for DiKTa-OBuIm and DiKTa-DPA-OBuIm, respectively. The emission spectrum of DiKTa-OBuIm is slightly blue-shifted and narrower than that in MeCN, which is expected due to the higher polarity of the solvent than mCP. Surprisingly, for DiKTa-DPA-OBuIm the emission is red-shifted by 14 nm, and with negligible change in the FWHM. This suggests that the conformation of the emitter in the solid state is slightly more conjugated than that in solution or that there are specific host-guest interactions with the DPA unit that perturbs the energy of the excited state. The emission is broader than that of a structurally similar emitter, QAD-mTDPA, a derivative of DiKTa containing two DPA substituents, reported by Zhang et al. The structure of QAD-mTDPA (λPL = 587 nm, FWHM = 62 nm, ΦPL = 97%, ΔEST = 0.33 eV, τD = 269 μs, 1.5 wt% CBP) is shown in Figure . Both emitters showed red-shifted and broadened emission compared to that of DiKTa (λPL = 466 nm, FWHM = 40 nm, ΦPL = 70%, ΔEST = 0.20 eV, τD = 168 μs, 2 wt% mCP) in the same host. Both emitters exhibited high ΦPL values in the mCP film at 71% and 61%, and these reduced to 57% and 53% in for DiKTa-OBuIm and DiKTa-DPA-OBuIm, respectively. As neat thin films, the emission for both compounds are red-shifted and significantly quenched (Figure ); indeed, the ΦPL for the neat film of DiKTa-OBuIm is only 9% while we could not ascertain a reliable value for DiKTa-DPA-OBuIm. The S1 and T1 levels were measured from the onsets of fluorescence and phosphorescence spectra in the 1 wt% doped mCP film at 77 K (Figure ). The corresponding ΔEST values are 0.20 eV and 0.19 eV, respectively, for DiKTa-OBuIm and DiKTa-DPA-OBuIm, which are nearly same to that reported for DiKTa (ΔEST = 0.20 eV). Experimental ΔEST values are smaller than those computationally predicted (0.27 eV and 0.24 eV, respectively for DiKTa-OBuIm and DiKTa-DPA-OBuIm). However, the trend of decreasing ΔEST is in line to the findings from DFT. The temperature dependent time-resolved PL decays in the 1 wt% doped mCP films are presented in Figures . Both emitters show prompt and delayed emission components with an enhancement of the delayed emission with increasing temperature, a feature of TADF. Unlike the delayed emission lifetime of DiKTa (15 μs in 3.5 wt% mCP, 23 μs in PhMe), and its derivatives such as Cz-DiKTa (τD = 196 μs, 2 wt% mCP), DMAC-DiKTa (τD = 6.6