and DPA-tBuPh-BN (22 nm) compared to that of DtBuCzB (14 nm) and tDPA-DtCzB (14 nm) indicate a larger degree of structural relaxation between ground and excited states due to the introduction of the tert-butylphenyl groups. tBuPh-BN and DPA-tBuPh-BN have contrasting FPL values in degassed toluene of 85 and 54%, respectively. A solvatochromic study on the impact of solvent polarity on the PL spectra of tBuPh-BN and DPA-tBuPh-BN is shown in Figure and. tBuPh-BN shows a very small degree of positive solvatochromism, which confirms that the SRCT character of the S1 state is conserved across all these solvents. By contrast, there are two regimes of behavior for DPA-tBuPh-BN, where in low polarity solvents emission from the SRCT state dominates but in higher polarity solvents dual emission is observed. This is due to emission originating from both the SRCT state and a long-range chargetransfer state (LRCT) from the DPA donor to the CzBN acceptor as this latter state becomes increasingly stabilized in higher polarity solvents, a behavior that was observed previously in donor-substituted DiKTa derivatives. HF OLED using 5CzBN as the assistant dopant. The absolute FPL values at different doping concentrations were measured under a nitrogen atmosphere (Table ). Both compounds show a propensity to aggregate that is reflected in their lower FPL (67% to 42% for tBuPh-BN and 47% to 30% for DPA-tBuPh-BN) and slightly broader and red-shifted emission (490 to 496 nm for tBuPh-BN and 484 to 491 nm for DPA-tBuPh-BN) of the 1, 2, 4 and 10 wt% doped films compared to in the results from toluene solution (FPL of 85% and 54%, lPL of 490 and 477 nm, respectively) (Figure ). The 2 wt% doped films of tBuPh-BN and DPA-tBuPh-BN in SF3-TRZ emit at lPL of 496 and 487 nm (FWHMs of 31 nm/0.16 eV and 31 nm/0.16 eV), respectively (Figure ). At 300 K under vacuum, the time-resolved PL decays of tBuPh-BN and DPA-tBuPh-BN show biexponential kinetics, with a dominant prompt lifetime, τp, of 5.9 and 4.6 ns and long τd of 41 and 60 ms, respectively. An analysis of the temperaturedependent time-resolved PL decays under vacuum reveal an increasing intensity of the delayed emission with increasing temperature that is characteristic of TADF (Figure ).