Host Guest Molecules Research Articles

Observation of the energy transfer sequence in an organic host–guest system of a luminescent polymer and a phosphorescent molecule

We used steady state optical spectroscopies such as photoluminescence and photoinduced absorption (PA), and magnetic-field PA (MPA) for studying the energy transfer dynamics in films and organic light emitting diodes (OLED) based on host–guest blends with different guest concentrations of the fluorescent polymer poly-[2-methoxy, 5-(2′-ethyl-hexyloxy)phenylene vinylene] (MEHPPV-host), and phosphorescent molecule PtII-tetraphenyltetrabenzoporphyrin [Pt(tpbp); guest]. We show that the energy transfer process between the excited states of the host polymer and guest molecule takes a ‘ping-pong’ type sequence, because the lowest guest triplet exciton energy, ET(guest), lies higher than that of the host, ET(host). Upon photon excitation the photogenerated singlet excitons in the host polymer chains first undergo a Förster resonant energy transfer process to the guest singlet manifold, which subsequently reaches ET(guest) by intersystem crossing. Because ET(guest)>ET(host) there is a subsequent Dexter type energy transfer from ET(guest) to ET(host). This energy transfer sequence has profound influence on the photoluminescence and electroluminescence emission spectra in both films and OLED devices based on the MEHPPV-Pt(tpbp) system.

Stepwise Host–Guest [2 + 2] Photoreaction in a Hydrogen-Bonded One-Dimensional Coordination Polymer to a Two-Dimensional Layered Structure

Two hydrogen-bonded interdigitated one-dimensional (1D) coordination polymers (CPs) were synthesized from Cd(II) nitrate and 1,4-benzenedicarboxylate (bdc) with two 4-styrylpyridine (4spy) derivatives. In the solid state structure of 1D CP containing 2′-fluorostyrylpyridine (2F-4spy), a guest 2F-4spy was sandwiched between the terminal 2F-4spy ligands and the C═C bonds of all the 2F-4spy are well-aligned in host–guest-host–host-guest–host- sequence infinitely. From time-dependent photoreaction studies, it has been found that the [2 + 2] photocycloaddition occurs between host–guest molecules first followed by host–host pairs in a two-step process, which resulted in the quantitative conversion of C═C bond pairs to cyclobutane rings. It is a rare quantitative photoreaction between the host and guest molecules. Further, solvothermal crystallization of the final photoproduct furnished an interesting three-dimensional CP with bcg topology. Interestingly, 2NO2-4spy also furnished a similar hydrogen-bonded 1D CP ...

Theoretical study on supramolecular chemistry of alkali-metal cations with crown ether derivatized thiophenes

This paper describes systemically a theoretical research on the interaction of alkali-metal cations ( Li +, Na +, K + and Rb +) with five different crown ether derivatized thiophenes using density functional theory (DFT). The fully optimized geometries have been performed with real frequencies which indicate the minima states. The optimized structures and electronic properties, such as HOMO and LUMO energies, bandgaps of the free ligands L (L1-L5), the complexes L/M+ ( Li +, Na +, K + and Rb +) have been performed at B3LYP/6-31+G(d,p) and Lanl2DZ level. Natural bond orbital (NBO) and frequency analysis are discussed on the basis of the optimized geometric structures. The main driving forces of the coordination in host–guest molecules are investigated, the electron-donating O offers lone pair electrons to the contacting LP* (1-center valence antibond lone pair) of alkali-metal cations. In addition, the transition energies are calculated by the time-dependent density functional theory (TD-DFT).

HYDROGEN-BONDED STRUCTURES OF TRIMESIC AND MELAMINE ON HIGHLY ORIENTED PYROLYTIC GRAPHITE

The assembled structures of triangular symmetrically shape molecules with three functional groups, trimesic acid (TMA) and melamine, have been investigated by scanning tunneling microscopy on Highly Oriented Pyrolytic Graphite (HOPG) in ambient. Hexagonal porous structures are formed by these molecules and the cavity of the assembled structure becomes bigger when larger molecules are used as building blocks. The cavity within the assembled structure formed by TMA is bigger enough to host guest molecule.

