?-Conjugated systems have been the focus of study in recent years in order to understand their charge transport and optical properties for use in organic electronic devices, fluorescence bioimaging, sensors, and 3D optical data storage (ODS), among others. As a result, several molecular building blocks have been designed, allowing new frontiers to be realized. While various successful building blocks have been fine-tuned at both the electronic and molecular structure level to provide advanced photophysical and optoelectronic characteristics, the azulene framework has been under-appreciated despite its unique electronic and optical properties. Among several attributes, azulenes are vibrant blue naturally occurring hydrocarbons that exhibit large dipolar character, coupled with stimuli-responsive behavior in acidic environments. Additionally, the non-toxic nature and the accompanying eco-friendly feature of some azulenes, namely guaiazulene, may set the stage to further explore a more "green" route towards photonic and conductive materials. The first part of this dissertation focuses on exploiting guaiazulene as a natural building block for the synthesis of chromophores with varying stimuli-responsiveness. Results described in Chapter 1 show that extending the conjugation of guaiazulene through its seven-membered ring methyl group with aromatic substituents dramatically impacts the optical properties of the guaiazulenium carbocation. Study of these ?–stabilized tropilium ions enabled establishing photophysical structure-property trends for guaiazulene-terminated ?-conjugated analogs under acidic conditions, including absorption, emission, quantum yield, and optical band gap patterns. These results were exploited in the design of a photosensitive polymeric system with potential application in the field of three dimensional (3D) optical data storage (ODS). Chapter 2 describes the use of guaiazulene reactive sites (C-3 and C-4 methyl group) to generate a series of cyclopenta[ef]heptalenes that exhibit strong stimuli-responsive behavior. The approach presents a versatile route that allows for various substrates to be incorporated into the resulting cyclopenta[ef]heptalenes, especially after optimization that led to devising a one-pot reaction toward such tricyclic systems. Examining the UV-vis absorption profiles in neutral and acidic media showed that the extension of conjugation at C(4) of the cyclopenta[ef]heptalene skeleton results in longer absorption maxima and smaller optical energy gaps. Additionally, it was concluded that these systems act as sensitizers of a UV-activated (< 300 nm) photoacid generator (PAG), via intermolecular photoinduced electron transfer (PeT), upon which the PAG undergoes photodecomposition resulting in the generation of acid. In a related study, the guaiazulene methyl group at C-4 was employed to study the linear and nonlinear optical properties of 4-styrylguaiazulenes, having the same ?–donor with varying ?-spacer. It was realized that the conjugation length correlates with the extent of bathochromic shift of the protonated species. On the other hand, a trend of decreasing quantum yield was established for this set of 4-styrylguaiazulenes, which can be explained by the increasingly higher degree of flexibility. The second part of this dissertation presents a comprehensive investigation of the linear photophysical, photochemical, and nonlinear optical properties of diketopyrrolopyrrole (DPP)-based derivatives, including two-photon absorption (2PA), femtosecond transient absorption, stimulated emission spectroscopy, and superfluorescence phenomena. The synthetic feasibility, ease of modification, outstanding robustness, and attractive spectroscopic properties of DPPs have motivated their study for fluorescence microscopy applications, concluding that the prepared DPP's are potentially suitable chromophores for high resolution stimulated emission depletion (STED) microscopy.

