The electronic and molecular structures of the two-dimensional septet-state molecules(S=3), 1,3,5-tris(p-methoxyphenylmethylene)benzene A and 1,3,5-tris(phenylmethylene)benzene B, were investigated by single-crystal ESR spectroscopy. A was designed for a prototypical core model for discotic liquid-crystalline organic high-spin molecules. We determined the fine-structure constants of A by the ESR spectroscopy and analyzed the crystal and molecular structures of B by X-ray diffraction. The effect of the methoxy functional group on the fine-structure constants was found : the effect showed up in the significant reduction of the fine-structure constant D value for A, indicating the modification of the topologically controlled robust pi spin polarization. The observed non-vanishing E value is in accord with the molecular symmetry and crystal structure of the host molecule, 1,3,5-tris(alpha-benzoyl)benzene.