Design of Functional Fullerenes: Investigation of Their Photophysical and Dna Self-Assembly Properties for Optoelectronic Applications

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DESIGN OF FUNCTIONAL FULLERENES: INVESTIGATION OF THEIR PHOTOPHYSICAL AND DNA SELF-ASSEMBLY PROPERTIES FOR OPTOELECTRONIC APPLICATIONS

THESIS SUBMITTED TO ACADEMY OF SCIENTIFIC AND INNOVATIVE RESEARCH (AcSIR) FOR THE AWARD OF THE DEGREE OF DOCTOR OF PHILOSOPHY IN CHEMISTRY UNDER THE FACULTY OF SCIENCE

By SANDEEPA K. V. Enrollment No: 10CC12A39010

Under the Supervision of Dr. JOSHY JOSEPH

PHOTOSCIENCES AND PHOTONICS SECTION CHEMICAL SCIENCES AND TECHNOLOGY DIVISION CSIR-NATIONAL INSTITUTE FOR INTERDISCIPLINARY SCIENCE AND TECHNOLOGY (CSIR-NIIST) THIRUVANANTHAPURAM - 695019, KERALA

APRIL 2018

PREFACE

The functional, monomeric fullerene derivatives are synthetically accessible and exhibit comparable optoelectronic properties to that of the parental [6,6]-phenyl-C61- butyricmethyl ester (PCBM), which can be polymerized via covalent or supramolecular techniques. Developing covalently modified polyfullerenes and non- covalently functionalized supramolecular fullerene polymers are promising strategies to create dimension-controlled nanomaterials. The programmed fullerene architectures constructed by intermolecular fullerene cross-linking might be useful in optoelectronic applications where routine self-assembly approach fails. Moreover, the ordered self-assembly of fullerene and their analogous into regular DNA nanostructures may prove to be an effective DNA based template for the selective localization of organic chromophores, inorganic ions, nanoparticles and nanoclusters etc. Therefore, the fullerene-DNA interactions and structural design presented here could be applied to the development of mutual assisted supramolecular nanodevices and nano-machines which can solve the next, most challenging problems of supramolecular chemistry and nanotechnology. The collaboration between these rapidly growing fields - fullerene chemistry and DNA nanotechnology- will open up new avenues in the field of supramolecular architectures and their applications.

In this regard, the present thesis has been divided into four chapters. In the Chapter 1, general fullerene functionalization strategies by using aromatic, aliphatic and core substituted moieties towards various applications such as bulk heterojunction polymer solar cells (PSCs), organic field effect transistor, charge separation/stabilization and the construction of various supramolecular nanostructures were discussed. Also various covalent approaches for the construction of fullerene polymers such as main-chain, side-chain and polymer grafted fullerene with relevant applications were discussed. Special emphasis was given to non-covalent approaches used for developing supramolecular fullerene polymers such as polymer assisted, host-guest induced and DNA-templated self-

assembly techniques. Furthermore, the objectives of the thesis are also briefly presented in this chapter. In the Chapter 2, the rational design and synthesis of a novel cross-linkable fullerene derivative, [6,6]-phenyl-C61-butyricbenzoxazine ester (PCBB) functionalised with benzoxazine moiety were described which exhibited dual stimuli responsiveness towards temperature and visible light. The heat triggered ring opening polymerisation of benzoxazine moiety in PCBB was investigated through differential scanning calorimetry and infrared spectroscopy which resulted in the formation of cross-linked solvent resistive adhesive films upon heating at 200 oC for 15 min. An inverted bulk hetero junction solar cell device using cross-linked, C-PCBB as electron transport layer based on the configuration ITO/ZnO/C-

PCBB/P3HT:PCBM/V2O5/Ag achieved 4.27% PCE compared to the reference device

ITO/ZnO/P3HT:PCBM/V2O5/Ag (PCE=3.28%). Moreover, under visible light PCBB showed time dependent morphology transformation from core-shell nanoparticle into nanonetwork structure in THF/H2O (9/1, v/v) solvent mixture. This transformation of self-assembled nanostructure is believed to involve the [2+2] cycloaddition reaction of fullerene as evident from NMR and mass spectral analysis. The aniline appended covalent modification of fullerene derivative, 3- aminobenzyl-phenyl-C61-butyrate (PCBAn), its polymer (P-PCBAn) obtained through

FeCl3 induced oxidative polymerisation and their photophysical studies towards application in PSCs were discussed in Chapter 3. The morphology of PCBAn, P- PCBAn alone and the polymer blend with Poly (3-hexylthiophene), P3HT donor was investigated using AFM and TEM analysis. Also, the square wave voltammetry revealed the upshifted LUMO levels of PCBAn and P-PCBAn (0.1 eV and 0.12 eV respectively) compared to the parental PCBM. Moreover, the fluorescence quenching experiments, life-time studies and Stern-Volmer plots indicated the efficient electron transfer from P3HT to P-PCBAn compared to PCBAn. The fabrication of inverted BHJ-PSC device using PCBAn and P-PCBAn as acceptors, in combination with P3HT showed 0.9% and 1.1% PCE, respectively.

Three monosubstituted fullerene derivatives having pyridinium, aniline or phenothiazine end groups (F-Py, F-An and F-PTz, respectively) were synthesized and their differential interaction with calf thymus DNA (CT-DNA) was probed via spectroscopic and imaging techniques, the details of which are presented in Chapter 4: Part A. The pyridinium derivative, F-Py gets molecularly dissolved in 10% DMSO- PBS and interacts with CT-DNA via groove binding and electrostatic interactions leading to initial condensation of CT-DNA into micrometer sized aggregates as evidenced through DLS, Zeta potential and microscopic studies. On the other hand, the aniline derivative F-An, which form nanoclusters of 3-5 nm size, interact with DNA through ordered, chiral assemblies on CT-DNA template perturbing the highly networked structure of CT-DNA to form nanonetworks, which eventually transform to condensed aggregates. In contrast, the phenothiazine derivative, F-PTz, which forms larger nanoclusters of ~70 nm in 10% DMSO-PBS showed only weak interactions with CT-DNA without affecting its network structure. Programmable, hierarchical assembly of DNA nanostructures with precise organisation of functional components have been demonstrated previously with tiled assembly and DNA origami. However, building organised nanostructures with random oligonucleotide strands remains as an elusive problem. In Chapter 4: Part B, we describe a simple and general strategy in which unique nanoclusters of a fullerene derivative act as stapler motifs in bringing ordered nanoscale assembly of short oligonucleotide duplexes and DNA three way junctions into micrometer-sized nanofibres, nanosheets and nanonetwork respectively. Moreover, the horizontal conductivity measurements using c-AFM confirmed the charge transport properties of these nanowires. Furthermore, the silver nanoclusters (AgNCs) templated over 3WJ-overhang/F-An nano-construct exhibited 40% fluorescence enhancement compared to that of 3WJ-OH. We demonstrate that the optimum cluster size, availability of DNA anchoring motifs and the nature of the DNA strands control the structure of these nanomaterials. Note: The abbreviations of various compounds used here correspond to those given under the respective chapters.