Microscopic organization of long-chain rhodamine molecules in monolayers at the air/water interface

The monolayer behavior of a rhodamine derivative with two C₁₈ aliphatic chains attached to the nitrogen atoms of the xanthene ring system (RhC₁₈) was studied by epifluorescence microscopy, second-harmonic generation (SHG), and absorption and fluorescence spectroscopy. The isotherm of RhC₁₈ exhibits a plateau which, presumably, corresponds to a slow collapse of the monolayer. As observed by epifluorescence microscopy, the RhC₁₈ monolayer remained homogeneous for a large range of molecular areas, and an abrupt change in the monolayer morphology occurred at the end of the surface pressure isotherm. Spectroscopic data showed that both fluorescent J-aggregates and nonfluorescent H-aggregates with a coplanar-inclined configuration were formed within the RhC₁₈ monolayer upon compression. The orientational SHG measurements revealed that, in the expanded region of the isotherm, the RhC₁₈ chromophore was oriented in a way where its xanthene plane made an angle of 36° with respect to the interface. The azimuthal angle SHG measurements revealed that compression induced an anisotropic arrangement of the S₀-S₁ transition moment of RhC₁₈ chromophores perpendicular to the compression direction. At the end of the isotherm prior to and after the collapse point, the film possessed a fairly regular structure characterized by a C_2v symmetry packing and a long-range order in the parallel alignment of the RhC₁₈ chromophores. Microscopic organization of the RhC₁₈ molecules in monolayers is discussed.