Recent years have shown a dramatic growth of research activity in searching of materials that exhibit nonlinear optical (NLO) behaviours. Later investigations have shown that molecule-based organic materials and biomaterials possess advantages in NLO characteristics. This thesis is devoted to the theoretical and experimental study of the second-order nonlinear optical properties of native and modified bacteriorhodopsins. A hybrid quantum mechanical/molecular mechanical-configuration interaction (QM/MM-CI) method has been used to determine the nonlinear behaviour of modified BR. The calculated structures are the ground state and M1 state of wild type BR, several BR variants, the BR mutants W86F, W189F, W182F, W138C, Y185F, and Y83F as well as a photoreceptor NpSRII. The first hyperpolarizability of two available BR mutants W86F and D85C has been determined by means of the HRS technique. The experimentally determined first hyperpolarizability value for the mutant W86F is in good agreement with that obtained theoretically. Simultaneously with the time-resolved absorption spectroscopy measurements, the time-dependent HRS measurements were performed. Changes in the HRS signal during the photocycle were detected. The kinetics of time-resolved HRS and flash spectroscopy measurements of BR and the BR mutants T46C, R225C, R227C, D96C, and D85C were compared. The obtained kinetic constant are comparable with those determined by means of absorption spectroscopy. However, an unexpected decrease in the HRS intensity during the M to N transition was observed. The program "MultiFit" (written in the Delphi environment) as well as data acquisition software (written in the C environment) can be found online at: http://repositorium.uni-osnabrueck.de/bitstream/urn:nbn:de:gbv:700-2008020119/4/MultiFit.zip and: http://repositorium.uni-osnabrueck.de/bitstream/urn:nbn:de:gbv:700-2008020119/5/TRAS.zip

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Cette thèse a pour objectif l’étude des propriétés optiques non linéaires (ONL) de second ordre de molécules de référence et de composés organiques photochromes, via des calculs de chimie quantique et des expériences de diffusion Hyper-Rayleigh (HRS). L’étude des réponses optiques de second harmonique d’une série de solvants organiques met tout d’abord en évidence la nécessité d’utiliser des méthodes théoriques intégrant les effets de la corrélation électronique, les effets d’environnement, ainsi que des bases très étendues pour évaluer quantitativement les premières hyperpolarisabilités moléculaires. La décomposition du tenseur de première hyperpolarisabilité en contributions dipolaires et octupolaires via le formalisme des harmoniques sphériques permet également d’illustrer l’effet de la symétrie des composés sur leurs réponses ONL. La seconde étude porte sur la caractérisation des effets de solvant sur les propriétés ONL d’un dérivé anil présentant un équilibre entre une forme énol et une forme cétone. Il est démontré que l’équilibre énol/cétone peut être déplacé vers la forme cétone en augmentant la proportion d’éthanol dans le mélange binaire de solvants éthanol/cyclohexane, conduisant à une augmentation de l’intensité HRS du mélange. La dernière partie de la thèse propose une étude théorique systématique des effets de substitution sur les propriétés ONL de sels de flavylium, dans le but d’optimiser le contraste des réponses ONL entre les différentes formes des composés. Il est démontré qu’une combinaison optimale de substituants permet d’obtenir de forts contrastes, démontrant le potentiel de ces composés comme systèmes multi-états pour des applications ONL. / This PhD addresses the nonlinear optical (NLO) properties of the reference organic molecules and molecular switches, by means of quantum chemical calculations and Hyper-Rayleigh Scattering (HRS) measurements. The thorough investigation of the NLO responses of a series of organic solvents first evidences the need of using theoretical methods including environment and electron correlation effects, as well as highly extended basis sets to quantitatively evaluate the molecular first hyperpolarizabilities. Decomposing the first hyperpolarizability tensor into dipolar and octupolar contributions via the spherical harmonics formalism also highlights the impact of the molecular symmetry on the NLO responses. The second part addresses the solvent effects on the NLO properties of an anil derivative, which exhibits an equilibrium between a enol and a keto form. It is shown that the enol/keto equilibrium is shifted towards the keto form when increasing the ethanol ratio in ethanol/cyclohexane solvent mixtures, leading to an increase of the HRS intensity. The last part addresses the effects of the chemical substituents on the NLO properties of flavylium salts, with the aim of optimizing the NLO contrasts between the various forms of the compounds. It is found that optimal combinations of chemical substituents lead to high NLO contrats, which demonstrates the potential of these compounds as multi-states systems for NLO applications

