*3.3.1.2 Photophysical properties and OLEDs based on monomer emission*

The cyclometalated [O^N^C^N] ligand system is useful for the construction of robust and highly efficient Pt(II) emitters. Che et al. developed a panel of platinum(II) [O^N^C^N] emitters, i.e., **Pt-15**–**Pt-20** (**Figure 6**) [39], and systematically investigated their photophysical and electroluminescent properties, which are summarized in **Tables 3** and **4**.

The intense absorption bands of **Pt-15**–**20** at <300 nm are assigned to intraligand π-π\* transitions, and the moderately intense bands at 430–450 nm with weak absorption at >460 nm are assigned to transitions with mixed MLCT and ILCT character. In degassed CH2Cl2 solutions, complexes **Pt-15**–**18** exhibit strong green to yellow luminescence (λem = 522–570 nm) with emission quantum yields and lifetimes in the range of 0.23–0.95 and 2.3–5.5 μs, respectively. With an additional t-Bu group at the *ortho*-position of the phenolate unit, **Pt-18** shows a much lower emission quantum yield (0.23) than **Pt-15**–**Pt-17** (0.77–0.95), indicating that the free rotation of this t-Bu group contributes to the increased non-radiative decay of the emissive state. Congeners **Pt-19** and **Pt-20**, with 6-5-6 metallacycles, are also highly efficient yellow (λem = 551 nm) and green (λem = 517 nm) phosphors, respectively. Their solution emission quantum yields (>0.80) and lifetimes (<5.1 μs) are similar to those of **Pt-15**–**17**. Femtosecond time-resolved fluorescence (fs-TRF) measurements suggested that **Pt-19** and **Pt-20** undergo an ultrafast intersystem crossing process with time constants of 0.44 and 0.15 ps, respectively. The emission origin for **Pt-15**–**20** was assigned to excited states with mixed 3 MLCT and 3 [L → π\*] characters. Notably, **Pt-17**–**Pt-20** do not display excimer emissions in CH2Cl2 even at high concentrations (1.0 × 10<sup>−</sup><sup>4</sup> M), while **Pt-15** and **Pt-16** showed excimer emissions as a low-energy band (ca. 650 nm) under the same conditions. This finding suggests that the incorporation of a bridging tertiary amine or a spiro-fluorene linkage in the ligand framework would be as effective in suppressing the intermolecular

**Figure 6.** *Chemical structures of platinum(II) complexes Pt-15–20.*

*Liquid Crystals and Display Technology*

*3.3.1.1 Molecular design strategies*

tional lifetimes.

*3.3.1 Pt(II) emitters with [O^N^C^N] ligands*

OLEDs have been reported by the same group [22, 37–39].

In 2013, Che et al. developed the first phosphorescent platinum(II) complexes

In general, for monochromic blue to yellow OLEDs, emission from monomeric Pt(II) complexes should be dominant, and aggregate emission should be minimized for achieving a high color purity (**Figure 5**). Early works showed that platinum(II) [O^N^C^N] complexes are prone to excimeric emission at elevated concentration. In attempts to address the excimer issues for realizing monchromic green and yellow OLEDs, Che and co-workers proposed a strategy to bolster the 3D configuration of the moelcular strucutre of the complexes to supress the intermolecular interactions via the introduction of rigid and bulky substituents, such as t-Bu groups and a norbornene moiety, to the ligand periphery, and the incorporation of bridging tertiary arylamine units or biphenyl groups with spiro linkages to the ligand frameworks. These modifications were found to effectively disfavor intermolecular interactions, evident by the diminished emission self-quenching and excimeric emissions in solution and in thin film at high concentrations. In addition, the corresponding devices showed improved device efficiencies and diminished efficiency roll-offs. Additionally, Pt(II) [O^N^C^N] emitters bearing a cross-shaped molecular structure (i.e., a spiro linkage) may also cause molecular entanglement in the amorphous state, which help prevent recrystallization of the emissive layer and prolong opera-

