1,721,055 research outputs found
Supramolecular Organization in Confined Nanospaces
No full textEmpty spaces are abhorred by nature, which immediately rushes in to fill the void. Humans have learnt pretty well how to make ordered empty nanocontainers, and to get useful products out of them. When such an order is imparted to molecules, new properties may appear, often yielding advanced applications. This review illustrates how the organized void space inherently present in various materials: zeolites, clathrates, mesoporous silica/organosilica, and metal organic frameworks (MOF), for example, can be exploited to create confined, organized, and self-assembled supramolecular structures of low dimensionality. Features of the confining matrices relevant to organization are presented with special focus on molecular-level aspects. Selected examples of confined supramolecular assemblies – from small molecules to quantum dots or luminescent species – are aimed to show the complexity and potential of this approach. Natural confinement (minerals) and hyperconfinement (high pressure) provide further opportunities to understand and master the atomistic-level interactions governing supramolecular organization under nanospace restrictions
Structure of Nanochannel Entrances in Stopcock-Functionalized Zeolite L Composites
Full text linkAbstract
Multifunctional hybrid materials are obtained by modifying zeolite L (ZL) with stopcock molecules, consisting of a tail group that can enter the ZL nanochannels and a head group too large to pass the channel opening. However, to date no microscopic-level structural information on modified ZL materials has been reported. Herein we draw atomistic pictures of channel openings and stopcock-functionalized ZL based on first-principles calculations. We elucidate the interactions of the tail group with the inner surface of ZL channels and the space-filling properties of the stopcocks, revealing cork- or lid-sealing modes. Water is essential to obtain stable modifications. Al[BOND]OH groups are the preferred modification sites, bipodal modifications suffer from strain, and tripod binding is ruled out. Our results suggest the viability of recursive functionalization by cross-linking.Neues vom Flaschenhals: Strukturinformationen zur ortsspezifischen Funktionalisierung von Zeolith L (ZL) auf Mikrometerebene sind rar. Deshalb wurde anhand von First-principles-Rechnungen untersucht, wie Stopfen-Moleküle ZL irreversibel modifizieren, indem sie mit OH-Gruppen an dessen Kanalöffnungen kondensieren. Die Wechselwirkungen zwischen Schwanzgruppe und Kanalinnenwand sowie Kopfgruppe und Kanaleingang werden beschrieben
Interactions, Behavior, And Stability of Fluorenone inside Zeolite Nanochannels
No full textThe development of functional materials based on the supramolecular organization of photoactive species in nanosized porous matrices requires a deep knowledge of host−guest interactions and of their influence on material properties and stability. Extensive first-principles investigations on the fluorescent dye fluorenone inside zeolite L, both at dry conditions and in the presence of water, have unraveled the molecular origin of the peculiar stability of this composite in humid environments, a fundamental prerequisite for practical applications. Results of first-principles molecular dynamics simulations, structural optimizations, and TDDFT calculations, validated by comparison with experimental data, provide a comprehensive picture of the structure, energetics, electronic excitation properties, and room-temperature behavior of the fluorenone/zeolite L composite and predict a substantial optical anisotropy for this material also maintained upon contact with water. The interaction of the fluorenone carbonyl group with the zeolite extraframework potassium cations is responsible for the dye stabilization in zeolite L nanochannels and features itself as a general leitmotiv regarding important properties of carbonyl functionalized photoactive species in hydrophilic matrices
Structure and dynamics of a Bronsted acid site in a zeolite: An ab initio study of hydrogen sodalite
No full textWe present the results of ab initio molecular dynamics simulations on the structural and dynamical properties of the Bronsted acid site in a zeolitic cage. The protonated sodalite H[AlSi11O24] is the case studied, and the sodium sodalite is the reference unprotonated zeolite. The comparison between the geometrical parameters calculated for these two sodalites shows that the formation of an O-H bond gives rise to a significant local distortion of the framework geometry. Moreover, an equilibrium between partially ionic and covalent structures in the T-O bonds of the acid site is deduced from our calculations. We have found that the OH bond oscillates in and out of the plane of the 6-T-ring. The analysis of the simulated vibrational spectra has allowed us to assign the bands relative to the O-H in-plane and out-of-plane bending modes and confirms that these bands overlap with the framework modes
