<p>The sun provides us daily with large quantities of energy in the form of light. With the world’s increasing demand of electrical energy the prospect of converting this solar light into electricity is highly tempting. In the strive towards mass-production and low cost solar cells, new types of solar cells are being developed, e.g. solar cells completely based on organic molecules and polymers. These materials offer a promising potential of low cost and large scale manufacturing and have the additional advantage that they can be produced on flexible and light weight substrate which opens for new and innovating application areas, e.g. integration with paper or textiles, or as building materials. In polymer solar cells a combination of two materials are used, an electron donor and an electron acceptor. The three dimensional distribution of the donor and acceptor in the active layer of the device, i.e. the morphology, is known to have larger influence of the solar cell performance. For the optimal morphology there is a trade-off between sometimes conflicting criteria for the various steps of the energy conversion process. The dissociation of photogenerated excitons takes place at an interface between the donor and acceptor materials. Therefore an efficient generation of charges requires a large interface between the two components. However, for charge transport and collection at the electrodes, continuous pathways for the charges to the electrodes are required.</p><p>In this thesis, results from morphology studies by atomic force microscopy (AFM) and dynamic secondary ion mass spectrometry (SIMS) of spin-coated blend and bilayer thin films of polyfluorene co-polymers, especially poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-5,5-(4´,7´-di-2-thienyl-2´,1´,3´-benzothiadiazole)] APFO-3, and the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) are presented. It is shown that by varying the blend ratio, the spin.-coating solvent, and/or the substrate, different morphologies can be obtained, e.g. diffuse bilayer structures, spontaneously formed multilayer structures and homogeneous blends. The connection between these different morphologies and the performance of solar cells is also analysed. The results indicate that nano-scale engineering of the morphology in the active layer may be an important factor in the optimization of the performance of polymer solar cells.</p>

The sun provides us daily with large quantities of energy in the form of light. With the world’s increasing demand of electrical energy the prospect of converting this solar light into electricity is highly tempting. In the strive towards mass-production and low cost solar cells, new types of solar cells are being developed, e.g. solar cells completely based on organic molecules and polymers. These materials offer a promising potential of low cost and large scale manufacturing and have the additional advantage that they can be produced on flexible and light weight substrate which opens for new and innovating application areas, e.g. integration with paper or textiles, or as building materials. In polymer solar cells a combination of two materials are used, an electron donor and an electron acceptor. The three dimensional distribution of the donor and acceptor in the active layer of the device, i.e. the morphology, is known to have larger influence of the solar cell performance. For the optimal morphology there is a trade-off between sometimes conflicting criteria for the various steps of the energy conversion process. The dissociation of photogenerated excitons takes place at an interface between the donor and acceptor materials. Therefore an efficient generation of charges requires a large interface between the two components. However, for charge transport and collection at the electrodes, continuous pathways for the charges to the electrodes are required. In this thesis, results from morphology studies by atomic force microscopy (AFM) and dynamic secondary ion mass spectrometry (SIMS) of spin-coated blend and bilayer thin films of polyfluorene co-polymers, especially poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-5,5-(4´,7´-di-2-thienyl-2´,1´,3´-benzothiadiazole)] APFO-3, and the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) are presented. It is shown that by varying the blend ratio, the spin.-coating solvent, and/or the substrate, different morphologies can be obtained, e.g. diffuse bilayer structures, spontaneously formed multilayer structures and homogeneous blends. The connection between these different morphologies and the performance of solar cells is also analysed. The results indicate that nano-scale engineering of the morphology in the active layer may be an important factor in the optimization of the performance of polymer solar cells.

