Increasing the flexoelastic ratio of liquid crystals using highly fluorinated ester-linked bimesogens.

We present experimental results on the bulk flexoelectric coefficients e and effective elastic coefficients K of non-symmetric bimesogenic liquid crystals when the number of terminal and lateral fluoro substituents is increased. These coefficients are of importance because the flexoelastic ratio e/K governs the magnitude of flexoelectro-optic switching in chiral nematic liquid crystals. The study is carried out for two different types of linkage in the flexible spacer chain that connects the separate mesogenic units: these are either an ether or an ester unit. It is found that increasing the number of fluorine atoms on the mesogenic units typically leads to a small increase in e and a decrease in K, resulting in an enhancement of e/K. The most dramatic increase in e/K, however, is observed when the linking group is changed from ether to ester units, which can largely be attributed to an increase in e. Increasing the number of fluorine atoms does, however, increase the viscoelastic ratio and therefore leads to a concomitant increase in the response time. This is observed for both types of linkage, although the ester-linked compounds exhibit smaller viscoelastic ratios compared with their ether-linked counterparts. Highly fluorinated ester-linked compounds are also found to exhibit lower transition temperatures and dielectric anisotropies. As a result, these compounds are promising materials for use in electro-optic devices.

Printed red-green-blue liquid crystal lasers

We present printable laser devices formed by dispersing dye-doped chiral nematic liquid crystals in solution-processible polymers. Unlike current technology, this allows lasers to be formed on a wide variety of surfaces, e.g. paper, plastic, metal. © 2012 OSA.

Printed red-green-blue liquid crystal lasers

We present printable laser devices formed by dispersing dye-doped chiral nematic liquid crystals in solution-processible polymers. Unlike current technology, this allows lasers to be formed on a wide variety of surfaces, e.g. paper, plastic, metal. © OSA 2012.

Numerical modelling of chirality-Induced bi-Directional swimming of artificial flagella

Biomimetic micro-swimmers can be used for various medical applications, such as targeted drug delivery and micro-object (e.g. biological cells) manipulation, in lab-on-a-chip devices. Bacteria swim using a bundle of flagella (flexible hair-like structures) that form a rotating cork-screw of chiral shape. To mimic bacterial swimming, we employ a computational approach to design a bacterial (chirality-induced) swimmer whose chiral shape and rotational velocity can be controlled by an external magnetic field. In our model, we numerically solve the coupled governing equations that describe the system dynamics (i.e. solid mechanics, fluid dynamics and magnetostatics). We explore the swimming response as a function of the characteristic dimensionless parameters and put special emphasis on controlling the swimming direction. Our results provide fundamental physical insight on the chirality-induced propulsion, and it provides guidelines for the design of magnetic bi-directional micro-swimmers. © 2013 The Author(s) Published by the Royal Society. All rights reserved.

Effect of catalyst pretreatment on chirality-selective growth of single-walled carbon nanotubes

We show that catalyst pretreatment conditions can have a profound effect on the chiral distribution in single-walled carbon nanotube chemical vapor deposition. Using a SiO2-supported cobalt model catalyst and pretreatment in NH3, we obtain a comparably narrowed chiral distribution with a downshifted tube diameter range, independent of the hydrocarbon source. Our findings demonstrate that the state of the catalyst at the point of carbon nanotube nucleation is of fundamental importance for chiral control, thus identifying the pretreatment atmosphere as a key parameter for control of diameter and chirality distributions. © 2014 American Chemical Society.

Stretchable liquid-crystal blue-phase gels.

Liquid-crystalline polymers are materials of considerable scientific interest and technological value. An important subset of these materials exhibit rubber-like elasticity, combining the optical properties of liquid crystals with the mechanical properties of rubber. Moreover, they exhibit behaviour not seen in either type of material independently, and many of their properties depend crucially on the particular mesophase employed. Such stretchable liquid-crystalline polymers have previously been demonstrated in the nematic, chiral-nematic, and smectic mesophases. Here, we report the fabrication of a stretchable gel of blue phase I, which forms a self-assembled, three-dimensional photonic crystal that remains electro-optically switchable under a moderate applied voltage, and whose optical properties can be manipulated by an applied strain. We also find that, unlike its undistorted counterpart, a mechanically deformed blue phase exhibits a Pockels electro-optic effect, which sets out new theoretical challenges and possibilities for low-voltage electro-optic devices.

