Recognition and dissociation kinetics in the interfacial molecular recognition of barbituric acid by amphiphilic melamine-type monolayers

Progress in the understanding of interfacial molecular recognition kinetics is obtained by use of the sweeping technique for experimental studies of the reaction kinetics between a host monolayer and a non-surface-active species dissolved in the aqueous subphase. The experimental results show that the interfacial recognition reaction between a 2C(11)H(23)-melamine (2,4-di(n-undecylamino)-6-amino-1,3,5-triazine) monolayer and dissolved barbituric acid is reversible when the 2C(11)H(23)-melamine/barbituric acid monolayer is transferred back onto a pure water subphase. The kinetics of the recognition and dissociation reaction is experimentally and theoretically investigated. The approximate additive theoretical model developed recently is extended to consider the dissociation kinetics of the interfacial supramolecular complex. The kinetic constants for the recognition and dissociation reactions in the mixed monolayer consisting of 2C(11)H(23)-melamine and 2C(11)H(23)-melamine/barbituric acid complex are determined. It is shown that the kinetic constant of the recognition reaction is nearly independent of temperature, whereas that of the dissociation reaction increases with increasing temperature.     

Analytical applications of organized molecular assemblies

Organized molecular assemblies have great potential utility in many types of analytical method. This review is concerned with recent of micelles, reversed micelles and micro-emulsions in shifting acid-base equilibria, and in electrochemical measurements, ultraviolet-visible spectrophotometry, micellar-echanced phosphorimetry and fluorimetry, liquid-liquid extraction, flame and plasmas atomic spectrometry, and high-performance and thin-layer liquid chromatography.     

Self-assembled spiral nanoarchitecture and supramolecular chirality in Langmuir-Blodgett films of an achiral amphiphilic barbituric acid

An amphiphilic barbituric acid derivative was found to form stable monolayers showing a clear phase transition at the air/water interface. It is interesting to find that the deposited Langmuir-Blodgett (LB) films of the compound showed circular dichroism (CD) although the molecule itself was achiral. AFM measurements on the transferred one-layer LB film revealed that spiral nanoarchitectures were formed. It was further found that the supramolecular chirality of the LB films was related to symmetry breaking at the interface. Hydrogen bonding and the pi-pi stacking between the neighboring molecules resulted in chiral fibers which formed the spiral structures. To the best of our knowledge, this is the first report on the chirality of the molecular assemblies and spiral nanostructures formed through the air/water interface by achiral molecules.     

The development of zeolite-entrapped organized molecular assemblies

A brief summary is presented of the development of organized molecular assemblies entrapped within the supercages of Y-zeolite. Emphasis is placed on work originating in the author's laboratory, although a discussion of some of the important contributions made by other workers, which inspired and facilitated this work, are included. Following pioneering studies by Lunsford and co-workers, which demonstrated the feasibility of encapsulating the common photosensitizer [Ru(bpy)3]2+ within the Y-zeolite supercage, Dutta and co-workers documented efficient photoinduced electron transfer to viologen acceptors occupying neighboring supercages. We have extended the range of available materials by developing synthetically versatile methods to permit the incorporation of heteroleptic complexes, including constituent ligands which contain peripheral nitrogen donor groups; for example, 2,2'-bipyrazine. In an impressive study employing zeolite-excluded acceptors, Dutta and co-workers showed that the reducing equivalents available from photoinduced electron transfer from the zeolite entrapped sensitizer to intra-zeolite acceptors could be transferred to the extra-zeolite acceptors in aqueous suspensions, although the net charge-separation efficiency was low, presumably because of a persistent relatively efficient back-electron transfer process involving the primary photoproduct; that is, the entrapped sensitizer-acceptor dyad. Exploiting the susceptibility of certain heteroleptic complexes to add reactive ruthenium reagents, methods were developed to construct spatially organized donor-sensitizer-acceptor triads within the supercage framework of Y-zeolite. Such assemblies exhibit dramatically improved net charge-separation efficiencies, presumably as a consequence of inhibiting the wasteful back-electron transfer reaction between the initial sensitizer-acceptor couple.     

