|Wachsmann-Hogiu, Sebastian: Vibronic coupling and ultrafast electron transfer studied by picosecond time-resolved resonance Raman and CARS spectroscopy |
Electronic photoexcitation of a molecular system initiates a complex sequence of dynamical and kinetical processes in which the molecule can either change or preserve its chemical identity. The processes are called photophysical if the chemical identity of the molecule is preserved, and photochemical if not. Frequently, the nuclei do not move on a single adiabatic electronic potential energy surface, but transit between different adiabatic potential surfaces. In such transition points (actually hypersurfaces in the multidimensional nuclear coordinate space) of exact degeneracy of adiabatic potential energy surfaces, also called conical intersections, the Born-Oppenheimer approximation breaks down, and vibronic coupling related to these nonadiabatic events occurs [1-6]. Photodissociation, charge-transfer, isomerization, and spin-changing reactions are, generally speaking, typical examples of events occurring at conical intersections . In particular, in the mechanism of vision and photosynthesis, a sequence of processes taking place at conical intersection occur.
Photophysical and photochemical processes cover a wide range of timescales. They can be very slow, like the oxidation of iron in air, which can take even years, or very fast, like the photoisomerization of the visual pigment rhodopsin, which occurs in femtoseconds . New developments of picosecond and femtosecond laser technology allow a view into the ultrafast photochemical and photophysical processes which occur on the timescale of a period of the nuclear vibration. Conversion of light energy into chemical energy in photosynthesis and vision can thus be studied on a real time scale. The knowledge gained from these studies can be used, for example, for developing materials and components for artificial photosynthesis. Controlling chemical reactions could become reality and new chemical components with new properties could be obtained. Developing fast electronic components based on light-driven processes is another important direction of studies using picoseconds and femtosecond methods.
Nonadiabatic interactions have long been known in molecular spectroscopy as being responsible for spectral perturbations, Jahn-Teller and Renner-Teller effects [6, 9]. The role of conical intersections as funnels for radiationless decay has also been recognized . There are three aspects related to the mechanism of the processes induced by conical intersection(s) : (i) the path to the conical intersection(s) on the electronically excited potential energy surface, (ii) the nonadiabatic transition near the conical intersection(s), which is determined by the geometrical structure of the conical intersection(s) and provides the opportunity for branching to different regions of the ground state potential energy surface, i.e. for multiple products, and (iii) the subsequent motion on the ground state potential energy surface.
This thesis deals with photophysical elementary processes occurring at/near conical intersections. The interactions responsible for them are strong intramolecular couplings. Among them, strong electron-vibration interactions (gradients of excited state potential energy surfaces with respect to the Condon active normal modes) is the key for photoisomerization (see paragraph 1.1), and strong mode-mode couplings play an important role in the ultrafast radiationless decay (paragraph 1.2).
Photoisomerization of polyene-like molecules (see Fig. 1.1 for the structure of polyene-like molecules) such as rhodopsin and bacteriorhodopsin is of great importance for the role they play in the nature. They exploit the high efficiency and speed of cis-trans isomerization about polyene double bonds for signal transduction [8, 11]. Another role of polyenes in nature is light-harvesting involving singlet-singlet energy transfer from carotenoids to chlorophylls and bacteriochlorophylls as well as photoprotection by quenching the triplet state of either chlorophyll or bacteriochlorophyll. These properties are believed to arise, at least partially, from the interaction of two energetically close lying excited electronic states . The nature of this interaction is still the subject of many studies. Its understanding requires elucidation of the coupling mechanism between these two excited states.
Fig. 1.1: Polyene sequence (left) and diphenylpolyene (right) general structure. By increasing n, longer (diphenyl)polyene chains are obtained.
A great number of experimental and theoretical work has been done to describe the spectroscopic properties and to identify the electronic states of polyenes ([13-15] and references therein). In particular, the coupling mechanism between their excited states has been a main topic in the last years. Ultrafast radiationless decay of the optically bright B excited state into the dark A excited state in trans-butadiene has been explained by a vibronic-coupling model describing the conical intersection of the A and B states . Woywod et. a. developed a model of vibronic coupling in the first two excited states of trans-hexatriene . Nevertheless, experimental work on this topic is scarce.