In the field of electronic organic materials, conjugated polymers (CPs) have attracted much attention in recent years. It has been well-established that performances of thin-film devices based on π-conjugated polymers, such as light-emitting diodes, field-effect transistors and photovoltaic cells, are strongly dependent on the organisation of the polymer molecules and their interactions with other constituents in multicomponent devices. The use of CPs in integrated circuits, solar cells, light-emitting diodes or sensors often requires their covalent fixation and patterning on various surfaces. CPs can be grafted to functionalized surfaces by (electro)chemical cross-linking; however, it is difficult to control a structural order within the cross-linked films. The attachment of CP chains to substrates by their end-points to form polymer brushes would be an interesting alternative, and could possibly be crucial for many devices requiring charge injection and charge transport processes. The main aim of this work, which was the synthesis of covalently grafted conjugated polymer brushes on solid substrates using a "grafting from" approach, was successfully performed. During the course of this work, the process of surface-initiated polycondensation was investigated. The newly developed method to selectively graft conjugated polymers from different substrates such as Si-wafers, quartz slides or modificated nanoparticles allowed us to produce different architectures which were earlier possible to prepare only non-conductive polymers. Exposure of the substrate with an activated surface layer into the monomer solution produced polymer brushes in a very economical way. Since only monomer was consumed for grafting from the surface. The grafting process was extensively investigated by different methods, and the thickness of the obtained poly(fluorene) films was elucidated by Null-ellipsometry and confirmed by the AFM scratch-test. Preliminary characteristics of the device, based on PS(Br)-core poly(octylfluorene)-shell nanoparticles, showed satisfactory results (such as turn-on voltage and electroluminescence in a blue region). They could be improved by replacement of the insulating PS(Br)-core of nanoparticles with other substances (semiconductive, etc.). There is still plenty of room for further development and improvement of the synthesis of poly(fluorene)-based polymer brushes. The polymer structures developed in this work can be utilized as an active layer in lab-on-chip devices. Alkyl groups in the 9th position of the poly(fluorene) monomer unit can be replaced by tailored receptors to detect specific species including small molecules, metal ions and biomolecules due to enhanced sensitivity through sensory signal amplification. Post-polymerization modifications may lead to highly water-swellable conjugated polyelectrolyte brushes. Also, polymerization of initially optically active fluorene-monomers may be the crucial step to the generation of a light source devices with a large degree of circularly polarized electroluminescence. This is of great interest for utilization as backlight for liquid crystalline displays. We believe that the utilization of covalently surface-immobilized conjugated polymers may have a great impact on the development of present-day technological processes.

Polyfluorenes are polymers with outstanding properties: They are semi-conducting, relatively rigid, quite stable chemically and thermally, easily substituted and therefore potentially soluble in numerous solvents and more importantly, they exhibit blue electro- and photoluminescence. For all these reasons, these polymers are the subjects of numerous academic and industrial researches.<p>The first subject of this work deal with the design, the synthesis and the characterisation of polyfluorenes end-capped with perylene dicarboximide derivatives. These perylene moieties are able to interact by energy transfer under specific conditions of illumination, proximity and orientation. Their observation by single molecule spectroscopy permitted therefore to gain valuable information concerning the three-dimensional folding of single polyfluorene chains. To complete this study, the synthesis and characterisation of a perylene end-capped trimer of fluorene was performed. This structure being monodisperse, a finer analysis of the energy-transfer occurring between both perylene dyes could be accomplished, which confirmed the structural hypothesis made for the polymer. During these