Effect of atomic scale differences on the self-assembly of thiophene-based polycatenars in liquid crystalline and organogel states, B. Aromatic π-π-driven supergelation, aggregation-induced emission and columnar self-assembly of star-shaped 1,2,4- oxadiazole derivatives, S.
CONTENTS
Introduction to liquid crystals and organogelators
Star-shaped fluorescent liquid crystals derived from s- triazine and 1,3,4-oxadiazole moieties
Effect of atomic-scale differences on the self-assembly of thiophene-based polycatenars in liquid crystalline and
Columnar self-assembly of luminescent bent-shaped hexacatenars with a central pyridine core connected with
List of abbreviations used in the text
GENERAL REMARKS
PREFACE
This is an introductory chapter to LCs in general describing their classification and their important significance in material science. The chapter focuses in greater detail
The number, length and location of substitution of the peripheral tails and the type of the heterocyclic moiety adjacent to the thiophene ring greatly affected the self-assembly of the molecules in the LC and gel states. The type of the heterocyclic group and the number of peripheral tails had a significant influence on the stabilization of the liquid crystalline and organogel self-assembly as well as the photophysical properties.
Introduction to liquid crystals and organogelators
The liquid crystal state
In the case of a liquid, there is no specific location for the center of mass of the molecules as it varies as the molecules diffuse in all directions as shown in fig. The organization of the molecules in a well-defined way gives anisotropic properties to the liquid crystals.
History of liquid crystals
The degree of organization of liquid crystal is less than that of the solid crystalline state, but has more degree of organization than that in the liquid state. Similar behavior of cloudy liquid phenomenon observed from 116 oC to 134 oC in para-azoxisole (II) (Fig. 1.2) by Gattermann.3b As a result of this preliminary work, Reinitzer is often credited with the discovery of the new intermediate phase of matter - the liquid crystal phase.
Classification of liquid crystals
7 etc.), molecular size (as low and high molecular weight) and with regard to the type of mesophase formed (nematic, cholesteric, smectic and columnar etc.).
Thermotropic liquid crystals
Conventional liquid crystals
In some special cases, lateral units X and Y (eg F, Cl, CN, CH3, etc.) are also incorporated into the main molecular structure. The search for such a mesophase is mostly governed by subtle changes in the number, size, and nature of the lateral chains beyond the central core.
Non-conventional liquid crystals
9 mesomorphism results from the microsegregation of the two constituents: the surface at the interactions between the conjugated rigid cores promotes the crystalline character, while the liquid character originates from the melting of the flexible peripheral alkyl chains in the mesophase. Most disc-shaped molecules have only one type of mesophase, but a few examples are known to show polymorphism.
Star-shaped mesogens (Hekates)
The presence of the voids between the arms of the molecular structure helps to avoid crystallization of mesophases and promote the glassy state. Thus, one would not expect these molecules to form mesophases based on nanosegregation of incompatible moieties, due to the presence of large voids, which may promote the mixing of core and peripheral chains, especially in the subgroup of shape-persistent molecules (Subgroup III) .
Polycatenars
There is a large void between the individual arms that must be filled in condensed phases. These molecules self-assemble into different LC phases due to the nano-segregation of aromatic units and flexible chains.
Mesophase morphologies of thermotropic columnar liquid crystals
Since polycatenars share the structural features of both calamitic and discotic LCs, they exhibit various LC phases such as nematic, smectic (lamellar), cubic, and Col phases, depending on the number of peripheral tails.
Types and structures of columnar phase
- Columnar hexagonal phase (Col h )
- Columnar tetragonal (square) phase (Col t )
- Columnar rectangular phase (Col r )
- Columnar oblique phase (Col ob )
- Columnar plastic phase (Col p )
- Columnar helical phase (H)
- Columnar lamellar phase (Col L )
The hexagonal packing of the molecular columns is characteristic of the columnar hexagonal mesophase, as shown in Table 1.1. The tetragonal packing of molecular columns is characteristic of the columnar tetragonal mesophase, as shown in Table 1.1.