Building on Cram's legacy: stimulated gating in hemicarcerands.

ConspectusDonald Cram’s pioneering Nobel Prize-winning work on host–guest molecules led eventually to his creation of the field of container molecules. Cram defined two types of container molecules: carcerands and hemicarcerands. Host–guest complexes of carcerands, called carceplexes, are formed during their synthesis; once a carceplex is formed, the trapped guest cannot exit without breaking covalent bonds. Cram defined a quantity called constrictive binding, arising from the mechanical force that prevents guest escape. The constrictive binding in carceplexes is high. In contrast, hemicarcerands have low constrictive binding and are able to release the incarcerated guests at elevated temperatures without breaking covalent bonds. We have designed molecules that can switch from carcerand to hemicarcerand through a change in structure that we call gating.The original discovery of gating in container molecules involved our computational studies of a Cram hemicarceplex that was observed to release a guest upon heating. We found that the side portals of this hemicarceplex have multiple thermally accessible conformations. An eight-membered ring that is part of a portal changes from a “chair” to a “boat” structure, leading to the enlargement of the side portal and the release of the guest. This type of gating is analogous to phenomena often observed with peptide loops in enzymes. We refer to this phenomenon as thermally controlled gating.We have also designed and synthesized redox and photochemically controlled gated hemicarceplexes. Gates are built onto host molecules so that the opening or closing of such gates is stimulated by reducing or oxidizing conditions, or by ultraviolet irradiation. In both cases, the appropriate stimuli can produce a carceplex (closed gates) or hemicarceplex (open gates). A hemicarceplex with closed gates behaves like a carceplex, due to its very high constrictive binding energy. When the gates are opened, constrictive binding is dramatically lowered, and guest entrance and exit become facile. This stimulated switching between open and closed states controls access of the guest to the binding site. The experimental and computational investigations of gated hemicarcerands and several potential applications of gated hemicarceplexes are described in this Account.

Density functional theory study on a fluorescent chemosensor device of aza-crown ether

Three derivatives of alkyl anthracene covalently bonded to aza-18-crown-6 at the nitrogen position, anthracene(CH2)n, (n = 1–3) which act as an on–off fluorogenic photoswitch have been theoretically studied using a computational strategy based on density functional theory at B3LYP/6-31 + G(d,p) method. The fully optimized geometries have been performed with real frequencies which indicate the minima states. The binding energies, enthalpies and Gibbs free energies have been calculated for aza-18-crown-6 (L) and their metal complexes. The natural bond orbital analysis is used to explore the interaction of host–guest molecules. The absorption spectra differences between L and their metal ligands, the excitation energies and absorption wavelength for their excited states have been studied by time-dependent density functional theory with the basis set 6-31 + G(d,p). These fluorescent sensors and switchers based on photoinduced electron transfer mechanism have been investigated. The PET process from aza-crown ether to fluorophore can be suppressed or completely blocked by the entry of alkali metal cations into the aza-crown ether-based receptor. Such a suppression of the PET process means that fluorescence intensity is enhanced. The binding selectivity studies of the aza-crown ether part of L indicate that the presence of the alkali metal cations Li+, Na+ and K+ play an important role in determining the internal charge transfer and the fluorescence properties of the complexes. In addition, the solvent effect has been investigated. Copyright © 2014 John Wiley & Sons, Ltd.

