## 2 Results and Discussion
### Triazine-Based TADF Light-Emitting Materials  
Beside diphenylsulfone, triazine is another popular acceptor building block that is largely used in elaboration of blue TADF emitters. To assess this, OLEDs based on a triazine acceptor/azasiline donor combination could display an unprecedented EQE value of 22.3% [83]. As discussed in the previous section, azasiline is a 6-membered heterocycle in which a carbon atom is replaced by a silicon atom. Azasiline has thus a larger HOMO-LUMO energy gap and a lower HOMO energy level than the molecules comprising a carbon atom. The silicon atom has an sp\({}^{3}\) hybridization. Therefore, it is possible to introduce various groups on the silicon-bridge that can provide \"on demand\" a steric hindrance, improve the solubility and rigidify the structure to the electron donor. This strategy also efficiently prevents intermolecular interactions and drastically reduces the conformation disorder. [PERSON] et al. synthetized two triazine/azasiline emitters (**12** and **13**, Figure 3) [79]. Structurally, **12** differs from **13** by the length of the \(\pi\)- conjugated bridge. **12** has a \(\Delta E_{\rm ST}\) of 0.14 eV, a lifetime for the delayed luminescence of 25.4 \(\mu\)s, and a prompt/delayed fluorescence ratio of 13/87. OLEDs were fabricated using **12** as the dopant and the co-deposited mCP:TSPO1 mixture as the cohost (TSPO1 is diphenyl-4-triphenylsilyl-phenylphosphine oxide). The electroluminescence was detected at 463 nm together with a small efficiency roll-off. However, the CIE coordinates of the emitted light were (0.15, 0.20), thus far from those of the deep blue standard. Another key element for the design of emitters with small \(\Delta E_{\rm ST}\) is the physical separation of the electron-donating and accepting part without completely interrupting the \(\pi\)-conjugation. If the internal torsion of the emitter is a strategy, which has been long detailed above, another strategy consists in elongating the \(\pi\)-connecting spacer that was introduced between the two partners. On the basis of this second approach, [PERSON] et al. replaced the phenyl bridge of **12** by a biphenyl system leading to **13**, which is longer than **12**. As anticipated by the authors, the biphenyl bridge increased the distance between the donor and the acceptor and decrease of the electronic communication between the HOMO and LUMO orbitals could be obtained. As a result, \(\Delta E_{\rm ST}\) significantly decreased (0.04 eV, Table 1). In OLEDs, **13** furnished an EQE of 4.7% with a blue emission of chromaticity coordinates (0.15, 0.09), which is close to the NTSC blue standard (0.14, 0.08). However, despites the more favourable \(\Delta E_{\rm ST}\) for **13** (0.04 eV for **13** vs. 0.14 eV for **12**), OLEDs that were fabricated with this emitter exhibited an that was EQE five times lower than that of **12**. This problem is commonly observed with TADF emitters having long donor/acceptor structure. Indeed, the elongation of the spacer between the donor and the acceptor directly impacts the intramolecular charge transfer, which is weaker due to the distance, reducing the oscillator strength the PLQY, and thus the EQE that is directly correlated to the PLQY of the light-emitting material. As discussed in the previous paragraphs, carbazole is a popular electron donor, largely employed for the design of TADF emitters. However, its five-membered central ring approaches the two lateral aromatic rings so that the steric hindrance that this group can generate is considerably reduced. To address this issue and in order to maintain a sufficiently large dihedral angle to reduce the electronic conjugation with the electron acceptor, several strategies have been envisioned, consisting in the substitution of the carbazole at the more favourable positions (i.e., the 1,8-positions), or conversely, of the spacer ensuring the linkage between the donor and the acceptor. [PERSON] et al.