Organic π-conjugated solids are promising candidates for new optoelectronic materials. The large body of evidence points at their advantageous properties such as high charge-carrier mobility, large nonlinear polarizability, mechanical flexibility, simple and low cost fabrication and superior luminescence. They can be used as nonlinear optical (NLO) materials with large two-photon absorption (2PA) and as electronic components capable of generating nonlinear neutral (excitonic) and charged (polaronic) excitations. In this work, we investigate the appropriate theoretical methods used for the (a) prediction of 2PA properties for rational design of organic materials with improved NLO properties, and (b) understanding of the essential electronic excitations controlling the energy-transfer and charge-transport properties in organic optoelectronics. Accurate prediction of these electro-optical properties is helpful for structureactivity relationships useful for technological innovations. In Chapter 1 we emphasize on the potential use of the organic materials for these two applications. The 2PA process is advantageous over one-photon absorption for deep-tissue fluorescence microscopy, photodynamic therapy, microfabrication and optical data storage owing to the three-dimensional spatial selectivity and improved penetration depth in the absorbing or scattering media. The design of the NLO materials with large 2PA cross-sections may reduce the optical damage due to the use of the high intensity laser beams for excitation. The organic molecules also possess self-localized excited states which can decay radiatively or nonradiatively to form excitonic states. This suggests the use of these materials in the electroluminescent devices such as light-emitting diodes and photovoltaic cells through the processes of exciton formation or dissociation, respectively. It is therefore necessary to understand ultrafast relaxation processes required in understanding the interplay between the iv efficient radiative transfer between the excited states and exciton dissociation into polarons for improving the efficiency of these devices. In Chapter 2, we provide the detailed description of the various theoretical methods applied for the prediction as well as the interpretation of the optical properties of a special class of substituted PPV [poly (p-phenylene vinylene)] oligomers. In Chapter 3, we report the accuracy of different second and third order time dependent density functional theory (TD-DFT) formalisms in prediction of the 2PA spectra compared to the experimental measurements for donor-acceptor PPV derivatives. We recommend a posteriori Tamm-Dancoff approximation method for both qualitative and quantitative analysis of 2PA properties. Whereas, Agren's quadratic response methods lack the double excitations and are not suitable for the qualitative analysis of the state-specific contributions distorting the overall quality of the 2PA predictions. We trace the reasons to the artifactual excited states above the ionization threshold. We also study the effect of the basis set, geometrical constraints and the orbital exchange fraction on the 2PA excitation energies and cross-sections. Higher exchange (BMK and M05-2X) and range-separated (CAM-B3LYP) hybrid functionals are found to yield inaccurate predictions both quantitatively and qualitatively. The failure of the exchangecorrelation (XC) functionals with correct asymptotic is traced to the inaccurate transition dipoles between the valence states, where functionals with low HF exchange succeed. In Chapter 4, we test the performance of different semiempirical wavefunction theory methods for the prediction of 2PA properties compared to the DFT results for the same set of molecules. The spectroscopic parameterized (ZINDO/S) method is relatively better than the general purpose parameterized (PM6) method but the accuracy is trailing behind the DFT methods. The poor performances of PM6 and ZINDO/S methods are attributed to the incorrect description of excited-to-excited state transition and 2PA energies, respectively. The different v semiempirical parameterizations can at best be used for quantitative analysis of the 2PA properties. The ZINDO/S method combined with different orders of multi-reference configuration interactions provide an improved description of 2PA properties. However, the results are observed to be highly dependent on the specific choice for the active space, order of excitation and reference configurations. In Chapter 5, we present a linear response TD-DFT study to benchmark the ability of existing functional models to describe the extent of self-trapped neutral and charged excitations in PPV and its derivative MEH-PPV