A quantum chemistry study of natural gas hydrates

The structure and properties of natural gas hydrates containing hydrocarbons, CO₂, and N₂ molecules were studied by using computational quantum chemistry methods via the density functional theory approach. All host cages involved in I, II, and H types structures where filled with hydrocarbons up to pentanes, CO₂ and N₂ molecules, depending on their size, and the structures of these host-guest systems optimized. Structural properties, vibrational spectra, and density of states were analyzed together with results from atoms-in-a-molecule and natural bond orbitals methods. The inclusion of dispersion terms in the used functional plays a vital role for obtaining reliable information, and thus, B97D functional was shown to be useful for these systems. Results showed remarkable interaction energies, not strongly affected by the type of host cage, with molecules tending to be placed at the center of the cavities when host cages and guest molecules cavities are of similar size, but with molecules approaching hexagonal faces for larger cages. Vibrational properties show remarkable features in certain regions, with shiftings rising from host-guest interactions, and useful patterns in the terahertz region rising from water surface vibrations strongly coupled with guest molecules. Likewise, calculations on crystal systems for the I and H types were carried out using a pseudopotential approach combined with Grimme's method to take account of dispersion.

Conjugated organic framework with three-dimensionally ordered stable structure and delocalized π clouds.

Covalent organic frameworks are a class of crystalline organic porous materials that can utilize π–π-stacking interactions as a driving force for the crystallization of polygonal sheets to form layered frameworks and ordered pores. However, typical examples are chemically unstable and lack intrasheet π-conjugation, thereby significantly limiting their applications. Here we report a chemically stable, electronically conjugated organic framework with topologically designed wire frameworks and open nanochannels, in which the π conjugation-spans the two-dimensional sheets. Our framework permits inborn periodic ordering of conjugated chains in all three dimensions and exhibits a striking combination of properties: chemical stability, extended π-delocalization, ability to host guest molecules and hole mobility. We show that the π-conjugated organic framework is useful for high on-off ratio photoswitches and photovoltaic cells. Therefore, this strategy may constitute a step towards realizing ordered semiconducting porous materials for innovations based on two-dimensionally extended π systems.

Phase and Glass Transitions Observed by Adiabatic Calorimetry of Host p-tert-Butylcalix[4]arene and Guest Toluene Inclusion Crystal, Suggesting the Progress of the Combined Order–Disorder Process of the Host–Guest Molecules

Heat capacities of crystalline p-tert-butylcalix[4]arene·toluene host-guest compound were measured in a temperature range between 3 K and 345 K by adiabatic calorimetry. The crystal showed one second-order phase transition at 256.4 K accompanied by wide heat-capacity tails on both the high and low temperature sides and two first-order phase transitions with heat-capacity peaks at 127 K and 139 K. The total entropy of a sequence of the phase transitions was assessed experimentally to be 47 J K(-1) mol(-1), indicating that the number of allowed distinguishable molecular configurations is over 100 in the high-temperature disordered state. Many such distinguishable configurations are understood to be produced by a special intermolecular interaction of C-H···(π-electron) bonds formed between p-tert-butylcalix[4]arene and toluene molecules, and the transitions were interpreted as originating from orientational order-disorder of both the guest toluene molecule and the tert-butyl groups of host arene in their combination.

Glutathione--hydroxyl radical interaction: a theoretical study on radical recognition process.

Non-reactive, comparative (2×1.2 μs) molecular dynamics simulations were carried out to characterize the interactions between glutathione (GSH, host molecule) and hydroxyl radical (OH•, guest molecule). From this analysis, two distinct steps were identified in the recognition process of hydroxyl radical by glutathione: catching and steering, based on the interactions between the host-guest molecules. Over 78% of all interactions are related to the catching mechanism via complex formation between anionic carboxyl groups and the OH radical, hence both terminal residues of GSH serve as recognition sites. The glycine residue has an additional role in the recognition of OH radical, namely the steering. The flexibility of the Gly residue enables the formation of further interactions of other parts of glutathione (e.g. thiol, α- and β-carbons) with the lone electron pair of the hydroxyl radical. Moreover, quantum chemical calculations were carried out on selected GSH/OH• complexes and on appropriate GSH conformers to describe the energy profile of the recognition process. The relative enthalpy and the free energy changes of the radical recognition of the strongest complexes varied from −42.4 to −27.8 kJ/mol and from −21.3 to 9.8 kJ/mol, respectively. These complexes, containing two or more intermolecular interactions, would be the starting configurations for the hydrogen atom migration to quench the hydroxyl radical via different reaction channels.

