Molecular Orientation of Rhodamine Dyes on Surfaces of Layered Silicates

Abstract

Films of the layered silicates fluorohectorite (FH) and saponite (Sap) with various rhodamine dyes were prepared. The dyes with acidic as well as large hydrophobic groups in their molecule were not adsorbed on the surface of FH, which was interpreted in terms of high charge density on the surface of this silicate. All adsorbed dyes formed similar forms, such as isolated cations and H-type molecular aggregates, which were characterized by different spectral properties. Polarized ultraviolet-visible (UV-vis) spectroscopy was used for the characterization of the molecular orientation of dye chromophores on the silicate surface. The isolated dye cations and species, which absorbed light at the low energy part of the spectra, were only slightly tilted with respect to the plane of the silicate surface. The cations forming H-aggregates and absorbing light at low wavelengths were oriented in a nearly perpendicular fashion. The nearly perpendicular orientation was observed as a strong increase of dichroic ratio with film tilting. The orientation of the cations in H-aggregates depends partially on the structure of the dye molecule, namely, on the type of amino group (primary, secondary, or tertiary) in the dye molecule. The type of amino groups probably plays a role in the suitable orientation of dye cations for effective electrostatic interaction between the cations and the negatively charged siloxane surface. X-ray powder diffraction could not distinguish dye phases of dye monomers and molecular aggregates.

Fluorescence resonance energy transfer between two cationic laser dyes in presence of the series of reduced-charge montmorillonites: Effect of the layer charge

“…The intensity of the band assigned to the H-aggregates increased with time. This trend is similar to that observed for rhodamine dyes [38] but opposite to the trends described for methylene blue/RCM dispersions [12,32]. For MB cations, the H-aggregates are formed at the very beginning of the reaction, probably in the zones of the colloidal electric double layer, and are thermodynamically unstable.…”

“…There are two possible explanation of this fact: (1) The amount of the aggregates is very small and not distinguished by absorption spectroscopy. The small tendency to form molecular assemblies with variable optical properties can be due to the complicated molecular structure and shape of R3B cations [38]. (2) R3B cations form aggregates in amounts similar to Ox4, but with an imperfect sandwich-type association (twisted H-type assemblies).…”

Energy transfer between rhodamine 3B and oxazine 4 in synthetic-saponite dispersions and films

“…These changes could be assigned to negligible molecular aggregation. The tendency of the aggregation in the smectite dispersions depends on the hydrophobic character of the dye and the properties of the smectite [21]. In contrast to the aggregation of methylene blue [28], the aggregation of the xanthene dyes increases with dispersion aging [21].…”

“…The tendency of the aggregation in the smectite dispersions depends on the hydrophobic character of the dye and the properties of the smectite [21]. In contrast to the aggregation of methylene blue [28], the aggregation of the xanthene dyes increases with dispersion aging [21]. On the other hand, Sum is a synthetic smectite, which is characterized by a rather low-density negative charge, which generally suppresses the aggregation of cationic dyes [28].…”

“…One could mention materials prepared using silica gels [14], glass [15], or more complex polymer-filled microporous glass [16]. Numerous studies have dealt with the interaction of rhodamine dyes with clay minerals in dispersions (e.g., [17]) or in self-assembled films (e.g., [18][19][20][21]). Ras et al [22] studied the molecular organization of rhodamine with hydrophobic octadecyl groups in the molecules in hybrid Langmuir-Blodgett layers also including single-layer coverage by clay mineral particles.…”