Probing molecular recognition in the solid-state by use of an enolizable chromophoric barbituric acid

Complex formation of the enolizable chromophor 1-n-butyl-5-(4-nitrophenyl)barbituric acid 1 with multiple binding sites for supramolecular assemblies and its corresponding adducts produced with the Proton Sponge (1,8-bis(dimethylamino)naphthalene), PS) and the adenine-mimetic 2,6-diacetamidopyridine (DAC) have been studied by means of solid-state proton NMR spectroscopy under fast magic-angle spinning, X-ray analysis, and UV/vis spectroscopy. Both NMR data and X-ray results reveal that the enolic chromophor undergoes self-aggregation to hydrogen-bonded dimers which are involved in stacked arrangements. Depending on the nature of the added base, this dimeric assembly is preserved in the formed enolate anion but can be broken in the presence of complementary hydrogen-bonding pattern leading to supramolecular complexes. Molecular recognition of these structural different bases significantly influences the chromophoric pi-system of 1.     

Enantiomeric recognition of amino acid derivatives by chiral schiff bases of calix[4]arene

Novel calix[4]arene Schiff bases bearing chiral substituents both on the upper and the lower rims have been developed. These chiral receptors exhibit good chiral recognition ability towards α-amino acid ester hydrochlorides (up to K_D/K_L = 4.36, ΔΔG₀ = − 3.65 kJ mol⁻¹) in CHCl₃. The molecular recognition abilities and enantioselectivities for guests are also discussed from a thermodynamic point of view.     

Interaction of flavonoids within organized molecular assemblies of anionic surfactant

The interaction of various flavonoids (compounds having C6-C3-C6 configuration) with sodium dodecyl sulfate (SDS) an anionic surfactant was studied through absorption spectroscopy as a function of the concentration of surfactant above and below the critical micelle concentration (CMC) of the surfactant. A mechanism was proposed for the interaction between these flavonoids and anionic surfactants. The approximate number of additive molecules (flavonoids) incorporated per micelle was estimated at a particular concentration of SDS. Incorporation of additive in micelles shifts the UV absorption bands towards higher wavelengths of different magnitude. The spectral shift also depends upon the nature of the surfactant head group. The absorption spectra of the flavonoids in aqueous solution and in methanol are also reported.     

Supramolecular assemblies with tunable morphologies from homopolymeric and small organic molecular building blocks

This work demonstrates the formation of micrometer-sized supramolecular assemblies with tunable morphologies using a homopolymer, poly(4-vinylpyridine), and a small organic acid, 5,7-dodecadiynedioic acid, as the building molecules. Three different morphologies (hollow spheres, solid spheres, and rods) were obtained, depending on the molar ratio of the building molecules. It is proposed that hydrogen bonding between P4VP and DCDA and the pi-pi stacking of the diacetylenic moieties are responsible for the formation of these assemblies. Interestingly, ordered hexagonal and lamellar mesostructures were also formed within the microstructure during the co-assembly process. As a result, UV irradiation of the supramolecular assemblies polymerized the diacetylenic moieties, resulting in cross-linked and responsive blue polydiacetylenic assemblies that can change color to red upon external stimuli (e.g., thermal stimuli). This work provides a novel concept of the synthesis of responsive supramolecular assemblies from a homopolymer and small organic molecules.     

Handedness inversion of chiral amphiphilic molecular assemblies evidenced by supramolecular chiral imprinting in mesoporous silica assemblies

Antipodal twisted helical ribbons with lamellar bilayer structure were obtained by self-assembly of chiral amphiphilic molecules in water and water/ethanol. The handedness inversion of the molecular arrangement in these antipodal helical ribbons was investigated by using chiroptical spectroscopy and molecular probes in their antipodal mesoporous silica assemblies synthesized through pairing interaction between the head group of the chiral amphiphilic molecules and a co-structure-directing agent. The supramolecular chirality is imprinted in the pore surface through the organic group of the co-structure-directing agent. The mirror-image diffuse-reflectance circular dichroism spectra of the conjugated discotic probing molecule introduced into their supramolecular chiral imprinted mesoporous silica demonstrated the origin of inverse chirality from the antipodal helical stacking of the molecules.     

