No description available.

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 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.