Polyacene molecules are constrained to rotate about the principal axis associated with the diagonal component of the molecular polar-izability tensor that is intermediate in size. The rotation axes are oriented vertically ('azimuthal rotors') and are attached to a covalent monolayer grid such as that recently reported in the literature. We calculate the value of the relevant component of the third-order electric susceptibility of the system for a variety of grid spacings and determine that the optical nonlinearity of such a system is not particularly large, at best approaching that of carbon disulfide. Published by Elsevier B.V.