Magnetic quantum oscillations for the surface states of topological insulator Bi2Se3

We study quantum oscillations of the magnetization in Bi2Se3 (111) surface system in the presence of a perpendicular magnetic field. The combined spin-chiral Dirac cone and Landau quantization produce profound effects on the magnetization properties that are fundamentally different from those in the conventional semiconductor two-dimensional electron gas. In particular, we show that the oscillating center in the magnetization chooses to pick up positive or negative values depending on whether the zero-mode Landau level is occupied or empty. An intuitive analysis of these features is given and the subsequent effects on the magnetic susceptibility and Hall conductance are also discussed.

A lattice dynamical treatment for the total potential energy of single-walled carbon nanotubes and its applications: relaxed equilibrium structure, elastic properties, and vibrational modes of ultra-narrow tubes

In this paper, we propose a lattice dynamic treatment for the total potential energy of single-walled carbon nanotubes (SWCNTs) which is, apart from a parameter for the nonlinear effects, extracted from the vibrational energy of the planar graphene sheet. The energetics, elasticity and lattice dynamics are treated in terms of the same set of force constants, independently of the tube structures. Based upon this proposal, we have investigated systematically the relaxed lattice configuration for narrow SWCNTs, the strain energy, the Young's modulus and Poisson ratio, and the lattice vibrational properties with respect to the relaxed equilibrium tubule structure. Our calculated results for various physical quantities are nicely in consistency with existing experimental measurements. In particular, we verified that the relaxation effect makes the bond length longer and the frequencies of various optical vibrational modes softer. Our calculation provides evidence that the Young's modulus of an armchair tube exceeds that of the planar graphene sheet, and that the large diameter limits of the Young's modulus and Poisson ratio are in agreement with the experimental values of graphite; the calculated radial breathing modes for ultra-narrow tubes with diameters ranging between 2 and 5 angstrom coincide with the experimental results and the existing ab initio calculations with satisfaction. For narrow tubes with a diameter of 20 angstrom, the calculated frequencies of optical modes in the tubule's tangential plane, as well as those of radial breathing modes, are also in good agreement with the experimental measurements. In addition, our calculation shows that various physical quantities of relaxed SWCNTs can actually be expanded in terms of the chiral angle defined for the corresponding ideal SWCNTs.

Aharonov-Bohm oscillation and chirality effect in optical activity of single-wall carbon nanotubes

We study the Aharonov-Bohm effect in the optical phenomena of single-wall carbon nanotubes (SWCN) and also their chirality dependence. Especially, we consider the natural optical activity as a proper observable and derive its general expression based on a comprehensive symmetry analysis, which reveals the interplay between the enclosed magnetic flux and the tubule chirality for arbitrary chiral SWCN. A quantitative result for this optical property is given by a gauge invariant tight-binding approximation calculation to stimulate experimental measurements.

Correlation effects for semiconducting single-wall carbon nanotubes: A density matrix renormalization group study

In this paper we report the applicability of the density matrix renormalization group (DMRG) approach to the cylindrical single wall carbon nanotube (SWCN) for the purpose of its correlation effect. By applying the DMRG approach to the t+U+V model, with t and V being the hopping and Coulomb energies between the nearest neighboring sites, respectively, and U the on-site Coulomb energy, we calculate the phase diagram for the SWCN with chiral numbers (n(1)=3, n(2)=2), which reflects the competition between the correlation energy U and V. Within reasonable parameter ranges, we investigate possible correlated ground states, the lowest excitations, and the corresponding correlation functions in which the connection with the excitonic insulator is particularly addressed.