Les interrupteurs moléculaires sont des composés capables de commuterréversiblement entre deux ou plusieurs états stables en réponse à un stimulusextérieur. Lʼobjectif de la thèse est la conception dʼinterrupteurs moléculairesprésentant des contrastes optiques non-linéaires (ONL) et la mise évidence desparamètres structuraux et électroniques menant à dʼimportants contrastes depremière hyperpolarisabilité (β) via une approche multidisciplinaire qui combine lasynthèse de nouveaux composés, la caractérisation de leurs réponses optiqueslinéaires et non-linéaires par spectroscopie dʼabsorption UV-visible et par diffusionhyper-Rayleigh et lʼutilisation des méthodes de la chimie théorique afin de prédire etdʼinterpréter les propriétés moléculaires. Ces phénomènes de commutationsréversibles et les changements de propriétés qui les accompagnent présentent denombreux intérêts, tant technologiques comme lʼélaboration dʼordinateursmoléculaires quʼau niveau des organismes vivants où de nombreuses fonctionsbiologiques sont basées sur un phénomène de commutation. Les principaux résultatsde nos travaux se situent au niveau de lʼinterprétation des réponses ONL et de leurscontrastes en fonction de la nature, de la position et du caractère donneur/accepteurdes substituants présents sur le squelette des interrupteurs moléculaires. / Molecular switches are compounds presenting the ability to commutereversibly between two or more states in response to external stimuli. The goal of thework is the design of molecular switches exhibiting contrasts of their second-ordernonlinear optical (NLO) properties and the highlight of the structural and electronicparameters leading to large contrasts of first hyperpolarizability (β) via amultidisciplinary approach combining the synthesis of new compounds, thecharacterization of their linear (by UV-Visible absorption spectroscopy) and nonlinearoptical properties (by hyper Rayleigh scattering), and the theoretical simulations inorder to predict and interpret molecular properties. These reversible switchingprocesses and the resulting variations of molecular properties have many interests intechnological area such as the development of molecular computers or in lifesciences since many biological functions are based on commutation mechanisms.The major results of our investigations are the interpretation of the NLO responsesand contrasts as a function of the nature, the positioning, and the donor/acceptorcharacter of the substituents.

This thesis provides an extensive study of the first molecular hyperpolarizability b of charge-transfer chromophores using hyper-Rayleigh scattering (HRS). The charge-transfer chromophores used in present work involve the tricyanohydrofuran¡]TCF¡^group as an electron acceptor, and/or thiophene in the pi-electron bridge. TCF is a very strong electron acceptor and thiophene greatly lowers the resonance energy. Their presence significantly increases the beta value of the chromophore, therefore enhancing potentials in applications. In hyper-Rayleigh scattering experiments, the laser radiation with tunable wavelengths is used as an excitation source for measuring the frequency dependence of beta. The experiment shows beta exhibiting a significant dispersion in the two-photon resonance region. Using the linear absorption spectrum in coordination with theory, we show that it is possible to use Kramers-Kronig (K-K) transform to reproduce the experimental beta value in the two-photon resonance region. The K-K approach provides an extension to the conventional Oudar-Chemla equation, which is invalid in the spectral region in which two-photon resonance occurs. Using the new approach, it is shown that reliable values of intrinsic hyperpolarizabilities beta_zero of charge-transfer chromophores can be extracted. The coordination of beta_zero with molecular structure provides one with an insight for the origin of the enhancement of the first molecular hyperpolarizability of charge-transfer chromophores. This thesis examines the variation of beta_zero with molecular structure. The same technique is also applied to a dendrimer that has charge-transfer nonlinear optical chromophores incorporated in the dendritic structure. The measured frequency dependent hyperpolarizability of the dendrimer is compared with that calculated from the linear absorption spectrum by the KK transform technique. The intrinsic hyperpolarizability beta_zero of the dendrimer obtained is compared with that of the single chromophore having a structure similar to that incorporated in the dendrimer. The comparison shows that the 3D dendritic structure is effective in reducing the interaction between chromophores by providing sufficient space between them, hence avoiding the possibility of aggregation formation due to attractive interactions between chromophores. The topic of two-photon fluorescence (TPF), which is related to HRS, is also investigated. The intensity of TPF is generally proportional to the square of the incident excitation intensity. Careful measurements of the TPF intensity of a nonlinear optical chromophore in conjunction with required auxiliary parameters have been used as a technique for determining the two-photon absorption cross-section. The TPF intensity measurement carried out in this thesis uses a variety of intensities. At low intensity excitation, the TPF intensity follows the usual quadratic intensity law (QIL), whereas deviations from the QIL are observed at higher incident intensities. The observation of similar lineshape associated with one- and two-photon fluorescence spectra suggests a 3-level model for the description of TPF excited by the incident intensity at various strengths. It is shown that by fitting the observed TPF intensity to an equation developed from the three-level model, it is possible to deduce the two-photon