541 Physikalische Chemie
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- Fraunhofer-Institut für Silicatforschung ISC (3)
- Institut für Optik und Atomare Physik, Technische Universität Berlin, 10623 Berlin, Germany (2)
- Laboratory for Chemistry and Life Science, Institute of Innovative Research, Tokyo Institute of Technology, Yokohama 226-8503, Japan (2)
- Center for Nanosystems Chemistry (CNC), Universität Würzburg (1)
- Center for Nanosystems Chemistry (CNC), Universität Würzburg, Am Hubland, 97074 Würzburg, Germany (1)
- Center of Excellence for Science and Technology - Integration of Mediterranean region (STIM), Faculty of Science, University of Split, Poljička cesta 35, 2100 Split, Croatia (1)
- Charles University, Faculty of Mathematics and Physics, Ke Karlovu 5, 121 16 Prague, Czech Republic (1)
- Departamento de Química, Facultad de Ciencias, Universidad Autónoma de Madrid, 28049 Madrid, Spain (1)
- Department of Chemistry, Humboldt Universität zu Berlin, Brook-Taylor-Strasse 2, 12489 Berlin, Germany (1)
- Department of Chemistry, Sungkyunkwan University, 440-746 Suwon, Republic of Korea (1)
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- M-1240-2017 (1)
The design of ordered arrays of metal nanoclusters such as for example 2D cluster organic frameworks might open a new route towards the development of materials with tailored optical properties. Such systems could serve as plasmonically enhanced light-harvesting materials, sensors or catalysts. We present here a theoretical approach for the simulation of the optical properties of ordered arrays of metal clusters that is based on the ab initio parametrized Frenkel exciton model. We demonstrate that small atomically precise silver clusters can be assembled in one- and two-dimensional arrays on suitably designed porphyrin templates exhibiting remarkable optical properties. By employing explicit TDDFT calculations on smaller homologs, we show that the intrinsic optical properties of metal clusters are largely preserved but undergo J- and H-type excitonic coupling that results in controllable splitting of their excited states.
Furthermore, ab initio parameterized Frenkel exciton model calculations allow us to predict an energetic splitting of up to 0.77 eV in extended two-dimensional square arrays and 0.79 eV in tilted square aggregates containing up to 25 cluster-porphyrin subunits.
We present a theoretical approach for the simulation of the electric field and exciton propagation in ordered arrays constructed of molecular-sized noble metal clusters bound to organic polymer templates. In order to describe the electronic coupling between individual constituents of the nanostructure we use the ab initio parameterized transition charge method which is more accurate than the usual dipole-dipole coupling. The electronic population dynamics in the nanostructure under an external laser pulse excitation is simulated by numerical integration of the time-dependent Schrodinger equation employing the fully coupled Hamiltonian. The solution of the TDSE gives rise to time-dependent partial point charges for each subunit of the nanostructure, and the spatio-temporal electric field distribution is evaluated by means of classical electrodynamics methods. The time-dependent partial charges are determined based on the stationary partial and transition charges obtained in the framework of the TDDFT. In order to treat large plasmonic nanostructures constructed of many constituents, the approximate self-consistent iterative approach presented in (Lisinetskaya and Mitric in Phys Rev B 89:035433, 2014) is modified to include the transition-charge-based interaction. The developed methods are used to study the optical response and exciton dynamics of Ag-3(+) and porphyrin-Ag-4 dimers. Subsequently, the spatio-temporal electric field distribution in a ring constructed of ten porphyrin-Ag-4 subunits under the action of circularly polarized laser pulse is simulated. The presented methodology provides a theoretical basis for the investigation of coupled light-exciton propagation in nanoarchitectures built from molecular size metal nanoclusters in which quantum confinement effects are important.
We present a joint theoretical and experimental study of excited state dynamics in pure and hydrated anionic gold clusters Au\(^-_3\)[H\(_2\)O]\(_n\) (n = 0-2). We employ mixed quantum-classical dynamics combined with femtosecond time-resolved photoelectron spectroscopy in order to investigate the influence of hydration on excited state lifetimes and photo-dissociation dynamics. A gradual decrease of the excited state lifetime with the number of adsorbed water molecules as well as gold cluster fragmentation quenching by two or more water molecules are observed both in experiment and in simulations. Non-radiative relaxation and dissociation in excited states are found to be responsible for the excited state population depletion. Time constants of these two processes strongly depend on the number of water molecules leading to the possibility to modulate excited state dynamics and fragmentation of the anionic cluster by adsorption of water molecules.
We have investigated the photodynamics of \(\beta\)-D-glucose employing our field-induced surface hopping method (FISH), which allows us to simulate the coupled electron-nuclear dynamics, including explicitly nonadiabatic effects and light-induced excitation. Our results reveal that from the initially populated S\(_{1}\) and S\(_{2}\) states, glucose returns nonradiatively to the ground state within about 200 fs. This takes place mainly via conical intersections (CIs) whose geometries
in most cases involve the elongation of a single O-H bond, while in some instances ring-opening due to dissociation of a C-O bond is observed. Experimentally, excitation to a distinct excited electronic state is improbable due to the presence of a dense manifold of states bearing similar oscillator strengths. Our FISH simulations explicitly including a UV laser pulse of 6.43 eV photon energy reveals that after initial excitation the population is almost equally spread over several close-lying electronic states. This is followed by a fast nonradiative decay on the time scale of 100-200 fs, with the final return to the ground state proceeding via the S\(_{1}\) state through the same types of CIs as observed in the field-free simulations.
