Smart Nanocages as a Tool for Controlling Supramolecular Aggregation

An important aspect in the field of supramolecular chemistry is the control of the composition and aggregation state of supramolecular polymers and the possibility of stabilizing out-of-equilibrium states. The ability to freeze metastable systems and release them on demand, under spatiotemporal control, to allow their thermodynamic evolution toward the most stable species is a very attractive concept. Such temporal blockage could be realized using stimuli-responsive "boxes" able to trap and redirect supramolecular polymers. In this work, we report the use of a redox responsive nanocontainer, an organosilica nanocage (OSCs), for controlling the dynamic self-assembly pathway of supramolecular aggregates of a luminescent platinum compound (Pt_AC). The aggregation of the complexes leads to different photoluminescent properties that allow visualization of the different assemblies and their evolution. We discovered that the nanocontainers can encapsulate kinetically trapped species characterized by an orange emission, preventing their evolution into the thermodynamically stable aggregation state characterized by blue-emitting fibers. Interestingly, the out-of-equilibrium trapped Pt species (Pt_AC@OSCs) can be released on demand by the redox-triggered degradation of OSCs, re-establishing their self-assembly toward the thermodynamically stable state. To demonstrate that control of the self-assembly pathway occurs also in complex media, we followed the evolution of the supramolecular aggregates inside living cells, where the destruction of the cages allows the intracellular release of Pt_AC aggregates, followed by the formation of microscopic blue emitting fibers. Our approach highlights the importance of "ondemand" confinement as a tool to temporally stabilize transient species which modulate complex self-assembly pathways in supramolecular polymerization.

X-Ray-Induced Growth Dynamics of Luminescent Silver Clusters in Zeolites

Herein, AlK_α X-rays are used to drive the growth of luminescent silver clusters in zeolites. The growth of the silver species is tracked using Auger spectroscopy and fluorescence microscopy, by monitoring the evolution from their ions to luminescent clusters and then metallic, dark nanoparticles. It is shown that the growth rate in different zeolites is determined by the mobility of the silver ions in the framework and that the growth dynamics in calcined samples obeys the Hill-Langmuir equation for noncooperative binding. Comparison of the optical properties of X-ray-grown silver clusters with silver clusters formed by standard heat treatment indicates that the latter have a higher specificity toward the formation of luminescent clusters of a specific (small) nuclearity, whereas the former produce a wide distribution of cluster species as well as larger nanoparticles.

Luminescence resonance energy transfer in hybrid materials based on terbium(iii) complex, rhodamine B and nanoclay

We prepared a novel kind of luminescent hydrogel based on the combination of terbium(iii) complexes and rhodamine B dye with the LAPONITE® nanoclay.

Entropy in multiple equilibria, compounds with different sites

The influence of entropy in multiple chemical equilibria is investigated for systems with different types of sites for the condition that the binding enthalpy of the species is the same within each type of sites and independent of those species that are already bonded. This allows splitting of the free reaction enthalpy into the particle distribution term and all other contributions for each type of sites separately and, hence, to evaluate this entropy contribution to the free reaction enthalpy. The situations for which this applies can be chemically very different, e.g. acid base, ligand exchange, isomerisation, conformational change, rearrangement of a ligand, ion exchange, adsorption of a species on the surface of a particle or a dendrimer, insertion of charged or neutral species into the cavities of a microporous or mesoporous host. We provide physical insight by discussing Xrc1{n1ABn2}Xrc2 systems. The number of coordination sites A and B are n1 and n2, respectively. The indices rc1 = 1, 2,…,n1 and rc2 = 1, 2,…,n2 count the number of X bonded to sites A and sites B, respectively. An important result is that the large number of equilibrium constants needed to describe those situations can be expressed as a function of two constants only. This allows studying systems quantitatively by experimental and theoretical means which otherwise might be difficult to handle. It has also implication for theoretical studies in the sense that it is sufficient to model only two reactions instead of many in order to describe a system. The results remain valid for systems with more than two types of different sites. The description of the entropy driven development of the fractional equilibrium coverage of the sites provides a new tool for understanding adsorption and ion exchange isotherms. The fractional equilibrium coverage of the sites can be described as a linear combination of individual Langmuir isotherms despite of the fact that such a linear combination has never the shape of the original Langmuir isotherm. This is remarkable and very useful. It provides us with new tools for describing and testing isotherms based on well defined, transparent physical ideas. Explicit solution for systems with 2, 3, 4, 5, 6, and 12 coordination sites are reported. Applications to a system with 12 coordination sites serve to illustrate information that can be obtained for complex situations.

Supramolecular Organization in Confined Nanospaces

Empty 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.

One-Directional Antenna Systems: Energy Transfer from Monomers to J-Aggregates within 1D Nanoporous Aluminophosphates

A cyanine dye (PIC) was occluded into two 1D-nanopoporus Mg-containing aluminophosphates with different pore size (MgAPO-5 and MgAPO-36 with AFI and ATS zeolitic structure types, with cylindrical channels of 7.3 Å diameter and elliptical channels of 6.7 Å × 7.5 Å, respectively) by crystallization inclusion method. Different J-aggregates are photophysically characterized as a consequence of the different pore size of the MgAPO frameworks, with emission bands at 565 nm and at 610 nm in MgAPO-5 and MgAPO-36, respectively. Computational results indicate a more linear geometry of the J-aggregates inside the nanochannels of the MgAPO-36 sample than those in MgAPO-5, which is as a consequence of the more constrained environment in the former. For the same reason, the fluorescence of the PIC monomers at 550 nm is also activated within the MgAPO-36 channels. Owing to the strategic distribution of the fluorescent PIC species in MgAPO-36 crystals (monomers at one edge and J-aggregates with intriguing emission properties at the other edge) an efficient and one-directional antenna system is obtained. The unidirectional energy transfer process from monomers to J-aggregates is demonstrated by remote excitation experiments along tens of microns of distance.

