Cyclodextrin complexation as a probe of molecular photophys1cs

Three examples are described in which complexation of organic guests by a cyclodextrin (CD) alters the photophysical paths for excited state decay of the host-guest complex. The fluorescence properties ol the complexes between β- and γ-CD and the bichromophoric molecule 1.1'-bis-(α-naphthylmethyl) dithiane are presented to illustrate how complexation alters the dynamics of conformational interconversion. The complexation of β-naphthol by β-CD is shown by time-correlated emission spectroscopy to prevent the adiabatic deprotonation of the excited alcohol normally observed in aqueous solution. Lastly, solid complexes of CD's and aromatic donor-acceptor molecules such as p-nitroaniline (PNA) are shown to be active for generation of second harmonic radiation (SHG) when irradiated by 1.06 μ light from a Nd-YAG laser. Since PNA itself crystallizes in a centrosymmetric space group incapable of producing SHG, the complexation method effectively induces SHG activity in this polarizable, but nonpolar material.     

A molecular chalice with hydrophobic walls and a hydrophilic rim: self-assembly and complexation properties

We describe the synthesis of a diphenylglycoluril/dibenzo-crown-6 molecular chalice, the self-assembly at the air/water interface and its complexation properties in solution and at the water/chloroform interface.     

Cyclodextrin knowledgebase a web-based service managing CD-ligand complexation data

Cyclodextrins are cyclic oligosaccharides that are able to form water-soluble inclusion complexes with small molecules. Because of their complexing ability, they are widely applied in food, pharmaceutical and chemical industries. In this paper we describe the development of a free web-service, Cyclodextrin KnowledgeBase: (http://www.cyclodextrin.net). The database contains four modules: the Publication, Interaction, Chirality and Analysis Modules. In the Publication Module, almost 50,000 publication details are collected that can be retrieved by text search. In the Interaction and Chirality Modules relevant literature data on cyclodextrin complexation and chiral recognition are collected that can be retrieved by both text and structural searches. Moreover, in the Analysis Module, the geometries of small molecule-cyclodextrin complexes can be predicted using molecular docking tools in order to explore the structures and interaction energies of the inclusion complexes. Complex geometry prediction is made possible by the built-in database of 95 cyclodextrin derivatives, where the 3D structures as well as the partial charges are calculated and stored for further utilization. The use of the database is demonstrated by several examples.     

Cyclodextrin-based inclusion complexation bridging supramolecular chemistry and macromolecular self-assembly

Research into macromolecular self-assembly has been progressively developing since the 1970s but with a little affect from the achievements of supramolecular chemistry. In recent years, this situation has changed as more and more factors and concepts in supramolecular chemistry have been introduced into studies of the self-assembly of polymers. In this respect, inclusion complexation based on cyclodextrins plays a remarkable role. In this tutorial review, we address how inclusion complexation has been employed and used to promote the recent developments in macromolecular self-assembly. These include the amphiphilicity adjustment of macromolecules, non-covalent linkages for forming pseudo block copolymers and micelles, surface modification and functionalization of polymeric micelles and vesicles, and the combination of synthetic polymeric assemblies with biological moieties. Furthermore, the realization of the reversible stimuli-responsiveness of polymeric assemblies and materials, particularly hydrogels by means of controllable inclusion complexation is discussed as well.     

Switching between supramolecular dimer and nonthreaded supramolecular self-assembly of stilbene amide-alpha-cyclodextrin by photoirradiation

3-trans-Stilbene amide-alpha-cyclodextrin (3-trans-Sti-alpha-CD) formed a double-threaded dimer in aqueous solutions. In contrast, the photoisomerization of the stilbene moiety in 3-Sti-alpha-CD from trans to cis leads to the structural changes from the double-threaded dimer to nonthreaded supramolecular assemblies in aqueous solutions. The structures of these supramolecular complexes have been found to be controllable by photoirradiation.     

Indistinguishability of the models of molecular self-assembly

Numerous applications dealing with molecular aggregation at the interface of biology, physics and chemistry use either the dimer or the indefinite equilibrium constant models even though there is the well-known property of indistinguishability of the models with respect to fitting experimental data by various experimental techniques. The problem of indistinguishability is uncovered in the present work and the way in which the existing paradigm of how these models should be applied to analysis of molecular self-association is suggested.     

