Compression induced chirality in dense molecular films at the air-water interface probed by second harmonic generation

Surface second harmonic generation was used to study the nonlinear optical properties of a two-dimensional film of 4-(4-(dihexadecylamino)styryl)-N-methylpyridinium iodide (DiA) formed at the air-water interface in a Langmuir trough. The second harmonic intensity was measured as a function of the incident fundamental and outgoing harmonic wave polarization angles during the monolayer compression. Below a critical average density of 0.55 nmol/cm(2), the film is in a liquid-expanded state and the DiA molecules are strongly tilted on the interface. Above this critical average surface density, the film reveals chirality arising from the formation of molecular aggregates at the interface. It is demonstrated through a full analysis of the data that the origin of this chiral property of the film arises from the coupling of the electric and magnetic fields at the fundamental frequency.