We introduce a general theoretical approach for the simulation of photochemical dynamics under the influence of circularly polarized light to explore the possibility of generating enantiomeric enrichment through polarized-light-selective photochemistry. The method is applied to the simulation of the photolysis of alanine, a prototype chiral amino acid. We show that a systematic enantiomeric enrichment can be obtained depending on the helicity of the circularly polarized light that induces the excited-state photochemistry of alanine. By analyzing the patterns of the photoinduced fragmentation of alanine we find an inducible enantiomeric enrichment up to 1.7%, which is also in good correspondence to the experimental findings. Our method is generally applicable to complex systems and might serve to systematically explore the photochemical origin of homochirality.
Coherent Multidimensional Spectroscopy in Molecular Beams and Liquids Using Incoherent Observables
(2018)
The aim of the present work was to implement an experimental approach that enables coherent two-dimensional (2D) electronic spectroscopy of samples in various states of matter. For samples in the liquid phase, a setup was realized that utilizes the sample fluorescence for the acquisition of 2D spectra. Whereas the liquid-phase approach has been established before, coherent 2D spectroscopy on gaseous samples in a molecular beam as developed in this work is in fact a new method. It employs for the first time cations in a time-of-flight mass spectrometer for signal detection and was used to obtain the first ion-selective 2D spectra of a molecular-beam sample. Additionally, a new acquisition concept was developed in this thesis that significantly decreases measurement times in 2D spectroscopy using optimized sparse sampling and a compressed-sensing reconstruction algorithm.
Characteristic for the variant of 2D spectroscopy presented in this work is the usage of a phase-coherent sequence of four laser pulses in a fully collinear geometry for sample excitation. The pulse sequence was generated by a custom-designed pulse shaper that is capable of rapid scanning by changing the pulse parameters such as time delays and phases with the repetition rate of the laser. The sample's response was detected by monitoring incoherent observables that arise from the final-state population, for instance fluorescence or cations. Phase cycling, i.e., signal acquisition with different combinations of the relative phases of the excitation pulses, was applied to extract nonlinear signal contributions from the full signal during data analysis.
Liquid-phase 2D fluorescence spectroscopy was established with the laser dye cresyl violet as a sample molecule, confirming coherent oscillations previously observed in literature that are originating from vibronic coherences in specific regions of the 2D spectrum.
The data set of this experiment was used subsequently to introduce optimized sparse sampling in 2D spectroscopy. An optimization algorithm was implemented in order to find the best sampling pattern while taking only one quarter of the regular time-domain sampling points, thereby reducing the acquisition time by a factor of four. Signal recovery was based on a new and compact representation of 2D spectra using the von Neumann basis, which required about six times less coefficients than the Fourier basis to retain the relevant information. Successful reconstruction was shown by recovering the coherent oscillations in cresyl violet from a reduced data set.
Finally, molecular-beam coherent 2D spectroscopy was introduced with an investigation of ionization pathways in highly-excited nitrogen dioxide, revealing transitions to discrete auto-ionizing states as the dominant contribution to the ion signal. Furthermore, the advantage of the time-of-flight approach to obtain reactant and product 2D spectra simultaneously enabled the observation of distinct differences in the multiphoton-ionization response functions of the nitrogen dioxide cation and the nitrogen oxide ionic fragment.
The developed experimental techniques of this work will facilitate fast acquisition of 2D spectra for samples in various states of matter and permit reliable direct comparison of results. Therefore, they pave the way to study the properties of quantum coherences during photophysical processes or photochemical reactions in different environments.
Ziel dieser Arbeit war es, ein Schichtsystem auf Basis des Sol-Gel-Prozesses zu entwickeln, um Lighttrapping in Si-Dünnschichtsolarzellen zu erzeugen. Die Grundlage dieses Schichtsystems bilden SiO2-Partikel, die über den Stöber-Prozess hergestellt werden. Es zeigte sich, dass sich die Rauheit und der Haze der Schichten über die Partikelgröße und Schichtdicke einstellen lassen.
Um die mechanische Stabilität der reinen Stöber-Schichten zu verbessern, kamen verschiedene Binder zum Einsatz. Beste Ergebnisse zeigten Binder basierend auf löslichen Vorstufenpulvern, da diese dem Stöber-Sol beigemischt werden konnten und so Binder und Partikel gleichzeitig aufgebracht werden konnten.
Auf diese Weise entstehen mechanisch stabile, lichtstreuende Schichten. Zum Einsatz kam zunächst ein TiO2-Binder. Durch eine anschließende Glättung der Stöber-TiO2-Streuschichten mit SiO2 entsteht eine defektfreie, aber dennoch raue Oberfläche. Zusätzlich wird ein beträchtlicher Teil des Lichts in große Winkel gestreut.
