Orientation and aggregation behavior of rhodamine dye in insoluble film at the air-water interface under compression. Second harmonic generation and spectroscopic studies

The absorption and fluorescence spectra and second harmonic generation (SHG) of the insoluble monolayer of bis-(N-ethyl,N-octadecyl)rhodamine (RhC18) at the air-water interface have been measured. These spectra were affected significantly by compression, and the observed changes were ascribed to the formation and structural rearrangement of aggregated species on the water surface during compression. The spectroscopic behavior of the monolayer was explained in accordance with its rheological properties, and the transition from disordered monomers to dimers, from dimers to aggregates, and from aggregates to two-dimensional arrays was proposed. SHG studies revealed that the RhC₁₈ molecules in the expanded film region are oriented with their C₂-axis tilted away from the surface normal on angle θ distributed in the range of 31-39°. The rotational distribution around the C₂-axis was assumed to be 45-60° according to preferable intermolecular interactions with the water subphase and surrounding molecules. The θ angle distribution became slightly narrow because of the increase of molecular ordering caused by two-dimensional external pressure. The sharp increase of SHG intensity and the phase shift observed at high compression were ascribed to the formation of blue-shifted aggregates with their electronic transition being in resonance with the incident laser frequency. The results of spectroscopic and SHG studies were jointly analyzed, and the structural rearrangement within the monolayer during compression was described.