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 C 2-axis tilted away from the surface normal on angle θ distributed in the range of 31−39°. The rotational distribution around the C 2-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.

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Mixed insoluble monolayers of bis(N-ethyl,N-octadecyl)rhodamine perchlorate (RhC18) and arachidic acid (ArAc) at the air−water interface were characterized by surface pressure−area isotherm, second harmonic generation (SHG), and reflection spectroscopy studies. The analysis of surface pressure−area isotherms of mixed RhC18/ArAc layers provided evidence that there was a high degree of miscibility of film components and a strong interaction among them. A new band was found to appear on the blue side of the reflection spectrum after an increase in both the concentration of ArAc in the layer and the applied surface pressure. This new band was assigned to dye aggregates, where spontaneous formation in the gaslike region was promoted by the presence of ArAc and induced by compression in the region of the continuous monolayer. Both in gaslike and in compressed film, the formation of blue-shifted aggregates coincided with the increase of the SHG response of the monolayer. Resonance SHG enhancement was proven to be the main reason for this observation due to electronic resonance of aggregates with the incident light frequency.

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