Es konnte gezeigt werden, dass sich auf den SiO2-geglätteten Stöber-TiO2-Streuschichten ZnO:Al deponieren lässt, wobei die elektrischen Eigenschaften von der Dicke der Glättung abhängen. Auch die elektrischen Eigenschaften der Si-Dünnschichtsolarzellen hängen von der Glättung bzw. der Dicke der Glättung ab. Dies gilt insbesondere für die von der Materialqualität abhängigen Parameter Füllfaktor FF und offen Klemmenspannung VOC. Insgesamt fallen die Parameter jedoch noch gegenüber Referenzzellen auf geätztem Frontkontakt zurück. Vor allem aber wurde die hohe Zellreflexion aufgrund der Glas-TiO2-Grenzfläche als primäres Problem identifiziert. Dennoch konnte bei einer Glättungsdicke von 200 nm sehr gutes Lighttrapping beobachtet werden. Verantwortlich hierfür ist sehr wahrscheinlich die Großwinkelstreuung der Stöber-TiO2-Streuschichten.
Um die Zellreflexion zu verringern, wurde der Brechungsindex des Binders und der Glättungsschichten an den Stack aus Substrat, Streuschicht und ZnO:Al-Schicht angepasst. Idee war es, durch Einbringen eines Al2O3-Vorstufenpulvers eine niedrigbrechende Komponente bereitzustellen, um durch Mischung von Al2O3- und TiO2-Vorstufenpulver freie Hand über den Brechungsindex des Binders und der Glättung zu erhalten.
Da sich das Volumenverhältnis von SiO2-Partikeln zu Binder bei verschiedenen Al2O3-TiO2-Verhältnissen nur schwer bestimmen lässt, wurde lediglich ein reiner Al2O3-Binder in den Streuschichten eingesetzt. Die Einstellung des Brechungsindex beschränkte sich allein auf die Glättungsschichten. Um Stöber-Al2O3-Streuschichten mit hoher Rauigkeit und geringen Defekten zu erzielen, muss das Binder-zu-Partikel-Verhältnis angepasst werden. Beste Ergebnisse ergaben sich bei einem Al2O3-Gehalt von 2% im Sol. Aufgrund der hohen Rauigkeit besitzen die Streuschichten einen hohen Haze und wegen des geringen Brechungsunterschied zwischen Glas und Binder eine hohe Transmission. Die Glättung der Streuschichten im Al2O3-TiO2-System ist nur mit Hilfe einer zusätzlichen SiO2-Glättungsschicht und einer reduzierten Dicke auf 50 nm möglich. Auf den reinen defektreichen Streuschichten tendieren die Al2O3-TiO2-Schichten selbst zu Rissbildung.
Zur Untersuchung der ZnO:Al-Deposition wurde eine Glättungsdicke von 200 nm gewählt. Die erwies sich als zu gering. Die aufgebrachten ZnO:Al-Schichten wiesen größere Poren und kleinere Oberflächendefekte auf. Die Anpassung des Brechungsindex der Glättungsschichten an die ZnO:Al-Schicht erwies sich nicht als vorteilhaft. Die reine Al2O3-Glättung zeigt auch nach der ZnO:Al-Deposition die höchste Transmission. Die Winkelverteilung des Streulichts der Stöber-Al2O3-Streuschichten ist gegenüber den Stöber-TiO2-Streuschichten zu kleineren Winkeln verschoben. Dennoch wird ein größerer Anteil des Lichts in große Winkel gestreut, als es bei der geätzten ZnO:Al-Referenz der Fall ist.
Trotz der Defekte in den ZnO:Al-Schichten konnten auf den Stöber-Al2O3-Streuschichten funktionierende Tandemzellen hergestellt werden. Der Füllfaktor und die offene Klemmenspannung fallen nur geringfügig hinter die der Referenzzelle zurück. In der Kurzschlussstromdichte machen sich die verringerte Zellreflexion und das sehr gute Lighttrapping bemerkbar, so dass das Niveau der Referenz erreicht werden konnte. Zu beachten ist allerdings, dass gerade im langwelligen Lighttrapping-Spektralbereich die gleiche EQE erreicht wurde, trotz immer noch leicht erhöhter Zellreflexion.
Die letzte Versuchsreihe konnte zeigen, dass die entwickelten Schichten sich sehr gut zur Erzeugung von Lighttrapping in Si-Dünnschichtsolarzellen eignen.
In der vorliegenden Arbeit wurde erfolgreich eine neue Gasphasen-Apparatur für
Photoelektronen-Imaging-Experimente simuliert, aufgebaut und in Verbindung mit einem ps-Lasersystem in Betrieb genommen.
Neben dem Aufbau der Apparatur stand die Aufklärung der Dynamik angeregter Zustände von aromatischen Heterocyclen und Pyrenen im Fokus dieser Arbeit. Die untersuchten Moleküle wurden durch Resonanzverstärkte Mehrphotonenionisation in einem Molekularstrahlexperiment sowohl zeit-, als auch frequenzaufgelöst untersucht.
