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The first is via direct dissociation and the second likely involves a barrier slowing down dissociation.
Chlorine-containing hydrocarbons pose a great risk for the environment and especially for the atmosphere. In this thesis I present the photodissociation dynamics of multiple chlorine-containing molecules. The method of velocity map imaging was utilized for gaining information on the kinetic energy distribution of the fragments generated in the photodissociation reactions.
First, the photodissociation of benzoyl chloride after excitation to the S1, S2 and the S3 state between 279 nm and 237 nm was studied. This stable molecule was an ideal candidate for demonstrating a new ionization scheme for chlorine atoms. It was shown that benzoyl chloride dissociates statistically from the ground state.
Afterwards, the results from experiments on the radicals trichloromethyl and dichlorocarbene are presented in the range of 230 to 250 nm. These radicals remain after the dissociation of carbon tetrachloride and have not been studied in detail because of their instability. Trichlormethyl dissociates via two paths: The loss of a chlorine atom to dichlorocarbene and by decaying to CCl and a chlorine molecule. The dissociation to dichlorocarbene involves a barrier. If the photon exciting the molecule has enough energy to surpass the barrier, which is the case starting at around 235 nm, trichlormethyl dissociates rapidly resulting in an anisotropic VMI. However, if the the excitation energy is lower, the dissociation takes longer than a rotational period and the anisotropy is lost.The path to CCl is a statistical dissociation.
Dichlorocarbene dissociates to CCl and Cl via to separate channels. The first is via direct dissociation and the second likely involves a barrier slowing down dissociation.
This work presents excited state investigations on several systems with respect to experimental
spectroscopic work. The majority of projects covers the temporal evolution of
excitations in thin films of organic semiconductor materials. In the first chapters, thinfilm
and interface systems are build from diindeno[1,2,3-cd:1’,2’,3’-lm]perylene (DIP)
and N,N’-bis-(2-ethylhexyl)-dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDIR-CN2)
layers, in the third chapter bulk systems consist of 4,4’,4”-tris[(3-methylphenyl)phenylamino]
triphenylamine (m-MTDATA), 4,7-diphenyl-1,10-phenanthroline (BPhen) and
tris-(2,4,6-trimethyl-3-(pyridin-3-yl)phenyl)borane (3TPYMB). These were investigated
by aggregate-based calculations. Careful selection of methods and incorporation
of geometrical relaxation and environmental effects allows for a precise energetical assignment
of excitations. The biggest issue was a proper description of charge-transfer
excitations, which was resolved by the application of ionization potential tuning on
aggregates. Subsequent characterization of excitations and their interplay condenses
the picture. Therefore, we could assign important features of the experimental spectroscopic
data and explain differences between systems.
The last chapter in this work covers the analysis of single molecule spectroscopy on
methylbismut. This poses different challenges for computations, such as multi-reference
character of low-lying excitations and an intrinsic need for a relativistic description.
We resolved this by combining complete active space self-consistent field based methods
with scalarrelativistic density-functional theory. Thus we were able to confidently
assign the spectroscopic features and explain underlying processes.
The development of ligands capable of effectively stabilizing highly reactive main‐group species has led to the experimental realization of a variety of systems with fascinating properties. In this work, we computationally investigate the electronic, structural, energetic, and bonding features of proximity‐enforced group 13–15 homodimers stabilized by a rigid expanded pincer ligand based on the 1,8‐naphthyridine (napy) core. We show that the redox‐active naphthyridine diimine (NDI) ligand enables a wide variety of structural motifs and element‐element interaction modes, the latter ranging from isolated, element‐centered lone pairs (e.g., E = Si, Ge) to cases where through‐space π bonds (E = Pb), element‐element multiple bonds (E = P, As) and biradical ground states (E = N) are observed. Our results hint at the feasibility of NDI‐E2 species as viable synthetic targets, highlighting the versatility and potential applications of napy‐based ligands in main‐group chemistry.
Isolated 2‐phenylallyl radicals (2‐PA), generated by pyrolysis from a nitrite precursor, have been investigated by IR/UV ion dip spectroscopy using free electron laser radiation. 2‐PA is a resonance‐stabilized radical that is considered to be involved in the formation of polycyclic aromatic hydrocarbons (PAH) in combustion, but also in interstellar space. The radical is identified based on its gas‐phase IR spectrum. Furthermore, a number of bimolecular reaction products are identified, showing that the self‐reaction as well as reactions with unimolecular decomposition products of 2‐PA form several PAH efficiently. Possible mechanisms are discussed and the chemistry of 2‐PA is compared with the one of the related 2‐methylallyl and phenylpropargyl radicals.
Radiationless energy transfer is at the core of diverse phenomena, such as light harvesting in photosynthesis\(^1\), energy-transfer-based microspectroscopies\(^2\), nanoscale quantum entanglement\(^3\) and photonic-mode hybridization\(^4\). Typically, the transfer is efficient only for separations that are much shorter than the diffraction limit. This hampers its application in optical communication and quantum information processing, which require spatially selective addressing. Here, we demonstrate highly efficient radiationless coherent energy transfer over a distance of twice the excitation wavelength by combining localized and delocalized\(^5\) plasmonic modes. Analogous to the Tavis-Cummings model, two whispering-gallery-mode antennas\(^6\) placed in the foci of an elliptical plasmonic cavity\(^7\) fabricated from single-crystal gold plates act as a pair of oscillators coupled to a common cavity mode. Time-resolved two-photon photoemission electron microscopy (TR 2P-PEEM) reveals an ultrafast long-range periodic energy transfer in accordance with the simulations. Our observations open perspectives for the optimization and tailoring of mesoscopic energy transfer and long-range quantum emitter coupling.
Simulationen zur transienten Absorptionsspektroskopie an Energie- und Ladungstransfersystemen
(2022)
Anregungsinduzierte Ladungstransferprozesse gemischtvalenter Verbindungen in einem, bzw. zwei Vibrationsfreiheitsgraden werden mithilfe vibronischer Modellsysteme untersucht. Anhand transienter und linearer Absorptionsspektren werden die berechneten mit experimentell bestimmten Daten verglichen. Eine detailliertere theoretische Analyse erfolgt unter den Gesichtspunkten der Populations- und Wellenpaketdynamik.
Darüber hinaus wird der Prozess der Exziton-Exziton-Annihilierung mithilfe eines elektronischen Modellsystems untersucht. Zu diesem Zweck werden, zusätzlich zu den oben genannten Methoden, spektroskopische Signale unterschiedlicher Emissionsrichtungen zum Vergleich herangezogen.
In this thesis, several contributions to the understanding and modeling of chemical phenomena using computational approaches are presented. These investigations are characterized by the usage of non-standard computational modeling techniques, which is necessitated by the complex nature of the electronic structure or atomic fluctuations of the target molecules.
Multiple biradical-type molecules and their spectroscopic properties were modeled. In the course of the investigation, it is found that especially the impact of correct molecular geometries on the computationally predicted absorption properties may be critical. In order to find the correct minimum geometries, Multi-Reference methods may have to be invoked.
The impact of geometry relaxation on the excitonic properties of Perylene Bisimide dimers were investigated. Oftentimes, these geometry factors are neglected in Organic Semiconductor modeling as an approximation. This present investigation suggests that this approximation is not always valid, as certain regimes are identified where geometrical parameters have critical impact on the localization and energetic properties of excitons.
The mechanism of the Triazolinedione (TAD) tyrosine bioconjugation reaction is investigated using quantum-chemical methods. By comparison of different conceivable mechanisms and their energetic ordering, the TAD tyrosine bioconjugation is found to proceed by means of a base-mediated electrophilic aromatic substitution reaction.
The kth nearest neighbor entropy estimation protocol is investigated. This estimator promises accurate entropy estimates even for flexible molecules with multiple structural minima. Our granular investigation of formal and practical properties of the estimator suggests that the uneven variance of a molecule’s vibrational modes is the cause of the observed slow convergence of the estimator. A rescaling procedure to reestablish fast convergence is suggested and benchmarks are performed.
The facile synthesis and detailed investigation of a class of highly potent protease inhibitors based on 1,4-naphthoquinones with a dipeptidic recognition motif (HN-l-Phe-l-Leu-OR) in the 2-position and an electron-withdrawing group (EWG) in the 3-position is presented. One of the compound representatives, namely the acid with EWG = CN and with R = H proved to be a highly potent rhodesain inhibitor with nanomolar affinity. The respective benzyl ester (R = Bn) was found to be hydrolyzed by the target enzyme itself yielding the free acid. Detailed kinetic and mass spectrometry studies revealed a reversible covalent binding mode. Theoretical calculations with different density functionals (DFT) as well as wavefunction-based approaches were performed to elucidate the mode of action.
Electrophilic (het)arenes can undergo reactions with nucleophiles yielding π- or Meisenheimer (σ-) complexes or the products of the S\(_N\)Ar addition/elimination reactions. Such building blocks have only rarely been employed for the design of enzyme inhibitors. Herein, we demonstrate the combination of a peptidic recognition sequence with such electrophilic (het)arenes to generate highly active inhibitors of disease-relevant proteases. We further elucidate an unexpected mode of action for the trypanosomal protease rhodesain using NMR spectroscopy and mass spectrometry, enzyme kinetics and various types of simulations. After hydrolysis of an ester function in the recognition sequence of a weakly active prodrug inhibitor, the liberated carboxylic acid represents a highly potent inhibitor of rhodesain (K\(_i\) = 4.0 nM). The simulations indicate that, after the cleavage of the ester, the carboxylic acid leaves the active site and re-binds to the enzyme in an orientation that allows the formation of a very stable π-complex between the catalytic dyad (Cys-25/His-162) of rhodesain and the electrophilic aromatic moiety. The reversible inhibition mode results because the S\(_N\)Ar reaction, which is found in an alkaline solvent containing a low molecular weight thiol, is hindered within the enzyme due to the presence of the positively charged imidazolium ring of His-162. Comparisons between measured and calculated NMR shifts support this interpretation
Quantenchemische Untersuchungen von Umgebungseinflüssen bei offen- und geschlossenschaligen Systemen
(2022)
In dieser Dissertation werden die Umgebungseinflüsse auf die strukturellen und elektronischen Eigenschaften von verschiedenen offen- und geschlossenschaligen Systemen mittels quantenchemischer Methoden berechnet.
Ein Kernpunkt umfasst die Untersuchung von verdreht angeordneten, biradikalischen Diborylalkenen, welche eine ungesättigte C2R2-Brücke (R = Et, Me) besitzen und durch cyclische (Alkyl)(amino)carbene (CAACs) stabilisiert werden. Quantenchemische Berechnungen zeigen, dass hauptsächlich sterische Effekte für die Ausbildung einer verdrehten Molekülanordnung verantwortlich sind, während bei geringen sterischen Wechselwirkungen (R = H) die Delokalisationseffekte überwiegen, wodurch eine planare Struktur begünstigt wird. Die Bevorzugung einer offenschaligen Singulettkonfiguration anstelle eines Tripletts ist auf den großen Energieunterschied der beiden einfach besetzten Molekülorbitale zurückzuführen. Durch die Berechnung der Lösungsmitteleffekte mithilfe von polarisierbaren Kontinuumsmodellen kann gefolgert werden, dass mit zunehmender statischer Dielektrizitätskonstante eine planare und geschlossenschalige Struktur stärker stabilisiert wird als eine verdrehte Anordnung.
Ein weiteres Thema dieser Dissertation befasst sich mit der quantenchemischen Analyse eines makrozyklischen Perylenbisimid-Trimersystems, welches eingebettet in einer Polymethylmethacrylat-Matrix bei Temperaturen nahe dem absoluten Nullpunkt eine Lokalisierung der ersten drei angeregten Zustände zeigt. Quantenchemische Vakuumberechnungen ergeben, dass unabhängig von der gegenseitigen geometrischen Orientierung der drei Perylenbisimid-Chromophore der Übergang vom Grundzustand in den S1-Zustand verboten ist und dass die ersten drei angeregten Zustände delokalisiert vorliegen. Mithilfe von expliziten Lösungsmittelmodellen kann jedoch gezeigt werden, dass das Auftreten dieser Lokalisierungen auf eine inhomogene Polymethylmethacrylat-Umgebung zurückzuführen ist, die zu einem Symmetriebruch und somit zu einer Zunahme der Oszillatorstärke für S1 und der Lokalisierungsgrade für S1, S2 und S3 führt.
Darüber hinaus wird der Lösungsmitteleinfluss auf die angeregten Zustände des Azulens mittels impliziter und expliziter Lösungsmittelmodelle berechnet. Bei einer Erhöhung der dynamischen Dielektrizitätskonstante im impliziten Modell nehmen die Anregungsenergien der vertikalen Singulettzustände ab, wobei der Effekt mit steigender Oszillatorstärke zunimmt. Die Auswirkung der statischen Dielektrizitätskonstante auf die Anregungsenergien ist dagegen deutlich schwächer ausgeprägt. Im expliziten Modell bewirkt das Lösungsmittel ebenfalls eine Abnahme der Anregungsenergie des hellen Singulettzustands, wenn auch in geringerem Umfang als im impliziten Modell.
Als letztes Thema wird der Inhibitionsmechanismus der Cysteinprotease Rhodesain durch zwei modifizierte 1,4-Naphthoquinone untersucht. Während beide Naphthoquinone an der 2-Position eine Dipeptideinheit aufweisen, besitzen sie an der 3-Position entweder einen Nitril- oder Chloridsubstituenten. Zwar erfolgt bei beiden Derivaten die Inhibition über einen kovalent-reversiblen Mechanismus, jedoch verläuft die Hemmung im Falle des Nitrilderivats erheblich effektiver. Die quantenchemischen Berechnungen eines vereinfachten Modells zeigen, dass die Cysteineinheit (HS-R) bevorzugt in einer exothermen und reversiblen Additionsreaktion an die elektronenarme C-C-Doppelbindung der Naphthoquinone anlagert. Dabei werden kleinere Reaktionsenergien für die Reaktion des Chlorderivats als für die Reaktion des Nitrilderivats erhalten. Durch die Berücksichtigung von Wasser in einem impliziten Lösungsmittelmodell kommt es bei fast allen Reaktionsprodukten zu einer Energiezunahme, die bei der Reaktion des Nitrilderivats stärker ausfällt als bei der Reaktion des Chlorderivats.