手性吡啶醇基联萘酚衍生物的合成及其在不对称催化反应中的应用

C2-对称性的1,1’-联萘-2-酚(BINOL)及其衍生物在不对称催化等领域受到了广泛的关注。联萘酚自身能够与具有路易斯酸性的亲氧过渡金属配位完成不对称催化，但往往不能在不对称催化反应中给出满意的结果；可以通过对其骨架中多个可修饰位点的改造和修饰，从而影响金属中心周围的环境，达到不对称诱导的目的。因此对联萘酚的结构改造成为人们始终关注的焦点和努力的方向。 本论文以天然的手性酮(-)-葑酮、(+)-樟脑、(-)-薄荷酮为原料合成两个系列的手性吡啶醇类化合物2a-c，12a-b，并通过与3-或3,3’-单或双(硼酸嚬呐醇酯)- 2,2’-双(甲氧甲基)-1,1’-联-2-萘酚的Suzuki偶联反应在联萘酚的3,3’-位引入手性吡啶醇取代基，合成了两个系列手性联萘酚衍生物，十对非对映异构体8a-c，9a-c，19a-b，20a-b，所有反应中间体及最终产物均已用1H NMR，13C NMR，IR和ESI-MS表征和验证结构。 探讨了两个系列手性配体在芳香醛不对称ZnEt2加成反应、苯甲硫醚不对称氧化反应中的应用。其中，第一系列六对非对映异构体8a-c，9a-c在芳香醛的不对称ZnEt2加成反应中表现出中等的反应活性及较好的对映选择性，最高可达96% e.e.对映选择性；在苯甲硫醚的不对称环氧化中，这些配体表现出较好的反应活性和中等的对映选择性，最好可达51％e.e.对映选择性。第二系列四对非对映异构体19a-b，20a-b在芳香醛的不对称ZnEt2加成反应中也表现出中等的反应活性及较好的对映选择性(91% e.e.值)，但较第一系列配体要低一些；在苯甲硫醚的不对称环氧化中，这些配体同样表现出较好的反应活性和中等的对映选择性，但较第一系列配体显示出较高的手性诱导能力，e.e.值最高可达71％。 同时，将部分配体与金属钼(Ⅵ)配位合成手性金属配合物，对部分配合物进行单晶结构的表征，确定配合物中具有分子内氧桥键的结构。并初步探讨了金属配合物在非官能团化烯烃的不对称环氧化反应中的应用。

手性有机小分子催化烯胺不对称还原

手性胺是合成天然产物和手性药物的重要中间体，亚胺和烯胺的不对称催化还原是制备手性胺最直接有效的方式之一。手性有机小分子催化的亚胺不对称还原已取得了可喜的进展，但到目前为止，有机小分子催化的烯胺不对称还原，尤其是环状烯胺的不对称还原还少有报道。 本研究从手性叔丁基亚磺酰胺出发，设计并合成了一系列含有叔丁基亚磺酰基的新型脲类及硫脲类催化剂，并将其用于催化三氯硅烷对烯胺的不对称还原，尤其是1, 4-二氢吡啶酯类环状烯胺的不对称还原。通过对催化反应条件的优化，发现当添加1eq H2O时，反应收率和对映选择性明显提高，获得高达99% 的收率和88% ee，同时也取得了很好的非对映选择性（dr = 8:92）。首次实现了三氯硅烷对1, 4-二氢吡啶酯类环状烯胺的高立体选择性还原。 通过机理方面的研究，我们推测反应过程中可能是：首先，底物1, 4-二氢吡啶酯与催化剂形成氢键而被活化，当加入添加剂后，添加剂与三氯硅烷反应释放出一个质子，然后受活化的1, 4-二氢吡啶酯捕获该质子转变成更活泼的亚胺正离子的中间体。随后，在催化剂上的手性硫氧的活化下，三氯硅烷的负氢加成到受活化的亚胺正离子的中间体上，最后生成比较有利的反式产物1, 4, 5, 6-四氢吡啶乙酯。 Calalytic enantioselective reduction of imines and enamines represents one of the most straightforward and efficient methods for the preparation of chiral amines, which is an important class of intermediates for the synthesis of natural products and chiral drugs. Significant progresses have been made in organocatalytic enantioselective reduction of imines. However, asymmetric reduction of enamines, especially of cyclic enamines catalyzed by small organocatalysts has scarcely been reported. In this study, starting from chiral tert-butanesulfinamide, a series of structurally simple tert-butanesulfinyl urea and thiourea organocatalysts were developed and employed in asymmetric reduction of enamines by triclorosilane, particularly in the reduction of cyclic enamines such as Hantzsch 1, 4-dihydropyridines. During the optimization of reaction condictions, we found that the addition of one equivalent of H2O could significantly improve the yields and enatioselectivities. Under optimal condictions, 99% yield, up to 88% ee, and 8:92 diastereomeric ratio were obtained. Thus, we have for the first time realized the highly stereoselective reduction of Hantzsch 1, 4-dihydropyridines catalyzed by triclorosilane. As for the mechanism, we speculate that the Hantzsch 1, 4-dihydropyridine was firstly engaged with the catalyst through hydrogen bond. The proton released from the reaction of the additive and triclorosilane next added to one of the C=C bond to make an active iminium intermediate, which was then attacked by the nucleophlic hydrogen of HSiCl3 activated by the Lewis basic sulfinyl function of the catalyst to provide superior trans-1, 4, 5, 6-tetrahydropyridine products.