absorption cross section of the nonlinear optical chromophore in solution. The new technique developed using the three-level model is tested on a Rhodamine B/Chloroform solution. The two-photon absorption cross-section obtained by using the new technique is found in agreement with that reported in the literature. Having demonstrated the suitability of the new technique, it is used to determine the two-photon absorption cross-section of a novel nonlinear optical chromophore. The two-photon absorption cross-section using the new technique is then compared with that obtained by the nonlinear transmittance method. The two results are in good agreement, indicating the applicability of the new technique. The new technique is more convenient than the conventional low excitation TPF method as it does not require various auxiliary parameters, some of them are difficult to obtain. The second harmonic generation (SHG) of a chromophore/polymer film which is optically poled by using a coherent superposition of a fundamental and its second harmonic beams. The growth rate of the SHG intensity is found to be proportional to the fourth power of the incident intensity of the fundamental beam, and the plateau intensity SHG is proportional to the square of the incident intensity. These observations are not in agreement with the published theory. While the reason for disagreement is yet to be clarified, the information obtained is useful for the development of nonlinear optical devices.

In recent years, coinage metal nanoparticles have emerged as materials with largest quadratic optical nonlinearity. Their first hyperpolarizabilities (β) are very high (105-106 x 10-30 esu) but such large values were quite unexpected because of their apparently centrosymmetric bulk structure. Only a small second harmonic generation (SHG) from coinage metal nanoparticles is expected through higher order multipolar (e.g., quadrupolar) polarization mechanisms. Various possible reasons have been attributed to the observation of large β values in coinage metal nanoparticles. They are: 1) Particles may not be overall centrosymmetric (as appears from the TEM pictures) which, in turn, can make SHG electric dipole allowed, 2) Several polarization mechanisms (dipolar, quadrupolar, retardation, etc.) may be operating simultaneously to render SHG very efficient, 3) SHG can be resonance enhanced if the incident or SH photons fall within the surface plasmon resonance (SPR) absorption bands or higher energy interband transitions in the metal particles, and 4) Surface capping agents used for stabilization of the nanoparticles in solution alter the SH response. It is, therefore, important to experimentally find out which of the above mentioned possibilities are dominant and under what conditions we can identify the contribution of various mechanisms to the overall SHG response of the coinage metal nanoparticles. In this thesis work, the origin of SHG from copper (one of the coinage metals) nanoparticles has been investigated using hyper-Rayleigh scattering (HRS). In chapter 1, an introduction to metal nanoparticles and their optical properties have been presented. A general introduction to second order nonlinear optics and various methods for the determination of first hyperpolarizability are provided. A literature survey on the second order NLO properties of metal nanoparticles is also done. At the end of the chapter, the motivation of the work done is outlined. In chapter 2, the experimental set-ups for unpolarized and polarization resolved hyper-Rayleigh scattering (HRS) measurements at different wavelengths are described. Generation of IR wavelength of 1543 and 1907 nm using stimulated Raman scattering in gases have been presented in this chapter. In chapter 3, synthesis and characterization of copper nanoparticles are described. Four different size copper nanoparticles (5, 9, 25, and 55 nm) were prepared by laser ablation. Size dependencies of first hyperpolarizability were investigated at different wavelengths and it was found that β increases with increasing size of the particle and that the SHG originates mainly from the surface of the particle. Dispersion in first hyperpolarizabilities of the copper nanoparticles has also been investigated and we find that at incident and SH wavelengths far from the SPR absorption band, the hyperpolarizability is large compared to molecular hyperpolarizabilities. In chapter 4, the results of polarization resolved HRS measurements on copper nanoparticles of five different sizes at four different wavelengths (738, 1064, 1543 and 1907 nm) are reported. Polarization analyses show that at small particle size to wavelength (d/λ) ratio the dipolar contribution to SHG is dominant whereas the quadrupolar and retardation effects become important at larger d/λ values. The “small particle limit” in the SHG from coinage metal nanoparticles has been assessed based on our results on copper and others’ results on silver and gold nanoparticles. In chapter 5, the effect of surface capping on the first hyperpolarizability of copper nanoparticles is investigated. Polyvinyl pyrrolidone (PVP) has been used as a capping agent. The results obtained for bare and capped copper nanoparticles show that capping enhances the hyperpolarizability by a factor of 2. In the last chapter 6, general conclusions drawn on SHG from coinage metal nanoparticles based on this work are presented along with future perspectives.

