## 2 Results and Discussion
### Tariylborane Emitters  
Since a decade, boron-based materials have been the focus of numerous researches, especially devoted to Organic Electronics as these materials exhibit a remarkable electron mobility due to the vacant _p_-orbital of the boron atom [74; 75; 76; 89; 90]. Triarylboron derivatives are also characterized by their bulkiness and these materials are also strong electron withdrawing groups. Interestingly, triarylboron derivatives have often been opposed to donors, such as 9,10-dihydro, triphenylamine of the popular carbazole group [91; 92; 93]. Some of them even show CIE coordinates of (0.15, 0.09) and (0.15, 0.08) for the PL of emitters in toluene but none of them gave OLEDs with colour coordinates satisfying or being close to the NTSC blue standard of CIE coordinates (0.14, 0.08) [94]. Recently, [PERSON] et al. reported a new strategy to design effective deep blue triarylboron based emitters [95]. The chemical structures of boron emitters (**28-29**) are given in Figure 6, while their novel design strategy is displayed in Figure 7.  
Figure 5: Cyano based emitters. Figure 6: Boron-containing TADF emitters (**28–29**). It is well known that efficient TADF-based OLEDs need a fluorophore with a small \(\Delta E_{\text{ST}}\). As illustrated through the review, the spatial separation of the HOMO and LUMO orbitals is a really popular strategy for minimizing \(\Delta E_{\text{ST}}\) involves. As a direct consequence of this, the exchange interaction between occupied molecular orbitals in the excited states is reduced, as illustrated in Figure 7A. Adversely, the Stokes shift is increased and the EL peaks are broadened. Presently, most of the commercially available OLEDs employ a colour filter [96; 97] to produce a pure blue emission. In this context, [PERSON] et al. overcame these issues by establishing a new design way to minimize \(\Delta E_{\text{ST}}\) of the materials without enlarging of the FL peaks. The authors obtained pure blue TADF molecules by using multiple resonance effect to efficiently separate HOMO-LUMO. The first molecule (**28**) showed in the Figure 6 comprises a triphenylboron donor fused with two triphenylamine acceptors. The second molecule (**29**) is designed by increasing the length of the \(\pi\)-conjugated system as well as the number of electron donors. Since the nitrogen atoms are electron donors and the boron atoms are electron acceptors, the _para_-substitution of the aromatic ring optimize the electronic communication between the two partners (Figure 7b). This rationality is supported by DFT calculations with the LUMOs being localized on the triphenylboron moiety, whereas the HOMOs are being localized on the triphenylamine parts (Figure 8). Figure 7: Comparative representation of energy diagram for a traditional TADF emitter (**a**) and the design developed to generate a multiple resonance effect (**b**). The curve corresponding to the emission spectrum of a benchmark blue OLED is provided as the green dotted line. Figure reproduced with permission from [PERSON] et al. [95] (reproduced with permission from [95] Copyright [PERSON], 2016). DFT calculations showed the extension of the \(\pi\)-conjugation of the donor in **29** when compared to that in **28** to induce a more delocalized HOMO and LUMO energy levels. As a direct consequence of this, the intramolecular charge transfer as well as the oscillator strength was greatly increased in **29** compared to **28** (Figure 8). As expected, OLEDs employing **28** as the emitter produced a deep blue light centred at 459 nm with a satisfying FWHM of 28 nm, a high EQE of 13.5%, and colour coordinates of (0.13, 0.09), which is very close to that defined by NTSC. Extending the \(\pi\)-conjugation of the donor improved the oscillator strength (\(f=0.415\) in **29** vs. \(f=0.205\) in **28**) without modifying the positions of the frontier orbitals in these molecules. This increased the OLED device performance and an EQE as high as 20.2% (emission at 467 nm, FWHM = 28 nm, CIE (0.12, 0.13)) was measured for **29**.