The PL spectra in solution state at RT reveal deep-blue and near UV emission (Figure ) for DIDOBNA-N and MesB-DIDOBNA-N, respectively. DIDOBNA-N shows an unstructured emission peaking at lPL = 426 nm and a moderate FWHM of 43 nm (0.29 eV). The emission from this compound is among the bluest D-A compounds containing a DOBNA acceptor moiety. MesB-DIDOBNA-N emits in the violet at lPL = 399 nm, with a very small FWHM of 23 nm (0.17 eV); and a low energy shoulder at 417 nm. This emission is narrower than that reported for DOBNA (lPL = 398 nm, FWHM = 34 nm in DCM, Figure ). The respectively. The FPL values decrease to 61% and 26%, respectively, upon exposure to air. Both emitters showed a small decrease in FPL upon exposure to air that can be attributed to singlet quenching as there was no delayed emission detected in the transient PL measurements (Figure ). showed the best performance in terms of FPL (Table ) and device EQE that are due to its high ET (3.36 eV) and tendency to better bipolar carrier transport than the other host materials. The FPL values for 1.5 wt% doped TSPO1 films containing DIDOBNA-N and MesB-DIDOBNA-N are 81% and 75%, respectively (Table ). Figure shows the steady state photophysical properties of evaporated thin films with a doping concentration of 1.5 wt% in TSPO1. DIDOBNA-N shows a deep blue emission at 444 nm with FWHM of 64 nm (0.38 eV), which is red-shifted and broader than that in dilute THF solution [lPL= 426 nm and FWHM = 43 nm (0.29 eV)]. The red-shift and spectral broadening of the emission are attributed to the polarization effect of the host matrix on the different excited state properties as CT-type TADF emitters are sensitive to the host polarity. By contrast, MesB-DIDOBNA-N showed negligible changes in its steady-state PL spectrum in the thin film [lPL at 402 nm and a FWHM of only 19 nm (0.15 eV)] compared to that in solution [lPL= 399 nm and FWHM = 23 nm (0.17 The S1/T1 energies for DIDOBNA-N and MesB-DIDOBNA-N in 1.5 wt% doped TSPO1 films were determined from the onsets of the respective prompt fluorescence and phosphorescence spectra at 77 K, which are 3.12 eV/2.89 eV and 3.18 eV/2.94 eV, respectively. The corresponding DEST values are 0.23 eV and 0.24 eV, respectively, which are consistent with those determined in 2-MeTHF.