Theory predicts peculiar features for excited-state dynamics in one dimension (1D) that are difficult to be observed experimentally. Single-walled carbon nanotubes (SWNTs) are an excellent approximation to 1D quantum confinement, due to their very high aspect ratio and low density of defects. Here we use ultrafast optical spectroscopy to probe photogenerated charge-carriers in (6,5) semiconducting SWNTs. We identify the transient energy shift of the highly polarizable S\(_{33}\) transition as a sensitive fingerprint of charge-carriers in SWNTs. By measuring the coherent phonon amplitude profile we obtain a precise estimate of the Stark-shift and discuss the binding energy of the S\(_{33}\) excitonic transition. From this, we infer that charge-carriers are formed instantaneously (<50 fs) even upon pumping the first exciton, S\(_{11}\). The decay of the photogenerated charge-carrier population is well described by a model for geminate recombination in 1D.
In the first part of his work, the causes for the sudden degradation of useable capacity of lithium-ion cells have been studied by means of complementary methods such as computed tomography, Post-Mortem studies and electrochemical analyses. The results obtained point unanimously to heterogeneous aging as a key-factor for the sudden degradation of cell capacity, which in turn is triggered by differences in local compression.
At high states of health, the capacity fade rate is moderate but some areas of the graphite electrode degrade faster than others. Still, the localized changes are hardly noticeable on cell level due to averaging effects. Lithium plating occurs first in unevenly compressed areas, creating patterns visible to the human eye. As lithium plating leads to rapid consumption of active lithium, a sudden drop in capacity is observed on cell level. Lithium plating appears to spread out from the initial areas over the whole graphite electrode, quickly consuming the remaining useful lithium and active graphite. It can be hypothesized that a self-amplifying circle of reciprocal acceleration of local lithium loss and material loss causes rapid local degradation.
Battery cell designers can improve cycle life by homogeneous pressure distribution in the cell and using negative active materials that are resilient to elevated discharge potentials such as improved carbons or lithium titanate. Also, a sufficiently oversized negative electrode and suitable electrolyte additives can help to avoid lithium plating. When packs are designed, care must be taken not to exert local pressure on parts of cells and to avoid both very high and low states of charge.
In the second part of this dissertation the resilience of cylindrical and pouchbag cells to shocks and different vibrations was investigated. Stresses inflicted by vibration and shock tests according to the widely recognized UN38.3 transport test were compared to a long-time test that exposed cells to a 186 days long ordeal of sine sweep vibrations with a profile based on real-world applications. All cells passed visual and electric inspection performed by TU München after the vibration tests. Only cylindrical cells subjected to long-term vibrations in axial direction showed an increase in impedance and a loss of capacity that could be recuperated in part.
The detailed analyses presented in this thesis gave more details on the damages inflicted by vibrations and shocks and revealed drastic damages in some cases. In cylindrical cells, only movement in axial direction caused damage. Long term vibrations were found to be especially detrimental.
No damage whatsoever could be detected for pouch cells, regardless of the test protocol and the direction of movement. The extreme resilience of pouchbag cells shows that the electrode stack of lithium-ion cells is resistant to vibrations, and that damages are caused by design imperfections that can be improved at low cost.
The findings of this work, and the general state of research show that it is most crucial to control the lithiation and thus potential of the graphite electrode.
In the last part of this work, a new, direct method for charge estimation based on changing transmission is presented. A correlation between transmission of short ultrasonic pulses and state of charge is found. This new technology allows direct measurement of the state of charge. The method is demonstrated for batteries with different positive active materials, showing its versatility. As the observed changes can be traced to the lithiation of graphite, it can be determined without a reference electrode. Already at this early stage of development, the found correlations allow estimation of state of charge. The present hysteresis in the signal height of the slow wave, which is unneglectable especially during discharging at higher currents, will be subject to further investigation.
The observed effects can be explained by effects on different length scales. Biot’s theory explains the second wave’s slowness based on the active material particles size in the range of 0.01 mm and electrolyte-filled pores. Lithiation of graphite changes the porosity of the electrode and thereby the velocity and wavelength of the impulse. When the wavelength approaches the length scale of the layers, 0.1 mm, scattering effects dampen the transmitted signal. Finally, the wavelength of the pulse should be shorter than the transducers diameter to obtain a homogeneous wave front.
To conclude, the new method allows the control of each individual cell in a pack independent from the electrical connections of the cells.
As the method shows great promise, further studies regarding factors such as long-term behavior, temperature and current rates should be conducted. In this thesis hysteresis was observed and a deeper understanding of the reasons behind it may allow further improvements of measurement precision.
In this thesis the excited-state dynamics of radicals and biradicals were characterized with femtosecond pump-probe spectroscopy.
These open-shell molecules play important roles as combustion intermediates, in the formation of soot and polycyclic aromatic hydrocarbons, in atmospheric chemistry and in the formation of complex molecules in the interstellar medium and galactic clouds. In these processes molecules frequently occur in some excited state, excited either by thermal energy or radiation. Knowledge of the reactivity and dynamics of these excited states completes our understanding of these complex processes.
These highly reactive molecules were produced via pyrolysis from suitable precursors and examined in a molecular beam under collision-free conditions. A first laser now excites the molecule, and a second laser ionizes it. Time-of-flight mass spectrometry allowed a first identification of the molecule, photoelectron spectroscopy a complete characterization of the molecule - under the condition that the mass spectrum was dominated by only one mass. The photoelectron spectrum was obtained via velocity-map imaging, providing an insight in the electronic states involved. Ion velocity map imaging allowed separation of signal from direct ionization of the radical in the molecular beam and dissociative photoionization of the precursor. During this thesis a modified pBasex algorithm was developed and implemented in python, providing an image inversion tool without interpolation of data points. Especially for noisy photoelectron images this new algorithm delivers better results.
