## 2 Results and Discussion  
### Molecular Design of Deep Blue TADF Emitters  
TADF is a mechanism of fluorescence relying on a specific molecular design of the light-emitting materials where the first excited singlet state (\(S_{1}\)) can be thermally repopulated by reverse intersystem crossing (RISC) from the first excited triplet state (\(T_{1}\)). This process is only possible when the energy difference between the triplet and the singlet-excited states (\(\Delta E_{\text{ST}}\)) is small enough. In most fluorescent molecules, the TADF process is negligible because \(\Delta E_{\text{ST}}\) is too big (0.5-1.0 eV). Comparative presentation of conventional fluorescence, phosphorescence, and TADF is given in Figure 1.  
TADF has been recently used to efficiently produce fluorescent materials. It is now widely recognized as a promising route to harvest triplet excitons in OLEDs, without taking recourse to expensive transition metal complexes. Getting TADF to work therefore took some clever molecular engineering, guided by quantum mechanics to minimize \(\Delta E_{\text{ST}}\) by forcing each frontier orbital (highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO)) to remain  
Figure 1: Light generation processes in (**a**) fluorescence and phosphorescence-based organic light-emitting diodes (OLEDs) and (**b**) thermally activated delayed fluorescence (TADF)-based OLEDs. Figure reproduced with permission from [PERSON] et al. [54] (reproduced with permission from [54]. Copyright The Japan Society of Applied Physics, 2014). confined on different parts of the molecule [54]. The general strategy to design TADF molecules has been well developed by [PERSON] et al. [55]. Structurally, these molecules are classically made of electron-donating and electron-accepting moieties that are localized on different parts of the molecules, enabling the triplet states to be extremely localized, and the excited states to exhibit a marked charge-transfer character. As abovementioned, the key parameter enabling initiating the delayed fluorescence is the thermal RISC, enabling the electrons from the triplet state to repopulate the singlet state. To reach this goal, emitters should be designed so the energy gap between the lowest T\({}_{1}\) to the lowest S\({}_{1}\) is small enough. By this specific mechanism, the non-radiative triplet states (that are lost under the form of non-radiative decays with the traditional fluorescent materials) can repopulate the singlet state and participate to light emission, enabling OLEDs to reach an internal quantum efficiency of 100%. Thus, the efficiency of the TADF process is largely dependent on the efficacy of the RISC. It is known that the RISC rate increases when \(\Delta\)E\({}_{\text{ST}}\) decreases [56], so minimizing \(\Delta\)E\({}_{\text{ST}}\) is crucial to promote the TADF process and this goal can be achieved if both HOMO and LUMO energy levels are spatially separated. Beyond the simple spatial separation of the two orbitals, a careful selection of both the electron donors and acceptors with adapted energy levels is also of crucial importance to minimize \(\Delta\)E\({}_{\text{ST}}\). However, in order the charge transfer to occur, a minimal interaction between the two orbitals should still exist and the complete isolation of the two orbitals onto two different parts of the molecules is definitely not a good strategy to give rise to a delayed fluorescence. The twist angle (\(\alpha\)) between the donor and the acceptor largely affects the \(\Delta\)E\({}_{\text{ST}}\). A large dihedral angle allows for the spatial separation of the HOMO and LUMO to be sufficient to realize a small \(\Delta\)E\({}_{\text{ST}}\). The twist angle also impacts the photophysical properties of light-emitting materials containing low-lying charge transfer excited states, including the rates of S\({}_{1}\)\(\rightarrow\)S\({}_{0}\) radiative and nonradiative decays.