considered in their trans-isomeric forms. The electronic excitations in question include the lowest singlet (S1) and triplet (T1 † ) excitons, positive (P+ ) and negative (P- ) polarons and the lowest triplet (T1) states. Use of the long-range-corrected DFT functional, such as LC-wPBE, is found to be crucial in order to predict the physically correct spatial localization of all the electronic excitations in agreement with experiment. The inclusion of polarizable dielectric environment play an important role for the charged states. The particlehole symmetry is preserved for both the polymers in trans geometries. These studies indicate two distinct origins leading to self-localization of electronic excitations. Firstly, distortion of molecular geometry may create a spatially localized potential energy well where the state wavefunction self-traps. Secondly, even in the absence of geometric and vibrational dynamics, the excitation may become spatially confined due to energy stabilization caused by polarization effects from surrounding dielectric medium. In Chapter 6, we aim to separate these two fundamental sources of spatial localization. We observe the electronic localization of P + and Pis determined by the polarization effects of the surrounding media and the character of the DFT functional. In contrast, the self-trapping of the electronic wavefunctions of S1 and T1(T1 † ) mostly follows their lattice distortions. Geometry vi relaxation plays an important role in the localization of the S1 and T1 † excitons owing to the nonvariational construction of the excited state wavefunction. While, mean-field calculated P + , Pand T1 states are always spatially localized even in ground state S0 geometry. Polaron P+ and Pformation is signified by the presence of the localized states for the hole or the electron deep inside the HOMO-LUMO gap of the oligomer as a result of the orbital stabilization at the LCwPBE level. The broadening of the HOMO-LUMO band gap for the T1 exciton compared to the charged states is associated with the inverted bond length alternation observed at this level. The molecular orbital energetics are investigated to identify the relationships between state localization and the corresponding orbital structure. In Chapter 7, we investigate the effect of various conformational defects of trans and cis nature on the energetics and localization of the charged P + and Pexcitations in PPV and MEHPPV. We observe that the extent of self-trapping for P+ and Ppolarons is highly sensitive on molecular and structural conformations, and distribution of atomic charges within the polymers. The particle-hole symmetry is broken with the introduction of trans defects and inclusion of the polarizable environment in consistent with experiment. The differences in the behavior of PPV and MEH-PPV is rationalized based on their orbital energetics and atomic charge distributions. We show these isomeric defects influence the behavior and drift mobilities of the charge carriers in substituted PPVs.

Time Dependent Density Functional Theory offers a good accuracy/computational cost ratio among different methods used to predict the electronic structure for molecules of practical interest. The Coupled Electronic Oscillator (CEO) formalism was recently shown to accurately predict Nonlinear Optical (NLO) properties of organic chromophores when combined with Time Dependent Density Functional Theory. Unfortunately, CEO does not lend itself easily to interpretation of the structure activity relationships of chromophores. On the other hand, the Sum Over States formalism in combination with semiempirical wavefunction methods has been used in the past for the design of simplified essential states models. These models can be applied to optimization of NLO properties of interest for applications. Unfortunately, TD-DFT can not be combined directly with SOS because state-to-state transition dipoles are not defined in the linear response TD approach. In this work, a second order CEO approach to TD-DFT is simplified so that properties of double excited states and state-to-state transition dipoles may be expressed through the combination of linear response properties. This approach is termed the a posteriori Tamm-Dancoff approximation (ATDA), and validated against high-level wavefunction theory methods. Sum over States (SOS) and related Two-Photon Transition Matrix formalism are then used to predict Two-Photon Absorption (2PA) profiles and anisotropy, as well as Second Harmonic Generation (SHG) properties. Numerical results for several conjugated molecules are in excellent agreement with CEO and finite field calculations, and reproduce experimental measurements well.

No description available.

