分子陀螺的设计、合成与组装

近来,科学家设计和合成了系列分子水平的陀螺。类似于儿童的玩具陀螺仪,这种分子陀螺由一个转子、一个定子框架和连接定子和转子的轴组成。定子框架通过自身的刚性结构为中心转子的转动提供足够的内在自由度,得以对内部的转子实施保护。并使得分子陀螺成为一个理想的分子转子。当转子上有偶极距时,则可能在外来电、磁、光的刺激下进行定向转动,成为分子马达。化学家们通过X射线晶体衍射技术、动态核磁技术、理论计算化学、热力学分析等方法表征了分子陀螺的各种特征,并积极探索其潜在的应用价值。本文着重介绍分子陀螺,以及超分子陀螺仪的发展历史以及研究进展。     

从分子构造到分子构型和分子构象（中）

我们从(上)文看到,化学家们最初认为物质的性质只决定于物质的分子组成,后来逐渐认识到物质的性质除决定于物质的分子组成外,还决定于分子构造.     

从分子构造到分子构型和分子构象（上）

分子构造(constitution)是指分子中原子相互联结的方式和次序,过去长期以来称为分子结构((structure),根据国际纯粹和应用化学联合会的建议,改为“构造”。“结构”一词应用在广泛的范围,例如物质结构、原子的电子结构等等。     

从分子构造到分子构型和分子构象（下）

范特霍夫和列贝尔分别各自提出碳原子的四个价键指向一个正四面体的四个顶点概念确立后,1885年德国化学家拜尔(Baeyer,Adolphvon 1835-1917)发表价键的张力学说。他根据碳原子四个价键的正四面体模型,任何两个价键之间的正常角度应当是109o28',如图I所示。     

卟啉超分子化合物在分子器件中的应用

分子电子器件已成为近年来的一个研究热点，卟啉类化合物因为光敏性好、性能稳定、易于修饰等优点成为分子器件研究的理想模型化合物。本文着重介绍了它在分子器件中的最新应用进展。     

基于C=O…HN分子间氢键的超分子液晶研究进展

基于C=O…HN的分子间氢键能够自组装形成具有精确分子排列和很好稳定性的有序结构,在设计构造液晶功能材料方面具有重要的不可替代的地位.分子形状是设计小分子热致液晶的一个主要考虑因素,它对液晶态的结构有至关重要的影响.以分子形状与液晶态相互关系为主线,重点介绍了目前文献报道的基于C=O…HN分子间氢键的盘状和楔形分子形成液晶的研究进展,以期为新型液晶材料的分子设计提供一些借鉴.     

分子调控的概念及其意义

在分子识别的基础之上提出了分子调控的新概念，指出分子调控是外界因素对分子某些性质的指令性干预，是超分子体系所持有的功能，通过这种调控作用，可以有意识、有目的地控制分子的行为，并列举若干实例加以说明。     

分子识别在分析化学中的应用

本文概述了分子模板理论的产生和发展，总结了分子模板技术在分析化学领域中的应用和发展趋势，同时对分子印渍技术的理论进行概述，并指出分子印渍技术在分析化学领域中的应用和发展情况，阐述了分子模板和分子印渍技在分子识别分析方面的应用前景，其将为各种物质的超微量分析提供更加讯捷，准确，方便的分析方法。     

药物头孢氨苄分子模板聚合物水中结合性质的研究

采用分子模板技术合成了以头孢氨苄为模板分子以三氟甲基丙烯酸和４－乙烯基吡啶同时为功能单体的分子模板聚合物。将得到的棒状聚合物研磨过筛后,运用平衡结合实验研究了头孢氨苄分子模板聚合物的结合性质,Ｓｃａｔｃｈａｒｄ分析表明,在所研究的浓度范围内,在聚合物中形成了两类不同的结合位点。头孢氨苄分子模板聚合物与其化学组成相同的非模板聚合物相比,有很高的结合容量。底物选择性实验表明,与其它结构相似的药物相比,     

分子逻辑器件研究进展

分子器件具有尺寸小、设计合成可控、存储量大、反应速度快、人工智能等诸多优点,是当今化学、物理和材料等领域研究最为重要的一个交叉领域.综述了近些年来分子逻辑器件领域的研究进展.介绍了各种类型的分子逻辑门、半(加)减法器、分子逻辑线路以及DNA分子和固态分子计算.最后提出了分子器件存在的问题并展望了其应用前景.     

