Non-equilibrium dynamics of di-platinum complexes and molecular dyes in solution: Insights from transient absorption and two-dimensional Fourier transform spectroscopy

Unraveling the interplay between electronic- and vibrational degrees of freedom on the earliest time scales of physical, chemical and biological processes is crucial to gaining insight into the mechanisms that govern the world around us, since it is during these primary steps that the fate of the excitation energy - be it solar, chemical or physical - is decided. A tiny structural change during the first 10s of femtoseconds can for example predetermine the dynamics of a system over microseconds, and newly developed techniques in time-domain spectroscopy have proved to successfully capture exactly these pivotal mechanisms.

In this thesis I use ultrafast spectroscopy to investigate the non-equilibrium dynamics of a prototypical di-platinum complex. Transient absorption spectroscopy is complemented with transient absorption anisotropy and fluorescence up-conversion to identify the spectral features of the intermediate excited states. The results show that the intersystem crossing from the singlet- to the triplet manifold of states happens on a 0.6 to 0.9 picosecond time scale, slower than previously assumed. As a consequence, the observable coherence is associated with a singlet excited state, rather than a triplet, and dephases prior to the intersystem crossing. The study further illustrates the importance of having a good understanding of the transient spectral signatures and yields insight into the structural dynamics of di-platinum complexes.

In addition, I have built and commissioned a two-dimensional photon echo experiment that is capable of acquiring broadband excitation- and detection frequency resolved transient spectra in the visible with a temporal resolution of approx. 10 femtoseconds. All relevant steps of the development are outlined in this thesis and 2D spectra of four different dye solutions highlight the capabilities of the experiment. The temporal evolution of the 2D spectra of nile blue is analyzed as an example and shows the information content that is contained within the data.

Effect of electronic and nuclear factors on the dynamics of dye-to-semiconductor electron transfer

Bernard Wenger

Dye-to-semiconductor electron transfer is the initial step in many processes where light is used for the storage of information (e. g. color photography) or converted into electricity as in dye-sensitized solar cells. In the latter, interfacial charge injection occurs on a timescale spanning from tens of femtoseconds (10-15 s) to several picoseconds (10-12 s), with an efficiency approaching 100 %. In standard electron transfer theory, the parameters controlling charge transfer dynamics are divided into two categories: a) electronic factors, depending essentially upon the overlap between the electronic wavefunctions of the donor and the acceptor, and b) nuclear factors, characterized by the reorganization energies of both the reactants and the surrounding solvent molecules. Because of the ultrafast injection rates observed for dye/semiconductors couples typically used in photovoltaic applications, the hypotheses leading to these models are likely to be invalid. In this work, we make use of time-resolved spectroscopic techniques to study the influence of some electronic and nuclear parameters influencing charge injection dynamics into wide bandgap semiconductors. As a preliminary study, we investigated reductive quenching of excited dyes by iodide at the surface of nanocrystalline TiO2. Our experiments show that the quantum yield of this reaction, that is in competition with electron injection, depends strongly upon the I- concentration, but also upon the preparation and aging of the samples. We deduce that a fraction of the dye molecules are not adsorbed directly onto the surface of the semiconductor, but rather aggregated in a second adsorption layer. In the light of these results we reconsidered the issue of charge injection from the standard N3 dye into TiO2. This reaction, largely studied due to its importance for dye-sensitized solar cells, has previously been reported to take place with a fast (< 100 fs) phase, followed by a slower (1-100 ps) component. Our measurements show that the slow part is actually related to the presence of weakly adsorbed dye molecules. When aggregation is minimized, we observe monophasic injection dynamics (τ < 20 fs). This result suggests the process is beyond the scope of vibration-mediated electron transfer models and is controlled by electron dephasing in the solid. Next, we applied two different approaches to investigate the influence of the distance over which charge transfer takes place. First, forward and back electron transfer kinetics were measured for a series of bridged-sensitizers containing p-phenylene spacer groups. For both reactions the rate decays exponentially with distance. However, the damping factor is much larger for the back transfer (β = 0.5 Å-1) than for injection (β = 0.19 Å-1), suggesting that nuclear reorganization plays an important role for the former reaction. In the second approach, a thin layer of Al2O3 of controlled thickness is inserted between the sensitizer and TiO2. In this case the distance parameter β is similar for both reactions. The weak estimated value (β = 0.15 Å-1) indicates that thin layers of aluminum oxide is much less insulating that pure, crystalline Al2O3, and that electron tunneling is mediated by empty states in the thin layer according to the superexchange mechanism. Finally, the influence of the density of acceptor states has been examined by using a ruthenium complex adsorbed on Nb2O5 films. Varying the excitation wavelength made it possible to probe the bottom of the conduction band where the density of states decays exponentially. Despite the spectral width of the pump laser pulses, a weak retardation (2-3 times) of electron injection was observed. In summary, our experiments show that electronic factors, in particular variations of electronic coupling with distance, mainly control electron injection dynamics at dye/semiconductor interfaces. By revealing the role of aggregation for standard dyes adsorbed on TiO2, these results have important consequences for the development of dye-sensitized solar cells. Finally, in the case of ultrafast monophasic kinetics, standard electron transfer models are not obeyed, and the dynamics is rather controlled by electron dephasing and relaxation in the continuum of acceptor states in the solid.

EPFL2006

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.

EPFL2011

Ultrafast Excited-State Dynamics ef [Re(L)(CO)(3)(bpy)](n) Complexes: Involvement of the Solvent

Andrea Cannizzo, Majed Chergui, Amal El Nahhas, Frank van Mourik

Ultrafast excited-state dynamics of Re(L)(CO)(3)(bpy) (L = Cl, Br, n = 0; L = 4-ethyl-pyridine (Etpy), n = 1+; bpy = 2,2'-bipyridine) have been investigated in dimethylformamide (DMF) solution by fluorescence up-conversion (FIUC) and UV - vis transient absorption (TA) with similar to 100 is time,resolution. TA was also measured in the [1-ethyl-3-methyl-imidazolium]BF4 ionic liquid. The complexes show a very broad fluorescence band at 540-550 nm at zero time delay, which decays with 100-140 Is (depending on L) by intersystem crossing (ISC) to pi pi* intraligand ((IL)-I-3) and a Re(L)(CO)(3) -> bpy charge-transfer ((CT)-C-3) excited states. A second emission decay component (1.1-1.7 ps). apparent in the red part of the spectrum, is attributed to (IL)-I-3 -> (CT)-C-3 conversion, leaving phosphorescence from the lowest (CT)-C-3 state as the only emission signal at longer time delays. The triplet conversion is slower in DMF? than acetonitrile, commensurate with solvation times. Full assignment of the excited-state absorption at long delay times is obtained by TD-DFT calculations on the lowest triplet state, showing,2 that the 373 nm band is the sole diagnostics of bpy reduction in the CT excited state. Bands in the visible are clue to Ligand-to-Metal-Charge-Transfer (LMCT) transitions. Time-resolved UV - vis absorption spectra exhibit a units-of-ps rise of all absorption features attributed to (IL)-I-3 -> (CT)-C-3 conversion as well as electronic and vibrational relaxation, and a similar to 15 ps rise of only the 373 nm pi pi*(bpy(center dot-)) band which slows down to similar to 1 ns in the ionic liquid solvent. It is proposed that this slow relaxation originates mainly from restructuring of solvent molecules that are found very close to the metal center, inserted between the ligands. The solvent thus plays a key role in controlling the intramolecular charge separation, and this effect may well be operative other classes of metal-based molecular complexes.

American Chemical Society2010