Rapid self-assembly of core-shell organosilicon microcapsules within a microfluidic device

The preparation of hierarchically structured organosilicon microcapsules from commercially available starting materials is described. Using a microfluidic device, an emulsion of dichlorodiphenylsilane is formed in a continuous phase of aqueous glycerol. The silane droplets undergo hydrolysis, condensation, and crystallization within minutes to form self-assembled, core-shell microcapsules. The microparticles have been characterized with light and electron microscopy, nuclear magnetic resonance spectroscopy (NMR), diffusion-ordered NMR spectroscopy (DOSY), Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and powder X-ray diffraction (XRD). The characterization data show that the microcapsule walls consist of amorphous, oligomeric poly(diphenylsiloxane) surrounded by a spiny layer of crystalline diphenylsilanediol. Glycerol is occluded within the wall material but is not covalently bound to the silicon components. Glycerol is a crucial element for producing low-dispersity microcapsules with well-ordered surface spines, as the use of methyl cellulose as viscomodifier yields amorphous surfaces.     

Designed self-assembly of molecular necklaces

This paper reports an efficient strategy to synthesize molecular necklaces, in which a number of small rings are threaded onto a large ring, utilizing the principles of self-assembly and coordination chemistry. Our strategy involves (1) threading a molecular "bead" with a short "string" to make a pseudorotaxane and then (2) linking the pseudorotaxanes with a metal complex with two cis labile ligands acting as an "angle connector" to form a cyclic product (molecular necklace). A 4- or 3-pyridylmethyl group is attached to each end of 1,4-diaminobutane or 1,5-diaminopentane to produce the short "strings" (C4N4(2+), C4N3(2+), C5N4(2+), and C5N3(2+)), which then react with a cucurbituril (CB) "bead" to form stable pseudorotaxanes (PR44(2+), PR43(2+), PR54(2+), and PR53(2+), respectively). The reaction of the pseudorotaxanes with Pt(en)(NO(3))(2) (en = ethylenediamine) produces a molecular necklace [4]MN, in which three molecular "beads" are threaded on a triangular framework, and/or a molecular necklace [5]MN, in which four molecular "beads" are threaded on a square framework. Under refluxing conditions, the reaction with PR44(2+) or PR54(2+) yields exclusively [4]MN (MN44T or MN54T, respectively), whereas that with PR43(2+) or PR53(2+) produces exclusively [5]MN (MN43S or MN53S, respectively). The products have been characterized by various methods including X-ray crystallography. At lower temperatures, on the other hand, the reaction with PR44(2+) or PR54(2+) affords both [4]MN and [5]MN. The supermolecules reported here are the first series of molecular necklaces obtained as thermodynamic products. The overall structures of the molecular necklaces are strongly influenced by the structures of pseudorotaxane building blocks, which is discussed in detail on the basis of the X-ray crystal structures. The temperature dependence of the product distribution observed in this self-assembly process is also discussed.     

The mechanism of hydrogen randomization in the stilbene molecular ion

Hydrogen randomization of the stilbene molecular ion preceding the formation of the [M—methyl]-ion is discussed.     

The foundation of a light driven molecular muscle based on stilbene and alpha-cyclodextrin

The rotaxane serves as the basis of a light driven molecular muscle, where reversible photoisomerisation of the stilbene units causes the cyclodextrins to move off and on the stilbene units, contracting and extending the distance between the blocking groups.     

Controlled self-assembly of quantum dots and block copolymers in a microfluidic device

The controlled self-assembly of polymer-stabilized quantum dots (QDs) into mesoscale aqueous spherical assemblies using microfluidics is described. In a flow-focusing configuration, self-assembly is initiated by the addition of water to a blended solution of polystyrene-coated QDs and amphiphilic polystyrene-block-poly(acrylic acid) stabilizing chains and terminated in a downstream quench step. The on-chip evolution of assemblies is monitored through fluorescence microscopy, and particle size distributions are determined off-chip by transmission electron microscopy. On-chip size control of the assemblies is demonstrated via both the average water concentration in the channel and the flow rate.     

Syntheses and molecular self-assembly of chiral phosphorami-dates

Two chiral phosphoramidates,(R)-(-)-1,1'-binaphthyl-2,2'-dihydroxy-N-[α-(S)-methylbenzyl] phosphoramidate and (-)-1,1'-biphenyl-2,2'-dihydroxy-N-[α-(S)-methylbenzyl]-phosphoramidate were synthesized.Their crystal structures were determined by X-ray single crystal diffraction analysis.The phosphoramidate molecules are self-associated by inter-molecular N-H...O = P hydrogen bonds and aromatic edge to face interactions.     