O poli (p-fenilenevinileno), ou PPV, é um polímero de grande relevância tecnológica devido a suas propriedades eletroluminescentes, que têm sido exploradas em diodos emissores de luz orgânicos, displays flexíveis e outros dispositivos optoeletrônicos. Embora o PPV seja um material de importância para muitas aplicações, a sua síntese na nano/microescala não pode ser obtida através do método padrão, o qual utiliza o aquecimento de um polímero precursor poli (cloreto de xileno tetrahidrotiofenio) (PTHT). Este trabalho mostra como a microestruturação com pulsos de femtosegundo pode ser empregada para a síntese de PPV em regiões pré-determinadas, empregando três diferentes abordagens, permitindo uma nova metodologia para a fabricação precisa de microcircuitos poliméricos complexos, (i) na primeira abordagem, o processo de conversão é obtido pela irradiação de filmes de PTHT com pulsos laser ultracurtos em regiões previamente determinadas, o que leva ao controle espacial da formação de PPV em microescala, (ii) na segunda abordagem, microestruturas tridimensionais dopadas com PTHT foram fotopolimerizadas por absorção de dois fótons. A conversão de PTHT para PPV nestas microestruturas dopadas foi obtida após um tratamento térmico, (iii) na terceira abordagem, a transferência direta induzida por laser (LIFT) com pulsos de femtossegundos permite a deposição controlada de PPV com alta resolução espacial, fornecendo micropadrões 2D, preservando sua estrutura e propriedades ópticas. As estruturas foram caracterizadas por microscopia eletrônica de varredura, microscopia óptica de transmissão, microscopia de fluorescência e microscopia confocal de fluorescência. Suas propriedades ópticas foram analisadas através de sistemas de micro-fotoluminescência e micro-absorção implementadas em um microscópio invertido. Medidas de espectroscopia Raman, microscopia de força atômica e medidas elétricas também foram realizadas. Este trabalho mostra como a microestruturação com laser de fs pode ser explorada para a síntese de PPV em regiões pré-determinadas para fabricar uma variedade de microdispositivos, abrindo novos caminhos na optoeletrônica baseada em polímeros. / Poly(p-phenylenevinylene), or PPV, is a polymer of great technological relevance due to its electroluminescent properties, which have been exploited in organic light emitting diodes, flexible displays and other optoelectronic devices. Although PPV is a material of foremost importance for many applications, its synthesis at the nano/micro scale cannot be achieved through the standard method that uses heating of a precursor polymer poly(xylene tetrahydrothiophenium chloride)(PTHT). This work demonstrates the use of direct laser writing with femtosecond pulses to obtain the synthesis of PPV in pre-determined regions, by applying three different approaches, allowing the precise fabrication of complex polymeric microcircuits, (i) in the first approach the conversion process is achieved by irradiating PTHT films with ultra-short laser pulses in previously determined regions, which leads to the spatial control of PPV formation at microscale, (ii) in the second approach, three-dimensional microstructures doped with PTHT were photopolymerized by two photons absorption. The conversion of PTHT to PPV in these doped microstructures was obtained by a subsequent thermal treatment, (iii) in the third approach, laser-induced forward transfer (LIFT) with femtosecond pulses enables the controlled deposition of PPV with high spatial resolution, providing 2D micropatterns, while preserving its structure and optical properties. The structures were characterized by scanning electron, fluorescence, transmission and confocal fluorescence microscopies. Their optical properties were analyzed by micro-photoluminescence and micro-absorption setups assembled on an inverted microscope. Raman spectroscopy, electrical measurements and atomic force microscopy were also performed. This thesis shows the use of fs-laser writing methods for the synthesis of PPV in pre-determined regions, to fabricate a variety of microdevices, thus opening new avenues in polymer-based optoelectronics.

