Some of the work described in this thesis has been reported in the following publications. It is interesting to note that the number of flexible tails in the periphery dictated the self-assembly (liquid crystallinity and gelation) and photophysical behavior.
Introduction to liquid crystals
The liquid crystal state
However, if the same solid melts into an isotropic liquid state, the molecules fall in all possible directions, resulting in the loss of all positional and orientational order. The discovery of ferroelectricity in non-chiral banana-shaped molecules has led to very intense research activity in this area.
Classification of liquid crystals
In 1986, liquid crystals formed from plate-like (lattice-like) molecules were reported.6 Depending on the relative size of the principal axes, these molecules may originate from rod-like or disc-like molecules. Among them, a class that has attracted particular attention is the oligomeric liquid crystals [OLCs].9 Liquid crystalline oligomers are composed of more than two similar or different mesogenic moieties connected to each other via flexible spacers.
Thermotropic liquid crystals
Conventional liquid crystals
In other words, the short molecular axes of the molecules orient themselves more or less parallel to each other, while their centers of mass are isotropically distributed in the nematic phase. Like chiral nematic or cholesteric phase, chiral discotic nematic phase ND* also exists.5c The mesophase occurs in mixtures of discotic nematic and mesomorphic or non-mesomorphic chiral dopants as well as in pure chiral discotic molecules.5d The helical structure of the chiral discotic nematic phase is shown in fig.
Non-conventional liquid crystals
Since the thesis deals with unconventional liquid crystals, we will limit our discussion only to this topic.
Bent-core mesogens
Oligomers
The mesogenic units can be rod-shaped or disk-shaped, or an unconventional shape such as curved systems. Schematic representation of different LOLCs: (a) symmetric dimer, (b) asymmetric dimer, (c) symmetric trimer, (d) C2 symmetric or partially identical trimer, (e) asymmetric trimer, (f) symmetric tetramer, (g) C2 symmetric tetramer and (h) asymmetric tetramer. Rectangular blocks generally represent the mesogenic segments).
Star-shaped mesogens (Hekates)
Depending on the similarity of the molecular structure of the individual mesogenic units, these LOLCs can be classified as (i) symmetric LOLCs - all the mesogenic units are identical, (ii) partially identical or C2 symmetric LOLCs - some of the mesogenic units are identical and (iii) unsymmetrical LOLCs – none of the mesogenic units are identical (Fig. 1.9). The void can be used to advantage by incorporating larger chromophores into the arm scaffold through covalent bonds or gas molecules through supramolecular interactions.!Complementary hydrogen bond donors and acceptors can interact to give planar disc-shaped aggregates due to the available void between the arms of the three-arm structure.
Polycatenars
Thus, a priori one would not expect that these molecules would form mesophases based on nanosegregation, since filling the space would probably lead to a mixing of the core and the periphery of the stars, especially in the subgroup of rigid molecules (subgroup III ). Incorporating an extended π-conjugated aromatic system into the polycatenar molecular structure is very important from the point of view of charge carrier and luminescence properties.28 Gin et.
Mesophase morphologies of thermotropic liquid crystals
Columnar phases of conventional and non-conventional liquid crystals: phase types and structures
Columnar hexagonal mesophase is characterized by a hexagonal packing of the molecular columns as shown in Fig. Similar to columnar hexagonal phase, this phase also exhibits spontaneous homeotropic alignment of the columns.
Identification of thermotropic mesophases
S can be easily calculated from the cell parameters, bearing in mind that its expression depends on the cell geometry (see Table 1.1). The number of discoids present in the unit cell (Zdisc) is characteristic of the lattice geometry and the proposed model (see Table 1.1).
Application and prospects of columnar phases
The carrier mobility along the surface is supported by fluctuation and the tunneling rates are exponentially dependent on the molecular core spacing. The core-core separation fluctuates with the surface and changes as soon as the surface is disturbed.
1,3,4-Oxadiazole/thiadiazole based star-shaped mesogens
Introduction
All these properties in turn affect the electronic behavior and the mesophase.15d Therefore, mesogenic heterocyclic derivatives are finding a strong basis in the area of. This leads to properties that are remarkably different from those of 1,3,4-oxadiazole derivatives; e.g., higher melting and clearing temperatures, higher viscosity, efficient packing, and larger dipole moments.15c, 27 The larger atomic size of sulfur in the case of hexahexylthiotriphenylene (HHTT), gave an order of added to the columnar packing and thus increases the conductivity compared to that exhibited by hexaalkoxytriphenylene.28 In contrast to the large number of 1,3,4-oxadiazole-based mesogens, reports on 1,3,4-thiadiazole-based mesogens are few. .
