## 2 Results and Discussion
### Cyanide Based TADF Emitter  
Cyano and carbazolyl are also typical acceptor and donor groups used to make efficient TADF materials [51]. [PERSON] et al. recently used this donor/acceptor couple to prepare new TADF molecules, employing a new design approach to improve the colour purity and broad emission spectrum of common blue TADF emitters [86]. The authors designed 22 (see Figure 5) comprising a biphenyl core decorated with electron-donating and accepting groups. Comparison of **22** having the interlocked donor based molecular design with a reference molecule (**21**) revealed that the interlocked donor structure is crucial to reduce the FWHM and to obtain deep blue emission colour as well as high EQE. Both compounds have \(\Delta E_{\text{ST}}\) of 0.27 eV (Table 1), while having different geometrical structures. The PL of **21-22** clearly showed the effect of the interlocked carbazole units. The FL spectrum of **22** was sharper than that of **21**. The FWHM of **22** was only 55 nm, which was much smaller than that of other TADF emitters. This value is even smaller than that of common blue phosphorescent emitters. The large FWHM of 71 nm of **21** certifies that **22** shows a very narrow emission spectrum as a TADF emitter. As expected, deep blue TADF OLEDs with a FWHM of 48 nm, a high EQE of 14.0%, and chromaticity coordinates of (0.14, 0.12) were obtained using the newly designed deep blue TADF emitter (**22**).  
Figure 4: TADF emitters comprising a phosphine oxide moiety.  
[PERSON] et al. also used cyano acceptor to design TADF materials (**23**, Figure 5) [87]. OLEDs comprising **23** as the emitter gave deep blue devices emitting a light with colour coordinates of (0.16, 0.06). However, due to its low TADF characteristics (high \(\Delta E_{\rm ST}\) of 0.403 eV, short delayed time of 0.01 us, Table 1), the EQE device was poor (1.6%). Finally, a rationalization of the molecular design to produce a deep blue, while combining the carbazole donor with the benzonitrile acceptor seems to have been found with **24-27**[88]. Without impacting the dihedral angle between the donor and the acceptor and by tuning the electron donating ability of carbazoles by a convenient substitution, OLEDs producing a deep blue electroluminescence could be obtained. Four substituents were investigated, namely H, Me, _t_-Bu, and OMe. By improving the electron ability, a clear decrease of \(\Delta E_{\rm ST}\) was evidenced, decreasing from 0.31 eV to 0.14 eV for 24 and 27, respectively. Jointly, a decrease of the lifetimes of the delayed components was evidenced, reducing from 18 to 5.5 us for 24 and 27 respectively. While fabricating OLEDs, deep blue EL emission could be obtained with 24 (\(\lambda_{\rm EL}\) = 418 nm), 26 (\(\lambda_{\rm EL}\) = 428 nm), and a blue emission with 25 (\(\lambda_{\rm EL}\) = 436 nm). When considering the CIE coordinates of the light produced by 24 (0.15, 0.05) and consistent with the low PLQY of the film, an EQE of 2.5% was however obtained, despites the wide \(\Delta E_{\rm ST}\) of 0.31 eV that adversely the exciton upconversion. On the opposite, by the more efficient upconversion of the triplet excitons (\(\Delta E_{\rm ST}\) = 0.26 eV) and the higher PLQY of the emitting film (76%) for 26, EQE of OLEDs could be increased to 10.3%, with an emission still being located in the deep blue region (0.16, 0.06). These OLEDs are the deepest-blue TADF OLED to be reported to date with a y-coordinate below 0.07 and with an EQE higher than 10%.