## 2 Results and Discussion
### Diphenylsulfone Based Emitters  
Diphenylsulfone (DPS) is one of the most popular acceptors used for designing deep blue TADF emitters, followed by triarylboron and triazine derivatives. The high electronegativity of the oxygen atoms results from the electron withdrawing ability of the DPS moiety. In addition, the DPS group exhibits a tetrahedral geometry and the S atom has an isolating nature, which efficiently limits the conjugation within the compounds [65]. For this reason, DPS has become a popular electron-accepting group for researchers that are aiming at designing TADF emitters. [PERSON] et al. reported for the first time in 2012 DPS emitters (**1-3**) with pure blue emissions [66]. The chemical structures of **1-3** are given in Figure 2 and the corresponding OLED characteristics are summarized in Table 1.  
All of the compounds have a donor-acceptor-donor (D-A-D) configuration where the electron donor is a diphenylamine moiety in case of **1** and **2** and a carbazole group in case of **3**. Examination of their luminescence spectra revealed that their T\({}_{1}\) states could be assigned to \({}^{3}\pi\)-\(\pi^{\star}\) transitions and mainly localized onto their donor moieties. The \(\Delta E_{\text{ST}}\) energies are 0.54, 0.45, and 0.32 eV for **1-3**, respectively. The introduction of bulky _tert_-butyl groups on the diphenylamine unit in the molecule **2** increased the electron donating properties of the donor compared to that of **1**, shifting the charge transfer band towards the visible region and lowering the charge transfer energy and \(\Delta E_{\text{ST}}\). When the carbazole group replaced the diphenylamine donor, a destabilization of the \({}^{3}\pi\pi^{\star}\) state in **3** was demonstrated, increasing its energy level and lowering \(\Delta E_{\text{ST}}\). TD-DFT calculations also evidenced a greater separation of the HOMO-LUMO orbitals in **3** as compared to **1** and **2** originating from the large dihedral angle in **3** (49\({}^{\circ}\) instead of 32\({}^{\circ}\) for **1** and **2**). As a result of this, a clear decrease of \(\Delta E_{\text{ST}}\) was evidenced for **3** relative to that of **1** and **2**. As expected, contribution of the delayed fluorescence in the overall luminescence decreased with increasing \(\Delta E_{\text{ST}}\), so that it could be detected anymore for **1**. When employed as dopants for multilayer OLEDs, maximum EQEs for **1-3** based OLEDs followed the order: EQE (**1**) < EQE (**2**) < EQE (**3**) (2.9%, 5.6%, and 9.9% for **1-3**, respectively). This experimental results coincides the order for the contribution of the delayed component of fluorescence for these compounds. If the highest EQE was obtained for **3**-based OLED, a severe efficiency roll-off was evidenced for these devices at high current density. This unexpected decrease of EQE was assigned to the long decay time of the delayed fluorescence (270 \(\upmu\)s). Under optimized conditions, a deep blue emission with CIE coordinates of (0.15, 0.07) was obtained with **3**. To overcome the efficiency roll-off issue, the emitter **4** was designed with substituents of improved electron-donating ability and the methoxy groups were selected as candidates [67]. By introducing these substituents, \(\Delta E_{\text{ST}}\) decreased from 0.21 eV for **4** to 0.32 eV for **3**. A reduction of the TADF lifetime as well as the efficiency roll-off could also be demonstrated. This is a direct consequence of the improved electron-donating ability and the elongation of the \(\pi\)-conjugated system of the 3,6-dimethoxycapazole when compared to that of the 3,6-di-_tert_-butylcarbazole. By mean of this convenient substitution, the S\({}_{1}\) state as well as the T\({}_{1}\) state of **4** could be lowered compared to that of **3**, reducing in turn \(\Delta E_{\text{ST}}\). As a beneficial effect for OLEDs, a delayed luminescence lifetime of 93 \(\upmu\)s was measured for **4**, considerably reduced as compared to **3** (270 \(\upmu\)s).