*3.3.1.4 WOLEDs based on a single emitter*

WOLED devices typically employ two or more co-dopants with different emission colors in the EML. Nevertheless, broad-band white light emission with a single Pt(II) emitter could be achieved when both the high-energy monomer emission and low-energy aggregation emission are harvested. In this case, a fine balance of the concentration of excited state monomers and excited state aggregation species is desired. Complex **Pt-22** displays both high-energy monomer emission at 482 nm and low-energy emission at 633–650 nm with emission quantum yields of up to 0.78 when doped into a solid matrix beyond 1.5 wt% [41]. This complex was first reported and used as a single emitter in white PLEDs by Che et al., and white

**177**

attributable to <sup>1</sup>

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

emission with an EQE of 11.51%, CIE coordinates of (0.41, 0.45), and a CRI of 74

which is comparable to the best reported values for a single emitter.

the performance of WOLEDs, devices with a structure of [ITO/MoO3 (5 nm)/TAPC (50 nm)/host:7 wt% 7 (10 nm)/EML (50 nm)/LiF (1.2 nm)/Al (150 nm)] were fabricated. In these devices, 9-[3-[6-(3-carbazol-9-ylphenyl)pyridin-2-yl]phenyl] carbazole (26DCzppy) or TCTA/26DCzppy (1:1 in weight) was used as a single or double host, while TmPyPB or Tm3PyBPZ was used as the ETL. The EQE of the TmPyPB device dropped slightly with increasing luminance. To decrease the driving voltage, 26DCzppy was replaced with TCTA/26DCzppy (1:1 w/w), and the turn-on voltage was decreased from 3.5 to 3.0 V, resulting in an EQE of 23.2%. The turn-on voltage was further decreased to 2.7 V by replacing TmPyPB with Tm3PyBPZ. Consequently, the PE of the device reached a high value of 55.5 lm W<sup>−</sup><sup>1</sup>

In 2013, Li et al. developed two efficient blue-emitting tetradentate platinum complexes with a carbazolyl-pyridine motif integrated into the ligand scaffold. These complexes show emission quantum yields of up to 0.89, and the corresponding devices achieved excellent EQEs of up to 25%, highlighting the potential of these platinum emitters for blue OLED applications [42]. Subsequent works by the same group demonstrated that the carbazolyl-pyridine entity is also a versatile modular building block for various tetradentate dianionic cyclometalated N^C^C^N ligands, providing access to several new classes of efficient blue-, green-,

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 modulat-

MLCT characters with 3

π−π\* transitions localized on the cyclometalated tetradentate ligand.

**Pt-24** is a representative red-emitting complex in this family [44]. This com-

IL state [48].

cm<sup>−</sup><sup>1</sup>

 M<sup>−</sup><sup>1</sup> )

were realized with 16 wt% **Pt-22** as the dopant. To further optimize

,

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

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

and red-emitting platinum(II) complexes [43–46].

*3.3.2.1 Molecular design strategies*

ing the degree of mixing of 1

*3.3.2.2 Red-emitting complexes and devices*

MLCT/3

plex shows strong absorption bands at 250–400 nm (ε = 2.4–6.4 × 104

at 1000 cd m<sup>−</sup><sup>2</sup>

**Figure 7.** *Chemical structures of platinum(II) complexes Pt-21–Pt-23.*

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

emission with an EQE of 11.51%, CIE coordinates of (0.41, 0.45), and a CRI of 74 at 1000 cd m<sup>−</sup><sup>2</sup> were realized with 16 wt% **Pt-22** as the dopant. To further optimize the performance of WOLEDs, devices with a structure of [ITO/MoO3 (5 nm)/TAPC (50 nm)/host:7 wt% 7 (10 nm)/EML (50 nm)/LiF (1.2 nm)/Al (150 nm)] were fabricated. In these devices, 9-[3-[6-(3-carbazol-9-ylphenyl)pyridin-2-yl]phenyl] carbazole (26DCzppy) or TCTA/26DCzppy (1:1 in weight) was used as a single or double host, while TmPyPB or Tm3PyBPZ was used as the ETL. The EQE of the TmPyPB device dropped slightly with increasing luminance. To decrease the driving voltage, 26DCzppy was replaced with TCTA/26DCzppy (1:1 w/w), and the turn-on voltage was decreased from 3.5 to 3.0 V, resulting in an EQE of 23.2%. The turn-on voltage was further decreased to 2.7 V by replacing TmPyPB with Tm3PyBPZ. Consequently, the PE of the device reached a high value of 55.5 lm W<sup>−</sup><sup>1</sup> , which is comparable to the best reported values for a single emitter.