Cyclocondensation of 3-aryl-2-cyano-2-butenoic acid esters with schiff bases

The condensation of ethyl 3-aryl-2-cyano-2-butenoates with benzylideneanilines in the presence of aluminum chloride affords 1,4,6-triaryl-2-oxo-1,2,5,6-tetrahydropyridine-2-carbonitriles; the reaction with hydrobenzamide gives N-unsubstituted tetrahydropyridine.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 209–211, February, 1988.     

Photoinduced intra- and intermolecular electron transfer in solutions and in solid organized molecular assemblies

The present paper highlights results of a systematic study of photoinduced electron transfer, where the fundamental aspects of the photochemistry occurring in solutions and in artificially or self-assembled molecular systems are combined and compared. In photochemical electron transfer (ET) reactions in solutions the electron donor, D, and acceptor, A, have to be or to diffuse to a short distance, which requires a high concentration of quencher molecules and/or long lifetimes of the excited donor or acceptor, which cannot always be arranged. The problem can partly be avoided by linking the donor and acceptor moieties covalently by a single bond, molecular chain or chains, or rigid bridge, forming D-A dyads. The covalent combination of porphyrin or phthalocyanine donors with an efficient electron acceptor, e.g. fullerene, has a two-fold effect on the electron transfer properties. Firstly, the electronic systems of the D-A pair result in a formation of an exciplex intermediate upon excitation both in solutions and in solid phases. The formation of the exciplex accelerates the ET rate, which was found to be as fast as >10(12) s(-1). Secondly, the total reorganization energy can be as small as 0.3 eV, even in polar solvents, which allows nanosecond lifetimes for the charge separated (CS) state. Molecular assemblies can form solid heterogeneous, but organized systems, e.g. molecular layers. This results in more complex charge separation and recombination dynamics. A distinct feature of the ET in organized assemblies is intermolecular interactions, which open a possibility for a charge migration both in the acceptor and in the donor layers, after the primary intramolecular exciplex formation and charge separation in the D-A dyad. The intramolecular ET is fast (35 ps) and efficient, but the formed interlayer CS states have lifetimes in microsecond or even second time domain. This is an important result considering possible applications.     

Reaction of schiff bases with α-halo ketones

1-Halomethyl-3-arylbenzo[f]quinoline derivatives were obtained by condensation of Schiff bases with α-halo ketones.     

Supramolecular chirality of the hydrogen-bonded complex Langmuir-Blodgett film of achiral barbituric acid and melamine

Complex monolayers of barbituric acid and melamine were formed by spreading a chloroform solution of amphiphilic barbituric acid on the subphase of melamine solution. It was confirmed that the complex monolayer was formed through in situ complementary hydrogen bonding at the air-water interface. It was interesting to find that the complex LB films showed supramolecular chirality although both of the molecules were achiral, as verified by the circular dichroism spectral measurements. It was suggested that the pi-pi stacking of the neighboring barbituric acid and melamine group in a helical sense resulted in the chirality of the molecular assemblies. Due to the directionality of the hydrogen bonding, the BA-M film could form regular aligned nanofibers on the AFM images. Increasing the subphase temperature will lead to the decrease of CD intensity and the change of the morphologies. We suggested that the strength of the hydrogen bonding resulted in the difference.     

Ortho effects in schiff bases of diaminodicyanoethene

In the electron impact mass spectra of azomethines derived from various substituted aromatic aldehydes and diarninodicyanoethene the superposition of two ortho effects concurring with the azomethine group is apparent: one involving the amino group of the diaminodicyanoethene part accounts for the cyclization to [C₅H₃N₄]⁺ ions and the other involving ortho substituents of the benzylidene part which can interact with the azomethine moiety is responsible for specific fragment ions, suppressing the typical fragmentations of azomethines. The ortho effect was studied for the o-nitro derivative by labelling experiments, analysis of metastable transitions and collisional activation comparing model ions, demonstrating that the specific [M-H₂O]⁺˙ and [C₇H₅NO₂]⁺˙ ions are the result of cyclization processes.     

