The LOUVRE Laboratory: State-of-the-Art Ultrafast Ultraviolet Spectroscopies for Molecular and Materials Science

We describe the facilities for ultraviolet studies in the femtosecond to nanosecond time domain. These facilities consist of: i) a set-up for deep-ultraviolet spectroscopy in the 260–380 nm range in both pump and probe pulses for transient absorption/reflectivity or two-dimensional spectroscopy studies; ii) a set-up for ultrafast fluorescence measurements with detection down to 300 nm. The capabilities of these set-ups are demonstrated by examples on molecular systems, biosystems, nanoparticles and solid materials.

Edoardo Baldini, Benjamin Bauer, Majed Chergui, Lars-Hendrik Mewes, Natalia Nagornova Boyarkine, Malte Oppermann, Aurelio Oriana, Tania Palmieri, Thomas Charles Henry Rossi, Frank van Mourik

We describe the facilities for ultraviolet studies in the femtosecond to nanosecond time domain. These facilities consist of: i) a set-up for deep-ultraviolet spectroscopy in the 260-380 nm range in both pump and probe pulses for transient absorption/reflectivity or two-dimensional spectroscopy studies; ii) a set-up for ultrafast fluorescence measurements with detection down to 300 nm. The capabilities of these set-ups are demonstrated by examples on molecular systems, biosystems, nanoparticles and solid materials.

Schweizerische Chemische Gesellschaft2017

Ultrafast Photoelectron and Optical Spectroscopies of Photoinduced Processes in Molecular Systems in Solution

Luca Longetti

Investigating molecular excitations with femtosecond time resolution is of pivotal importance to understand the out-of-equilibrium processes taking place in molecular systems upon light absorption. The photochemistry of solvated species is heavily determined by the early steps of the relaxation dynamics. In this thesis, ultrafast electronic spectroscopies are exploited to study relaxation dynamics of photoexcited molecules in solution by probing the transient electronic structure. Vacuum-ultraviolet (VUV) ultrashort monochromatized light pulses (HARMONIUM, 15-100 eV) are exploited for steady-state photoelectron spectroscopy (PES) and time-resolved PES in a pump-probe scheme. This is a powerful tool for investigating electronic structure and ultrafast processes of solvated molecules with high time (50fs) and energy (0.12eV) resolution. PES retrieves binding energies of valence and core electronic states with elemental specificity and without the constraints of selection rules, and is here applied to liquids and solutions by means of a liquid micro-jet. With the aim of extending this technique to a larger class of solutes, we demonstrate the capability to perform studies on volatile liquids such as organic solvents. PES spectra of gas phase and liquid phase aromatic compounds are presented and their valence orbitals are compared and identified. The solvation is discussed and the vertical ionization energies are reported. This retrieves important parameters for electrochemistry and opens new perspectives towards PES studies of molecules in organic solvents. Upon photoexcitation, molecules can undergo several relaxation processes such as nonradiative relaxation processes which include dissociation, internal conversion and intersystem crossing. Energy can also be released to the solvent environment by intermolecular processes such as vibrational energy transfer. All of these phenomena are reflected in a transient electronic configuration. PES can hence probe electronic and structural dynamics along the entire reaction coordinates. The ferric trisoxalate complex represents an ideal coordination compound showing an intriguing cascade of processes upon photoexcitation. We have directly observed the metal photoreduction due to an instantaneous intramolecular charge transfer. We then suggest a branching between photodissociation and a back-electron transfer channels, which completes the picture of the compoundâs photochemistry. This experiment shows PES sensitivity to oxidation state changes, providing an interesting perspective for photoredox reaction studies in solution and for intra- and inter-molecular charge-transfer phenomena. Vibrational wave packets (WP) are interesting observables to map non-adiabatic dynamics, relaxation pathways and coupling to solvent of the excited molecule. We have investigated molecular iodine in ethanol, which we have studied first by optical transient absorption spectroscopy using a white light continuum probe. The electronic absorption spectrum of I2 in ethanol is strongly modified compared to the case of other organic solvents, due to a strong coupling of covalent and ionic states. We have mapped the vibrational WP dynamics, in particular its trajectory and damping along the I-I bond axis. This study is intended to prepare for an ultrafast PES experiment aimed at linking the response of inner shells to the WP launched in the valence states. Preliminary steady-state PES spectra are reported and discussed.

EPFL2020

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

Olivier Christian Bräm

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.