路易斯碱催化还原胺化反应研究

胺及其衍生物是很多重要生物活性分子的结构单元，是合成天然产物和手性药物的重要中间体。 直接还原胺化由于其合成步骤简单而成为制备二级胺和三级胺的简便方法。为了发展一种较为简便的直接还原胺化反应，我们把研究的重点放在开发一种简便实用的有机小分子催化方法上。由文献调研可知，现已报道的直接还原胺化方法大多是催化醛或酮与一级胺或者脂肪二级胺的直接还原胺化，而醛或酮与芳香二级胺的直接还原胺化却尚无报道。在本文中，我们发现用简单的四甲基乙二胺(TEMED)在室温下以二氯甲烷为溶剂即可催化三氯氢硅对酮和芳香二级胺之间的直接还原胺化反应，并取得了高达92%的收率。该反应条件温和，底物普适性广，各种类型的酮均可以与芳香二级胺进行直接还原胺化，并且得到比较满意的收率。 同时，我们从手性Sulfoximine出发，设计和合成了一系列的Sulfoximine类新衍生物，并将其应用于间接还原胺化反应中。遗憾的是我们并没有得到预期的不对称催化效果。 Amines and their derivatives are basic structural motifs in natural products and pharmaceuticals and highly versatile building blocks for various organic substrates. Direct reductive amination (DRA) is a convenient method for the preparation of secondary and tertiary amines owing to its operational simplicity. In an effort to develop a simple and convenient procedure for direct reductive amination reaction, we focused our study on search for a mild and efficient organocatalytic system. In the literature, there are many reports concerning DRA between aldehydes or ketones and either primary amines or secondary aliphatic amines. But there are no reports concerning DRA between aldehydes or ketones and secondary aromatic amines. In this study, we have developed a highly practical method for the synthesis of tertiary amines by the direct reductive amination of ketones and secondary aromatic amines with tetramethylethylenediamine (TEMED) as the catalyst using HSiCl3 as the reducing agent in dichloromethane (affording up to 92% yield). This method can be carried out under mild conditions and is compatible with many functional groups. A variety of ketones were efficiently aminated with secondary aromatic amines to afford the corresponding amines in good to excellent yields. Starting from chiral sulfoximine, we designed and synthesized a series of new sulfoximine derivatives and tested their efficiencies as asymmetric organocatalysts for the reduction of imines, which, unfortunately, only exhibited low catalytic activity and enantioselectivity.

