## 2 Results and Discussion
### Diphenylsulfone Based Emitters
From a synthetic viewpoint, reducing the number of synthetic step can facilitate the design of TADF emitters. Azasiline is thus a promising electron donor for constructing TADF emitters. However, at present, only few azasiline-based blue TADF emitters have been reported in the literature. The extensive works of [PERSON] et al. concerning TADF emitters could deduce an interesting design rules. Notably, they demonstrated the decay rate of the delayed fluorescence to be greatly improved by extending the \(\pi\)-conjugation of both the electron-donating and electron-accepting parts. By elongating the \(\pi\)-conjugation, the oscillator strength could be increased and \(\Delta E_{\text{ST}}\) simultaneously lowered even for molecules where a small overlap is observed between the HOMO and LUMO energy level [80]. These design rules constitute a second key-element for the design of TADF emitters. In the work of [PERSON] et al. [79], the highly twisted structure of **8** contributed to reduce \(\Delta E_{\text{ST}}\), improve the colour purity, and enhance the electroluminescence device performances. Benefiting from these favourable features, **8** furnished deep blue OLEDs with colour coordinates of (0.15, 0.11). Conversely, the exceptional colour purity of the emitted light resulted in a low EQE of only 2.3% for OLEDs, but it should be kept in mind that this value is directly to the wavelength at which the light is emitted. As described above, DPS based TADF emitters generally exhibit a large dihedral angle between the phenyl rings of the DPS acceptor and the electron donor. This leads to the effective spatial separation of the HOMO and LUMO wavefunctions. Intrinsically, larger \(\Delta E_{\rm ST}\) values can be expected for blue-emitting materials when compared to green or yellow emitters, simply due to the fact that the HOMO-LUMO gap is wider. Resulting from this, a less efficient RISC process can be attended from these materials. Consequently, a careful selection of the \(\pi\)-conjugation length as well as the electronic properties of the donor and acceptor should be done to reduce at best \(\Delta E_{\rm ST}\). Faced with these considerations, [PERSON] et al. proposed an interesting strategy consisting in introducing one or several DPS groups in the emitters [81]. By introducing between the donors one or several DPS, by varying the number of acceptors, by modifying the connection between donors and acceptors, the electronic properties of the final molecules could be finely tuned while maintaining a wide bandgap and a high triplet energy level. Authors could even drastically reduce \(\Delta E_{\rm ST}\). They effectively synthetized two blue TADF molecules (**9-10**) that were based on _bis_(phenylsulfonyl)benzene with very small \(\Delta E_{\rm ST}\) by covalently linking substituted carbazoles to _bis_-sulfone systems (Figure 2). Theoretical and experimental results revealed that the introduction of _bis_-sulfone groups in the emitters could drastically reduce \(\Delta E_{\rm ST}\) as compared to molecules possessing a _mono_-sulfone spacer (**3**, _cf._ Table 1). However, authors also demonstrated for compound **9**, the _para_-substitution of the _bis_(phenylsulfonyl)benzene core to induce a red-shift of the emission. Conversely, EL performance of **9** was higher than that obtained with its _meta_-isomer. \(\Delta E_{\rm ST}\) values of 0.05 and 0.24 eV were, respectively, determined for **9** and **10**, whereas a value of 0.32 eV was previously determined for **3**. These values  
Figure 2: Diphenylsulfone-based (**1–8**) and _bis_(phenylsulfonyl)benzene-based (**9–11**) emitters. are consistent with the values determined by DFT (\(\Delta E_{\rm ST}\) values of 0.19 and 0.26 eV for **9** and **10**) and the small values of \(\Delta E_{\rm ST}\) prove the design strategy of these authors to be effective for the design of TADF emitters with small energy splitting. Devices based on **10** produced a deep blue EL with CIE coordinates of (0.15, 0.08), a maximum CE of 4.4 cd A\({}^{-1}\), and an EQE of 5.5%.