexes each adopt a distorted octahedral geometry with the expected trans-N, cis-C arrangement of the cyclometalated ligands. Electrochemical studies confirmed subtle perturbation of the Ir(III/IV) redox couple as a function of ligand variation. Luminescence studies showed the significant contribution of (3)MLCT to the phosphorescent character with predictable and modestly tunable emission wavelengths between 618 and 636 nm. DFT studies provided approximate qualitative descriptions of the HOMO {located over the Ir(5d) center (11-42%) and the phenylquinoxaline ligand (54-87%)} and LUMO {located over the ancillary bipyridine ligands (ca. 93%)} energy levels of the five complexes, confirming significant MLCT character. TD-DFT calculations indicate that UV-vis absorption and subsequent emission has substantial MLCT character, mixed with LLCT. Predicted absorption and emission wavelengths are in good general agreement with the UV-vis and luminescence experiments.

A Comprehensive Survey of Cationic Iridium(III) Complexes Bearing Nontraditional Ligand Chelation Motifs

“…[66] It should be noted that Φ PL values are reported relative to fac-[Ir(ppy) 3 ] with its quoted Φ PL of 40 %, which is incorrect and should be 97 % instead. [46] These complexes emit between 618 and 636 nm, with Φ PL values of around 2 % in aerated CH 3 CN with mixed MLCT/LLCT character. The emission is unstructured and results from a CT process.…”

“…The first oxidation potential could be tuned through decoration of the aryl substituent with values from 1.89 V for [( 28b ) 2 Ir( 9a )](PF 6 ) to 2.35 V for [( 28c ) 2 Ir( 9a )](PF 6 ), but the process is irreversible on account of a large orbital contribution from 28a – c . Quasi‐reversible reduction of 9a occurs at around –0.69 V. More recently, 2,3‐diarylquinoxalines 29a – d were used as C∧N ligands on iridium in combination with either bpy or 4,4′‐dioctylamido‐2,2′‐bipyridine 46. These complexes emit between 618 and 636 nm, with Φ PL values of around 2 % in aerated CH 3 CN with mixed MLCT/LLCT character.…”

Ligand‐Tuneable, Red‐Emitting Iridium(III) Complexes for Efficient Triplet–Triplet Annihilation Upconversion Performance

“…Our own work into luminescent Ir III complexes has included the development of low‐energy emitting species that luminesce in the red part of the visible spectrum. The requisite cyclometallating ligands are based upon core ligand structures of 2‐phenylquinoline or 2‐phenylquinoxaline, and these can provide interesting species with related pyrene derivatives that also show capability as potent photo‐oxidation sensitisers . Other studies have reported extending the emission wavelengths of Ir III complexes into the near‐infrared (NIR) region…”

Deep-Red Phosphorescent Iridium(III) Complexes Containing 1-(Benzo[b] Thiophen-2-yl) Isoquinoline Ligand: Synthesis, Photophysical and Electrochemical Properties and DFT Calculations

“…Although many investigations about efficient pure blue and green light-emitting complexes have been reported, the search for true red emitters are more difficult because their luminescent quantum yields tend to be lower due to the smaller energy gap [6][7][8][9]. Therefore, a great amount of efforts have been devoted to the development of red-emitting materials [10][11][12].…”