Some highlighted results:
• The 2-methylallyl radical was excited in the ππ*-state with different internal energies using three different pump wavelengths (240.6 , 238.0 and 236.0 nm). Ionized with 800 nm multi-photon probe, the photoelectron spectra shows a s-Rydberg fingerprint spectrum, a highly positive photoelectron anisotropy of 1.5 and a bi-exponential decay ( τ1= 141\pm43 fs, τ2= 4.0\pm0.2 ps for 240.6 nm pump), where the second time-constant shortens for lower wavelengths. Field-induced surface hopping dynamics calculations confirm that the initially excited ππ*-state relaxes very fast to an s-Rydberg state (first experimentally observed time-constant), and then more slowly to the first excited state/ground state (second time-constant). With higher excitation energies the conical intersection between the s-Rydberg-state and the first excited state is reached faster, resulting in shorter life-times.
• The benzyl radical was excited yith 265 nm and probed with two wavelengths, 798 nm and 398 nm. Probed with 798 nm it shows a bi-exponential decay (\tau_{1}=84\pm5 fs, \tau_{2}=1.55\pm0.12 ps), whereas with 398 nm probe only the first time-constant is observed (\tau_{1}=89\pm5 fs). The photoelectron spectra with 798 nm probe is comparable to the spectrum with 398 nm probe during the first 60 fs, at longer times an additional band appears. This band is due to a [1+3']-process, whereas with 398 nm only signal from a [1+1']-process can be observed. Non-adiabatic dynamic on the fly calculations show that the initially excited, nearly degenerate ππ/p-Rydberg-states relax very fast (first time-constant) to an s-Rydberg state. This s-Rydberg state can no longer be ionized with 398 nm, but with 798 nm ionization via intermediate resonances is still possible. The s-Rydberg state then decays to the first excited state (second time-constant), which is long-lived.
• Para-xylylene, excited with 266 nm into the S2-state and probed with 800 nm, shows a bi-exponential decay (\tau_{1}=38\pm7 fs, \tau_{2}=407\pm9 fs). The initially excited S2-state decays quickly to S1-state, which shows dissociative photoionization. The population of the S1-state is directly visible in the masses of the dissociative photoionization products, benzene and the para-xylylene -H.
• Ortho-benzyne, produced via pyrolysis from benzocyclobutendione, was excited with 266 nm in the S2 state and probed with 800 nm. In its time-resolved mass spectra the dynamic of the ortho-benzyne signal was superposed with the dynamics from dissociative photoionization of the precursor and of the ortho-benzyne-dimer. With time-resolved ion imaging gated on the ortho-benzyne these processes could be seperated, showing that the S2-state of ortho-benzyne relaxes within 50 fs to the S1-state.
Polypeptoids are an old but recently rediscovered polymer class with interesting synthetic, physico-chemical and biological characteristics. Here, we introduce new aromatic monomers, N-benzyl glycine N-carboxyanhydride and N-phenethyl glycine N-carboxyanhydride and their block copolymers with the hydrophilic polysarcosine. We compare their self-assembly in water and aqueous buffer with the self-assembly of amphiphilic block copolypeptoids with aliphatic side chains. The aggregates in water were investigated by dynamic light scattering and electron microscopy. We found a variety of morphologies, which were influenced by the polymer structure as well as by the preparation method. Overall, we found polymersomes, worm-like micelles and oligo-lamellar morphologies as well as some less defined aggregates of interconnected worms and vesicles. Such, this contribution may serve as a starting point for a more detailed investigation of the self-assembly behavior of the rich class of polypeptoids and for a better understanding between the differences in the aggregation behavior of non-uniform polypeptoids and uniform peptoids.
Diese Arbeit befasst sich mit zeitaufgelösten Prozessen in molekularen Systemen. Dabei wurde sowohl die Wellenpaketdynamik nach Photoanregung betrachtet als auch spektrale Eigenschaften mittels Absorptions- und zweidimensionaler Spektroskopie untersucht.
Zunächst widmet sich die Arbeit der Wellenpaket- und Populationsdynamik in zwei diabatischen, gekoppelten Zuständen. Nach impulsiver Anregung aus dem zu Beginn besetzten Zustand treten in der Populationsdynamik zwei deutlich verschiedene Oszillationen auf. Der langsamer variierende Populationstransfer besitzt die Periodendauer der Vibrationsbewegung und ist auf einen Wechsel der Zustände beim Durchlaufen des Wellenpakets durch die Kreuzungsregion der diabatischen Potentiale zurückzuführen. Die ultraschnelle Komponente mit einer Periodendauer von etwa 4 fs lässt sich als eine Art Rabi-Oszillation beschreiben, die durch die (zeitunabhängige) Kopplung hervorgerufen wird. Sie wurde mit Hilfe von analytischen Berechnungen ausführlich charakterisiert. Damit dieser Prozess auftreten kann müssen mehrere Bedingungen erfüllt werden: Das Wellenpaket muss über die Dauer der Oszillationen annähernd örtlich lokalisiert bleiben; dies ist an den Umkehrpunkten der Wellenpaketsbewegung der Fall. Die Amplitude der Oszillationen in den Populationen ist proportional zum Verhältnis der Kopplung zum Energieabstand der Zustände. Deshalb muss an den stationären Stellen die Kopplung groß im Vergleich zum Energieabstand sein. Die Amplitude der Oszillationen hängt außerdem von dem Populationsverhältnis und den Phasen der Komponenten des Wellenpakets in den beiden Zuständen ab. Die ultraschnellen Oszillationen bleiben auch in mehrdimensionalen Systemen mit unterschiedlichen Vibrationsfrequenzen je Freiheitsgrad erhalten.
