Oxidation effects on the hyper-Rayleigh scattering (HRS) second-order nonlinear optical responses of homo- and hetero-helicenes have been investigated using the time-dependent Hartree-Fock approach and the Austin Model I semi-empirical Hamiltonian. Upon oxidation, this approach predicts a substantial increase of the HRS response, which, with the exception of tetrathia-[7]-helicene and pentathia-[9]-helicene, is accompanied by a decrease of the depolarization ratio. In the case of helicenes bearing thiophene units, the HRS increase is associated with the presence of a substantial axial contribution to the first hyperpolarizability, whereas the radial component is otherwise dominant. (c) 2005 Elsevier B.V. All rights reserved.