Through deliberate molecular design and variations in the doping concentration, the extent of intermolecular interactions and aggregations of platinum(II) [O^N^C^N] emitters could be controlled and manipulated for red and NIR as well as white OLED applications based on aggregation and monomer/aggregation emissions, respectively (**Figure 5**). Instead of tuning the 3D configuration to limit intermolecular interactions, for these applications, adopting a relatively planar ligand scaffold and introducing fluorine substituent(s) at specific position(s),

supported by tetradentate [O^N^C^N] ligands for white OLED and polymer organic light-emitting diode (PLED) applications [36]. These complexes were found to possess desirable physical properties as OLED emitters including high thermal stability with Td > 400°C and ease of sublimation for vacuum deposition. Several follow-up studies on this family of Pt(II) emitters for high-efficiency

**172**

**Figure 5.**

*Molecular design strategies for various OLED applications.*


#### **Table 3.**

*Photophysical data of Pt-15–Pt-20.*

interactions/aggregation as introducing multiple bulky t-Bu substituents at the ligand periphery.

The EL properties of **Pt-15**–**Pt-18** were investigated in OLEDs based on the device structure [ITO/MoO3 (5 nm)/di-[4-(N,N-ditolylamino)-phenyl]cyclohexane (TAPC, 50 nm)/4,4′,4″-tris(carbazole-9-yl)triphenylamine (TCTA):platinum complex (10 nm)/1,3,5-tri(m-pyrid-3-yl-phenyl) (TmPyPB, 50 nm)/LiF (1.2 nm)/ Al (150 nm)], in which the emission layer was doped with **Pt-15**–**18** at different concentrations. The EL performance data are summarized in **Table 4**. At low doping concentrations, the EL spectra of all devices matched well with the corresponding solution-phase monomer emissions. With increasing dopant concentrations, aggregation emission was observed for **Pt-15**–**Pt**-**17**. Notably, aggregation emission was not observed for **Pt-18**, even at a high doping concentration of 15 wt%, revealing that the self-aggregation of **Pt-18** in EML is negligible. At an optimized doping concentration of 10 wt%, **Pt-18** achieved a maximum EQE of 27.1%, and this value dropped to 16.8% at a luminance of 10,000 cd m<sup>−</sup><sup>2</sup> .

The EL properties of **Pt-19** and **Pt-20** were studied in a device with the structure [ITO/MoO3 (5 nm)/TAPC (50 nm)/TCTA:platinum(II) complex (10 nm)/TmPyPB or 2,4,6-tris(3-(3-(pyridin-3-yl)phenyl)phenyl)-1,3,5-triazine (Tm3PyBPZ, 50 nm)/ LiF (1.2 nm)/Al (150 nm)] in which the doping concentration of the complexes ranged from 2 to 30 wt%. Similar to **Pt-15**–**Pt-18**, at a low doping concentration of 2 wt%, the emissions of both complexes are identical to the corresponding monomer emissions in solution. Increasing the doping concentration to 30 wt% caused a slight redshift for the **Pt-19**-based device, whereas only monomer emission was observed for **Pt-20** at the same doping level. This phenomenon could be rationalized by **Pt-19** to be more prone to undergo intermolecular interactions than those of **Pt-20**. In TmPyPB devices, a maximum EQE of 27.6%, CE of 104.2 cd A<sup>−</sup><sup>1</sup> , and PE of 109.4 lm W<sup>−</sup><sup>1</sup> have been achieved with 10 wt% **Pt-20**, and a maximum EQE of 26.0%, CE of 100.0 cd A<sup>−</sup><sup>1</sup> , and PE of 105.5 lm W<sup>−</sup><sup>1</sup> were achieved with **Pt-19** under the same conditions. At a high luminance (10,000 cd m<sup>−</sup><sup>2</sup> ), the EQE of the devices based on **Pt-19** (30 wt%) or **Pt-20** (10 wt%) remained above 20%. To further optimize the PE of the OLEDs, TmPyPB was replaced with Tm3PyBPZ as the ETL. In Tm3PyBPZ devices, the driving voltage was significantly decreased. Consequently, the maximum PEs were improved to 118

