Efficient singlet-singlet energy transfer in a novel host-guest assembly composed of an organic cavitand, aromatic molecules, and a clay nanosheet

Spectroscopic Insights of an Emissive Complex between 4'-N,N-Diethylaminoflavonol in Octa-Acid Deep-Cavity Cavitand and Rhodamine 6G

Excited-state chemistry relies on the communication between molecules, making it a crucial aspect of the field. One important question that arises is whether intermolecular communication and its rate can be modified when a molecule is confined. To explore the interaction in such systems, we investigated the ground and excited states of 4'-N,N-diethylaminoflavonol (DEA3HF) in an octa acid-based (OA) confined medium and in ethanolic solution, both in the presence of Rhodamine 6G (R6G). Despite the observed spectral overlap between the flavonol emission and the R6G absorption, as well as the fluorescence quenching of the flavonol in the presence of R6G, the almost constant fluorescence lifetime at different amounts of R6G discards the presence of FRET in the studied systems. Steady-state and time-resolved fluorescence indicate the formation of an emissive complex between the proton transfer dye encapsulated within water-soluble supramolecular host octa acid (DEA3HF@(OA)₂) and R6G. A similar result was observed between DEA3HF:R6G in ethanolic solution. The respective Stern-Volmer plots corroborate with these observations, suggesting a static quenching mechanism for both systems.

Pyrene-based metal–organic framework NU-1000 photocatalysed atom-transfer radical addition for iodoperfluoroalkylation and (Z)-selective perfluoroalkylation of olefins by visible-light irradiation

Iodoperfluoroalkylation or (Z)-selective perfluoroalkylation of olefins is mediated through energy transfer processes by using pyrene-based MOF NU-1000 under visible-light irradiation.

Preparative scale and convenient synthesis of a water-soluble, deep cavitand

Cavitands are established tools of supramolecular chemistry and molecular recognition, and they are finding increasing application in sensing and sequestration of physiologically relevant molecules in aqueous solution. The synthesis of a water-soluble, deep cavitand is described. The route comprises six (linear) steps from commercially available precursors, and it relies on the fourfold oligomeric cyclization reaction of resorcinol with 2,3-dihydrofuran that leads to the formation of a shallow resorcinarene framework; condensation with aromatic panels, which deepens the hydrophobic binding cavity; construction of rigid urea functionalities on the upper rim; and the introduction of the water-solubilizing methylimidazolium groups on the lower rim. Late intermediates of the synthesis can be used in the preparation of congener cavitands with different properties and applications, and a sample of such a synthetic procedure is included in this protocol. Emphasis is placed on scaled-up reactions and on purification procedures that afford materials in high yield and avoid chromatographic purification. This protocol provides improvements over previously described procedures, and it enables the preparation of sizable amounts of deep cavitands: 7 g of a water-soluble cavitand can be prepared from resorcinol in 13 working days.

Supramolecular Surface Photochemistry: Cascade Energy Transfer between Encapsulated Dyes Aligned on a Clay Nanosheet Surface

Three coumarin derivatives (7-propoxy coumarin, coumarin-480, and coumarin-540a, 2, 3, and 4, respectively) having different absorption and emission spectra were encapsulated within a water-soluble organic capsule formed by the two positively charged ammonium-functionalized cavitand octaamine (OAm, 1). Guests 2, 3, and 4 absorb in ultraviolet, violet, and blue regions and emit in violet, blue, and green regions, respectively. Energy transfer between the above three coumarin@(OAm)2 complexes assembled on the surface of a saponite clay nanosheet was investigated by steady-state and time-resolved emission techniques. Judging from their emission and excitation spectra, we concluded that the singlet-singlet energy transfer proceeded from 2 to 3, from 2 to 4, and from 3 to 4 when OAm-encapsulated 2, 3, and 4 were aligned on a clay surface as two-component systems. Under such conditions, the energy transfer efficiencies for the paths 2* to 3, 2* to 4, and 3* to 4 were calculated to be 33, 36, and 50% in two-component systems. When all three coumarins were assembled on the surface and 2 was excited, the energy transfer efficiencies for the paths 2* to 3, 2* to 4, and 3* to 4 were estimated to be 32, 34, and 33%. A comparison of energy transfer efficiencies of the two-component and three-component systems revealed that excitation of 2 leads to emission from 4. Successful merging of supramolecular chemistry and surface chemistry by demonstrating novel multi-step energy transfer in a three-component dye encapsulated system on a clay surface opens up newer opportunities for exploring such systems in an artificial light-harvesting phenomenon.

