Tetradentate Platinum Complexes for Efficient and Stable Excimer‐Based White OLEDs

Abstract

wileyonlinelibrary.comlayers, or through the combination of fl uorescent and phosphorescent materials. [9][10][11] With these structures, typically employing iridium complexes, WOLEDs have achieved external quantum effi ciencies over 20%, color rendering index (CRI) over 80, and power effi ciencies over 100 lm/W when advanced outcoupling techniques are employing. [ 12 ] However, the strategy of using multiple emissive materials depends on the precise control over various energy transfer processes within the device which can signifi cantly complicate the device fabrication and have significant tradeoffs between device effi ciency and emission color. [ 13 ] Furthermore, the precise color balance of WOLEDs containing multiple emissive materials can be signifi cantly perturbed by variations in the driving conditions, or through different aging processing of the various materials. [ 14 ] Thus, it is strongly desired to achieve an effi cient WOLED containing a single emissive material which is effi cient, stable, and can be fabricated within a single emissive layer.One major approach to achieve single doped white OLEDs is through the exploitation of the excimer emission properties of square planar complexes for a broad white emission. [ 15 ] In excimer based OLEDs, white emission is achieved through the combination of blue emission from an isolated dopant molecule and orange-red emission of two or more closely stacked dopant molecules. Much of the existing reports of excimer based white OLEDs employ either bidentate or tridentate cyclometalating ligands, both of which have typically demonstrated external quantum effi ciency (EQE) less than 20% and often poor CRI or Commission internationale de l'éclairage (CIE) coordinates. [ 16,17 ] One exception is the recent development of platinum(II) bis(methyl-imidazolyl)benzene chloride (Pt-16) resulted in a device with peak EQE of 20.1%, CRI of 80, and CIE of (0.33,0.33). [ 18 ] However, it was demonstrated that the monomer species of Pt-16 was ineffi cient, leading to a lower overall effi ciency and an unavoidable tradeoff between optimal color and highest effi ciency. Furthermore, the N^C^N complexes and other tridentate analogs require Cl − or other monoanionic ligands as the fourth coordinating ligand which may be potentially unstable so a new molecular design motif is needed. [ 19 ] Here, we report the synthesis of a series of tetradentate Pt complexes based on a phenyl methyl-imidazole emissive ligand ( Figure 1 ), which demonstrate effi cient emission from both the excimer and monomer achieving a peak EQEs of 24% ± 2% for all three emitters at concentrations from 2% to 16%. The effect A series of tetradentate platinum complexes that exhibit both effi cient monomer and excimer emission are synthesized. Via small modifi cations to the cyclometalating ligands, both the monomer and excimer emission energy can be separately tuned. Devices employing all of the developed emitters demonstrate impressively high external quantum effi ciencies (EQEs) within the range of 22% to 27%...

Functionalization of phosphorescent emitters and their host materials by main-group elements for phosphorescent organic light-emitting devices

“…86) which can show an intense blue emission in a degassed solution with a peak at 470 nm and a tail extended to the green region (Table 9). 333,334 Blue emitter Pt-Az12 (Fig. As a result, all the green PHOLEDs based on Pt-Az9 to Pt-Az11 exhibit EQE of over 10% with PE exceeding 26 lm W À1 , indicating that Pt-Az9 to Pt-Az11 are superior candidates as triplet emitters for green-emitting PHOLEDs.…”

“…Li et al designed a series of novel tetradentate Pt(II) complexes containing imidazole moieties. 333 At a low doping level of 2%, the device based on Pt-Az13 shows primarily green emission (l em = 496 nm) with a remarkably high EQE of 25.7%, a CE of 70.6 cd A À1 and a PE of 57.1 lm W À1 . 87) possesses a very high quantum yield of 0.64 in solution and 0.81 in doped PMMA films (Table 9).…”

“…318,320,[331][332][333] Li et al reported a simple imidazole-based N^C^N Pt(II) complex Pt-Az8 (Fig. 318,320,[331][332][333] Li et al reported a simple imidazole-based N^C^N Pt(II) complex Pt-Az8 (Fig.…”

Strongly phosphorescent platinum(ii) complexes supported by tetradentate benzazole-containing ligands

“…4,4 0 ,4 00 -Tris(carbazol-9-yl)-triphenylamine (TCTA) was applied as the host material. 7,[16][17][18] Pt2 also showed excimer-based emission at a high doping level. The device data are summarized in Table 2.…”

“…2 Recently platinum(II) complexes bearing rigid tetradentate ligands have emerged as an important class of phosphorescent materials because their rigid structure can reduce the probability of nonemissive decay from ligand-centered excited states ( 3 p-p*) and metal-to-ligand charge-transfer (MLCT) excited states through suppressing the vibration and rotation around metal ions, thus leading to high luminescence efficiency. 7 Our interest in this area is to develop robust phosphorescent Pt(II) complexes supported by benzazole-containing ligands. By employing rigid tetradentate Pt(II) complexes, Li et al demonstrated efficient pure blue OLEDs with an external quantum efficiency (EQE) of 22.7% at 100 cd m À2 and Commission Internationale de L'Eclairage (CIE) coordinates of (0.148, 0.079).…”