Effect of molecular structure on mesomorphism XX. Unsymmetrical dimeric mesogens

A series of unsymmetrical dimeric mesogens was prepared, members of which were composed of a 4'-pentyloxybiphenyl-4-carboxyl core and a 4'-substituted-benzylidene-4-oxyaniline core joined via an alklyene spacer. The structural variants were cyano, nitro and trifluoromethyl. Comparison of mesophase behaviour of the unsymmetrical dimers with that of corresponding monomeric 'halves' and also with that of corresponding symmetrical dimers indicated a tendency for the unsymmetrical dimers to form smectic phases although not as highly ordered as those of some of the monomers corresponding to a relevant core group.     

In situ photochemical conversion from cinnamoyl-functionalized liquid-crystalline monomers to liquid-crystalline dimers

Liquid-crystalline (LC) monomers, which were functionalized with a cinnamoyl group on their extremity, were synthesized and irradiated with UV light in their LC phases. In the presence of a triplet sensitizer, most LC monomers were converted into the corresponding dimers, which were produced by the cycloaddition reaction of the cinnamoyl group. The photodimerization reaction could proceed while the LC phases were maintained, because the dimers showed LC phases whose temperature ranges were wider than those of the corresponding monomers. A ¹H NMR study of the LC dimers indicated that the cyclobutane unit dominantly had an anti-head-to-head configuration, that is, δ-truxinate. As the LC monomers, which had a phenyl biphenyl-4-carboxylate moiety as a mesogen, showed smectic A phases and the corresponding dimers also exhibited smectic A phases, we estimated the smectic layer distances by X-ray diffraction analysis and found that the dimers adopted the structure in which the two mesogens aligned laterally and existed in the same smectic layer in the LC phases.     

Effect of molecular structure on mesomorphism XX. Unsymmetrical dimeric mesogens

Abstract

A series of unsymmetrical dimeric mesogens was prepared, members of which were composed of a 4′-pentyloxybiphenyl-4-carboxyl core and a 4′-substituted-benzylidene-4-oxyaniline core joined via an alklyene spacer. The structural variants were cyano, nitro and trifluoromethyl. Comparison of mesophase behaviour of the unsymmetrical dimers with that of corresponding monomeric ‘halves’ and also with that of corresponding symmetrical dimers indicated a tendency for the unsymmetrical dimers to form smectic phases although not as highly ordered as those of some of the monomers corresponding to a relevant core group.     

In situ photochemical conversion from cinnamoyl‐functionalized liquid‐crystalline monomers to liquid‐crystalline dimers

Liquid‐crystalline (LC) monomers, which were functionalized with a cinnamoyl group on their extremity, were synthesized and irradiated with UV light in their LC phases. In the presence of a triplet sensitizer, most LC monomers were converted into the corresponding dimers, which were produced by the cycloaddition reaction of the cinnamoyl group. The photodimerization reaction could proceed while the LC phases were maintained, because the dimers showed LC phases whose temperature ranges were wider than those of the corresponding monomers. A ¹H NMR study of the LC dimers indicated that the cyclobutane unit dominantly had an anti‐head‐to‐head configuration, that is, δ‐truxinate. As the LC monomers, which had a phenyl biphenyl‐4‐carboxylate moiety as a mesogen, showed smectic A phases and the corresponding dimers also exhibited smectic A phases, we estimated the smectic layer distances by X‐ray diffraction analysis and found that the dimers adopted the structure in which the two mesogens aligned laterally and existed in the same smectic layer in the LC phases.     

Supramolecular chemistry in the structure direction of microporous materials from aromatic structure-directing agents

A combination of fluorescence spectroscopy, thermogravimetric analysis, and molecular mechanics calculations has been used to study the structure-directing effect of the aromatic benzylpyrrolidine (BP) molecule (and its monofluorinated derivatives), and (S)-(-)-N-benzylpyrrolidine-2-methanol (BPM) in the synthesis of the microporous AFI structure. The results clearly show that, while all molecules form supramolecular aggregates in concentrated water solution, BPM molecules have a much more pronounced trend to aggregate as dimers within the AFI structure due to the development of interdimer H-bond interactions. Instead, BP (and its ortho- and meta-fluorinated derivatives) SDAs tend to incorporate in the AFI structure as monomers but with the simultaneous occlusion of water molecules, while para-fluorinated BP derivatives do not form compact dimers able to be accommodated in the AFI structure. We propose a crystallization mechanism where the presence of dimers is required for the nucleation step to occur, while crystal growth takes place through the simultaneous occlusion of SDA monomers and water (when the synthesis is performed with BP and derivatives) or through the occlusion of SDA dimers (in the synthesis with BPM).     