studies, it has been observed that, in addition to the energy transfer occurring between both perylene derivatives, another energy transfer occurs between the polyfluorene backbone and the perylene derivatives upon excitation of the first. This led to the idea of the synthesis of a polyfluorene bearing perylenes dicarboximide as side chains. This perylene-rich polyfluorene has been used to build a photovoltaic cell efficient in the wavelengths of both polyfluorene absorption and perylene carboximide absorption. <p>Another subject of this work was the design, synthesis and characterisation of polyfluorenes bearing bulky phenoxy groups as side-chains. These polymers, due to their lower tendency toward aggregation, exhibited a better stability of their emission colour upon annealing. Similarly, a series of homo- and copolymers of fluorene bearing bulky and hole accepting triphenylamine substituants was synthesised and characterised. In addition to their improved colour stability in comparison with dialkylpolyfluorenes, the LEDs build with these materials exhibited a very low turn on voltage. <p> / Doctorat en sciences, Spécialisation chimie / info:eu-repo/semantics/nonPublished

In the field of electronic organic materials, conjugated polymers (CPs) have attracted much attention in recent years. It has been well-established that performances of thin-film devices based on π-conjugated polymers, such as light-emitting diodes, field-effect transistors and photovoltaic cells, are strongly dependent on the organisation of the polymer molecules and their interactions with other constituents in multicomponent devices. The use of CPs in integrated circuits, solar cells, light-emitting diodes or sensors often requires their covalent fixation and patterning on various surfaces. CPs can be grafted to functionalized surfaces by (electro)chemical cross-linking; however, it is difficult to control a structural order within the cross-linked films. The attachment of CP chains to substrates by their end-points to form polymer brushes would be an interesting alternative, and could possibly be crucial for many devices requiring charge injection and charge transport processes. The main aim of this work, which was the synthesis of covalently grafted conjugated polymer brushes on solid substrates using a "grafting from" approach, was successfully performed. During the course of this work, the process of surface-initiated polycondensation was investigated. The newly developed method to selectively graft conjugated polymers from different substrates such as Si-wafers, quartz slides or modificated nanoparticles allowed us to produce different architectures which were earlier possible to prepare only non-conductive polymers. Exposure of the substrate with an activated surface layer into the monomer solution produced polymer brushes in a very economical way. Since only monomer was consumed for grafting from the surface. The grafting process was extensively investigated by different methods, and the thickness of the obtained poly(fluorene) films was elucidated by Null-ellipsometry and confirmed by the AFM scratch-test. Preliminary characteristics of the device, based on PS(Br)-core poly(octylfluorene)-shell nanoparticles, showed satisfactory results (such as turn-on voltage and electroluminescence in a blue region). They could be improved by replacement of the insulating PS(Br)-core of nanoparticles with other substances (semiconductive, etc.). There is still plenty of room for further development and improvement of the synthesis of poly(fluorene)-based polymer brushes. The polymer structures developed in this work can be utilized as an active layer in lab-on-chip devices. Alkyl groups in the 9th position of the poly(fluorene) monomer unit can be replaced by tailored receptors to detect specific species including small molecules, metal ions and biomolecules due to enhanced sensitivity through sensory signal amplification. Post-polymerization modifications may lead to highly water-swellable conjugated polyelectrolyte brushes. Also, polymerization of initially optically active fluorene-monomers may be the crucial step to the generation of a light source devices with a large degree of circularly polarized electroluminescence. This is of great interest for utilization as backlight for liquid crystalline displays. We believe that the utilization of covalently surface-immobilized conjugated polymers may have a great impact on the development of present-day technological processes.