能同时高对映选择性还原酮和亚胺的有机小分子催化体系的设计与合成

不对称催化还原反应是当今有机化学研究中最具活力的研究方向之一。在这一研究领域中，已有不少催化剂体系能够对前手性酮或亚胺进行高对映选择性还原，但极少能够对酮和亚胺同时具有很好的催化效果。本论文从光学纯的哌啶酸出发，设计与合成了一系列结构简单、合成方便的手性有机小分子路易斯碱催化剂，并研究了该系列催化剂在酮和亚胺不对称还原本课题组前期研究中开发出L-哌啶酸和（1S,2R）-1,2-二苯基氨基乙醇衍生的路易斯碱催化剂，在三氯氢硅对亚胺的还原反应中显示出很高的催化活性和对映选择性，但对酮的还原却没有得到很理想的催化效果。本研究对该催化剂进行一定的结构改造和修饰，获得了能够同时高对映选择性催化还原酮和亚胺两类底物的有机小分子催化剂。通过优化条件，取得了很好的收率（高达99%）和对映选择性（高达93%的ee 值）。而且，其底物普适性也是前所未有的，对芳香酮和脂肪酮，芳香亚胺和脂肪亚胺都具有很好的催化效果。通过机理方面的探讨，我们推测该催化体系对酮的还原和对亚胺的还原反应可能分别采用了七配位和六配位过渡态模型。Asymmetric reduction is one of the most active research areas in modernasymmetric synthesis. A number of highly efficient methods have been developed forthe asymmetric reduction of either ketones or ketimines. However, there have beenextremely rare examples of catalytic systems that allow for highly enantioselectivereductions of both ketones and ketimines.In this study, starting from optically pure pipecolinic acid, we designed andsynthesized a series of structurally simple and easily accessible chiral organic Lewisbasic catalysts and employed them in asymmetric reduction of ketones and ketimines.Previously our group has developed a highly enantioselective Lewis basiccatalyst for the asymmetric reduction of ketimines by trichlorosilane starting fromL-piperdine-2-carboxylic acid and (1R,2S)-2-amino-1,2-diphenylethanol. But thiscatalyst was found not to be very effective in the asymmetric reduction of ketones.Slight modifications of this catalyst has led to a new highly enantioselective catalystapplicable for the reduction of both ketones and ketimines by trichlorosilane. Underthe optimal conditions, this catalyst afforded excellent yields (up to 99%) andenantioselectivities (up to 93% ee). Moreover, an unprecedented substrate spectrumwas observed with this catalyst, which are highly effective for aromatic and aliphaticketones as well as aromatic and aliphatic ketimines. A heptacoordinate silicontransition structure and a hexacoordinate one were proposed for the reduction of ketones and ketimines, respectively.

