At the outset, with the deepest sense of gratitude, I would like to express my sincere thanks to my supervisor, Dr. Achalkumar for his able guidance, inspiration, creative and scholarly ideas that helped me explore the scope of my work gathered in this thesis. Apart from my supervisor, I would like to express my deepest gratitude and heartfelt thanks to my PhD committee members, Dr. Subhas Chandra Pan and Dr. Chandan Kumar Jana for their periodic evaluation of my work, valuable suggestions, constant support and encouragement.
Papers presented in National/International conferences: (Oral/Poster)
Self-assembly of acidochromic, luminescent, star-like molecules with cyanovinylene chromophores in columnar liquid crystal form at room temperature, R. Poster presented at the Fourth International Symposium on Advances in Sustainable Polymers (ASP-17) held at IIT Guwahati.
CONTENTS
Introduction to perylene based discotic liquid crystals and their applications
Columnar self-assembly of electron deficient dendronized bay-annulated perylene bisimides
List of abbreviations used in the text
GENERAL REMARKS
PEDOT:PSS film was spin-coated on the ITO substrates from aqueous dispersion and dried at 120 °C for 30 min. After that, separate solutions of PVC and organic materials were made in chlorobenzene solution and mixed in a 90:10 ratio and the mixture was then stirred for 30 minutes to ensure efficient doping.
PREFACE
- Introduction
- Application of Columnar Phases in Organic Electronics
- Perylene as a core for the liquid crystalline organic semiconductor
- Liquid crystals based on perylene tetraesters and their extended derivatives
- Liquid crystals based on perylene diesters
- Liquid crystals based on perylene bisimides
- Applications of perylene derivatives in organic electronics 1. Perylene based LCs for organic solar cells
- Perylene based LCs for organic light emitting diodes
- Perylene based LCs for organic field effect transistors
- Conclusion and perspective
- References
- Introduction
- Results and Discussion
- Synthesis and molecular structural characterization
- Thermal behavior
- Photophysical properties
- Electrochemical behavior
- Density functional theory (DFT) calculation
- Relative Quantum Yield Calculation
- Conclusion
- Experimental section
- References
The increased clearing point in the case of 4d is due to the increased number of intermolecular hydrogen bonds. However, the width of the Colh phase was reduced with the decrease in clarification and an increase in the melting points with respect to the parent compound. This may be due to the increased disorder in the case of branched chain derivatives.
With the increase in chain length, the mesophase range was narrowed due to the reduction in clarification temperatures. Compared to the perylene tetraester 5b, the band gap was found to be larger in the case of 3b. In the case of canceled derivatives, LUMO was found to spread across the heteroatom.
With the increase of the flexible chain length, the mesophase range decreases due to the decrease of the clarification temperature.
Chapter 3a
Introduction
This self-assembly is driven by shape anisotropy of the molecule, strong π-π interaction and nano-segregation of incompatible molecular subunits as central rigid core and peripheral flexible tails. Incorporation of substituents at the bay position often changes the photophysical properties of these molecules, but also leads to twisting of the perylene core, disrupting the columnar packing. Thus, one has to change the physical properties of this class of compounds by expanding the central core while keeping the planarity intact.
Thus, in order to improve the processability of the material for applications, we envisaged the synthesis of LC materials at room temperature (RT) based on bayed perylene tetraesters. It is well known in the field of liquid crystals that the melting point refers to the melting of the flexible chains, while the clarification point refers to the loss of positional order of the rigid nuclei in the mesophase.
Results and discussion
- Synthesis and molecular structural characterization
- Thermal behavior
- Photophysical and electrochemical studies
The decrease in transition temperature may be due to the disorder caused by branched chains and stereoheterogeneity. It is interesting to note the dramatic increase in mesophase width and reduced melting point of compound 1 due to the introduction of branched peripheral tails (Figure 3.1). Further cooling led to an increase in the brightness of the sheet-like pattern, which expanded and coalesced (Figure 3.3b).
This can be attributed to chain melting or possible interdigitation of peripheral chains in neighboring columns.12 The number of molecules (Z) forming a 3.51 Å thick columnar slice (hc) is 1. However, it should be noted that they also have a stabilized Colh phase at room temperature temperature together with an increase in the purification temp., which resulted in an overall increase in the width of the mesophase relative to the equichain analogs 2c and 3c.
Electroluminescence properties
Key device properties are summarized in Table 3.6 and efficiency and brightness graphs are shown in Figure 3.10a-c. Figure 3.10d shows that the turn-on voltages (at a luminance of 1 cd/m2) decreased for all three devices compared to the devices with a PVK:LC emitting layer. Similarly, Figure 3.12d, e and f show the AFM images obtained for the films prepared from the mixture of PVK and 10 wt% compounds 1, 2 and 3, respectively.
The thin films obtained in the first case (Figure 3.12a-c) were found to have smooth and amorphous surfaces, with a root mean square (RMS) roughness of 1.72 nm, 2.19 nm and 2.81 nm for the compounds 1, 2 and 3 thin films respectively. Then the excitations are formed directly on the guest molecule without the need for Förster energy transfer from host to guest molecule.46 In Figure 3.11a we can clearly distinguish between the energy levels of LC 1, 2 and 3 in relation to the energy levels of PVK.
