The vertical excitation energies and oscillator strengths of the S1 and T1 excited states of the ribbons CBN_1-m and CBN_2-m calculated with the ADC(2) method are listed in Table for m = 1 -9. These results are also presented graphically in Fig. . The corresponding results obtained with the CC2 method are given in Table and Fig. in the SI. Table . Vertical excitation energies of the S1 and T1 electronic states (in eV), S1-T1 energy gap ∆ST, and oscillator strengths f of the S0-S1 transition of the CBN_1-m and CBN_2-m ribbons, computed at the Notably, the dependence of the S1 and T1 vertical excitation energies on the number of units is qualitatively different for the CBN_1-m and CBN_2-m ribbons, see Fig. . While the vertical excitation energies of the CBN_2-m ribbons decrease monotonically with increasing m, the dependence of the vertical excitation energies of the CBN_1-m ribbons on m is non-monotonic. CBN_1-1 exhibits an unusually low excitation energy of about 0.9 eV and the excitation energy increases up to m = 6, followed by a slow decrease for m > 6. Despite the different unit cells, the S1 and T1 energies of the CBN_1-m and CBN_2-m ribbons converge towards a common limit of about 1.2 eV for large m. It is also notable that the S1-T1 energy gap ΔST is clearly negative for low-m members of CBN_1-m ribbons, while ΔST is close to zero for the CBN_2 -m ribbons, see Fig. (a). The CC2 method yields qualitatively similar results, see Table and Fig. in the SI. Considering the fact that ADC(2) tends to slightly overestimate the magnitude of negative singlet-triplet gaps, it may be concluded that the ribbons CBN_1-1, CBN_1-2 and CBN_1-3 are inverted singlet-triplet (IST) systems, while the remaining ribbons exhibit singlet-triplet degeneracy or very small positive singlet-triplet gaps. They are thus predicted to be typical TADF systems.