*3.2.2 Chemical and thermal stability*

Platinum(II) N2O2 complexes are generally stable in the solid state under ambient conditions. When dissolved in solution and exposed to light and air, **Pt-9** and **Pt-10** gradually decompose. By contrast, all Schiff base complexes are stable in common organic solvents such as EtOH, 2-propanol, DMSO, and CH3CN under ambient conditions. All platinum(II) N2O2 complexes exhibit high thermal stability as assessed by thermal gravimetric analysis; **Pt-9** and **Pt-10** are stable up to 440 and 530°C, respectively. The decomposition temperatures of Pt(II) Schiff base complexes, including **Pt-11** and **Pt-12**, are in the range of 315–495°C. The introduction of ▬CH3, *t*-Bu, or -F to the phenoxide moieties positively influenced the thermal stability of Pt(II) Schiff base complexes.

#### *3.2.3 Electroluminescent properties*

Devices with bis(2-(2-hydroxyphenyl)pyridine)beryllium (Bepp2) as the host and **Pt-9** or **Pt-10** dopant as the emitting layer were fabricated: [ITO/N,N′-di(αnaphthyl)-N,N-diphenyl-(1,1-biphenyl)-4,4-diamine (NPB, 30 nm)/Bepp2:**Pt-9** (or **Pt-10**) (30 nm)/LiF (0.5 nm)/Al (250 nm)]. All of the devices exhibited turn-on voltages ranging from 5 to 7 V, with yellow to yellow-green emissions. **Pt-10** showed a maximum luminance and power efficiency of 9330 cd m<sup>−</sup><sup>2</sup> (at 330 mA cm<sup>−</sup><sup>2</sup> ) and 1.44 lm W<sup>−</sup><sup>1</sup> (at 40 mA cm<sup>−</sup><sup>2</sup> ), respectively. Notably, although **Pt-9** exhibited a much higher emission quantum yield than **Pt-10** in solution, the EL performance of the former was inferior to that of the latter, which was attributed to the strong intermolecular quenching processes in **Pt-9**. Therefore, the bulky *t*-Bu groups in **Pt-10** are thought to play a vital role in suppressing intermolecular interactions.

The EL properties of platinum(II) Schiff base complexes were investigated. **Figure 4** shows two additional complexes, **Pt-13** and **Pt-14** [33, 34], discussed below, together with **Pt-11** and **Pt-12**. The EL spectra of devices with 4,4′-bis(carbazol-9-yl)biphenyl (CBP) as the host closely matched the corresponding PL spectra, suggesting that the EL originated from the same triplet excited states. The best device performances were obtained with dopant concentrations ranging from 1.5 to 4.5 wt%. At low dopant concentrations (<5.0 wt%), the devices exhibited yellow-green emission, and the efficiency was improved with increasing dopant concentrations. Additionally, the profile of the emission spectra remained unchanged. With doping concentration >5 wt%, the current efficiency (CE) was found to decrease, and the emission color changed due to the formation of excimers or aggregates. A maximum luminance of 9370 cd m<sup>−</sup><sup>2</sup> was achieved by optimizing the dopant concentration to 3 wt%. Notably, devices with simple structures, with Bepp2 as the host and **Pt-13** as the dopant, can generate white emission, and the maximum luminance reached 3045 cd m<sup>−</sup><sup>2</sup> . Additionally,

**171**

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

the CIE coordinates of (0.33, 0.35) are close to those of white light (0.33, 0.33). Unlike **Pt-13**, no aggregate or excimer formation was observed for devices with 6.0 wt% of **Pt-11**, presumably due to the steric bulk of the (tetramethyl)ethylene bridges. Consequently, the performance of **Pt-11** was superior to that of **Pt-13**, with current and power efficiencies and luminance values up to 31 cd A<sup>−</sup><sup>1</sup>

For red light-emitting materials, **Pt-12** achieved a current efficiency of 10.8 cd

molecular interactions as well as further optimize EL performance, a norbornenebased platinum(II) Schiff base complex, **Pt-14** (**Figure 4**) [34], was prepared. Sterically hindered norbornene moieties are highly effective in mitigating emission

approximately 50% higher than that of **Pt-12** with the same device structure. In addition, the efficiency roll-off was reduced by 35%, benefiting from the lower selfquenching rate constant. By incorporating a wide bandgap iridium(III) complex as

respectively, were realized. In addition, the current efficiency could be maintained

demonstrating that platinum(II) Schiff base complexes are promising red emitters

Ease of synthesis, relatively short emission lifetime, high thermal stability, and decent emission quantum yield are traits that make platinum(II) N2O2 emitters attractive phosphorescent dopants, particularly for red OLEDs. Further research efforts in assessing and optimizing their operational stability in devices are

