Molecular arrangement of rhodamine 6G cations in the films of layered silicates:: the effect of the layer charge

Montmorillonite nanocor saturated with Li+ cations was used for the synthesis of reduced charge montmorillonites (RCMs). The films of RCMs with intercalated rhodamine 6G (R6G) were prepared by adsorption of the dye on montmorillonite film deposited on a quartz slide. Penetration of the dye into the interlayer spaces of silicates was monitored by elemental analysis and X-ray diffraction measurements of the prepared R6G/silicate films. Polarized UV-Vis spectroscopy was used for the characterization of the molecular orientation of dye cations on silicate surface. There were several species of R6G cations in the interlayer spaces of the inorganic host, which was easily resolvable by the polarized spectra. The cations forming H-aggregates and absorbing light at low wavelengths were oriented in nearly perpendicular fashion, which was observed as a strongly positive dichroism, i.e., the increase of the dichroic ratio with film tilting. Adsorbed isolated dye cations were inclined at low angles with respect to the plane of the silicate surface. The layer charge of the inorganic host affected only partially the orientation of dye cations in the H-aggregates, but rather it controlled the extent of aggregation and in this way the number of nearly perpendicularly oriented cations.