Atomistic Insight Into the Host–Guest Interaction of a Photoresponsive Metal–Organic Framework

Photoresponsive functional materials have gained increasing attention due to their externally tunable properties. Molecular switches embedded in these materials enable the control of phenomena at the atomic level by light. Metal–organic frameworks (MOFs) provide a versatile platform to immobilize these photoresponsive units within defined molecular environments to optimize the intended functionality. For the application of these photoresponsive MOFs (pho-MOFs), it is crucial to understand the influence of the switching state on the host–guest interaction. Therefore, we present a detailed insight into the impact of molecular switching on the intermolecular interactions. By performing atomistic simulations, we revealed that due to different interactions of the guest molecules with the two isomeric states of an azobenzene-functionalized MOF, both the adsorption sites and the orientation of the molecules within the pores are modulated. By shedding light on the host–guest interaction, our study highlights the unique potential of pho-MOFs to tailor molecular interaction by light.     

Carbazole endcapped heterofluorenes as host materials: theoretical study of their structural, electronic, and optical properties

By mimicking the molecular structure of 4,4'-bis(N-carbazolyl)-2,2'-biphenyl (CBP), which is a widely used host material, a new series of host molecules (carbazole-endcapped heterofluorenes, CzHFs) were designed by linking the hole-transporting carbazole to the core heterofluorene molecules in either meta or para positions of the heterofluorene. The aromatic cores considered in this study are biphenyl, fluorene, silafluorenes, germafluorenes, carbazole, phosphafluorene, oxygafluorene, and sulfurafluorene. To reveal their molecular structures, optoelectronic properties and structure-property relationships of the proposed host materials, an in-depth theoretical investigation was elaborated via quantum chemical calculations. The electronic structures in the ground states, cationic and anionic states, and lowest triplet states of these designed molecules have been studied with emphasis on the highest occupied molecular orbitals (HOMOs), the lowest unoccupied molecular orbitals (LUMOs), energy gaps (E(g)), triplet energy gaps ((3)E(g)), as well as some other electronic properties including ionization potentials (IPs), electron affinities (EAs), reorganization energies (λ), triplet exciton generation fraction (χ(T)), spin density distributions (SD), and absorption spectra. These photoelectronic properties can be tuned by chemical modifications of the heteroatom and the carbazole substitution at different positions. This study provides theoretical insights into the nature of host molecules, and shows that the designed CzHFs can meet the requirements of the host materials for triplet emitters.     

Cooperative molecular recognition of dyes by dyad and triad cyclodextrin-crown ether conjugates

Three beta-cyclodextrin (beta-CyD) derivatives with crown ether units, that is N-(4'-benzo-15-crown-5)-6-imino-6-deoxy-beta-CyD (2), 6,6'-[N-(4,4'-dibenzo-18-crown-6)-imino]-bridged bis(beta-CyD)(3), and 2,2'-[O-(4',5'-benzo-15-crown-5)-ethyl]-bridged bis (beta-CyD)(5), were synthesized as cooperative recognition receptor models. Their molecular binding behavior with four representative fluorescent dyes, i.e., ammonium 8-anilino-1-naphthalenesulfonate (ANS), sodium-6-toluidino-2-naphthalene-sulfonate (TNS), Acridine Red (AR) and Rhodamine B (RhB), was investigated in buffer solutions (pH = 7.20) at 25 degreesC by means of circular dichroism, NMR and fluorescence spectroscopy. 2D-ROESY experiments showed that dyad host 2 and triad host 3 adopted a CyD-guest-crown ether binding mode, while triad host 5 adopted a CyD-guest-CyD binding mode, upon inclusion complexation with guest molecules. Therefore, hosts 2 and 3 showed high molecular recognition ability towards charged guests, giving an enhanced binding ability up to 115 times for ANS by 3 and fairly high molecular selectivity up to 1450 times for the ANS/AR pair by 2 as compared with native beta-CyD in an aqueous phosphate buffer solution. On the other hand, host 5 was found to be able to effectively recognize the shape of a guest molecule, showing significantly higher binding ability towards linear guests. The binding affinities and molecular recognition abilities of these CyD-crown ether conjugates towards guest molecules are discussed from the viewpoint of electrostatic and/or hydrophobic interactions, size/shape-fit concept, and multiple recognition mechanism between host and guest.     

