Density functional theory (DFT) calculations were performed on DtCzBN-CNBT1 and DtCzBN-CNBT2 at the M062X/6-31G(d,p) level to provide insight into the nature and energies of the low-lying excited states and the frontier molecular orbitals. This level of theory was chosen as the M06X functional has a high ratio (54%) of Hartree-Fock-like exchange, which has been shown to correlate with more accurate excited-state energy predictions in D-A typed TADF emitters. As shown in Figure , the HOMOs of DtCzBN-CNBT1 and DtCzBN-CNBT2 are located on the MR-TADF DtCzBN core and have comparable energies of -6.10 and -6.12 eV, respectively, while the HOMO-1 orbitals are stabilized by 0.11 eV and localized on one of the peripheral benzothienocarbazole donor moieties. The LUMOs of DtCzBN-CNBT1 and DtCzBN-CNBT2 are also located on the MR-TADF core, and the electron density also extends to the pendant benzonitrile group. The orbital distribution pattern is complementary to that of the HOMO, residing largely on adjacent atoms within the DtCzBN core. Time-dependent DFT calculations within the Tamm-Dancoff approximation (TDA-DFT) were employed to interrogate both the nature and the energies of the excited states; additionally, spin-orbit coupling matrix elements were determined between different combinations of low-lying singlet/triplet excited states at the optimized T1 state geometry to assess the magnitude of the spin-orbit coupling (SOC) between these states and thus the likely operational mechanism for RISC. The S1/T1 energies of DtCzBN-CNBT1 and DtCzBN-CNBT2 are predicted to be 2.95/2.46 eV and 2.97/2.50 eV, respectively, resulting in a ΔEST of 0.49 and 0.47 eV, respectively. These calculations are known to overestimate the ΔEST values because the TDA-DFT calculations do not take into account multiple excitations. Due to the too large number of heavy atoms in DtCzBN-CNBT1 and DtCzBN-CNBT2 neither of the more accurate spin-component scaling second-order approximate coupled cluster (SCS-CC2) or double hybrid calculations converged. Based on the HOMO-LUMO transition that describes both the S1 and T1 states, both possess significant SRCT located on the MR-TADF core, while the S2 states (3.48 eV for both emitters) is much better described as having LRCT character transition from the pendant benzothienocarbazole to the benzonitrile acceptor. The S2 states of DtCzBN-CNBT1 and DtCzBN-CNBT2 are also close in energy to those predicted for the S1/T1 states of, CNBT1 (3.68/3.40 eV, Figure ) and CNBT2 (3.66/3.41 eV, Figure ), which are the corresponding compounds containing only the pendant D-A moieties of DtCzBN-CNBT1 and DtCzBN-CNBT2. This result indicates that the twisted structure can reduce the electronic communication between the MR-TADF DtCzBN core and the pendant D-A moiety. For DtCzBN-CNBT1, the S2 state (3.48 eV) lies close to three triplet states, T5 to T7 (3.38~3.45 eV, LRCT), with spin-orbital coupling matrix element (SOCME) 𝐻 !"# !$%& of 0.53 cm - 1 , 𝐻 !"# !$%' of 0.51 cm -1 , and 𝐻 !"# !$%( of 0.49 cm -1 , respectively. For DtCzBN-CNBT2, T2 (3.00 eV) with hybrid LRCT/SRCT character is degenerate with the S1 (3.06 eV) state. There is a moderately large SOCME 𝐻 !"# !)%$ of 0.45 cm -1 . The LRCT S2 state (3.43 eV) is degenerate with T5 (3.41 eV, SRCT) and T6 (3.46 eV, LRCT), with corresponding 𝐻 !"# !$%& and 𝐻 !"# !$%' of 1.97 and 1.00 cm -1 , respectively. The larger 𝐻 !"# !$%& and 𝐻 !"# !