The meta-and ortho-analogues, by contrast, are efficient TADF emitters. CzTRZ (renamed as o-CzTRZ; λPL = 455 nm, τd = 3.90 μs in 5 wt% mCP film) possesses a much smaller overlap between the HOMO and LUMO as a result of the more strongly twisted conformation for the dihedral angle between carbazole and phenylene bridge determined from the crystal structure ), which translates into a compound with a DEST of 0.1 eV in 5 wt% mCP film. By doping 5 wt% o-CzTRZ in mCP host as the emitting layer, the OLED exhibited a blue emission with a λEL at 470 nm and an EQEmax of 9.3% despite the FPL of o-CzTRZ being only 16.7% (5 wt% mCP), which likely implies that the FPL has been underestimated, considering the device performance. Interestingly, the full width at half maximum (FWHM) of the blue TADF device is only 66 nm, which we attribute to a much reduced geometric reorganization in the excited state to the small degree of conformational freedom present in the ortho-linked system. Indeed, many other compounds containing ortho-linked donors and acceptors also show narrow emission spectra (in section 9 related to TSCT). Normally, meta-disposed electron-donors and electron-acceptors in D-A type emitters show the weakest conjugation compared to their orthoand para-analogues, which should lead to a correspondingly smaller DEST. However, it was observed by Wu et al. that the DEST of m-CzTRZ (0.22 eV in 10 wt% in mCP film) is indeed larger than 0.03 eV for o-CzTRZ (determined again by Wu et al. although these values are extracted from the energy difference between the PL at room temperature and the phosphorescence spectra at 77 K in 2-MeTHF and so differ from those of Gong et al. ). The authors contended that the smaller observed DEST for o-CzTRZ resulted from the near orthogonal conformation of the donor, leading to near complete suppression of the conjugation between Cz and TRZ. The DFT calculations (Figure ) from their work suggest that the 3 LE state of m-CzTRZ is much more stabilized than the 3 CT state, while the 3 LE state of o-CzTRZ is much closer in energy to its oscillator strengths (f) for o-CzTRZ and m-CzTRZ. Reproduced with permission. Copyright © 2019, Royal Society of Chemistry.