Sensitivity of the NTB phase formation to the molecular structure of imino-linked dimers

ABSTRACT

Here we report on the synthesis and mesomorphic properties of a series of imino-linked dimeric molecules. In order to improve our understanding of the structure–N_TB phase correlations, we have studied the impact of geometric and electronic factors arising from varying mesogenic units, different spacer lengths and from the ratio (n/m) between the lengths of terminal chains (n) and spacer (m). From the perspective of the molecular geometry, the results show that the stability of the N_TB phase results from increasing effective molecular bending and with the broadening of the mesogenic unit, in particular near the spacer, and that the n/m ratio plays a substantial role in conjunction with the specific mesogenic unit. A computational study of the electronic properties shows that a broadening of the mesogenic core in the vicinity of the spacer is associated with an increased anisotropy of the electrostatic potential distribution. Within a given series of materials our study suggests that the incidence of the N_TB phase and its thermal stability are governed by the synergy of specific geometrical factors and the anisotropy of the electrostatic potential distribution of the mesogenic core.     

Twist-bend nematics,liquid crystal dimers,structure–property relations

ABSTRACT

One of the current challenges in liquid crystal science is to understand the molecular factors leading to the formation of the intriguing twist-bend nematic phase (N_TB) and determine its properties. During our earlier hunt for the N_TB phase created on cooling directly from the isotropic phase and not the nematic phase, we had prepared 30 symmetric liquid crystal dimers. These had odd spacers and methylene links to the two mesogenic groups; desirable but clearly not essential features for the formation of the N_TB. Here, we report the phases that the dimers exhibit and their transition temperatures as functions of both the lengths of the spacer and the terminal chains. In addition we describe the transitional entropies, their optical textures, the X-ray scattering patterns and the ²H NMR spectra employed in characterising the phases. All of which may lead to important properties of the twist-bend nematic phase.     

吡啶基桥连双卟啉的合成及其与有机小分子的相互作用

通过将2，6-吡啶二羟酸或2，6-二溴甲基吡啶分别与5-（4-羟基苯基）-10，15，20-三苯基卟啉（1）和5-（4-氨基苯基）-10，15，20-三苯基卟啉（2）反应，合成了3种新型的双卟啉化合物3～5．用紫外-可见光谱（UV-Vis）、红外光谱（IR）、质子核磁共振光谱（1^HNMR）、质谱（MS）对中间体和目标化合物的结构进行了表征；用紫外-可见光谱考察了双卟啉化合物4与2-羟基-苯乙酸（苦杏仁酸）的相互作用。结果表明，双卟啉与苦杏仁酸之间形成1：2的络合物，其结合位点在卟啉环内。     

Deep-ultraviolet surface plasmon resonance of Al and Al_(core)/Al_2O_(3shell) nanosphere dimers for surface-enhanced spectroscopy

The localized surface plasmon resonance properties of Al and Alcore/Al2O3 shell nanosphere dimers with Al and Al core nanosphere radii of 20 nm and Al2O3 shell of 2 nm in the deep-ultraviolet region have been studied using the finite difference time domain method. The extinction spectra and the electric field distribution profiles of the two dimers for various gap distances between two individual nanospheres are compared with those of the corresponding monomers to reveal the extent of plasmon coupling. It is found that with the interparticle distance decreasing, a strong plasmon coupling between two Al or Alcore/Al2O3 shell nanospheres is observed accompanied by a significant red shift in the extinction spectra at the parallel polarization direction of the incident light related to the dimer axis, while for the case of the perpendicular polarization direction, a weak plasmon coupling arises characterized by a slight blue shift in the extinction spectra. The electric field distribution profiles show that benefiting from the dielectric Al2O3 shell, the gap distance of Alcore/Al2O3 shell nanosphere dimers can be tailored to < 1 nm scale and results in a very high electric field enhancement. The estimated surface-enhanced Raman scattering enhancement factors suggests that the Alcore/Al2O3 shell nanosphere dimers with the gap of < 1 nm gave rise to an enhancement as high as 8.1 × 107 for interparticle gap = 0.5 nm. Our studies reveal that the Alcore/Al2O3 shell nanosphere dimers may be promising substrates for surface-enhanced spectroscopy in the deep-ultraviolet region.     

Stationary Coverage of a Stochastic Adsorption–Desorption Process with Diffusional Relaxation

We show that it is possible to derive the stationary coverage of an adsorption–desorption process of dimers with diffusional relaxation with a very simple ansatz for the stationary distribution of the process supplemented by a hypothesis of global balance. Our approach is compared to the exact result and we seek to understand its validity within an instance of the model.     