Results and discussion
- Synthesis and Characterization
In this study we report the syntheses, characterization and thermal behavior of star-shaped LCs based on 1,3,4-oxadiazole and 1,3,4-thiadiazole (Fig. 2.1). As representative cases from each series, the 1H NMR and 13C NMR spectra of star-shaped 1,3,4-oxadiazole SO3 and star-shaped 1,3,4-thiadiazole ST3 are shown in Figs.
Col h
Thermal behavior
As with the compound SO2, we assign the Colh structure at both temperatures. In the case of compound SO4, with nine branched tails, we expected a huge drop in melting point. Thus, it is clear that the presence of sulfur in the molecular structure is responsible for the enhanced mesophase range.
Photophysical properties
This is most evident in the case of compound ST1 with a mesophase interval of 31 degrees; compared to the monotropic compound SO1. Such a large shift in the emission maximum is due to the large delocalization of the electron cloud compared to that in the case of the SO1 compound. The emission spectra of compounds ST1-4 obtained by exciting them at their absorption maximum showed an emission maximum centered in the range 416-471 nm (Fig. 2.14b).
Electrochemical properties
Similar to compounds SO1–4, we observed a bathochromic shift in the emission with an increase in the number of tails as we move from ST1 to ST3, whereas the emission of compound ST4 does not differ much from that of compound ST3. The observed red shift in the absorption and emission of the thiadiazole derivatives is due to the higher polarizability and basic nature of the sulfur in the thiadiazole moiety. Oxadiazole-based compounds SO1-4 showed quantum yields in the range of 0.43 to 0.48, whereas thiadiazole-based compounds showed slightly smaller quantum yields in the range of 0.25 to 0.31.
Conclusions
The optical band gap Eg,opt was estimated from the red edge of the absorption spectra. The number and type of the peripheral tails have a significant effect on the stabilization of the liquid crystallinity of these molecules. The number and type of the peripheral tails did not have much effect on the absorption maxima, although it did show a significant shift in the emission maxima of these compounds.
Experimental Section
Excess thionyl chloride was removed by distillation, the crude product (1,3,5-benzenetricarbonyl trichloride) was dried in vacuo and. Excess thionyl chloride was removed by distillation and the crude product (1,3,5-benzenetricarbonyl trichloride) was dried in vacuo. Excess thionyl chloride was removed by distillation, the crude product (1,3,5-benzenetricarbonyl trichloride) was dried in vacuo and used for the next reaction without further purification.
1,3,4-Thiadiazole based polycatenar mesogens
Introduction
Incorporating an extended π-conjugated aromatic system into the polycatenar molecular structure is very important from the point of view of charge carrier and luminescence properties.14 Gin et. Recently, there has been a renewed interest in the integration of heterocycles into the molecular design of mesogens, due to the wide variety of structures and thus resulting properties.23 Heteroatoms such as nitrogen, oxygen and sulfur provide reduced molecular symmetry, strong lateral and/ or or longitudinal dipoles and a donor-acceptor interaction within the molecule, which in turn influences the LC self-assembly and the electronic behavior of the mesogens.23 Mesogens with heterocyclic moieties in their molecular structures provide tunability of the emission color and also make a polarized emission possible. This is an important property, which finds application in the field of OLEDs.24 Thus, many polycatenar LCs carrying a heterocyclic group in their molecular structures have been reported.
Results and discussion
- Synthesis and molecular structural characterization
- Thermal behavior
- Photophysical properties
- Electrochemical properties
In the wide-angle range, a relatively diffuse peak corresponding to the d-spacing of 4.47 Å was observed. This shows that the π-electron cloud is delocalized to a greater extent for the p-substituted compounds (PT1-3) compared to the m-substituted compounds (MT1-3). As a result, there is a reduction in the band gap of the polycatenars compared to their star-shaped analogue33b (Fig. 3.23).