Neste trabalho, apresentamos uma investigação sistemática da microestrutura e da dinâmica de dois polímeros emissores de luz derivados do polifluoreno, a saber, Poly(9,9-dioctylfluorenyl-2,7-diyl) (BE 329) and Poly[(9,9-dioctyl-2,7-divinylene-fluorenylene)-altco-{ 2-methoxy-5-(2-ethyl-hexyloxy)-1,4-phenylene}] (GE 108), utilizando técnicas de Difração de Raios X em Alto Ângulo (WAXD), Espalhamento de Raios X em Baixo Ângulo (SAXS), Ressonância Magnética Nuclear (RMN) no estado sólido, Análise Térmica Dinâmico-Mecânica (DMTA) e Espectroscopia de Fotoluminescência (PL). Em relação as propriedades estruturais, diferenças notáveis entre a microestrutura de ambos os polímeros foram observadas. As medidas de WAXD revelaram a presença de cristalinidade nos filmes da amostra BE 329 (cristalinidade de aproximadamente 47 %) e, também, que o polímero sofre uma modificação estrutural em 433 K. Em contrapartida, as mesmas medidas na amostra GE 108 não revelaram a presença de cristalinidade, mas sim a de estruturas agregadas similares a fases mesomórficas típicas de cristais líquidos. A interpretação dos dados foi feita assumindo um modelo estrutural, onde a distância entre anéis aromáticos coplanares (empilhados) é de d1 = ~ 4.5 Å, e o espaçamento lateral entre eles é de d2 = ~ 18 Å. Estas distâncias possibilitaram a associação com os picos de WAXD, que foram obtidos em diversas temperaturas e revelaram que até aproximadamente 380 K, o parâmetro d1 não muda consideravelmente, entretanto, a partir desta temperatura começa a mudar com intensidade, até atingir 5.5 Å em 413 K. Em contrapartida, o parâmetro d2 varia intensamente até 330 K, permanecendo praticamente estável até 413 K. Por DMTA, ambos os polímeros apresentaram uma relaxação em aproximadamente 210 K e outra em aproximadamente 370 K. A natureza microscópica das relaxações foi elucidada via métodos de RMN capazes de detectar dinâmica molecular de grupos químicos individuais. Mostramos que a relaxação em baixa temperatura é associada a movimentos da cadeia lateral, que apresentaram energia de ativação de aproximadamente 20 kJ/mol para ambos os polímeros. Mesmo apresentando energias de ativação similares, pudemos observar que a fração móvel das cadeias laterais do GE 108 é maior, indicando a presença de cadeias laterais rígidas no BE 329, provavelmente relacionado a fase cristalina. Medidas de RMN também revelaram a presença de movimentos lentos (com tempo de correlação de ms) na cadeia principal, principalmente acima de 370 K, indicando que a relaxação em mais alta temperatura observada por DMTA esta associado a este tipo de dinâmica. Analisados em conjunto, esses resultados indicam que o início dos movimentos das cadeias laterais produz um aumento do espaçamento lateral dos anéis aromáticos nas estruturas agregadas, facilitando o início de movimentos de torção na cadeia principal. Além disso, em 370 K, o aumento da amplitude dos movimentos na cadeia principal induz o aumento da distância entre anéis agregados. Por fim, o entendimento da evolução estrutural e da dinâmica dos diferentes grupos químicos foi aplicado para explicar mudanças observadas nos espectros de PL em função da temperatura. Particularmente as variações de amplitude e deslocamentos para o azul dos espectros de PL foram diretamente associadas com processos de dissociação (aumento da distância) dos anéis agregados e com o aumento dos movimentos de torção dos anéis favorecidos pelo ganho de dinâmica na cadeia lateral. / In this work we present an investigation of the microstructure and dynamics of two Polyfluorene based polymers, said Poly(9,9-dioctylfluorenyl-2,7-diyl) (BE 329) and Poly[(9,9-dioctyl-2,7-divinylene-fluorenylene)-alt-co-{2-methoxy-5-(2-ethyl-hexyloxy)-1,4- phenylene}] (GE 108), using Wide-Angle X-ray Scattering (WAXS), Solid-State Nuclear Magnetic Resonance (NMR), Dynamic Mechanical Thermal Analysis (DMTA) and Fluorescence Spectroscopy (PL). Concerning the structural properties, remarkable differences between the microstructure of both polymers were found. The WAXS measurements revealed that cast films prepared with BE 329 are semi-crystalline (with crystalinity ration of ~ 47 %) and undergo a structural modification at ~ 433 K. In contrast, WAXS measurements revealed that Ge108 as cast films do not present crystallinity, but forms aggregated structures similar to a mesomorphic liquid crystal phase, such as observed in MEH-PPV like polymers. Assuming a model for the aggregated structure, where the phenyl rings planes are stacked parallel to each other with an average distance of