Singlet-triplet (S0 → T1) excitation energies of the [4 × n] rectangular graphene nanoribbon series (n = 2-6): A comparative theoretical study

► [4 × n] rectangular graphene nanoribbon singlet-triplet excitation energies calculated. ► Range of model chemistries and basis sets employed. ► Significant variability dependent on model chemistry. ► Only B2PLYP density functional yields positive ES-T for all nanoribbons. ► Vanishingly small ES-T at polymeric limit for any [m × n] graphene nanoribbon.