Conclusions
Experimental Section
A solution of crude product 7a (0.4 mmol, 1 equiv) in dry toluene (8 mL) was added dropwise to a solution of Lawesson's reagent (1 mmol, 2.4 equiv) in dry toluene at room temperature under argon. A solution of crude product 6a (0.6 mmol, 1 equiv) in dry toluene (12 mL) was added dropwise to a solution of Lawesson's reagent (1.4 mmol, 2.4 equiv) in dry toluene at room temperature under an argon atmosphere and Reflux for 24 hours. A solution of crude product 6b (0.6 mmol, 1 equiv) in dry toluene (12 mL) was added dropwise to a solution of Lawesson's reagent (1.4 mmol, 2.4 equiv) in dry toluene at room temperature under an argon atmosphere and Reflux for 24 hours.
A solution of crude product 6c (0.2 mmol, 1 equiv) in dry toluene (4 mL) was added dropwise to a solution of Lawesson's reagent (0.5 mmol, 2.4 equiv) in dry toluene at room temperature under an argon atmosphere and Reflux for 24 hours. . After removing the solvent in vacuo, the crude product was further purified by column chromatography on neutral aluminum oxide.
1,2,4-Oxadiazole based star-shaped mesogens
Introduction
There is increasing interest in the design of highly π-conjugated liquid crystals bearing heterocycles13 due to the enormous possibilities they offer. This is because the aggregation-induced quenching of luminescence in the Col phase is detrimental to device performance. In this chapter, we report the first examples of star-shaped molecules containing three 1,2,4-oxadiazole heterocyclic moieties attached to the 1, 3, and 5 positions of the central benzene ring.
Results and discussion
- Synthesis and Characterization
- Thermal behavior
- Photophysical and electrochemical studies
- Gelation studies
- Rheological studies
- Single crystal XRD studies
Thus, increasing the length of the peripheral chain decreased the thermal range of the mesophase. The luminescence remained the same despite the change in gel structure. POM image of a thin film of compound SI2 in xerogel state under parallel polarizers (obtained by dripping 4 mM dodecane solution) (a); and fluorescence microscopy image of a xerogel thin film obtained for compound SI2 (4 mM solution in dodecane, scale bar 100 µm) (b).
Conclusions
Given the lack of organic materials that emit blue light with a wide band gap, these star-shaped molecules are promising, due to their emissive nature in the aggregated state and their columnar self-assembly.
Experimental Section
The reaction mixture was filtered over celite bed and concentrated to give the crude product, which was further purified by column chromatography on silica gel (60-120) with 10% EtOAc-hexanes as eluent. To this is added pyridinium chlorochromate (11.3 mmol, 1 equiv.) adsorbed over an equal amount of silica gel and stirred at room temperature for 1 hour. The reaction mixture was filtered over celite bed and concentrated to get the crude product, which was further purified by column chromatography on silica gel (60-120) with 10% EtOAc-hexanes as eluent. After cooling to room temperature, water was added to the reaction mixture, which was then extracted with chloroform (5× 20mL).
This data can be retrieved free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html.
1,2,4-Oxadiazole based polycatenar mesogens
Introduction
This is an exceptional combination of order and mobility that gives stimuli responsive, self-healing and adaptive behavior, leading to their technological importance in the current era. Polycatenar mesogens or phasmids are considered the link between conventional rod-like (calamitics) and disk-like (discotic) mesogens, as they possess the structural properties and exhibit rich mesomorphic behavior common to both classes of conventional LCs.1 With regard to stabilization of Col phase this class of molecules are advantageous due to their lower melting/clearing point, which aids in the tuning and the inherent synthetic flexibility to incorporate different functionalities, compared to discotics. This compound exhibited weak emission in the solution state (solvated monomers) and exhibited technologically important aggregation-induced blue light emission.
Results and discussion
- Synthesis and Characterization
- Thermal behavior
The mosaic texture showed a change in color upon further cooling, as the homeotropic domains coalesced, which persisted until crystallization (as seen in Figure 5.4b), while DSC did not show any peaks corresponding to this transition. Photomicrograph of texture as seen by POM for the Colh phase of compound PO2 at 85 °C (a); Compound PO3 with six n-dodecyloxy tails showed a mosaic texture mixed with homeotropic regions of dark field of view upon cooling from the isotropic melt (Fig. 5.5a).
