Ultrafast Visible and Ultraviolet Fluorescence Studies of Molecular and Biological Sytems in Solution

A wide variety of physical and chemical processes at the molecular level, as charge or energy transfer, solvation, electronic as well as vibrational relaxation, is at the origin of the biological functionality of proteins. The work reported in this thesis is devoted to the study of these molecular dynamics in different chromophore systems. More specifically, this study is focused on the role of these dynamics in the ultrafast photophysics and photochemistry of haemoproteins. This important class of proteins has been widely studied through various spectroscopic techniques, in order to understand their functionality. These proteins contain several chromophore moieties, among which one aminoacid residue, Tryptophan, and the haem prosthetic group, which are of particular interest in this work. While Tryptophan is used as a probe of the local environment via intermolecular relaxation dynamics, we investigated the role of haem intramolecular dynamics on the protein functionality. While different kind of ultrafast spectroscopic techniques are indicated to tackle these issues, like pump probe or photon-echo peak shift experiments, we choose a different approach to probe directly the energetic relaxation, which consists in following the ultrafast changes in the photoluminescence spectrum. The dynamics are then extracted from the spectral evolution of the emission. With this purpose, we implemented a fluorescence up-conversion set-up, which allows us resolving temporally and spectrally the photoluminescence of the sample under investigation, excited with an ultrashort light pulse, and with a temporal resolution on the femtosecond timescale. The thesis is structured in 9 chapters. Chapter 1 resumes the theoretical background required for the interpretation of the experimental results. Chapter 2 describes the photophysical properties of the different systems investigated. Chapter 3 presents the experimental technique of fluorescence up-conversion, used to achieve broadband femtosecond detection of photoluminescence. In Chapter 4, we present a study on the role of ultrafast vibrational and structural dynamics on the time-resolved fluorescence spectra from two UV dyes. The measurements allow us exploring the capabilities of the set-up in revealing subtle relaxation dynamics with an unequaled accuracy. In Chapter 5, we move on to the study of the relaxation dynamics of Tryptophan (Trp) in water. This aminoacid served as an in vivo probe of solvation dynamics, owing to its high dipolar moment in its excited state. Our study focuses on water reorganization around the chromophore and on the distinction of this important process from the electronic internal conversion occurring on a similar timescale. In the framework of the study of protein dynamics, Chapter 6 reports the investigation of a family of molecules extensively found in nature, namely porphyrins. In these molecular systems, a quite complex pattern of intramolecular relaxation mechanisms occurs, which needed to be clarified. A systematic study on the free base and on a wide series of metallo-porphyrins definitely clarified the picture of their electronic relaxation pathways. We finally elaborate a comprehensive scheme of this relaxation, including substituent's influence. The mechanisms observed are intrinsically related to the photophysical properties used by nature, as will be illustrated in the next Chapter. Trp and metallo-porphyrin constitute the key natural chromophore molecules for the non-invasive spectroscopic investigation of complex biological systems as haemoproteins. In Chapter 7, we follow relaxation dynamics of photo-excited Tryptophan and Haem (Fe-porphyrin) in ferro- and ferricytochrome c and in Myoglobin in its met form. In the former protein, an ultrafast energy transfer from Tryptophan to Heme is evidenced, the efficiency of which is found to depend on the oxidation state. The energetic relaxation pathway of the haem, also dependent on the oxidation state, is characterized by a porphyrin to metal charge transfer that triggers the ligand dissociation. In metMyoglobin, the fluorescence contribution of the two Tryptophans (W14 and W7), also quenched by resonant energy transfer to the haem, are temporally and spectrally separated, and the solvation dynamics of W7, located in the water-protein interface, is followed. The haem of metMb, which is in the ferric form, shows an electronic relaxation similar to that of ferricytochrome c. In parallel to the biology oriented investigations presented above, we extended our study to the metal-polypiridine complexes. Due to their specific photophysics, mainly characterized by a photo-induced Metal-to-Ligand Charge Transfer (MLCT), they are extensively used in photochemical applications involving charge transfer dynamics. In particular, they are used as sensitizer in the Dye-Sensitized-Solar-Cells (DSSCs). In Chapter 8, we present studies of these dyes, both in solution and adsorbed on a substrate, in order to understand the role of intra- and intermolecular dynamics on the electron injection process occurring upon absorption of light. Finally, Chapter 9 summarizes the conclusions of the investigations of the various systems studied.

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