**3.2 Platinum(II) complexes supported by dianionic N2O2 ligands**

#### *3.2.1 Ligand systems and photophysical properties*

Using "one-metal-one-ligand" approach to construct a stable luminescent platinum material, Che and co-workers developed the first non-porphyrin tetradentate aromatic N2O2 chelates, **Pt-9** and **Pt-10** (**Figure 3**), in 2003 [31]. The photophysical data of **Pt-9**-**12** are summarized in **Table 2**. These aromatic diimine-based Pt(II) complexes exhibit intense absorption bands at λ < 375 nm, which are attributed to


**169**

ligand-centered 1

mixed with 1

**Table 2.**

**Figure 3.**

500 nm are assigned to the <sup>1</sup>

*Photophysical data of Pt-9–12.*

and 1.9 μs and 0.1 for **Pt-10**. Owing to the 3

while those at λ > 400 nm are attributed to <sup>1</sup>

of the aforementioned platinum(II) porphyrin complexes.

absorption bands at λ < 400 nm are dominated by ligand-based <sup>1</sup>

**Pt-11**, bearing a (tetramethyl)ethylene bridge, shows yellow-green emissions (λem = 541–546 nm) in solution with τem values of 3.4–3.9 μs and *Ф*PL values of 0.18–0.27. When the nonconjugated bridge was replaced with a conjugated phenylene unit, as in **Pt-12**, a significant redshift in the emission λmax to 608–628 nm in

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

*Chemical structures of the platinum(II) complexes supported by dianionic N2O2 ligands, Pt-9–12.*

**dm3 cm<sup>−</sup><sup>1</sup> )**

420 (0.52), 488 (sh, 0.67), 504 (0.72)

0.252)

503 (sh, 0.86), 535 (0.99)

**Complex UV-Vis absorption in CH2Cl2, λabs (nm) (298 K) (ԑ, ×104 mol<sup>−</sup><sup>1</sup>**

**Pt-9** 291 (3.92), 315 (3.40), 325 (3.23), 352 (2.58), 375 (2.47),

**Pt-10** 253 (4.10), 313 (1.84), 397 (0.840), 479 (0.294), 504 (sh,

**Pt-12** 253 (4.16), 318 (2.49), 366 (3.61), 382 (3.41), 462 (0.93),

π-π\* transitions. The low-energy absorptions between 400 and

MLCT [dπ → π\*(diimine)] character. **Pt-9** and **Pt-10** display strong

MLCT/3

MLCT and 1

orange-red phosphorescence in CH2Cl2 at 298 K with λmax values of 586 and 595 nm, respectively. The emission lifetimes and quantum yields are 5.3 μs and 0.6 for **Pt-9**

**Pt-11** 319 (1.31), 344 (1.67), 420 (0.58), 440 (0.54) 542 3.7 0.27

state, the emission lifetimes of **Pt-9** and **Pt-10** are significantly shorter than those

Schiff base ligands constitute another important class of N2O2 systems. The facile synthesis of Schiff base ligands, which can be prepared via one-pot multi-gram scale condensation reactions between substituted salicylic aldehydes and alkyl/ aryl diamines, makes them an attractive ligand system for use in the synthesis of Pt(II) emitters. To elucidate structure-property relationships, Che and co-workers conducted a detailed investigation of a panel of Pt(II) Schiff base complexes with alkylene and arylene bridges (e.g., **Pt-11** and **Pt-12**; **Figure 3**) [32, 33]. The photophysical parameters are listed in **Table 2**. Similar to **Pt-9** and **Pt-10**, the

ILCT transition (L → π\*, L = lone pair/phenoxide)

ILCT nature of the emissive

π-π\* transitions,

**Emission**

586 5.3 0.60

595 1.9 0.12

618 3.6 0.20

**τem (μs)** *Ф***PL**

**λem (nm)**

ILCT [L → π\*] transitions.

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

*In CH2Cl2. <sup>c</sup>*

*.*

**Table 1.** *Photophysical and OLED performance data for Pt-1, Pt-2, and Pt-6–8.*

*Max PE = 0.90 lm W<sup>−</sup><sup>1</sup> d In PVK:PBD.*

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

#### **Figure 3.**

*Liquid Crystals and Display Technology*

correlate with the emission lifetime in the solid matrix.

*3.2.1 Ligand systems and photophysical properties*

**Complex** *Ф***PL toluene τem (μs) toluene τem (μs)** 

*Photophysical and OLED performance data for Pt-1, Pt-2, and Pt-6–8.*

**3.2 Platinum(II) complexes supported by dianionic N2O2 ligands**

The data from devices prepared by spin coating these dendrimers as the non-doped emissive layer are better than those of small molecule Pt(II) porphyrin analogs.

