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

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.

Influence of the protein environment on spectroscopy and ultrafast dynamics of retinal in bacteriorhodopsin

Selma Schenkl

The environment is important for the exact course of a chemical reaction. As an example of the strong influence of the environment on the reaction, this work studies the isomerization reaction of the chromophore retinal in the binding pocket of the protein bacteriorhodopsin. Three selected distance scales with reference to the chromophore are addressed: The distant protein surface at a length scale of 3-5 nm, the fuzzy environment of the binding pocket (1-2 nm) and the immediate neighborhood represented by the amino acid tryptophan Trp86 (5 Å). First, static absorption and fluorescence spectroscopies are applied to three dimensionally crystallized bacteriorhodopsin. An especially adapted set-up was developed for each task. The analysis of data is carried out in comparison with the measured spectra of the native protein membrane in solution. While the absorption spectrum shows a broadening on the high-energy side, the fluorescence spectrum deviates in the general shape of that measured in solution. The synoptic analysis of all spectra reveals the existence of three spectroscopically distinct species in the crystal: BR490 (absorption maximum at 490 nm) with a relative contribution of 28%, BR570 (native spectrum of solution, 68%), and BR610 ("blue membrane", 4%). BR490 appears particularly in the additional fluorescence band at 550 nm and is assigned to the dehydrated species of bacteriorhodopsin. The presence of that particular species indicates the suppression of rehydration in a partially dehydrated crystal and shows that the three dimensional arrangement hinders water diffusion. A simple model, close domains are assumed to form, eventually suppressing free water diffusion in a crystal. The lack of water on the protein surface modifies the protonation states of the amino acids in the binding pocket and therefore the optical properties of the retinal. The chromophore actually reacts on the alteration of its distant environment upon incorporation in the protein crystal. The second distance scale - the fuzzy environment - is addressed by a time-integrated fluorescence spectroscopy on native bacteriorhodopsin as function of the excitation wavelength (430 nm to 650 nm). The optimized experimental set-up guaranteed that the fluorescence of only the excited state I460 contributed. The measured spectra show maxima at 740 nm, and exhibit a Stokes shift of 5000 cm-1, independent of the excitation wavelength. The similarity of the fluorescence excitation spectrum with the spectrum of absorbed photons excludes energy-dependent loss channels. However, the spectra show a growing asymmetric broadening on the high-energy side with decreasing excitation wavelength. The additional fluorescence originates from vibrational hot states, also supported by the decomposition by Gaussian functions. A comparison with the fluorescence spectrum of blue membranes shows that the low-energy part also arises from vibrational hot states. Consequently, the first fast intramolecular vibrational energy relaxation does not entirely distribute the access-energy to other modes. The remaining energy is found in a quasi-static occupation of vibrational modes. The subsequent second vibrational relaxation — probably not only involving the retinal but also the protein — takes place on a time scale similar to that of the isomerization. The fast isomerization reaction prevents dissipation of energy relaxation of the excited state of the retinal into the protein environment. On a third distance scale, femtosecond spectroscopy in the UV offers a direct view into the electrostatic interaction of the chromophore with its immediate neighborhood. For the first time, the retinal isomerization was studied within the spectral window of the tryptophan absorption from 265 nm to 310 nm. For this, a set-up adapted to UV wavelengths was realized, basing on non-linear optics. The developed single-shot detection system yields a sensitivity of 10-5 in optical density. In the transient absorption measurements, a time resolution of 90 fs was achieved. At short probe wavelengths (265 nm - 294 nm), the measured transients show first a fast reduction of the absorption which subsequently recovers on three time scales (380 fs, 3 ps, > 16 ps). With increasing probe wavelength an induced transient absorption becomes dominant (282 nm - 294 nm). It has a rise time of 280 fs, and decays with a time constant of 4.3 ps. With further increasing probe wavelength the absorption signal decays faster. In response to the complex signature of the transients, an iterative and model-independent transient-decomposition is introduced. A set of three basis elements were identified: one bleach, and two absorption transients. The two induced absorption transients are assigned to retinal. The dynamics of the excited state (I460) in the first retinal-transient is described by two time constants (280 fs and 1 ps). The fast time constant leads to the photoproduct, while the slower one leads back to the ground state, presumably by internal conversion. The 250 fs delayed onset of the second transient (photo-intermediate J) is connected to a wave packet motion in the excited state. This delayed onset of the transient was observed for the first time. It demands a rise time of the J photo-intermediate of only 280 fs. This contrasts common estimates of 500 fs given in literature. For the first time, the temporal behavior of the permanent dipole moment of retinal is observed experimentally. Its evolution is imprinted in the dominant bleach feature (265 nm - 294 nm), originating from tryptophan Trp86. In the excited state, the permanent dipole moment increases strongly within the first 250 fs, and decays exponentially afterwards, following the formation of the photoproduct. However, the related time constant of 380 fs is slightly greater than that of 280 fs of the photoproduct J. This discrepancy may indicate a remaining long-lived polarization of the binding pocket. In brief, by using the intrinsic tryptophan as a probe, the local evolution of the electric field during the isomerization reaction was investigated. Indeed the isomerization reaction is reflected in the immediate environment. The variety of optical spectroscopy provides methods especially adapted for studying the influences of the environment on a chemical reaction. The results achieved here highlight the importance of these influences.

