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Summer Research Fellowship Programme of India's Science Academies

Incorporation of functionalised donor-acceptor molecules and its co-assembly within the block copolymer domains

Sujith Nadarajan

Central Institute of Plastic Engineering and Technology-Institute of Plastic Technology (CIPET-IPT), Udyogamandal P.O, Eloor, Kochi, Kerala 683501 

Dr. E. Bhoje Gowd

Materials Science and Technology Division, CSIR-National Institute for Interdisciplinary Science and Technology (NIIST), Industrial Estate P.O, Pappanamcode, Thiruvananthapuram, Kerala 695019

Abstract

Block copolymer (BCP)-based supramolecular systems were propounded to be the most effective and momentous tool designed for the creation of materials having hierarchical morphologies and invigorates for the desired properties. In this work, we describe a three component hierarchical self-assembly approach to generate stable alternate donor-acceptor (D-A) assemblies within block copolymer microdomains via non-covalent specific interactions. The D-A interaction within the block copolymer domain leads to the formation of the charge transfer (CT) complex, and this often helps in the modification of the electronic structure of system. Here we report block copolymer supramolecules composed of two small molecules with a carboxylic functional group (donor and acceptor) and polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP). Here the stoichiometry has a crucial role for the formation of hierarchical assembly. In the current work, we chose the stoichiometry between P4VP: PBA (donor) is as 0.5, and for the donor: acceptor is as 1. Due to the presence of carboxylic functionality, both the molecules such as 1- Pyrenebutyric acid (PBA, donor) and naphthalene diimide (NDI, acceptor) forms hydrogen bonding with P4VP and also forms aromatic interactions to generate CT complexes within the block copolymer domains in the solid state. Using FTIR, NMR, UV/vis spectroscopy, photoluminescence and mass spectroscopy measurements, we demonstrate the formation of hierarchical structures and CT complexes between PBA and NDI. This work keenly observes whether the three component hierarchical assembly permanently retains the D-A stacking within the block copolymer domain and as a structural motif. The organization of donor and acceptor molecules within the block copolymer microdomains opens up new vision into the areas of electronic devices because of its advantages such as solution processibility, controlled formation of hierarchical assemblies, and the CT interaction in the solid state.

Keywords: supramolecular assembly, non-covalent interaction, hierarchical structure, charge transfer complex

Abbreviations

Abbreviations
                  BCP                                             Block co-polymer
            CT complex                                    Charge Transfer complex
FTIR Fourier Transform Infrared Spectroscopy 
1H NMRProton Nuclear Magnetic Resonance Spectroscopy 
PS-b-P4VP Polystyrene-block-Poly(4-vinyl pyridine) 
 PBA1-pyrene butyric acid 
 FNDIFunctionalised Napthalenediimide 
PL Photoluminescence
 SMASupramolecular Assembly 
 DMFDimethyl formamide 

INTRODUCTION

 Ongoing interest in the field of supramolecular assembly in π conjugated systems (organic semiconductors, liquid crystalline mesogens)1-6 are originating from the ambition for generating the functional nanomaterial with tailored properties focus on the interdisciplinary areas ranging from biology to materials science. The hierarchical structures obtained by the organized supramolecular assembly of these π conjugated chromophores achieving the desired photo physical,7-9 charge transport properties,10-12 etc. It is an exciting area of research due to its relevant applications in organic electronic devices.13, 14 In recent years, special attention has been made to chromophores with stacked donor (D)-acceptor (A) functionalities, because of their charge transport and electron transport properties possess a fundamental role in organic solar cells,15, 16 artificial photosynthesis,17 and organic light emitting diodes,18 etc. In most of the cases, the energy transport or charge transport between donor and acceptor is considerably influenced by the morphology or crystalline packing and spatial relationship of D-A chromophores.19-21 One way to control the distance between donor and acceptor molecule is by covalently linking the donor and acceptor molecule in a single polymer chain.22, 23 The most convenient method to attain this by the block copolymer approach because it has received more attention due to their capability to produce ordered nanostructures.24, 25

When two or more distinct blocks were chemically bound together there would be some degree of incompatibility. In order to minimise this incompatibility, these BCP will self-assemble into various micro phase separated structures and thereby having many useful properties in both thin film and bulk state. Generally, in a BCP, the domain of the minor components is found to be dispersed in a matrix of the major phase. The variation in the molar ratio of the polymer blocks outcomes in a change in the morphology of the equilibrium domain.26, 27 Figure 1 shows the theoretical phase diagram of a diblock copolymer with various morphologies. In accordance with the self-consistent mean field theory, for a linear AB diblock copolymer, there are four equilibrium morphologies: spherical (S), cylindrical (C), gyroid (G) and lamellar (L) ,which are determined by volume fraction of blocks (f) , Flory-Huggins interaction parameter (χ) and degree of polymerisation (N).28