We investigate NCl\(_{3}\) and the NCl\(_{2}\) radical by photoelectron-photoion coincidence spectroscopy using synchrotron radiation. The mass selected threshold photoelectron spectrum (ms-TPES) of NCl\(_{3}\) is broad and unstructured due to the large geometry change. An ionization energy of 9.7±0.1 eV is estimated from the spectrum and supported by computations. NCl2 is generated by photolysis at 213 nm from NCl\(_{3}\) and its ms-TPES shows an extended vibrational progression with a 90 meV spacing that is assigned to the symmetric N−Cl stretching mode in the cation. An adiabatic ionization energy of 9.94 ± 0.02 eV is determined.
Twisted boron-based biradicals featuring unsaturated C\(_2\)R\(_2\) (R=Et, Me) bridges and stabilization by cyclic (alkyl)(amino)carbenes (CAACs) were recently prepared. These species show remarkable geometrical and electronic differences with respect to their unbridged counterparts. Herein, a thorough computational investigation on the origin of their distinct electrostructural properties is performed. It is shown that steric effects are mostly responsible for the preference for twisted over planar structures. The ground-state multiplicity of the twisted structure is modulated by the σ framework of the bridge, and different R groups lead to distinct multiplicities. In line with the experimental data, a planar structure driven by delocalization effects is observed as global minimum for R=H. The synthetic elusiveness of C\(_2\)R\(_2\)-bridged systems featuring N-heterocyclic carbenes (NHCs) was also investigated. These results could contribute to the engineering of novel main group biradicals.
We introduce a new approach to transient spectroscopy, fluorescence‐detected pump–probe (F‐PP) spectroscopy, that overcomes several limitations of traditional PP. F‐PP suppresses excited‐state absorption, provides background‐free detection, removes artifacts resulting from pump–pulse scattering, from non‐resonant solvent response, or from coherent pulse overlap, and allows unique extraction of excited‐state dynamics under certain conditions. Despite incoherent detection, time resolution of F‐PP is given by the duration of the laser pulses, independent of the fluorescence lifetime. We describe the working principle of F‐PP and provide its theoretical description. Then we illustrate specific features of F‐PP by direct comparison with PP, theoretically and experimentally. For this purpose, we investigate, with both techniques, a molecular squaraine heterodimer, core–shell CdSe/ZnS quantum dots, and fluorescent protein mCherry. F‐PP is broadly applicable to chemical systems in various environments and in different spectral regimes.
Die ersten Beispiele für Lewis-Basen-Addukte des Stammboraphosphaketens H\(_{2}\)B-PCO und ihre cyclischen Dimere wurden hergestellt. Eines dieser Addukte zeigt unter milden Bedingungen eine Decarbonylierung und anschließende Insertion des Phosphinidens in die B-C-Bindung eines Borols, was in der Bildung sehr seltener Beispiele für 1,2-Phosphaborinine, B,P-Isostere von Benzol, resultiert. Die starken Donoreigenschaften dieser 1,2-Phosphaborinine wurden durch die Synthese ihrer π-Komplexe mit Metallen der Gruppe 6 bestätigt.
Bei der Einelektronenreduktion eines durch eine cyclisches (Alkyl)(amino)carben (CAAC) stabilisierten Arylborylen-Carbonylkomplexes erfolgt die Bildung eines dimeren Borylketyl-Radikalanions, bedingt durch eine intramolekulare Arylmigration zum CO Kohlenstoffatom. Computergestützte Analyse liefert Hinweise auf eine radikalanionische [(CAAC)B(CO)Ar]\(^{.-}\) Zwischenstufe. Weiterführende Reduktion des entstandenen Komplexes liefert ein hoch nukleophiles (Boranyliden)methanolat.
The concepts of aromaticity and antiaromaticity have a long history, and countless demonstrations of these phenomena have been made with molecules based on elements from the p, d, and f blocks of the periodic table. In contrast, the limited oxidation‐state flexibility of the s‐block metals has long stood in the way of their participation in sophisticated π‐bonding arrangements, and truly antiaromatic systems containing s‐block metals are altogether absent or remain poorly defined. Using spectroscopic, structural, and computational techniques, we present herein the synthesis and authentication of a heterocyclic compound containing the alkaline earth metal beryllium that exhibits significant antiaromaticity, and detail its chemical reduction and Lewis‐base‐coordination chemistry.
Coherent two-dimensional (2D) optical spectroscopy has revolutionized our ability to probe many types of couplings and ultrafast dynamics in complex quantum systems. The dynamics and function of any quantum system strongly depend on couplings to the environment. Thus, studying coherent interactions for different environments remains a topic of tremendous interest. Here we introduce coherent 2D electronic mass spectrometry that allows 2D measurements on effusive molecular beams and thus on quantum systems with minimum system-bath interaction and employ this to identify the major ionization pathway of 3d Rydberg states in NO2. Furthermore, we present 2D spectra of multiphoton ionization, disclosing distinct differences in the nonlinear response functions leading to the ionization products. We also realize the equivalent of spectrally resolved transient-absorption measurements without the necessity for acquiring weak absorption changes. Using time-of-flight detection introduces cations as an observable, enabling the 2D spectroscopic study on isolated systems of photophysical and photochemical reactions.
Diese Dissertation beschäftigt sich mit der Exzitonendynamik molekularer Aggregate, die nach Mehrphotonen-Anregung auf ultrakurzer Zeitskala stattfindet. Hierbei liegt der Fokus auf der Charakterisierung der Exziton-Exziton-Annihilierung (EEA) mithilfe von zweidimensionaler optischer Spektroskopie fünfter Ordnung. Dazu werden zwei verschiedene Modellsysteme implementiert: Das elektronische Homodimer und das elektronische Homotrimer-Modell, wobei Letzteres eine Erweiterung des Dimer-Modells darstellt. Die Kopplung des quantenmechanischen Systems an die Umgebung wird mithilfe des Quantum-Jump-Ansatzes umgesetzt. Besonderes Interesse kommt der Analyse des Signals fünfter Ordnung in Abhängigkeit der Populationszeit T zu.
Anhand des Dimer-Modells als kleinstmögliches Aggregat lassen sich bereits gute Vorhersagen auch über das Verhalten größerer molekularer Aggregate treffen. Der Zerfall des oszillierenden Signals für lange Populationszeiten korreliert mit der EEA. Dies zeigt, dass die zweidimensionale optische Spektroskopie genutzt werden kann, um den Annihilierungsprozess zu charakterisieren. Innerhalb des Modells des Dimers wird weiterhin der Einfluss der Intraband-Relaxation untersucht. Zunehmende Intraband-Relaxation verhindert den Austausch zwischen den lokalen Zuständen, der essentiell für den Annihilierungsprozess ist, und die EEA wird blockiert.
Das elektronische Trimer-Modell erweitert das Dimer-Modell um eine Monomereinheit. Somit befinden sich die Exzitonen im Anschluss an die Anregung nicht mehr unvermeidlich nebeneinander. Es gibt somit eine Konfiguration, bei der sich die Exzitonen zunächst zueinander bewegen müssen, bevor die Startbedingung des Annihilierungsprozesses gegeben ist. Dieser zusätzliche Schritt wird auch Exzitonendiffusion genannt. Die Ergebnisse dieser Arbeit legen nahe, dass das erwartete Verhalten nur zu sehr kurzen Zeiten im Femtosekundenbereich auftritt und somit die Zeitskala der Exzitonendiffusion im Falle des Trimers nicht sichtbar wird. Es bedarf demnach eines größeren Modellsystems, bei dem sich der Effekt der zeitverzögert eintretenden EEA deutlich in der Zerfallsdynamik manifestieren kann.
We report infrared spectra of xylylene isomers in the gas phase, using free electron laser (FEL) radiation. All xylylenes were generated by flash pyrolysis. The IR spectra were obtained by monitoring the ion dip signal, using a IR/UV double resonance scheme. A gas phase IR spectrum of para‐xylylene was recorded, whereas ortho‐ and meta‐xylylene were found to partially rearrange to benzocyclobutene and styrene. Computations of the UV oscillator strength for all molecules were carried out and provde an explanation for the observation of the isomerization products.
Within this work, an additive and a subtractive QM/MM interface were implemented into CAST. The interactions between QM and MM system are described via electrostatic embedding. Link atoms are used to saturate dangling bonds originating from the separation of QM and MM system. Available energy evaluation methods to be combined include force fields (OPLSAA and AMBER), semi-empirical programs (Mopac and DFTB+), and quantum-chemical methods (from Gaussian, Orca, and Psi4). Both the additive and the subtractive interface can deal with periodic boundary conditions. The subtractive scheme was extended to enable QM/QM, three-layer, and multi-center calculations. Another feature only available within the subtractive interface is the microiteration procedure for local optimizations.
The novel QM/MM methods were applied to the investigation of the reaction path for the complex formation between rhodesain and K11777. Benchmark calculations show a very good agreement with results from Gaussian-ONIOM. When comparing the relative energies obtained with different options to a computation where the whole system was treated with the “QM method” DFTB3, the electrostatic embedding scheme with option “delM3” gives the best results. “delM3” means that atoms with up to three bonds distance to the QM region are ignored when creating the external charges. This is done in order to avoid a double counting of Coulomb interactions between QM and MM system. The embedding scheme for the inner system in a three-layer calculation, however, does not have a significant influence on the energies. The same is true for the choice of the coupling scheme: Whether the additive or the subtractive QM/MM interface is applied does not alter the results significantly. The choice of the QM region, though, proved to be an important factor. As can be seen from the comparison of two QM systems of different size, bigger is not always better here. Instead, one has to make sure not to separate important (polar) interactions by the QM/MM border.
After this benchmark study with singlepoint calculations, the various possibilities of CAST were used to approximate the solution of a remaining problem: The predicted reaction energy for the formation of the rhodesain-K11777 complex differs significantly depending on the starting point of the reaction path.
The reason for this is assumed to be an inadequate adjustment of the environment during the scans, which leads to a better stabilization of the starting structure in comparison to the final structure. The first approach to improve this adjustment was performing the relaxed scan with a bigger QM region instead of the minimal QM system used before. While the paths starting from the covalent complex do not change significantly, those starting from the non-covalent complex become more exothermic, leading to a higher similarity of the two paths. Nevertheless, the difference of the reaction energy is still around 15 kcal/mol, which is far from a perfect agreement. For this reason, Umbrella Samplings were run. Here, the adjustment of the environment is not done by local optimizations like in the scans, but by MD simulations. This has the advantage that the system can cross barriers and reach different local minima. The relative free energies obtained by Umbrella Samplings with suitable QM regions are nearly identical, independently of the starting point of the calculation. Thus, \(\Delta A\) evaluated by these computations can be assumed to reproduce the real energy change best. An MD simulation that was started from the transition state in order to mimic a “real-time” reaction indicates a very fast adjustment of the environment during the formation of the complex. This confirms that Umbrella Sampling is probably better suitable to describe the reaction path than a scan, where the environment can never move strong enough to leave the current local minimum.
Besides their widespread use in coordination chemistry, 2,2’‐bipyridines are known for their ability to undergo cis–trans conformational changes in response to metal ions and acids, which has been primarily investigated at the molecular level. However, the exploitation of such conformational switching in self‐assembly has remained unexplored. In this work, the use of 2,2’‐bipyridines as acid‐responsive conformational switches to tune supramolecular polymerization processes has been demonstrated. To achieve this goal, we have designed a bipyridine‐based linear bolaamphiphile, 1, that forms ordered supramolecular polymers in aqueous media through cooperative aromatic and hydrophobic interactions. Interestingly, addition of acid (TFA) induces the monoprotonation of the 2,2’‐bipyridine moiety, leading to a switch in the molecular conformation from a linear (trans) to a V‐shaped (cis) state. This increase in molecular distortion along with electrostatic repulsions of the positively charged bipyridine‐H\(^{+}\) units attenuate the aggregation tendency and induce a transformation from long fibers to shorter thinner fibers. Our findings may contribute to opening up new directions in molecular switches and stimuli‐responsive supramolecular materials.
Using Photoionization to Investigate Reactive Boron Species and the Kinetics of Hydrocarbon Radicals
(2021)
This thesis highlights the importance of isomer-selective approaches for the complete analysis of chemical processes. The method of choice is photoelectron/photoion coincidence spectroscopy, which allows simultaneous detection of electrons and ions coming from a single ionization event. Ionization techniques are sensitive and can record multiple species simultaneously, rendering them ideal tools to probe molecular transformations. Coupling these setups to synchrotron radiation allows one to analyze complex mixtures with isomer selectivity, based on ionization energies and vibrational structure in the cation, without any prior separation steps. Only few setups exist that can be used to gather these data, although their impact and applicability is growing steadily in various fields. For closed-shell species an easier and more widely used method is gas-chromatography, but most open shell species would not survive the separation process. Due to the reactivity of radicals they have to be created by selectively converting stable precursor molecules. Depending on the radical generation method different properties can be investigated ranging from thermodynamic data, over concentrations in high temperature environments, to chemical kinetics.
The first part of this thesis deals with the determination of bimolecular rate constants. Isomeric hydrocarbon radicals were generated by a high intense UV light pulses and their kinetics with oxygen was measured. The pressure dependence of different isomers in the falloff region was compared to theoretical models, and their reactivity could be explained.
