Triplet states of zigzag edged hexagonal graphene molecules C6m**2H6m (m=1, 2, 3, ..., 10) and carbon based magnetism

The geometry and magnetization (spin distribution) of the series of flat hexagonal zigzag edged molecules C6m**2H6m (m = 1,2,..., 10) in their lowest triplet state (S-z = 1) has been calculated using density functional theory and a connection established from the known benzene (m = 1) triplets to the triplets and singlet ground state of the largest molecules (m = 9, 10). The triplet state potential energy surface has two minima corresponding to distortions from the ground state geometry, such that CC bonds bisected by a C-2('') rotation axis are either longer or shorter. For both geometries, the spin on the carbon atoms forms a pattern that peaks at the middle of an edge and for large index (m) values is the same (apart from sign) as the edge pattern of the hexagonally sectored singlet radical ground state of the largest member C600H60. This similarity suggests that the singlet ground state of the larger (m = 9, 10) zigzag edged hexangulenes is possibly a hex-radical, in some ways analogous to the di- and higher multiradical ground state of the linear acenes C4m+2H2m+4 starting around m >= 8 and 9. The spin patterns provide guidance in interpreting the multiradical nature of ground and low lying excited states of large hexangulenes and how magnetism evolves with size in molecules with graphene cores. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3569135]