Compostos orgânicos constituem uma classe interessante de materiais para aplicações em óptica por apresentarem boa processabilidade, relativa facilidade para integração em dispositivos e, principalmente, pela possibilidade de otimização de suas propriedades ópticas através da engenharia molecular. Várias estratégias têm sido empregadas para sintetizar moléculas orgânicas, que exibam singificativos efeitos ópticos não lineares. Nesta direção, nos últimos anos moléculas multi-ramificadas vêm sendo produzidas com o objetivo de intensificar efeitos não lineares, já que estas podem exibir um forte efeito cooperativo entre seus ramos. Nesta tese estudamos a relação da absorção de dois fótons (A2F) e da primeira hiperpolarizabilidade com a estrutura molecular, para um conjunto de nove derivados de trifenilamina com diferentes grupos aceitadores de elétrons arranjadas em geometrias dipolar, quadrupolar e octopolar. O processo A2F foi estudados através da técnica de Varredura-Z, enquanto que a primeira hiperpolarizabilidade foi caracterizada pela técnica de espelhamento hiper Rayleigh. Os dados experimentais para a absorção de dois fótons revelaram espectros bem definidos, com valores razoáveis de seção de choque na região do visível e infravermelho próximo. Observamos ainda um engrandecimento para a seção de choque de A2F para as moléculas quadrupolares. Os resultados obtidos para a primeira hiperpolarizabilidade (&beta;) mostraram que moléculas quadrupolares apresentam maior &beta; do que as dipolares e octopolares, portanto, nossos resultados permitem concluir que o acoplamento eletrônico entre os ramos contribuem fortemente para a seção de choque por A2F e &beta; nas moléculas quadrupolares, não sendo este processo relevante nas moléculas octopolares. Todos os resultados foram interpretados por meio de estudos teóricos empregando a teoria do funcional da densidade (DFT). / Organic compounds constitute an interesting class of materials for optical applications due to their excellent processability, easy integration into devices and, mainly, the possibility of optimizing its optical properties through molecular engineering. Several strategies have been employed to synthesize organic molecules, which exhibit significant nonlinear optical effects. In this direction, in the last few years multi-branched molecules have been obtained aiming at intensifying nonlinear optical effects, since they may exhibit a strong cooperative effect among their branches. On this thesis we have studied the relationship of two-photon absorption (2PA) and first hyperpolarizability with the molecular structure of a group of nine triphenylamine derivatives attached to distinct electron acceptor groups arranged in dipole, quadrupole and octopolar geometries. The 2PA process was studied by Z-scan technique, while the first hyperpolarizability was characterized by the hyper-Rayleigh scattering technique. The experimental data for two-photon absorption revealed well-defined spectra with reasonable cross section magnitude in the visible and near infrared range. We also observed an enhancement of the 2PA cross-section for the quadrupolar molecules in comparison to the dipolar and octopolar ones. The results obtained for the first hyperpolarizability (&beta;) shown that the quadrupolar molecules present higher &beta; than the dipolar and octopolar, suggesting that the electronic coupling between the branches strongly contribute to the 2PA cross-section and &beta; in quadrupolar molecules, being not relevant in the octopolar molecules. All results were interpreted through theoretical studies based on the density functional theory (DFT).

Neste trabalho realizamos o estudo da cinética e formação de agregados em cristais iônicos halogenetos alcalinos dopados com impurezas divalentes, através da técnica de espectroscopia de absorção de dois fótons (ADF). Analisamos a formação dos agregados em cristais envelhecidos a temperaturas fixas (50 e 250&#176C), através de medidas de ADF das transições 4f7 &#8594 4f7 do Eu2+, proibidas por um fóton. A evolução do espectro de ADF em função do tempo, mostrou a existência de três fases distintas de agregação. A primeira fase consiste na formação de dímeros e é observada no espectro de ADF através de três absorções muito próximas às absorções do dipolo isolada. As outras duas fases (II e III) evoluem a partir dos dímeros, dependendo do tratamento térmico a que a amostra é submetida. Estas fases são observadas no espectro de ADF através de absorções relativamente afastadas das absorções do dipolo isolado. Todo sistema experimental foi montado em nosso laboratório, consistindo de um sistema de aquisição de dados controlado por um micro-computador; laser de corante bombeado por um YAG:Nd3+; motores de passos para a varredura do comprimento de onda do laser de corante; sistema de detecção; etc. / In this work we realize the study of the Kinetic and formation of precipitates in ionic crystals doped with divalent impurities by the TPA (two photon absorption) spectroscopy technique. We analyze the formation of the aggregates in crystals annealed at temperatures of 50 and 250&#176C. We measured the TPA of the transition 4f7 &#8594 4f7 of the Eu2+, which is forbidden by one photon. The evolution of the TPA spectra at the time showed us three phases of the precipitates. The dimmers formation was observed in the first phase, by three absorptions bands near the one of the impurity vacancy dipole. The others phases (II and III) involve from the dimmers, depending on the annealing temperature that the samples were submitted. Those phases are observed in the TPA spectra through absorptions which are relatively far from the absorptions due to the isolated (I-V) dipole. All experimental system employed is home made. It consists of a data acquisition system controlled by a micro-computer; a Dye laser pumped by a YAG:Nd3+ laser; step motors to scan the wavelength of the Dye laser; detection system and others devices.