Sequential energy and electron transfer in a three-component system aligned on a clay nanosheet

To achieve the goal of energy transfer and subsequent electron transfer across three molecules, a phenomenon often utilized in artificial light harvesting systems, we have assembled a light absorber (that also serves as an energy donor), an energy acceptor (that also serves as an electron donor) and an electron acceptor on the surface of an anionic clay nanosheet. Since neutral organic molecules have no tendency to adsorb onto the anionic surface of clay, a positively charged water-soluble organic capsule was used to hold neutral light absorbers on the above surface. A three-component assembly was prepared by the co-adsorption of a cationic bipyridinium derivative, cationic zinc porphyrin and cationic octaamine encapsulated 2-acetylanthracene on an exfoliated anionic clay surface in water. Energy and electron transfer phenomena were monitored by steady state fluorescence and picosecond time resolved fluorescence decay. The excitation of 2-acetylanthracene in the three-component system resulted in energy transfer from 2-acetylanthracene to zinc porphyrin with 71% efficiency. Very little loss due to electron transfer from 2-acetylanthracene in the cavitand to the bipyridinium derivative was noticed. Energy transfer was followed by electron transfer from the zinc porphyrin to the cationic bipyridinium derivative with 81% efficiency. Analyses of fluorescence decay profiles confirmed the occurrence of energy transfer and subsequent electron transfer. Merging the concepts of supramolecular chemistry and surface chemistry we realized sequential energy and electron transfer between three hydrophobic molecules in water. Exfoliated transparent saponite clay served as a matrix to align the three photoactive molecules at a close distance in aqueous solutions.

Room temperature phosphorescence from a guest molecule confined in the restrictive space of an organic–inorganic supramolecular assembly

Supramolecular double walls composed of organic capsules and inorganic nanosheets realized stable room-temperature phosphorescence of enclosed guest molecules due to the effective suppression of oxygen quenching.

In Situ Crystallization of Al-Containing Silicate Nanosheets on Monodisperse Amorphous Silica Microspheres

The fine crystals of an Al-containing layered silicate, whose negative layer charge is generated by an isomorphous substitution in the tetrahedral SiO4 framework, successfully grew on monodisperse amorphous silica microspheres with diameters of 1.0 and 2.6 μm. The fine, plate-like crystals were observed to thoroughly cover the surface of the silica spheres, irrespective of their size, by the hydrothermal reactions of the silica powder in aqueous alkali solution containing Al and Mg ions in a rotating Teflon-lined autoclave. The crystal size increased when the concentration of the precursors was low. The presence of fluorine in the reaction media enlarged the crystalline phase in the direction of the layer stacking while reducing the plate size. The difference in the crystal size affected the kinetics on the hinokitiol desorption in n-hexane from the layered silicates modified with organoammonium ions. The organically modified layered silicate behaved as an exfoliated nanosheet in the nonpolar solvent. The less harmful elements in this hybrid suggest that it can be used in cosmetic and pharmaceutical applications as a drug support, without flaking off the fine layers on the microspherical substrates.

Shining light on clay-chromophore hybrids: layered templates for accelerated ring closure photo-oxidation

Templates with specific microenvironments have been long employed to facilitate specialized reactions. From enzymes to metal organic frameworks (MOFs), various systems have exerted their prowess to affect specific chemical reactions. Here we report, for the first time, the acceleration of a ring closure photo-oxidation reaction due to the specific structural constraints provided by layered materials. A stilbene derivative has been used as a prototype reactant and the di-hydrophenanthrene intermediate has been isolated and characterized en route to the complete photo-oxidation. Combining the gathered evidence, a possible mechanism for the chemical transformation has been proposed. Kinetic analysis showed that layered materials help to manipulate the rate of the electrocyclic ring closure and, in turn, accelerate the complete reaction sequence. The structural microenvironment induced by layered materials could be a unique platform to probe and stabilize a plethora of photo-oxidative reactions and intermediates.

Supramolecular Photochemistry in Solution and on Surfaces: Encapsulation and Dynamics of Guest Molecules and Communication between Encapsulated and Free Molecules

