Organic π-conjugated molecules and materials with large real parts and small imaginary parts of the third-order polarizability are of great interest for all-optical switching applications. In this dissertation, we use quantum-chemical and molecular-dynamics approaches to investigate the structure-property relationships that influence the nonlinear optical properties of π-conjugated molecules and materials. We begin with an overview of nonlinear optics, focusing in particular on the electronic properties of linear π-conjugated systems and some of the important problems that have limited device applications of these molecules to date. This is followed by a brief review of the computational methods employed in these studies. We then turn to the main results of the dissertation. Chapter 3 describes the structural dependence of the transition dipole moment between the first two polymethine excited states. Chapter 4 discusses the relationship between BLA, which depends on the geometric structure, and BOA, which probes electronic structure. Chapter 5 describes the benchmarking of computational methods to describe the symmetry-breaking of long polymethines and preliminary evidence regarding the role of vibrational modes in symmetry-breaking. Chapter 6 explains the negative third-order polarizability of tetraphenylphosphate and analogous systems. Chapter 7 focuses on molecular-dynamics studies of polymethine aggregation, particularly the relationships between chemical structure and the geometric and electronic structures of aggregates. Finally, Chapter 8 provides a synopsis of the work and discussion of further directions.

Material development is necessary before all-optical signal processing (AOSP) can be realized. Traditional AOSP will require materials with a large magnitude of the real part of the third-order polarizability, while having a small imaginary magnitude of third-order polarizability. The aim of this thesis is to investigate the potential for polymethine dyes to be used for AOSP applications. The basic structure of a polymethine dye was synthetically modified in a variety of positions to observe the effects upon the linear and nonlinear optical properties. The modifications included variation in the terminal group, substituents in the polymethine bridge, and exchange of the counterion. The dyes were generally synthesized using the Vilsmeyer-Haack reaction to form simple polymethine precursors, and then complexity was added by performing the Knoevenagel condensation with various acceptors and the polymethine precursor. Ion metathesis was often employed to exchange the initial counterion for a counterion that provided increased solubility in common organic solvents. The third-order nonlinear optical polarizabilities were characterized by Dr. Joel Hales who used the open- and closed-aperture Z-scan technique at 1300 nm and non-degenerate two-photon absorption experiments to identify the position of the two-photon absorption bands.

L’objectif de cette thèse consistait à développer des colorants de la famille des polyméthines absorbant dans le proche infrarouge et présentant des propriétés d’optique non-linéaire (ONL) prononcées. De nouveaux chromophores ont été préparés et leurs propriétés spectroscopiques ont été systématiquement étudiées afin d’établir des relations structures-propriétés. Cette stratégie a permis l’identification des facteurs permettant d’exalter la réponse ONL des polyméthines et de prédire leur comportement face à de subtiles modifications structurales. Trois familles de colorants ont été préparées : des monométhines aza-dipyrrométhènes de bore, des heptaméthines symétriques ainsi que des heptaméthines dissymétriques.Dans un premier temps, des aza-bodipys possédant des groupements donneurs périphériques sont synthétisés. Une étude spectroscopique approfondie permet d’identifier les structures présentant une absorption à deux photons importante en vue d’applications en limitation optique aux longueurs d’onde des télécommunications (1500 nm). Le chapitre suivant a pour but de rationaliser les équilibres de la limite cyanine. L’étude de la substitution centrale de nombreux dérivés heptaméthines a permis de moduler leurs propriétés optiques sur une large gamme spectrale et de mettre en évidence une nouvelle structure électronique limite de type bis-dipôle. Parmi cette famille de colorants, des dérivés keto-heptaméthines ont été utilisées en imagerie biologique (cellules et cerveau) par microscopie non-linéaire. Enfin, l’élaboration de structures heptaméthines dissymétriques originales est détaillée dans le dernier chapitre. Ces chromophores ont révélé des propriétés ONL du second ordre idéales pour des applications en modulation électro-optique. / The aim of this thesis consisted in developing dyes from the polymethine family absorbing in the near-infrared (NIR) region and featuring pronounced nonlinear optical (NLO) properties. Several new chromophores have been prepared and their spectroscopic properties have been systematically investigated in order to establish structure-properties relationships. This strategy has permitted the identification of the factors that enhance the NLO response of polymethines and allowed the prediction of their optical behaviour depending on subtle structural modifications. Three families of dyes have been prepared and investigated: boron aza-dipyrro-monomethines (aza-bodipys), symmetrical heptamethines and unsymmetrical heptamethines.In a first time, aza-bodipys possessing electro-donating peripherical substituants has been synthesized. A detailed spectroscopic study allow the identification of structures that present a high two-photon absorption (TPA), towards optical limiting applications at telecommunication wavelengths (1500 nm). The next chapter rationalizes the equilibrium surrounding the cyanine limit. The study of the central substitution of several heptamethines derivatives permitted the fine tuning of their optical properties upon a large spectral region and evidenced a new bis-dipolar electronic structure. Within this dye family, keto heptamethine derivatives has been tested in bio-imaging experiments (living cells and brain) using non-linear microscopy. Finally, the elaboration of original unsymmetrical heptamethines is detailed in the last chapter. These chromophores revealed ideal second order NLO properties for applications in electro-optic modulation.