The DFT calculations predict a 0.12 eV increase in the band gap from DIDOBNA-N to MesB-DIDOBNA-N, despite the apparent increase in conjugation length. Both DIDOBNA-N and MesB-DIDOBNA-N display similar electron density distribution patterns for their LUMOs, with a pattern that is reminiscent of the DOBNA moiety (Figure ). However, DIDOBNA-N and MesB-DIDOBNA-N show distinct electron density distribution patterns for their HOMOs. The electron density of the HOMO in DIDOBNA-N is reminiescent of triphenylamine, a good electron donor, while the electron density of the HOMO in MesB-DIDOBNA-N is localized on the DOBNA units, similar to the DOBNA core, and with a nodal plane passing through the central nitrogen and boron atoms of the molecule. Excited state calculations were performed using wavefunction based methods using the spincomponent scaling second-order approximate coupled-cluster (SCS-CC2) in tandem with the cc-pVDZ basis set, which we have previously shown to be a sufficient level of theory to accurately predict ΔEST in MR-TADF emitters. The S1 energy of DIDOBNA-N of 3.67 eV is slightly higher than that of MesB-DIDOBNA-N at 3.63 eV. Despite their similar S1 energies, the oscillator strengths (f) for the S0-S1 transition are starkly different at 0.63 and 0.05 for DIDOBNA-N and MesB-DIDOBNA-N, respectively. The much-attenuated f in MesB-DIDOBNA-N is likely the result of the higher symmetry of this compound (Figure ). Notably, a similar change in the properties of the singlet excited state was predicted for DABNA-1 compared to its more symmetric fused derivative TABNA. Identical T1 energies of 3.47 eV were predicted for DIDOBNA-N and MesB-DIDOBNA-N, resulting in ΔEST of 0.20 eV and 0.16 eV, respectively. An intermediate triplet state was predicted to exist in both compounds, which is acknowledged to facilitate RISC via spin-vibronic coupling in D-A TADF emitters. Difference density plots of the S1, S2, T1 and T2 states are shown in Figure . For MesB-DIDOBNA-N each excited state possesses the typical alternating pattern of increasing and decreasing electron density of MR-TADF compounds that is characteristic of an excited state possessing SRCT character. The magnitude of the CT character of the excited state was quantified using two separate metrics: (1) the distance of charge transfer (DCT) and (2) the amount of charge transferred (qCT). These two metrics reveal that there is a greater CT character in the excited states of DIDOBNA-N compared to that of MesB-DIDOBNA-N (Tables ), pointing towards a more distinct long-range charge transfer (LRCT) character for the former compound; however, the magnitude of the CT character of DIDOBNA-N is still modest compared to conventional D-A TADF emitters, with DCT reported to be 1.1 Å here while typical D-A TADF emitters show DCT of > 1.6 Å. Notably, the orbital types for the T2 states of both compounds are different from those of S1 and so spin-orbit coupling is expected to be non-negligible. The electrochemical properties of both emitters were investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) in deaerated DCM with 0.1 M tetra-n-butylammonium hexafluorophosphate as the supporting electrolyte (Figure ). The oxidation potential, Eox, determined from the peak value of the first oxidation wave of the DPV curve is 1.14 V and 1.39 V vs SCE, respectively, for DIDOBNA-N and MesB-DIDOBNA-N, which translate to respective HOMO levels of -5.48 eV and -5.73 eV. This is in line with the DFT calculations that predict a stabilization of the HOMO energy level of MesB-DIDOBNA-N compared to DIDOBNA-N. For DIDOBNA-N, there was a second oxidation peak in the DPV at 1.25 V. The reduction potential, Ered, determined from the peak value of the first reduction wave of the DPV curve is -1.94 V and -2.10 V vs SCE, respectively, for DIDOBNA-N and MesB-DIDOBNA-N, which correspond to respective LUMO levels of -2.40 eV and -2.24 eV. The DFT calculations corroborate the trend in LUMO levels. The LUMO energy level of DIDOBNA-N is similar to other reported D-A type deep blue emitters having a DOBNA fragment as the acceptor. Reductions for both emitters were broad, in the case of DIDOBNA-N a second reduction peak appeared at -2.10 V, which is likely due to the reduction of second DOBNA unit. The electrochemical band gap is calculated to be 3. a In THF solutions (10 -6 M). b Measured in evaporated thin films consisting of 1.5 wt% emitter in TSPO1 host under vacuum. lexc = 280 nm. c Lowest energy absorbance peak, Absorptivity (e) in parenthesis ( /10 4 M -1 cm -1 ). d Steady-state emission maximum at 300 K. lexc = 365 nm. e Full width at half maximum of emission peak. f S1 and T1 energies were obtained from the onsets of the respective prompt fluorescence (delay: 1 ns; gate time: 100 ns) and phosphorescence spectra (delay: 1 ms; gate time: 9 ms) at 77 K. λexc = 343 nm. g Solution samples for ΔEST measurements were prepared in 2-MeTHF (10 -6 M). h DEST = E(S1) -E(T1). i Relative ΦPL in solutions were measured by a comparative method using quinine sulfate as a standard (Φr = 54.6% in 1 N H2SO4), while absolute ΦPL of the thin films were measured using an integrating sphere. j Prompt and delayed lifetimes were obtained by TCSPC and MCS, respectively. λexc = 378 nm. k Intersystem and reverse intersystem crossing rates were calculated using steady-state approximation method as described in literature. The photophysical properties were studied by measurements of absorption and photoluminescence (PL), PL quantum yield (FPL) and time-resolved PL in 10 -6 M THF. Figure shows the absorption and PL properties of DIDOBNA-N and MesB-DIDOBNA-N and these together with other photophysical data are collated in Table . There is excellent agreement between the experimental and simulated absorption spectra (Figure ). Intense absorption bands were noted for DIDOBNA-N and MesB-DIDOBNA-N around 250 nm, reflected in the molar absorptivity (e) values of 92 × 10 4 M -1 cm -1 and 50 × 10 4 M -1 cm -1 , respectively. An intense band at 340 nm (e = 79 × 10 4 M -1 cm -1 ) was observed for MesB-DIDOBNA-N, which has a similar alternating increasing/decreasing electron density pattern to S1 that is indicative of a SRCT state (Figure ). For DIDOBNA-N there is an intense band at 384 nm (e = 66 × 10 4 M -1 cm -1 ), a feature also observed in the absorption spectra of other D-A systems containing DOBNA acceptor fragment. The lowest energy absorption band in DIDOBNA-N is blueshifted compared to that in MesB-DIDOBNA-N, which aligns with the predicted SCS-CC2 calculations (vide supra). The intensity at the lowest energy absorption band is higher for DIDOBNA-N than MesB-DIDOBNA-N. This is qualitatively consistent with the predicted higher oscillator strength of S1 and S2 of DIDOBNA-N compared to those of MesB-DIDOBNA-N, where the oscillator strength is weak (<0.1) for S1.