Identification of thermotropic mesophases
Where ρ is the density of the liquid crystal phase, NA is Avogadro's constant, S is the column cross-sectional area, hc is the height of the column slice, and M is the molecular weight of the constituent molecule. S can be easily calculated from the cell parameters, given that its expression depends on the cell geometry (Table 1.1).
Applications of columnar phases
However, there are limitations in mesophase characterization by either of these methods, as the optical textures of different smectic or Col phases are difficult to distinguish, and the enthalpy values may not be so characteristic of different phase transitions. 19 carriers in the Col phases also make them worthy of their use in fast and high-resolution xerographic and laser printing as active charge transport layers. 33Col LCs can be used as sensitive sensors for both polar and nonpolar molecules because they have a unique conductive surface that can changes due to the disturbance in the surface.34 In recent years, work has been done on using Col phases for the construction of the organic light emitting diodes (OLEDs), as they can function as good emitting and conducting materials.
The Gel state
Schematic representation of the self-assembly of low molecular weight gelators into one-dimensional aggregates and the subsequent formation of an entangled network. 43c. The formation of anisotropic phase-separated structures leads to the induction of new functions and the improvement of the properties.
Star-shaped fluorescent liquid crystals derived from s-triazine and 1,3,4-oxadiazole moieties
Introduction
Several 1,3,5-substituted benzene-based or heterocyclic cores have been used to prepare these star-shaped mesogens. 1,3,5-Triazine-based star-shaped mesogens have been prepared by one of the following methods, (i) the reaction of nucleophilic group-functionalized aromatics with cyanuryl chloride9; (ii) nucleophilic9 or palladium-catalyzed substitutions10 with trihalo-1,3,5-triazines and (iii) cyclotrimerization of nitriles.11, 12b, 17.
Results and discussion
- Synthesis and Characterization
- Thermal behavior
- Photophysical properties
- Electrochemical behavior
Results of the (hkl) indexation of the XRD profiles of the compounds TZ2-4 at a given temperature (T) of the mesophasesa. Results of the (hkl) indexation of the XRD profiles of the compounds TZ6-8 at a given temperature (T) of the mesophasesa. This observation is limited to the type of peripheral substitution, which is not observed in the other compounds of the respective series.
Conclusion
The lower LUMO level and band gaps of these star molecules can reduce the barriers to electrons while simultaneously blocking hole movement. In general, decreasing the chain length increased the core–core interaction, leading to stabilization of Colr. The preserved fluorescence in the solid state is explained by the formation of aggregates that do not quench the fluorescence.
Experimental Section
A mixture of ethyl 4-(hexadecyloxy)benzoate (19 mmol, 1 equiv), excess hydrazine hydrate (20 equiv) in ethanol was refluxed for 48 hours. A mixture of ethyl 4-(dodecyloxy)benzoate (19 mmol, 1 equiv), excess hydrazine hydrate (20 equiv) in ethanol was refluxed for 48 h. The excess thionyl chloride was removed by distillation, and then the crude product was dried in vacuo and used for the next reaction without further purification and characterization.
Effect of atomic-scale differences on the self- assembly of thiophene-based polycatenars in
Introduction
Thiophene-based molecular materials are an extensively studied type of heterocyclic materials due to their synthetic flexibility and applications as n-type (electron-rich) semiconductor systems.1 Thiophene-based oligomers and polymers are known for their unique optoelectronic properties and have found utility in the production of field-effect transistors,2 light-emitting diodes,3 and photovoltaic cells.4 Most oligothiophenes must be purified by vacuum sublimation techniques due to their poor solubility. Most of the thiophene-based liquid crystals reported stabilize nematic and smectic phases.5 A few oligomers with a greater number of flexible terminal tails have been reported to stabilize columnar (Col) phases.6 Stabilization of Col phases in this class of materials is advantageous because the stacking of molecules one above the other to form columns of indefinite length provides a pathway for one-dimensional charge migration. We envisioned that the presence of oxadiazole and thiadiazole could result in a drastic difference in the self-assembly and related macroscopic properties.