1. Babu, S. S.; Praveen, V. K.; Ajayaghosh, A. Functional π-Gelators and Their Applications. Chemical Reviews 2014, 114 (4), 1973-2129 DOI: 10.1021/cr400195e.​

Based on the capability of block copolymer for the formation of microstructures with morphological stability, the research community has been designed donor-block-acceptor copolymers for photovoltaic applications.29-31 Initially, they focused on the block copolymers with flexible aliphatic groups act as the main chain and functionalized acceptor molecule as a pendent block. Here the major issue was the non-conjugated backbone, which may inhibit the charge transport.32, 33 Later, several groups modified this system by altering with fully conjugated block copolymers like P3HT molecules.34, 35 In many instances, conjugated block copolymers demonstrate some advantages due to their ability for self-assembly and solution processing. However, the synthesis of conjugated blocks with precise control of molecular weight required for ordering and alignment is extremely challenging.

To overcome these difficulties, researchers got more attention in the field of block copolymer supramolecular assemblies (SMA). It can be generated by the incorporation of a low molecular weight additive to any one of the blocks of BCP by non-covalent interactions like metal coordination, hydrogen bonding, π -π interactions, van der Waals forces, etc. As a result, the volume fraction of the block gets increased, causing a transition in the block copolymer morphology. Hence, the BCP with desired morphology and properties could be developed with this supramolecular approach. By incorporating the small molecules with optical or electronic properties into the BCP having the advantage of getting the nanostructured assembly and reduce the burden of synthesis.36-40 Also, this approach gives another advantage of the easy removal of the incorporated low molecular weight additive by selective dissolution in a suitable solvent. Within the various kinds of supramolecular approaches, hydrogen bonded interactions have much importance because of their directionality and versatility. 41

Inspired from the recent works in the hierarchical self assembly of block copolymers(Gowd,2019,52,2889-2899), here in we report the block copolymer supramolecules composed of polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) and two small molecules (donor and acceptor) both possess functional group capable of forming H-bonding with BCP. In the current work, 1-pyrenebutyric acid (PBA) is taken as the donor molecule and carboxylic functionalized Naphthalene diimide (FNDI) as the acceptor molecule.42 Unlike the supramolecular approach discussed in Gowd's system, here the acceptor molecule also have an interaction with BCP. The stoichiometric ratio between the 4VP unit and PBA was fixed at 0.5 (PS-b-P4VP (PBA) 0.5) and for the donor: acceptor is as 1. Due to the presence of functionalized group in both molecules, they form hydrogen bonding with the P4VP and also generate π-π interactions to form CT complexes with in the BCP domain in solid state and this often helps in the modification of the electronic structure of the system. Using FTIR, TEM, UV/vis spectroscopy, and photoluminescence spectroscopy measurements, we demonstrate the formation of hierarchical structures and CT complexes between PBA and FNDI within the BCP domains.

OBJECTIVES

  • Synthesis of carboxyl functionalized naphthalene diimide (FNDI) from naphthalene anhydride.
  • Characterization of synthesised FNDI

 The first part the project involves the synthesis of acceptor molecule and involves the following objectives:

The second part of the project involves the preparation of supramolecular assembly of block copolymer with two small molecules (PBA donor and FNDI acceptor molecule) and has the following objectives:

  • To check for the formation of non-covalent interactions between the block copolymer and the small molecules, i.e.; generation of supramolecular assembly, using FTIR spectroscopy.
  • To investigate the charge -transfer complex formation (CT complex) between donor and acceptor molecule within the BCP domains by UV spectroscopy and photoluminescent spectra.

LITERATURE REVIEW

Block copolymer can self-assemble to form various microphase separated nanostructures such as sphere, cylinder, lamellae morphologies, etc. which has immensely attracted much attention because of their potential application in the field of sensors, drug delivery system, and functional nanomaterials. In recent years, a large number of reports are coming up in the field of block copolymer self-assembly, still it is an attractive subject for scientist to construct the nanostructure with well-defined morphologies and advantageous functions. Nowadays, there is growing interest in supramolecular polymeric materials which combine many of the attractive features of conventional covalent polymers with the properties resulting from the reversibility of noncovalent interactions.

