Comparative study of carbon and BN nanographenes: Ground electronic states and energy gap engineering

The relationship between stabilities and shape configurations of carbon and boron nitride (BN) nanographenes (NGs) was studied at the B3LYP/6-31G* level of theory. The HOMO-LUMO energy gaps of rectangular-shaped carbon nanographenes (CNGs) decrease as the graphene sizes increase with a direct inverse dependence on the length of zigzag edge. Due to the double zigzag edge boundaries, the CNGs with long zigzag edges have open-shell singlet ground states; in contrast, the HOMO-LUMO energy gaps of BN nanographenes (BNNGs) have a weak dependence with size; all BNNGs have closed-shell singlet ground states, and those with long zigzag edges have slightly larger energy gaps. CNGs with long zigzag edges are less favorable energetically than their structural isomers with long armchair edges, while the BNNGs have the opposite preference. Chemical modifications that change the long zigzag edge into armchair type can efficiently stabilize the kinetically unstable CNGs (with open-shell singlet ground states) and modify their energy gaps.