Influence of silanols condensation on surface properties of micelle-templated silicas: A modelling study
No full textThe dehydration of model mesoporous silicas has been studied via molecular dynamics simulations. By progressively dehydrating the system, initially characterized by an even distribution of surface silanols, patches with hydrophobic or hydrophilic character are formed on the pores surface. It is found that the local concentration of silanols is strictly correlated to the local structure of pores surface, namely hydrophobic and hydrophilic patches are located in high- and low-curvature surface regions, respectively, in line with experimental evidences. Simulations suggest that the microscopic origin of both the uneven Si-OH distribution and the pores surface deformation should be related to the dehydroxylation process, which is governed by interactions among the silica matrix
TS-1 from First Principles
No full textFirst principles Studies on periodic TS-1 models at Ti content corresponding to 1.35% and 2.7% in weight of TiO2 are presented. The problem of Ti preferential siting is addressed by using realistic models corresponding to the TS-1 unit cell [TiSi95O192] and adopting for the first time a periodic DFT approach, thus providing an energy scale for Ti in the different crystallographic sites in nondefective TS-1. The structure with Ti in site T3 is the most stable, followed by T4 (+0.3 kcal/mol); the less stable structure, corresponding to Ti in T1, is 5.6 kcal/mol higher in energy. The work has been extended to investigate models with two Ti's per unit cell [Ti2Si94O192] (2.7%). The possible existence of Ti-O-Ti bridges, formed by two corner-sharing TiO4 tetrahedra, is discussed. By using Cluster models cut from the optimized periodic DFT structures, both vibrational (DFT) and electronic excitation spectra (TDDFT) have been calculated and favorably compared with the experimental data available on TS-1. Interesting features emerged from excitation spectra: (i) Isolated tetrahedral Ti sites show a Beer-Lambert behavior, with absorption intensity proportional to concentration. Such a behavior is gradually lost when two Ti's occupy sites close to each other. (ii) The UV-vis absorption in the 200-250 nm region can be associated with transitions from Occupied states delocalized on the framework oxygens to empty d states localized on Ti. Such extended-states-to-local-states transitions may help the interpretation of the photovoltaic activity recently detected in Ti zeolites
Orientation and Order of Xanthene Dyes in the One-Dimensional Channels of Zeolite L: Bridging the Gap between Experimental Data and Molecular Behavior
No full textSupramolecular organization in nanochannels is governed by both the nature of the channels and the size, shape, and charge of the guests, and may also depend on the cosolvent in the host. Oxonine (Ox+) and pyronine (Py+) cationic dyes in zeolite L (ZL) could provide important insight on this issue, but their orientation in the composite materials is not understood yet. Theoretical modeling of Ox+/ZL and Py+/ZL composites indicates that the 70–80° orientation, deduced from optical microscopy data but incompatible with host–guest geometry contraints, is not stable and reveals two possible orientations for these dyes in ZL nanochannels: perpendicular or parallel to the channel axis. In the hydrated composite, corresponding to optical microscopy experiments conditions, perpendicular Ox+/Py+ is the favored orientation, suggesting a larger portion of perpendicularly oriented dye molecules and a smaller fraction aligned. Optical microscopy data might therefore be reinterpreted as the vector sum of components arising from a population of molecules with nonuniform orientation. The stability order is reversed at dry conditions, indicating that the organization of xanthene dyes in ZL channels is governed by water. Our study explains this cosolvent-driven supramolecular organization regime, here unraveled for the first time, and highlights the role of molecular flexibility in switching the dye orientation
One-dimensional self-assembly of perylene-diimide dyes by unidirectional transit of zeolite channel openings
Full text linkConfined supramolecular architectures of chromophores are key components in artificial antenna composites for solar energy harvesting and storage. A typical fabrication process, based on the insertion of dye molecules into zeolite channels, is still unknown at the molecular level. We show that slipping of perylene diimide dyes into the one-dimensional channels of zeolite L and travelling inside is only possible because of steric-interaction-induced cooperative vibrational modes of the host and the guest. The funnel-like structure of the channel opening, larger at the entrance, along with a directionally asymmetric entrance–exit probability, ensures a favorable self-assembly process of the perylene units
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