The B-CMes bond is longer than other bonds in the system, because of this difference in bond lengths the nonacene skeleton is slightly bent as shown in Figure . The ground [excited] state optimizations were carried out using [Time Dependent-] Density Functional Theory [TD-](DFT) with the PBE0 functional and the 6-31G(d,p) basis set [within the Tamm-Dancoff approximation (TDA)] in the gas phase while the excited states excitation energies were modelled with the spin-component scaling second-order approximate coupled-cluster (SCS-CC2) method. The latter wavefunction-based method has been shown to be essential to model accurately the excited states of MR-TADF emitters. In the ground state (Figure ), the HOMO is mainly localized on the atoms of the central benzene ring and the DOBNA fragments while the LUMO is mainly localized on the DOBNA boron atoms with a very small contribution from the mesityl boranes. The calculated HOMO and LUMO values are -4.99 eV and -1.22 eV, respectively, resulting in a predicted HOMO-LUMO gap of 3.77 eV. This gap is smaller than both DOBNA itself (3.97 eV) and the structurally related deep blue B,N-doped heptacene derivatives, α-3BNOH (4.20 eV) and α-3BNMes (4.03 eV). The difference density plots for the first singlet and triplet excited states show the alternating pattern of increasing and decreasing electron density that is characteristic of MR-TADF emitters. The difference density plots reveal that the electron density in both the S1 and T1 states is mostly localized on the electron-rich central part of the molecule. However, the difference density patterns of S1 and T1, while being reminiscent of SRCT excited states, are distinct from each other because of the different oneelectron transitions contributions (see supporting information, Tables and). The electron density is more diffusely distributed across the entire molecular skeleton in the S2 and T2 states. A small ΔES1T1 value of 0.12 eV is noted for NOBNacene (Table ); however, owing to the different S1 and T1 difference density patterns, obtained from vertical excitations from the ground state, it is likely that the vertical ΔES1T1 would not be accurate, as these excited states would exhibit different relaxation energies. We therefore undertook SCS-CC2 calculations at the optimized S1 and T1 geometries and observed that the adiabatic ΔES1T1 increases to 0.18 eV (Figure and Table ), An intermediate T2 state is reported between S1 and T1, obtained from vertical excitation from the ground state, which suggests that RISC could occur through a spin-vibronic coupling mechanism. The S0-S1 transition has a high oscillator strength of 0.32 from calculations with vertical excitation from the ground state, which rises to 0.40 at the relaxed S1 geometry.