Les travaux présentés dans ce mémoire de thèse portent sur la synthèse et l’étude des propriétés spectroscopiques et électrochimiques de systèmes donneur-accepteur conçus pour des applications variées telles que l’électrofluorochromisme, l’absorption à deux photons et le photovoltaïque. La s-tétrazine a été choisie comme accepteur pour sa forte affinité électronique, ses propriétés émissives remarquables et sa capacité à s’organiser via des interactions intermoléculaires de type &#61552--stacking. La triphénylamine a été sélectionnée comme donneur pour son faible potentiel d’ionisation, ses propriétés spectroscopiques (fortes absorption et émission) et la modulation facile de ses propriétés par changement de substituants. Sept dérivés de triphénylamine ont été synthétisés ainsi que dix-huit nouveaux composés multichromophoriques à base de tétrazine et de triphénylamine présentant cinq liens différents et des substituants variés. Ils ont été caractérisés par électrochimie et spectroscopie (stationnaire et résolue en temps). L’étude de la modulation de leurs propriétés photophysiques par le changement de l’état rédox a ensuite été réalisée. Dix composés présentant un lien permettant la conjugaison entre la triphénylamine et la tétrazine ont été synthétisés et caractérisés par électrochimie et spectroscopie. Compte-tenu de leurs propriétés, six d’entre eux ont été testés en absorption à deux photons et deux ont étés retenus pour être utilisés comme donneurs dans une cellule photovoltaïque organique. Par ailleurs, deux réactions ont été étudiées en détail pour expliquer la formation des produits obtenus, inattendus à un premier abord. / This work deals with the synthesis and the spectroscopic and electrochemical studies of donor-acceptor systems which have been designed for electrofluorochromism, two-photon absorption and photovoltaics. s-Tetrazine has been chosen as the acceptor for its high electron affinity, its emission properties and its ability to structure a layer thanks to intermolecular interactions (&#61552--stacking). Triphenylamine has been selected as the donor for its low ionization potential, its spectroscopic properties (high absorption and emission) and the easy modulation of its properties by changing the substituents. Seven triphenylamine derivatives have been synthesized as well as eighteen new multichromophoric compounds based on tetrazine and triphenylamine which have five different links and various substituents. They have been characterized by electrochemistry and spectroscopy (stationary and time-resolved). The study of the modulation of the photophysic properties with the controle of the redox state has been then done. Ten compounds having a conjugating link between the tetrazine and the tetrazine have been synthesized and characterized by electrochemistry and spectroscopy. Given their properties, six of them have been tested in two-photon absorption and two of them have been selected to be used as a donor in an organic solar cell. Moreover two reactions have been examined in depth in order to explain some unexpected synthesis results.