The second part deals with boron containing compounds in various electronic situations. The corresponding precursors were successfully synthesized or could be bought. They were subjected to fluorine atoms in chemical reactors or destroyed pyrolytically at high temperatures. Most investigated species exhibited vibronic effects that could be elucidated using high level computations.
cAAC‐Stabilized 9,10‐diboraanthracenes—Acenes with Open‐Shell Singlet Biradical Ground States
(2020)
Narrow HOMO–LUMO gaps and high charge‐carrier mobilities make larger acenes potentially high‐efficient materials for organic electronic applications. The performance of such molecules was shown to significantly increase with increasing number of fused benzene rings. Bulk quantities, however, can only be obtained reliably for acenes up to heptacene. Theoretically, (oligo)acenes and (poly)acenes are predicted to have open‐shell singlet biradical and polyradical ground states, respectively, for which experimental evidence is still scarce. We have now been able to dramatically lower the HOMO–LUMO gap of acenes without the necessity of unfavorable elongation of their conjugated π system, by incorporating two boron atoms into the anthracene skeleton. Stabilizing the boron centers with cyclic (alkyl)(amino)carbenes gives neutral 9,10‐diboraanthracenes, which are shown to feature disjointed, open‐shell singlet biradical ground states.
Das erste Ziel der vorliegenden Dissertation bestand darin, ein bereits bestehendes TOF-MS-Setup dahingehend zu erweitern, um damit Velocity Map Imaging-Experimente durchführen zu können. Dies erforderte zunächst die Konzipierung und Programmierung einiger für die Datenaufnahme, -verarbeitung und -analyse benötigter LabView-Anwendungen. Anschließend konnten erste Kalibrierexperimente an Methyliodid, in denen wichtige experimentelle Parameter identifiziert und optimiert wurden, durchgeführt werden. Außerdem gelang es dadurch, die Messgenauigkeit des Setups auf 0.7 % und dessen Auflösungsvermögen auf 4.4 % zu bestimmen, was im Bereich für VMI-Apparaturen typischer Werte liegt. Zur weiteren Überprüfung der Funktionstüchtigkeit des Setups wurde in ersten zeitaufgelösten Experimenten im Folgenden die Desaktivierung des S1-Zustands von Pyridin untersucht. Neben der Reproduktion einiger bereits literaturbekannter Resultate konnten dabei zusätzlich die im Multiphotonen-Ionisationsschritt populierten Rydberg-Zustände identifiziert werden. Anschließend wurde mit Experimenten an bisher weniger gut untersuchten organischen Aromaten und Heteroaromaten fortgefahren. Das Ziel dieser Studien lag in der Aufklärung der photoinduzierten Dynamiken der Verbindungen, wobei das zur Verfügung stehende ps-Lasersystem die Möglichkeit bot, die Desaktivierung elektronisch angeregter Zustände gezielt in Abhängigkeit von deren Schwingungsenergie zu untersuchen. Der darin bestehende Vorteil zeigte sich vor allem in Studien an Tolan und Phenanthridin, deren erste angeregte, optisch aktive Zustände am Origin Lebensdauern im ns-Bereich aufweisen, die sich mit zunehmender vibronischer Anregung jedoch auf bis zu 10 ps verringern. Als Grund dafür konnten nichtstrahlende Desaktivierungsprozesse, für deren Eintreten eine energetische Barriere überwunden werden muss, identifiziert werden. Während in Tolan nach Photoanregung ein Übergang in einen (πσ∗)-Zustand, der zur Ausbildung einer trans-bent-Struktur führt, erfolgt, ist im Falle von Phenanthridin vermutlich ein El-Sayed-erlaubter ISC-Übergang in einen 3(nπ∗)-Zustand für die drastische Verkürzung der S1-Lebensdauer verantwortlich. Ein solcher konnte weder im zu Phenanthridin isomerischen Benzo[h]quinolin, noch in dessen PAH-Muttermolekül Phenanthren beobachtet werden, was auf die höhere energetische Lage bzw. die Abwesenheit des mittels ISC populierten 3(nπ∗)-Zustands in diesen Molekülen zurückgeführt werden kann. In weiteren im Rahmen der vorliegenden Arbeit durchgeführten Experimente wurden zudem die aromatischen Moleküle Acenaphthylen und 4-(Dimethylamino)benzethin (DMABE) untersucht. Zeitaufgelöste Studien zeigten dabei, dass die Desaktivierung der S2-Zustände beider Moleküle auf der sub-ps-Zeitskala stattfindet und mit dem vorhandenen Lasersystem daher nicht aufgelöst werden kann. In Acenaphthylen erfolgt die S2-Relaxation größtenteils über einen sequentiellen IC-Mechanismus, innerhalb dem der S1-Zustand des Moleküls intermediär besetzt wird. Dessen Lebensdauer konnte am Origin auf 380 ps bestimmt werden, fällt mit steigender Schwingungsanregung jedoch auf bis zu 55 ps ab. Für die Desaktivierung des S2-Zustands von DMABE konnte hingegen ein paralleles Relaxationsmodell, in dem neben dem S1-Zustand ein weiterer elektronisch angeregter Zustand populiert wird, nachgewiesen werden. Bei diesem könnte es sich möglicherweise um einen (πσ∗)-Zustand, dessen Besetzung die Ausbildung einer trans-bent-Geometrie innerhalb der Acetylen-Einheit des Moleküls zur Folge hat, handeln. Einen weiteren großen Teil der vorliegenden Dissertation nahmen Experimente an van-der-Waals-gebundenen Clustersystemen ein. Im Fokus der Studien standen dabei Moleküle mit ausgedehnten aromatischen π-Systemen, da solche eine hohe Relevanz für verschiedene materialwissenschaftliche Forschungsgebiete besitzen. Ein Beispiel hierfür ist Tetracen, welches als Modellsystem für die Untersuchung von Singlet Fission-Prozessen angesehen wird. In Kombination mit nichtadiabatischen Surface-Hopping-Simulationen zeigten Experimente an Tetracen-Dimeren, dass nach deren S2-Anregung zunächst ein schneller S1←S2-Übergang (τ < 1 ps), gefolgt von der Ausbildung einer Excimerstruktur, stattfindet. Letztere erfolgt mit einer Zeitkonstante von 62 ps und führt zu einem Anstieg des transienten Ionensignals, wohingegen die Desaktivierung des Excimer-Zustands von einem abklingenden Signalbeitrag mit τ = 123 ps repräsentiert wird. Wenngleich über die weitere Relaxation der Excimerspezies zum gegenwärtigen Zeitpunkt keine Aussage getroffen werden kann, besteht damit die Möglichkeit, dass Excimer-Zustände als Zwischenstufe im SF-Mechanismus isolierter Tetracen-Dimere auftreten. In zeitaufgelösten Experimenten an Phenanthren-Dimeren konnte ebenfalls ein Anstieg des transienten Signals mit einer vergleichbaren Zeitkonstante von τ = 86 ps, der jedoch auf einem konstanten Signaloffset endet, gefunden werden. Dies deutet darauf hin, dass auch Phenanthren-Dimere in der Lage sind, Excimerstrukturen, die im Gegensatz zu denen des Tetracens jedoch deutlich langlebiger sind, auszubilden. Studien an den Dimerspezies der Azaphenanthrene Benzo[h]quinolin und Phenanthridin offenbarten hingegen etwas schnellere Relaxationen mit Zeitkonstanten von 15 bzw. 40 ps. Zudem zeigten beide Spezies eine stark ausgeprägte Fragmentation, sodass für deren Untersuchung auf die VMI-Detektionsmethode zurückgegriffen werden musste. Dadurch wurde deutlich, dass sich Photoionen-Imaging-Experimente hervorragend für Studien an schwach gebundenen Clustersystemen eignen, da diese die Separation verschiedener Signalbeiträge innerhalb eines betrachteten Massenkanals ermöglichen.
Diplatinum A‐frame complexes with a bridging (di)boron unit in the apex position were synthesized in a single step by the double oxidative addition of dihalo(di)borane precursors at a bis(diphosphine)‐bridged Pt\(^{0}\)\(_{2}\) complex. While structurally analogous to well‐known μ‐borylene complexes, in which delocalized dative three‐center‐two‐electron M‐B‐M bonding prevails, theoretical investigations into the nature of Pt−B bonding in these A‐frame complexes show them to be rare dimetalla(di)boranes displaying two electron‐sharing Pt−B σ‐bonds. This is experimentally reflected in the low kinetic stability of these compounds, which are prone to loss of the (di)boron bridgehead unit.
The absence of fluorine from most biomolecules renders it an excellent probe for NMR spectroscopy to monitor inhibitor–protein interactions. However, predicting the binding mode of a fluorinated ligand from a chemical shift (or vice versa) has been challenging due to the high electron density of the fluorine atom. Nonetheless, reliable \(^{19}\)F chemical‐shift predictions to deduce ligand‐binding modes hold great potential for in silico drug design. Herein, we present a systematic QM/MM study to predict the \(^{19}\)F NMR chemical shifts of a covalently bound fluorinated inhibitor to the essential oxidoreductase tryparedoxin (Tpx) from African trypanosomes, the causative agent of African sleeping sickness. We include many protein–inhibitor conformations as well as monomeric and dimeric inhibitor–protein complexes, thus rendering it the largest computational study on chemical shifts of \(^{19}\)F nuclei in a biological context to date. Our predicted shifts agree well with those obtained experimentally and pave the way for future work in this area.
The reaction products of the picolyl radicals at high temperature were characterized by mass‐selective threshold photoelectron spectroscopy in the gas phase. Aminomethylpyridines were pyrolyzed to initially produce picolyl radicals (m /z =92). At higher temperatures further thermal reaction products are generated in the pyrolysis reactor. All compounds were identified by mass‐selected threshold photoelectron spectroscopy and several hitherto unexplored reactive molecules were characterized. The mechanism for several dissociation pathways was outlined in computations. The spectrum of m /z =91, resulting from hydrogen loss of picolyl, shows four isomers, two ethynyl pyrroles with adiabatic ionization energies (IE\(_{ad}\)) of 7.99 eV (2‐ethynyl‐1H ‐pyrrole) and 8.12 eV (3‐ethynyl‐1H ‐pyrrole), and two cyclopentadiene carbonitriles with IE′s of 9.14 eV (cyclopenta‐1,3‐diene‐1‐carbonitrile) and 9.25 eV (cyclopenta‐1,4‐diene‐1‐carbonitrile). A second consecutive hydrogen loss forms the cyanocyclopentadienyl radical with IE′s of 9.07 eV (T\(_0\)) and 9.21 eV (S\(_1\)). This compound dissociates further to acetylene and the cyanopropynyl radical (IE=9.35 eV). Furthermore, the cyclopentadienyl radical, penta‐1,3‐diyne, cyclopentadiene and propargyl were identified in the spectra. Computations indicate that dissociation of picolyl proceeds initially via a resonance‐stabilized seven‐membered ring.
Excitation energy transport in DNA modelled by multi-chromophoric field-induced surface hopping
(2020)
Absorption of ultraviolet light is known as a major source of carcinogenic mutations of DNA. The underlying processes of excitation energy dissipation are yet not fully understood. In this work we provide a new and generally applicable route for studying the excitation energy transport in multi-chromophoric complexes at an atomistic level. The surface-hopping approach in the frame of the extended Frenkel exciton model combined with QM/MM techniques allowed us to simulate the photodynamics of the alternating (dAdT)10 : (dAdT)10 double-stranded DNA. In accordance with recent experiments, we find that the excited state decay is multiexponential, involving a long and a short component which are due to two distinct mechanisms: formation of long-lived delocalized excitonic and charge transfer states vs. ultrafast decaying localized states resembling those of the bare nucleobases. Our simulations explain all stages of the ultrafast photodynamics including initial photoexcitation, dynamical evolution out of the Franck-Condon region, excimer formation and nonradiative relaxation to the ground state.
Tetraiododiborane(4) (B\(_2\)I\(_4\)) is a Polymer based on sp\(^3\) Boron in the Solid State
(2020)
Herein we present the first solid‐state structures of tetraiododiborane(4) (B\(_2\)I\(_4\)), which was long believed to exist in all phases as discrete molecules with planar, tricoordinate boron atoms, like the lighter tetrahalodiboranes(4) B\(_2\)F\(_4\), B\(_2\)Cl\(_4\), and B\(_2\)Br\(_4\). Single‐crystal X‐ray diffraction, solid‐state NMR, and IR measurements indicate that B\(_2\)I\(_4\) in fact exists as two different polymeric forms in the solid state, both of which feature boron atoms in tetrahedral environments. DFT calculations are used to simulate the IR spectra of the solution and solid‐state structures, and these are compared with the experimental spectra.
Iodine oxides appear as reactive intermediates in atmospheric chemistry. Here, we investigate IO and HOI by mass‐selective threshold photoelectron spectroscopy (ms‐TPES), using synchrotron radiation. IO and HOI are generated by photolyzing iodine in the presence of ozone. For both molecules, accurate ionization energies are determined, 9.71±0.02 eV for IO and 9.79±0.02 eV for HOI. The strong spin‐spin interaction in the 3Σ− ground state of IO+ leads to an energy splitting into the Ω=0 and Ω=±1 sublevels. Upon ionization, the I−O bond shortens significantly in both molecules; thus, a vibrational progression, assigned to the I−O stretch, is apparent in both spectra.
Schwingungsspektroskopie ist eine vielseitige spektroskopische Methode, mit der Molekülstrukturen und inter-/intramolekulare Wechselwirkungen untersucht werden können. Sie ist deshalb ein hervorragendes Mittel für die Identifikation von Molekülen. Die vorliegende Arbeit umfasst drei Projekte, in denen Schwingungsspektroskopie angewandt wurde, um reaktive Moleküle und ihre Hochtemperatur-Reaktionsprodukte zu untersuchen:
1. Die Aufklärung der Entstehungsmechanismen von polycyclischen aromatischen Kohlenwasserstoffen (PAKs) in Verbrennungsprozessen ist eines der Hauptanliegen der Verbrennungschemie.
