The studies presented in this thesis explore energy transfer and relaxation in excitonic systems using two types of third-order non-linear spectroscopy, transient grating (TG) and two-dimensional photon echo (2DPE). Two major excitonic systems are studied, colloidal CdSe nanocrystals and a photosynthetic system, the light harvesting antenna protein PE545 isolated from cryptophyte algae Rhodomonas CS24. The inhomogeneous broadening present in colloidal samples has long been an obstacle to the study of the fine structure of CdSe nanocrystals. This thesis will present two studies, each using a different spectroscopy to overcome this problem and gain insight into the fine structure of the ground state exciton and biexciton. In both cases, polarization dependent measurements are used to isolate particular types of pathways through the fine structure. Polarization dependent TG measurements and a detailed kinetic model have allowed us to identify major pathways for excitonic relaxation within the fine structure of the ground state exciton, despite the inhomogeneous broadening present in the sample. Conclusions from this work include an analogy to the internal conversion and intersystem crossing transitions in molecules, concepts which are familiar to most chemists. Polarization dependent 2DPE experiments and a simulation of the resulting spectra have allowed for the direct observation of the ground state biexcitonic fine structure. The high density of states in the spectral region of the biexciton obscures these states in most spectroscopies, but the combination of polarization dependence and the use of a two-dimensional technique have revealed these elusive fine structure states. This 2DPE technique is then used to inspect light harvesting antenna protein PE545, a system of eight chromophores covalently bonded to a protein scaffold. The 2DPE measurement is capable of identifying oscillatory components of the signal which are signatures of coherent energy sharing. Using this technique, coherent energy sharing is observed throughout the spectrum of the protein at ambient temperatures, and control experiments suggest that the observed coherences are electronic, not vibrational. A short study is also presented which compares the utility of two approximations for calculations of electronic coupling and energy transfer in elongated excitonic systems.

The studies presented in this thesis explore energy transfer and relaxation in excitonic systems using two types of third-order non-linear spectroscopy, transient grating (TG) and two-dimensional photon echo (2DPE). Two major excitonic systems are studied, colloidal CdSe nanocrystals and a photosynthetic system, the light harvesting antenna protein PE545 isolated from cryptophyte algae Rhodomonas CS24. The inhomogeneous broadening present in colloidal samples has long been an obstacle to the study of the fine structure of CdSe nanocrystals. This thesis will present two studies, each using a different spectroscopy to overcome this problem and gain insight into the fine structure of the ground state exciton and biexciton. In both cases, polarization dependent measurements are used to isolate particular types of pathways through the fine structure. Polarization dependent TG measurements and a detailed kinetic model have allowed us to identify major pathways for excitonic relaxation within the fine structure of the ground state exciton, despite the inhomogeneous broadening present in the sample. Conclusions from this work include an analogy to the internal conversion and intersystem crossing transitions in molecules, concepts which are familiar to most chemists. Polarization dependent 2DPE experiments and a simulation of the resulting spectra have allowed for the direct observation of the ground state biexcitonic fine structure. The high density of states in the spectral region of the biexciton obscures these states in most spectroscopies, but the combination of polarization dependence and the use of a two-dimensional technique have revealed these elusive fine structure states. This 2DPE technique is then used to inspect light harvesting antenna protein PE545, a system of eight chromophores covalently bonded to a protein scaffold. The 2DPE measurement is capable of identifying oscillatory components of the signal which are signatures of coherent energy sharing. Using this technique, coherent energy sharing is observed throughout the spectrum of the protein at ambient temperatures, and control experiments suggest that the observed coherences are electronic, not vibrational. A short study is also presented which compares the utility of two approximations for calculations of electronic coupling and energy transfer in elongated excitonic systems.

No description available.

The work summarizes basic theory of relaxation, energy transfer and decoher- ence in photosynthetic molecular aggregates described as open quantum systems and basic theory of third order coherent non-linear spectroscopy. The work presents two methods for calculation of photo-induced dynamics of molecular aggregates. The methods relax certain approximations of the theories commonly used to model the relaxation and energy transfer in the molecular systems on the sub-picosecond time scale. The first theory derived in the formalism of para- metric projection operators accounts for correlations in a second-order non-linear response-function that are usually neglected in the formalism of master equations. The second theory represents stochastic model of exact dynamics via the cumulant expansion. The work also presents an analysis of importance of the secular and the Markov approximations in the description of dynamics derived in the second-order perturbation theory in the system-bath coupling with emphasis on the excitonic coherence lifetime.

No description available.

No description available.