Das gleiche Modell wurde benutzt, um Ladungstransferprozesse mittels linearer und 2D-Spektroskopie zu untersuchen. Eine Kopplung an die Umgebung wurde, aufbauend auf einer Quanten-Master-Gleichung in Markov-Näherung, wellenfunktionsbasiert mittels eines Quantum-Jump-Algorithmus mit expliziter Dephasierung beschrieben. Dabei findet mit vorher definierten Wahrscheinlichkeiten zu jedem Zeitschritt einer von drei stochastischen Prozessen statt. Neben kohärenter Propagation können Sprünge in einen anderen Eigenzustand des Systems und Dephasierungen auftreten. Zwei Dissipationsparameter spielen dabei eine Rolle. Dies ist zum einen die Stärke der System-Bad-Kopplung, welche die Gesamtrate der Energierelaxation beschreibt. Weiterhin beeinflusst die Dephasierungskonstante den Verlust kohärenter Phasen ohne Energieänderung. Fallenzustände wurden identifiziert, die durch sehr geringe Sprungraten in niedrigere Zustände charakterisiert sind. Die Langlebigkeit kann durch die Form der Eigenfunktionen erklärt werden, die eine deutlich andere Wahrscheinlichkeitsverteilung als die der Nicht-Fallenzustände besitzen. Dadurch werden die in die Sprungraten eingehenden Matrixelemente klein. Das Absorptionsspektrum zeigt Peaks an der Stelle der Fallenzustände, da nur die Eigenfunktionen der Fallenzustände große Franck-Condon-Faktoren mit der Anfangswellenfunktion besitzen. Verschiedene Kombinationen der Dissipationsparameter führen zu Änderungen der relativen Peakintensitäten und der Peakbreiten.
Die 2D-Spektren des Ladungstransfersystems werden störungstheoretisch über die Polarisation dritter Ordnung berechnet. Sie zeigen viele eng nebeneinander liegende Peaks in einer schachbrettmusterförmigen Anordnung, die sich auf Übergänge unter Mitwirkung der Fallenzustände zurückführen lassen. Höhere System-Bad-Kopplungen führen aufgrund der effizienten Energiedissipation zu einer Verschiebung zu kleineren Energien. Peaks, die mit schneller zerfallenden Fallenzuständen korrespondieren, bleichen schneller aus. Höhere Dephasierungskonstanten resultieren in verbreiterten Peaks. Um den Einfluss der Dissipation genauer zu charakterisieren, wurden gefilterte 2D-Spektren betrachtet. Dazu wurden Ausschnitte der Polarisation dritter Ordnung zu verschiedenen Zeiten fouriertransformiert. Längere Zeiten führen zu einer effektiveren Energierelaxation entlang der entsprechenden Zeitvariablen. Die Entvölkerung der höher liegenden Zustände lässt sich somit zeit- und energieaufgelöst betrachten. Weiterhin wurde gezeigt, dass sich der Zerfall eines einzelnen Peaks mit dem Populationsabfall des damit korrespondierenden Eigenzustandes in Einklang bringen lässt, obwohl die Zuordnung der Peaks im 2D-Spektrum zu Übergängen zwischen definierten Eigenzuständen nicht eindeutig ist.
Mit dem benutzten eindimensionalen Modell können auch Ladungstransferprozesse in organischen gemischtvalenten Verbindungen beschrieben werden. Es wurde die Frage untersucht, welche Prozesse nach einem optisch induzierten Energietransfer in solchen Systemen ablaufen. Experimentelle Daten (aufgenommen im Arbeitskreis von Prof. Lambert) deuten auf eine schnelle interne Konversion (IC) gefolgt von Thermalisierung hin. Um dies theoretisch zu überprüfen, wurden Absorptionsspektren bei verschiedenen Temperaturen berechnet und mit den gemessenen transienten Spektren verglichen. Es findet sich, abhängig von der Stärke der elektronischen Kopplung, eine sehr gute bis gute Übereinstimmung, was die Annahme eines schnellen ICs stützt.
