Highly fluorescent conjugated polymers (CPs) are an important class of biomaterials used for various biological applications including labelling, sensing, and delivery of biological substances. Synthetic versatility and tunable emission make CPs a superior class of biomaterials. Understanding the structure-function relationship of CPs plays a vital role in designing high performing biomaterials. The cationic CPs are self-assembled to conjugated polymer nanoparticles (CPNs) in an aqueous environment due to their amphiphilicity. The physical and biophysical properties of CPNs are highly dependent on the chemical functionality and backbone structure of CPs. Modulation of the surface property and backbone structure of CPNs play an important role for efficient internalization of CPNs into cells. The goal of this dissertation is to understand the structure function relationship of CPNs in an aqueous environment and the change in their photo physical properties upon the self-assembly of CPNs with different backbone structure upon complexation with biologically significant polysaccharides and cell membrane. This work presents the self-assembly of a set of four cationic CPs with different connectivity and backbone structure upon complexation with a linear polyanion hyaluronic acid (HA). The study of photo physical properties changes upon the complexation with series of Glycosaminoglycans (GAGs) provides more insight about how the self-assembly behavior of cationic CPs changes upon the exposure to negatively charged polysaccharides. The understanding of the self-assembly of CPNs with negatively charged biologically important macromolecules under in vitro conditions can give us an idea of photophysical property changes of CPNs during the treatment of CPNs in the cellular environment. The study of the interaction of CPNs with cell membranes using scanning ion conductance microscopy (SICM)-based topography, potential mapping, and confocal microscopy imaging is presented. CPNs are able to induce transient pore like feature formation on the cell membrane during the cellular internalization process. A comparative study of cellular labelling and delivery of siRNA of five CPNs with guanidine motif is presented. The subcellular localization and delivery of siRNA were dependent on the side chain hydrophilicity. The CPNs fabricated with hydrophilic aminoethoxyethanol possesses excellent cellular imaging with higher siRNA delivery.

The study of conjugated polymers, in particular PPV derivatives, continues to fascinate researchers both from the standpoint of developing new routes to control their structure and photophysical properties; this is also driven by the immense potential since this class of polymers has demonstrated in the context of various devices, such as LEDs, photovoltaics, FETs, etc. Also, there has been great interest in utilizing conjugated polymers in developing sensory devices. This thesis has examined a few interesting aspects that could be used to control the structure and consequently the photophysical properties of PPV derivatives. The first chapter of this thesis provides a brief introduction to conjugated polymers, with an emphasis on PPV based polymers, different synthetic methodologies for their preparation, previous attempts to obtain PPV with controlled conjugation length, and also a brief discussion of conjugated polymer nanoparticles (CPNs) and their preparation by various methods. The second chapter deals with the preparation of conjugated polymer (MEHPPV) nanoparticles by reprecipitation method and utilization of these nanoparticles in detection of nitro explosives in aqueous medium. Nanoparticles of MEHPPV with different sizes were prepared by a simple precipitation method from a THF solution into water. Although these nanoparticles were prepared from very hydrophobic MEHPPV, these nanoparticles were reasonably stable in aqueous medium, especially when their sizes were relatively small; their UV-visible and fluorescence spectra could be readily recorded using simple solution methods. The sizes could be controlled by varying the concentration of the polymer solution used. The CPNs are spherical particles as confirmed by atomic force microscopy (AFM). The emission maximum of the nanoparticles is red shifted compared to a solution of the polymer. The fluorescence spectrum of this aqueous nanoparticle dispersion exhibited very high sensitivity to electron-deficient aromatic compounds, in particular the explosive TNT; the CPNs were able to sense nanomolar concentrations of the explosives. Stern-Volmer constant (KSV) is higher for 2,4,6-trinitro toluene (TNT) than any other analytes studied. Among the different sized nanoparticles studied the bigger one showed highest quenching efficiency. Electron-deficient aromatic molecules were shown to quench the fluorescence of the nanoparticles, possibly by excited state electron transfer mechanism; this hypothesis was supported by quenching experiments carried out using a variety of nitro-aromatic molecules with varying reduction potentials, in addition to a few electron-rich aromatic molecules. A fairly good correlation between the quenching efficiency of the analyte and its reduction potential was noticed; however, in a few instances this correlation failed. This suggested that a second factor, namely the solubility of the analyte in water (its hydrophobicity) also is a key factor as this governs the tendency of the analyte to adsorb on the nanoparticle surface, which clearly is the first step in the quenching process. In the third chapter synthesis and characterization of MEHPPV with reduced conjugation length by utilizing the concept of conjugation breaking using non-coplanar entities are presented. MEHPPV with reduced conjugation length was prepared by incorporating non-coplanar entities, such as biphenyl and binaphthyl units, along the polymer backbone. Both Gilch and Witting-Horner methods were successfully utilized to prepare copolymers; the former approach permitted the variation of the level of the twisted comonomer incorporation, whereas the latter approach only provided an alternating copolymer. Although biphenyl based monomers could not be homopolymerized by Gilch method, it was possible to prepare copolymers using xylylene type comonomers. The polymers prepared by the Gilch method are random copolymers; the solubility decreases with increase in the amount of biphenyl incorporation, which was ascribed to the presence of a substantial fraction of longer conjugated segments in such random copolymers. The non-coplanar entities cause truncation of conjugation length in MEHPPV, as evident from their absorption spectra. The copolymers prepared via the Gilch method exhibited a blue shift of about 57 and 20 nm in the absorption and emission, respectively; while the alternating copolymers prepared by Wittig-Horner-Emmons method exhibited a larger blue-shift of about 84 and 54 nm clearly implying a significantly larger reduction in the conjugation length. The copolymers prepared by Wittig-Horner-Emmons method are alternating in nature and therefore leads to a greater reduction in conjugation length; this was evident from the substantially higher blue-shift in the absorption and fluorescence spectra. An interesting feature in the alternating copolymers is the distinct difference in the relative changes in the absorption and emission spectra of the biphenyl and binaphthyl containing copolymers, which appear to suggest in the latter case there is a greater extent of planarization of the excited state. Both these systems provided some useful insights into the various factors that govern the photophysical properties of this class of truncated conjugated polymers. Further examination of this aspect could reveal some other unique features of these copolymers. In the fourth chapter, an approach to prepare copolymer precursors to unsubstituted PPV that holds the potential to control conjugation length is described. The precursor copolymers were prepared by changing the monomer feed ratio of the two monomers, viz. dithiocarbamate (DTC) and xanthate, using the Gilch copolymerization to generate the precursors having varying extents of DTC and xanthate groups. The percentage composition of the precursor copolymers was calculated using 1H NMR and compared with the values calculated from thermogram; the copolymer composition varies linearly with monomers feed suggesting that any desired composition can be readily accessed. Thermogravimetric analysis (TGA) of the precursors, as a preliminary study to examine the possibility of selective elimination of one of these groups to generate conjugated polymers with varying conjugation lengths, demonstrates that a certain level of selectivity in thermal elimination can be achieved because of the distinctly different thermal labilities of the xanthate and DTC groups. These studies clearly suggest that fine-tuning of the thermally eliminatable groups, specifically using xanthate and DTC, could serve as a useful approach to vary the conjugation length of unsubstituted PPVs, which could have important implication in device fabrication. Clearly further work is needed to characterize the selectively eliminated polymers using other spectroscopic methods, such as UV-visible and fluorescence, before device work could be taken up.