Vibrational Spectroscopic Studies on the Dicyclopropylaminecadmium(II)Tetracyanometallate(II) Benzene Clathrates

ABSTRACT New Hofmann–Td-type clathrates of the forms Cd(CPA)2M(CN)4.C6H6 (CPA = cyclopropylamine; M˭Cd or Hg) prepared in powder form and characterized by FT-IR, FT-Raman, far-IR spectra, X-ray diffraction, and elemental analyses are reported. Vibrational assignments are proposed for the bands of the host lattice and guest molecule. It is shown that the spectra are consistent with a proposed crystal structure for the compounds derived from X-ray diffraction measurements. The C, H, and N analyses were carried out for all the compounds. All the vibrational modes of coordinated CPA are characterized. The spectral features suggest that these compounds are similar in structure to the Hofmann–Td-type clathrates.

Molecular templates and nano-reactors: two-dimensional hydrogen bonded supramolecular networks on solid/liquid interfaces

With tunable cavities, the two-dimensional (2D) hydrogen bonded supramolecular networks are receiving increasing attention in surface supramolecular chemistry during the past decade. Herein, we give a brief account of our efforts on the 2D supramolecular host–guest chemistry based on molecular networks connected by hydrogen bonds, with an aid of scanning tunnelling microscopy (STM) technique. Following the molecular assembling, we have constructed a group of two-dimensional H-bonded supramolecular networks with cavities of different sizes, shapes, and symmetry characteristics on HOPG surface. Our researches have reported that these open porous structures could serve as molecular templates to host alien ensembles, as molecular sieves for molecular separation and recognition, as molecular sensors for ion detections, and as spatial confiners to control chemical reactions. It can be noticed that the interactions between host templates and guest molecules are emphasized throughout this review. On the basis of the confinement of supramolecular networks, it could be less problematic to explore the controllability, predictability, stability and applicability of the ordered structures.

Understanding adsorption-induced structural transitions in metal-organic frameworks: From the unit cell to the crystal

Breathing transitions represent recently discovered adsorption-induced structural transformations between large-pore and narrow-pore conformations in bi-stable metal-organic frameworks such as MIL-53. We present a multiscale physical mechanism of the dynamics of breathing transitions. We show that due to interplay between host framework elasticity and guest molecule adsorption, these transformations on the crystal level occur via layer-by-layer shear. We construct a simple Hamiltonian that describes the physics of host-host and host-guest interactions on the level of unit cells and reduces to one effective dimension due to the long-range elastic cell-cell interactions. We then use this Hamiltonian in Monte Carlo simulations of adsorption-desorption cycles to study how the behavior of unit cells is linked to the transition mechanism at the crystal level through three key physical parameters: the transition energy barrier, the cell-cell elastic coupling, and the system size.

A study of the interaction energy between a glycoluril molecular clip and dihydroxybenzenes or diaminobenzenes: a justification for easy separation by affinity chromatography

Theoretical calculations were performed to determine the interaction energy between a glycoluril (GL) molecular clip and hydroxybenzenes (HBs) and aminobenzenes (ABs). The theoretical calculations on the GL and its interactions were carried out using the hybrid functional closed-shell RB3LYP and the 6–31G* basis set, employing gaussian 03. The stability in energy of the guest inside the GL, ΔET(1), was in the following order: m-DHB-GL >o-DHB-GL >m-THB-GL >m-DAB-GL >o-THB-GL. The geometric parameters, in particular the bond lengths are discussed for the host molecule GL and guest molecules DHB and DAB and their parameters are compared with the host–guest molecules DHB-GL and DAB-GL, respectively.

