Confinement of dye molecules in nanopores: structural characterization of fluorenone in zeolite L by synchrotron X-ray powder diffraction.

Zeolite L (LTL framework type) is an appealing and excellent host for the supramolecular organization of different kinds of molecules and complexes. The capacity of this zeolite framework - characterized by monodimensional 12-ring channels - to induce an ordered geometrical arrangement of dye molecules, is used for the design of antenna systems and charge-transfer complexes. Relatively few structural information is available about the orientation and alignment of the dye molecules in the zeolite pores, hence a detailed structural characterization is of great importance for understanding the functionality of these host-guest systems. In this study, the neutral dye fluorenone (FL) was inserted - by using gas-phase adsorption under vacuum - into a synthetic potassium zeolite L, previously dehydrated so to assure that water molecules do not block the pathway for molecule entering. Synchrotron X-ray powder diffraction experiments were performed at Gilda beamline (ESRF, Grenoble) on both dehydrated and FL-loaded zeolite L (FL-L). The diffraction data clearly evidenced the embedding of the dye 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-L was determined by a two-phases Rietveld refinement in the hexagonal space group P6/mmm. The relevant incorporation of FL into the channels of dehydrated L 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. 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 FL-L hybrid is re-exposed to the air. Our experimental results are in agreements with those obtained by molecular dynamics simulations on a less loaded hybrid, and explain the high stability under ambient condition of this material.