We measured nonlinear optical activity in second harmonic generation (SHG) by reflection on a thin film of chiral molecules. We selected the structure of our molecules in order to obtain very large chiroptical effects. We indeed measured an angle of rotation of the second harmonic polarization as large as 66degrees for an acridine substituted Troger base. This is in good agreement with our theoretical expectations for such an excitonic coupling chirality. (C) 2002 Elsevier Science B.V. All rights reserved.