Chirality in molecular films at the air-water interface

Second Harmonic Generation was used to study the optical properties of molecular films formed at the air-water interface. The technique was first applied to a two-dimensional film of 4-(4-dihexadecylaminostyryl)-N-methylpyridinium iodide (DiA) formed at the air-water interface in a Langmuir trough. The SHG intensity was measured as a function of the incident fundamental and outgoing harmonic wave polarization angles at different states of the monolayer compression. The film revealed chirality arising from the formation of molecular aggregates and it was demonstrated that this chirality property arose from the coupling of the electric and magnetic fields at the fundamental frequency. In a second stage, we applied the SHG technique to follow the emergence of chirality during the compression of a molecular film of the amphiphilic compound 5-(octadecyloxy)-2-(2-thiazolylazo) phenol (TARC18). A similar behavior was observed and it was shown that the same origin was responsible for the appearance of chirality. Polarization resolved intensity measurements demonstrated that the magnetic dipole contribution was the dominant one.