## 1 Introduction
designed a hybridized local and charge transfer fluorescence by modifying the carbazole core with both of a triphenylamine donor and a cyano acceptor [31]. OLED fabricated with this material furnished an EQE of 6.5% together with an excellent CIE \(y\)-coordinate of 0.06 and a full-width at half maximum (FWHM) of 48 nm. Another example that can be cited here is a carbazole/coumarin based emitter that furnished deep blue OLEDs with CIE coordinates of (0.14, 0.11), a maximum luminance of 292 cm\({}^{-2}\), and an EQE of 0.4% [32]. [PERSON] et al. reported an anthracene emitter at CIE (0.15, 0.07) and maximum current and external quantum efficiencies (CE and EQE) up to 3.2 cd/A and 4.5%, respectively, whereas a dramatic efficiency roll-off was evidenced [33]. [PERSON] et al. reported deep-blue OLEDs with CIE coordinates of (0.14, 0.08) and a moderate EQE of 1.51% using 3,3_'-bis-p-_(2,2-diphenylvinyl)phenylbinestiyl as the emitter [34]. Parallel to purely organic materials, metal complexes, such as iridium complexes, perfectly matching the NTSC or EBU standard blues have also been elaborated with EQE, ranging to \(\sim\)6% [35]. Regardless the type of emitter investigated, another key issue in the development of blue OLEDs is the stability of the emitted light. Most of the chromophores that can exhibit excellent deep blue emissions in solution do not perform similarly in the solid state, thus do not give expected deep blue coordinates when tested in OLED devices. This is due to \(\pi\)-\(\pi\) intermolecular stacking or due to molecular aggregation. As a consequence, the emission in solid state and in device is often red-shifted to longer wavelengths, resulting in a blue-green emission and a decrease of efficiency. Molecular engineering and the conception of new emitters in which bulky side groups have been introduced can address this point. Alternatively, the conception of twisted structure can also address this issue [36; 37; 38; 30]. As immediate but adverse consequences, these two strategies often sacrifice the device efficiency or increase raises the turn-on voltage of OLEDs. Aggregation induced emission (AIE) is also a powerful tool to design blue or deep-blue emitters without adversely affecting color emission and device performances. For instance, deep blue tetraphenylethene based emitters have been obtained with CIE \(y\)-coordinate below 0.08 [39; 40; 41]. However, as far as we know, none of the phosphorescent emitters reported to date could display deep blue emissions approaching the blue standards. Recently, a new family of fluorescent emitters i.e., thermally activated delayed fluorescence (TADF) emitting materials has become the rising star of OLEDs by furnishing electroluminescence (EL) performances comparable or outperforming those that were obtained with phosphorescent materials. [PERSON] et al. were the first reported TADF based OLED in 2009 [42]. They developed new fluorescent materials with 100% internal quantum efficiency (IQE) and established TADF light emission principle. In 2012, they published the first purely organic TADF emitter based OLEDs with \(\eta_{\text{IEQE}}\) of 19 %, which is comparable to those of phosphorescent OLEDs [43]. Since these pioneer works, the great potential of TADF emitters in OLEDs has been clearly evidenced and recent results have even put these materials on par with phosphorescence emitting complexes. TADF materials are nowadays considered as the next generation materials, not only for OLEDs [44; 45; 46; 47; 48], but also for other applications, such as photoinitiators for polymerization [49; 50; 51]. In particular, TADF emitters can be metal-free and thus cheaper and less polluting than phosphorescent materials. However, this technology still suffers from the lack of deep blue TADF emitters. A huge number of TADF emitters have been developed, but most of them displayed only sky-blue light [52; 53].