Supramolecular assemblies that help to preorganize reactant molecules have played an important role in the development of concepts related to the control of excited-state processes. This has led to a persistent search for newer supramolecular systems (hosts), and this review briefly presents our work with octa acid (OA) to a host to control excited-state processes of organic molecules. Octa acid, a water-soluble host, forms 1:1, 2:1, and 2:2 (host-guest) complexes with various organic molecules. A majority of the guest molecules are enclosed within a capsule made up of two molecules of OA whereas OA by itself remains as a monomer or aggregates. Luminescence and (1)H NMR spectroscopy help to characterize the structure and dynamics of these host-guest complexes. The guest molecule as well as the host-guest complex as a whole undergoes various types of motion, suggesting that the guests possess freedom inside the confined space of the octa acid capsule. In addition, the confined guests are not isolated but are able to communicate (energy, electron, and spin) with molecules present closer to the capsule. The host-guest complexes are stable even on solid surfaces such as silica, clay, α-Zr phosphate, TiO2, and gold nanoparticles. This opens up new opportunities to explore the interaction between confined guests and active surfaces of TiO2 and gold nanoparticles. In addition, this allows the possibility of performing energy and electron transfer between organic molecules that do not adsorb on inert surfaces of silica, clay, or α-Zr phosphate. The results summarized here, in addition to providing a fundamental understanding of the behavior of molecules in a confined space provided by the host OA, are likely to have a long-range effect on the capture and release of solar energy.

Molecular containers assembled through the hydrophobic effect

This review focuses on molecular containers formed by assembly processes driven by the hydrophobic effect, and summarizes the progress made in the field over the last ten years. This small but growing facet of supramolecular chemistry discusses three classes of molecules used by researchers to investigate how self-assembly can be applied to form discrete, mono-dispersed, and structurally well-defined supramolecular entities. The approaches demonstrate the importance of preorganization of arrays of rigid moieties to define a specific form predisposed to bind, fold, or assemble. As the examples demonstrate, studying these systems and their properties is teaching us how to control supramolecular chemistry in water, shedding light on aspects of aqueous solutions chemistry, and illustrating novel applications that harness the unique properties of the hydrophobic effect.

Manipulation of supramolecular 2D assembly of functional dyes toward artificial light-harvesting systems

In recent years, excellent research has revealed that light-harvesting systems (LHSs) are composed of beautifully aligned chlorophyll molecules; the regulated alignment of chlorophylls is responsible for the efficient and selective light-harvesting energy transfer processes in purple bacteria. This finding led to the construction of a regularly arranged assembly of functional dyes as a step toward fabricating artificial LHSs. While most approaches toward the construction of dye assemblies have depended on molecular interactions such as covalent, coordination, and hydrogen bonds, my approach involves guest–host interactions using an inorganic nanosheet as the host material. This short review presents the construction of a 2D dye assembly and its effective utilization in artificial light-harvesting applications. Owing to the highly stable and uniform 2D alignment of functional dyes on inorganic nanosheets, nearly 100 % singlet–singlet energy transfer and efficient light-harvesting were achieved. I believe that the results presented herein will contribute to the construction of efficient photochemical reaction systems in supramolecular host–guest assemblies, which may facilitate the realization of artificial photosynthesis.

Organoclays in water cause expansion that facilitates caffeine adsorption

This study investigates the adsorption of caffeine in water on organically modified clays (a natural montmorillonite and synthetic saponite, which are smectite group of layered clay minerals). The organoclays were prepared by cation-exchange reactions of benzylammonium and neostigmine with interlayer exchangeable cations in the clay minerals. Although less caffeine was uptaken on neostigmine-modified clays than on raw clay minerals, uptake was increased by adding benzylammonium to the clays. The adsorption equilibrium constant was considerably higher on benzylammonium-modified saponite (containing small quantities of intercalated benzylammonium) than on its montmorillonite counterpart. These observations suggest that decreasing the size and number of intercalated cations enlarges the siloxane surface area available for caffeine adsorption. When the benzylammonium-smectite powders were immersed in water, the intercalated water molecules expanded the interlayer space. Addition of caffeine to the aqueous dispersion further expanded the benzylammonium-montmorillonite system but showed no effect on benzylammonium-saponite. We assume that intercalated water molecules were exchanged with caffeine molecules. By intercalating benzylammonium into smectites, we have potentially created an adaptable two-dimensional nanospace that sequesters caffeine from aqueous media.

Synthesis, characterization, guest inclusion, and photophysical studies of gold nanoparticles stabilized with carboxylic acid groups of organic cavitands

Water-soluble gold nanoparticles (AuNP) stabilized with cavitands having carboxylic acid groups have been synthesized and characterized by a variety of techniques. Apparently, the COOH groups similar to thiol are able to prevent aggregation of AuNP. These AuNP were stable either as solids or in aqueous solution. Most importantly, these cavitand functionalized AuNP were able to include organic guest molecules in their cavities in aqueous solution. Just like free cavitands (e.g., octa acid), cavitand functionalized AuNP includes guests such as 4,4'-dimethylbenzil and coumarin-1 through capsule formation. The exact structure of the capsular assembly is not known at this stage. Upon excitation there is communication between the excited guest present in the capsule and gold atoms and this results in quenching of phosphorescence from 4,4'-dimethylbenzil and fluorescence from coumarin-1.