There have been a great number of reports that are deliberating the prominence of block copolymer based supramolecules. Especially the pioneered group of Ikkala and ten Brinke et al. conversed the hierarchical structures formed by the interaction of pentadecyl phenol (PDP) with block copolymers. 43 They discussed about the terrace formation and phase behavior of this system, which is obtained after annealing under chloroform vapors.37, 44 Stamm and co-workers have investigated the synthesis of SMA formed between PS-b-P4VP and 2-(4′-hydroxyphenylazo) benzoic acid (HABA), which possess both carboxyl and hydroxyl functional group for H-bonding with P4VP. They found that depending on the molar ratio between HABA and 4VP units, the SMA exhibit a morphological transition from cylinders to lamellae both in the case of bulk as well as in thin films. Furthermore, the selectivity of the solvent as well as the degree of swelling plays a crucial role for switching the orientation of cylindrical microdomains of P4VP/HABA.45

Xu and co-workers have investigated the supramolecular system based on PS-b-P4VP block copolymer by incorporating suitably functionalized organic semiconductor. They construct a nanostructured semiconductor composite with charge carrier mobilities which is comparable to the existing semiconductors.46-48 Kuila et al. illustrated the synergetic coassembly of the block copolymer with pyrene butyric acid (PBA), which is a fluorescent probe molecule, and suggests that the molecular orientation of the luminescent molecules is efficiently controlled in the thin film by block copolymer supramolecular assembly.49 Recently, Asha and coworkers reported the well-defined nano organization of n-type semiconductors based on perylene bisimide as well as naphthalene bisimide with the polymer poly(4-vinyl pyridine) P4VP, arising in an ordered lamellar structure with less domain spacing and exhibit a higher conductance compared to the pristine PBI/ NBI molecule.50, 51

To date, very limited work has been done to establish supramolecular nanostructures based on the donor (D)–acceptor (A) dyad architectures. Recently, Gowd and co-workers introduced a three component assembly via BCP supramolecular approach for the first time, and demonstrated an alternate D-A stacking within the BCP domains in the solid state. In their work, the donor, PBA molecules are hydrogen-bonded with poly (4-vinyl pyridine) of polystyrene-b-poly (4-vinyl pyridine) (PS-b-P4VP) and further addition of NDI molecules (acceptor) to form CT complexes by π-π interactions without disturbing the hydrogen bonding between the PBA and P4VP. 42

 

METHODOLOGY

Materials

PS-b-P4VP with number-averaged molecular masses (Mn): PS 35500 g mol-1, P4VP 4400 g mol-1, and PDI (Đ) = 1.06, was purchased from Polymer Source, Inc.1-Pyrene butyric acid (PBA), 1,4,5,8- naphthalenetetracarboxylic acid dianhydride(NDA), n-hexyl amine, potassium hydroxide (KOH), and 4-aminobutanoic acid were purchased from Sigma Aldrich. The solvents HPLC-grade dimethylformamide (DMF), 1, 4-dioxane, and methanol were also purchased from Sigma Aldrich. The solvents used were of analytical grade and carefully dried before use.

Synthesis of Functionalized NDI (FNDI)

Pic 2.png
    Figure 2. Schematic representation for the synthesis of FNDI

    Synthesis of compound 1a

    2.0 g of 1,4,5,8- naphthalenetetracarboxylic acid dianhydride (7.46 mmol) is dissolved in 350 ml water taken in a RB flask. About 1 M aq. KOH solution (35 mL) is added to the above solution with continues stirring until the compound completely dissolved and then the obtained solution is heated. In order to adjust the pH around 6.4, 1 M H3PO4 was added. 0.98 ml of n-hexyl amine (7.46 mmol) was added and again the pH was readjusted to 6.4 with 1 M H3PO4. The solution thus obtained is refluxed overnight and it is allowed to cool and filtered. The filtrate was then acidified with 5ml acetic acid and the obtained precipitate was then filtered and repeatedly washed with water. It is then dried using a vacuum oven to get 1.02 g (51% yield) of compound 1 as an off-white solid. ESI-HRMS (m/z): calc for C20H17NO5+ [M + H]+ : 352.35, found: 352.27.