Im letzten Teil der Arbeit wurden vibronische 2D-Spektren von molekularen Aggregaten betrachtet. Dazu wurde die zeitabhängige Schrödingergleichung für ein Monomer-, Dimer- und Trimersystem mit der Multi-Configuration Time-Dependent Hartree-Methode gelöst und die Polarisation nicht-störungstheoretisch berechnet. Der Hamiltonoperator des Trimers umfasst hierbei sieben gekoppelte elektronische Zustände und drei bzw. sechs Vibrationsfreiheitsgrade. Der betrachtete Photonenecho-Beitrag der Polarisation wurde mittels phasencodierter Laserpulse extrahiert. Die resultierenden Spektren sind geometrieabhängig, ein Winkel zwischen den Übergangsdipolmomenten der Monomere von 0° (180°) resultiert in einem H-Aggregat (J-Aggregat). Die Lage und Intensität der Peaks im rein elektronischen Trimer wurde analytisch erläutert. Die Spektren unter Einbeziehung der Vibration zeigen eine ausgeprägte vibronische Struktur. Es wurde gezeigt, wie die Spektren für höhere Aggregationsgrade durch die höhere Dichte an vibronischen Zuständen komplexer werden. Im J-Aggregat ist mit zunehmender Aggregation eine stärkere Rotverschiebung zu sehen. Das Spektrum des H-Aggregats zeigt eine im Vergleich zum J-Aggregat kompliziertere Struktur. Die Verwendung zweier Vibrationsfreiheitsgrade je Monomer führt zu Spektren mit überlappenden Peaks und einer zusätzlichen vibronischen Progression. Der Vergleich von Spektren verschiedener Mischungen von Monomer, Dimer und Trimer, entsprechend einem von Temperatur und Konzentration abhängigen Aggregationsgrad, zeigt den Einfluss dieser experimentellen Faktoren. Schließlich wurden mögliche Ansätze aufgezeigt, anhand der Spektren auf den Aggregationsgrad zu schließen.
Time-resolved spectroscopy allows for analyzing light-induced energy conversion and
chromophore–chromophore interactions in molecular systems, which is a prerequisite in
the design of new materials and for improving the efficiency of opto-electronic devices.
To elucidate photo-induced dynamics of complex molecular systems, transient absorption
(TA) and coherent two-dimensional (2D) spectroscopy were employed and combined
with additional experimental techniques, theoretical approaches, and simulation models
in this work.
A systematic series of merocyanines, synthetically varied in the number of chromophores
and subsitution pattern, attached to a benzene unit was investigated in cooperation with
the group of Prof. Dr. Frank Würthner at the University of Würzburg. The global analysis
of several TA experiments, and additional coherent 2D spectroscopy experiments, provided
the basis to elaborate a relaxation scheme which was applicable for all merocyanine
systems under investigation. This relaxation scheme is based on a double minimum on the
excited-state potential energy surface. One of these minima is assigned to an intramolecular
charge-transfer state which is stabilized in the bis- and tris-chromophoric dyes by
chromphore–chromophore interactions, resulting in an increase in excited-state lifetime.
Electro-optical absorption and density functional theory (DFT) calculations revealed a
preferential chromophore orientation which compensates most of the dipole moment of
the individual chromophores. Based on this structural assignment the conformationdependent
exciton energy splitting was calculated. The linear absorption spectra of the
multi-chromophoric merocyanines could be described by a combination of monomeric and
excitonic spectra.
Subsequently, a structurally complex polymeric squaraine dye was studied in collaboration
with the research groups of Prof. Dr. Christoph Lambert and Prof. Dr. Roland Mitric
at the University of Würzburg. This polymer consists of a superposition of zigzag and
helix structures depending on the solvent. High-level DFT calculations confirmed the previous
assignment that zigzag and helix structures can be treated as J- and H-aggregates,
respectively. TA experiments revealed that in dependence on the solvent as well as the
excitation energy, ultrafast energy transfer within the squaraine polymer proceeds from
initially excited helix segments to zigzag segments or vice versa. Additionally, 2D spectroscopy
confirmed the observed sub-picosecond dynamics. In contrast to other conjugated
polymers such as MEH-PPV, which is investigated in the last chapter, ultrafast
energy transfer in squaraine polymers is based on the matching of the density of states
between donor and acceptor segments due to the small reorganization energy in cyanine-like
chromophores.
Finally, the photo-induced dynamics of the aggregated phase of the conjugated polymer
MEH-PPV was investigated in cooperation with the group of Prof. Dr. Anna Köhler at the University of Bayreuth. Our collaborators had previously described the aggregation of MEH-PPV upon cooling by the formation of so-called HJ-aggregates based on exciton
theory. By TA measurements and by making use of an affiliated band analysis distinct
relaxation processes in the excited state and to the ground state were discriminated. By
employing 2D spectroscopy the energy transfer between different conjugated segments
within the aggregated polymer was resolved. The initial exciton relaxation within the
aggregated phase indicates a low exciton mobility, in contrast to the subsequent energy
transfer between different chromophores within several picoseconds.
This work contributes by its systematic study of structure-dependent relaxation dynamics
to the basic understanding of the structure-function relationship within complex
molecular systems. The investigated molecular classes display a high potential to increase
efficiencies of opto-electronic devices, e.g., organic solar cells, by the selective choice of
the molecular morphology.
