Blue‐Emitting Cationic Iridium Complexes with 2‐(1H‐Pyrazol‐1‐yl)pyridine as the Ancillary Ligand for Efficient Light‐Emitting Electrochemical Cells

Abstract

Two blue‐emitting cationic iridium complexes with 2‐(1H‐pyrazol‐1‐yl)pyridine (pzpy) as the ancillary ligands, namely, [Ir(ppy)2(pzpy)]PF6 and [Ir(dfppy)2(pzpy)]PF6 (ppy is 2‐phenylpyridine, dfppy is 2‐(2,4‐difluorophenyl) pyridine, and PF6− is hexafluorophosphate), have been prepared, and their photophysical and electrochemical properties have been investigated. In CH3CN solutions, [Ir(ppy)2(pzpy)]PF6 emits blue‐green light (475 nm), which is blue‐shifted by more than 100 nm with respect to the typical cationic iridium complex [Ir(ppy)2(dtb‐bpy)]PF6 (dtb‐bpy is 4,4′‐di‐tert‐butyl‐2,2′‐bipyridine); [Ir(dfppy)2(pzpy)]PF6 with fluorine‐substituted cyclometalated ligands shows further blue‐shifted light emission (451 nm). Quantum chemical calculations reveal that the emissions are mainly from the ligand‐centered 3π–π* states of the cyclometalated ligands (ppy or dfppy). Light‐emitting electrochemical cells (LECs) based on [Ir(ppy)2(pzpy)]PF6 gave green‐blue electroluminescence (486 nm) and had a relatively high efficiency of 4.3 cd A−1 when an ionic liquid 1‐butyl‐3‐methylimidazolium hexafluorophosphate was added into the light‐emitting layer. LECs based on [Ir(dfppy)2(pzpy)]PF6 gave blue electroluminescence (460 nm) with CIE (Commission Internationale de L'Eclairage) coordinates of (0.20, 0.28), which is the bluest light emission for iTMCs‐based LECs reported so far. Our work suggests that using diimine ancillary ligands involving electron‐donating nitrogen atoms (like pzpy) is an efficient strategy to turn the light emission of cationic iridium complexes to the blue region.

Recent Developments in the Application of Phosphorescent Iridium(III) Complex Systems

“…[227] Apart from the intensively studied LECs based on [Ru(bpy) 3 ] 2þ and closely related complexes, [1,11,225,230] iridium(III)-based complexes have turned out to be highly promising candidates for LEC devices due to their high quantum efficiencies. [9,[77][78][79][80][231][232][233][234] As a highlight in this respect, the green-emitting complex [(ppy-F 2 ) 2 Ir( t Bu 2 -bpy)](PF 6 ) (Scheme 2) should be mentioned. It showed in thin solid film [5 wt% in poly(methyl methacrylate)matrix] a PLQY close to unity.…”

Heteroleptic Ir(iii) complexes based on 2-(2,4-difluorophenyl)-pyridine and bisthienylethene: structures, luminescence and photochromic properties

“…Compared to L1H and L2H, compounds , and reveal significantly red shifted emission, with broad bands at 508 and 494 nm, respectively. 5 These broad and relatively featureless emissions are characteristic of predominant 3 MLCT character, 17 which is confirmed by their luminescence spectra measured at 77 K (Fig. S10).…”

“…18a The luminescence spectra of , and were further measured at room temperature in solvents with different polarity, including toluene, CH 2 Cl 2 and CH 3 CN (Fig. S13, S14 17 In addition, the luminescence intensities of , and in CH 2 Cl 2 are relatively higher than those in toluene and in CH 3 CN under the same experiment condition. Previously, we reported the photochromic behavior of L1H.…”

Luminescent Iridium Complexes Used in Light-Emitting Electrochemical Cells (LEECs)

“…5. Further blueshifting of the emission can be achieved by incorporating electron-rich heterocycles within the ligand frameworks of either the N^N (3) or C^N (4) ligands [33,44,45]. This strategy is most effective when this structural modification occurs within the ancillary ligand, as exemplified by the greater blue shift in emission observed for 3 (k PL = 451, 484 nm in MeCN) compared to 4 (k PL = 492 nm in DCM).…”