In der vorliegenden Arbeit wurde IR/UV-Ion-Dip-Spektroskopie in Verbindung mit DFT-Frequenzrechnungen und FTIR-Messungen angewandt, um Produkte von Radikal-Radikal-Reaktionen in einem Mikroreaktor bei hohen Temperaturen zu identifizieren. Als IR-Laserquelle für die IR/UV-Ion-Dip-Experimente diente der Freie-Elektronen-Laser FELIX (Free-Electron Laser for Infrared eXperiments) in Nijmegen (Niederlande).
In einem Teilprojekt wurde der A 1A´ (S1) <- X 1A´ (S0) Übergang in 1-(Phenylethinyl)naphthalin (1-PEN), einem mutmaßlich verbrennungsrelevanten Molekül, mit [1+1]-REMPI-Spektroskopie untersucht.
2. Die Identifikation von gasförmigen Reaktionsprodukten bei der thermischen Analyse (EGA: Emissionsgasanalyse) kann als komplementäre Methode zur DTA/TG zusätzliche Informationen für die Aufklärung von Reaktionsmechanismen liefern.
Der Aufbau eines elementaren EGA/FTIR-Experiments, basierend auf einer heizbaren IR-Gaszelle, ermöglichte in der vorliegenden Arbeit die Durchführung dynamischer IR-Messungen, mit denen thermische Umsetzungen von Übergangsmetall-Precursorkomplexen zu Koordinationspolymeren untersucht wurden.
3. Die Synthese des ersten bei Raumtemperatur stabilen Diborins, einer Verbindung mit einer Bor-Bor-Dreifachbindung, stellte einen Meilenstein in der elementorganischen Chemie dar. Dies implizierte eine umfassende Untersuchung der Eigenschaften der BB-Bindung und hatte die Synthese einer Reihe ähnlicher Bor-Bor-Mehrfachbindungssysteme mit variierenden Bindungseigenschaften zur Folge.
In der vorliegenden Arbeit wurde Raman-Spektroskopie in Verbindung mit DFT-Frequenzrechnungen angewandt, um für diese Bor-Bor-Systeme die strukturellen/elektronischen Eigenschaften der zentralen CBBC-Einheit zu untersuchen.
Mechanistic Insights into the Inhibition of Cathepsin B and Rhodesain with Low-Molecular Inhibitors
(2019)
Cysteine proteases play a crucial role in medical chemistry concerning various fields reaching from more common ailments like cancer and hepatitis to less noted tropical diseases, namely the so-called African Sleeping Sickness (Human Arfican Trypanosomiasis). Detailed knowledge about the catalytic function of these systems is highly desirable for drug research in the respective areas. In this work, the inhibition mechanisms of the two cysteine proteases cathepsin B and rhodesain with respectively one low-molecular inhibitor class were investigated in detail, using computational methods. In order to sufficiently describe macromolecular systems, molecular mechanics based methods (MM) and quantum mechanical based method (QM), as well as hybrid methods (QM/MM) combining those two approaches, were applied.
For Cathespin B, carbamate-based molecules were investigated as potential inhibitors for the cysteine protease. The results indicate, that water-bridged proton-transfer reactions play a crucial role for the inhibition. The energetically most favoured pathway (according to the calculations) includes an elimination reaction following an E1cB mechanism with a subsequent carbamylation of the active site amino acid cysteine.
Nitroalkene derivatives were investigated as inhibitors for rhodesain. The investigation of structurally similar inhibitors showed, that even small steric differences can crucially influence the inhibition potential of the components. Furthermore, the impact of a fluorination of the nitroalkene inhibitors on the inhibition mechanism was investigated. According to experimental data measured from the working group of professor Schirmeister in Mainz, fluorinated nitroalkenes show – in contrast to the unfluorinated compounds – a time dependent inhibition efficiency. The calculations of the systems indicate, that the fluorination impacts the non-covalent interactions of the inhibitors with the enzymatic environment of the enzyme which results in a different inhibition behaviour.
Diese Arbeit befasst sich mit der störungstheoretischen Berechnung von zweidimensionalen Photonen-Echo-Spektren für das elektronische und vibronische Modell eines Homo- und Hetero-Dimers sowie für ein vibronisches Modell eines Monomers unter dem Einfluss einer System-Bad-Wechselwirkung. Bei der Analyse der Dimerspektren steht neben der Orientierungsmittelung der Polarisation dritter Ordnung der Unterschied zwischen elektronischen und vibronischen Spektren sowie der Vergleich der Spektren von Homo- und Hetero-Dimeren im Zentrum des Interesses. Bei der Analyse der Monomer-Spektren steht die Behandlung einer dissipativen Dynamik bzw. des vibrational-coolings innerhalb eines stochastischen Ansatzes im Vordergrund.
Der erste Teil dieser Arbeit konzentriert sich auf die störungstheoretische Berechnung der Polarisation dritter Ordnung in Dimeren. Dabei werden alle Aspekte und Ergebnisse für verschiedene Geometrien der Übergangsdipolmomente analysiert und diskutiert. Die Berechnungen berücksichtigen dabei auch die zufällige Anordnung der Moleküle in der Probe. Die Zusammenhänge zwischen den 2D-Spektren und den Eigenschaften der Monomereinheiten, die Abhängigkeit der Intensitäten mancher Peaks von der zeitlichen Abfolge der Pulse sowie der Einfluss der elektronischen Kopplung und verschiedener Übergangsdipolmomente ermöglichen ein grundlegendes Verständnis der elektronischen Photonen-Echo-Spektren. Im elektronischen Dimer wird der Hetero-Dimer-Charakter durch verschiedene Monomeranregungsenergien sowie unterschiedliche Übergangsdipolmomente der Monomereinheiten bestimmt. Der Einfluss dieser Größen auf die Photonen-Echo-Spektren kann durch die Kombination einer detaillierten analytischen Betrachtung und numerischen Rechnungen anschaulich nachvollzogen werden. In der vibronischen Betrachtungsweise zeigt sich, dass die Spektren deutlich an Komplexität gewinnen. Durch die Vibrationsfreiheitsgrade vervielfachen sich die möglichen Übergänge im System und damit die möglichen Peakpositionen im Spektrum. Jeder Peak spaltet in eine Vibrationssubstruktur auf, die je nach ihrer energetischen Position mit anderen überlagern kann. Der Vergleich zwischen Homo- und Hetero-Dimer-Spektren wird durch die Wahl verschiedener Vibrationsfrequenzen und unterschiedlicher Gleichgewichtsabstände entlang der Vibrationskoordinaten erweitert.
Die Berechnung des Orientierungsmittels erfolgt mit zwei verschiedenen Ansätzen. Zum einen wird das Mittel durch den numerischen sampling-Ansatz berechnet. Dabei werden Azimutal- und Polarwinkel in kleinen Winkelinkrementen abgetastet und für jede Kombination ein 2D-Spektrum berechnet. Die Einzelspektren werden anschließend gemittelt. Diese Methode erweist sich im Dimer als sehr effektiv. Zum anderen erlaubt die analytische Auswertung der Polarisation dritter Ordnung, das gemittelte Spektrum direkt in einer einzelnen Rechnung durch winkelgemittelte Gewichtungsfaktoren zu bestimmen. Bei der Berechnung der elektronischen 2D-Spektren ist diese Methode sehr leistungsfähig, da alle Ausdrücke analytisch bekannt sind. Für vibronische Systeme ist dieser Ansatz ebenfalls sehr leistungsstark, benötigt aber eine einmalige aufwendige Analyse vor der Berechnung. Trotz der deutlich erhöhten Anzahl an Zustandsvektoren, die propagiert werden müssen, ist diese Methode circa zweimal schneller als die direkte Mittelung mit der sampling-Methode.
Im zweiten Teil konzentriert sich die Arbeit auf die Beschreibung eines Monomers, das sich in einer dissipativen Umgebung befindet. Dabei wird auf die Lösung einer stochastischen Schrödingergleichung zurückgegriffen. Speziell wird die sogenannte quantum-state-diffusion-Methode benutzt. Dabei werden nicht nur die Erwartungswerte für die Energie und den Ort, sondern auch die Polarisation dritter Ordnung – eine phasensensitive Größe – bestimmt. In der theoretischen Fragestellung wird dabei, ausgehend von der von-Neumann Gleichung, die Zeitentwicklung der reduzierten Dichtematrix durch die Integration einer stochastischen zeitabhängigen Schrödingergleichung reproduziert. In Rechnungen koppelt die Stochastik über die Erwartungswerte von Ort und Impuls die verschiedenen störungstheoretischen Korrekturen der Wellenfunktion miteinander. Die Spektren, die aus den numerischen Simulationen erhalten werden, spiegeln das dissipative Verhalten des Systems detailliert wider. Eine Analyse der Erwartungswerte von Ort und Energie zeigt, dass sich die einzelnen elektronischen Zustände wie gedämpfte harmonische Oszillatoren verhalten und jeweils einen exponentiellen Zerfall abhängig von der Dissipationskonstante zeigen. Dieser Teil der Arbeit erweitert vorausgehende Untersuchungen, bei denen ein vereinfachter Ansatz zu Einsatz kam, der die korrelierte Stochastik nicht berücksichtigte.
Describing the light-to-energy conversion in OSCs requires a multiscale understanding of the involved optoelectronic processes, i.e., an understanding from the molecular, intermolecular, and aggregate perspective. This thesis presents such a multiscale description to provide insight into the processes in the vicinity of the organic::organic interface, which are crucial for the overall performance of OSCs. Light absorption, exciton diffusion, photoinduced charge transfer at the donor-acceptor interface, and charge separation are included. In order to establish structure-property relationships, a variety of different molecular p-type semiconductors are combined at the organic donor-acceptor heterojunction with fullerene C60, one of the most common acceptors in OSCs. Starting with a comprehensive analysis of the accuracy of diverse ab initio, DFT, and semiempiric methods for the properties of the individual molecules, the intermolecular, and aggregate/device stage are subsequently addressed. At all stages, both methodological concepts and physical aspects in OSCs are discussed to extend the microscopic understanding of the charge generation processes.
Investigation of Nanostructure-Induced Localized Light Phenomena Using Ultrafast Laser Spectroscopy
(2017)
In recent years, the interaction of light with subwavelength structures, i.e., structures that are smaller than the optical wavelength, became more and more interesting to scientific research, since it provides the opportunity to manipulate light-induced dynamics below the optical diffraction limit. Specifically designed nanomaterials can be utilized to tailor the temporal evolution of electromagnetic fields at the nanoscale. For the investigation of strongly localized processes, it is essential to resolve both their spatial and their temporal behavior. The aim of this thesis was to study and/or control the temporal evolution of three nanostructure-induced localized light phenomena by using ultrafast laser spectroscopy with high spatial resolution.
In Chapter 4, the absorption of near-infrared light in thin-film a-Si:H solar cells was investigated. Using nanotextured instead of smooth interfaces for such devices leads to an increase of absorption from < 20% to more than 50% in the near-infrared regime. Time-resolved experiments with femtosecond laser pulses were performed to clarify the reason for this enhancement. The coherent backscattered radiation from nanotextured solar cell devices was measured as a function of the sample position and evaluated via spectral interferometry. Spatially varying resonance peaks in the recorded spectra indicated the formation of localized photonic modes within the nanotextured absorber layers. In order to identify the modes separately from each other, coherent two-dimensional (2D) nanoscopy was utilized, providing a high spatial resolution < 40 nm. In a nanoscopy measurement on a modified device with an exposed nanotextured a-Si:H absorber layer, hot-spot electron emission was observed and confirmed the presence of localized modes. Fitting the local 2D nanospectra at the hot-spot positions enabled the determination of the resonance frequencies and coherence lifetimes of the modes. The obtained lifetime values varied between 50 fs and 130 fs. Using a thermionic emission model allowed the calculation of the locally absorbed energy density and, with this, an estimation of the localization length of the photonic modes (≈1 μm). The localization could be classified by means of the estimated localization length and additional data evaluation of the backscattered spectra as strong localization ─ the so-called Anderson localization.
Based on the experimental results, it was concluded that the enhanced absorption of near-infrared light in thin-film silicon solar cells with nanotextured interfaces is caused by the formation of strongly localized photonic modes within the disordered absorber layers. The incoming near-infrared light is trapped in these long-living modes until absorption occurs.
In Chapter 5, a novel hybridized plasmonic device was introduced and investigated in both theory and experiment. It consists of two widely separated whispering gallery mode (WGM) nanoantennas located in an elliptical plasmonic cavity. The goal was to realize a periodic long-range energy transfer between the nanoantennas. In finite-difference time-domain (FDTD) simulations, the device was first optimized with respect to strong coupling between the localized antenna modes and the spatially-extended cavity mode. The geometrical parameters of the antennas and the cavity were adjusted separately so that the m="0" antenna mode and the cavity mode were resonant at λ="800 nm" . A high spatial overlap of the modes was achieved by positioning the two antennas in the focal spots of the cavity, leading to a distance between the antenna centers of more than twice the resonant wavelength of the modes. The spectral response of the optimized device revealed an energy splitting of the antenna and the cavity mode into three separated hybridized eigenmodes within an energy range of about 90 meV due to strong coupling. It could be well reproduced by a simple model of three coupled Lorentzian oscillators. In the time domain, an oscillatory energy transfer between both antennas with a period of 86 fs and an energy transfer efficiency of about 7% was observed for single-pulse excitation. For the experiments, devices with cavities and antennas of varying size were fabricated by means of focused-ion-beam (FIB) milling. Time-resolved correlation measurements were performed with high spatial and temporal resolution by using sequences of two femtosecond laser pulses for excitation and photoemission electron microscopy (PEEM) for detection. Local correlation traces at antennas in resonant devices, i.e., devices with enhanced electron emission at both antenna positions, were investigated and reconstructed by means of the coupled-oscillator model. The corresponding spectral response revealed separated peaks, confirming the formation of hybridized eigenmodes due to strong coupling. In a subsequent simulation for single-pulse excitation, one back-and-forth energy transfer between both antennas with an energy transfer efficiency of about 10% was observed.