Ce manuscrit décrit le processus optique non linéaire de Génération de Second Harmonique (acronyme anglais SHG pour Second Harmonic Generation) réalisé en phase liquide. En particulier, la propriété de cohérence de ce processus est étudiée en détail. En effet, en raison de la parité du processus SHG, cette cohérence est perdue dans les liquides. Ces études portent ainsi sur plusieurs géométries afin d’accéder aux réponses de volume et de surface. Les avantages d’une méthode combinée sont aussi discutés. Dans une première partie, la configuration classique de la Diffusion Hyper Rayleigh (acronyme anglais HRS pour Hyper Rayleigh Scattering ou SHS Second harmonic scattering) est utilisée pour l’étude de systèmes simples comme les solvants purs. Cette étude porte en particulier sur les méthodes de normalisation des hyperpolarisabilités moléculaires. Une voie alternative est proposée basée sur l’introduction d’une section efficace HRS ou SHS. Par la suite, les effets liés aux ordres non linéaires supérieurs sont démontrés et interprétés, ceux-ci pouvant altérer les valeurs absolues qui peuvent être mesurées.La seconde partie porte sur l’introduction de plusieurs améliorations dans les mesures HRS ou SHS. En particulier, l’évolution vers des géométries non conventionnelles s’éloignant de la géométrie standard ainsi que sur des évolutions permettant d’accéder aux aspects dynamiques sont proposées. Des expériences sont réalisées sur des systèmes simples comme les solvants purs, des composés moléculaires et des nanoparticules, tous présentant des spécificités propres. Cette partie finit sur l’exploration des systèmes aléatoires diffusants et les problèmes associés. Enfin, dans une dernière partie, la réponse HRS ou SHS de solutions aqueuses de sels est discutée. Les mesures portent plus particulièrement sur une étude comparée des aspects cohérents et incohérents et les propriétés qu’il est possible de mesurer dans ces systèmes / This manuscript describes the nonlinear optical process of Second Harmonic Generation (SHG acronym for Second Harmonic Generation) carried out in liquid phase. In particular, this process coherence property is studied in detail. Indeed, due to the parity of the SHG process, this coherence is lost in liquids. These studies covers several geometries to access the volume and surface responses. The benefits of a combined method are also discussed. In the first part, the typical configuration of the Hyper Rayleigh Scattering(HRS or SHS for Second harmonic scattering) is used for the study of simple systems like pure solvents. This particular study focuses on methods of standardization of molecular hyperpolarizabilities. An alternative route is proposed based on the introduction of a HRS or SHS cross. Subsequently, the effects of the higher nonlinear orders are demonstrated and interpreted, they can alter the absolute values that can be measured.The second part deals with the introduction of several improvements in the HRS or SHS measurement. In particular, the trend towards unconventional geometries away from the standard geometry as well as developments for accessing dynamic aspects are proposed. Experiments are performed on simple systems such as pure solvents, molecular compounds and the nanoparticles, all presenting specificities. This part ends on exploring the random scattering systems and associated problems. Finally, in the last part, the HRS or SHS response of aqueous salt solutions is discussed. The measures focus specifically on a comparative study of coherent and incoherent aspects and properties that can be measured in these systems

