Inhomogeneous and Spontaneous Formation of Chirality in the Langmuir Monolayer of Achiral Molecules at the Air/Water Interface Probed by In Situ Surface Second Harmonic Generation Linear Dichroism

Abstract

The chirality of the Langmuir monolayer at different surface densities formed with achiral 5-octadecyloxy-2-(2-pyridylazo)phenol (PARC18) molecules at the air/water interface was measured with the in situ optical second harmonic generation linear dichroism (SHG-LD) technique. Using the s-polarization detection curve in SHG-LD, the degree of chiral excess (DCE) in the Langmuir monolayer can be determined as accurately as 2%. We found that the PARC18 Langmuir monolayer was chiral and the degree of chirality varied randomly between the two opposite chiral states in the same monolayer at different surface densities and at different locations, suggesting coexistence of the various enantiomers simultaneously in the PARC18 Langmuir monolayer. This inhomogeneity of the monolayer chirality was independent of the laser power and wavelength as well as the surface density and the surface pressure, and the chiral domains distributed randomly in the Langmuir monolayer. We also found that the chirality in the PARC18 Langmuir monolayer was spontaneously formed upon molecular aggregation, instead of being compression induced. There was no apparent magnetic dipole contribution observed throughout the different surface densities in the PARC18 Langmuir monolayer. These results indicate that the SHG-LD technique is not only an effective in situ probe of the interface chirality but also an effective tool to study the dynamic processes in the close-packed monolayer or in the self-assembled films.