    Synthesis of compound 1b (FNDI)

    The 1g of compound 1a (2.85 mmol) synthesised was dissolved in dimethylformamide (DMF, 15 mL) followed by heating at 60°C. To this, 0.605 g of γ-aminobutyric acid (5.87 mmol) and 1.02 ml of DIPEA (5.87 mmol) were added one after the other. The reaction mixture was heated at 90°C for 12 h. It is allowed to cool and the solvent was evaporation was carried out using rota evaporator. The crude residual matter was suspended in 100 ml water/methanol (2:1).The pH was adjusted to 3 by the addition of hydrochloric acid (6 N). The solid obtained was repeatedly washed with water by centrifugation and then kept it in a vacuum oven for drying to get 0.8 g (80 % yield) of compound 2 as a light brown solid.

    Preparation of Block Copolymer Supramolecular Assembly with Small Molecules

    Block copolymer (PS-b-P4VP) solution and 1-pyrene butyric acid (PBA) solution (1 mol 4-vinylpyridine monomer unit: 0.5 mol PBA) were prepared separately in 1,4-dioxane. To the PBA solution, block copolymer solution was added dropwise with continuos stirring and keeping the mixing temperature close to the boiling point of the solvent. The resulting solution was kept for the complete formation of hydrogen bonding for 3 days. To this solution, the FNDI solution prepared separately was added with continuosly stirring and the resulting solution was kept for 2 days. The prepared SMA solution was transferred into the petri dish and the solvent was allowed to slowly evaporate for a week. These samples were kept for further drying in a vacuum oven at 40 °C for 12 h. The physical blends of PBA and FNDI were prepared by solution blending by mixing 1:1 weight ratio of small molecules in 1,4-dioxane.

    Characterization Methods

    Nuclear magnetic resonance spectroscopy

    The structure of the synthesized FNDI was confirmed by proton nuclear magnetic resonance spectra 1H-NMR (deuterated DMSO as the solvent) using Bruker NMR spectrometer operated at 500 MHz. Chemical shifts for 1H NMR spectra are reported as δ in units of parts per million (ppm) downfield from SiMe4 (δ 0.0) and relative to the signal of chloroform-d (δ 7.25, singlet). Multiplicities were given as: s (singlet); d (doublet); t (triplet); q (quartet); dd (double doublet); m (multiplet).

    Mass spectroscopy

    Mass spectra were recorded under EI/HRMS at 60,000 resolution using Thermo Scientific Exactive mass spectrometer.

    Fourier-transform infrared (FTIR) spectroscopy

    FTIR measurements were performed on a PerkinElmer series Spectrum Two FTIR spectrometer. The FTIR spectra were recorded over the wavenumber range of 4000–400 cm−1. The FTIR spectra were obtained by averaging 32 scans at a resolution of 2 cm−1.

    UV-visible spectroscopy

    UV-visible spectra were recorded with a Shimadzu UV-3600, UV-visible-NIR spectrophotometer.

    Photoluminescent spectroscopy

    The photoluminescent spectra of the solid samples were performed in a Spex-Fluorolog FL22 spectrofluorimeter equipped with a double grating 0.22 m Spex 1680 monochromator and a 450W Xe lamp as the excitation source operating in the front face mode. The photoexcitation was made at an excitation wavelength of 340 nm.

    RESULTS AND DISCUSSION

    The supramolecular assembly of two small molecules (donor and acceptor) with block copolymer involving the non-covalent secondary interaction such as hydrogen-bonding and the π – π stacking between the small molecules were investigated. In the first part, PBA, the donor molecules will form hydrogen bonding with the pyridine units of P4VP to form the PS-b-P4VP(PBA)0.5 supramolecules. To this, the functionalized acceptor molecule (FNDI) is added, which is capable of forming hydrogen bonding with pyridine units and as well as π – π interaction with the donor molecule within the P4VP domain. The COOH functionalized naphthalene diimide molecules (FNDI) were synthesized and characterized in this study. The synthesis was carried out using reported literature elsewhere as describe in the methodology section. The structure of the synthesized product was confirmed by using 1H NMR and mass spectroscopy.

    Picture 3a_1.png
      Picture 3b_1.png
        Figure 3: The 1H NMR spectra of synthesized compound 1a and compound 1b (FNDI).

        Figure 3 compares the 1H NMR spectra of intermediate product and final product (FNDI) during the synthesis, which is recorded in deuterated DMSO at room temperature (25 °C). The details from the 1H NMR spectra and mass analysis was given below

        Compound 1a: δ/ppm = 8.56-8.54 (d, 2H), 8.19-8.18 (d, 2H), 4.03-4.01 (t, 2H), 1.65-1.61 (m, 2H), 1.35-1.28 (m, 6H), 0.86- 0.84 (t, J = 6Hz, 3H).

