Toward fluorine-free blue-emitting cationic iridium complexes: to generate emission from the cyclometalating ligands with enhanced triplet energy

Abstract

A route toward fluorine-free blue-emitting cationic iridium complexes, to generate emission from the cyclometalating ligands with enhanced triplet energy, has been proposed and demonstrated. Attaching electron-donating groups to the pyridine moieties of the ppy-type cyclometalating ligands (Hppy is 2-phenylpyridine) enhances the triplet ((3)π-π*) energy of the ligand, and the use of electron-rich or non-conjugated ancillary ligands ensures that the emission is generated from the (3)π-π* states of cyclometalating ligands. By this molecular design, [Ir(buoppy)2(pzpy)]PF6 (1) and [Ir(buoppy)2(bim-cb)]PF6 (2) have been developed, with 4-butoxy-2-phenylpyridine (buoppy) as the cyclometalating ligand and electron-rich 2-(1H-pyrazol-1-yl)pyridine (pzpy) or non-conjugated N-heterocyclic dicarbene (bim-cb) as the ancillary ligands. Complexes 1 and 2 give emission with major emission peaks around 465 nm, which is among the bluest reported for fluorine-free cationic iridium complexes. For both complexes, the emission is generated from the (3)π-π* states centered on buoppy. For complex 1, the charge-transfer (Ir/buoppy → pzpy) state is dominated by non-radiative deactivation and it behaves as a non-radiative deactivation channel for the emissive buoppy-centered (3)π-π* states which lies close to the charge-transfer state in energy. Such a non-radiative deactivation channel is largely suppressed in the rigid matrix, and is eliminated in complex 2 with a non-conjugated dicarbene ancillary ligand.

Cationic Ir(III) Complexes Featuring Phenylimidazole-type Cyclometalated Ligands: Fluorine-Free Blue Phosphorescent Emitters for Light-Emitting Devices

“…In such cases, the emission originates from the mixed 3 π−π* (C ∧ N-centered)/ 3 MLCT (Ir → C ∧ N) state (MLCT is short for metal-toligand charge transfer), and as a consequence, the 3 π−π* energy of the C ∧ N ligand essentially determines the emission energy. 20,36 The combination of fluorine-free C ∧ N ligands featuring high 3 π−π* energy and appropriate unconjugated/ electron-rich A ∧ A ligands has been regarded as an effective strategy to prepare blue emissive cationic Ir(III) complexes. 20,35,36,38 With this molecular design principle, fluorine-free deep blue emissive Ir(III) complexes were prepared by using high-triplet-energy C ∧ N ligands like 5-phenyl-1H-1,2,4-triazole-type ligands and electron-rich A ∧ A ligands like 4dimethylamino-2-(1H-pyrazol-1-yl)pyridine (dmapzpy).…”

Scrutinizing Design Principles toward Efficient, Long‐Term Stable Green Light‐Emitting Electrochemical Cells

“…In consequence, the use of F and/or F‐containing groups as electron withdrawing substituents has been dropped as a design concept for green emitting LECs and the design and synthesis of fluorine‐free Ir‐iTMCs for LEC application has been pursued. Other electron withdrawing (EW) groups such as sulfur containing groups (e.g., sulfanyl and sulfone) as well as other EW C ^ N ligands like methoxy/methyl substituted 2,3′‐bipyridines have been explored in the attempt to get fluorine‐free green emitting Ir‐iTMCs. Unfortunately, these fluorine‐free Ir‐iTMCs did not lead to an improvement in LEC device lifetime, thereby raising the question whether fluorine indeed is the primary cause of the short lifetime of green LEC devices based on fluorinated iTMCs.…”

Synthesis, Spectroscopic, and Photophysical Studies of Phosphorescent Bis(2-(2,4-difluorophenyl)pyridine)Iridium(III) Complex Containing Derivative of 1<i>H</i>-1,2,4-Triazole Anchillary Ligand

“…3), as well as our previously synthesized [Ir(2,4-F 2 ppy) 2 (mbpyta)]PF 6 and [Ir(2,4-F 2 ppy) 2 (hpyta)]PF 6 [15]. The aromatic signals of pyridyltriazole and phenylpyridine [18] protons were detected between 10.00 and 7.00 ppm [18]. F 2 bpyta showed two singlet peaks at 11.15 ppm H(8) and 9.53 ppm H (7), which were akin to the pyta [14] and were utilized as reference peaks in numerous studies [17].…”

“…with the addition of Ag 2 O (0.049 g, 2.4 mol equiv.). The mixture was refluxed under continuous N 2 for 24 h and prepared following literature report [18]. [Ir(2,4-F 2 ppy) 2 (F 2 bpyta)]PF 6 complex was achieved from the purification using CH 2 Cl 2 /acetone (4.5:0.5) as eluent of the column chromatography.…”

“…The EWG effect of a fluorine atom bonded to phenyl ring at 2and/or 4-carbon position led to stabilizing the HOMO greater than the LUMO and consequently enlarged their energy distances between HOMO and LUMO [27]. Furthermore, adding a pyridyltriazole base anchillary ligand was expected to raise the LUMO energy and enhance the synthesized complexes' luminescence properties [3,5,18].…”