▎ 摘　　要

Singlet-triplet (S-0 -> T1) well-to-well (WWES-T) and vertical (VES-T) excitation energies of the [4 x n] rectangular graphene nanoribbon series (n = 2-6) were estimated using various semiempirical, Hartree-Fock (HF), density functional (DFT), and second order Moller-Plesset perturbation theory methods with the assumption of a closed-shell singlet state. Significant model chemistry dependent variability in theoretically obtained WWES-T/VES-T is evident for the rectangular graphene nanoribbons. With the exception of the B2PLYP density functional (which, along with the mPW2PLYP functional, combines exact HF exchange with an MP2-like correlation to the DFT calculation), all DFT, semiempirical, and HE methods investigated predict the onset of a negative WWES-T/VES-T (ground state triplet) starting somewhere between the [4 x 3] through [4 x 61 derivatives, with most functionals predicting a transition from a singlet to triplet ground state between the [4 x 4] and [4 x 5] rectangular graphene nanoribbons. Consistent with previous work on the n-acene series. MP2 WWES-T/VES-T estimates have a significant positive systematic bias and HF estimates have substantial negative systematic biases. Extrapolation of the B2PLYP results, which are in excellent agreement with prior FPA-QZ VES-T estimates, for any [in x n] rectangular graphene nanoribbon derivatives predicts a vanishingly small singlet-triplet gap at the polymeric limit (m -> infinity and/or n -> infinity). (C) 2011 Elsevier B.V. All rights reserved.