Time-resolved Raman or Coherent Anti-Stokes Raman Scattering (CARS) spectroscopy are powerful methods to study the role of vibrational modes in coupling between the two excited electronic states of polyenes and diphenylpolyenes. They enable to monitor the formation of photoproducts as a function of time, and to determine the specific modes responsible for vibronic coupling. One goal of this thesis is the study of the mechanism of electron-vibration interaction in excited states of a polyene-like molecule, diphenylhexatriene. A time-resolved picosecond CARS spectrometer has been constructed to determine the changes and instabilities in molecular structure due to vibronic coupling.
Electron transfer (ET) reactions represent an elementary chemical process which occurs in a large variety of molecules, ranging from small ion pairs up to large biological systems. ET can be optically or/and thermally activated and triggers photosynthesis, metabolism, polymerization reactions, electrochemical reactions, etc. Understanding and control of electron transfer reactions comprises one of the broadest and most active research areas of physical chemistry [18-20]. Photoinduced ET occurs in nature in connection with energy transduction. Many coupled ET events in photosynthesis and respiratory chain are crucial to the respective function. The kinetics of specific charge-transfer processes dictate the efficiency of photosynthetic and energy conversion systems (natural or artificial). The overall rates of ET are determined by intramolecular mechanisms as well as by the response of the environment to the photoinduced changes of the charge distribution . The intramolecular mechanisms include redistribution of electronic charge and relevant vibrational excitations due to electron-vibrational interactions described by the interaction Hamiltonian Hif in Fig. 1.2. Excitation of high-frequency molecular vibrations could be the basic physical mechanism allowing to control ET. However, few experiments have been performed to identify the strongly contributing modes and the research is still at the beginning.
Because Resonance Raman spectroscopy is very sensitive to the vibrational modes with high Franck-Condon factors, it allows to monitor directly the relevant coupling modes in the ET process. This method gives thus an insight into the ET mechanism. The goal of this study is to determine the role of vibrational modes in the back-ET reaction in Betaine-30 (B-30). A highly sensitive time-resolved picosecond Raman spectrometer has been developed for this purpose.
Fig. 1.2: Schematic diagram of vibrational excitation of high frequency modes of the product during ET. Electron-vibrational coupling described by the Hamiltonian Hif, and vibrational manifold of the reactant R (i) and product P (f) are also shown.
Chapter 2 contains the theoretical background that is used for the interpretation of the experimental results. Mechanisms of vibronic coupling in organic molecules will be discussed, followed by the introduction of the time-dependent approach of light-matter
4interaction. In addition, a review of the models used most frequently in describing ET processes will be presented.
In Chapter 3, general considerations of the experiments will be presented, including the Raman effect, Coherent Anti-Stokes Raman Scattering (CARS) and the pump-probe technique. The picosecond Raman/CARS experimental set-up consisting of a 50 Hz amplified dye laser and a highly sensitive detection system will also be described in detail.
In Chapter 4 a study of the vibronic coupling between the first and second excited electronic states of diphenylhexatriene (DPH) will be presented. This should offer the possibility to gain new information about the relaxation mechanism between the excited states of polyenes and about their role in the intermolecular energy transfer in photosynthesis.
First, some photophysical properties of the molecule will be introduced. Secondly, the experimental results of the CARS measurements after excitation in the optically allowed excited state will be presented. Based on the CARS spectra of the excited states and semiempirical calculations, the changes of molecular geometry in the excited state will be discussed. These information reveal a new effect of vibronic coupling between the two excited states. Two mechanisms of vibronic coupling will be finally discussed.
In Chapter 5 a study of the mode specific vibrational kinetics after intramolecular back-ET in B-30 will be reported. This will enable us to determine the role of the vibrational modes in back-ET and subsequent relaxation.
The importance of the work is discussed in a general context, followed by the presentation of stationary and transient Raman spectra. These spectra allow to discern the acceptor modes in the back-ET process. The transient spectra give evidence of selective vibrational excitation and nonequilibrium vibrational population for a few ps after back-ET. The interplay between vibrational excitation and intramolecular vibrational relaxation will be discussed. Quantum-chemical calculations will be presented, which allow to assign most of the observed vibrational frequencies.
Chapter 6 contains final conclusions and remarks.
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