Based on the theoretical and experimental results, it was demonstrated that in the presented plasmonic device a periodic long-range energy transfer between the two nanoantennas is possible. Furthermore, the coupled-oscillator model enables one to study in depth how specific device properties impact the temporal electric-field dynamics within the device. This can be exploited to further optimize energy transfer efficiency of the device. Future applications are envisioned in ultrafast plasmonic nanocircuitry. Moreover, the presented device can be employed to realize efficient SPP-mediated strong coupling between widely separated quantum emitters.
In Chapter 6, it was investigated in theory how the local optical chirality enhancement in the near field of plasmonic nanostructures can be optimized by tuning the far-field polarization of the incident light. An analytic expression was derived that enables the calculation of the optimal far-field polarizations, i.e., the two far-field polarizations which lead to the highest positive and negative local optical chirality, for any given nanostructure geometry. The two optimal far-field polarizations depend on the local optical response of the respective nanostructure and thus are functions of both the frequency ω and the position r. Their ellipticities differ only in their sign, i.e., in their direction of rotation in the time domain, and the angle between their orientations, i.e., the angle between the principal axes of their ellipses, is ±π/"2" . The handedness of optimal local optical chirality can be switched by switching between the optimal far-field polarizations. In numerical simulations, it was exemplarily shown for two specific nanostructure assemblies that the optimal local optical chirality can significantly exceed the optical chirality values of circularly polarized light in free space ─ the highest possible values in free space. The corresponding optimal far-field polarizations were different from linear and circular and varied with frequency. Using femtosecond polarization pulse shaping provides the opportunity to coherently control local optical chirality over a continuous frequency range. Furthermore, symmetry properties of nanostructures can be exploited to determine which far-field polarization is optimal.
The theoretical findings can have impact on future experimental studies about local optical chirality enhancement. Tuning the far-field polarization of the incident light offers a promising tool to enhance chirally specific interactions of local electromagnetic fields with molecular and other quantum systems in the vicinity of plasmonic nanostructures. The presented approach can be utilized for applications in chiral sensing of adsorbed molecules, time-resolved chirality-sensitive spectroscopy, and chiral quantum control.
In conclusion, each of the localized light phenomena that were investigated in this thesis ─ the enhanced local absorption of near-infrared light due to the formation of localized photonic modes, the periodic long-range energy transfer between two nanoantennas within an elliptical plasmonic cavity, and the optimization of local optical chirality enhancement by tuning the far-field polarization of the incident light ─ can open up new perspectives for a variety of future applications.
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Diese Arbeit befasst sich mit verschiedenen Aspekten der Dynamik von Kernen, Elektronen und gekoppelten Kern-Elektron-Systemen, wobei je nach System unterschiedliche Herangehensweisen gewählt wurden.
Zentrale Punkte sind bei allen drei Kapiteln einerseits die Lokalisierung von Teilchen und Energie und andererseits eine hohe Sensitivität in Bezug auf die Wahl der Anfangsbedingungen.
Im ersten Teil wurden von der Carrier-Envelope-Phase (CEP) abhängende, laser-induzierte Lokalisierungen betrachtet. Das zentrale Element ist dabei das entwickelte Doppelpulsschema, mit welchem eine CEP-Abhängigkeit in beobachtbaren Größen erzeugt wird. Als Beispielsysteme wurden die Fragmentation im D₂⁺-Modellsystem und eine Isomerisierung im Doppelminimumpotential (DMP) untersucht. Als Observable wird die Asymmetrie betrachtet
Im DMP kann die Asymmetrie mit dem Entantiomeren/Isomerenüberschuss gleich gesetzt werden kann und im D₂⁺-Modellsystem mit der Lokalisierung des Elektrons auf einem der beiden dissoziierenden Kerne.
Eine Phasenabhängigkeit der Asymmetrien besteht nur für die CEP des zweiten Pulses φ₂, für welchen keine Begrenzungen für die Anzahl an Laserzyklen auftreten.
Im DMP wurde die CEP-Abhängigkeit der Asymmetrien auch bei unterschiedlichen Startkonfigurationen untersucht. Für alle untersuchten Startkonfigurationen konnte ein Laserparametersatz gefunden werden, der für zumindest eine der beiden Asymmetrien eine CEP-Abhängigkeit liefert. Aufgrund der aufgehobenen energetischen Entartung der Paare gerader und ungerader Symmetrie ist die resultierende Lokalisierung zeitabhängig. Zur Messung der vorhergesagten Dynamiken ist z.B. die Aufnahme eines Photoelektronen-Spektrums denkbar.
In nächsten Kapitel wurden unterschiedliche Dynamiken innerhalb eines 4d Kern-Elektron-Modells in der Nähe einer konischen Durchschneidung (CI) zweier Potentiale betrachtet. Hierbei ist hervorzuheben, dass eine solche gleichzeitige Untersuchung von Kern- und Elektron-Dynamik in Systemen mit CIs in der Literatur, nach Wissen des Autors, bisher nicht veröffentlicht ist. Das 4d-Potential wurde mit Hilfe des sogenannten Potfit-Algorithmus gefittet. Dieser Fit wurde anschließend verwendet, um die Dynamik des gekoppelten Systems mit Hilfe der ”Multi-Configuration Time-Dependent Hartree”(MCTDH)-Methode zu berechnen. Aus der Analyse der gekoppelten Kern-Elektron-Wellenfunktion ergaben sich zwei grundlegend unterschiedliche Klassen von Dynamiken:
• Diabatisch: Kern- und Elektrondynamik sind nahezu entkoppelt. Der Kern bewegt sich und das Elektron bleibt statisch.
• Adiabatisch: Kern- und Elektrondynamik sind stark gekoppelt. Die Kerndynamik findet auf Kreisbahnen statt. Mit der Rotation der Kerndichte um den Winkel φ geht eine Rotation der Elektron-Dichte einher.
Die diabatische Bewegung entspricht der Dynamik durch die konische Durchschneidung und die adiabatische Bewegung der Dynamik auf der unteren Potentialfläche. Welche der beiden Dynamiken stattfindet, wird durch die Wahl der Anfangsbedingung bestimmt. Der wesentliche Unterschied zwischen den beiden Startzuständen ist dabei die Lage des Knotens im elektronischen Anteil der Wellenfunktion. In den diabatischen Bewegungen bleibt z.B. der pₓ -artige Charakter der elektronischen Wellenfunktion konstant, wohingegen sich bei der adiabatischen Dynamik der Charakter mit der Kernbewegung ändert. Die Zeitersparnis durch die Verwendung des MCTDH-Ansatzes im Vergleich zur Split-Operator-Methode liegt etwa bei einem Faktor 5.
Das letzte Kapitel widmet sich der mikroskopischen Beschreibung von Exziton-Exziton-
Annihilierung (EEA). Dabei werden numerische Lösungen der aus einem mikro-
skopischen Modell hergeleiteten Ratengleichungen mit Messungen ( transienter Absorption) verglichen. Es wurden zwei Systeme untersucht: ein Squarain-basiertes Heteropolymer (SQA-SQB)ₙ und ein [2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenvinylen]-Polymer, auch bekannt als MEH-PPV. In beiden Fällen gelang die systematische Parameterbestimmung mit Hilfe einer Aufteilung in lokalisierte Subsysteme. Diese Subsysteme werden einzeln gewichtet und anschließend aufsummiert, wobei die Gewichte optimiert werden können. Aus den so erhaltenen Parametern ergibt sich für beide Systeme ein ähnliches Bild:
• Durch ultraschnelle Lokalisierung der Anregung im fs-Bereich auf kleinere Aggregateinheiten bilden sich voneinander getrennte Subsysteme.
• Die in den Subsystemen lokalisierten Exzitonen können sich nur innerhalb dieser Bereiche frei bewegen. Es ist ausreichend, direkt benachbarte Mono-, Bi-, Tri- und Tetra-Exzitonen in bis zu zwei Dimensionen zu berücksichtigen.
• Auf einer fs-Zeitskala annihilieren direkt benachbarte Exzitonen.
• Im MEH-PPV ergibt sich der Signalzerfall im fs-Bereich als Mittelwert aus einer schnellen (zwischen Ketten) und einer langsamen (innerhalb von Ketten) Annihilierung.
• Im ps- bis ns-Bereich wird sowohl durch Diffusion vermittelte Annihilierung, also auch der Zerfall der ersten angeregten Zustände bedeutsam.
Two thematic complexes were addressed within this work. One part is related to improvements and new implementations into the CAST program package. Thereby the main focus laid on the delivery of a tool which can be used to characterize complex reactions and their mechanisms. But also within the new force field (FF) method (SAPT-FF) within the CAST program, several improvements were made. The second topic is related to the description of dye molecules and their spectral properties. The main focus within these studies was set on the influence of the environment on these properties. In the first topic improvements of the local acting NEB (nudged elastic band) methods were included and the number of available methods was extended. The initial pathway generation was improved by implementing the IDPP (image dependent pair potential) method and a new method was implemented for describing temperature dependent pathways. Additionally, improvements have been made to the optimization routines (global NEB). As a second part the Pathopt (PO) method was considerably improved. In the beginning of the work the original PO idea was used. In this approach one starts with a global optimization on one n-1 dimensional hyperplane which divides the reaction into two sub-areas for obtaining guesses of TSs (transition states). These found TS guesses were used to optimize to the ”true” TS. Starting from the optimized ones a relaxation to the next connected minima is done. This idea has been automatically implemented and extended to several number of hyperplanes. In this manner a group of pathsegments is obtained which needs to be connected, but within this work it was realized that such a procedure might be not very efficient. Therefore, a new strategy was implemented which is founded on the same constrained global optimization scheme (MCM) for which the user defines the number of hyperplanes generated. The number of such generated hyperplanes should be large enough
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to describe the space between the concerning reactants in a sufficient way. The found minima are directly used to built up the reaction pathway. For this purpose a RMSD (root mean square deviation) criterion is used to walk along ways of minimal change from one to another hyperplane. To prove the implementations various test calculations were carried out and extensions included to prove the capabilities of the new strategy. Related to these tests a new strategy for applying the move steps in MCM (Monte Carlo with minimization) was realized which is also related to the question of the coordinates representation. We were able to show that the hopping steps in MCM can be improved by applying Cartesian steps in combination of random dihedral moves with respect to the constraint. In this way it was possible to show that a large variety of systems can be treated. An additional chapter shows the improvements of the SAPT-FF implementation and related test cases. It was possible to treat benzene dimer and cluster systems of different sizes consistently also in accordance with high level ab initio based approaches. Furthermore, we showed that the SAPT-FF with the right parameters outperforms the standard AMOEBA implementation which is the basis of the SAPT-FF implementation. In the last three chapters deal with the description of perlyene-based dyes. In the first smaller chapter ground state chemistry description of macro cycles of PBI (perylene bisimide) derivatives were investigated. Therefore, AFM (atomic force microscopy) based pictures were explained within our study. The methods to explain aggregation behavior in dependency of the ring size were MD simulations and configuration studies. The last two chapters deal with opto-electronic or photo-physical properties of PBI and PTCDA (perylene-3,4,9,10-tetracarboxylic dianhydride). In detail, we investigated the role of the environment and the aggregate or crystal surrounding by applying different models. In that way implicit and explicit solvation models, the size of aggregates and vibration motions were used. In the case of PBI the recent work is found on preliminary studies related to my bachelor thesis and extends it. It was shown that the direct influence of a polarizable surrounding, as well as explicit inclusion of solvent molecules on the overall description of the excitations and nature of the excited states is weaker as one might expect. However the inclusion of intra-molecular degrees of freedom showed a stronger influence on the state characteristics and can induce a change of the order of states within the dimer picture. For the PTCDA molecule the main focus was set on the description of the absorption spectrum of crystalline thin films. Related to this older works exist which already gave a description and assignment of the absorption band, but are based on different approaches compared to the one used in this work. We used the supermolecule ansatz, whereas the environment and different aggregate sizes were investigated. Within the dimer based approach we were able to show that using continuum solvation (IEFPCM/COSMO) based description for the environment the relative order of states remains unchanged. Similar to the PBI calculations the influence of the vibrational motions /distortions is larger. The simulation of the crystal environment by using QM/MM (quantum mechanics/molecular mechanics) approaches delivered that an asymmetric charge distribution might induce a localization of the excitation and a stronger mixing of states. For obtaining further insights we go beyond the dimer picture and aggregates of different sizes were used, whereas the simulations up to the octadecamer mono- and even dual-layer stack were carried out. Within these calculations it was shown that the H-coupling is dominating over a weaker J-coupling between different stacks. Additionally the calculations based on DFT (density functional theory) and semi-empirics showed that the lowest state in terms of energy are mostly of Frenkel type, whereas the higher lying states are CT ones which mix with embedded Frenkel type states. The first band of the absorption spectrum was explained by inclusion of vibrational motions within the stacks which induce an intensity gain of the first excited state. This intensity was not explainable by using the undistorted stacks. Also relaxations at the crystal surface might play a role, but are experimentally not explainable.
In the present contribution, we describe the synthesis of highly dispersed silver nanorods (NRs) of different aspect ratios using a chemical route. The shape and size of the synthesized NRs were characterized by Transmission Electron Microscopy (TEM) and UV-visible spectroscopy. Longitudinal and transverse absorptions bands confirm the rod type structure. The experimentally recorded UV-visible spectra of NRs solutions were fitted by using an expression of the extinction coefficient for rod like nano structures under the dipole approximation. Simulated and experimentally observed UV-visible spectra were compared to determine the aspect ratios (R) of NRs. The average values of R for NR1, NR2 and NR3 solutions are estimated to be 3.0 ± 0.1, 1.8 ± 0.1 and 1.2 ± 0.1, respectively. These values are in good agreement with those obtained by TEM micrographs. The silver NRs of known aspect ratios are used to study antimicrobial activities against B. subtilis (gram positive) and E. coli (gram negative) microbes. We observed that the NRs of intermediate aspect ratio (R = 1.8) have greater antimicrobial effect against both, B. subtilis (gram positive) and E. coli (gram negative). The NRs of aspect ratio, R = 3.0 has better antimicrobial activities against gram positive than on the gram negative.