A molecular approach to Cu doped ZnO nanorods with tunable dopant content

A novel molecular approach to the synthesis of polycrystalline Cu-doped ZnO rod-like nanostructures with variable concentrations of introduced copper ions in ZnO host matrix is presented. Spectroscopic (PLS, variable temperature XRD, XPS, ELNES, HERFD) and microscopic (HRTEM) analysis methods reveal the +II oxidation state of the lattice incorporated Cu ions. Photoluminescence spectra show a systematic narrowing (tuning) of the band gap depending on the amount of Cu(II) doping. The advantage of the template assembly of doped ZnO nanorods is that it offers general access to doped oxide structures under moderate thermal conditions. The doping content of the host structure can be individually tuned by the stoichiometric ratio of the molecular precursor complex of the host metal oxide and the molecular precursor complex of the dopant, Di-aquo-bis[2-(methoxyimino)-propanoato]zinc(II) 1 and -copper(II) 2. Moreover, these keto-dioximato complexes are accessible for a number of transition metal and lanthanide elements, thus allowing this synthetic approach to be expanded into a variety of doped 1D metal oxide structures.     

Application of a high-performance,special-purpose computer,GRAPE-2A,to molecular dynamics

The special-purpose computer GRAPE-2A accelerates the calculation of pairwise interactions in many-body systems. This computer is a back-end processor connected to a host computer through a Versa Module Europe (VME) bus. GRAPE-2A receives coordinates and other physical data for particles from the host and then calculates the pairwise interactions. The host then integrates an equation of motion by using these interactions. We did molecular dynamics simulations for two systems of liquid water: System 1 (1000 molecules), and System 2 (1728 molecules). The time spent for one step of molecular dynamics was 3.9 s (System l), and 10.2 s (System 2). The larger the molecular system, the higher the performance. The speed of GRAPE-2A did not depend on the formula describing the pairwise interaction. The cost performance was about 20 times better than that of the fastest workstations available today, and GRAPE-2A cost only $22,000. © 1994 by John Wiley & Sons, Inc.     

Enthalpy-entropy compensation reveals solvent reorganization as a driving force for supramolecular encapsulation in water

A chiral self-assembled M4L6 host assembly has been shown to be a suitable host for the supramolecular encapsulation of a series of guests in polar solvents, ranging from simple organic ammonium cations to more complex organometallic species. This molecular recognition process creates highly selective reactivity within the host cavity. In order to understand the factors driving the molecular recognition process, the standard thermodynamic parameters for encapsulation were determined for a series of protiated and fluorinated iridium guests in a variety of polar solvents using van't Hoff analysis. The encapsulation process for these guests exhibited enthalpy-entropy compensation effects. In solvents such as water and methanol, error analysis suggests a chemical origin for this behavior. In contrast, error analysis of this compensation behavior in polar aprotic solvents such as dimethyl sulfoxide reveals that this correlation is due to an artifact inherent in the intrinsic correlation between the enthalpy and entropy terms in the van't Hoff analysis. Guest encapsulation in polar protic solvents such as water appears to be driven by initial desolvation of the guest with concomitant rearrangement of the hydrogen bond networks in solution. This behavior shares common characteristics with other synthetic and natural host-guest and molecular recognition processes in aqueous solution, ranging from simple crown ether to complex enzyme-ligand interactions.     

Origins of selectivity for the [2+2] cycloaddition of alpha,beta-unsaturated ketones within a porous self-assembled organic framework

This article studies the origins of selectivity for the [2+2] cycloadditions of alpha,beta-unsaturated ketones within a porous crystalline host. The host, formed by the self-assembly of a bis-urea macrocycle, contains accessible channels of approximately 6 A diameter and forms stable inclusion complexes with a variety of cyclic and acyclic alpha,beta-unsaturated ketone derivatives. Host 1 crystals provide a robust confined reaction environment for the highly selective [2+2] cycloaddition of 3-methyl-2-cyclopentenone, 2-cyclohexenone, and 2-methyl-2-cyclopentenone, forming their respective exo head-to-tail dimers in high conversion. The products are readily extracted from the self-assembled host and the crystalline host can be efficiently recovered and reused. Molecular modeling studies indicate that the origin of the observed selectivity is due to the excellent match between the size and shape of these guests to dimensions of the host channel and to the preorganization of neighboring enones into favorable reaction geometries. Small substrates, such as acrylic acid and methylvinylketone, were bound by the host and were protected from photoreactions. Larger substrates, such as 4,4-dimethyl-2-cyclohexenone and mesityl oxide, do not undergo selective [2+2] cycloaddition reactions. In an effort to understand these differences in reactivity, we examined these host-guest complexes by thermogravimetric analysis (TGA), NMR, powder X-ray diffraction (PXRD) and molecular modeling.     