Dans cette thèse, les propriétés optiques de biomolécules importantes ont été étudiées en utilisant une approche théorique et, dans un cas, aussi expérimentale. La Théorie de la fonctionnelle de la densité (DFT) et la timedependent-DFT (TD-DFT) sont les principales méthodes de chimie quantique utilisées dans cette thèse. Plusieurs spectroscopies ont été étudiées (au niveau théorique et, dans certains cas, également au niveau expérimental) : absorption électronique linéaire (absorption à un photon, OPA) et non-linéaire (absorption à deux ou trois photons, TPA et 3PA), dichroïsme circulaire électronique (DCE) et spectroscopie de fluorescence. Les effets de l’environnement, particulièrement importants dans des systèmes biologiques, ont été pris en compte, pour les propriétés de l’état fondamental et des états excités en utilisant une méthode multi-échelles QM/MM appelée Polarizable Embedding (PE). L’échantillonnage des conformations a été pris en compte avec des simulations de dynamique moléculaire (MD) qui sont basées sur la mécanique classique. Deux thématiques ont été étudiées dans cette thèse : le cholestérol et le design in silico de ses analogues fluorescents ainsi que la caractérisation des coudes de type β dans différentes conformations grâce à la simulation des spectres DCE. La simulation de plus d’une spectroscopie a été importante dans l’étude des états excités du cholestérol dans des solutions organiques. Le design in silico a suggéré un nouveau stérol-polyénique (P-stérol) qui montre despropriétés optiques améliorées pour le mécanisme d’excitation à trois photons par rapport au déhydroergostérol (DHE), une sonde du cholestérol déjà très utilisée. Ce nouveau P-stérol a été suggéré pour la synthèse. L’étude des spectres de DCE des coudes β en différentes conformations a mené une double conclusion : même si deux allures de DCE pour les conformations des coudes β étudiées (4) ont été trouvées (dans la majorité des cas), la spectroscopie de DCE doit toujours être associée à d’autres techniques spectroscopiques dans la caractérisation en solution des coudes β. / In this thesis, the optical properties of important biomolecules were studied using a theoretical approach and, in one case, also an experimental one. Density Functional Theory (DFT) and time-dependent-DFT (TD-DFT), were the principal quantum chemical methods adopted in this thesis.Various spectroscopies were studied (theoretically and, in some cases, also experimentally) : linear (one-photon, OPA) and non-linear (two- and three-photon, TPA and 3PA) electronic absorption, electronic circular dichroism (ECD) and fluorescence spectroscopy. The environment effects, which are particularly important in biological systems, were taken into account, for both ground and excited states properties, using a multiscale QM/MM method called Polarizable Embedding (PE). The sampling of conformations was addressed by Molecular Dynamic (MD) simulations based on classical mechanics. Two topics were studied in this thesis: cholesterol and the in-silico design of its fluorescent analogues, and the characterization of β-turns in different conformations by simulations of their ECD spectra in aqueous solutions. The simulation of more than one spectroscopy resulted to be important when studying the electronic excited states of cholesterol in organic solutions. The in-silico design study suggested a novel polyene-sterol (P-sterol) which shows improved optical properties for the three-photon excitation mechanism with respect to dehydroergosterol (DHE), an already widely used cholesterol probe. This new P-sterol was thus suggested for synthesis. The achievement from the study of ECD spectra for different β-turn conformations is two-fold: even if two ECD patterns for the β-turn conformations studied (4) were found (in most of cases), ECD spectroscopy should be always associated with other spectroscopic techniques when trying to characterize the β-turn conformations in solutions.

Nous avons mis en place un dispositif permettant l’acquisition d'échos de photons stimulés infrarouges à l’échelle femtoseconde. Le but est d'examiner la dynamique vibrationnelle aux temps courts de métaux carbonyles (W(CO)₆ and Fe(CO)₅) piégés en matrice cryogénique (4-50 K). Cet environnement solide, issu de la condensation d'un mélange gazeux contenant une impureté et un gaz inerte (N₂, CH₄, Ar, etc.), est propice à l’étude de systèmes dans leur état fondamental. L’excitation d’une vibration moléculaire s’atténue toujours temporellement, ce qui correspond dans le domaine spectral à un élargissement des raies d’absorption. L’étude de la dynamique vibrationnelle vise à examiner les causes physiques à l’origine de cet élargissement spectral. Typiquement, elles sont de trois sortes : phénomènes intramoléculaires, interactions entre molécules piégées et interactions entre la molécule piégée et l’environnement. Les échos de photons permettent de distinguer les contributions homogènes et inhomogènes de l’élargissement spectral et de caractériser les processus de déphasage, de relaxation des populations et de diffusion spectrale. Parmi les résultats obtenus, nous avons mis en évidence l'influence des phonons spécifiques aux matrices moléculaires (ex : libration de N₂ et rotation de CH₄ ) sur le déphasage vibrationnel ainsi que l’influence de la transition de phase du méthane solide à 20 K sur la dynamique vibrationnelle. Nous avons également montré que la dynamique vibrationnelle était dépendante du site cristallographique dans lequel est piégée la molécule. Enfin, en excitant plusieurs modes de vibration simultanément, nous avons pu examiner les couplages intramoléculaires. / We built an experimental set-up in order to generate infrared stimulated photon echoes at the femtosecond timescale. The purpose is to examine the short time vibrational dynamics of metal carbonyls (W(CO)₆ and Fe(CO)₅) trapped in cryogenic matrices (4-50 K). This environment, resulting from the condensation of a gas mixture containing the impurity and an inert gas (N₂, CH₄, Ar, etc.), is well suited to study systems in their ground state. An excited molecular vibration is always damped in the time domain. It corresponds in the frequency domain to a broadening of the absorption line. The study of the vibrational dynamics aims at examining the physical causes of this spectral broadening. Typically, there are three kinds of causes: intramolecular phenomena, interactions between trapped molecules and interactions between the impurity and the environment. Photon echoes allow distinguishing between the homogeneous and the inhomogeneous contributions of the spectral broadening and characterizing dephasing process, population relaxation and spectral diffusion. Among the obtained results, we highlighted the influence of phonons that are specific to molecular matrices (ex: N₂ libration and CH₄ rotation) on the vibrational dephasing. Moreover, we observed the influence of the phase transition of solid methane at 20 K on the vibrational dynamics. We also showed that the vibrational dynamics depends on the site in which the molecule is trapped. Finally, when exciting several vibrational modes, we are also able to study intramolecular couplings.