        ESI-HRMS (m/z): calc for C20H17NO5+ [M + H]+ : 352.35, found: 352.27.

        Compound 1b (FNDI): δ/ppm = 12.02 (s, 1H), 8.62 (s, 4H), 4.08 (t, 2H), 4.02 (t, 2H), 2, 32 (br. 1H), 1.99 (t, 2H), 1.65 (t, 2H), 1.30-1.25(m, 6H), 0.86-0.84 (t, 3H).

        ESI-HRMS (m/z): calc for C20H17NO5+ [M + H]+ : 437.46, found: 437.17.

        The type and the position of the corresponding peaks in 1H NMR and the mass obtained from the mass spectroscopy confirms the formation COOH functionalized naphthalein diimide molecule.

        Figure 4 shows the molecular structure of the block copolymer (PS-b-P4VP), donor molecule (PBA) and acceptor molecule (FNDI).

        Pic4.png
          Figure 4: Molecular structure of (a) PS-b-P4VP diblock copolymer (b) donor molecule (PBA) and (c) acceptor molecule (FNDI).

          Figure 5a and Figure 5b show the FTIR spectra of PS-b-P4VP, PBA, FNDI, PS-b-P4VP(PBA)0.5 and PS-b-P4VP(PBA+FNDI) in different regions. In the PS-b-P4VP block copolymer, free pyridine groups show the absorption at 993 cm-1 and 1001 cm-1 corresponding to the symmetric ring stretching mode. The free pyridine ring absorption band at 993 cm−1 becomes broad after the addition of PBA and the peak intensity reduces substantially owing to H-bonding. Simultaneously, the absorption band at 1001 cm-1 shifted to 1004 cm-1 indicating the formation of hydrogen bonding between pyridine units and PBA. After the addition of acceptor molecule, the free pyridine ring absorption band at 993 cm-1 completely disappears as shown in Figure 5a and the peak corresponding to hydrogen bonded pyridine (1004 cm-1) remains intact which means the functionalized acceptor molecule also form hydrogen bonding with pyridine. The H-bonding is further confirmed by shift in the characteristic peaks of P4VP at 1597 cm-1 to 1601 cm-1 shown in Figure 5b.

          Pic5.png
            Figure 5: FTIR spectra of FNDI, PBA, PS-b-P4VP, PS-b-P4VP(PBA)0.5, and PS-b-P4VP(PBA+FNDI) in the region of (a) 1030-980 cm-1 (b) 1610-1585 cm-1.

            The FNDI (acceptor) was added into the SMA solution of the block copolymer supramolecule (with PBA (donor)) and then solvent was allowed to evaporate. Solidification results in a visible colour change from pale yellow (SMA solution) to red colour (Figure 6a and 6b). This may be due to the charge transfer complex formation between the PBA and FNDI molecules within the block copolymer microdomain in solid state. In order to further confirm this, UV/Vis spectroscopy study was carried out, which shows an absorption band in the range of 400–700 nm. The UV/Vis absorption spectra of PS-b-P4VP(PBA+FNDI) (figure 6c)shows a broad band around 540 nm is associated with the charge transfer (CT) complex formation between the π-electron-rich PBA and π-electron-deficient FNDI moiety. For the comparison, the physical blend of PBA and FNDI (without block copolymer) was prepared in a 1:1 ratio and analysed. Surprisingly, the physical blend in the solid state also shows reddish colour and exhibits the characteristic band corresponding to CT complexation around 540 nm which is completely different from the previous report.42 This may be due to the presence of COOH group in both the molecules leads to hydrogen bonding which stabilizes the D-A stacking and inhibiting the self-sorting behavior. However, compared to the physical blend, the intensity of CT band is prominent in the block copolymer containing system, suggesting the D-A stacking is more effective within the block copolymer domains.

            Pic6.png
              Figure 6: The visual appearance of (a) PS-b-P4VP(PBA)0.5, (b) PS-b-P4VP(PBA+FNDI) and (c) UV/Vis spectra of PS-b-P4VP(PBA+FNDI), physical blend (PBA+FNDI), FNDI and PBA in solid state.