The aim of the present work is the development and implementation of new simulation
possibilities for the CAST program package. Development included, among other
things, the partial parallelization of the already existing force fields, extension of the
treatment of electrostatic interactions and implementation of molecular dynamics and
free energy algorithms.
The most time consuming part of force field calculations is the evaluation of the nonbonded
interactions. The calculation of these interactions has been parallelized and
it could be shown to yield a significant speed up for multi-core calculations compared
to the serial execution on only one CPU. For both, simple energy/gradient as well as
molecular dynamics simulations the computational time could be significantly reduced.
To further increase the performance of calculations employing a cutoff radius, a linkedcell
algorithm was implemented which is able to build up the non-bonded interaction
list up to 7 times faster than the original algorithm.
To provide access to dynamic properties based on the natural time evolution of a system,
a molecular dynamics code has been implemented. The MD implementation features
two integration schemes for the equations of motion which are able to generate stable
trajectories. The basic MD algorithm as described in Section 1.2 leads to the sampling
in the microcanonical (NVE) ensemble. The practical use of NVE simulations is limited
though because it does not correspond to any experimentally realistic situation.
More realistic simulation conditions are found in the isothermal (NVT) and isothermalisobaric
(NPT) ensembles. To generate those ensembles, temperature and pressure
control has been implemented. The temperature can be controlled in two ways: by direct
velocity scaling and by a Nose-Hoover thermostat which produces a real canonical
ensemble. The pressure coupling is realized by implementation of a Berendsen barostat.
The pressure coupling can be used for isotropic or anisotropic box dimensions with the
restriction that the angles of the box need to be 90. A crucial simulation parameter in
MD simulations is the length of the timestep. The timestep is usually in the rang of 1fs.
Increasing the timestep beyond 1fs can lead to unstable trajectories since the fastest
motion in the system, usually the H-X stretch vibration can not be sampled anymore.
A way to allow for bigger timesteps is the use of a constraint algorithm which constrains the H-X bonds to the equilibrium distance. For this the RATTLE algorithm has been
implemented in the CAST program. The velocity Verlet algorithm in combination with
the RATTLE algorithm has been shown to yield stable trajectories for an arbitrary
length of simulation time. In a first application the MD implementation is used in conjunction
with the MOPAC interface for the investigation of PBI sidechains and their
rigidity. The theoretical investigations show a nice agreement with experimentally obtained
results. Based on the MD techniques two algorithms for the determination of free
energy differences have been implemented. The umbrella sampling algorithm can be
used to determine the free energy change along a reaction coordinate based on distances
or dihedral angles. The implementation was tested on the stretching of a deca-L-alanine
and the rotation barrier of butane in vacuum. The results are in nearly perfect agreement
with literature values. For the FEP implementation calculations were performed
for a zero-sum transformation of ethane in explicit solvent, the charging of a sodium
ion in explicit solvent and the transformations of a tripeptide in explicit solvent. All
results are in agreement with benchmark calculations of the NAMD program as well
as literature values. The FEP formalism was then applied to determine the relative
binding free energies between two inhibitors in an inhibitor-protein complex.
Next to force fields, ab-initio methods can be used for simulations and global optimizations.
Since the performance of such methods is usually significantly poorer than force
field applications, the use for global optimizations is limited. Nevertheless significant
progress has been made by porting these codes to GPUs. In order to make use of these
developments a MPI interface has been implemented into CAST for communication
with the DFT code TeraChem. The CAST/TeraChem combination has been tested
on the $H_2 O_{10}$ cluster as well as the polypeptide met-Enkephalin. The pure ab-initio
calculations showed a superior behavior compared to the standard procedure where the
force field results are usually refined using quantum chemical methods.
Theoretical Investigations on the Interactions of Small Compounds with their Molecular Environments
(2015)
In the first part of this work, a combination of theoretical methods for the rational design of covalent inhibitor is presented. Starting from the crystal structure of the covalent complex of a lead compound, quantum mechanical and QM/MM calculations were used to derive the exact geometry of the preceeding non-covalent enzyme inhibitor complex. The geometry of the latter mainly determines the reactivity of the inhibitor against its target enzyme concerning the formation of the covalent bond towards an active site residue. Therefore, this geometry was used as starting point for the optimization of the substitution pattern of the inhibitor such as to increase its binding affinity without loosing its ability to covalently bind to the target protein. The optimization of the chemical structure was supported by using docking procedures, which are best suited to estimate binding affinities that arise from the introduced changes. A screening of the novel substitution patterns resulted in a first generation of model compounds which were further tested for their reactivity against the target. Dynamic simulations on the novel compounds revealed that the orientation that compounds adopt within the active site are such that a covalent interaction with the enzyme is no longer possible. Hence, the chemical structure was further modified, including not only changes in the substituents but also within the core of the molecule. Docking experiments have been conducted to assure sufficiently high binding affinities and to obtain the most favored binding poses. Those have then again been used for dynamic simulations which resulted in structures, for which the bond formation process appeared feasible. A final series of QM/MM calculations considering various protonation states was computed to estimate the reaction energies for the covalent attachment of the inhibitor to the enzyme. The theoretical results indicate a reasonable high inhibition potency of the novel compounds.
The second part concentrates on the environmental influences on the electron density of an inhibitor molecule. Therefore, a vinylsulfone-based model compound was selected for which an experimental crystal structure for the pure compound as well as a theoretically determined enzyme-inhibitor complex have been available. To provide reference data for the larger systems, the conformational space of the isolated molecule was screened for favorable geometries which were later compared to those within the crystal and protein surrounding. The geometry of the crystal structure could readily be taken from the experimental data whereas calculations on the protein complex revealed four potential non-covalent complexes exhibiting different arrangements of the molecule within the active site of the protein as well as two possible protonation states of the catalytic dyad. Hence, all four protein complexes have been compared to the crystal structure of the molecule as well as against the more favorable geometries of the isolated molecule being determined within vacuum or aqueous surrounding. Whereas the molecule itself was found to adopt comparable geometries within all investigated environments, the interactions pattern between the crystal surrounding and the protein differed largely from each other. The favorable formation of dimers within the crystal has a strong stabilizing effect and explains the extraordinarily good quality of the crystal. Within the protein however, repulsive forces have been found between the protein and the inhibitor. The origin of the repulsion could be traced back to effect of on of the substituents to the vinyl scaffold. The difference in the chemical structure in comparison to a well known inhibitor might also explain the experimentally found loss of activity for the model compound in comparison to K11777.
The photoionization of several nitrogen-containing reactive intermediates relevant in combustion processes was investigated in the gas phase employing VUV synchrotron radiation. The intermediates were either freshly prepared and stored under cryogenic temperatures during the experiment or generated in situ by vacuum flash pyrolysis of suitable precursor molecules. The iPEPICO (imaging photoelectron photoion coincidence) setups of the VUV beamlines at the Swiss Light Source and Synchrotron SOLEIL were then used to record mass-selected threshold photoelectron (TPE) spectra. TPE spectra reveal the ionization energy and vibrational structure in the cationic states can often be resolved, which enables to distinguish different isomers. Accurate ionization energies for the radicals carbonyl amidogen, pyrrolyl, and 3-picolyl, and for the closed shell molecules isocyanic acid and cyanovinylacetylene were obtained. The analysis of the dissociative photoionization of the pyrolysis precursors enables in some cases to retrieve thermochemical data. Beyond, the absolute photoionization cross section of the cyclic carbene cyclopropenylidene was determined, NEXAFS and normal Auger spectra of isocyanic acid were recorded and analyzed at the O1s, N1s, and C1s edges, and the dissociative photoionization and pyrolysis of 1,4-di-tert-butyl-1,4-azaborinine was studied.
The scope of computational chemistry can be broadened by developing new methods and more efficient algorithms. However, the evaluation of the applicability of the methods for the different fields of chemistry is equally important. In this thesis systems with an unusual and complex electronic structure, such as excitonic states in organic semiconductors, a boron-containing bipolaron and the excited states of pyracene were studied and the applicability of the toolkit of computational chemistry was investigated. Concerning the organic semiconductors the focus was laid on organic solar cells, which are one of the most promising technologies with regard to satisfying the world's need for cheap and environmentally sustainable energy. This is due to the low production and material costs and the possibility of using flexible and transparent devices. However, their efficiency does still not live up to the expectations. Especially the exciton diffusion lengths seem to be significantly too short. In order to arrive at improved modules, a fundamental understanding of the elementary processes occurring in the cell on the molecular and supramolecular level is needed. Computational chemistry can provide insight by separating the different effects and providing models for predictions and prescreenings. In this thesis, the focus was laid on the description of excitonic states in merocyanines and perylene-based dyes taking the influence of the environment into account.
At first, the photochemical isomerization between two configurations of 6-nitro BIPS observed experimentally was studied by first benchmarking several functionals against SCS-ADC(2) in the gas phase and subsequently calculating the excited-state potential energy surface. The geometries obtained from a relaxed scan in the ground state as well as from a scan in the excited state were used. The environment was included using different polarizable continuum models. It was shown that the choice of the model and especially the question of the state specificity of the approach is of vital importance. Using the results of the calculations, a two-dimensional potential energy surface could be constructed that could be used to explain the experimental findings. Furthermore, the importance of the excited-state isomerization as a potential deactivation channel in the exciton transport was pointed out.
Then the assessment of the suitability of different merocyanines for optoelectronic applications with quantum-chemical methods was discussed. At first, the effect of the environment on the geometry, especially on the bond length alternation pattern, was investigated. It was shown that the environment changes the character of the ground-state wave function of several merocyanines qualitatively, which means that the results of gas-phase calculations are meaningless - at least when a comparison with solution or device data is desired. It was demonstrated that using a polarizable continuum model with an effective epsilon, a qualitative agreement between the calculated geometry and the geometry in the crystal structure can be obtained. Therefore, by comparing the bond length alternation in solution and in the crystal, a rough estimate of the effect of the crystal environment can be made.
It was further shown that the connection between the HOMO energy and the open-circuit voltage is not as simple as it is often implied in the literature. It was discussed that it is not clear whether the HOMO of a single molecule or a $\pi$-stack containing several monomers should be used and if the environmental charges of the bulk phase or the interface should be included. Investigating the dependence of the HOMO energy on the stack size yielded no definitive trend. Furthermore, it was discussed that the effect due the optimization of the modules (solvent, bulk heterojunction) during the production masks any potential correlation between the HOMO energy and measured open-circuit values. Therefore, a trend can only be expected for unoptimized bilayer cells. It was concluded that ultimately, the importance of the HOMO energy should not be overestimated.
The correlation between the exciton reorganization energy and the so-called cyanine limit, which is predicted by a simple two-state model, was also discussed. By referring to the results of VB calculations, it was discussed that the correlation indeed exists and is non-negligible, although the effect is not as strong as one might have expected. In this context, a potential application of a VB/MM approach was covered briefly. The importance of the molecular reorganization energy and the device morphology was also discussed.
It was concluded that the optimization of merocyanines for organic optoelectronic devices is inherently a multiparameter problem and one cannot expect to find one particular parameter, which solely controls the efficiency.
The perylene-based dyes were studied with a focus on the description of a potential trapping mechanism involving an intermolecular motion in a dimer. The aim was to find methods which can be applied to larger model systems than a dimer and take the effect of the environment into account. As a test coordinate the longitudinal shift of two monomers against each other was used. At first, it was demonstrated how the character of an excited state in a dimer can be defined and how it can be extracted from a standard quantum-chemical calculation. Then several functionals were benchmarked and their applicability or failure was rationalized using the character analysis. Two recipes could be proposed, which were applied to a constraint optimization (only intermolecular degrees of freedom) in the excited states of the PBI dimer and to the description of the potential energy surfaces of ground and excited states along a longitudinal displacement in the perylene tetramer, respectively.
It was further demonstrated that the semi-empirical OMx methods fail to give an accurate description of the excited-state potential energy surfaces as well as the ground-state surface along the test coordinate. This failure could be attributed to an underestimation of overlap-dependent terms. Consequently, it could be shown that the methods are applicable to large intermolecular distances, where the overlap is negligible. The results of DFT calculations with differently composed basis sets suggested that adding an additional single p-function for each atom should significantly improve the performance.
QM/MM methods are ideally suited to take the effect of the environment on a a dimer model system into account. However, it was shown that standard force fields also give an incorrect description of the interaction between the monomers along the intermolecular coordinate. This failure was attributed to the isotropic atom-atom interaction in the repulsion term of the Lennard-Jones potential. This was corroborated using two simple proof-of-principle anisotropy models. Therefore, a novel force field called OPLS-AA_O was presented that is based on OPLS-AA, but uses an anisotropic model for the repulsion. The model involves the overlap integral between the molecular densities, which are modeled as a sum of atom-centered p-type Gaussian functions. It was shown that using this force field an excellent agreement with the DFT results can be obtained when the correct parameters are used. These parameters, however, are not very generalizable, which was attributed to the simplicity of the model in its current state (using the same exponential parameter for all atoms). As a short excursion, the applicability of an MO-based overlap model was discussed.
It was demonstrated that the repulsion term based on the density overlap can be used to correct the failure of the OMx methods for the ground states. This is in accord with the assumption that an underestimation of the overlap terms is responsible for the failure.
It was shown that OPLS-AA_O also gives an excellent description of the longitudinal shift in a PBI tetramer. Using the tetramer as a test system and applying the recipe obtained in the TDDFT benchmark for the QM-part and OPLS-AA_O for the MM-part in conjunction with an electrostatic embedding scheme, a QM/MM description of the excited states of the PBI dimer including the effect of the environment could be obtained.
In the last chapter the theoretical description of the Bis(borolyl)thiophene dianion and the excited states of pyracene were discussed. The electronic structure of the Bis(borolyl)thiophene dianion - a negative bipolaron - was elucidated using DFT and CASPT2 methods. Furthermore, an estimation of the extent of triplet admixture to the ground state due to spin-orbit coupling was given.