d1 = ~ 4.5 Å and laterally spaced by d2 = ~ 18 Å, it was possible to associate these distances with specific peaks in the WAXS pattern. By doing so, the evolution of d1 and d2 as a function of temperature was probed, revealing that d1 remains mostly constant as a function of temperature until ~ 380 K and then start increasing, reaching 5.5 Å at 413 K. d2 also present a trend change at ~ 380 K, but in this case it first increases swiftly and than become more constant. The DMTA data show that both polymers have a low temperature relaxation at ~ 210 K and a high temperature relaxation at ~ 373 K. The microscopic nature of these relaxations was elucidated by solid-state NMR methods capable of detecting molecular dynamics of individual chemical groups[6]. It was show that the low temperature relaxation is associated with motion in the side-chain, occurring with activation energies of ~ 20 kJ/mol in both polymers. Despite the similar activation energies, it was also observed that the fraction of mobile side chains is higher in the GE 108 sample, pointing to the presence of rigid side-chains in BE 329, probably in the crystalline phase. Besides, NMR measurements also revealed the presence of slow motions (ms correlation time range) in the polymers backbones, mainly above 370 K, showing that the high temperature relaxation observed by DMTA is associated to this kind of motion. Again, the fraction of mobile segments was higher for the GE 108 sample, indicating that these dynamic processes mostly occurs in the amorphous part of the polymers. Put together, these results shows that the onset of the side chain motion produces the increase the lateral spacing of the phenyl ring in the aggregated structures, facilitating the onset of torsional motion in the backbone. Besides, at 370 K the increase in the amplitude of the backbone motions induces the increase in the distance between the stacked phenyl rings. This behavior explains many aspects of the temperature dependence of the Fluorescence spectra of the polymers. Particularly, the changes in the intensity and the blue shift of the PL spectra were associated with the dissociation of the phenyl rings and the increase of torsional motion in the main chain, which are facilitated by the onset of the side-chain motions.

Neste trabalho, apresentamos uma investigação sistemática da microestrutura e da dinâmica de dois polímeros emissores de luz derivados do polifluoreno, a saber, Poly(9,9-dioctylfluorenyl-2,7-diyl) (BE 329) and Poly[(9,9-dioctyl-2,7-divinylene-fluorenylene)-altco-{ 2-methoxy-5-(2-ethyl-hexyloxy)-1,4-phenylene}] (GE 108), utilizando técnicas de Difração de Raios X em Alto Ângulo (WAXD), Espalhamento de Raios X em Baixo Ângulo (SAXS), Ressonância Magnética Nuclear (RMN) no estado sólido, Análise Térmica Dinâmico-Mecânica (DMTA) e Espectroscopia de Fotoluminescência (PL). Em relação as propriedades estruturais, diferenças notáveis entre a microestrutura de ambos os polímeros foram observadas. As medidas de WAXD revelaram a presença de cristalinidade nos filmes da amostra BE 329 (cristalinidade de aproximadamente 47 %) e, também, que o polímero sofre uma modificação estrutural em 433 K. Em contrapartida, as mesmas medidas na amostra GE 108 não revelaram a presença de cristalinidade, mas sim a de estruturas agregadas similares a fases mesomórficas típicas de cristais líquidos. A interpretação dos dados foi feita assumindo um modelo estrutural, onde a distância entre anéis aromáticos coplanares (empilhados) é de d1 = ~ 4.5 Å, e o espaçamento lateral entre eles é de d2 = ~ 18 Å. Estas distâncias possibilitaram a associação com os picos de WAXD, que foram obtidos em diversas temperaturas e revelaram que até aproximadamente 380 K, o parâmetro d1 não muda consideravelmente, entretanto, a partir desta temperatura começa a mudar com intensidade, até atingir 5.5 Å em 413 K. Em contrapartida, o parâmetro d2 varia intensamente até 330 K, permanecendo praticamente estável até 413 K. Por DMTA, ambos os polímeros apresentaram uma relaxação em aproximadamente 210 K e outra em aproximadamente 370 K. A natureza microscópica das relaxações foi elucidada via métodos de RMN capazes de detectar dinâmica molecular de grupos químicos individuais. Mostramos que a relaxação em baixa temperatura é associada a movimentos da cadeia lateral, que apresentaram energia de ativação de aproximadamente 20 kJ/mol para ambos os polímeros. Mesmo apresentando energias de ativação similares, pudemos observar que a fração móvel das cadeias laterais do GE 108 é maior, indicando a presença de cadeias