Photonics of dimers of cyanine dyes

The results of study on the properties of dimers of thiamonomethine-and thiatrimethinecyanines (thiacarbocyanines) in the ground and electronically excited states in aqueous solutions are presented. Dimers of cyanine dyes have the sandwiched structure with near-parallel alignment of the polymethine chains of the monomers in the dimer. The formation of dimers is manifested by two absorption bands of different intensities due to splitting of the S* level of the monomers upon their resonance interaction. Dimers of thiacarbocyanines are characterized by a low fluorescence quantum yield φ_fl as compared to monomers; however, φ_fl of dimers of thiamonomethinecyanines are markedly higher than that of monomers. Dimers of cyanine dyes are also characterized by a relatively high quantum yield of intersystem crossing to the triplet state. In the triplet-triplet absorption spectra, two bands of different intensities are revealed, which are due to the splitting of the higher triplet level of the monomers that form the dimer. In the presence of electron donors (ascorbic acid, hydroquinone) and/or acceptors (p-benzoquinone, p-nitroacetophenone, methylviologen), the triplet state of dimers is quenched as a result of electron transfer yielding radical products. Dimers in the triplet state can serve as photosensitizers of redox reactions.     

N,N'-Linked biazoles : Part 7. Oxidative dimerization of n-unsubstituted tetrahydroindazolones

The structure of the products obtained by oxidative dimerization of tetrahydroindazolones was established as being N(1)-C(3a') dimers. In one case a C(3a)-C(3a') dimer was also isolated, but N-N'-Linked dimers were never found, contrary to a previous report in the literature. A complete 13-carbon nmr study of the monomers and their fixed methylated derivatives was performed to determine the most abundant tautomer in each case, since the tautomerism study of tetrahydroindazolone and of its 4-ethoxycarbonyl derivative was needed to fully determine the structure of the dimers.     

Fluorescence spectra and structure of the difluoro(dibenzoylmethanato)boron monomers and dimers absorbed on silica gel

It has been shown that difluoro(dibenzoylmethanato)boron ((dbm)BF₂) can be absorbed on silica gel in the form of fluorescent monomers and dimers with the emission properties that change in the presence of vapors of volatile organic compounds, such as ethanol, acetone, toluene, and meta-xylene. Fluorescence quenching was observed for the (dbm)BF₂ monomers and dimers in the case of ethanol and acetone, whereas the formation of fluorescent exciplexes with monomers and enhancement of the dimer fluorescence were observed in the case of toluene and meta-xylene. Results of the quantum-chemical calculations of the structure of the (dbm)BF₂ monomer complex with the matrix and toluene and (dbm)BF₂ dimers with matrix are presented.     

Polymorphic smectic A phases in perfluoroalkylated mesogenic dimers

New homologous symmetric dimers are presented in which two-ring monomers are connected by a flexible spacer and perfluorinated chains are introduced in terminal positions. The mesophase behaviour depends on the length and structure of the spacer and on the length of the terminal chains. The most interesting finding is the occurrence of dimorphic SmA phases in the longer chain homologues with odd-numbered spacer. The transition between the dimorphic SmA phases is accompanied by a relatively high enthalpy. Considering the results of XRD measurements and computer simulations, structural models are proposed, based on different conformations of the molecules.     

Polymorphic smectic A phases in perfluoroalkylated mesogenic dimers

New homologous symmetric dimers are presented in which two-ring monomers are connected by a flexible spacer and perfluorinated chains are introduced in terminal positions. The mesophase behaviour depends on the length and structure of the spacer and on the length of the terminal chains. The most interesting finding is the occurrence of dimorphic SmA phases in the longer chain homologues with odd-numbered spacer. The transition between the dimorphic SmA phases is accompanied by a relatively high enthalpy. Considering the results of XRD measurements and computer simulations, structural models are proposed, based on different conformations of the molecules.     

Group 12 Dihalides: Structural Predilections from Gases to Solids

Connections between the structures of Group 12 dihalides in their vapor and crystal phases are sought and discussed. The molecular structures of all monomers and dimers (MX₂: M=Zn, Cd, Hg and X=F, Cl, Br, I) were calculated at the density functional B3PW91 and MP2 computational levels. All the monomers are linear, with the mercury dihalide molecules having shorter bonds than their cadmium analogues; the ZnX₂ and CdX₂ structures are similar. The shorter Hg? X distances are traced back to relativistic effects. For the dimers, many possible geometrical arrangements were considered. The zinc and cadmium dihalide dimers have the usual D_2h‐symmetry geometry, whereas the mercury dihalide dimers are loosely‐bound units with C_2h symmetry. The origins of this C_2h structure are discussed from different points of view, including frontier orbital interactions. The crystals of Group 12 dihalides span a wide range of structure types, from three‐dimensional extended solids to molecular crystals. There is an obvious connection between the structures and characteristics of monomers, their dimers, and the crystals they form. The similarities as well as startling differences from the Group 2 dihalides are analyzed.     