Mechanistic studies of the phosphine-catalyzed homodimerization of ketoketenes

The mechanism of PBu₃-catalyzed homodimerization of ketoketenes has been explored and compared with that of the previously reported trialkylphosphite-mediated reactions. NMR studies of the PBu₃-catalyzed reaction implicated the involvement of tetravalent phosphonium intermediates. Phosphonium intermediates in the catalytic cycle were trapped through reaction with trimethylsilyl chloride and 4-chlorobenzaldehyde, and the resulting products were characterized. A method for the stoichiometric generation of phosphonium enolates was developed as a result of these studies. No evidence was obtained for the involvement of pentacovalent phosphorane intermediates in trialkylphosphine-catalyzed ketoketene homodimerization reactions, in contrast with the mechanism of the trialkylphosphite-mediated homodimerization of dimethylketene. An X-ray crystal structure analysis of methylphenylketene dimer showed that it possesses Z-geometry about the exocyclic olefin.     

单核锌双卟啉配合物构象平衡的理论研究及光谱分析

合成了一系列以不同长度柔韧链相连的p/p型单核锌双卟啉配合物。选用Tripos力场,利用分子动力学模拟退火和分子力学构象搜索相结合的方法对该系列双卟啉进行了能量优化和构象分析。理论计算结果表明:该类双卟啉稳定存在的最低能量构象为叠合式,最高能量构象为伸展式,并存在一系列的中间能量构象;双卟啉分子内π-π作用和能量转移与双卟啉存在的两种主要构象密切相关;分析了分子内π-π作用的本质。运用不同光谱测试手段验证了理论计算结果: 利用可见和相应二阶导数吸收光谱研究了双卟啉主要存在的叠合式和伸展式构象,通过红外光谱观察了对双卟啉构象和卟啉环间π-π作用较为敏感的吸收谱带;利用荧光光谱计算了双卟啉的分子内能量转移效率。     

A comparative study of the dimers of selected hydroxybenzenes

Hydroxybenzenes are the parent compounds of large classes of derivatives, many of which exhibit biological activities. This work presents the results of a comparative study of the dimers of selected hydroxybenzenes, considering all the possible mutual geometrical arrangements of the two monomers and comparing their relative stabilities and interaction energies. The OH···OH hydrogen bond between the two monomers is the dominant stabilizing factor, with frequent preference for mutual perpendicularity of the two aromatic rings. C? H···O unconventional H‐bonds, OH···π unconventional H‐bonds, H···π interactions and π··π interactions also may play significant roles. The factors stabilizing individual hydroxybenzenes (presence of intramolecular hydrogen bonds; number, positioning and orientation of the OH groups; symmetry features) have greater influence on the dimers' relative energy than on the interaction energy between monomers. While results from different calculations methods (HF, MP2, and DFT/B3LYP) show consistency for all the features just‐mentioned, they show some relevant differences in the way they take into account different types of interactions between monomers, resulting in some differences in the geometry arrangements of the monomers in the lowest energy dimers and in differences in the relative preferences among higher‐energy dimer geometries. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

Is Electrostatics Sufficient to Describe Hydrogen‐Bonding Interactions?

The stability and geometry of a hydrogen‐bonded dimer is traditionally attributed mainly to the central moiety A?H???B, and is often discussed only in terms of electrostatic interactions. The influence of substituents and of interactions other than electrostatic ones on the stability and geometry of hydrogen‐bonded complexes has seldom been addressed. An analysis of the interaction energy in the water dimer and several alcohol dimers—performed in the present work by using symmetry‐adapted perturbation theory—shows that the size and shape of substituents strongly influence the stabilization of hydrogen‐bonded complexes. The larger and bulkier the substituents are, the more important the attractive dispersion interaction is, which eventually becomes of the same magnitude as the total stabilization energy. Electrostatics alone are a poor predictor of the hydrogen‐bond stability trends in the sequence of dimers investigated, and in fact, dispersion interactions predict these trends better.     

Supramolecular Luminescent Lanthanide Dimers for Fluoride Sequestering and Sensing

Lanthanide complexes (Ln=Eu, Tb, and Yb) that are based on a C₂‐symmetric cyclen scaffold were prepared and characterized. The addition of fluoride anions to aqueous solutions of the complexes resulted in the formation of dinuclear supramolecular compounds in which the anion is confined into the cavity that is formed by the two complexes. The supramolecular assembly process was monitored by UV/Vis absorption, luminescence, and NMR spectroscopy and high‐resolution mass spectrometry. The X‐ray crystal structure of the europium dimer revealed that the architecture of the scaffold is stabilized by synergistic effects of the Eu? F? Eu bridging motive, π stacking interactions, and a four‐component hydrogen‐bonding network, which control the assembly of the two [EuL] entities around the fluoride ion. The strong association in water allowed for the luminescence sensing of fluoride down to a detection limit of 24 nM .