**175**

*a*

*b*

**Table 4.**

**Pt-19** (10)b

**Pt-20** (10)b

*TmPyPB is used as the ETL.*

*Tm3PyBPZ is used as the ETL.*

*OLED performance data for Pt-15–Pt-20.*

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

**) CE (cd A<sup>−</sup><sup>1</sup>**

**Max. At** 

**Pt-15** (2) 92.0 21.8 83.4 55.0 24.4 16.4 (0.32,

**Pt-15** (6) 25.2 5.2 25.5 16.4 13.5 8.6 (0.48,

**Pt-15** (12) 6.7 1.3 11.1 5.3 10.9 5.2 (0.60,

**Pt-16** (4) 61.5 10.5 68.3 31.4 19.7 8.6 (0.34,

**Pt-16** (8) 60.0 16.8 75.0 46.6 20.6 13.1 (0.35,

**Pt-16** (16) 48.6 16.1 51.0 43.4 20.4 17.1 (0.42,

**Pt-17** (2) 98.1 20.0 93.7 46.0 23.8 11.2 (0.39,

**Pt-17** (10) 94.3 32.1 90.0 68.1 24.8 18.8 (0.39,

**Pt-17** (15) 65.3 23.2 72.7 51.2 21.8 15.4 (0.40,

**Pt-18** (4) 82.1 7.4 86.1 22.8 22.7 6.6 (0.41,

**Pt-18** (10) 86.4 26.5 100.5 62.8 27.1 16.8 (0.41,

**Pt-18** (16) 91.0 32.6 94.0 73.8 26.3 19.1 (0.43,

**Pt-19** (2) 103.3 12.8 96.3 40.2 25.7 11.4 (0.42,

**Pt-19** (10) 105.5 20.6 100.0 55.4 26.0 14.4 (0.44,

**Pt-19** (16) 101.3 27.7 96.8 70.4 25.7 18.7 (0.45,

**Pt-19** (30) 80.7 27.8 82.5 68.4 24.8 20.5 (0.47,

**Pt-20** (2) 99.6 12.6 91.7 34.7 24.9 9.59 (0.29,

**Pt-20** (6) 106.7 31.1 101.1 73.1 26.9 19.1 (0.31,

**Pt-20** (10) 109.4 24.7 104.2 79.2 27.6 20.0 (0.31,

**Pt-20** (30) 95.7 28.4 90.0 66.9 24.0 17.9 (0.33,

118.0 22.5 94.3 48.0 25.3 12.5 (0.44,

126.0 24.4 98.8 52.0 26.4 13.6 (0.31,

**104 cd m<sup>−</sup><sup>2</sup>**

**) EQE (%)**

**Max. At** 

**104 cd m<sup>−</sup><sup>2</sup>**

**CIE (x, y)**

0.63)

0.50)

0.40)

0.62)

0.62)

0.56)

0.60)

0.60)

0.58)

0.57)

0.57)

0.56)

0.57)

0.55)

0.54)

0.52)

0.64)

0.64)

0.64)

0.63)

0.55)

0.63)

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

**Complex (wt%)a**

**PE (lmW<sup>−</sup><sup>1</sup>**

**104 cd m<sup>−</sup><sup>2</sup>**

**Max. At** 


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

#### **Table 4.**

*Liquid Crystals and Display Technology*

**Complex UV-Vis absorption in CH2Cl2, λabs (nm) (ԑ, ×104 mol<sup>−</sup><sup>1</sup>**

**Pt-15** 282 (4.5), 304 (sh, 3.3), 336 (sh, 1.8), 372 (1.9), 400

**Pt-16** 283 (4.4), 298 (sh, 3.7), 362 (sh, 1.6), 373 (1.7), 400

**Pt-17** 286 (4.4), 303 (sh, 3.2), 265 (sh, 1.5), 376 (1.8), 405

**Pt-18** 261 (5.1), 288 (5.4), 361 (sh, 1.5), 376 (2.2), 410 (sh,

**Pt-19** 262 (4.4), 295 (sh, 3.5), 330 (2.2), 370 (sh, 1.1), 450

**Pt-20** 261 (sh, 5.0), 279 (5.4), 301 (sh, 3.6), 329 (1.8), 356

**dm3 cm<sup>−</sup><sup>1</sup> )**

(sh, 1.1), 430 (sh, 0.8)

(sh, 1.0), 435 (sh, 0.69)

(sh, 0.98), 440 (sh, 0.75)

1.2), 450 (sh, 0.85)

(sh, 0.27), 481 (sh, 0.21)

(1.7), 393 (0.72), 431 (sh, 0.38)

ligand periphery.