In the second project the S1 and S2 states of pyracene were computed using SCS-CC2 and SCS-ADC(2) and an estimation for the balance between aromaticity and ring strain was given. This also involved computing the vibrational frequencies in the excited states.
In both studies the results of the computations were able to rationalize and complete experimental results.
A series of combustion relevant species like radicals, carbenes and polycyclic aromatic hydrocarbons were characterized in the gas phase by vacuum UV synchrotron radiation and their ionization energies (IE) and further spectroscopic details of the respective cations were retrieved from threshold photoelectron spectra. The reactive intermediates were generated by flash vacuum pyrolysis from stable precursor molecules. Furthermore three polycyclic aromatic hydrocarbons were investigated by threshold photoelectron spectroscopy, too. The experiment was performed at the VUV beamline of the Swiss Light Source in Villigen/Switzerland and the iPEPICO (imaging photoelectron photoion coincidence) setup was applied to correlate ions and electrons from the same ionization event. From the threshold photoelectron spectra and from quantum chemical computations the vibrational structure of the molecule cations and the geometry changes upon ionization were assigned. The ionization energies of the two C4H5 isomers 2-butyn-1-yl and 1-butyn-3-yl were assigned to 7.94±0.02 eV and 7.97±0.02 eV, respectively. The isomerization between the two isomers was computed to have a barrier of 2.20 eV, so a rearrangement between the two radicals cannot be excluded. From the threshold photoelectron spectra of the two constitutional C4H7 isomers 1-methylallyl and 2-methylallyl the ionization energies were assigned to 7.48±0.02 eV and to 7.59±0.02 eV for 1-E-methylallyl and 1-Z-methylallyl, as well as to 7.88±0.01 eV for 2-methylallyl. The two radicals 9-fluorenyl, C13H9, and benzhydryl, C13H11, were observed to ionize at 7.01±0.02 eV and 6.7 eV. The threshold photoelectron spectrum of benzhydryl also incorporated the signal of the diphenylmethyl carbene, C13H10, which has an IE at 6.8 eV. In addition, the head-to-head dimers of 9-fluorenyl and benzhydryl were observed as products in the pyrolysis. C26H18 has an IE at 7.69±0.04 eV and C26H22 has an IE at 8.13±0.04 eV. The three polycyclic aromatic hydrocarbon DHP (C14H16) 1-PEN (C18H22) and THCT (C22H16) were investigated in an effusive beam. The ionization energies were determined to IE(DHP)= 7.38±0.02 eV, IE(1-PEN)=7.58±0.05 eV and IE(THCT)=6.40±0.02 eV. Furthermore the thermal decomposition and the dissociative photoionization of diazomeldrum’s acid was investigated. The pyrolysis products yielded beside several other products the two not yet (by photoelectron spectroscopy) characterized molecules E-formylketene, C3O2H2 and 2-diazoethenone, N2C2O. The dissociative photoionization showed the Wolff rearrangement to occur at higher internal energies.
As organic semiconductors gain more importance for application, research into their properties has become necessary. This work investigated the exciton and charge transport properties of organic semiconducting crystals. Based on a hopping approach, protocols have been developed for the calculation of Charge mobilities and singlet exciton diffusion coefficients. The protocols do not require any input from experimental data except for the x-ray crystal structure, since all needed quantities can be taken from high-level quantum chemical calculations. Hence, they allow to predict the transport properties of yet unknown compounds for given packings, which is important for a rational design of new materials. Different thermally activated hopping models based on time-dependent perturbation theory were studied for the charge and exciton transport; i. e. the spectral overlap approach, the Marcus theory, and the Levich-Jortner theory. Their derivations were presented coherently in order to emphasize the different levels of approximations and their respective prerequisites. A short reference was made to the empirical Miller-Abrahams hopping rate. Rate equation approaches to calculate the stationary charge carrier mobilities and exciton diffusion coefficients have been developed, which are based on the master equation. The rate equation approach is faster and more efficient than the frequently used Monte Carlo method and, therefore, provides the possibility to study the anisotropy of the transport parameters and their three-dimensional representation in the crystal. The Marcus theory, originally derived for outer sphere electron transfer in solvents, had already been well established for charge transport in organic solids. It was shown that this theory fits even better for excitons than for charges compared with the experiment. The Levich-Jortner theory strongly overestimates the charge carrier mobilities and the results deviate even stronger from the experiment than those obtained with the Marcus theory. The latter contains larger approximations by treating all vibrational modes classically. The spectral overlap approach in combination with the developed rate equations leads to even quantitatively very good results for exciton diffusion lengths compared to experiment. This approach and the appendant rate equations have also been adapted to charge transport. The Einstein relation, which relates the diffusion coefficient with the mobility, is important for the rate equations, which have been developed here for transport in organic crystals. It has been argued that this relation does not hold in disordered organic materials. This was analyzed within the Framework of the Gaussian disorder model and the Miller-Abrahams hopping rate.
Zur Charakterisierung der Wechselwirkungen zwischen organischen Dispergiermitteln und nanoskaligen Oberflächen stellen Komplexe aus Kohlenstoffnanoröhren und (Bio-)Polymeren aufgrund der großen Oberfläche der Nanoröhren und der kommerziellen Verfügbarkeit fluoreszenzmarkierter DNA-Oligomere unterschiedlicher Länge sowie intrinsisch fluoreszierender Polymere ein vielversprechendes Modellsystem dar. Im Rahmen der vorliegenden Dissertation wurden verschiedene Methoden evaluiert, um die Stabilität derartiger Komplexe zu untersuchen und dadurch Rückschlüsse auf das Adsorptionsverhalten der (Bio-)Polymere zu ziehen. Dabei konnte gezeigt werden, dass das publizierte helikale Adsorptionsmodell der DNA auf Kohlenstoffnanoröhren die Resultate der durchgeführten Experimente nur unzureichend beschreiben kann und stattdessen andere Adsorptionskonformationen in Erwägung gezogen werden müssen.
Im Rahmen der vorliegenden Arbeit wurde durch einzelmolekülspektroskopischer bzw. -mikroskopischer Methoden in Kombination mit einer mikrofluischen Zel- le unter Potenzialkontrolle die Elektrochemie von einzelnen einwandigen (6,5)- Kohlenstoffnanoröhren untersucht. Hierfür wurde ein Nahinfrarot-Photolumineszenz- Mikroskop aufgebaut und eine speziell an die experimentellen Vorgaben angepasste elektrochemische Zelle entwickelt, insofern als drei Elektroden (Arbeits-, Gegen- und Referenzelektrode) in einen mikrofluidischen Chip integriert wurden. Darüber hinaus war für die Durchführung der Experimente unter Wasser- und Sauerstoffaus- schluss die Konstruktion eines Handschuhkastens notwendig, sowie eine allgemeine Vorbehandlung der Elektrolytlösungen zur Entfernung gelöster Gase und Wasserreste.
Ein weiteres Projekt umfasste den Aufbau einer chemischen Gasphasenabschei- dungsapparatur zur Synthese von Kohlenstoffnanoröhren. Die hierbei durchgeführten Experimente erbrachten Klarheit über den Einfluss der Prozessparameter Druck, Temperatur und Durchflussrate an Edukten.
Aus den PL-Intensitätsänderungen bei Potenzialvariation konnten Reduktions- und Oxidationspotenziale (ERed = 0.15 V; EOx = 1.34 V) einzelner (6,5)-SWNTs gegen- über einer Platin Referenzelektrode und einem daraus resultierenden Redoxpotenzial
von ∆ERedOx = 1.19 V ermittelt werden. Durch diese einzelmolekülspektroskopische
Methode konnte zum einen gewährleistet werden, dass nur dieser spezielle Chira- litätstyp untersucht wurde und zum anderen eine Verfälschung der Resultate durch einen Potenzialabfall wie er typischerweise in CNT-Filmen auftritt aussgeschlossen werden. Eine Kombination der PL-Daten mit der Ramanintensitätsabhängigkeit des (6,5)-SWNT-S2-Übergangs bei Potenzialvariation erlaubte eine genauere Analyse des Löschmechanismus der PL von Kohlenstoffnanoröhren. Mithilfe eines von Her- tel et al. entwickelten diffusionslimitierten Stoßdesaktivierungsmodells konnte eine invers-quadratische Proportionalität zwischen der (6,5)-SWNT-Emission und den spannungsinduzierten Ladungsträgern ausgemacht werden. Auf Grundlage dieses Ergebnisses folgt, dass die über Photolumineszenzänderungen ermittelten Reduktions-und Oxidationswerte nicht mit den Bandkanten der CNTs übereinstimmen müssen, und dass für deren Bestimmung vielmehr auf Raman- bzw. Absorptionsspektroskopi- sche Techniken zurückgegriffen werden muss.
Die einzelmolekülspektroskopische Herangehensweise ermöglichte ferner eine statis- tische Analyse der Verteilung der Reduktions- und Oxidationspotenziale im Vergleich zu den jeweiligen Erwartungswerten. Hierdurch konnte eine Einteilung der Modifika- tionseinflüsse auf das SWNT-Redoxverhalten in zwei Grenzfälle erfolgen. Es wurde angenommen, dass diese als “Dispergiermitteleffekte” und “CNT-Strukturdefekte” be- zeichneten Auswirkungen entweder das Resultat einer heterodispersen Verteilung an DOC auf der CNT-Oberfläche oder eine Folge von Defekten in der CNT-Gitterstruktur waren. In diesem Zusammenhang ergab sich aus der interpartikulären Analyse der Reduktions- und Oxidationswerte eine Korrelation, die einem dominierenden Einfluss der “CNT-Strukturdefekte” zugeordnet werden konnte. Dieser Beobachtung entgegen- gesetzt konnten aber auch über Untersuchungen der Redoxpotenziale innerhalb einer (6,5)-SWNT lokale Bereiche ausgemacht werden, die eine signifikante Abhängigkeit von “Dispergiermitteleffekte” aufwiesen.
Abgesehen von diesen Einflüssen auf den Emissionsverlauf wurde auch eine Be- trachtung der Breite des spannungsgesteuerten Emissionsabfall durchgeführt. Da- raus konnte ermittelt werden, dass diese Ausdehnung eine Konsequenz aus der PL- Löschungseffizienz der Ladungsträger ist und, dass bei einer Verteilung von 0.32 Löschzentren pro Nanometer eine vollständige Abnahme der Photolumineszenzinten- sität induziert wird.
Darüber hinaus wurde im Rahmen dieser Arbeit das redoxchemische Verhalten in- dividueller (6,5)-SWNTs in Wechselwirkung mit Ferrocenmolekülen untersucht. Die erhaltenen Ergebnisse ließen annehmen, dass die sich ausbildende Verbindung nicht-kovalenter Natur ist. Zwei verschiedene Gründe führten zu dieser Erkennt- nis: einerseits ließen sich die Ferrocenmoleküle von der CNT-Oberfläche durch ein Durchspülen des mikrofluidischen Kanals mit einer reinen DMF-Lösung entfernen und andererseits war keine dauerhafte Emissionsminderung durch die Ausbildung kovalenter Bindungen zu beobachten. Aus der potenzialabhängigen PL wurde zudem ein Elektronentransfer der Ferrocenmoleküle in die optisch generierten Löcher des CNT-Valenzbandes festgestellt und über eine anregungsintensitätsabhängige Messung die Zunahme dieses Ladungstransfers bei steigendem Photonenfluss nachgewiesen.
Hinsichtlich der Anwendung von Kohlenstoffnanoröhren zur Elektrolyse bzw. Photo- lyse von Wasser wurde auch die Redoxchemie von (6,5)-SWNTs in diesem Solvens untersucht. Bezüglich der Emissionsintensität konnte gezeigt werden, dass diese im Vergleich zu organischen Lösungsmitteln reduziert vorliegt. Außerdem wurde eine irreversible Reaktion nach anodischer Polarisation über eine dauerhafte Löschung der PL beobachtet. Die Bestimmung der hierfür notwendigen Reaktionsumstände erbrachte, dass Wasser, Exzitonen (erzeugt durch optische Anregung) und spannungs- induzierte Löcher im Valenzband zur Bildung einer [SWNT(Q)]-Spezies führen, welche die irreversible Minderung der CNT-Emission verursacht. Darüber hinaus konnte die Reaktionsgeschwindigkeit über eine Kinetik pseudo-nullter-Ordnung be- schrieben werden, unter der Voraussetzung, dass die soeben genannten Parameter konstant verblieben. Desweiteren zeigte sich in einer ferrocenhaltigen Lösung, dass der Löscheffekt der [SWNT(Q)]-Spezies im anodischen Potenzialbereich teilweise reduziert wird. Es wurde angenommen, dass diese Beobachtung auf eine Oxidation der Löschzentren durch die Fc+-Kationen gründet.
Mit Hilfe der CVD-Apparatur gelang es Kohlenstoffnanoröhren zu synthetisieren, wobei Ethanol als Kohlenstoffquelle und ein Eisen-Kobalt-Zeolith-Gemenge als Ka- talysator diente. Die Analyse der verschiedenen Prozessparameter zeigte, dass bei T = 750 °C das beste Verteilungsverhältnis zwischen den gewünschten (6,5)-SWNTs und anderen CNT-Chiralitäten bzw. dem amorphen Kohlenstoff vorliegt. Hierfür war, dass bei T < 750 °C die Verbrennung unerwünschter amorpher Kohlenstoffreste nur geringfügig stattfindet, und dass bei T > 750 °C die Bildung anderer Chiralitäten mit größerem Durchmesser als die (6,5)-SWNT bevorzugt wurde. Die Variation der Durchflussrate hingegen wirkte sich nur in einer absoluten Zunahme aller Chirali- täten aus. Die Steigerung des (6,5)-SWNT-Anteils für höhere Durchflüsse gelang trotzdem durch die geschickte Auswahl geeigneter Druck- und Temperaturwerte. Die Experimente zur Untersuchung der Druckabhängigkeit wiesen auf eine Relation mit dem Gesetz von Le Chatelier hin, insofern als bei einer Druckverringerung eine Verschiebung der Ethanol-Crackreaktion auf Produktseite stattfand. In diesem Zusam- menhang wurde angenommen, dass die damit verstärkt gebildeten Moleküle Ethan, Ethen und Methan den CNT-Anteil zwar erhöhen, jedoch auch eine Steigerung der amorphen Kohlenstoffkonzentration verursachen. Dementsprechend ergab ein Druck von p = 9 mbar das beste (6,5)-SWNT zu dem amorphen Kohlenstoffverhältnis.
