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

The understanding of the behavior of molecules inside porous materials under the combined effects of applied pressure, spatial confinement and constraints in morphology at the nanoscale is a challenge which has never been endeavored so far. One of the purposes of “ImPact” Project (financed by Italian MIUR, FIRB- Futuro in Ricerca 2013-2016) is to understand at atomistic level the supramolecular organization of different molecules in zeolites frameworks. In particular, the pressure-induced modifications of functional properties of hybrid materials (e.g. Zeolite L + organic colorants), currently applied in strategic areas, from sustainable energy technologies to biomedical sciences, are studied. Neutral dye fluorenone (FL) is able to form a stable host-guest complex with Zeolite L(ZL) - an appealing and excellent host for the supramolecular organization- and has received a considerable attention since it is a component of the energy relay system in artificial antennas [1,2]. Nevertheless, relatively few structural information, even at Pamb, is available about the orientation and alignment of the dye molecules in the zeolite pores. On the other hand, theoretical studies have shown that the orientation of fluorenone and the presence of water are parameters that strongly influence the light harvesting properties of this host-guest complex [3]. Hence, a detailed structural characterization is of great importance for understanding the functionality of these host-guest systems. In this study, three different FL/K-ZL materials have been synthesized by an increasing loading of FL in ratio of 0.5, 1.0, 1.5 and 2.0 molecules/unit cell. The samples obtained were characterized by means of X-ray powder diffraction, thermo-gravimetric analysis, IR and UV-vis spectroscopies at P amb. The incorporation of FL into the zeolite channels was confirmed by a significant change of the unit cell parameters and by drastic decrease in the ZL surface area also at low FL loading (Figure 1). The strong interaction between FL carbonyl group and the extraframework potassium cation, predicted by theoretical modelling [1], was confirmed by the short bond distances (2.77 Å), evidenced in the Rietveld refined structure, and by the shift of the C=O stretching frequency evidenced in the IR spectra. Such an interaction explains why FL is not displaced by water molecules when FL/ZL hybrid is re-exposed to the air [1]. Interestingly, although the UV-vis absorption spectrum was almost unaffected by the FL loading, the corresponding emission spectrum evidenced a strong influence: the optimum FL/K-LTL ratio was then determined in order to optimize the performances of the device as light harvester. The structural information obtained theoretically and from XRD allowed also to explain the loading dependence of the optical properties of the material and to correlate it with the relative orientation of the fluorenone molecules in the zeolite channels.