分子线电子输运特性的第一性原理研究

从第一性原理出发,利用密度泛函理论研究了SH-C8H16-SH分子和金表面的相互作用,并利用分子前线轨道理论和微扰理论定量地确定了该相互作用能常数,然后,利用弹性散射格林函数方法研究了该分子与金表面形成的分子线的伏-安特性.研究结果表明,当含有硫氢官能团的有机分子化学吸附于金表面时,硫原子将与金原子形成以共价键为主的混和键,此时,扩展的分子轨道使分子线的电导呈现出欧姆特性,而对于局域的分子轨道,电子的输运只能通过隧道效应来实现.对分子线伏-安特性的计算结果显示,在零偏压附近,存在一个电流禁区,随着偏压的增加,分子线的电导呈现出平台特征.     

Quantum chemical analysis of electronic structure and n- and p-type charge transport in perfluoroarene-modified oligothiophene semiconductors

Density-functional theory (DFT) is employed to investigate the structural, electronic, and transport properties of several isomeric fluoroarene-oligothiophene-based semiconductors. Three oligothiophene systems varying in the perfluoroarene group positions within the molecule are studied to understand the electronic structure leading to the observed mobility values and to the n- or p-type behavior in these structures. Analyses of both intermolecular interactions in dimers and extended interactions in crystalline structures afford considerable insight into the electronic properties and carrier mobilities of these materials, as well as the polarity of the charge carriers. From the calculated carrier effective masses, we find that sterically governed molecular planarity plays a crucial role in the transport properties of these semiconductors. Our calculations correlate well with experimentally obtained geometries, highest-occupied molecular orbital (HOMO)/lowest-unoccupied molecular orbital (LUMO) energies, and the experimental carrier mobility trends among the systems investigated.     

Single molecule electron transport junctions: charging and geometric effects on conductance

A p-benzenedithiolate (BDT) molecule covalently bonded between two gold electrodes has become one of the model systems utilized for investigating molecular transport junctions. The plethora of papers published on the BDT system has led to varying conclusions with respect to both the mechanism and the magnitude of transport. Conductance variations have been attributed to difficulty in calculating charge transfer to the molecule, inability to locate the Fermi energy accurately, geometric dispersion, and stochastic switching. Here we compare results obtained using two transport codes, TRANSIESTA-C and HUCKEL-IV, to show that upon Au-S bond lengthening, the calculated low bias conductance initially increases by up to a factor of 30. This increase in highest occupied molecular orbital (HOMO) mediated conductance is attributed to charging of the terminal sulfur atom and a corresponding decrease in the energy gap between the Fermi level and the HOMO. Addition of a single Au atom to each terminal of the extended BDT molecule is shown to add four molecular states near the Fermi energy, which may explain the varying results reported in the literature.     

Controlling Electrical Conductance through a π‐Conjugated Cruciform Molecule by Selective Anchoring to Gold Electrodes

Tuning charge transport at the single‐molecule level plays a crucial role in the construction of molecular electronic devices. Introduced herein is a promising and operationally simple approach to tune two distinct charge‐transport pathways through a cruciform molecule. Upon in situ cleavage of triisopropylsilyl groups, complete conversion from one junction type to another is achieved with a conductance increase by more than one order of magnitude, and it is consistent with predictions from ab initio transport calculations. Although molecules are well known to conduct through different orbitals (either HOMO or LUMO), the present study represents the first experimental realization of switching between HOMO‐ and LUMO‐dominated transport within the same molecule.     

Nonequilibrium electronic transport of 4,4'-bipyridine molecular junction

The electronic transport properties of a 4,4'-bipyridine molecule sandwiched between two Au(111) surfaces are studied with a fully self-consistent nonequilibrium Green's-function method combined with the density-functional theory. The 4,4'-bipyridine molecule prefers to adsorb near the hollow site of the Au(111) surface and distorts slightly. The modifications on the electronic structure of the molecule due to the presence of the electrodes are described by the renormalized molecular orbitals, which correspond well to the calculated transmission peaks. The average Fermi level lies close to the lowest unoccupied renormalized molecular orbital, which determines the electronic transport property of the molecular junction under a small bias voltage. The total transmission is contributed by a single channel. The transmission peaks shift with the applied bias voltage, and this behavior depends on the spatial distribution of the renormalized molecular orbitals and the voltage drop along the molecular junction. The shape of the calculated conductance curve of the equilibrium geometric configuration reproduces the main feature of the experimental results, but the value is larger than the measured data by about 6 times. Good agreement with the experimental measurements can be obtained by elongating the molecular junction. The electronic transport behaviors depend strongly on the interface configuration.     

