This master´s thesis deals with possibilities of application of new organic molecular materials for electronic devices. Nowadays it is a very attractive field of research, because of the tendencies in industry to miniaturize, reduce production costs and develop new, eco-friendlier, processes of production. The theoretical part of the thesis provides a short overview of organic materials suitable for smart applications and thin films issues including their characterization. The experimental part is dedicated to means how to prepare thin-film electronic components to silicon wafers for thin films field effect transistors. The obtained results in the last part of thesis are discussed about properties of prepared thin films, in the concrete about the electrical transport properties, in the connection with the condition of preparation.

This thesis is concerned with the material processing, photophysical and third-order nonlinear optical responses, and applications of a set of conjugated polymers in the telecommunication regions. Polyacetylene-based third-order nonlinear optical materials were chosen as candidates for all-optical signal and image processing. Substituted polyacetylenes were obtained using ring-opening metathesis polymerization of mono-substituted cyclooctatetraenes. Polymerization and processing conditions have been developed to generate thick, large-area films possessing large third-order nonlinearities in the telecommunication bands. The good optical quality of a 200 μm thick substituted polyacetylene film allowed for image correlation via off-resonant degenerated four-wave mixing with improved diffraction efficiency. Poly(2-methoxy-5-(2-ethyl-hexyloxy)-(phenylene vinylene)) (MEH-PPV) and (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) composites showed strong nonlinear absorption and potential as optical limiters in the region of 700-900 nm. High optical quality, thick film of MEH-PPV:PCBM with the plasticizer dioctylphthalate (DOP) were made. Optical limiting of femtosecond and nanosecond pulses in the near infrared on these composites showed strong power suppression over a broad temporal regime. Femtosecond and nanosecond transient studies on the same thick MEH-PPV:PCBM:DOP composite films and the experimental results showed evidence for the photogeneration of radical ions as being responsible for the enhanced nonlinear absorption and strong optical suppression in the near infrared. Dithienopyrrole-based donor-acceptor copolymers with narrow bandgap showed strong nonlinear absorption and potential as optical limiters in the telecommunication wavelengths. Molecular engineering was applied to manipulate the spectral overlap of two-photon absorption and subsequent nonlinear absorptions. Femtosecond transient spectroscopy showed near infrared transient absorption and 22 - 61% yields of photogenerated charge-transfer species depending on donor-acceptor coupling strength. Torsional fluctuations of the backbone structure potentially affected the excited state behavior. Evidence suggests that ultrafast relaxation occurs to ground state and to long-lived charge-transfer state from the initially excited state. The dispersion of nonlinear absorption measured using the Z-scan method revealed large two-photon absorption cross sections of these polymers in the telecommunication region. Large suppression of nanosecond pulses at 1064 nm was achieved.

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.