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

*Liquid Crystals and Display Technology*

*3.3.1.3 Aggregation-induced red and NIR OLEDs*

out-coupling enhancement.

current density of 100 mA cm<sup>−</sup><sup>2</sup>

devices would have longer lifetimes.

*3.3.1.4 WOLEDs based on a single emitter*

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

from that of monomers. Because of the increased metal character in the excited states (e.g., MMLCT) leading to the enhanced radiative decay rates, the emission lifetimes of aggregated Pt(II) emitters are usually short, in the range of 0.1–1 μs, which is fundamentally important for addressing the efficiency roll-off and the operational lifetime issues of phosphorescent OLEDs. In addition, this aggregation emission can be manipulated by tuning the doping concentration; this is particu-

Recently, two series of platinum [O^N^C^N] complexes (**Figures 6** and **7**), i.e., type-I (**Pt-21** and **Pt-22**) and type-II (**Pt-15**, **Pt-16** and **Pt-23**) [39, 40], which are prone to excited-state aggregation, were employed as emitting material in both doped and non-doped deep red and NIR devices; these complexes exhibited high EQE and low efficiency roll-off. For devices with neat complexes, high emission quantum yields were only realized with type-I complexes. For instance, when using a neat **Pt-21** film as the EML, the device demonstrated NIR emission with λmax exceeding 700 nm and with an EQE of 15.84%. In addition, the EQE remained at 11.19% even at a high

(26 wt%) exhibited a deep red emission with λmax of 661 nm, CIE coordinates of (0.63,

as the dopant were 59 and 374 h, respectively, demonstrating that aggregation-based

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

. Of the doped devices, the device based on **Pt-16**

. The operational

) with 10 and 30 wt% **Pt-23**

larly useful for the design of high-performance red and NIR OLEDs.

0.37), and an EQE value of 21.75% at a luminance of 1000 cd m<sup>−</sup><sup>2</sup>

lifetimes at 90% initial luminance (LT90, L0 = 100 cd m<sup>−</sup><sup>2</sup>

The emission of Pt(II) complexes in aggregation forms is dramatically redshifted

and 126 lm W<sup>−</sup><sup>1</sup>

**176**

**Figure 7.**

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

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