The recorded F PL values of PhCz-O-DikTa and PhCz-DiKTa in air are 56% and 73% (Table ), respectively, which improve to 80% and 91% in N 2 , respectively. Based on the contribution of the delayed component to the average t DF , the calculated F DF associated with the delayed fluorescence in N 2 are 59% and 78% for PhCz-O-DikTa and PhCz-DiKTa, respectively (Figure ). The DE S1T1 values estimated from the difference in energy between the onsets of the prompt fluorescence and delayed emission are almost identical (0.18 eV vs 0.19 eV) (Figures and). However, the calculated k RISC values of PhCz-O-DikTa and PhCz-DiKTa are 5.44 ´ 10 3 s -1 and 6.64 ´ 10 4 s -1 (Table ), respectively, representing a 12-fold enhancement in the latter despite the similar DE S1T1 . However, according to Marcus electron transfer theory, the k RISC of PhCz-DiKTa should be slower than that of PhCz-O-DikTa because the SOC constant of PhCz-DiKTa between S 1 and T 1 states is much smaller than that of PhCz-O-DiKTa (0.10 cm -1 vs 0.43 cm -1 , Figure ). Thus, a higher-lying triplet excited state T 2 probably serves as a bridge for boosting the spin vibronic coupling via second-order perturbation. Temperature-dependent PL measurements were further performed to evidence the presence of the T 2 state. As temperature decreases from 298 K to 200 K, the l PL of the steady-state PL spectrum of PhCz-O-DiKTa gradually red-shifts from 504 nm to 512 nm (Figure ), together with a remarkable enhancement in emission intensity. This excludes TADF as the origin for this spectral change as the intensity should weaken with decreasing temperature if the origin were TADF. Thus, this highly strengthened PL must originate from emission from triplet excitons. As the temperature further decreases to 77 K, the emission intensity at 512 nm diminishes accordingly, and a new concomitant T 1 emission at 540 nm emerges. Evidently, the intensity ratio between 540 nm and 512 nm increases due to the suppressed thermal activation of excitons from T 1 to T 2 states (Figure ). The picture of PhCz-DiKTa is similar as a function of temperature (Figure ). Next, temperature-dependent timegated measurements were conducted to isolate the T 1 and T 2 emissions. As the temperature decreases to 200 K, compared to the TADF emission at ~500 nm, the enhanced T 2 phosphorescence centered at 515 nm appears (as a result of the combined suppressed RISC and nonradiative transitions), and then gradually disappears. Alongside this, there is the emergence and enhancement of T 1 phosphorescence at 550 nm as temperature decreases to 77 K, which cuts off thermally activated reverse internal conversion. T 1 and T 2 phosphorescence can also be evidenced by green and yellow phosphorescence afterglows at 200 K and 77 K (Figure , inset), respectively. Notably, the T 2 emission recorded in mCP is blue-shifted compared to T 1 recorded in 2-MeTHF glass at 77 K, which removes the possibility that T 2 phosphorescence originates from emission from low-energy aggregates (Figure ). A similar phosphorescence evolution over temperature can be observed in PhCz-DikTa (Figure ). At 200 K, there is a strongly enhanced T 2 phosphorescence at 534 nm, while at 77 K T and Table ).