Compression of the dye monolayer at the air/water interface studied by second harmonic generation Original Research Article

The second harmonic (SH) generation efficiency upon compression of insoluble rhodamine C18 and fluorescein O322 monolayers at the air/water interface has been measured. The SH intensity dependence on area per molecule clearly varied for four main regions: expanded monolayer, expanded–condensed transition region, condensed monolayer and collapse point. This SH intensity dependence was similar with the π–A isotherm for both molecules. The SH intensity change would be explained with a model of interactions of dye chromophores based on the dipole–dipole correlation, CH-π interaction, aggregates of electronic transition moments and formation of large order π-conjugated system.