Computational design of B,N-substituted graphene ribbons exhibiting quasi-degenerate S1 and T1 states and high fluorescence rates

B,N-substituted graphene ribbons are computationally designed and their spectroscopic properties are systematically explored with wave-function based electronic-structure methods. All B,N-graphene ribbons exhibit exceptionally small S1-T1 energy gaps. The oscillator strength of the S1-S0 transition increases monotonically with the length of the ribbons. Some B,N-graphene ribbons of intermediate length (5 – 10 rings along the main axis) exhibit a negative singlet-triplet energy gap concurrently with a substantial oscillator strength. The calculated emission energies provide evidence of high rigidity of the ribbons and therefore narrow absorption/emission profiles and low radiationless quenching rates of the emissive S1 state.