## 2 Results and Discussion
### Triazine-Based TADF Light-Emitting Materials
[84] examined the two possibilities with a series of carbazole/triazine emitters **14-17** (Figure 3) where the methyl groups in charge to ensure the steric hindrance were introduced either on the carbazole group (**14**, **15**) or on the phenyl central bridge (**16**, **17**). On this basis, a structure-performance relationship could be carried out [84]. As first point, modification of the substitution does not significantly modify the optical properties of the different emitters and an almost similar twist angle could be determined for the different molecules. Dihedral angle values of 49.9, 86.8, 71.4, and 82.4\({}^{\circ}\) were calculated by DFT for **14-17**, respectively. A completely different behaviour was evidenced for \(\Delta E_{\text{ST}}\) (_cf._ Table 1). Notably, substitution of the carbazole group by methyl groups drastically modify both its HOMO energy level, but also its redox potential due to the electron-donating ability of the methyl groups, reducing the optical bandgap. On the opposite, the optical bandgap of carbazole was not affected when the methyl groups were introduced on the spacer, enabling the emitters to retain the wide bandgap of carbazole. Thus, **14** that is the less substituted molecule of the series, but also exhibits the more flat structure also showed the highest \(\Delta E_{\text{ST}}\) of the series. Due to its relatively flat structure, the HOMO level of **14** could extend over the donor, but also the phenylene bridge, increasing \(\Delta E_{\text{ST}}\). Upon introduction of methyl groups, the twisting angle between the carbazole and the phenylene spacer drastically increased, more efficiently separating the HOMO and LUMO wavefunctions. As expected, a reduction of the oxidation potential was evidenced for **15**, whereas similar oxidation potentials were measured for **14**, **16** and **17** (+0.87 V for 15 vs. +0.97V for **14**, **16**, and **17**). Photoluminescencest studies proved the first triplet-excited states of **14**, **16**, and **17** to be \({}^{3}\)LE states. Conversely, the excited state of 15 was determined as being a \({}^{3}\)CT state. This assignment could be possible by phosphorescence measurements carried out at 77K. PL spectra at low temperature showed well-resolved vibrational structures for **14-17**, evidencing their T\({}_{1}\) states to be \({}^{3}\)LE states. On the opposite, a broad spectrum was obtained for **15**, suggesting that the T\({}_{1}\) state is a \({}^{3}\)CT state. As anticipated from its large \(\Delta E_{\text{ST}}\), contribution of the delayed fluorescence in the overall photoluminescence of **14** was the lowest of the series. By their large twist angle, a clear contribution of the delayed fluorescence in the radiative decay of **15-17** was demonstrated. Lifetimes of the delayed components of luminescence were determined to be 3.5, 13.0, and 10.3 us for **15-17**, respectively. Due to the absence of delayed contribution in its photoluminescence, **14**-based devices only furnished a relatively low EQE of 7.2%. This performance is much poorer than that of **15-17**-based devices for which maximum EQEs of 22.0, 19.2, and 18.3% were determined, respectively. Similar CIE coordinates were determined for **16**- and **17**-based devices gave (0.15, 0.10), whereas less favourable chromaticity coordinates were determined for **15** as a result of its lower oxidation potential.