New fluorescent polarization method to evaluate the orientation of adsorbed molecules in uniaxial 2D layered materials

A new fluorescence polarization method is developed as a powerful tool to evaluate the preferential orientation of Rhodamine 6G (R6G) molecules adsorbed in the two-dimensional (2D) interlayer space of Laponite (Lap) clay films. The fluorescence anisotropy of a very low R6G loading film, around the 0.1% of the total cation exchange capacity (% CEC) of Lap, reveals that the dye is adsorbed as monomeric units with a preferential orientation angle of 62° with respect to the normal to the clay layers. This tilted angle agrees with that previously obtained by absorption spectroscopy with linearly polarized light [V. Martínez Martínez, F. López Arbeloa, J. Bañuelos Prieto, I. López Arbeloa, Chem. Mater. 17 (2005) 4134–4141], justifying the validity of the present emission technique. The fluorescent method can be applied to evaluate the orientation of any fluorescent molecules preferentially adsorbed along the normal of the plane of any rigid 2D host arrays.