$%' values for DtCzBN-CNBT2 are ascribed to the sulfur atom's participation in the S2 and T6 states (Figure ), which is not the case in DtCzBN-CNBT1 (Figure ), and this "heavy" atom effect from the sulfur atom in DtCzBN-CNBT2 boosts the SOC between the S2/T5 and S2/T6 states. The sufficiently higher-lying LRCT S2 states of DtCzBN-CNBT1 and DtCzBN-CNBT2 guarantee that these states, even in polar media, will not be so stabilized as to affect the narrowband SRCT emission from the S1 states. Meanwhile, the small energy gap plus the large SOCME values between S2 and T5/T6 in DtCzBN-CNBT2 make S2 serve as a mediating channel to open a route for a higher-lying RISC (hRISC) processes before internal conversion to the S1 state. With a view to employing these compounds as emitters in OLEDs, the solid-state photophysics of DtCzBN-CNBT1 and DtCzBN-CNBT2 were investigated in PhCzBCz as the host due to its suitably high triplet energy (ET = 2.96 eV) and balanced carrier transport ability. Benefiting from the engineered encapsulated structure, the emission of DtCzBN-CNBT1 and DtCzBN-CNBT2 shows a resistance to concentration quenching, as neither spectral broadening nor a decrease in ΦPL was observed at concentrations up to 50 wt% (Figure ). Across a doping concentration ranging from 1 to 50 wt%, the λPL of DtCzBN-CNBT1 only slightly red-shifts from 498 nm (1 wt%) to 509 nm (50 wt%), with a FWHM effectively remaining the same at 29 nm (0.14 eV) to 30 nm (0.15 eV), respectively. A similar behavior is observed for DtCzBN-CNBT2 where the λPL varies from 498 nm (1 wt%) to 511 nm (50 wt%) and the FWHM being 28 nm (0.13 eV) and 30 nm (0.15 eV), respectively. The modest red-shifting of the PL was attributed to the compounds experiencing a more polar environment at higher doping concentrations. The highest ΦPL values were achieved at 15 wt% at 92% for DtCzBN-CNBT1 and 97% for DtCzBN-CNBT2 (97%); further, the ΦPL values remain over 70% for DtCzBN-CNBT1 and DtCzBN-CNBT2 at a concentration as high as 50 wt% (Figure ). The S1/T1 state energies and ΔEST values of the 15 wt% doped films of DtCzBN-CNBT1 and DtCzBN-CNBT2 in PhCzBCz are 2.56/2.46/0.10 eV and 2.56/2.44/0.12 eV, respectively, determined from the difference in onsets of the steady-state PL and phosphorescence spectra, which are close to the results measured in frozen 2-MeTHF. The phosphorescence spectra of DtCzBN-CNBT1 and DtCzBN-CNBT2 show a high energy, broad emission (400 ~ 470 nm) in doped PhCzBCz films (Figure ), which overlaps with the phosphorescence spectra of the pedant CNBT1 and CNBT2 moieties (Figure ). The time-resolved PL decay traces of DtCzBN-CNBT1, DtCzBN-CNBT2, CNBT1 and CNBT2 are shown in Figures and. Compounds DtCzBN-CNBT1 and DtCzBN-CNBT2 have prompt lifetimes, τp, of 3.8 and 5.1 ns, and average delayed lifetimes, τd,avg, of 75.5 and 15.3 µs, respectively. Considering that both compounds possess the same MR-TADF moiety, the much shorter τd,avg of DtCzBN-CNBT2 must be due to the presence of the pendant donor-acceptor moiety. Notably, the donor-acceptor moieties CNBT1 and CNBT2 have similar τp of 2.7 and 3.5 ns, respectively, and only CNBT2 exhibits a delayed emission, with τd,avg of 159 µs. The origins of the TADF in CNBT2 are due to the smaller ΔEST of 0.28 eV in the 15 wt% doped film in PhCzBCz film (Figure ) and the larger predicted 𝐻 !"# !)%) of 0.73 cm -1 benefiting from the sulfur atom's direct participation in the S1 and T1 states (Figure ). For CNBT1, TADF is not present due to a combination of the larger ΔEST of 0.36 eV and smaller 𝐻 !"# !)%) of 0.24 cm - 1 (Figure ) in this compound. The TADF character of CNBT2 implies the presence of multiple RISC channels available in DtCzBN-CNBT2, evidenced by the faster RISC rate constant (kRISC = 1.1 ×10 5 s - 1 ) compared to the kRISC of 2.0 ×10 4 s -1 for DtCzBN-CNBT1; a complete kinetics analysis is provided in the ESI. -5.46/-2.69 /2.77 a Measured in diluted toluene (λexc = 340 nm), b measured in 15 wt% doped films in PhCzBCz (λexc = 379 nm), c measured in 15 wt% doped films in PhCzBCz using an integrating sphere under air/nitrogen (λexc =450 nm), d S1 obtained from the onset of the SS PL spectrum at 77 K, T1 obtained from the onset of the time-gated emission spectrum (1-10 ms) at 77 K (lexc = 340 nm), e measured in DMF solution (EHOMO/LUMO = -(Eox / Ered versus Fc/Fc + + 4.8) eV).