Molecular dynamics of host-guest complexes of small gas molecules with calix[4]arenes

The unexpected sorption of gases by a low-density p-tert-butylcalix[4]arene crystal polymorph raises fundamental questions about differential gas transport and sequestration in the organic solid state. To gain insight into the processes underlying these observations, we have used molecular dynamics simulations, augmented with calculations of potentials of mean force, to investigate the stability of isolated host-guest complexes and the relationship between the dynamics of these complexes and the dynamics of a solvated host molecule. Thermal fluctuations of the calixarenes themselves are found to be consistent with proposed mechanisms for gas entry into the host cavities, while relative host-guest stabilities correlate well with experimental absorption-desorption isotherms in some cases (CO2 and CH4) but not in others (C2H2). In these isolated systems, stable complexes characteristically form when the attractive interactions of the guest with the ring of negative charge density on the inner surface of the host cavity are not disrupted by thermal motion. The experimentally observed efficient uptake of gases such as C2H2 by the host crystals suggests, however, that stabilization of host-guest complexes in some systems may derive from dynamical constraints imposed by the crystal lattice.     

Guest–host systems containing anthraquinone dyes with multiple visible transitions giving positive and negative dichroic order parameters: an assessment of principal molecular axes and computational methods

ABSTRACT

Three 1,4-disubstituted anthraquinone dyes with bis(4-n-butylphenyl) substituents connected via amine or amide linking groups have been studied as guest molecules dissolved in the nematic host E7. UV-visible absorption spectroscopy has shown each of the dyes to exhibit multiple absorption bands in the visible region, and dichroic order parameters obtained from polarised spectra of aligned guest–host samples were shown to differ significantly between the bands for each dye, and between the dyes. Time-dependent density functional theory calculations indicated that each dye exhibits several transitions, giving transition dipole moment vectors with a range of orientations, and fully atomistic molecular dynamics simulations of the guest–host mixtures showed differences in the calculated molecular alignments of the dyes. Combining the results from these two sets of calculations enabled a comparison of molecular alignment models based on the moments of inertia and the surface tensors of the dyes. The match between calculated and experimental values was improved significantly when using the surface tensor rather than the moment of inertia model, indicating that the shapes of the molecular surfaces of these dyes are crucial to their alignment. A novel method of calculating polarised UV-visible absorption spectra of dyes in liquid crystal hosts is also presented.     

A Water‐Soluble Perylene Bisimide Cyclophane as a Molecular Probe for the Recognition of Aromatic Alkaloids

Herein, we report a water‐soluble macrocyclic host based on perylene bisimide (PBI) chromophores that recognizes natural aromatic alkaloids in aqueous media by intercalating them into its hydrophobic cavity. The host–guest binding properties of our newly designed receptor with several alkaloids were studied by UV/Vis and fluorescence titration experiments as the optical properties of the chromophoric host change significantly upon complexation of guests. Structural information on the host–guest complexes was obtained by 1D and 2D NMR spectroscopy and molecular modelling. Our studies reveal a structure–binding property relationship for a series of structurally diverse aromatic alkaloids with the new receptor and higher binding affinity for the class of harmala alkaloids. To our knowledge, this is the first example of a chromophoric macrocyclic host employed as a molecular probe for the recognition of aromatic alkaloids.     