Results and discussion
- Synthesis and Characterization
- Thermal behavior
- Photophysical properties
- Electrochemical behavior
- Gelation properties
- Rheological properties
Results of (hkl) indexing of XRD profiles of compounds TT3-4 at a given temperature (T) of the mesophase. Therefore, increasing the bent angle of the side rings helps to increase the width of the mesophase. The photophysical properties of polycatenars TO1-4 and TT1-4 were investigated in the state of solution and thin film (Table 3.4).
Conclusion
Experimental Section
The crude product 7a (0.5 mmol, 1 equiv.) was dissolved in dry toluene and added to Lawesson's reagent (2.2 equiv.) at room temperature. The crude product 7b (0.5 mmol, 1 equiv.) was dissolved in dry toluene and added to Lawesson's reagent (2.2 equiv.) at room temperature. The crude product 7c (0.5 mmol, 1 equiv.) was dissolved in dry toluene and added to Lawesson's reagent (2.2 equiv.) at room temperature.
Columnar self-assembly of luminescent bent- shaped hexacatenars with a central pyridine core
Introduction
Most of the reported polycatenars are known to have a central linear rod-like structure, while very few are known to have a bent structure.7 The polycatenars with central bent structure have shown lower transition temperature and an enhanced mesophase range.8 There is a growing interest in the research community , for the incorporation of heterocycles into the LC design, as they help vary the overall lateral and/or longitudinal dipole moment of the molecule along with the variations in their molecular shape.9,10 These changes affect the molecular self-assembly and macroscopic properties of such molecules, and can therefore be effectively used to modify the material properties. We were curious to compare the optical, photophysical and gelation properties of polycatenars based on 1,3,4-oxadiazole derivatives with their thiadiazole counterparts given the alternative arrangement of donor and acceptor moieties in the molecular system. We visualized that the incorporation of oxadiazole or thiadiazole into the polycatenar structure had an impact on the self-assembly and related macroscopic properties.
Results and discussion
- Synthesis and Characterization
- Thermal behavior
- Photophysical properties
- Solvatochromism and acidochromism
- Gelation properties
- Aggregation induced enhanced emission (AIEE) behavior
- Rheological properties
Indexing results (hkl) of XRD profiles of compounds PT2-4 at a certain temperature (T) of the mesophase. Again, deprotonation of compound PT3 by addition of TEA leads to displacement of the protons of the pyridine ring (Fig. 4.23a). Intensity versus 2θ profiles obtained from the XRD pattern for the Colr phase of the PO3 composite xerogel (a); Colr phase of xerogel of composite PT3 (b).
Conclusion
Two of the hexacatenar supergels investigated showed aggregation-induced enhanced emission with a several-fold increase in luminescence intensity in the gel state. The variation in the gelation ability of these molecules shows that although the π-π interactions play a major role in self-assembly, the nanosegregation of incompatible molecular segments such as peripheral flexible tails also contributes to organogelation and LC self-assembly. In a nutshell, this study explains the effect of molecular structural variations on the nature of supramolecular self-assembly and the resulting macroscopic properties, which are of substantial importance to the scientific community working in the broad area of supramolecular chemistry.
Experimental Section
The excess thionyl chloride was removed by distillation and the crude product (pyridinium chloride) was dried in vacuo and used for the next reaction without further purification and characterization. After removal of solvent in vacuo, the crude product was further purified by column chromatography on neutral alumina. The solution of crude product 12a (1.2 mmol, 1 equiv.) in dry toluene (8 mL) was added dropwise to a solution of Lawesson's reagent (3 mmol, 2.4 equiv.) in toluene at room temperature under argon atmosphere and refluxed. for 24 hours.
