Molecular Orientation, Aggregation, and Order in Rhodamine Films at the Fused Silica/Air Interface

Abstract

We report optical second harmonic generation studies of the organic dye molecule rhodamine 6G spin cast on fused silica surfaces. The concentration dependence of the second harmonic response demonstrates oscillatory behavior with a period corresponding to the concentration required for monolayer surface coverage. This behavior reflects the formation of ordered molecular adlayers which persist for approximately five periods. Polarized SHG studies confirm orientational anisotropy of the dye molecules and allow the orientation within adjacent layers to be determined. Optical absorbance measurements of the films indicate the onset of rhodamine 6G aggregate formation at surface coverages of approximately one monolayer. However, the onset of dimer or aggregate fluorescence is observed to occur only at much higher surface coverages, consistent with the loss of orientational order within the adlayers. Our results indicate strong adsorbate-substrate interaction which gives rise to orientational anisotropy within the first molecular layer. Well-defined order within subsequent layers is determined by interlayer adsorbate-adsorbate aggregation and decays on a length scale of several molecular diameters. These results provide a direct measure of the extent of interfacial ordering at the solid/air interface.

Highly fluorescent colloids based on rhodamine 6G, modified layered silicate, and organic solvent

“…Similar assemblies were observed after the adsorption of rhodamines on various substrates, that is, silica colloids [29,30], or in solid hybrid materials [31]. Recently, the formation of photoactive J-aggregates in hybrid systems with layered silicates, such as colloids and solids, was also reported [32,33].…”

Electropolymerization of Preadsorbed Layers of Some Azine Redox Dyes on Graphite

“…Figure 5 shows the changes in luminescence intensity, obtained by potential cycling at a low scan rate. In the anodic potential scan first a drop in luminescence intensity was observed in the potential region 0.20-0.25 V. This drop can be tentatively ascribed as associated with the formation of nonluminescent H-dimers (29,30). The second drop of luminescence intensity was observed in a sufficiently positive potential region from 0.65 V to 0.90 V. The potential value of 0.65 V coincides very well with the onset of a polymerization process observable in cyclic voltammograms (Fig.…”

“…It is a well-known tendency of azine dyes to form dimers and higher aggregates at higher concentrations (29). A new blue-shifted absorption band centered at approximately 580 nm appeared when the concentration of NB was increased up to 0.9 × 10 −4 M. This blue-shifted band is characteristic of the parallel, sandwich-type dimers (H-type dimers) (29,30). The luminescence spectrum from the solution with high NB concentration was essentially the same as that observed at low (0.9 × 10 −6 M) concentration, indicating that luminescence arises from the monomers, as H-type dimers do not luminescence (30).…”

“…A new blue-shifted absorption band centered at approximately 580 nm appeared when the concentration of NB was increased up to 0.9 × 10 −4 M. This blue-shifted band is characteristic of the parallel, sandwich-type dimers (H-type dimers) (29,30). The luminescence spectrum from the solution with high NB concentration was essentially the same as that observed at low (0.9 × 10 −6 M) concentration, indicating that luminescence arises from the monomers, as H-type dimers do not luminescence (30). The observation of a luminescence spectrum from the adsorbed NB at a graphite electrode identical with that of a low concentration of NB in the solution phase indicates the presence of monomers in the adsorbed state.…”

Single-molecule fluorescence spectroscopy in (bio)catalysis

“…In an experimental catalytic context, it is extremely challenging to attempt to relate catalytic or sorption behavior to local orientation at the single site because doing so requires that the orientation of the surrounding matrix, e.g., the protein structure, or the pore structure of a solid catalyst, can be controlled and measured as a framework of reference. Measurements of average orientation with respect to planar surfaces are possible by using, for instance, second harmonic generation, but clearly such results pertain to ensemble averages (61). For fluorescent single molecules in a polymer film, heterogeneities in rotational dynamics have been observed (62).…”