laterais rígidas no BE 329, provavelmente relacionado a fase cristalina. Medidas de RMN também revelaram a presença de movimentos lentos (com tempo de correlação de ms) na cadeia principal, principalmente acima de 370 K, indicando que a relaxação em mais alta temperatura observada por DMTA esta associado a este tipo de dinâmica. Analisados em conjunto, esses resultados indicam que o início dos movimentos das cadeias laterais produz um aumento do espaçamento lateral dos anéis aromáticos nas estruturas agregadas, facilitando o início de movimentos de torção na cadeia principal. Além disso, em 370 K, o aumento da amplitude dos movimentos na cadeia principal induz o aumento da distância entre anéis agregados. Por fim, o entendimento da evolução estrutural e da dinâmica dos diferentes grupos químicos foi aplicado para explicar mudanças observadas nos espectros de PL em função da temperatura. Particularmente as variações de amplitude e deslocamentos para o azul dos espectros de PL foram diretamente associadas com processos de dissociação (aumento da distância) dos anéis agregados e com o aumento dos movimentos de torção dos anéis favorecidos pelo ganho de dinâmica na cadeia lateral. / In this work we present an investigation of the microstructure and dynamics of two Polyfluorene based polymers, said Poly(9,9-dioctylfluorenyl-2,7-diyl) (BE 329) and Poly[(9,9-dioctyl-2,7-divinylene-fluorenylene)-alt-co-{2-methoxy-5-(2-ethyl-hexyloxy)-1,4- phenylene}] (GE 108), using Wide-Angle X-ray Scattering (WAXS), Solid-State Nuclear Magnetic Resonance (NMR), Dynamic Mechanical Thermal Analysis (DMTA) and Fluorescence Spectroscopy (PL). Concerning the structural properties, remarkable differences between the microstructure of both polymers were found. The WAXS measurements revealed that cast films prepared with BE 329 are semi-crystalline (with crystalinity ration of ~ 47 %) and undergo a structural modification at ~ 433 K. In contrast, WAXS measurements revealed that Ge108 as cast films do not present crystallinity, but forms aggregated structures similar to a mesomorphic liquid crystal phase, such as observed in MEH-PPV like polymers. Assuming a model for the aggregated structure, where the phenyl rings planes are stacked parallel to each other with an average distance of d1 = ~ 4.5 Å and laterally spaced by d2 = ~ 18 Å, it was possible to associate these distances with specific peaks in the WAXS pattern. By doing so, the evolution of d1 and d2 as a function of temperature was probed, revealing that d1 remains mostly constant as a function of temperature until ~ 380 K and then start increasing, reaching 5.5 Å at 413 K. d2 also present a trend change at ~ 380 K, but in this case it first increases swiftly and than become more constant. The DMTA data show that both polymers have a low temperature relaxation at ~ 210 K and a high temperature relaxation at ~ 373 K. The microscopic nature of these relaxations was elucidated by solid-state NMR methods capable of detecting molecular dynamics of individual chemical groups[6]. It was show that the low temperature relaxation is associated with motion in the side-chain, occurring with activation energies of ~ 20 kJ/mol in both polymers. Despite the similar activation energies, it was also observed that the fraction of mobile side chains is higher in the GE 108 sample, pointing to the presence of rigid side-chains in BE 329, probably in the crystalline phase. Besides, NMR measurements also revealed the presence of slow motions (ms correlation time range) in the polymers backbones, mainly above 370 K, showing that the high temperature relaxation observed by DMTA is associated to this kind of motion. Again, the fraction of mobile segments was higher for the GE 108 sample, indicating that these dynamic processes mostly occurs in the amorphous part of the polymers. Put together, these results shows that the onset of the side chain motion produces the increase the lateral spacing of the phenyl ring in the aggregated structures, facilitating the onset of torsional motion in the backbone. Besides, at 370 K the increase in the amplitude of the backbone motions induces the increase in the distance between the stacked phenyl rings. This behavior explains many aspects of the temperature dependence of the Fluorescence spectra of the polymers. Particularly, the changes in the intensity and the blue shift of the PL spectra were associated with the dissociation of the phenyl rings and the increase of torsional motion in the main chain, which are facilitated by the onset of the side-chain motions.