Infrared spectra of homogeneous and heterogeneous proton-bound dimers in the gas phase

Infrared multiphoton dissociation spectra of three homogeneous and two heterogeneous proton-bound dimers were recorded in the gas phase. Comparison of the experimental infrared spectra recorded in the fingerprint region of the proton-bound dimers with spectra predicted by electronic structure calculations shows that all modes which are observed contain motion of the proton oscillating between the two monomers. The O-H-O asymmetric stretch for the homogeneous dimers is shown to occur at around 800 cm-1. As expected, the O-H-O asymmetric stretching modes for the heterogeneous proton-bound dimers are observed to shift to significantly higher energy with respect to those for the homogeneous proton-bound dimers due to the asymmetry of the O-H-O moeity. This shift is shown to be predictable from the difference in proton affinities between the two monomers. Density functional predictions of the infrared spectra based on the harmonic oscillator model are demonstrated to predict the observed spectra of the homogeneous proton-bound dimers with reasonable accuracy. Calculations of the structure and infrared spectrum of protonated diglyme at the B3LYP/6-31+G** level and basis also agree well with an infrared spectrum recorded previously. For both heterogeneous proton-bound dimers, however, the predicted spectra are blue-shifted with respect to experiment.     

6-甲基-4-羟基嘧啶单体及二聚体质子转移过程的理论研究

应用密度泛函理论的B3LYP/6-311+G(d)方法研究了6-甲基-4-羟基嘧啶单体及二聚体质子转移的异构化反应.对反应势能面的研究发现,该化含物可能存在9种单体异构体,对其最稳定的单体构型进行分析.各单体间异构化反应的过渡态共有9种,反应的活化能最小为22.06 kJ/mol,最大为356.55 kJ/mol,最可能的反应路径在室温下即可进行. 研究了2种二聚体及其异构化反应的过渡态,发现二聚体均比其对应的单体稳定,而且质子转移所需要的活化能仅为20.13 kJ/mol,比单体低很多. 氢键在这种变化中起了主要作用,由单体和二聚体的总能量计算了氢键的键能.     

Non-symmetrical discotic liquid crystalline dimers: molecular design,synthesis and mesomorphic properties‡

The synthesis and mesomorphic properties of novel non-symmetrical discotic dimers have been investigated. Dimers have been prepared by the combination of electron-deficient (n-type) anthraquinone and electron-rich (p-type) triphenylene discotic monomers. The mesophases have been characterised using polarising optical microscopy, differential scanning calorimetry and X-ray diffraction. Most of the dimers have been shown to exhibit a rectangular columnar mesophase and one has a nematic columnar mesophase also. Charge transfer behaviour has been studied by UV–Vis spectroscopy.     

Thermolysis of vic-dihydroxybacteriochlorins: effect of the nature of substrates in directing the formation of chlorin-chlorin dimers with fixed and flexible orientations and their preliminary in vitro photosensitizing efficacy

The thermolysis products obtained by refluxing a series of vic-dihydroxychlorins in o-dichlorobenzene are characterized. Depending on the nature of substrates, this methodology provides an access for novel carbon-carbon linked chlorin-chlorin dimers and chlorin-porphyrin dimers with fixed and flexible orientations. The configuration of the linkers in the symmetrical and unsymmetrical dimers was confirmed by extensive NMR (COSY, ROESY) and molecular modeling studies. The molecular modeling studies of the energy-optimized dimers with flexible orientation confirmed that one of the chlorin units of the dimeric structure is tilted toward the opposite ring as evident by the shielding effect in the resonances of some of the protons in the (1)H NMR spectroscopy. Among the dimers with fixed orientation, compared to the free-base analogues, the related mono- and di-Zn(II) complexes produced a decreased fluorescence intensity, suggesting a possibility of the faster energy transfer via intersystem crossing (ISC) in the metalated derivatives than the corresponding free-base analogues to produce the corresponding excited triplet states. The photosensitizing efficacy of the monomers and the related dimers was also compared in radiation-induced fibrosarcoma (RIF) tumor cells at variable drug/light doses. In preliminary screening, compared to monomers, the corresponding carbon-carbon linked dimers produced enhanced photosensitizing efficacy.     