Le travail présenté dans ce mémoire de thèse avait pour but de synthétiser de nouveaux composés pour des applications en imagerie médicale. La première partie porte sur la synthèse de ligands hétéroleptiques pour la chélation de deux métaux différents en vue d’une utilisation dans deux types d’imagerie. Pour cela, nous avons d’abord synthétisé des porphyrines. Ces molécules sont connues pour la chélation de nombreux métaux de transition et notamment le cuivre, dont l’isotope cuivre-64 est un radioémetteur β+ utilisable en Tomographie par Émission de Positron (TEP). Ces porphyrines ont été couplées à un dérivé du DOTA, molécule connue pour son application, après métallation avec du gadolinium, en Imagerie par Résonance Magnétique (IRM). Ces composés possèdent des valeurs de relaxivité encourageantes pour une application en IRM. Enfin, des biomolécules ont été modifiées afin de vectoriser nos composés. La seconde partie porte sur la synthèse de composés pour l’imagerie médicale par optique non-linéaire. Nous avons dans un premier temps synthétisé des porphyrines amphiphiles et zwiterrioniques pour la Génération de Second Harmonique (GSH). Leur efficacité a été mesurée et celle-ci est suffisante pour pouvoir envisager l’obtention d’images. Dans un second temps, nous avons travaillé sur la synthèse d’un composé pour une application en imagerie biphotonique et en IRM. Pour cela, nous avons relié un dérivé du DOTA avec un fluorophore de type DPP. Le composé final a été métallé avec du gadolinium et sa relaxivité est supérieure à celle du DOTA(Gd). / The goal of my PhD studies was to synthesize new compounds for possible medical imaging applications. The first part of my thesis focused on the synthesis of heteroleptic ligands to achieve the chelation of two different metals aimed at addressing two types of medical imaging. We first synthesized porphyrins, which are well-known for the chelation of numerous transition metals. We focused on copper, whose copper-64 isotope is a β+ emitter usable in Positron Emission Tomography (PET). These porphyrins have been coupled with a DOTA derivative. This molecule, metallated with gadolinium, is well-known in Magnetic Resonance Imaging (MRI). Our compounds display encouraging relaxivities for MRI applications. At last, these molecular probes have been conjugated to a few biomolecules in order to vectorize our compounds. The second part of this work is related to the synthesis of fluorophores for nonlinear optical imaging. We first synthesized amphiphilic zwitterionic porphyrins for Second Harmonic Generation (SHG). The efficiency of these compounds has been measured and was satisying enough to consider the possibility to perform imaging studies. We also worked on the synthesis of compounds for Two Photon Emission Fluorescence (TPEF) imaging and MRI. We have linked a DOTA derivative with a diketopyrrolopyrrole (DPP). This conjugate has been metallated with gadolinium and its relaxivity has been measured. Interestingly, this value is superior to that of DOTA(Gd).

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 (β) mostraram que moléculas quadrupolares apresentam maior β 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 β 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 (β) shown that the quadrupolar molecules present higher β than the dipolar and octopolar, suggesting that the electronic coupling between the branches strongly contribute to the 2PA cross-section and β in quadrupolar molecules, being not relevant in the octopolar molecules. All results were interpreted through theoretical studies based on the density functional theory (DFT).