Color Tuning of Nanofibers by Periodic Organic–Organic Hetero-Epitaxy

We report on the epitaxial growth of periodic para-hexaphenyl (p-6P)/α-sexi-thiophene (6T) multilayer heterostructures on top of p-6P nanotemplates. By the chosen approach, 6T molecules are forced to align parallel to the p-6P template molecules, which yields highly polarized photoluminescence (PL)-emission of both species. The PL spectra show that the fabricated multilayer structures provide optical emission from two different 6T phases, interfacial 6T molecules, and 3-dimensional crystallites. By a periodical deposition of 6T monolayers and p-6P spacers it is demonstrated that the strongly polarized spectral contribution of interfacial 6T can be precisely controlled and amplified. By analyzing the PL emission of both 6T phases as a function of p-6P spacer thickness (Δdp–6P) we have determined a critical value of Δdp–6P ≈ 2.73 nm where interfacial 6T runs into saturation and the surplus of 6T starts to cluster in 3-dimensional crystallites. These results are further substantiated by UPS and XRD measurements. Moreover, it is demonstrated by morphological investigations, provided by scanning force microscopy and fluorescence microscopy, that periodical deposition of 6T and p-6P leads to a significant improvement of homogeneity in PL-emission and morphology of nanofibers. Photoluminescence excitation experiments in combination with time-resolved photoluminescence demonstrate that the spectral emission of the organic multilayer nanofibers is dominated by a resonant energy transfer from p-6P host- to 6T guest-molecules. The sensitization time of the 6T emission in the 6T/p-6P multilayer structures depends on the p-6P spacer thickness, and can be explained by well separated layers of host–guest molecules obtained by organic–organic heteroepitaxy. The spectral emission and consequently the fluorescent color of the nanofibers can be efficiently tuned from the blue via white to the yellow-green spectral range.

UCST-driven self-assembly and crosslinking of diblock copolymer micelles

The upper critical solution temperature (UCST) behaviour of poly[oligo(ethylene glycol) monomethylether methacrylate] (POEGMA) in 1-octanol was exploited to self-assemble and crosslink micellar aggregates. Four diblock copolymers, POEGMA-block-poly(N-isopropylacrylamide) (POEGMA-b-PNIPAM), POEGMA-b-poly(N,N-diethylacrylamide) (POEGMA-b-PDEAM), and two examples of POEGMA-b-[PNIPAM-co-poly(pentafluorophenyl acrylate)] (POEGMA-b-(PNIPAM-co-PPFPA)), containing different amounts of activated PFP esters, were found to reversibly self-assemble into well-defined spherical micelles upon cooling in 1-octanol, as evidenced by dynamic light scattering (DLS) and electron microscopy. Transition temperatures, measured by turbidity and DLS, were around room temperature and the PNIPAM and PDEAM blocks did not significantly influence the critical temperature of the POEGMA block compared to homo POEGMA. The aggregates exhibited an inverted morphology compared to PNIPAM core–POEGMA shell micelles accessible through a thermally triggered self-assembly in water exploiting the lower critical solution temperature (LCST) behaviour of PNIPAM. Inverted micelles with PNIPAM-co-PPFPA coronae in 1-octanol were shell crosslinked using a diamine and subsequently transferred into water. This procedure yielded cage-like structures with swollen POEGMA cores and crosslinked PNIPAM shells. The shells reversibly collapsed onto the cores when heated above the LCST of PNIPAM, providing particles with novel architecture and the potential to host and release guest molecules by a thermal trigger.

Mechanism of Breathing Transitions in Metal–Organic Frameworks

We present a multiscale physical mechanism and a stochastic model of breathing transitions, which represent adsorption-induced structural transformations between large-pore and narrow-pore conformations in bistable metal–organic frameworks, such as MIL-53. We show that due to interplay between host framework elasticity and guest molecule adsorption, these transformations on the level of the crystal occur via layer-by-layer shear. We construct a simple Hamiltonian that describes the physics of host–host and host–guest interactions and show that a respective Monte Carlo simulation model qualitatively reproduces the experimentally observed features of breathing transitions.

