Liquid crystalline dendrimer: Sythesis and Chracterization (original) (raw)
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A Review on Synthesis, Properties, of Liquid Crystal Dendrimers
Al-Nahrain Journal of Science, 2021
This review provides brief information concerning with the dendrimer. The supramolecular organization of selected examples of liquid-crystalline dendrimers within lamellar, columnar and nematic phases is reported. It is shown that tuning of the mesomorphic structure can be achieved by an appropriate molecular design depending upon the chemical nature of the terminal mesogenic groups, dendritic core and dendrimer generation. The pseudospherical shape of a dendrimer arises from its structure, which consists of an internal region (the core) which is connected to repeating units constituting a radial branching pattern.
Triazine-based Dendrimers as Liquid Crystals: Synthesis and Characterization
2009
D ifferent generations of dendritic macromolecules based on triazine were synthesized by the divergent growth approach and functionalization with mesogenic units based on peripherally located dihydroxybiphenyl derivatives. Thermotropic liquid crystal dendrimers, containing poly(ethylene glycol) (PEG) as a core and triazine dendrons with peripherals mesogenic groups were synthesized. First generation (G 1 ) was prepared by coupling of hydroxyl group of PEG with cyanuric chloride in dichloromethane at 0-5ºC. Reaction of compound G 1 with amino group of ethanolamine in dichloromethane resulted G 1.5 . Second generation of linear dendritic compounds (G 2 ) was synthesized using coupling reaction of hydroxyl groups of compound G 1.5 and cyanuric chloride in dichloromethane. Reaction of compound G 2 with amino group of ethanolamine in water/dioxane resulted G 2.5 . The growth of dendrons on the PEG core and their structures were investigated using common spectroscopy methods. In the next step, compounds C n with different alkyl tail groups were synthesized via nucleophilic displacement of bromine from 1-bromoalkane by potassium-4,4'-biphenoxide in DMF. The mesogenic compounds 4-bromo-(4-alkyloxybiphenyl-4'-oxy) butane (C n C 4 Br) were synthesized by reaction with C n and 1,4dibromobutane undergoing S N2 substitution. Liquid crystalline dendrimers (LCD) were synthesized via coupling of hydroxyl group of G 2.5 and bromine from mesogenic units, 4-bromo-(4-alkyloxy-biphenyl-4'-oxy) butane. The liquid crystal property of mesogenic compound has been studied in details with differential scanning calorimetry and optical polarization microscopy. The experiments showed that the thermal properties are influenced with the spacer length.
Liquid-Crystalline Dendrimers Designed by Click Chemistry
Macromolecules, 2016
Liquid-crystalline dendrimers have been prepared from second-generation Percec-type poly(benzyl ether) dendrons or second-generation poly(aryl ester) dendrons carrying cyanobiphenyl mesogens. The Janus dendrimer, which combines the two types of dendromesogens, has also been synthesized. Those compounds have been prepared under copper-catalyzed azide−alkyne cycloaddition conditions. The mesomorphic properties have been studied by thermal analysis (POM, DSC) and small-angle X-ray scattering. Smectic A, nematic, and columnar phases have been observed depending on the dendritic building blocks. The click reaction has proven to be a powerful and elegant synthetic tool for the design of complex dendritic liquid-crystalline architectures.
End-on and side-on nematic liquid crystal dendrimers
…, 2004
A group of nematic materials based on poly(propyleneimine) dendrimers (PPI) containing two types of mesogenic subunits were synthesized. The presence of different types of mesogenic group, the topology of the attachment to the core (end-on and side-on), and the chemical nature of the linking connection to the PPI scaffold (imine or amide) were used to tailor the mesomorphic properties. The materials were characterized by a variety of techniques including 1 H, 13 C NMR spectroscopy, GPC, MALDI-TOF MS, DSC, POM, and X-ray diffraction. The crystalline state, when it exists, was found to be suppressed for most of the materials once they had undergone the first transition to the isotropic phase. The majority of the compounds exhibited glass transitions near to or lower than room temperature and an accompanying wide temperature range mesophase. All of the side-on dendrimers synthesized exhibit a nematic mesophase whereas end-on dendrimers exhibit different mesophases depending on the length of the terminal chain of the mesogenic unit (nematic for n ) 2, 4, 5). The 2 H NMR technique was used to investigate the symmetry of the nematic phase formed by the side-on dendrimers (G ) 3). These studies revealed that the symmetry of the nematic phase is uniaxial. They also allow the investigation of the orientational order of the phase and the rate of field-induced director alignment.