Alignment of liquid crystals on ultrathin organized molecular films

Liquid crystal (LC) alignment techniques based on various kinds of ultrathin organized molecular films are reviewed. The mechanisms of LC alignment on the organized films are discussed. For the homeotropic alignment of LCs the main anchoring mechanism is due to the dipole–dipole interaction between polar groups of an aligning agent and LC molecules while the homogeneous alignment is mainly attributed to the orientation of polymer chains or polymer aggregates. An experimental system for an anchoring transition induced by a conformation change of aligning molecules is introduced. Finally the AFM experimental observations on the rubbed polymer films and its mechanisms are summarized.     

Template-induced and molecular recognition directed hierarchical generation of supramolecular assemblies from molecular strands

The linear oligo-isophthalamide strand 1 undergoes a conformational reorganization upon binding of a cyanuric acid template as effector to afford a helical disklike object possessing radially disposed alkyl residues. Solvophobic and stacking interactions, in turn, drive a "second level" self-assembly of the templated structure, the stacking of the helical disks, to yield fibers as revealed by electron microscopy. These data provide insight into the interplay of the different structural and interactional features of the molecular components towards the formation of supramolecular fibers through sequential hierarchical self-assembly events and suggest design strategies for the effector-controlled generation of related supramolecular assemblies.     

Crystal and molecular structures of ionophore-siderophore host-guest supramolecular assemblies relevant to molecular recognition

Ionophore-siderophore host-guest assemblies composed of 18-crown-6 and ferrioxamine B, benzo-18-crown-6 and ferrioxamine B, and cis-syn-cis-dicyclohexano-18-crown-6 and ferrioxamine B were successfully crystallized, and their structures were determined by single-crystal X-ray diffraction. All three crystal lattices also include solvated Mg(II) and perchlorate ions. The ionophore-siderophore host-guest assembly is noncovalently held together by a hydrogen bonding interaction between the pendant protonated amine in the second coordination sphere of ferrioxamine B and the hydrogen bond acceptor oxygen atoms in the crown ether. The crystals of 18-crown-6:ferrioxamine B host-guest assembly are monoclinic, with space group P2(1)/c, and four molecules per unit cell with dimensions a = 19.8327(11) A, b = 20.4111(11) A, c = 15.1698(8) A, and beta = 96.435(1) degrees. The crystals of benzo-18-crown-6:ferrioxamine B host-guest assembly are triclinic, with space group P(-)1, and two molecules per unit cell with dimensions a = 11.1747(10) A, b = 16.0580(15) A, c = 18.4175(17) A, alpha = 80.469(3) degrees, beta = 81.481(3) degrees and gamma = 70.212(2) degrees. The crystals of cis-syn-cis-dicyclohexano-18-crown-6:ferrioxamine B host-guest assembly are monoclinic, with space group P2(1)/c, and four molecules per unit cell with dimensions a = 20.1473(13) A, b = 21.5778(15) A, c = 14.8013(10) A, and beta = 94.586(2) degrees. The crystal structures of all three host-guest assemblies contain a racemic mixture of Lambda-N-cis, cis and Delta-N-cis, cis coordination isomers of ferrioxamine B. The crystal structures indicate that the steric rigidity of the benzo-18-crown-6 and cis-syn-cis-dicyclohexano-18-crown-6 cavity has a pronounced effect on the conformation of the crown ring and ultimately on the hydrogen bonding interactions between the crown ethers and ferrioxamine B. The structural parameters and the conformational features of the ferrioxamine B guests compare very well with each other and with those of the ferrioxamine B structure obtained in the absence of a host. Structural features relevant to siderophore molecular recognition are discussed.