EPFL2004

Scanning Tunneling Luminescence of Pentacene Nanocrystals

Alexander Kabakchiev

Organic semiconductors are promising materials for future electronic and electroluminescence applications. A detailed understanding of organic layers and nano-sized crystals down to single molecules can address fundamental questions of contacting organic semiconductors at the nanometer limit and obtaining luminescence from them. In this thesis, electroluminescence spectra from pentacene, a policyclic hydrocarbon (acene), are discussed. The luminescence is induced by the current from the tip of a scanning tunneling microscope (STM). Pentacene is an organic semiconductor which gained a lot of attention in technology because of its electronic properties suitable for applications in thin film devices. Moreover, recent fundamental studies employed pentacene as a standard to demonstrate sub-molecular resolution by scanning probe techniques. This work reports the first observation of light emission from nanometer-sized pentacene crystals grown on an ultrathin insulating layer on noble metal surfaces. Different STM-techniques are combined to characterize the individual systems studied with respect to topography, crystal structure, electronic properties and work function changes. The initial step of the project was the implementation of an optical system for light collection from the tunnel-junction of a low-temperature STM. In the set-up, a novel approach based on the use of in situ adjustable lenses has been realized. Three lenses placed in the vicinity of the tip-apex allow light collection into three independent channels which can be used for versatile optical analysis. An important part in the characterization of the organic system was to clarify the interaction between adsorbed molecules and substrates. We investigated individual pentacene molecules on different ultrathin insulator-metal systems. With the STM tip in tunnel contact, the molecules are situated in a double barrier junction formed by the insulating layer on one side and the vacuum gap on the other side. The metal surface and the STM-tip form the electrodes. The electronic properties of pentacene in this configuration have been characterized. Insulator-metal-systems, which provide a good electronic decoupling for pentacene from the metal, have been chosen as substrates for the growth of pentacene nanocrystals. Using the sub-molecular resolution of the STM we resolved the structure of the top-layer of the pentacene nanocrystals and found that the crystal phase agrees with the pentacene bulk structure. Moreover, the comparison of charge injection barriers between individual molecules and nanocrystals of pentacene indicates a significant change of electronic properties after the formation of ordered structure from the individual building blocks. Optical spectroscopy of the emitted light reveals an excitonic emission from the nanocrystals, which is in very good agreement with photoemission spectra of macroscopic crystals. Although a highly localized current-injection by the STM tip is used for excitation, it can be concluded that the source of light emission is delocalized. In contrast to luminescence measurements reported for other organic materials in STM, the excitation is not localized on the individual molecule, into which a charge is injected by the STM-tip. Our study indicates the importance of inter-molecular coupling leading to a high mobility of the excited state which has to be considered in the framework of STM-induced luminescence. A further result is the observation of the Stark shift of pentacene luminescence, that has so far not been measured at large electric fields of the order of 1V/nm. The evaluation of the Stark shift provides information on dipole and polarizability change during the decay of an exciton. The measured large dipole change of the lowest singlet exciton in pentacene indicates a mixing of the Frenkel exciton (FE) with a small contribution of charge-transfer (CT). Finally, we developed a set-up for photon correlation measurements in the STM. The properties of photon statistics is a means which can unambiguously prove that the luminescence is due to a single-photon source, i.e. a light source which emits no more than one photon at a time. A typical example of such a source is the emission from a single molecule. We performed photon correlations measurements of the luminescence from pentacene and C60 nanocrystals. While the excitonic bulk emission of pentacene is not expected to represent a single photon emitter, we also did not observe anticorrelations in the case of C60 indicating the importance of the local environment in the STM-junction, e.g. the short distance to the metal electrodes.

EPFL2010

Photoemission from non-polar aromatic molecules in the gas and liquid phase

Christopher Alexander Arrell, Majed Chergui, Jakob Grilj, Luca Longetti, Lars-Hendrik Mewes, José Javier Pancracio Ojeda Andara, Frank van Mourik

The photoelectron spectra of both liquid and gas phase aromatic molecules are reported. The spectra were obtained using a 34.1 eV source produced by high harmonic generation and analysed with the help of high-level ab initio simulations using the reflection principle combined with path integral molecular dynamics simulations accounting for nuclear quantum effects for the gas phase. We demonstrate the suitability of three trimethylbenzenes (1,3,5-trimethylbenzene, 1,2,3-trimethylbenzene and 1,2,4-trimethylbenzene) as a solvent for liquid photoelectron spectroscopy of solute species. We also discuss the electrokinetic charging of a non-polar liquid jet.