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.

Lors de ce travail de thèse, nous avons effectué des recherches sur des chromophores organiques. Notre étude s’est portée sur la commutation des propriétés optiques linéaires et non linéaires via l’étude d’interrupteurs organiques multi-adressables. Notre étude s’est portée plus particulièrement sur la famille des dérivés de l’indolino-oxazolidine. Ce type de molécule présente l’avantage d’avoir deux voix de commutations distinctes par photochromisme et acidochromisme. Nous avons recherché le lien entre la structure des molécules et leurs propriétés optiques par une étude combinée originale théorie/expérience avec l’utilisation de calculs quantiques sophistiqués et par diffusion hyper-Rayleigh de la lumière. / In this work we have investigated the commutation of linear and non linear optical properties in organic multifunctional switches. Indolino-oxazolidine compounds and their derivatives were chosen as example of multifunctional systems, because they combine both acidochromic and photochromic properties. The relationships between the molecular structure of these compounds and their optical properties were established by means of high level the quantum chemical approach and hyper-Rayleigh Scattering of light approaches.

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.

This thesis deals with the investigation of the first hyperpolarizabilities (β) of a large number of molecules linked to other molecules either covalently or noncovalently. Chapter 1 gives a brief introduction to supramolecular chemistry and Nonlinear Optics (NLO). A survey of literature pertinent to noncovalently interacting supramolecular assembly and their NLO properties as well as NLO properties of oligomeric systems has been presented. The scope of the present investigation has been described at the end of the chapter. Chapter 2 discusses all the methods used in carrying out this thesis work. The first hyperpolarizabilities (β) of all the compounds have been measured by the hyper Rayleigh scattering (HRS) technique; the experimental details of which are written in this chapter. Various spectroscopic techniques such as NMR, IR, UV-Vis, etc. that were used in the investigation have been presented. The subsequent chapters 3-5 deal with the actual results obtained in this work. In chapter 3 first hyperpolarizabilities of o-, m-, and p-aminobenzoic acids and their oligomers viz., dimer, trimer and tetramer (covalently linked) have been studied. The compounds are synthesized and characterized by various spectroscopic methods and their β values have been measured by HRS. The hyperpolarizability increases in going from the monomer to the dimer but decreases subsequently from the dimer to the trimer to the tetramer. This unexpected trend in β has been attributed to the formation of molecular aggregates in the trimers and tetramers. Further evidences of aggregation come from the results of1H NMR spectroscopy and conductivity measurements. In chapter 4, synthesis, characterization and HRS investigation to probe the formation, dissociation and binding constants of hydrogen bonded supramolecular complexes (noncovalent interaction) formed in solution between 6-amino-2-(pivaloylamino)pyridine and ferrocene functionalized barbituric acid and 5-methoxy-N,N′-bis(6-amino-2-pyridinyl)-1,3-benzenedicarboxamide and ferrocenyl barbituric acid have been described. From the HRS data the stoichiometry of the supramolecular complexes has been determined and compared to that from the NMR data. Some of the complex stoichiometries that are measured by HRS have not been seen in the NMR data and vice versa. The results have been rationalized in terms of the strengths and weaknesses of various spectroscopic methods as applied to this problem. Many fold increase in the β value has been realized in the supramolecular complex formation process. Depolarized HRS experiments have been carried out to obtain structural information on the complexes. In the last chapter the synthesis, characterization and measurements on the first hyperpolarizabilities of unsubstituted tetraphenylporphyrin and its metallated complexes have been presented. Synthesis of supramolecular complexes of ferrocenyl barbituric acid with functionalized porphyrin compounds has been carried out although the amount of the final complex was insufficient for HRS measurements. This chapter ends with a perspective for the future work in the direction.

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.