Incorporating anionic C-donor unit(s) into chromophoric ligands has been rec-

metal complexes [35]. The same principle generally holds for tetradentate Pt(II) emitters. The tetradentate cyclometalated Pt(II) emitters reported in the literature typically feature high phosphorescence quantum yields of up to unity, which could be attributed to the following combined effects: (i) the rigid tetradentate ligand scaffold may help suppress excited-state structural distortion, thereby disfavoring non-radiative deactivation of the emissive excited state, (ii) the strongly σ-donating carbanion may destabilize the antibonding Pt 5dx2-y2 orbitals to a great extent, thus

tional lifetime of 18,000 h was realized at an initial luminance of 1000 cd m<sup>−</sup><sup>2</sup>

, respectively, which are comparable to those of

,

,

,

transition

. To suppress the inter-

and 18.33 lm W<sup>−</sup><sup>1</sup>

and d8

d-d state, and (iii) the carbanion

MLCT) and hence the

, **Pt-14** showed a current efficiency

). More importantly, an opera-

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

, and 23,000 cd m<sup>−</sup><sup>2</sup>

tris-cyclometalated iridium(III) complexes.

self-quenching. At a luminance of 1000 cd m<sup>−</sup><sup>2</sup>

and an operational lifetime of >20,000 h at 100 cd m<sup>−</sup><sup>2</sup>

*Chemical structures of platinum(II) Schiff base complexes Pt-13 and Pt-14.*

a co-dopant, high current and power efficiencies of 20.43 cd A<sup>−</sup><sup>1</sup>

at a high luminance (1000 cd m<sup>−</sup><sup>2</sup>

**3.3 Platinum(II) complexes supported by cyclometalated ligands**

ognized as an effective strategy to enhance the luminescence of d6

reducing the quenching of emissive states via the 3

radiative decay rate of the emissive excited states.

donor atom may also increase the metal character (e.g., 3

14 lm W<sup>−</sup><sup>1</sup>

**Figure 4.**

at 14.69 cd A<sup>−</sup><sup>1</sup>

anticipated.

for OLED displays.

A<sup>−</sup><sup>1</sup>

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

#### **Figure 4.**

*Liquid Crystals and Display Technology*

3

14 and 28 cm<sup>−</sup><sup>1</sup>

emitters with ZFS < 1 cm<sup>−</sup><sup>1</sup>

*3.2.2 Chemical and thermal stability*

stability of Pt(II) Schiff base complexes.

) and 1.44 lm W<sup>−</sup><sup>1</sup>

*3.2.3 Electroluminescent properties*

intermolecular interactions.

330 mA cm<sup>−</sup><sup>2</sup>

and Ir(III) <sup>3</sup>

and the emissive states were assigned to have mixed 3

MLCT emitters (60–170 cm<sup>−</sup><sup>1</sup>

.

various solvents with τem values of 1.4–3.6 μs and *Ф*PL of 0.10–0.26 was observed. In addition, the emission color can be finely tuned by attaching electron-donating or electron-withdrawing substituent(s) to the phenolate moieties of Schiff base ligands. The emission of these complexes displays moderate solvatochromic shift,

MLCT [d → π\*(diimine)] characters. This assignment was further corroborated by the intermediate magnitude of their total zero-field spitting (ZFS) values between

Platinum(II) N2O2 complexes are generally stable in the solid state under ambient conditions. When dissolved in solution and exposed to light and air, **Pt-9** and **Pt-10** gradually decompose. By contrast, all Schiff base complexes are stable in common organic solvents such as EtOH, 2-propanol, DMSO, and CH3CN under ambient conditions. All platinum(II) N2O2 complexes exhibit high thermal stability as assessed by thermal gravimetric analysis; **Pt-9** and **Pt-10** are stable up to 440 and 530°C, respectively. The decomposition temperatures of Pt(II) Schiff base complexes, including **Pt-11** and **Pt-12**, are in the range of 315–495°C. The introduction of ▬CH3, *t*-Bu, or -F to the phenoxide moieties positively influenced the thermal

Devices with bis(2-(2-hydroxyphenyl)pyridine)beryllium (Bepp2) as the host and **Pt-9** or **Pt-10** dopant as the emitting layer were fabricated: [ITO/N,N′-di(αnaphthyl)-N,N-diphenyl-(1,1-biphenyl)-4,4-diamine (NPB, 30 nm)/Bepp2:**Pt-9** (or **Pt-10**) (30 nm)/LiF (0.5 nm)/Al (250 nm)]. All of the devices exhibited turn-on voltages ranging from 5 to 7 V, with yellow to yellow-green emissions. **Pt-10** showed a maximum luminance and power efficiency of 9330 cd m<sup>−</sup><sup>2</sup>

(at 40 mA cm<sup>−</sup><sup>2</sup>

**Pt-9** exhibited a much higher emission quantum yield than **Pt-10** in solution, the EL performance of the former was inferior to that of the latter, which was attributed to the strong intermolecular quenching processes in **Pt-9**. Therefore, the bulky *t*-Bu groups in **Pt-10** are thought to play a vital role in suppressing

