Dye molecules confined in nanopores: structural characterization of fluorenone in zeolite L

Zeolite L (ZL) framework - characterized by one-dimensional 12-ring channels - is a versatile host material for the supramolecular organization of cromophore molecules. The inclusion of dyes molecules into the channels is particularly intriguing, as the resulting host-guest compounds may exhibit unique properties, such as efficient energy transfer [1-3]. The lack of detailed structural information about the orientation and alignment of the neutral dye fluorenone (FL) in the ZL pores suggested this study, aimed to understanding the functionality of these host-guest systems from the structural point of view. The ZL/FL hybrid complex was investigated by synchrotron X-ray powder diffraction experiments (Fig. 1) at GILDA beamline (ESRF, Grenoble). The loading of the dye on previously dehydrating the zeolite L was carried out by using gas-phase adsorption under vacuum, so to assure that water molecules did not block the pathway for molecule entering [4]. The diffraction data evidenced the embedding of FL into the channels, as well as the minor presence (7.4 %) of fluorenone molecules on the zeolite surface. As a consequence, the structure of FL-ZL hybrid was determined by a two-phases Rietveld refinement, in the hexagonal space group P6/mmm [5]. The relevant incorporation of FL into the channels of the dehydrated ZL was confirmed by a significant change of the unit cell parameters. 1.8 FL molecules per unit cell were located near the walls of the large 12-membered ring channel (Fig. 2). A strong interaction between FL carbonyl group and the extraframework potassium cation is suggested by the short bond distances, and explains why FL is not displaced by water molecules when the FL-ZL hybrid is re-exposed to the air. Our experimental results are in good agreement with those obtained by molecular dynamics simulations on a less loaded hybrid [6] and will be compared with new computational data presently in progress.