UV-vis absorption spectra of the two emitters in toluene reveal a relatively intense SRCT transition at ~ 450 nm (Figure ). Compared to DiKTa, PhCz-O-DiKTa possesses a similar absorption coefficient (e < 1 ´ 10 4 M -1 cm -1 ) around 300-400 nm, which is highly enhanced in PhCz-DiKTa due to the p-conjugated geometry. At 298 K, narrowband emissions were evidenced in the steady-state PL spectra of PhCz-O-DikTa and PhCz-DiKTa, with l PL at 478 nm and 490 nm and full widths at half maxima (FWHMs) of 39 nm and 55 nm (Figure ). In degassed toluene, steady-state PL intensities of the two emitters are both enhanced compared to those recorded in aerated toluene (Figure ). F PL values of PhCz-O-DikTa and PhCz-DiKTa in aerated toluene are 62% and 45%, which improve to 71% and 61%, respectively, demonstrating the contribution from triplet excitons (Figure ). The DE S1T1 values of PhCz-O-DikTa and PhCz-DiKTa were estimated to be 0.36 eV, from the difference in energy of the onset of prompt and delayed emission spectra in 2-methyltetrahydrofuran (2-MeTHF) glass at 77 K (Figures and). The photophysical data are summarized in Table . The magnitude of this DE S1T1 implies that TADF is not operational in these compounds. Reminiscent of the design model (Figure ) and theoretical calculations (Figure ), the higher-energy emission band probably originates from a higher degenerate triplet excited state, here defined as the T 2 state, which is expected to be the bridge for TADF occurrence in this design.