Close-Packed Dye Molecules in Zeolite Channels Self-Assemble into Supramolecular Nanoladders

Abstract

A tough challenge in nanomaterials chemistry is the determination of the structure of multicomponent nanosystems. Dye-zeolite L composites are building blocks of hierarchically organized multifunctional materials for technological applications. Supramolecular organization inside zeolite L nanochannels, which governs electronic properties, is barely understood. This is especially true for confined close-packed dye molecules, a regime not investigated in applications yet and that might have great potential for future development in this field. Here we realize for the first time composites of zeolite L with maximally-packed fluorenone molecules and elucidate their structure by integrated multi-technique analyses. By IR, thermogravimetric and X-ray diffraction we establish the maximum degree of dye loading obtained (1.5 molecules per unit cell) and by modeling we reveal that at these conditions fluorenone molecules form quasi 1-D supramolecular nanoladders running along the zeolite channels. Spatial and morphological control provided by the nanoporous matrix combined with a complex blend of strong dye-zeolite and weaker dye-dye van der Walls interactions lie at the origin of this unique architecture, which is also stabilized by the hydrogen bond network of co-adsorbed water molecules surrounding the dye nanoladder and penetrating between its rungs.

Unravelling the High-Pressure Behaviour of Dye-Zeolite L Hybrid Materials

“…These columns are connected to form larger circular 12-ring (12 MR) channels and smaller elliptical 8-ring (8 MR) The ZL/FL composites here investigated are those synthesized and previously characterized at ambient pressure (P amb ) by Gigli et al [56]. Specifically, three hybrids at different fluorenone loadings were investigated, containing 0.5, 1 and 1.5 molecules of colorant per unit cell (from now on ZL/0.5FL, ZL/1FL, ZL/1.5FL).…”

“…In the structures of ZL/0.5FL and ZL/1FL at P amb [56] the fluorenone molecules are sited in the 12 MR on the mirror planes parallel to the c axis, statistically occupying only one of the six equivalent positions (Figure 2a,b). The oxygen of the carbonyl group of the FL molecule is strongly interacting with potassium atoms KD located along the walls of the main channel.…”

“…The structure of ZL/1.5FL resulted to be too complex to be unravelled by structural refinement (i.e., due to high symmetry constrains imposed and to the low electronic density of the atoms), thus a theoretical approach was used. Density functional calculations were indeed able to provide a reliable structural model of the packing of fluorenone inside the composite at room pressure conditions: more specifically, the supramolecular organization of the confined dyes consisted of pairs of molecules-positioned roughly on top of each other, with their long axes nearly parallel to the channel axis-alternated by a dye molecule oriented at about 45 • with respect to the zeolite channel axis (Figure 3) [56]. Moreover, calculations evidenced that such a peculiar arrangement-originally called "dye-nanoladder"-was due not only to the structural constraints imposed by the ZL-nanochannel geometry but also to a complex network of intermolecular interactions, among which the dominant one was the coordination of the carbonyl oxygen to the K + extra framework cations (KD)).…”

“…Specifically, we selected the zeolite L (ZL)-fluorenone (FL) hybrid (ZL/FL from now on), subject of previous investigations performed by our group [56]. We are interested in exploring whether the compression could favour a more ordered distribution of the dye molecules in the zeolite L channels and if this process leads to an improvement of the optical properties of the hybrid material.…”

“…We are interested in exploring whether the compression could favour a more ordered distribution of the dye molecules in the zeolite L channels and if this process leads to an improvement of the optical properties of the hybrid material. In fact, it has been shown that the properties of dye-zeolite systems depend on the molecular orientation, arrangement and packing inside the channel [14,15,20,[44][45][46]56,58], which control the guest-guest and host-guest interactions. To unravel and understand at molecular level the compression behaviour of the inclusion composites we adopted an integrated experimental-theoretical approach, based on the use of high pressure (HP) in situ synchrotron X-ray powder diffraction (XRPD) and ab initio molecular dynamics [98] simulations, which already captured a nice example of pressure-induced supramolecular organization in zeolites [97].…”