Nesta tese, estudamos o processo de absorção multifotônica em polímeros e resinas poliméricas, abordando tanto aspectos fundamentais quanto aplicados. Com relação aos aspectos fundamentais, estudamos processos de absorção multifotônica (absorção de dois, três e quatro fótons) no polímero conjugado MEH-PPV (poly(2-methoxy-5-(2´-ethylhexyloxy)-1,4- phenylenevinylene)), utilizando a técnica de Varredura-Z com pulsos ultracurtos. Através desta técnica, determinamos o espectro da absorção de dois, três e quatro fótons do MEHPPV. As seções de choque de absorção de multi-fótons correspondentes a cada processo foram determinadas através do ajuste das curvas experimentais com um conjunto de equações desenvolvidas neste trabalho. Os resultados obtidos permitiram traçar relações entre os espectros não lineares e os níveis de energia do polímero. Na vertente mais aplicada do projeto, estudamos a fotopolimerização de resinas acrílicas através do processo de absorção de dois fótons. Devido ao confinamento espacial da polimerização, graças à absorção de dois fótons, este método permite a confecção de micro-estruturas complexas para diversas aplicações tecnológicas. Além da fabricação de microestruturas convencionais não dopadas, neste trabalho desenvolvemos uma metodologia que possibilita a fabricação de microestruturas dopadas com MEH-PPV, visando a produção de micro-elementos fluorescentes para dispositivos fotônicos, e microestruturas dopadas com quitosana, um polímero biocompatível que pode ser utilizado em aplicações médicas e biológicas. / In this thesis we have studied the multi-photon absorption process in polymers and polymeric resins, exploiting its fundamental as well as technological aspects. Regarding the fundamental aspects, we have studied the multi-photon absorption (two-, three- and four-photon absorption) in the conjugated polymer MEH-PPV (poly(2-methoxy-5-(2´-ethylhexyloxy)-1,4-phenylenevinylene)), by using the Z-scan technique with ultrashort laser pulses. Through this technique, we determined the two-, three- and four-photon absorption spectra of MEH-PPV. The multi-photon absorption cross-sections, corresponding to each specific process, have been determined by fitting the experimental data with a set of equations developed in this work. The results allowed us to correlate the nonlinear absorption spectra to the energy level of the polymer. On the technological side of this thesis, we have investigated the photopolymerization of acrylic resins by two-photon absorption. Because of the spatial confinement of the polymerization, resulting from the two-photon excitation, this method allows the fabrication of complex microstructures which can be used for several technological applications. In addition to the fabrication of undoped microstructures, in this work we have developed a methodology that allows the fabrication of microstructures doped with MEHPPV, aiming the production of fluorescent micro-elements for photonics applications, and microstructures doped with chitosan, a biocompatible polymer, that can be used for medical and biological applications.

This thesis concerns quadratic response properties and their application to properties in Jablonski diagrams such as resonant two-photon absorption and excited state absorption. Our main interest lies in optical power limiting applications, and in this context, molecules containing heavy metal atoms prove superior. Therefore, we are interested in how relativity affects these properties, and in order to assess this, a four-component relativistic framework is adopted. To properly address the molecular properties of interest, both relativistic effects and electron correlation need to be accounted for. These two properties are not additive, and, therefore, correlation needs to be incorporated into the four-component framework. We present the implementation of quadratic response properties at the four-component density functional level of theory. For second-harmonic generation, we have, with numerical examples, demonstrated that correlation and relativity are indeed not additive and that the inclusion of noncollinear magnetization is of little importance. We report that both electron correlation as well as relativity strongly affect results for second-harmonic generation. For example, relativity alone reduces the µβ-response signal by 62% and 75% for meta- and ortho-bromobenzene, respectively, and enhances the same response by 17% and 21% for meta- and ortho-iodobenzene, respectively. In the four-component framework, we present the implementations of single and double residues of the quadratic response function, which allows for the evaluation of resonant two-photon absorption cross sections and excited state properties. Using these tools, we discuss different levels of approximation to the relativistic Hamiltonian and we demonstrate that for two-photon absorption, a proper treatment of relativistic effects qualitatively alters the spectrum. For example, already for an element as light as neon, significant differences are seen between the relativistic and nonrelativistic spectra as triplet transitions acquire substantial absorption cross sections in the former case. Finally, quantum mechanics in conjunction with electrodynamics is applied to determine clamping levels in macroscopic samples. The microscopic properties of the optically active chromophores are determined by response theory, and then, electrodynamics is used to describe the interactions between the chromophores and incident laser pulses. Using this approach a series of molecules have been investigated and their performances have been compared and ranked in order to find novel materials for optical power limiting applications.