Orientation of adsorbed dyes in the interlayer space of clays.: 2 -: Fluorescence polarization of rhodamine 6G in laponite films

A new fluorescence polarization method is applied to evaluate the preferential orientation of rhodamine 6G (R6G) adsorbed into laponite (Lap) clay mineral in supported films. For this purpose, the fluorescence spectra for the horizontal and vertical directions of the emission polarizer were recorded for a common horizontally polarized excitation light as a function of the twist angle delta of the film with respect to the excitation (and emission) beam. The orientation of the fluorescent species adsorbed in the clay films can be evaluated from the linear relationship between the fluorescence dichroic ratio and the twist angle 6 recently established.(1.2) For low dye-loading R6G, molecules are adsorbed into the Lap surface as monomeric units with a tilt psi angle of 28 degrees with respect to the clay layer, in agreement with results obtained by visible absorption with polarized light. Fluorescent coplanar J type dimers of R6G are also observed in Lap films for moderate dye loadings with a similar orientation angle around 30 degrees. The presence of coplanar R6G H type dimers and high-order aggregates in moderate- and high-loading R6G/Lap films causes a depolarization for the fluorescence anisotropy of the J dimer, ascribed to an excitation energy-transfer process and reducing the applicability of this fluorescence method. However, present experimental results confirm a more-perpendicular disposition of these H aggregates with respect to the Lap layer. The new fluorescence polarization method can be extrapolated to determine the orientation of any adsorbed fluorophore in rigid and ordered 2D host materials.