Sequence modulation of tunneling barrier and charge transport across histidine doped oligo-alanine molecular junctions

Series tunneling across peptides composed of various amino acids is one of the main charge transport mechanisms for realizing the function of protein. Histidine, more frequently found in redox active proteins, has been proved to be efficient tunneling mediator. While how it exactly modulates charge transport in a long peptide sequence remains poorly explored. In this work, we studied charge transport of a model peptide junction, where oligo-alanine peptide was doped by histidine at different position, and the series of peptides were self-assembled into a monolayer on gold electrode with soft EGaIn as top electrode to form molecular junction. It was found that histidine increased the overall conductance of the peptide, meanwhile, its position modulated the conductance as well. Quantitative analysis by transport model and ultraviolet photoelectron spectroscopy (UPS) indicated a sequence dependent energy landscape of the tunneling barrier of the junction. Density-functional theory (DFT) calculation on the electronic structure of histidine doped oligo-alanine peptides revealed localized highest occupied molecular orbital (HOMO) on imidazole group of the histidine, which decreased charge transport barrier.     

外电场作用下寡聚苯分子导线的性质

利用Hartree-Fock 方法在6-31G*水平上对聚苯分子进行了计算研究. 分别从几何构型、分子轨道空间分布和分子轨道能级三个方面讨论了外电场对寡聚苯分子导线的影响, 给出了分子导线的性质与外电场的关系. 进一步, 连接硫原子于聚苯分子的两端, 并共价结合在金电极上. 利用非平衡格林函数方法对其在0-2.0 V 偏压下电子输运特征进行了深入研究.     

First-principles investigation of the asymmetric contact effect on current-voltage characteristics of a molecular device

The properties of electronic transport in an electronic device composed of a spatially symmetric phenyldithiolate molecule sandwiched between two gold electrodes with asymmetric contact are investigated by the first-principles study. It is found that the I-V and G-V characteristics of a device show significant asymmetry and the magnitudes of current and conductance depend remarkably on the variation of molecule-metal distance at one of the two contacts. Namely, an asymmetric contact would lead to the weak rectifying effects on the current-voltage characteristics of a molecular device. We also calculate self-consistently other microscopic quantities such as the local density of states, the total density of states, and the distribution of charges in the asymmetric molecular models under the applied bias. The results show that the highest-occupied molecular orbital (HOMO) is responsible for the resonant tunneling and the shifting of the HOMO due to the charging of the device under the bias voltage is the intrinsic origin of asymmetric I(G)-V characteristics.     

金钯二元小团簇的几何结构与电子性质

在UBP86/LANL2DZ和UB3LYP/def2-TZVP水平下详细研究了AumPdn(m+n≤6)团簇的几何结构和电子性质.阐明了团簇的结构特征、平均结合能、垂直电离势、垂直电子亲和能、电荷转移以及成键特征.除单取代混合团簇(AunPd和AuPdn,n=5或6)外,五和六原子混合团簇中钯原子趋于聚集到一起形成Pdcore,金原子分布在Pdcore周围形成PdcoreAushell结构.含一个和两个钯原子团簇的电子性质与纯金团簇类似,呈现一定奇偶振荡.混合团簇的电子性质,如最高占据分子轨道(HOMO),最低未占据分子轨道(LUMO),垂直电离势,垂直电子亲和能,Fermi能级和化学硬度等均与团簇空间结构和金、钯原子数之比直接相关.混合团簇中存在钯原子到金原子间的电荷转移,表明团簇中存在明显金钯间成键作用.分析团簇的电荷分布、前线轨道和化学硬度表明,金钯混合团簇对小分子如O2、H2和CO等的反应活性要强于纯金团簇.     

Switching mechanism of photochromic diarylethene derivatives molecular junctions

The electronic transport properties and switching mechanism of single photochromic diarylethene derivatives sandwiched between two gold surfaces with closed and open configurations are investigated by a fully self-consistent nonequilibrium Green's function method combined with density functional theory. The calculated transmission spectra of two configurations are strikingly distinctive. The open form lacks any significant transmission peak within a wide energy window, while the closed structure has two significant transmission peaks on both sides of the Fermi level. The electronic transport properties of the molecular junction with closed structure under a small bias voltage are mainly determined by the tail of the transmission peak contributed unusually by the perturbed lowest perturbed unoccupied molecular orbital. The calculated on-off ratio of currents between the closed and open configurations is about two orders of magnitude, which reproduces the essential features of the experimental measured results. Moreover, we find that the switching behavior within a wide bias voltage window is extremely robust to both substituting F or S for H or O and varying end anchoring atoms from S to Se and Te.