Photochemistry and Photophysics in Silica-Based Materials: Ultrafast and Single Molecule Spectroscopy Observation

Silica-based materials (SBMs) are widely used in catalysis, photonics, and drug delivery. Their pores and cavities act as hosts of diverse guests ranging from classical dyes to drugs and quantum dots, allowing changes in the photochemical behavior of the confined guests. The heterogeneity of the guest populations as well as the confinement provided by these hosts affect the behavior of the formed hybrid materials. As a consequence, the observed reaction dynamics becomes significantly different and complex. Studying their photobehavior requires advanced laser-based spectroscopy and microscopy techniques as well as computational methods. Thanks to the development of ultrafast (spectroscopy and imaging) tools, we are witnessing an increasing interest of the scientific community to explore the intimate photobehavior of these composites. Here, we review the recent theoretical and ultrafast experimental studies of their photodynamics and discuss the results in comparison to those in homogeneous media. The discussion of the confined dynamics includes solvation and intra- and intermolecular proton-, electron-, and energy transfer events of the guest within the SBMs. Several examples of applications in photocatalysis, (photo)sensors, photonics, photovoltaics, and drug delivery demonstrate the vast potential of the SBMs in modern science and technology.

Irreversible Conversion of a Water-Ethanol Solution into an Organized Two-Dimensional Network of Alternating Supramolecular Units in a Hydrophobic Zeolite under Pressure

Turning disorder into organization is a key issue in science. By making use of X-ray powder diffraction and modeling studies, we show herein that high pressures in combination with the shape and space constraints of the hydrophobic all-silica zeolite ferrierite separate an ethanol-water liquid mixture into ethanol dimer wires and water tetramer squares. The confined supramolecular blocks alternate in a binary two-dimensional (2D) architecture that remains stable upon complete pressure release. These results support the combined use of high pressures and porous networks as a viable strategy for driving the organization of molecules or nano-objects towards complex, pre-defined patterns relevant for the realization of novel functional nanocomposites.

Entropy in multiple equilibria, theory and applications

Entropy controls the dependence of the equilibrium constants in the synthesis of host–guest composites on the occupation rc for channels of different length.

Solvent free mechanochemical synthesis of Eu3+ complex and its luminescent sensing of trace water and temperature

A solid state Eu(iii) complex was synthesized by a mechanochemical method and used for luminescent sensing of trace water and temperature.

Cell Growth on ("Janus") Density Gradients of Bifunctional Zeolite L Crystals

Nanoparticle density gradients on surfaces have attracted interest as two-dimensional material surfaces that can mimic the complex nano-/microstructure of the native extracellular matrix, including its chemical and physical gradients, and can therefore be used to systematically study cell-material interactions. In this respect, we report the preparation of density gradients made of bifunctional zeolite L crystals on glass surfaces and the effects of the density gradient and biopolymer functionalization of zeolite L crystals on cell adhesion. We also describe how we created "Janus" density gradient surfaces by gradually depositing two different types of zeolite L crystals that were functionalized and loaded with different chemical groups and guest molecules onto the two distinct sides of the same glass substrate. Our results show that more cells adhered on the density gradient of biopolymer-coated zeolites than on uncoated ones. The number of adhered cells increased up to a certain surface coverage of the glass by the zeolite L crystals, but then it decreased beyond the zeolite density at which a higher surface coverage decreased fibroblast cell adhesion and spreading. Additionally, cell experiments showed that cells gradually internalized the guest-molecule-loaded zeolite L crystals from the underlying density gradient containing bifunctional zeolite L crystals.

Modulating the optical properties of the AIE fluophor confined within UiO-66's nanochannels for chemical sensing

Transformation of aggregation induced emission (AIE) molecules into functional materials can greatly tune their fluorescence properties and expand their related potential applications. Here, we demonstrate a facile strategy to modulate the fluorescence properties of AIE molecules by confining them within the nanochannels of metal-organic frameworks (MOFs). AIE molecules (4,4'-(hydrazine-1,2-diylidene bis(methanylylidene)) bis(3-hydroxybenzoic acid), HDBB) with pH-dependent emission were successfully integrated into UiO-66 resulting in a UiO-66⊃HDBB complex while maintaining the crystal structure by the host-guest process. The fluorescence properties of HDBB can be modulated by virtue of the unique nanospace confining effect. Furthermore, UiO-66⊃HDBB was used as a sensitive and selective fluorescent probe for Cu²⁺ detection in aqueous solution. This study provides a facile strategy for the construction of high performance AIE based luminescent materials with adjustable properties for potential applications.

One-dimensional self-assembly of perylene-diimide dyes by unidirectional transit of zeolite channel openings

Confined 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.

Functionalization of arrays of silica nanochannels by post-condensation

Functionalized arrays of silica nanochannels (ASNCs) were prepared by post-condensation. This synthetic approach allows control of the functional group distribution in a one-pot reaction, while preserving the defined nanoporous structure of the ASNCs.