三种金属硫蛋白动力学稳定性的理论研究

对三类金属硫蛋白（大鼠金属硫蛋白亚型Ⅱ,兔肝金属硫蛋白亚型Ⅰ和兔肝金属硫蛋白亚型Ⅱ）的单体和二聚体进行了水溶液条件下的分子动力学模拟。其中大鼠金属硫蛋白亚型Ⅱ的结构直接来自于晶体数据,兔肝金属硫蛋白亚型Ⅰ和Ⅱ的结构则通过同源蛋白模型搭建。动力学模拟的结果显示,这三种单体在水溶液中都具有相当大的柔性,柔性主要来源于柔性区的氨基酸残基。三类金属硫蛋白单体的动力学模拟均表明α结构域的动力学稳定性都要优于     

An example of an unsymmetrically substituted,monofunctionalized anthraquinone-based discotic liquid crystal

We report here the synthesis and characterization of an unsymmetrically substituted, monofunctionalized anthraquinone-based discotic liquid crystal, namely 5-hydroxy-1-(4-nitrobenzyloxy)2,3,6,7-tetrapentyloxy-9,10-anthraquinone. Interestingly, this compound, which has only four alkyloxy chains attached to the anthraquinone core, shows a hexagonal columnar phase (Col_h) of much wider temperature range compared with analogues having six alkyloxy chains. Incidentally, among the anthraquinone-based disc-like molecules exhibiting columnar phases known so far, this is the first molecule, which does not possess any C₂-axis of symmetry. Besides its unsymmetrical structure and interesting mesomorphic behaviour, monofunctionalized monomers of this kind, unlike the available C₂-symmetric difunctionalized tetraethers, can serve as potential synthons for obtaining dimers in a unique way. In addition, various anthraquinone-based oligomers, metallomesogens, and side group liquid crystalline polymers can be obtained from these monomers.     

An example of an unsymmetrically substituted, monofunctionalized anthraquinone-based discotic liquid crystal

We report here the synthesis and characterization of an unsymmetrically substituted, monofunctionalized anthraquinone-based discotic liquid crystal, namely 5-hydroxy-1-(4-nitrobenzyloxy)2,3,6,7-tetrapentyloxy-9,10-anthraquinone. Interestingly, this compound, which has only four alkyloxy chains attached to the anthraquinone core, shows a hexagonal columnar phase (Col_h) of much wider temperature range compared with analogues having six alkyloxy chains. Incidentally, among the anthraquinone-based disc-like molecules exhibiting columnar phases known so far, this is the first molecule, which does not possess any C₂-axis of symmetry. Besides its unsymmetrical structure and interesting mesomorphic behaviour, monofunctionalized monomers of this kind, unlike the available C₂-symmetric difunctionalized tetraethers, can serve as potential synthons for obtaining dimers in a unique way. In addition, various anthraquinone-based oligomers, metallomesogens, and side group liquid crystalline polymers can be obtained from these monomers.     

Twisted perylene stereodimers reveal chiral molecular assembly codes

Unique perylene diastereomeric linear and cyclic dimers were synthesized from twisted perylene monomers, revealing that pi-stacking stereoisomerism imparted specific intermolecular self-assembly and intramolecular folding. Only the homochiral twisted tetrachloroperylene monomers cyclized via a cooperative reaction, forming the homochiral diastereomers. The heterochiral tetrachloroperylene monomers proceeded through a stepwise reaction and yielded a linear heterochiral dimer, which equilibrated with the linear homochiral dimers. The linear homochiral dimers cyclized to produce the same cyclic homochiral diastereomers. These results demonstrated that homochiral and heterochiral self-assemblies were two distinct molecular codes, directing two specific chemical pathways. The homochiral cyclic dimers remain isomerically pure at -20 degrees C but can be interconverted to the heterochiral cyclic dimer meso compound at room temperature. The diastereomers were readily separated by HPLC. While driven by solvophobic forces, foldable linear dimers synthesized from the same twisted monomers using phosphoramidite chemistry folded into homodimer and heterodimer, confirming the inherent molecular codes, which were dictated by the perylene chirality, ultimately gauged the weak pi-stack forces, and directed self-assembly and folding.     

核酸碱基的非谐性振动:Ⅲ,DNA叠加二聚体的一维和二维红外光谱

用量子化学计算方法研究了DNA碱基单体及其15个B型和2个G-四链体的叠加二聚体的非谐性振动光谱特征.研究发现,从碱基单体到二聚体,C=O伸缩模式之间的相互作用很强,表现在其非谐性振动频率和非谐性常数都发生了明显的改变,特别是在含碱基G的叠加体中.这些变化能在一维和二维红外光谱中很好地表现出来.利用振动模式的势能分布和非谐性常数的组成分析,讨论了叠加二聚体中C=O模式的离域化程度.