Computational Design of B,N-Substituted Graphene Ribbons Exhibiting S1 and T1 States with Near-Degeneracy and High Oscillator Strengths

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 S₀–S₁ 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.