Anhand der Arbeiten in dieser Dissertation sind neue Erkenntnisse zwischen der PL-Sensitivität von (6,5)-SWNTs und deren Ladungszustand erhalten worden. Insbe- sondere die genaue Bestimmung der Korrelation zwischen der Photolumineszenz und den induzierten Ladungsträgern ermöglicht einen gezielteren Einsatz von Kohlenstoff- nanoröhren – so zum Beispiel im Bereich der Sensorik. In diesem Zusammenhang zeigen auch die interpartikulären Analysen der Redoxpotenzialverteilung die genau- en Auswirkungen vom Lösungsmittel und der Defektdichte auf die elektronische Struktur der CNTs auf. Darüber hinaus kann aus der Ursachenbestimmung für die Varianz der literaturbekannten Reduktions- bzw. Oxidationspotenziale fortan die ge- eignete spektroskopische Methode zur Evaluierung der Position von Leitungs- und Valenzband in Kohlenstoffnanoröhren besser eingegrenzt werden. Die spektroelektro- chemischen Analysen von (6,5)-SWNTs im Lösungsmittel Wasser und speziell die Bestimmung der Kinetik für die auftretende Reaktion liefern einen tieferen Einblick in die Wechselwirkung (6,5)-SWNT-H2O. Diese Ergebnisse sind insbesondere bei der Verwendung von Kohlenstoffnanoröhren als Elektrodenmaterial für die photolytische bzw. elektrolytische Spaltung von Wasser in Wasserstoff und Sauerstoff von Bedeu- tung. Neben der Untersuchung der SWNT-Wasser Interaktion unter andoischer und optischer Anregung, die zu einer kovalenten Bindung führte, wurde mit Hilfe der (6,5)- SWNT-Ferrocen Wechselwirkung ein Beispiel für eine nichtkovalente Redoxreaktion dargestellt, womit ein Vergleich dieser beiden Spezies und ihrer unterschiedlichen Auswirkungen auf die elektronische Struktur aufgezeigt werden konnte.
Die vorliegende Dissertation untersucht fünf unterschiedliche Moleküle hinsichtlich ihrer Geometrien im Grund- und angeregten Zustand sowie deren Dynamik
nach elektronischer Anregung. Der Fokus liegt dabei unter anderem auf Pi-konjugierten Systemen, die über eine zusätzliche aliphatische Einheit verbrückt (Paracyclophan-
Derivate) oder erweitert (Pyracen) sind. Die Paracyclophan-Derivate sind ein ideales Modellsystem um Einsicht in Pi-Pi-Wechselwirkungen zu erlangen. Ein
weiterer Schwerpunkt dieser Arbeit beschreibt die Dynamik des resonanzstabilisierten 2-Methylallyl-Radikals. Die Forschung an solchen kleinen Kohlenwasserstoff-Radikalen ist wichtig, da auf deren Grundlage Modelle entwickelt werden können, die zum Beispiel helfen, den Verbrennungsprozess aufzuklären. Aufgrund ihrer Instabilität sind solche kleinen Kohlenwasserstoff-Radikale nicht einfach zu handhaben und das spektroskopische Vermessen stellt immer eine Herausforderung dar.
Plasmonic modes supported by noble-metal nanostructures offer strong subwavelength electric-field confinement and promise the realization of nanometer-scale integrated optical circuits with well-defined functionality. In order to measure the spectral and spatial response functions of such plasmonic elements, we combine a confocal microscope setup with spectral interferometry detection. The setup, data acquisition, and data evaluation are discussed in detail by means of exemplary experiments involving propagating plasmons transmitted through silver nanowires. By considering and experimentally calibrating any setup-inherent signal delay with an accuracy of 1 fs, we are able to extract correct timing information of propagating plasmons. The method can be applied, e.g., to determine the dispersion and group velocity of propagating plasmons in nanostructures, and can be extended towards the investigation of nonlinear phenomena.
We present polarimetry, i.e. the detection of optical rotation of light polarization, in a configuration suitable for femtosecond spectroscopy. The polarimeter is based on common-path optical heterodyne interferometry and provides fast and highly sensitive detection of rotatory power. Femtosecond pump and polarimeter probe beams are integrated into a recently developed accumulative technique that further enhances sensitivity with respect to single-pulse methods. The high speed of the polarimeter affords optical rotation detection during the pump-pulse illumination period of a few seconds. We illustrate the concept on the photodissociation of the enantiomers of methyl p-tolyl sulfoxide. The sensitivity of rotatory detection, i.e. the minimum rotation angle that can be measured, is determined experimentally including all noise sources to be 0.10 milli-degrees for a measurement time of only one second and an interaction length of 250 μm. The suitability of the presented setup for femtosecond studies is demonstrated in a non-resonant two-photon photodissociation experiment.
The experimental technique predominantly employed within the scope of this Thesis constitutes one subarea of femtochemistry: the time-resolved spectroscopy of photoin-
duced chemical reactions in the liquid phase by means of molecular signatures in the mid-infrared (MIR) spectral range. Probing transient vibrational states, i.e., dynamic
changes in the vibrational motion of specic molecular subunits or functional Groups allows for a distinct separation and assignment of measured signals to emerging molecular species. For this purpose, one key building block is indispensable, which most of the investigations carried out within the eld of femtochemistry have in common: a coherent light source delivering ultrashort laser pulses with a temporal duration that matches the femtosecond time scale on which molecular motions typically occur. This instrumentation enables the observation of photoinduced chemical reactions from the
starting point|the excitation event to the appearance of intermediates to the nal formation of stable photoproducts after several pico- or nanoseconds.
This work comprises the acquisition and presentation of time-resolved spectroscopic data related to promising molecular systems upon photoexcitation as well as the im-
plementation and testing of experimental optical techniques both for the presented experiments but as well for experiments conceivable in the future. In addition, linear spectroscopy measurements and quantum-chemical simulations on the emerging chemical species have been carried out. In so doing, the primary processes and subse-
quently emerging reaction products of two compounds on a timescale of several nanoseconds after photoexcitation have been elucidated in great detail. Both compounds, the
[Mn(CO)3(tpm)]+ (tpm = tris(2-pyrazolyl)methane) CO-releasing molecule (CORM) and the 5-diazo Meldrum's acid (DMA), are of academic interest but in addition belong
to molecular classes that might be utilized in the near future as dark-stable prodrugs under physiological conditions or that are already utilized in industrial chemistry procedures, respectively. The ndings of both studies gave rise to implement and examine two techniques for prospective transient absorption experiments, namely the shaping and characterization of ultraviolet (UV) laser pulses and the recording of two-photon excitation spectra. Beyond that, since each of the depicted experiments is based on the detection of weak transient absorption signals in the MIR spectral region, two dif-
ferent detection schemes, via chirped-pulse upconversion (CPU) on the one hand and via direct multichannel MCT detection on the other hand, have been juxtaposed at the
conclusion of this work. Since both techniques are suitable in femtosecond pump-probe measurements but thereby exhibit individual strengths and weaknesses, a comparative study provides clarication of the respective pros and cons. The first study introduced within this work investigates the complex photochemistry
of DMA, a photoactive compound used in lithography and industrial chemistry. By femtosecond MIR transient absorption spectroscopy covering several nanoseconds, the
light-induced dynamics and ultrafast formation of several photoproducts from the manifold of reaction pathways have been disclosed to form a coherent picture of the overall
reaction scheme. After UV excitation of DMA dissolved in methanol to the second excited state S2, 70% of excited molecules relax back to the S0 ground state. In compet-
ing processes, they can either undergo an intramolecular Wolff rearrangement to form ketene, which reacts with a solvent molecule to an enol intermediate and further to carboxylate ester, or they rst relax to the DMA S1 state, from where they can isomerize to a diazirine. The third competing reaction channel, having the lowest quantum efficiency with respect to the rst two channels, is the formation of a singlet carbene out of the S1 state. From there an ylide can arise or, via an intersystem crossing, the triplet form of the carbene follows. Whereas the primary reaction steps occur on a picosecond timescale, the subsequently arising intermediates and stable photoproducts are formed
within a few hundreds to thousands of picoseconds. For a reliable identication of the involved compounds, density functional theory calculations on the normal modes and
Fourier-transform infrared spectroscopy of the reactant and the photoproducts in the chemical equilibrium accompany the analysis of the transient spectra. Additional experiments in ethanol and isopropanol led to slight spectral shifts as well as elongated time
constants due to steric hindrance in transient spectra connected with the ester Formation channel, further substantiating the assignment of the occurring reaction pathways and photoproducts.
The study demonstrated that the combination of linear and time-resolved spectroscopic measurements in conjunction with quantum-chemical calculations constitutes a powerful tool to unravel even highly complex photoreactions exhibiting multiple consecutive intermediate states within parallel reaction pathways. Although some of the individual reaction steps, for example the ketene formation via Wolff rearrangement, have been observed on ultrashort time scales before, this work encompassed the Observation of the whole set of appearing photoproducts of DMA in different alcohol solutions within several nanoseconds. In this sense, the ultrafast photochemistry of DMA represents a prototype example for a multisequential reaction scheme, elucidated by the capabilities of femtosecond MIR spectroscopy.
With a modified instrumentation concerning amongst others the system delivering the fundamental laser pulses or the generation of the UV pump pulses, the next ob-
jective within this work was to elucidate the primary processes upon UV Irradiation of a manganese tricarbonyl CORM in aqueous environment. The time-resolved
experiment was performed with two different pump wavelengths and furthermore supported by linear spectroscopy methods and time-dependent density functional theory (TDDFT) calculations on the excited states as well as DFT calculations on the ground
states. The measurements revealed that irradiating the compound with UV excitation pulses primarily leads to ultrafast photolysis of one CO ligand. Geminate recombination may occur within one picosecond but it remains a minor process as the photolyzed CO
group is liberated and the unoccupied coordination site is predominantly filled by an incoming solvent molecule. There was no evidence for hot CO bands, i.e., the remaining CO ligands|in the dicarbonyl photoproduct as well as in the intact CORM are not vibrationally excited through the UV excitation of the CORM. According to this, the excess energy merges into low-frequency vibrational modes associated with the molecule as a whole. Since studies on a macroscopic scale at irradiation times of several minutes prove that UV irradiation eventually leads to the release of two or even all three CO
ligands, further loss of CO most likely necessitates manganese oxidation or another interaction with light. To clarify the latter, a consecutive UV pulse was employed in order to excite the photoproducts subsequent to the initial pump interaction. However, the data obtained was not instructive enough to denitely exclude the manganese oxidation being responsible for the loss of further CO groups. Besides the exchange of a CO Group by a solvent molecule or the geminate recombination, the employment of two different excitation wavelengths in combination with ndings derived from the TDDFT calculations suggested another reaction process, namely the possibility that the excitation does not lead to any bond cleavage at all. As the CORM under investigation is tissue-selective and cytotoxic against cancer cells, knowledge of these rst photoinduced reaction steps is essential for a full understanding of its biological activity. Inspired by these two studies, experimental techniques for prospective transient absorption measurements have been implemented and tested within preparative measure-
ments. First, in the course of a UV-pump-MIR-probe experiment with specically tailored excitation pulses, one could pursue the aim of coherently controlling the outcome of a photoreaction in the liquid phase. Out of the rich photochemistry of DMA the vibrational signature of a particular molecular species might thereby serve as a feedback signal, which is a central part of a learning loop that adaptively determines the pulse shape that steers the quantum mechanical system upon photoexcitation into a desired direction. This motivated the installation and testing of devices by means of which the shaping and characterization of ultrashort laser pulses in the UV could be performed. Second, motivated by the biological applications of CORMs, one can imagine a scenario where a certain amount of CORMs is deposited inside cancerous tissue. Since the activation of CO loss by means of UV pulses is not possible due to the absorption characteristics of biological tissue, the simultaneous excitation via two photons from the visible spectral regime seems appealing. However, success or failure of such an application depends on whether the deposited compound efficiently absorbs two photons simultaneously, i.e., whether the two-photon absorption cross section is large enough. Therefore, a setup to record two-photon excitation spectra under full consideration of
the crucial laser pulse parameters like the pulse duration, energy and central wavelength was arranged and tested. The rst results were obtained with a commercially available reference system (Mn2CO10) but the setup as well as the described measurement and
data analysis procedure can easily be applied to record the two-photon absorption cross section of more promising molecular systems. Third, as the detection of probe pulses
in the MIR spectral region is part of each time-resolved measurement throughout this thesis, a comparison between the newly established technique of CPU and direct multi-
channel MCT detection is presented by means of pump{probe experiments on Mn2CO10 and Co4CO12 with a 1 kHz shot-to-shot data acquisition. It was shown that the CPU detection technique scores with its high spectral resolution and coverage of the easy-to-handle and more cost-effective CCD detectors. On the other hand, in the course of the additional nonlinear upconversion process intensity fluctuations of the chirped fundamental pulses are transferred to the probe spectrum in the visible regime. This entails a lower signal-to-noise ratio than the direct MCT detection, which can be compensated by an additional normalization procedure applied to the CPU probe pulses. As a consequence, the CPU detection scheme offers more flexibility for future investigations
employing MIR probe pulses. This is of great importance for many applications within the presented eld of femtochemistry as a huge variety of time-resolved investigations on a multitude of systems in the liquid phase is based on the detection of weak transient
absorption signals in the MIR spectral region.