The emission of platinum(II) porphyrin complexes can be further shifted to the NIR region by extending the π conjugation of the porphyrin ligand [30]. Schanze and co-workers developed a series of NIR-emitting platinum(II) di- and tetrasubstituted benzoporphyrin complexes, **Pt-6**–**8** (**Figure 2**), to investigate structureproperty relationships and the link between photophysical parameters and OLED performances. The photophysical parameters are listed in **Table 1**. Although the di-substituted porphyrin complex gives a higher ФPL (0.49) and longer τem (53.0 μs) than the tetra-substituted complexes (ФPL = 0.33–0.35; τem = 29.9–32.0 μs) in solution, the same trend does not hold in solid matrices, in which the emission lifetimes of **Pt-6**–**8** are comparable (45.7–57.5 μs). The EQEs of all the devices (8.0–9.2%) were similar. Consequently, the authors concluded that (i) the major non-radiative decays are associated with out-of-plane distortion of the porphyrin ligands, (ii) the rigid polymer matrix could help suppress the enhanced non-radiative decay of the more distorted complexes (**Pt-6** and **Pt-7**) in solid matrices, and (iii) the performance of platinum(II) benzoporphyrin-based OLEDs was observed to strongly

Pt(II) porphyrins as OLED emitters generally exhibit high thermal stability and outstanding performance in saturated red and NIR devices in terms of color purity and EL efficiencies attributed to their narrow emission band and high emission quantum yields. Nevertheless, the practical interest of this class of Pt(II) emitters is limited by the long emission decay times, which would result in substantial efficiency loss at higher luminance. It is hypothesized that careful device design and the use of appropriate auxiliary materials to mitigate TTA, e.g., by using a double host to broaden the recombination zone, could be a strategy for improving the practicality of

Using "one-metal-one-ligand" approach to construct a stable luminescent platinum material, Che and co-workers developed the first non-porphyrin tetradentate aromatic N2O2 chelates, **Pt-9** and **Pt-10** (**Figure 3**), in 2003 [31]. The photophysical data of **Pt-9**-**12** are summarized in **Table 2**. These aromatic diimine-based Pt(II) complexes exhibit intense absorption bands at λ < 375 nm, which are attributed to

**Pt-1** 0.42 80.5 91.0a 650 4.0 **Pt-2** 0.09b 60.0b — 655 —<sup>c</sup> **Pt-6** 0.35 29.9 45.7d 773 8.0 ± 0.5 **Pt-7** 0.33 32.0 49.8d 773 9.2 ± 0.6 **Pt-8** 0.49 53.0 57.5d 777 7.8 ± 0.5

**film**

**λmax EL (nm) Max EQE (%)**

**168**

*a*

*b In CH2Cl2. <sup>c</sup>*

*d*

**Table 1.**

*In polystyrene.*

*In PVK:PBD.*

*Max PE = 0.90 lm W<sup>−</sup><sup>1</sup>*

*.*

these materials.

*Chemical structures of the platinum(II) complexes supported by dianionic N2O2 ligands, Pt-9–12.*


#### **Table 2.**

*Photophysical data of Pt-9–12.*

ligand-centered 1 π-π\* transitions. The low-energy absorptions between 400 and 500 nm are assigned to the <sup>1</sup> ILCT transition (L → π\*, L = lone pair/phenoxide) mixed with 1 MLCT [dπ → π\*(diimine)] character. **Pt-9** and **Pt-10** display strong orange-red phosphorescence in CH2Cl2 at 298 K with λmax values of 586 and 595 nm, respectively. The emission lifetimes and quantum yields are 5.3 μs and 0.6 for **Pt-9** and 1.9 μs and 0.1 for **Pt-10**. Owing to the 3 MLCT/3 ILCT nature of the emissive state, the emission lifetimes of **Pt-9** and **Pt-10** are significantly shorter than those of the aforementioned platinum(II) porphyrin complexes.

Schiff base ligands constitute another important class of N2O2 systems. The facile synthesis of Schiff base ligands, which can be prepared via one-pot multi-gram scale condensation reactions between substituted salicylic aldehydes and alkyl/ aryl diamines, makes them an attractive ligand system for use in the synthesis of Pt(II) emitters. To elucidate structure-property relationships, Che and co-workers conducted a detailed investigation of a panel of Pt(II) Schiff base complexes with alkylene and arylene bridges (e.g., **Pt-11** and **Pt-12**; **Figure 3**) [32, 33]. The photophysical parameters are listed in **Table 2**. Similar to **Pt-9** and **Pt-10**, the absorption bands at λ < 400 nm are dominated by ligand-based <sup>1</sup> π-π\* transitions, while those at λ > 400 nm are attributed to <sup>1</sup> MLCT and 1 ILCT [L → π\*] transitions. **Pt-11**, bearing a (tetramethyl)ethylene bridge, shows yellow-green emissions (λem = 541–546 nm) in solution with τem values of 3.4–3.9 μs and *Ф*PL values of 0.18–0.27. When the nonconjugated bridge was replaced with a conjugated phenylene unit, as in **Pt-12**, a significant redshift in the emission λmax to 608–628 nm in

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, and the emissive states were assigned to have mixed 3 ILCT [L → π\*(diimine)] and 3 MLCT [d → π\*(diimine)] characters. This assignment was further corroborated by the intermediate magnitude of their total zero-field spitting (ZFS) values between 14 and 28 cm<sup>−</sup><sup>1</sup> . These ZFS values lie between those of conventional Ru(II), Os(II), and Ir(III) <sup>3</sup> MLCT emitters (60–170 cm<sup>−</sup><sup>1</sup> ) and those of Pd(II) and Rh(III) 3 IL emitters with ZFS < 1 cm<sup>−</sup><sup>1</sup> .