Observations of Host Guest Interactions Specific to Molecular Matrices: Water Monomers and Dimers in Hydrogen Matrices

Water monomers and dimers have been studied at low temperatures in matrices of solid p-H(2), o-D(2), n-H(2), and n-D(2) using infrared spectroscopy. Our data demonstrate interaction mechanisms between host matrix and guest molecules that are different from the ones observed in atomic noble gas matrices. Notably both guest/host rotational--rotational interaction and matrix induced modifications of the guest libration modes are observed. We also show that different types of interaction influence the relaxation times of some of guest modes. Water rotates freely in p-H(2) and o-D(2) but librates in n-H(2) and n-D(2). Rotational relaxation is faster in o-D(2) than in p-H(2) and faster in p-H(2) than in Ne. This is attributed to interactions between water rotation and matrix molecule rotation in p-H(2) and o-D(2). In n-H(2) and n-D(2), a strong water libration band is observed in the far-infrared, and strong water monomer vibration bands have libration satellites. Water dimer bands, close to matrix rotation bands, are perturbed by the matrix motions. The H-bonded isomer H(2)O--HOD rapidly converts to the D-bonded form H(2)O-DOH in p-H(2) and in o-D(2) but slowly in n-H(2) and n-D(2).

2-(4-Phenyl-5-pyridin-2-yl-4H-1,2,4-triazol-3-yl)cyclohexanecarboxylic Acid and Its DMSO Solvate: Synthesis, Crystal Structure and Biological Activity

A new 1,2,4-triazole derivative, 2-(4-phenyl-5-pyridin-2-yl-4H-1,2,4-triazol-3-yl)cyclohexanecarboxylic acid, C20H20N4O2 (I), and its dimethyl sulfoxide solvate 1:1 (II) have been synthesized and their crystal structure was established. Compound (I) was screened for its antiproliferative and antiinflammatory activity. Structural analysis indicated the substantial difference between two symmetry independent molecules in (I) and this in (II), it manifests in the relative orientation of pyridine/phenyl and triazole rings, as well as in the orientation of carboxyl group with respect to cyclohexane ring. The molecules A and B in the crystal (I) form two hydrogen-bonded chains through O–Hcarboxyl and Ntriazole atoms, giving separate catemers of symmetry independent molecules. The catemer of (IA) running along the 21 axis is homochiral, while the catemer (IB) is racemic—formed about the c glide plane. In the crystalline solvate (II) complexation of (I) with DMSO induced enantiomeric self-resolution. Obtained crystals are racemic twins, in which each part is built of one enantiomer of (I) having the relative configuration 11S,12R or 11R,12S. A pair of host–guest molecules is linked by the O–Hcarboxyl⋯ODMSO hydrogen bond.

Reverse Vesicles from a Salt-Free Catanionic Surfactant System: A Confocal Fluorescence Microscopy Study

We give a detailed confocal fluorescence microscopy study on reverse vesicles from a salt-free catanionic surfactant system. When tetradecyltrimethylammonium laurate (TTAL) and lauric acid (LA) are mixed in cyclohexane at the presence of a small amount of water, stable reverse vesicular phases form spontaneously. The reverse vesicular phases can be easily labeled with dyes of varying molecular size and hydrophobicity while the dyes are nearly insoluble in cyclohexane without reverse vesicles. This indicates the reverse vesicular phases can be good candidates to host guest molecules. With the help of a fluorescence microscope combined a confocal method, the features of these interesting reverse supramolecular self-assemblies were revealed for the first time. Because of the absence of electrostatic repulsions and hydration forces between adjacent vesicles, the reverse vesicles have a strong propensity to aggregate with each other and form three-dimensional clusters. The size distributions of both individual reverse vesicles and clusters are polydisperse. Huge multilamellar reverse vesicles with closely stacked thick walls (giant reverse onions) were observed. Besides the spherical reverse vesicles and onions, other supramolecular structures such as tubes have also been detected and structural evolutions between different structures were noticed. These interesting supramolecular self-assemblies form in a nonpolar organic solvent may serve as ideal micro- or nanoreaction centers for biological reactions and synthesis of inorganic nanomaterials.