Chirally Homogeneous and Heterogeneous Dendritic Liquid Crystals
Macromolecules, 2013
We have coupled methyl α-D-glucoside (G) and methyl α-D-mannoside (M) as the chiral structural variant moieties in the core and at the branching sites within dendritic scaffolds surrounded by 12 cyanobiphenyl mesogens (CB). Systematic studies of the influence of the chiral moieties on thermal and mesomorphic properties were carried out by positional permutation approach of the pyranose units G and M within the core and branching points. The mesomorphic properties of the chirally homogeneous dendrimers GG 4 /MM 4 and the corresponding chirally heterogeneous dendritic homologues GM 4 /MG 4 were investigated by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). Remarkably, the thermal profile and the mesophase structure of the four dendrimers appear to be significantly independent of the nature of the central chiral core. The outer chirality at the periphery significantly dominates the liquid crystalline properties.
Using the controlled layer by layer experimental technique via reiterative sequence of chemical reactions carbosilane LC dendrimers with terminal cyanobiphenyl mesogenic groups of generations 1 - 5 were synthesized. Molecular structure and purity of all new compounds were characterized by 1H-NMR spectroscopy and GPC analysis. Thermal behavior of LC dendrimers was investigated by means of polarizing optical microscopy and DSC methods. All LC dendrimers synthesized have mesophases of smectic types over wide temperature region. Values of glass transition temperatures of LC dendrimers do not depend on generation number, but isotropisation temperature raises with increasing of generation number of LC dendrimers. LC dendrimer of generation 5 bearing 128 cyanobiphenyl mesogenic groups displays unusual type of structural polymorphism, which is under investigation.
Chemistry of Materials, 2001
The first photosensitive liquid crystalline (LC) dendrimer with terminal cinnamoyl groups was synthesized. A new approach to the synthesis of photochromic carbosilane LC dendrimers was elaborated. The method consists of synthesis of dendrimer with terminal hydroxyl groups and then coupling of 4-methoxycinnamoyl chloride to it. It was shown that the LC dendrimer of the first generation forms a smectic A (SmA) phase. The photochemical behavior of the dendrimer in dilute solution and in films was studied. The experimental data show that at least two processes, E-Z photoisomerization and [2+2] photocycloaddition leading to the formation of the three-dimensional network, take place. 10.
Chemistry - A European Journal, 2014
A ribbon-shaped chiral liquid crystalline (LC) dendrimer with photochromic azobenzene mesogens and an isosorbide chiral center (abbreviated as AZ 3 DLC) was successfully synthesized and its major phase transitions were studied by using differential scanning calorimetry (DSC) and linear polarized optical microscopy (POM). Its ordered structures at different temperatures were further identified through structure-sensitive diffraction techniques. Based on the experimental results, it was found that the AZ 3 DLC molecule exhibited the low-ordered chiral smectic (Sm*) LC phase with 6.31 nm periodicity at a high-temperature phase region. AZ 3 DLC showed the reversible photoisomerization in both organic solvents and nematic (N) LC media. As a chiralinducing agent, it exhibited a good solubility, a high helicaltwisting power, and a large change in the helical-twisting power due to its photochemical isomerization in the commercially available N LC hosts. Therefore, we were able to reversibly "remote-control" the colors in the whole visible region by finely tuning the helical pitch of the spontaneously formed helical superstructures.