PhCz-O-DiKTa and PhCz-DiKTa were synthesized via Ullmann condensation and Suzuki-Miyaura coupling reactions using PhCz and DiKTa as building blocks. The synthetic routes are outlined in Scheme S1. All samples were purified via silica column chromatography followed by temperaturegradient vacuum sublimation. A combination of 1 H & 13 C nuclear magnetic resonance spectroscopy (NMR), high-resolution mass spectrometry (HRMS), melting point determination, high-performance liquid chromatography (HPLC), elemental analysis (EA), and single-crystal X-ray diffraction were used to confirm the molecular structures and assess their purity (Figures , Table ). First, the photophysical properties of PhCz-O-DiKTa and PhCz-DiKTa were modelled using density functional theory at the PBE0/6-31G(d,p) level. As shown in Figure Tamm-Dancoff approximation (TDA) performed at the same level of theory predict that PhCz-O-DiKTa has a larger spin-orbit coupling constant between the S 1 and T 1 states at the optimized S 1 geometry than PhCz-DiKTa (0.43 cm -1 vs 0.10 cm -1 ) but similar oscillator strength (0.11 vs 0.12), and both exhibit a relatively large energy gap between S 1 and T 1 , DE S1T1 > 0.4 eV, (Figure ), which implies that neither compound will show TADF. Calculated vertical excitations to the S 1 state predict that the character of this state in both compounds is hybridized local and CT (Figure ). We have previously demonstrated the importance of using wavefunction-based methods to accurately model the excited state energies and DE S1T1 of MR-TADF emitters, thus, we computed the energies of low-lying singlet and triplet excited states using spin-component scaling second-order algebraic diagrammatic construction (SCS-ADC( )) functional with the cc-pVDZ16 basis set (Figure ). The calculation data are summarized in Table . As anticipated, a somewhat reduced DE S1T1 (~ 0.25 eV) exists in both PhCz-O-DiKTa and PhCz-DiKTa. The S 1 oscillator strength of both compounds is over 0.24, reflecting a highly emissive state. The calculated distance (d CT ) of the S 1 state of DiKTa is 0.803 Å (Table ). A similar d CT exists in PhCz-O-DiKTa (0.944 Å), but a larger value is recorded for PhCz-DiKTa (1.292 Å) due to the p-conjugated geometry. Nevertheless, difference density plots of the four lowest-lying excited states reveal a remarkably SRCT localized on the DiKTa core (Figure ). Such a small reorganization of the electron density in the S 1 state is expected to translate into a narrowband emission. The difference density plots for PhCz-O-DiKTa and PhCz-DiKTa, are similar to that of DiKTa (Figure ), thus demonstrating that the incorporation of PhCz only causes a perturbation of the excited-state energies without remarkably influencing the nature of the low-lying excited-states. Notably, although a DE S1T1 of ~ 0.25 eV has been predicted, it remains challenging to achieve fast k RISC at 298 K with this energy gap as according to the Boltzmann distribution, the approximate exciton ratio between the S 1 and T 1 is 6.0 ´ 10 -5 : 1. Thus, to promote RISC in these compounds, a higher-lying triplet excited state T 2 of different orbital type to the S 1 state must be implicated to enable second-order coupling. The calculations predict that the S 1 and T 2 states of the two emitters are dominated by SRCT and n-p* transitions (Figure ), respectively. PhCz-DiKTa possesses a smaller DE S1T2 than that of PhCz-O-DiKTa (0.21 eV vs 0.08 eV) (Figure ). Therefore, we can reasonably expect that faster k RISC and shorter TADF lifetime (t DF ) occur in PhCz-DiKTa than in PhCz-O-DiKTa via second-order spin-vibronic coupling. As a result, it is vital to experimentally detect the T 2 energy to clarify the RISC processes. as the supporting electrolyte and Fc/Fc + as the internal reference (0.46 V vs. SCE).