The discovery of metal-free organic chromophores which can serve as efficient emitters in organic light emitting diodes (OLEDs) has transformed the materials science of optoelectronic devices in recent years. In OLED emitters, the lowest singlet (S1) and triplet (T1) excited electronic states are populated by the recombination of injected electrons and holes in the ratio 3:1 according to spin statistics. The high population of the T1 state is unfavorable for achieving high fluorescence quantum yields. However, if the S1-T1 energy gap, ΔST = ES1 -ET1, is sufficiently small (that is, in the range of thermal energies), the S1 state can be populated from the lower-lying T1 state by the process of reverse intersystem crossing (RISC) at room temperature. The thermal conversion of the triplet population to the singlet population enhances the fluorescence yield. Emitters relying on the RISC process were pioneered by Adachi and coworkers and are known as thermally activated delayed fluorescence (TADF) emitters. A particularly versatile class of TADF emitters was synthesized and spectroscopically characterized by Hatakeyama and coworkers. In these planar heteroaromatic compounds, B and N heteroatoms are arranged in a manner such that the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are localized on alternate atoms, which was termed the "multiple resonance effect". The exceptionally small spatial overlap of HOMO and LUMO in these compounds results in a small exchange matrix element and therefore in a small S1-T1 gap. The prototypical Hatakeyama compound (DABNA-1) is a diazaboranaphtoanthracene and exhibits a ΔST of the order of 0.15 eV. The suppression of the HOMO-LUMO spatial overlap by the multiple resonance effect necessarily also suppresses the oscillator strength of the S1-S0 radiative transition, which is unfavorable for efficient fluorescence from the S1 state. Numerous modifications and extensions of DABNA-1 were synthesized, spectroscopically characterized and tested in OLED devices. In computational studies, several multiple-resonance compounds were found which exhibit a good compromise between a low ΔST and a reasonably large S1-S0 oscillator strength. In recent extensive computational studies, the vertical electronic excitation energies, singlet-triplet gaps and S1-S0 transition moments of a large variety of multiple-resonance-type chromophores were explored by varying the size of the compounds or the distribution of B,N heteroatoms. Graphene nanoribbons are narrow sheets of graphene with nanoscale widths. They have attracted considerable attention in the fields of nanoelectronics and optoelectronics. It has been shown that graphene ribbons (GRs) can be synthesized in a bottom-up manner from organic precursors. Atomically precise B-substitution of GRs also has been demonstrated. The electronic properties and optical spectra of GRs were investigated with computational methods and were shown to be sensitive to the width and edge structure of the GRs. In the present work, the effects of B,N substitutions on the spectroscopic properties of GRs are explored with electronic-structure calculations. The distribution of the B and N heteroatoms and the size (length) of the sheets are systematically varied. The excitations energies, singlet-triplet gaps and S1-S0 oscillator strengths are computed in order to provide insight into the suitability of these so far unexplored class of compounds as TADF emitters. Four types of carbon-boron-nitride (CBN) ribbons are considered in the present work. The structures are denoted as CBN_n-m, where n = 1 … 4 denotes the type of the GR and m = 1, 2, 3 … denotes the length of the GR (the number of unit cells). The GRs are terminated at both ends to generate closed-shell systems. For the CBN_1 and CBN_3 ribbons, the left and right termini are different, while they are the same for the CBN_2 and CBN_4 ribbons. The CBN_3 and CBN_4 ribbons differ from the CBN-1 and CBN_2 ribbons by the replacement of the N-atoms in the upper rim by CH groups. The structures of the first two members of the four types of ribbons are shown in Scheme 1. The complete list of structures considered in the present work is displayed in Schemes S1 and S2 in the supporting information (SI).