Tuning the Excited State of Tetradentate Pd(II) Complexes for Highly Efficient Deep-Blue Phosphorescent Materials

Abstract

Deep-blue-light-emitting materials are urgently desired in high-performance organic light-emitting diodes (OLEDs) for full-color display and solid-state lighting applications. However, the development of stable and efficient deep-blue emitters remains a great challenge. Herein, a series of stable and efficient tetradentate Pd(II)-complex-based deep-blue emitters with rigid 5/6/6 metallocycles and no F atom were designed and synthesized. These deepblue emitters employ various isoelectronic five-membered heteroarylring-containing ligands to exhibit extremely narrow emission spectra peaking at 439−443 nm with a full width at half-maximum (fwhm) of only 22−38 nm in 2-methyltetrahydrofuran at room temperature. In particular, the design of an intramolecular hydrogen bond enabled the 1-phenyl-1,2,3-trazole-based Pd(II) complexes to achieve CIE y < 0.1 (0.069−0.078; CIE is Commission Internationale de L'Eclairage). Theoretical calculation and natural transition orbital analysis reveal that these deep-blue materials emit light exclusively from their ligand (carbazole)-centered ( 3 LC) states. Moreover, the triplet excited-state property can be efficiently regulated through ligand modification with isoelectronic oxazole and thiazole rings or pyridine rings, resulting in sky-blue-to-yellow materials, which emit light originating from an admixture of metal-to-ligand chargetransfer ( 3 MLCT) and intraligand charge-transfer states. The newly developed Pd(II) complexes are strongly emissive in various matrixes with a quantum efficiency of up to 51% and also highly thermally stable with a 5% weight-reduction temperature (ΔT 5% ) of up to 400 °C. Deep-blue OLEDs with CIE y < 0.1 employing Pd(II) complexes as emitters were successfully fabricated for the first time. This study demonstrates that the Pd(II) complexes can act as excellent phosphorescent light-emitting materials through rational molecular design and also provide a valuable method for the development of Pd(II)-complex-based efficient and stable deepblue emitters.

Luminescent Pd(II) Complexes with Tridentate − Aryl‐pyridine‐(benzo)thiazole Ligands

“…Similar behaviour is observed for the protoligands H at potentials lying about 0.7 V lower than those corresponding to the complexes (Figures S6 to S17, Tables S2 and S3). Consistent with reports of similar complexes, [8,9,13,43–46,48–53,57–63] we thus assume that the lowest unoccupied molecular orbital (LUMO) is centred largely on the pyridine‐thiazole/‐benzothiazole moiety.…”

“…Within the series of varying X coligands [Pd(ph(py)tz)X], with X = Cl, Br, I, the oxidation potentials decrease markedly from Cl to I (Table 1, Figures S11 to S13) as was recently also observed for the series [Pd(Phbpy)X] (HPhbpy = 6-(phenyl)-2,2'bipyridine). [63] Data of similar complexes [8,9,13,45,[48][49][50][59][60][61][62][63] suggest essentially metal (Pd II /Pd III ) based oxidation processes. However, the marked impact of the halide co-ligand and the sensitivity of the potentials to the solvent (THF vs. MeCN) suggest a contribution from the halide or the MÀ X bond to the oxidation processes.…”

“…From the viewpoint of the choice of design cyclometalated tetradentate ligand fit perfectly to the square planar geometry of d 8 ‐configured metal cations and conform with the requested rigidity [2,4,7–9,14,15,21,27,49–53] . Nevertheless, tridentate cyclometalating ligands have been very successfully used in luminescent Pt(II), [2,13,20,26,28–31,45,51–53] Pd(II) [5,12,19,20,24,28,31,43,45] and especially Au(III) complexes, [2,51,54–56] as their synthesis is easier than for the complex scaffolds related to tetradentate ligands, whereas the fourth coordination site allows for fine‐tuning of the electronic properties.…”

“…As a rule‐of‐thumb, from a pair of isoleptic (having the same ligands) Pd(II) and Pt(II) complexes, the Pt derivative should show superior photophysical properties with higher PL quantum yields. Also, the absorption and emission energies should be red‐shifted for a Pt(II) complex compared with its Pd(II) derivative [2,4,11,14,20,25,28,31–50] . Nevertheless, a growing number of Pd(II) complexes with high PL quantum yields have been reported [4,5,9,12,14,18–21] …”

“…[4,5,9,12,14,[18][19][20][21] From the viewpoint of the choice of design cyclometalated tetradentate ligand fit perfectly to the square planar geometry of d 8 -configured metal cations and conform with the requested rigidity. [2,4,[7][8][9]14,15,21,27,[49][50][51][52][53] Nevertheless, tridentate cyclometalating ligands have been very successfully used in luminescent Pt(II), [2,13,20,26,[28][29][30][31]45,[51][52][53] Pd(II) [5,12,19,20,24,28,31,43,45] and especially Au(III) complexes, [2,51,[54][55][56] a...…”