手性有机小分子路易斯碱催化剂的设计、合成及其在亚胺不对称还原中的应用

手性胺是合成天然产物和手性药物的重要中间体，亚胺的不对称催化还原是制备光学活性手性胺的最直接有效的方法之一。但是，由于C＝N双键的反应活性较弱以及容易发生E/Z异构等问题，亚胺的不对称催化还原具有很大的挑战性，既具有高对映选择性又具有宽广底物普适性的催化剂很少。 本文分别由手性脯氨酸、哌啶酸、哌嗪酸以及氨基醇出发，设计和合成了一系列结构新颖、合成简便、性能优良的酰胺类有机小分子路易斯碱催化剂，以廉价的三氯氢硅为氢源，用这些催化剂催化亚胺不对称还原，得到了非常优良的收率、对映选择性和前所未有的底物普适性。 文献研究认为，除N-甲酰基外，分子内含有芳香酰胺是能催化亚胺还原的有机小分子路易斯碱催化剂具有较高对映选择性的必要条件，我们研究发现N-甲酰脯氨酸非芳香酰胺类催化剂（包括结构简单的C2-对称型脯氨酰胺类催化剂），对N-芳基酮亚胺的还原可获得达86%的对映选择性，远高于同类芳香酰胺催化剂，证明N-甲酰非芳香酰胺类路易斯碱催化剂在亚胺还原中也能得到高的对映选择性。 在进一步研究中，我们以手性六元哌啶酸为模板，分别设计合成了N-甲酰哌啶酸芳香酰胺和N-甲酰哌啶酸非芳香酰胺两类催化剂，其中芳香酰胺催化剂（S）-N-（甲酰基）哌啶-2-酸-1-萘基酰胺（28）和非芳香酰胺催化剂（2S,1'S,2'S）-N-（甲酰基）-哌啶-2-酸（1',2'-二苯基-2'-乙酰氧基-乙基）酰胺（30）显示出非常优良的催化活性和对映选择性，对于N-芳基芳香酮亚胺的还原，无论是缺电子体系还是富电子体系，绝大部分都能得到很高的收率（达98%）和对映选择性（达96% ee)。特别值得一提的是30对一些脂肪族亚胺和α,β-不饱和亚胺的还原，虽然底物为E/Z混合物，也能得到很高的收率（达93%）和对映选择性（达95% ee)，这样的底物普适性在过渡金属催化体系中也是前所未有的。 现有的催化亚胺还原的高对映选择性催化体系大多仅适用于甲基酮亚胺底物，对位阻较大的非甲基酮亚胺很难获得好的结果。我们以L-哌嗪酸为模板设计和合成出的（S）-N-（甲酰基）-哌嗪-2-酸-4-对叔丁基苯磺酰基-苯基酰胺不但对N-芳基甲基酮亚胺有很好的对映选择性（达90% ee)，而且对于大位阻的N-芳基非甲基酮亚胺有更好的对映选择性（达97% ee)。该催化剂与30在底物普适性方面具有很好的互补性。 我们还设计了基于1,2-二苯基氨基醇为模板的新型N-甲酰路易斯碱有机小分子催化剂，首次发现结构简单的N-甲酰（1S,2R）二苯基氨基醇能较好的催化N-芳基酮亚胺，最高可以得到82%的对映选择性。 针对我们设计合成的结构新颖、性能优良的催化剂，我们对催化机理进行了探讨和解释，提出了几个假想的机理模型。 Catalytic enantioselective reduction of imines represents one of the most straightforward and efficient methods for the preparation of chiral amines, an important intermediate for the synthesis of natural products and chiral drugs. However, asymmetric reduction of imines remains a big challenge and highly enantioselective catalysts with a satisfactorily broad substrate scope remain elusive. Factors contributing to the difficulty of this transformation include the weak reactivity of the C=N bond and the existence of inseparable mixtures of E/Z isomers. Starting from chiral proline, pipecolinic acid, piperazine-2-carboxylic acid and 1,2-diphenyl amino alcohol, a series of structurally simple and easily prepared amides were developed as highly effective Lewis basic organocatalysts for the asymmetric reduction of imines with trichlorosilane as the reducing agent, which promoted the reduction of N-aryl imines with high yields and excellent enantioselectivities with an unprecedented substrate spectrum. In the literature, it has been believed that besides the N-formyl group, the existence of an arylamido group in the structure of Lewis basic organocatalysts is a prerequisite for obtaining high enantioselectivity in the catalytic reduction of imines. However, we found that the N-formyl-L-prolinamides bearing non-arylamido groups, including structurally simple C2-symmetric tetraamides, could also work as effective Lewis basic catalysts to promote the asymmetric reduction of ketimines with high enantioselectivities (up to 86% ee), which are even more enantioselective than the analogues with arylamido groups. In further studies, we developed novel N-formamides with arylamido groups and non-arylmido groups as Lewis basic catalysts using the commercially available L-pipecolinic acid as the template. The catalysts (S)-1-formyl-piperidine-2-carboxylic acid naphthylamide 28 and (2S,1'S,2'S)-acetic acid 2-[(1-formyl-piperidine-2-carbonyl) -amino]-1,2-diphenyl-ethyl ester 30 were found to promote the reduction of a broad range of N-aryl imines in high yields (up to 98%) and excellent ee values (up to 96%) under mild conditions. Furthermore, catalyst 30 also exhibited high enantioselectivities (up to 95% ee) for the challenging aliphatic ketimines and α,β-unsaturated imines despite that these imines exist as E/Z isomeric mixtures. The broad substrate spectrum of this catalyst is unprecedented in catalytic asymmetric imine reduction, including transition-metal-catalyzed hydrogenation processes. Many of the currently available highly enantioselective catalytic systems only tolerate methyl ketimines, which gave poor results for bulkier non-methyl ketimines. Starting from L-piperazine-2-carboxylic acid, we developed (S)-4-(4-tert- butylbenzenesulfonyl)-1-formyl-N-phenyl-piperazine-2-carboxamide as highly enantioselective Lewis basic catalysts for the hydrosilylation of both methyl ketimines and steric bulky non-methyl ketimines. Moreover, higher enantioselectivities were obtained for non-methyl ketimines than methyl ketimines under the catalysis of this catalyst. Thus, this catalyst system complements with 30 in terms of the substrate scope. We also found that easily accessible (1R,2S)-N-formyl-1,2-diphenyl- 2-aminoethanol worked as an effective Lewis basic catalyst in the enantioselective hydrosilylation of ketimines, affording high enantioselectivities (up to 82% ee) for a broad range of ketimines. To rationalize the high efficiencies of the structurally novel catalysts we developed, several catalytic models have been proposed.