              In order to further confirm the formation of CT complex, solid-state photoluminescence (PL) measurements were carried out which shows a substantial quenching of the PBA emission in the block copolymer supramolecules (PS-b-P4VP(PBA+FNDI)) and also in the physical blend (PBA+FNDI) . All samples were excited at 340 nm and the corresponding PL spectra were recorded. Figure 7a, 7b, 7c and 7d show the visual images (under UV illumination) and figure 7e shows PL spectra of PS-b-P4VP(PBA)0.5, PS-b-P4VP(PBA+FNDI) and physical blend (PBA+FNDI) and FNDI in the solid state. The pure PBA shows yellowish-green fluorescence which is blue shifted when the PBA molecule was added to the PS-b-P4VP. As shown in Figure 7d and inset of 7e, up on the addition of FNDI to PS-b-P4VP(PBA)0.5, there is a drastic quenching of fluorescence of PBA indicating the FNDI molecules are sandwiched in between PBA molecules to generate the alternate D-A stacks within the P4VP domains of the block copolymer. Consistent with the UV results, the physical blend also exhibit D-A stacking, which is confirmed from the quenching of PBA emission in the PL spectra. (Figure 7b and inset of 7e).

              Pic7.png
                Figure 7: The visual appearance of (a) PBA, (b) physical blend (PBA+FNDI), (c) PS-b-P4VP(PBA)0.5, (d) PS-b-P4VP(PBA+FNDI) under UV light illumination and (e) PL spectra of PS-b-P4VP(PBA)0.5, PS-b-P4VP(PBA+FNDI) and physical blend (PBA+FNDI) and FNDI in the solid state.

                Based on the observations, a stable charge transfer complex was formed in both block copolymer based system as well as in the physical blend in solid state. This result is slightly different from our previous report where the donor molecule only having the functional group and shows stable charge transfer complex in block copolymer containing system and a self-sorted assembly in physical blend.42 By comparing these two observations, it is clear that, the functional group of the small molecules has a crucial role in stabilizing the D-A stacking. In the current work, due to the presence of functional group in both the donor and acceptor molecule, there is a competition between hydrogen bonding and π – π stacking, which stabilizes the D-A stacking in both system. However, a detailed study is necessary to understand the competition between the non covalent interactions within the molecule as well as with the block copolymer domain.

                CONCLUSION

                In conclusion, we have successfully demonstrated the formation of three-component hierarchical self-assembly to generate the alternate co-assembly of donor and acceptor molecules within the block copolymer domains in the solid state through the non-covalent interactions. Here both the donor as well as the acceptor molecules form effective complexation with P4VP block of the block copolymer domain as evident from the FTIR. At the same time, noncovalent aromatic donor acceptor interactions leads to the formation of charge transfer complex confirmed by UV and PL results. The competition between hydrogen bonding and π – π stacking stabilizes the D-A stacking in both system, which confirms the effect of functional group of small molecules in the assembly. The major advantages of this process are solution processability and the controlled formation of hierarchical assemblies in bulk, which are very much useful for device applications such as organic photovoltaic (OPV), light harvesting, optical waveguides, etc.

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                ACKNOWLEDGEMENTS

                First and foremost, I would like to express my deep sense of gratitude to Indian Academy of Sciences for providing me a golden opportunity to do IASc-INSA-NASI Summer Research Fellowship 2019 at CSIR – National Institute for Interdisciplinary Science and Technology (NIIST), Trivandrum.

                I wish to express my deepest gratitude is to my guide, Dr. E Bhoje Gowd, Principal Scientist, Materials Science & Technology Division, CSIR-NIIST, for his guidance, inspiration, constant support, scholastic suggestions and corrections that laid the foundation stone for my work. I have been extremely fortunate to have worked under his supervision.

                I am deeply indebted to Mrs. Deepthi Krishnan,PhD scholar, for her immense support, motivation and genuine encouragement throughout the period of my project which actually helped me to learn and improve.

                I am deeply indebted to Mr. Amal Raj R B, Research Intern, for his invaluable help, stimulating suggestions and encouragement and helped me to complete my work.

                I would like to extend my heartiest thanks to our group, especially Dr. Lakshmi V(RA-CSIR), Mrs.Sijila Rosely. C.V (Ph.D. scholar),Ms.Praveena N.M (Ph.D. scholar), Mr.Vipin G Krishnan (Ph.D. scholar), Mr.G.Virat (Ph.D. scholar) and Mrs. Sruthi Suresh(Ph.D. scholar)for their inspirational guidance, enthusiasm, deep concern, suggestions and discussions throughout my project.

                I owe my sincere thanksto Author Café for providing such an excellent space to write my report and collaboration with my guide.

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