*3.3.2.3 Green-emitting complexes and devices*

*Liquid Crystals and Display Technology*

The moderately intense absorption, which can be assigned to the <sup>1</sup>

shows red emission at 602 nm in CH2Cl2 with an emission quantum yield of 34% at room temperature. A significant rigidochromic blueshift by ca. 30 nm to 574 nm is observed in the glassy solution (2-MeTHF) at 77 K, which is a sign of strong mixing

The EL properties of **Pt-24** were studied with a device structure of [ITO/ dipyrazino[2,3-f:2′,3′-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile (HATCN, 10 nm)/NPB (40 nm)/10% **Pt-24**:CBP (25 nm)/BAlq (10 nm)/Alq3 (40 nm)/LiF (1 nm)/Al (100 nm)], where BAlq is bis(2-methyl-8-quinolinolato)(biphenyl-4-olato)aluminum. The device showed an orange-red emission band at 606 nm, and this band was broader than that in solution. The EL spectrum also included a weak blue emission between 450 and 550 nm, originating from the hole transporting layer NPB. The device displayed a maximum EQE of 8.2% and an EQE of 7.8% at a

ting metal complexes. However, the operational lifetime was encouraging. At an

nance (LT97) was approximately 534 h, which is comparable to that of well-known iridium complexes with similar device structures, e.g., Ir(ppy)3 and (pq)2Ir(acac). To remove the NPB emission as well as improve the efficiency, a 10-nm thick layer of 9,9′,9″-triphenyl-9H,9′H,9″H-3,3′:6′3″-tercarbazole (TrisPCz), with a higher LUMO level and triplet energy than the CBP host, was disposed between the HTL and EML. The maximum EQE was further increased to 11.8%, and the operational

lifetime of LT97 was estimated to be 542 h at a luminance of 1000 cd m<sup>−</sup><sup>2</sup>

showed a driving voltage of 3.6 V at a current density of 1 mA cm<sup>−</sup><sup>2</sup>

.

tion, by replacing Alq3 with 2,7-di(2,2′-bipyridin-5-yl)triphenylene (BPyTP), a device with the structure of [ITO/HATCN (10 nm)/NPB (40 nm)/TrisPCz (10 nm)/10% **Pt-24**:CBP (25 nm)/BAlq (10 nm)/BPyTP (40 nm)/LiF (1 nm)/ Al (100 nm)] was fabricated to decrease the driving voltage. Notably, the device

1.6 V lower than that of the above devices with Alq3 as the ETL. Importantly, the operational lifetime was also substantially improved, with an LT97 value of 638 h at

appears at a longer wavelength of 450–550 nm (ε = 3900 cm<sup>−</sup><sup>1</sup>

triplet absorption is located beyond 560 nm (ε = 120 cm<sup>−</sup><sup>1</sup>

MLCT characters in the T1 state.

MLCT transition

) in CH2Cl2. **Pt-24**

). Spin-forbidden

. In addi-

, which was

M<sup>−</sup><sup>1</sup>

M<sup>−</sup><sup>1</sup>

, which is not outstanding among the reported red-emit-

, the operational lifetime at 97% of the initial lumi-

**178**

of 1

**Figure 8.**

MLCT/3

luminance of 100 cd m<sup>−</sup><sup>2</sup>

initial luminance of 1000 cd m<sup>−</sup><sup>2</sup>

*Molecular design strategies for color tuning.*

a luminance of 1000 cd m<sup>−</sup><sup>2</sup>

**Pt-25** displays strong green phosphorescence at 491 nm in CH2Cl2 at room temperature with emission quantum yields of 0.81 in CH2Cl2 and 0.90 in doped poly(methyl methacrylate (PMMA) films along with an exceptionally narrow spectral bandwidth with an full-width-at-half-maximum (FWHM) of 18 nm, which is comparable to those of quantum dots (25–40 nm) [46]. The authors attributed this phenomenon to localization of the T1 state on the chromophoric unit.

