Both compounds display green emission in dilute toluene with emission maxima, lPL, of 540 nm and 541 nm for Sym-DiDiKTa and Asym-DiDiKTa, respectively, which are ca. 90 nm red-shifted from that of DiKTa (lPL = 453 nm). This bathochromic shift is corroborated by the SCS-CC2 calculations. Both compounds display narrow PL spectra at room temperature (FWHM = 29 nm), and small Stokes shifts of ca. 25 nm, which confirms the small degree of geometrical reorganization in the excited state owing to their conformationally rigid structure (Figures and). The small degree of positive solvatochromism (Figure and Tables and) reflects the SRCT character of the emissive excited state. The S1 excited state energy levels were determined to be 2.26 and 2.25 eV, respectively, for Sym-DiDiKTa and Asym-DiDiKTa and the T1 excited state energy levels are identical at 2.02 eV, each obtained from the lPL of the respective prompt fluorescence and phosphorescence spectra in toluene glass at 77 K. We note that the nature of T1 and S1 are identical based on the difference density plots (Figure ). The corresponding DEST values are 0.24 eV and 0.23 eV, respectively, for Sym-DiDiKTa and Asym-DiDiKTa. These values match with those predicted by SCS-CC2 calculations (DEST = 0.24 eV for both compounds). The PL quantum yield, FPL, in toluene is 70% under N2, which decreases to 64% in air for Sym-DiDiKTa and 53% under N2 and 50% in air for Asym-DiDiKTa. The FPL values were next measured for vacuum-deposited 1 wt% doped thin films in 1,3-bis(N-carbazolyl)benzene (mCP). The FPL values were determined to be 64% and 57% for Sym-DiDiKTa and Asym-DiDiKTa, respectively (Table ). The FPL values closely resemble those obtained in dilute toluene, which implies that non-radiative decay due to vibrations is not significant in these compounds. UV-vis absorption of CT transition; b) Prompt emission in toluene degassing with N2; c) Photoluminescence quantum yield in toluene relative to quinine sulfate in 1N H2SO4 (FPL = 54.6%); d) Full width at half maximum; e) Obtained using an integrating sphere under N2; f) Energy gap between S1 and T1 calculated from the difference of the peaks of the fluorescence and phosphorescence spectra in toluene glass at 77 K g) 1 wt% Sym-DiDiKTa and Asym-DiDiKTa doped in mCP. We next investigated the solid-state photophysical properties of spin-coated 1 wt% Sym-DiDiKTa and Asym-DiDiKTa doped thin films in mCP. The low doping concentration was selected to mitigate the potential for undesired aggregation in the films. As shown in Figures and, at 300 K, Sym-DiDiKTa and Asym-DiDiKTa show a similar emission profile with emission maxima, lPL, at 542 and 547 nm, respectively, values that are close to the prompt emission maximum in toluene. A small FWHM of 35 nm (0.13 eV) was calculated for both compounds in the films, which is slightly broader than the FWHM of 29 nm (0.12 eV) determined in toluene, indicating the influence of the matrix on the conformational stabilization of the excited states. The average τd values are 4.6 µs and 3.0 µs for Sym-DiDiKTa and Asym-DiDiKTa, respectively (Figure , Table ). We then compared the oxygen dependence of the PL spectrum at room temperature. The PL spectrum in air shows a small decrease in intensity compared to the spectrum under vacuum (Figures and), indicating a weak involvement of triplet excited states. The temperature-dependent PL spectra (Figures and) and temperature-dependent time-resolved PL decays (Figures and) of both emitters document the characteristic decrease in intensity and in the contribution of the delayed emission, respectively, with decreasing temperature. This could be due to the MR-TADF behavior only being apparent in a suitable host matrix due to exciplex-like host-emitter interactions.