*3.3.2.1 Molecular design strategies*

The emission energies of the complexes in this family can be rationally and readily tuned by modifying the modular ligand scaffold, which consists of a cyclometalated chromophoric C^N unit and an auxiliary carbazolyl-pyridine group connected by a heteroatom or the heteroatom itself may be part of the chromophoric unit, as shown in **Figure 8**. Complex **Pt-24**, bearing a 4-phenylpyridine ring, shows red emission at 602 nm in solution at rt. Upon switching the 4-phenylpyridine group in the chromophoric unit to a pyrazole moiety to raise the LUMO energy, the emission of **Pt-25** is considerably blueshifted to 491 nm. The emission energy of the complexes can be further increased by reducing or breaking the π conjugation via manipulation of the chromophoric unit and/or the tethered group (**Pt-26**–**28**). For instance, by replacing carbazole with a 9,10-dihydroacridine group to interrupt the π conjugation, the emission maximum of **Pt-28** is blueshifted by 8 nm to 483 nm with respect to **Pt-25**.

This class of Pt[N^C^C^N] complexes was reported to be free from excimer-based emission, which was proposed to be a consequence of distortion of the molecular structure from planarity that disfavors intermolecular interactions [47]. Recently, Li and co-workers conducted a systematic photophysical study on derivatives of **Pt-25** and found that introducing substituents on the auxiliary unit dramatically influenced the emission spectral bandwidth and the nature of the emissive T1 state through modulating the degree of mixing of 1 MLCT/3 MLCT characters with 3 IL state [48].

### *3.3.2.2 Red-emitting complexes and devices*

**Pt-24** is a representative red-emitting complex in this family [44]. This complex shows strong absorption bands at 250–400 nm (ε = 2.4–6.4 × 104 cm<sup>−</sup><sup>1</sup> M<sup>−</sup><sup>1</sup> ) attributable to <sup>1</sup> π−π\* transitions localized on the cyclometalated tetradentate ligand.

**Figure 8.** *Molecular design strategies for color tuning.*

The moderately intense absorption, which can be assigned to the <sup>1</sup> MLCT transition appears at a longer wavelength of 450–550 nm (ε = 3900 cm<sup>−</sup><sup>1</sup> M<sup>−</sup><sup>1</sup> ). Spin-forbidden triplet absorption is located beyond 560 nm (ε = 120 cm<sup>−</sup><sup>1</sup> M<sup>−</sup><sup>1</sup> ) in CH2Cl2. **Pt-24** 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 of 1 MLCT/3 MLCT characters in the T1 state.

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 luminance of 100 cd m<sup>−</sup><sup>2</sup> , which is not outstanding among the reported red-emitting metal complexes. However, the operational lifetime was encouraging. At an initial luminance of 1000 cd m<sup>−</sup><sup>2</sup> , the operational lifetime at 97% of the initial luminance (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> . In addition, 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 showed a driving voltage of 3.6 V at a current density of 1 mA cm<sup>−</sup><sup>2</sup> , which was 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 a luminance of 1000 cd m<sup>−</sup><sup>2</sup> .

**179**

of the ligand.

*Tetradentate Platinum(II) Emitters: Design Strategies, Photophysics, and OLED Applications*

**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

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

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

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

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

was estimated to be 375 h at the same luminance, which corresponds to an LT70

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%,

the electrons inside the EML, the device with structure 2 demonstrated a slightly

1 exhibited a maximum EQE of 8.2% at a luminance of 1000 cd m<sup>−</sup><sup>2</sup>

value of 18,806 h at an initial luminance of 100 cd m<sup>−</sup><sup>2</sup>

improved peak EQE of 10.1% at 1000 cd m<sup>−</sup><sup>2</sup>

), corresponding to an LT70 of

, and the LT70

. Using TrisPCz to confine

, and the LT70 was estimated to be

. Additionally, in an optimized device

.

phenomenon to localization of the T1 state on the chromophoric unit.

70% of the initial luminance (LT70, L0 = 2200 cd m<sup>−</sup><sup>2</sup>

of ca. 60,000 h were achieved at a luminance of 100 cd m<sup>−</sup><sup>2</sup>

32,000 h at an initial luminance of 100 cd m<sup>−</sup><sup>2</sup>

*3.3.2.4 Blue-emitting complexes and devices*

*DOI: http://dx.doi.org/10.5772/intechopen.93221*

*3.3.2.3 Green-emitting complexes and devices*

*Tetradentate Platinum(II) Emitters: Design Strategies, Photophysics, and OLED Applications DOI: http://dx.doi.org/10.5772/intechopen.93221*