To investigate the EL properties of **Pt-25**, OLEDs with the structure [ITO/ PEDOT:PSS/NPB (30 nm)/TAPC (10 nm)/x% **Pt-25**:2,6-bis(N-carbazolyl)pyridine (26 mCPy, 25 nm)/2,8-bis(diphenylphosphoryl)-dibenzothiophene (PO15, 10 nm)/1,3-bis[3,5-di(pyridin-3-yl)phenyl] benzene (BmPyPB, 30 nm)/LiF (1 nm)/ Al (90 nm)] were fabricated with dopant concentrations (x) ranging from 2 to 14%. The device with a doping concentration of 14% demonstrated a maximum EQE of 25.6%. Additionally, **Pt-25** was employed as the emitter in a device with a structure of [ITO/HATCN (10 nm)/NPB (40 nm)/x% **Pt-25**:CBP (25 nm)/BAlq (10 nm)/Alq3 (40 nm)/LiF/Al] (x = 6, 10, and 20) to probe the operational stability. The device with a dopant concentration of 10% exhibited an operational lifetime of 70 h at 70% of the initial luminance (LT70, L0 = 2200 cd m<sup>−</sup><sup>2</sup> ), corresponding to an LT70 of 32,000 h at an initial luminance of 100 cd m<sup>−</sup><sup>2</sup> . Additionally, in an optimized device structure of [ITO/HATCN (10 nm)/NPB (40 nm)/9-phenyl-3,6-bis(9-phenyl-9H-carbazol-3-yl)-9H-carbazole (TrisPCz; 10 nm)/10% **Pt-25**:3,3-di(9H-carbazol-9-yl)biphenyl (mCBP; 25 nm)/9,9′-(2,8-dibenzothiophenediyl)bis-9H-carbazole (mCBT; 8 nm)/BPyTP (40 nm)/LiF/Al], a maximum EQE of 22.1% and LT70 value of ca. 60,000 h were achieved at a luminance of 100 cd m<sup>−</sup><sup>2</sup> .

## *3.3.2.4 Blue-emitting complexes and devices*

Breaking the π conjugation of ligand scaffolds can increase the T1 energy for harvesting blue emission. By having all-six-membered chelate rings to interrupt the π conjugation, the O-bridged carbazolyl-pyridyl complex **Pt-26** shows a sky blue emission at 473 nm in a PMMA film with a high emission quantum yield of 0.83 and an emission lifetime of 3.8 μs [43]. A subtle disruption of π conjugation could also blueshift the emission. **Pt-28**, featuring a 9,10-dihydro-9,9-dimethylacridine subunit, displays a structured emission at 476 nm at 77 K [45], corresponding to CIE coordinates of (0.11, 0.30), which is 8 nm blueshifted from that of its carbazole analog, **Pt-25**. The **Pt-28-**doped PMMA film showed a high emission quantum yield of 0.68. Interestingly, the emission spectrum of **Pt-28** in CH2Cl2 at room temperature is dramatically broader than that of **Pt-25,** possibly due to the higher flexibility of the ligand.

Devices with the structure [ITO/HATCN (10 nm)/NPB (40 nm)/EBL/10% **Pt-28**:mCBP (25 nm)/HBL/BPyTP (40 nm)/LiF (1 nm)/Al (100 nm)] were fabricated and EL properties and operational lifetimes were examined. The EBL and HBL were arranged as follows: structure 1: no EBL/EML/BAlq (10 nm); structure 2: TrisPCz (10 nm)/EML/BAlq (10 nm); structure 3: no EBL/EML/mCBT (8 nm); and structure 4: TrisPCz (10 nm)/EML/mCBT (8 nm). The device with structure 1 exhibited a maximum EQE of 8.2% at a luminance of 1000 cd m<sup>−</sup><sup>2</sup> , and the LT70 was estimated to be 375 h at the same luminance, which corresponds to an LT70 value of 18,806 h at an initial luminance of 100 cd m<sup>−</sup><sup>2</sup> . Using TrisPCz to confine the electrons inside the EML, the device with structure 2 demonstrated a slightly improved peak EQE of 10.1% at 1000 cd m<sup>−</sup><sup>2</sup> , and the LT70 was estimated to be 416 h. Notably, when BAlq in structure 1 was replaced with a higher bandgap material (mCBT), the device with structure 3 displayed a peak EQE of 15.9%,

and the LT70 was significantly prolonged to 635 h at 1000 cd m<sup>−</sup><sup>2</sup> and 31,806 h at 100 cd m<sup>−</sup><sup>2</sup> . Considering the advantages of TrisPCz (EBL) and mCBT (HTL) in the above devices (i.e., structures 2 and 3), these materials were employed in structure 4. As expected, this device achieved the best efficiency, namely, a peak EQE of 17.8%. However, the operational lifetime of LT70 decreased to 482 h at a luminance of 1000 cd m<sup>−</sup><sup>2</sup> .

Overall, tetradentate cyclometalated Pt(II) emitters have been demonstrated to exhibit high versatility in emission color tuning across RGB colors and white light, as well as superior photophysical and electroluminescent efficiencies and respectable operational lifetimes at practical luminance levels. While the performance metrics of this class of Pt(II) emitters are comparable to that of the best reported Ir(III) emitters in many aspects, more focused efforts should be directed at reducing the radiative lifetimes of these emitters by careful molecular design, which will be instrumental in further improving the operational stability of these complexes to meet the stringent standards required for commercialization.