Catalytic epoxidation of stilbene with FePt@Cu nanowires and molecular oxygen

A novel FePt@Cu nanowire catalyst was prepared by the reduction of Cu(acac)(2) on the surface of FePt nanowires, in oleylamine (OAm). This nanowire catalyst efficiently epoxidised stilbene in the presence of molecular oxygen, and the conversion and selectivity were maintained with repeated use of the catalyst, compared with recycled catalyst.     

Cyclodextrin polyrotaxanes assembled from a molecular construction kit in aqueous solution

We describe a molecular construction kit in which amphiphilic polymers and functionalized cyclodextrins are arranged into sophisticated molecular architectures in aqueous solution without the need to perform chemical reactions. Therefore, these systems are highly biocompatible and show programmable lifetimes. The kinetic stabilities of our polyrotaxane structures are tunable using sterically demanding groups that hinder dissociation. These cyclodextrin‐based polymer systems are applicable in principle for the detection of analytes at the level of single molecules. These systems may also serve well in targeted drug delivery and gene transfection. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6333–6341, 2009     

丙烯腈与芳烃分子配位作用的研究

报导了用UV法和NMR法共同测量丙烯腈与某些芳烃的分子配合常数,以Rose-Drago方程处理数据.并比较了二种方法测定得到的配位常数.结果表明UV法和NMR法测量所得相关系数都很高,大都在0.99以上,认为体系以1:1形式配位的假设是合理的,两种方法得到的配位常数的误差也大体相当.     

Nanorings from the self-assembly of amphiphilic molecular dumbbells

We have prepared amphiphilic dumbbell molecules consisting of hydrophobic alkyl chains and hydrophilic oligoether dendrons at each end of the rod segment. The molecular dumbbells, in aqueous solution, self-assemble into toroids as an intermediate nanostructure between spherical and long cylindrical micelles. The formation of toroidal structure is likely to originate from side by side connections of discrete bundles through the combination of strong hydrophobic interactions and anisotropic aggregation of rod segments.     

Controlled self-assembly of triphenylene-based molecular nanostructures

Molecular nanostructures of the disc-shaped molecule hexapentyloxytriphenylene have been fabricated on length scales ranging from 30 nm to 1.5 mum following self-assembly arising from pi-pi interactions in organic solvents. The size and density of the molecular nanostructures deposited onto glass and indium tin oxide-coated glass substrates were characterized by atomic force microscopy. Dynamic light scattering and spectroscopic evidence of predeposition aggregation in solution are presented, suggesting that the nanostructures are organized in solution and then deposited onto the substrate. Correlations between the relative solvent polarity and the size of molecular nanostructures as well as between the solute concentration in dilute solutions and their density on the substrate are discussed.     

Hydrogen bonding control of molecular self-assembly

In this short review we describe approaches to the design and construction of synthetic molecules that mimic the process of self organization that is at the heart of biological complexity. Multi-subunit enzymes, viruses, and higher order DNA structures are formed by the non-covalent association of many smaller components. This self-assembly is controlled by the nature, number and orientation of interacting groups on the surface of the subunits. The central problem lies in overcoming the unfavorable entropy of multi-subunit association by significant enthalpic contribution from the binding of complementary regions on the subunits. We will place particular emphasis on the design of synthetic molecules that use hydrogen bonding interactions to control the formation of aggregates of well-defined structure.     

Cyclization and catenation directed by molecular self-assembly

We report here that molecular self-assembly can effectively direct and enhance specific reaction pathways. Using perylene pi-pi stacking weak attractive forces, we succeeded in synthesizing perylene bisimide macrocyclic dimer and a concatenated dimer-dimer ring from dynamic self-assembly of monomeric bis-N,N'-(2-(2-(2-(2-thioacetylethoxy)ethoxy)ethoxy)ethyl)perylenetetracarboxylic diimide. The monocyclic ring closure and the dimer-dimer ring concatenation were accomplished through formation of disulfide bonds, which was readily triggered by air oxidization under basic deacetylation conditions. The perylene cyclic dimer and its concatenated tetramer were characterized using both structural methods (NMR, mass spectroscopy) and photophysical measurements (UV-vis spectroscopy). Kinetic analyses offer informative insights about reaction pathways and possible mechanisms, which lead to the formation of complex concatenated rings. Molecular dynamic behaviors of both the monocyclic dimer and the concatenated dimer-dimer ring were modeled with the NWChem molecular dynamics software module, which shows distinct stacking activities for the monocyclic dimer and the concatenated tetramer.