*Photophysical data of Pt-15–Pt-20.*

**Table 3.**

interactions/aggregation as introducing multiple bulky t-Bu substituents at the

dropped to 16.8% at a luminance of 10,000 cd m<sup>−</sup><sup>2</sup>

a maximum EQE of 27.6%, CE of 104.2 cd A<sup>−</sup><sup>1</sup>

The EL properties of **Pt-15**–**Pt-18** were investigated in OLEDs based on the device structure [ITO/MoO3 (5 nm)/di-[4-(N,N-ditolylamino)-phenyl]cyclohexane (TAPC, 50 nm)/4,4′,4″-tris(carbazole-9-yl)triphenylamine (TCTA):platinum complex (10 nm)/1,3,5-tri(m-pyrid-3-yl-phenyl) (TmPyPB, 50 nm)/LiF (1.2 nm)/ Al (150 nm)], in which the emission layer was doped with **Pt-15**–**18** at different concentrations. The EL performance data are summarized in **Table 4**. At low doping concentrations, the EL spectra of all devices matched well with the corresponding solution-phase monomer emissions. With increasing dopant concentrations, aggregation emission was observed for **Pt-15**–**Pt**-**17**. Notably, aggregation emission was not observed for **Pt-18**, even at a high doping concentration of 15 wt%, revealing that the self-aggregation of **Pt-18** in EML is negligible. At an optimized doping concentration of 10 wt%, **Pt-18** achieved a maximum EQE of 27.1%, and this value

The EL properties of **Pt-19** and **Pt-20** were studied in a device with the structure [ITO/MoO3 (5 nm)/TAPC (50 nm)/TCTA:platinum(II) complex (10 nm)/TmPyPB or 2,4,6-tris(3-(3-(pyridin-3-yl)phenyl)phenyl)-1,3,5-triazine (Tm3PyBPZ, 50 nm)/ LiF (1.2 nm)/Al (150 nm)] in which the doping concentration of the complexes ranged from 2 to 30 wt%. Similar to **Pt-15**–**Pt-18**, at a low doping concentration of 2 wt%, the emissions of both complexes are identical to the corresponding monomer emissions in solution. Increasing the doping concentration to 30 wt% caused a slight redshift for the **Pt-19**-based device, whereas only monomer emission was observed for **Pt-20** at the same doping level. This phenomenon could be rationalized by **Pt-19** to be more prone to undergo intermolecular interactions than those of **Pt-20**. In TmPyPB devices,

achieved with 10 wt% **Pt-20**, and a maximum EQE of 26.0%, CE of 100.0 cd A<sup>−</sup><sup>1</sup>

(10 wt%) remained above 20%. To further optimize the PE of the OLEDs, TmPyPB was replaced with Tm3PyBPZ as the ETL. In Tm3PyBPZ devices, the driving voltage was significantly decreased. Consequently, the maximum PEs were improved to 118

.

, and PE of 109.4 lm W<sup>−</sup><sup>1</sup>

), the EQE of the devices based on **Pt-19** (30 wt%) or **Pt-20**

were achieved with **Pt-19** under the same conditions. At a high

have been

**Emission**

**λem (nm) τem (μs)** *Ф***PL**

522 4.0 0.77

522 4.0 0.77

543 5.5 0.95

570 2.3 0.23

551 4.3 0.90

517 5.1 0.80

, and

**174**

PE of 105.5 lm W<sup>−</sup><sup>1</sup>

luminance (10,000 cd m<sup>−</sup><sup>2</sup>

*OLED performance data for Pt-15–Pt-20.*

and 126 lm W<sup>−</sup><sup>1</sup> for the devices with **Pt-19** and **Pt-20** as dopants, respectively, and these values are comparable to those of the best iridium(III) OLED devices without out-coupling enhancement.