Under the electric dipole approximation, second harmonic of the incident light is scattered by a collection of randomly oriented molecular dipoles in solution due to instantaneous orientational fluctuation which is directional. If two such dipoles are correlated in space through intermolecular or other interactions, the intensity of the second harmonic scattered light (SHSL) will be related to the extent of such interactions. If two dipoles are arranged in a particular geometry by design, the geometry will determine the intensity of the SHSL. If a molecule has no dipole moment, the intensity of the SHSL will be less and is only allowed by higher order electric multipoles. If two such zero-dipole molecules interact with each other and transfer some amount of electronic charge from one to the other, the induced dipole moment will give rise to an enhanced SHSL. However, along with the direction of the dipole moment from the donor to the acceptor, the actual geometry of such molecular dimer/complex should also play an important role to determine the nature of the SHSL response. If all the isotropic nonzero components of first hyperpolarizability (β) are taken into account, from the measurement of β and related quantities such as depolarization ratios, in solution it should be possible to derive information about the geometry of the dimer/complex. This is precisely the motivation behind this thesis. Chapter 1 gives a brief introduction of 1:1 charge transfer (CT) complexes between a donor and an acceptor and their importance in chemistry. It also contains an introduction to nonlinear optics, various spectroscopic techniques to characterize CT complexes, etc. The motivation of extracting the geometry of such complexes from hyper-Rayleigh scattering (HRS) measurements in solution is presented in this chapter. In Chapter 2, all the experimental details of the unpolarized and polarization resolved HRS measurements at various excitation wavelengths have been described. Generation of infrared wavelengths (1543 nm and 1907 nm) using stimulated Raman scattering in gases have also been discussed. In Chapter 3, the first hyperpolarizability (βHRS) for two series of 1:1 molecular complexes between methyl substituted benzene donors with tetrachloro-p-benzoquinone (CHL) and dicyanodichloro-p-benzoquinone (DDQ) acceptors in solution at 1543 nm have been presented. Enhancement of βHRS due to charge transfer from the donor to the acceptor molecule which was predicted theoretically has been verified. Using linearly (electric field vector along X direction) and circularly polarized incident light, respectively, two macroscopic depolarization ratios D = I2ω,X,X/I2ω,Z,X and D' = I2ω,X,C/I2ω,Z,C in the laboratory fixed XYZ frame by detecting the SHSL in a polarization resolved fashion have been measured. The experimentally obtained first hyperpolarizability (βHRS), D and D' values, are then matched with the theoretically calculated values from single and double configuration interaction calculations using the Zerner’s intermediate neglect of differential overlap and the self-consistent reaction field (ZINDO–SDCI– SCRF) approach by adjusting the geometrical parameters. It has been found that in most of the CT complexes studied here, there exists a significant twist in the equilibrium geometry at room temperature which is not a simple slipped parallel geometry as was believed. In chapter 4, the βHRS, D and D' values of 1:1 pyridine (PY)-chloranil (CHL) complex at 1064 nm have been described. Previous theoretical studies have shown that there is a tilt angle of 77.9 degree in the gas phase PY-CHL complex. In this chapter, this prediction about the geometry of 1:1 PY-CHL complex has been probed. The experimentally found βHRS, D and D' are matched well with theoretically calculated values, using ZINDO–SDCI–SCRF, for a cofacial geometry of PY-CHL complex in solution indicating that the solution geometry is different from the gas phase geometry. In Chapter 5, the βHRS, D and D' for a series of 1:1 complexes of tropyliumtetrafluoroborate and methyl-substituted benzenes in solution at 1064 nm have been reported. The measured D and D' values vary from 1.36 to 1.46 and 1.62 to 1.72, respectively and are much lower than the values expected from a typical sandwich or a T-shaped geometry. The lowering in D and D' indicates that these complexes have higher symmetry than C2v. The value of D close to 1.5 indicates there is a significant octupolar contribution in such complexes. In order to probe it further, βHRS, D and D' were computed using the ZINDO-SDCI-SCRF technique in the presence of BF4-anion. By arranging the three BF4-ions in a C3 symmetry around the complex in such a way that electrical neutrality is maintained, the computed values are brought to agreement with experiments. This unprecedented influence of the anion on the HRS, D and D' values of these complexes are discussed in this chapter. In Chapter 6, the effect of dipolar interactions, within a multichromophoric system, on the second order nonlinear optical properties have been studied. It has been found that the βHRS response of the multichromophoric system is always larger than expected for uncorrelated chromophores demonstrating that the dipole moment of individual chromophores are not merely additive within the multichromophoric system but contribute cooperatively to the SHSL signal. Also the relative orientation and nature of the chromophores and the angle of interaction between them alter the HRS values. Chapter 7 is the concluding chapter in which all the work done in the thesis has been summarized and future direction has been proposed.

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

No description available.

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.