X-ray diffraction studies on the thin film of PVK:10 wt% LC1
In order to understand the microstructure of the thin film formed by the spin-coated film of PVK:10 wt% LC1, we performed powder XRD studies. This revealed the presence of a single low-angle peak along with several peaks in the mid-angle region, indicating the crystalline nature of the sample. These diffraction patterns were almost similar regardless of temperature with a single low-angle peak with a d-spacing of 12.4 Å and a wide-angle diffuse peak with a d-spacing of 4.71 Å .
Results of (hkl) indexing of XRD profiles of the PVK:10 wt.% LC1 and PVK alone at different temperature intervals during cooling. The thermal annealing as a driving force therefore helps in the reorganization of the polymer chains and DLCs.
Relative Quantum Yield Calculation
It should be noted that the intercolumnar distance (a) observed only in the case of LC1 is 23-24 Å (Table 3.2), while for the composite material, the observed intercolumnar distance is a = 26-27 Å. Since there is not much variation in the intercolumnar distance, we can conclude that the polymer chains are arranged along the columnar axis, with the carbazole pendants arranged in a hexagonal columnar lattice. The absolute values were calculated according to the following equation: QS = QR × (mS / mR) × (nS / nR)2.
Where, V: Quantum yield; m: Slope of the plot of integrated fluorescence intensity vs absorbance; n: refractive index (1.407 for THF). The subscript R refers to the reference fluorophore ie compound 1b solution in THF and subscript S refers to the sample under investigation.
Conclusion
Another major problem with current materials is that they do not show homeotropic stretching when they are very thin and especially on a single substrate.
Experimental section
A mixture of compound 4 (0.6 mmol, 1 equiv.) and triethyl phosphite (7 mL) was refluxed for 4 h under argon atmosphere. The reaction mixture was cooled to room temperature and the triethylphosphite was removed in vacuo, the residue was diluted with DCM and washed with water, brine and dried over sodium sulfate. The product was dissolved in DCM and a solution of this concentrate in cold methanol was added giving the desired product as a waxy solid.
Further purification was carried out by the addition of a concentrated solution of the compound in cold methanol, which gave a waxy product. Further purification was carried out by the addition of a concentrated solution of the compound in cold methanol, which gave a waxy product.
Chapter 3b
Introduction
Due to the radially branched structures, star-shaped D-A molecules reduce the intermolecular interactions in the solid state, thus leading to improved solid-state emission with good film-forming properties. In addition, this design can be an alternative to polymers, which suffer from batch-to-batch variation in the molecular weight, purity and polydispersity issues, which affect the reproducibility of the device performance. There is a report where the star-shaped truxene derivative with fluorene arms exhibited higher values for electroluminescence (EL) and external quantum efficiency (EQE) compared to its linear counterpart.2 There are several reports with fluorene-containing star-shaped molecules exhibiting improved device characteristics. compared to the linear polymers.3 The better balanced charge injection and transport are important factors, which affect the OLED performance.
The integration of the N-canceled perylene tetraesters to form a star-shaped architecture was motivated by their columnar self-assembly and high luminescence quantum yield. The presence of three N-cancelled perylene tetraesters is believed to increase the steric hindrance of the arms, leading to a propeller-shaped molecule (Figure 3.19).
Results and Discussion
The thin film of the sample was prepared by annealing compound PT-T (Figure 3.20e) and showed red-shifted absorption and emission bands in the thin film state, suggesting the formation of aggregates. The fluorescence lifetime of the excited species formed in the thin film of compound PT-T was measured by monitoring their emission maxima. The bay cancellation of PTE to obtain composite N-PTE increases HOMO and LUMO levels along with an increase in energy gap (Figure 3.24).
Chromaticity diagram of the OLEDs. a) Overlay of UV spectrum of PT-T over the fluorescence spectrum of PVK in thin film mode; (b). In the solution state, the fluorescence lifetime is reduced due to the interaction between solute and solvent.
Conclusion
Experimental section
Introduction
This has led to transformed aromatic systems with fascinating properties. The incorporation of sulfur and selenium into organic semiconductors is widely explored due to the intermolecular S···S and Se···Se interactions, which also help stabilize molecular self-assembly and thereby improve 1D conductivity. Motivated by our success in stabilizing the self-assembly of luminescent Col into heteroatom canceled PTEs over a broad thermal range, we envisioned the synthesis of their bisimide counterparts.
Results and discussion
- Synthesis and molecular structural characterization
- Thermal behavior
- Density functional theory (DFT) calculation
- Photophysical and electrochemical studies
Overlay of the extended range 1H NMR spectra (CDCl3, 600 MHz) of PBI and bay-annulled PBIs. Usually, the phase becomes more ordered, due to the reduced mobility of the molecules together with a lowered nucleus-nucleus distance. This must be due to the increasing influence of the ethyl group logging in the core region.
It is possible that the rarely observed transition between two hexagonal columnar phases (Colh1-Colh2) is caused by the presence of the ethyl group in the core region. In the case of selenium (PBI-Se), the perturbation likely leads to tilting of the columnar lattice (Colob phase), although only after three-dimensional ordering (as visible by a number of sharp peaks) occurs.
Conclusion
Experimental section
The reaction mixture was cooled to room temperature and methanol was added to this solution to precipitate the compound. The resulting mixture is extracted with dichloromethane, the extract is washed with water and brine. After cooling to room temperature, the mixture is filtered, washed with methanol and dried under vacuum.
The reaction mixture was extracted with methylene chloride and the organic layer was washed with saturated brine. The combined organic layer was washed with 0.2 N KOH, water, saturated NH 4 Cl, dried over water.