The EL properties of platinum(II) Schiff base complexes were investigated. **Figure 4** shows two additional complexes, **Pt-13** and **Pt-14** [33, 34], discussed below, together with **Pt-11** and **Pt-12**. The EL spectra of devices with 4,4′-bis(carbazol-9-yl)biphenyl (CBP) as the host closely matched the corresponding PL spectra, suggesting that the EL originated from the same triplet excited states. The best device performances were obtained with dopant concentrations ranging from 1.5 to 4.5 wt%. At low dopant concentrations (<5.0 wt%), the devices exhibited yellow-green emission, and the efficiency was improved with increasing dopant concentrations. Additionally, the profile of the emission spectra remained unchanged. With doping concentration >5 wt%, the current efficiency (CE) was found to decrease, and the emission color changed due to the formation of excimers or aggregates. A maximum luminance of 9370 cd m<sup>−</sup><sup>2</sup>

achieved by optimizing the dopant concentration to 3 wt%. Notably, devices with simple structures, with Bepp2 as the host and **Pt-13** as the dopant, can generate

white emission, and the maximum luminance reached 3045 cd m<sup>−</sup><sup>2</sup>

. These ZFS values lie between those of conventional Ru(II), Os(II),

) and those of Pd(II) and Rh(III) 3

ILCT [L → π\*(diimine)] and

IL

(at

was

. Additionally,

), respectively. Notably, although

**170**

*Chemical structures of platinum(II) Schiff base complexes Pt-13 and Pt-14.*

the CIE coordinates of (0.33, 0.35) are close to those of white light (0.33, 0.33). Unlike **Pt-13**, no aggregate or excimer formation was observed for devices with 6.0 wt% of **Pt-11**, presumably due to the steric bulk of the (tetramethyl)ethylene bridges. Consequently, the performance of **Pt-11** was superior to that of **Pt-13**, with current and power efficiencies and luminance values up to 31 cd A<sup>−</sup><sup>1</sup> , 14 lm W<sup>−</sup><sup>1</sup> , and 23,000 cd m<sup>−</sup><sup>2</sup> , respectively, which are comparable to those of tris-cyclometalated iridium(III) complexes.

For red light-emitting materials, **Pt-12** achieved a current efficiency of 10.8 cd A<sup>−</sup><sup>1</sup> and an operational lifetime of >20,000 h at 100 cd m<sup>−</sup><sup>2</sup> . To suppress the intermolecular interactions as well as further optimize EL performance, a norbornenebased platinum(II) Schiff base complex, **Pt-14** (**Figure 4**) [34], was prepared. Sterically hindered norbornene moieties are highly effective in mitigating emission self-quenching. At a luminance of 1000 cd m<sup>−</sup><sup>2</sup> , **Pt-14** showed a current efficiency approximately 50% higher than that of **Pt-12** with the same device structure. In addition, the efficiency roll-off was reduced by 35%, benefiting from the lower selfquenching rate constant. By incorporating a wide bandgap iridium(III) complex as a co-dopant, high current and power efficiencies of 20.43 cd A<sup>−</sup><sup>1</sup> and 18.33 lm W<sup>−</sup><sup>1</sup> , respectively, were realized. In addition, the current efficiency could be maintained at 14.69 cd A<sup>−</sup><sup>1</sup> at a high luminance (1000 cd m<sup>−</sup><sup>2</sup> ). More importantly, an operational lifetime of 18,000 h was realized at an initial luminance of 1000 cd m<sup>−</sup><sup>2</sup> , demonstrating that platinum(II) Schiff base complexes are promising red emitters for OLED displays.

Ease of synthesis, relatively short emission lifetime, high thermal stability, and decent emission quantum yield are traits that make platinum(II) N2O2 emitters attractive phosphorescent dopants, particularly for red OLEDs. Further research efforts in assessing and optimizing their operational stability in devices are anticipated.

#### **3.3 Platinum(II) complexes supported by cyclometalated ligands**

Incorporating anionic C-donor unit(s) into chromophoric ligands has been recognized as an effective strategy to enhance the luminescence of d6 and d8 transition metal complexes [35]. The same principle generally holds for tetradentate Pt(II) emitters. The tetradentate cyclometalated Pt(II) emitters reported in the literature typically feature high phosphorescence quantum yields of up to unity, which could be attributed to the following combined effects: (i) the rigid tetradentate ligand scaffold may help suppress excited-state structural distortion, thereby disfavoring non-radiative deactivation of the emissive excited state, (ii) the strongly σ-donating carbanion may destabilize the antibonding Pt 5dx2-y2 orbitals to a great extent, thus reducing the quenching of emissive states via the 3 d-d state, and (iii) the carbanion donor atom may also increase the metal character (e.g., 3 MLCT) and hence the radiative decay rate of the emissive excited states.
