We study second-order nonlinear optical properties of Langmuir-Blodgett films of two chiral helicenebisquinones. We compare the second harmonic efficiency of both materials and we demonstrate that the second harmonic signal generated by a Langmuir-Blodgett film from the nonracemic compounds is three orders of magnitude higher than the signal from a similar film of the racemic compound. We explain this by demonstrating that the chiral contributions to the nonlinearity are dominant. In addition, we discuss the symmetry of the samples and its consequence on the analysis of the second harmonic response. Furthermore, we study the thermal and temporal stability of the Langmuir-Blodgett films of both helicenes. We also show that one of the materials is potentially useful for constructing quasi-phase-matched structures for frequency conversion. (C) 2000 Elsevier Science S.A. All rights reserved.