For a more detailed investigation of the electronic structure, we selected the ribbons CBN_1-2 and CBN_2-2. For both systems, the S1 and T1 states are nearly pure HOMO-LUMO (H-L) excitations. In CBN_1-2, the weight of the H-L configuration is 96 % in the S1 state and 98 % in the T1 state. In CBN_2-2, the corresponding weights are 93 % and 98 %. The lower weight of the H-L configuration in the S1 states reflects the higher contribution of doubly excited configurations in the S1 states, which stabilizes the energies of the singlet states relative to the triplet states, leading to singlet-triplet inversion. The HOMO and LUMO of CBN_1-2 and CBN_2-2 are displayed in Fig. . The orbitals of the ribbons with five units (CBN_1-5 and CBN_2-5) are shown in Fig. . It can be seen that the HOMO and the LUMO are preferentially localized on alternate atoms, as is typical for CBN compounds (so-called multi-resonance effect). This property of HOMO and LUMO is responsible for an exceptionally small exchange integral which is a necessary requirement for S1-T1 degeneracy. Upon closer inspection, it can be seen, however, that the spatial overlap of HOMO and LUMO is not exactly zero. There exist small overlaps especially on the left and right termini. This feature is more pronounced for CBN_2-2 than for CBN_1-2, which explains why the singlet-triplet inversion is more pronounced in the CBN_1-m series than in CBN_2-m series (Fig. ). On the other hand, a nonzero spatial overlap of HOMO and LUMO is favourable for the transition dipole moment and therefore enhances the oscillator strength of the S1-S0 transition. This explains the larger oscillator strengths of the CBN_2-m series compared to the CBN_1-m series (Fig. ).