**7. Other types of tetradentate platinum(II) complexes**

Besides the four series of tetradentate platinum(II) complexes discussed above, there were also some other new types. In 2015, a series of sky-blue emitters based on 3-(trifluoromethyl)- 5-(2-pyridyl)pyrazole or 3-(trifluoromethyl)-5-(2-pyridyl)-1,2,4-triazole containing spiroarranged tetradentate ligands were developed. The peak EQE of one blue OLED could reach 15.3% and CIE values of (0.190, 0.342) [58]. In 2017, Liao, Fan, and co-workers developed three 1-isopropyl-2-phenyl-benzo[*d*]imidazole-based emitters with decomposition temperature above 400°C, and one device exhibited a peak EQE of 22.3% [59].

Very recently, Fukagawa and co-workers reported great progress in ultrapure green OLEDs based on a NHC emitter PtN7N [60], which was developed by Li′s group before 2014 [61]. The optimized OLED showed CIE coordinates of (0.18, 0.74) using a top-emitting OLED with a microcavity structure and also using a boron-based host material [60]. Fukagawa's work demonstrated that the narrowband emitter PtN7N was superior to the iridium(II)-based emitter Ir(mppy)<sup>3</sup> for the development of ultrapure green emitter to satisfy the BT.2020 for ultrahighdefinition displays [60], owing to the very small vibrational structures of PtN7N that could be well suppressed by microcavity technology. Similar phenomenon was also observed in the previous report of narrowband green emitter PtN1N vs. PtOO3 [62]. Moreover, Fukagawa's work also demonstrated that the operational stability of PtN7N-based OLEDs could be comparable to that of the Ir(mppy)<sup>3</sup> -based ones, indicating the promise for the practical application of PtN7N by employing suitable host and charge-transporting materials [60, 63].