Propagation of Enzyme‐Induced Surface Events inside Polymer Nanoassemblies for a Fast and Tunable Response

We report a new molecular design strategy that allows for the propagation of surface enzymatic events inside a supramolecular assembly for accelerated molecular release. The approach addresses a key shortcoming encountered with many of the currently available enzyme‐induced disassembly strategies, which rely on the unimer–aggregate equilibria of amphiphilic assemblies. The enzymatic response of the host to predictably tune the kinetics of guest‐molecule release can be programmed by controlling substrate accessibility through electrostatic complexation with a complementary polymer. Accelerated guest release in response to the enzyme is shown to be accomplished by a cooperative mechanism of enzyme‐triggered supramolecular host disassembly and host reorganization.     

新型Schiff碱分子钳对中性分子的识别性能研究

采用差紫外光谱法考察了3种新型Schiff碱分子钳对一系列二苯甲酮、芳香二胺的识别性能.测定了主客体间的结合常数(Ka)和自由能变化(ΔG0).结果表明,分子钳对所考察的客体显示良好的识别作用,主客体间形成1:1型超分子配合物.讨论了识别作用的推动力与形状、大小匹配和几何互补等因素对形成主客体配合物的影响,并利用核磁氢谱与计算机模拟作为辅助手段对主要的实验结果与现象进行了解释.     

手性锌卟啉的非线性光学性质及对咪唑类客体分子识别的构象研究

在纳秒(ns)领域中，利用Z-扫描技术测定了Zn(o-BocTyr)TAPP(主体1)和Zn(p-BocTyr)TAPP(主体2)两种手性锌卟啉的三阶非线性光学性质以及对咪唑类客体的分子识别行为的构象研究. 结果表明，(1) 两种手性锌卟啉都具有反饱和吸收效应和自散焦效应；(2) 由于两种主体中侧链位置的差异，造成两种主体分子极化程度的不同，主体1具有较大的三阶非线性折射率(n2)值；(3) 主体1中侧链苯环与卟啉环之间存在一定的相互作用；(4) 分子识别出现了配位方向的选择性，客体咪唑(Im)与主体1侧链中的苯环能够形成π-π相互作用，选择从有侧链一方配位于主体1，而2-甲基咪唑(2-MeIm)选择从没有侧链一方进攻主体1，但对于主体2，则没有出现配位方向的选择性.     

胆甾类分子钳对氨基酸衍生物的对映选择性识别

用差紫外光谱滴定法考察了以脱氧胆酸作spacer的手性分子钳1～3对一系列α-氨基酸甲酯的对映选择性识别性能。结果表明,分子钳1和2与客体氨基酸甲酯形成1:1型超分子配合物,并显示较好的手性识别能力。分钳3对所考察的氨基酸甲酯均没有明显的识别作用。讨论了主-客体间尺寸/形状匹配、几何互补等因素对形成超分子配合物的影响,并利用计算机模拟作辅助手段对实验结果和现象进行了解释。     

Incorporation dynamics of molecular guests into two-dimensional supramolecular host networks at the liquid-solid interface

The objective of this work is to study both the dynamics and mechanisms of guest incorporation into the pores of 2D supramolecular host networks at the liquid-solid interface. This was accomplished by adding molecular guests to prefabricated self-assembled porous monolayers and the simultaneous acquisition of scanning tunneling microscopy (STM) topographs. The incorporation of the same guest molecule (coronene) into two different host networks was compared, where the pores of the networks either featured a perfect geometric match with the guest (for trimesic acid host networks) or were substantially larger than the guest species (for benzenetribenzoic acid host networks). Even the moderate temporal resolution of standard STM experiments in combination with a novel injection system was sufficient to reveal clear differences in the incorporation dynamics in the two different host networks. Further experiments were aimed at identifying a possible solvent influence. The interpretation of the results is aided by molecular mechanics (MM) and molecular dynamics (MD) simulations.     

超分子体系中轴－轮状D.D.主体分子的立体异构现象

组装了轴一轮状主体分子1,1,6,6-四基乙－2,4－二炔-1,6二醇(1)与天然主物异补骨脂素(2)。茴香醚(3)形成的两种超分子异构体的包结物晶体,它们的主客体分子摩尔比分别为1：2和2：1单晶X射线衍射分析了游离主体分子以及超分子包结物晶体的结构,结果表明在主客体分子摩尔比1：2的晶体中,主体分子与异补骨脂素形成氢键,主体分子采取对位交叉式构象;在主客体分子摩尔比为2：1的晶体中,主体分子这间形成氢键,主体分子采取邻位交叉式构,主体分子所取的构象取决于客体分子的性质,当客体分子为氢键好的受体时,可与主体分子生成1：2的包结物;当客体分子为氢键差的受体时,生成2：1有包结物,本文还对三种晶体是的主体分子的立体构苯环两面角,C(1)和C(6)所连基团的夹角和能量变化规律进行了比较和分析。     