This thesis deals with theoretical studies of electronic properties of organic conjugated molecules. The first chapter introduces different classes of organic conjugated molecules which possess high hole mobility, large quadratic non-linear response and low band gap. In this chapter, we further describe different photo-physical processes and the basic principles of various opto-electronic devices. The second chapter provides an introduction to various many-body techniques, which are employed in studying ground and excited state properties of organic conjugated systems. First, we describe the Hartree-Fock theory and the Density Functional (DFT) method. These are followed by full Configuration-Interaction (CI) methods and various semi-empirical methods (CNDO, INDO and NDDO). The INDO method is used in subsequent chapters to obtain the ground and excited state properties of organic conjugated molecules. In addition, we describe the restricted CI (SCI and SDCI) and the Density Matrix Renormalization Group (DMRG) methods. The third chapter of this thesis deals with a time evolution study to ascertain the role of the triplet state in the green emission of the ethyl-hexyl substituted poly-fluorene (PF2/6) films. To understand this phenomenon, we have modeled various non-radiative processes like (i) Inter-System Crossing (ISC), (ii) electron-hole Recombination (e-hR) and (iii) Triplet Quenching (TQ). These studies conclusively prove the contribution of triplet states to the 500 nm EL peak. In chapter four, we describe the origin of the unusual EL in tri-p-tolylamine (TTA) based hole conductors. In order to model this phenomenon, we have performed SCI calculations on TTA, its radical ions and allied hole conductors (TAPC and TPD). These calculations indicate that the unusual EL is due to low-lying charge-transfer (CT) state, which is stabilized by charge-dipole and charge-induced-dipole interactions. In chapter five, we turn our attention to the calculation of ground and excited state properties of a class of donor-acceptor (DA) system using ab-initio DFT and INDO methods. In these systems, DFT calculations along with INDO-SCI calculation, show strong intramolecular charge transfer interaction between the D and the A units. We have further calculated various properties like permanent dipole moments, oscillator strengths, Stoke’s shifts in various solvents etc. In chapter six, we focus on studying linear and non-linear optical properties of first generation nitrogen based dendrimers, using DMRG method. A novel scheme which includes the weights of the dipole allowed states in the computation of the density matrix is developed to obtain accurate dipole allowed excited states as well as the linear and nonlinear optical responses. Chapter seven deals with non-linear optical properties of weak donor-acceptor (DA) complexes formed between methyl substituted phenylenes (donor) and Chloranil or DDQ (acceptors). We have calculated the ground and the low-lying excited states of these DA complexes using INDO-SDCI method. The first hyperpolarizability (β) response coefficients are calculated using the Correction Vector (CV) technique, which are further used to obtain macroscopic depolarization ratios. By comparing the theoretical results with experimental findings, it can be shown that the slipped parallel configuration with a slight twist is the most preferred geometry of these weak DA complexes in solution.