We present a fast and sensitive polarimeter combining common-path optical heterodyne interferometry and accumulative spectroscopy to detect rotatory power. The sensitivity of rotatory detection is determined to be 0.10 milli-degrees for a measurement time of only one second and an interaction length of 250 µm. Its suitability for femtosecond studies is demonstrated in a non-resonant two-photon photodissociation experiment.
We use pump-repump-probe transient absorption spectroscopy to investigate the role of higher-lying electronic states in the photochemistry of a molecular switch. Moreover, replacing the pump pulse by a pulse-shaper-generated phase-stable double pulse, triggered-exchange two-dimensional (TE2D) electronic spectroscopy is established in the visible regime.
Ultraviolet irradiation of CO-releasing molecules (CORMs) in water eventually leads to the loss of several carbon monoxide ligands.We show for an exemplary manganese tricarbonyl CORM that only one ligand is photolyzed off on an ultrafast timescale and that some molecules may undergo geminate recombination.
Three spectroscopic techniques are presented that provide simultaneous spatial and temporal resolution: modified confocal microscopy with heterodyne detection, space-time-resolved spectroscopy using coherent control concepts, and coherent two-dimensional nano-spectroscopy. Latest experimental results are discussed.
Organic semiconductors are attractive for optical sensing applications due to the effortless processing on large active area of several \(cm^2\), which is difficult to achieve with solid-state devices. However, compared to silicon photodiodes, sensitivity and dynamic behavior remain a major challenge with organic sensors. Here, we show that charge trapping phenomena deteriorate the bandwidth of organic photodiodes (OPDs) to a few Hz at low-light levels. We demonstrate that, despite the large OPD capacitances of similar to 10 nF \(cm^{-2}\), a frequency response in the kHz regime can be achieved at light levels as low as 20 nW \(cm^{-2}\) by appropriate interface engineering, which corresponds to a 1000-fold increase compared to state-of-the-art OPDs. Such device characteristics indicate that large active area OPDs are suitable for industrial sensing and even match medical requirements for single X-ray pulse detection in the millisecond range.
Within the framework of this thesis, photolysis reactions in the liquid phase were investigated by means of ultrafast optical spectroscopy. Apart from molecular studies dealing with the highly spin-dependent reactivity of diphenylcarbene (DPC) in binary solvent
mixtures and ligand dissociation reactions of so-called CO-releasing molecules (CORMs),
special emphasis was put on the implementation and characterization of methods improving
and extending the signal detection in conventional pump–probe transient absorption setups.
The assumption of DPC being an archetypal triplet-ground-state arylcarbene was recently questioned by matrix-isolation studies at low temperatures. DPC embedded in argon matrices revealed a hitherto unknown reactivity when the carbene environment was modified by small amounts of methanol dopant molecules. To complement these findings with liquid-phase experiments at room temperature, femtosecond pump–probe transient absorption spectroscopy with probing in the visible and ultraviolet regime was employed to unravel primary reaction processes of DPC in solvent mixtures. Supported by quantum chemical simulations conducted by our collaborators, it was shown that a competition between the reaction pathways occurs that not only depends on the solvent molecule near-by but also on its interaction with other solvent molecules. In-depth analysis of the solvation dynamics and the amount of nascent intermediates corroborates the importance of a hydrogen-bonded complex with a protic solvent molecule, in striking analogy to complexes found at cryogenic temperatures.
Probing the transient absorption of molecules in the mid-infrared spectral range benefits from the high chemical specificity of molecules’ vibrational signatures. The technique of chirped-pulse upconversion (CPU) constitutes a promising alternative to standard direct multichannel MCT detection when accessing this spectral detection window. Hence, one chapter of this thesis is dedicated to a direct comparison between both detection methods. By conducting an exemplary pump–probe transient absorption experiment, it became evident, that the additional nonlinear interaction step is responsible for increased noise levels when using CPU. However, a correction procedure capable of removing these additional noise contributions—stemming from the fundamental laser radiation used for upconversion—was successfully tested. Perhaps most importantly for various spectroscopic applications, CPU scored with a significantly extended detection bandwidth owing to the high pixel numbers of modern CCD cameras.
Transition-metal complexes capable of releasing small molecular messengers upon photoactivation are promising sources of gasotransmitters such as carbon monoxide (CO) or nitric oxide (NO) in biological applications. However, only little is known about the characteristic time scales of ligand dissociation in this class of molecules. For this purpose, two complexes were investigated with femtosecond time resolution: [Mn(CO)3(tpm)]Cl with tpm=tris(2-pyrazolyl)methane, a manganese tricarbonyl complex which has proven to be selective and cytotoxic to cancer cells, and [Mo(CO)2(NO)(iPr3tacn)]PF6 with iPr3tacn=1,4,7-triisopropyl-1,4,7-triazacyclononane, a molybdenum complex containing both carbonyl and nitrosyl ligands. By conducting pump–probe transient absorption measurements in different spectral probing windows supported by quantum chemical calculations and linear absorption spectroscopy, it was shown that both complexes are able to release one CO ligand within the first few picoseconds after UV excitation. The results complement existing studies which focused on the molecules’ ligand-releasing properties upon long-term exposure. The additional information gained on an ultrafast time scale provides a comprehensive understanding of individual reaction steps connected with ligand release in this class of molecules. Hence, the studies might create new incentives to develop modified molecules for specific applications.