Spectrophotometric and calorimetric titration studies on molecular recognition of camphor and borneol by nucleobase-modified beta-cyclodextrins

A series of modified beta-cyclodextrins with nucleobase substituents, that is, mono(6-ade-6-deoxy)-beta-cyclodextrin (2) and mono(6-ura-6-deoxy)-beta-cyclodextrin (3) as well as mono(6-thy-6-deoxy)-beta-cyclodextrin (4), were selected as molecular receptors to investigate their conformation and inclusion complexation behaviors with some chiral molecules, that is, (+)-camphor, (-)-camphor, (+)-borneol, and (-)-borneol, by spectrophotometric and microcalorimetric titrations in aqueous phosphate buffer solution (pH 7.2) at 298.15 K. Circular dichroism and NMR studies demonstrated that these nucleobase-modified beta-cyclodextrins adopted a co-inclusion mode upon complexation with guest molecules; that is, the originally self-included nucleobase substituents of the host did not move out from the beta-cyclodextrin cavity, but coexisted with guest molecule in the beta-cyclodextrin cavity upon inclusion complexation. Significantly, these nucleobase-modified beta-cyclodextrins efficiently enhanced the molecular binding ability and the chiral recognition ability of native beta-cyclodextrin, displaying enantioselectivity up to 3.7 for (+)-camphor/(-)-camphor pair by 2 and 3.5 for (-)-borneol/(+)-borneol pair by 3. The enhanced molecular/chiral recognition abilities of 2-4 toward (+/-)-camphor were mainly attributed to the increased entropic gains due to the extensive desolvation effects, while the favorable enthalpic gains originating from the good size-fit relationship as well as the hydrogen bond interactions between host and guest result in the enhanced molecular/chiral recognition abilities of 2-4 toward (+/-)-borneol.     

Coherence transfer processes in vibrational relaxation of polyatomic molecules in condensed media

The vibrational relaxation of a polyatomic molecule in a condensed host is studied by a consideration of two molecular vibrations. Relaxation processes, intermode coupling terms and vibrational frequency fluctuation contributions are retained. Population decay (T₁), dephasing (T₂), and coherence transfer rates are evaluated through second order in the limit where the host bath dynamics are rapid compared to these molecular timescales. The rates are expressed in terms of temperature and frequency dependent bath correlation functions. For the special case of a three level system (the ground state and ones where one of the two vibrational modes is excited) the important effects of anharmonicity are incorporated. It is shown that certain coherence transfer terms involve zero frequency bath correlation functions, so they should be larger than the high frequency ones which obey modified energy gap laws. A discussion is presented of the types of interactions which may contribute to these coherence transfer processes.     

Writing with molecules on molecular printboards

Nanotechnology aspires to create functional materials with characteristic dimensions of the order 1-100 nm. One requirement to make nanotechnology work is to precisely position molecules and nanoparticles on surfaces, so that they may be addressed and manipulated for bottom-up construction of nanoscale devices. Here we review the concept of a "molecular printboard". A molecular printboard is a monolayer of host molecules on a solid substrate on which guest molecules can be attached with control over position, binding strength, and binding dynamics. To this end, cyclodextrins were immobilized in monomolecular layers on gold, on silicon wafers and on glass. Guest molecules (for example, adamantane and ferrocene derivatives) bind to these host surfaces through supramolecular, hydrophobic inclusion interaction. Multivalent interactions are exploited to tune the binding strength and dynamics of the interaction of guest molecules with the printboard. Molecules can be positioned onto the printboard using supramolecular microcontact printing and supramolecular dip-pen nanolithography due to the specific interaction between the 'ink' and the substrate. In this way, nanoscale patterns can be written and erased on the printboard. Currently, the molecular printboard is exploited for nanofabrication, for example in combination with electroless deposition of metals and by means of supramolecular layer-by-layer deposition.