The knowledge of geometry of molecular complexes formed via molecular association in solution through weak interactions is always important to understand the origin of stability and function of an array of molecules, supramolecular assemblies, and macromolecular networks. Simple 1:1 molecular complexes are very useful in this regard as they provide a model to understand both the nature of these interactions and their structural implications. Several weak noncovalent forces from long range (van der Waal’s, electrostatic, induction, dispersion) to short range (charge transfer) govern the geometry, that is, relative orientation of the two molecules in such a complex. On one hand, we find 1:1 electron donor acceptor (EDA) complexes such as naphthalene-tetracyanobenzene, hexamethylbenzene-chloranil etc. which stack parallel or in slipped parallel geometry in their crystals. On the other, benzene dimer has been found to stabilize in T shaped geometry in all its three physical states. In this thesis, I focus on 1:1 EDA complexes in solution. A good volume of literature is available which deals with the optical studies on the formation of such complexes. It has been suggested that the nature of the intermolecular interactions stabilizing these complexes in the gas phase or in their crystals is modified by the presence of solvent-solute interactions in solution thus bringing in difference in the solution geometry. However, the existing experimental techniques, both optical and magnetic, are unable to determine the exact geometries of 1:1 EDA complexes in solution. This opens an opportunity to probe their geometry in solution. The quadratic nonlinearity or first hyperpolarizability (β) of a molecule is a measure of the change in dipole moment (or polarization) in the second order of the applied electrical field and thus has a purely electronic origin. It is a tensorial property and can be resolved in components along the three dimensions. The number of β components and the nonlinear optical anisotropies in a typical donor-acceptor type dipolar molecule, defined as (equation) (where1, 2, 3 axes define the molecular frame, 1 being the direction along the principal axis of symmetry and pointing from the acceptor toward the donor), are determined by the symmetry /structure of the molecule. It has been shown theoretically that the 1:1 EDA complexes possess large hyperpolarizabilities. In the case of pNA dimers calculation revealed that the geometry of the dimer and its symmetry is important for obtaining the correct estimate of β from its tensorial components. Therefore, it should be possible to use the values of tensorial β components to construct the unknown geometry of such complexes. Experimentally macroscopic depolarization ratios (D and D′) in the laboratory fixed frame (XYZ, X being the direction of polarization and Z the direction of propagation of the incident light), are measured from the polarization resolved intensities of second harmonic scattering from molecules in solution using the hyper-Rayleigh scattering technique. The depolarization ratios are correlated to the anisotropy parameters, u and v through a co-ordinate transformation. In this thesis I, have first, characterized the quadratic nonlinear optical property of a variety of 1:1 electron donor acceptor complexes and used the values of u and v obtained from depolarized hyper-Rayleigh scattering to deduce their geometry in solution. Chapter 1 provides an introduction to the 1:1 electron donor acceptor complexes, their relevance to chemistry and biology. It also contains an introduction to nonlinear optical processes in molecules. The objective of the present work and scope of the investigation carried out in this thesis is presented in this chapter. Chapter 2 describes the details of the experimental polarization resolved HRS technique. The geometrical model adopted for the analysis of the HRS data has also been introduced and the method of analysis has been described in detail in this chapter. Chapter 3 presents the measurement of β values of two series of 1:1 EDA complexes of variously substituted methylbenzenes donors with tetrachloro-p-benzoquinone (CHL) and dicyanodichloro-p-benzoquinone (DDQ) acceptors at 1064 nm. In agreement with recent theoretical results we find large first hyperpolarizabilities for these complexes. The β values are greater than that of the typical push-pull molecule p-nitroaniline (pNA). We also find that in general β decreases with decrease in the donor strength. Chapter 4 presents the β values for the two series of EDA complexes of CHL and DDQ acceptors at 1907 nm. The values of β are less in magnitude at 1907 nm than that at 1064 nm which is due to the dispersion effect in β. In Chapter 5 and 6, it is described how depolarized hyper-Rayleigh scattering can be utilized to probe geometries of 1:1 complexes in solution. Chapter 5 concentrates mainly on 1:1 EDA complexes of CHL and DDQ and TCNB (tetracyanobenzene), while chapter 6 contains examples of other 1:1 molecular complexes where the noncovalent interactions are much weaker, such as in benzene-naphthalene, benzene-methoxybenzene, benzene-hexafluorobenzene and benzene-chlorobenzene pairs. We find the geometry of 1:1 EDA complexes in solution in terms of tilt angle (θ) and twist angle (ϕ) between the donor and acceptor pairs. The angle θ varies from 29°-47° for different pairs of EDA complexes, while ϕ varies within 34° and 38°. We find that the geometry of 1:1 EDA complexes in solution is different (twisted and tilted cofacial and twisted ‘V’) from those in the crystalline or gaseous states (cofacial), if known. We find that both benzene-naphthalene and benzene-chlorobenzene pairs assume twisted ‘T’ shape geometry with θ = 82° and 85°, respectively, and φ = 38°, while benzene-hexafluorobenzene assumes a twisted ‘V’ shape. A strong solvent effect is seen in the geometry of the benzene- methoxybenzene complex. The tilt angle is 55° when chloroform is used as a solvent and it is 82° without chloroform. Chapter 7 is the concluding chapter where the main work done in this thesis is summarized and future directions are presented.