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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.
Die vorliegenden Arbeit behandelt VUV Valenz-Photoionisations-Experimente in der Gasphase. Zunächst wird die Photoionisation von stickstoffhaltigen Radikalen und deren Pyrolyseprodukten untersucht. Im Anschluss werden molekulare Biradikale betrachtet. Da in der Literatur bislang nur wenige solcher Biradikale als Intermediate experimentell zugänglich waren, war es das Ziel dieser Arbeit, neue reaktive Spezies dieser Substanzklassen in der Gasphase zu isolieren und deren Struktur, Eigenschaften und Reaktivität besser zu verstehen. Im Mittelpunkt stehen dabei Intermediate, die als echte Biradikale, Biradikaloide oder Triplett Carbene auftreten. Zu letzteren zählen das Methylbismut sowie die Pentadiinylidene. Biradikale bilden in Verbrennungsprozessen sehr effizient Ruß(vorläufer), was anhand des ortho-Benz-ins dargelegt wurde, indem dessen Pyrolyseprodukte charakterisiert und mögliche PAH-Bildungswege aufgezeigt wurden. Vakuum Flash Pyrolyse wurde verwendet, um in situ aus den geeigneten Vorläufermolekülen die radikalischen und biradikalischen Intermediate zu erzeugen. Während für biradikalische Zwischenstufen meist spezielle Verbindungen als Vorläufer synthetisiert werden müssen, waren die verwendeten Vorläufer für die stickstoffhaltigen Radikale kommerziell erhältlich. Die reaktiven Spezies wurden alle mittels monochromatischer VUV Synchrotronstrahlung an der Swiss Light Source in Villigen/ Schweiz ionisiert. Die Ionisationsereignisse wurden mit der Schwellenphotoelektronen-Photoionen-Koinzidenz (TPEPICO) Technik detektiert und ausgewertet. Anhand der resultierenden massenselektiven Schwellenphotoelektronenspektren wurden die Ionisierungsenergien der (Bi)radikale bestimmt und die Schwingungsstruktur der jeweiligen Kationen analysiert. Die erhaltenen Spektren und Daten wurden in Zusammenarbeit mit der theoretischen Chemie interpretiert.
Wichtige Erkenntnisse
• Es wurde die Ionisierungsenergie der 2-, 3- und 4-Picolylradikale auf 7.70\pm
0.02 eV, 7.59\pm
0.01 eV und 8.01\pm
0.01 eV bestimmt. Diese wurden in der Pyrolyse selektiv aus ihren zugehörigen Picolylaminen erzeugt. Zudem wurde analog zum Benzyl-Radikal für alle drei Radikale eine ausgeprägte Schwingungsprogression ermittelt, die der totalsymmetrischen Deformationsmode des aromatischen Rings entspricht.
• Die Picolyl-Radikale dissoziieren in der Pyrolyse thermisch zu weiteren Produkten. Die Fragmentierung verläuft dabei isomerenunabhängig über ein stickstoffhaltiges Siebenringintermediat, dem Azepinyl-Radikal. Der Fragmentierungsmechanismus wurde mit dem von Benzyl verglichen. Die gewonnenen Erkenntnisse haben Relevanz für Verbrennungsprozesse, beispielsweise von Biokraftstoffen.Im ersten Schritt entstehen vier Isomere, das Cyclopenta-1,4-dien-1-carbonitril, das Cyclopenta-1,3-dien-1-carbonitril, das 2-Ethynyl-1H-pyrrol und das3-Ethynyl-1H-pyrrol mit den zugehörigen Ionisierungsenergien von 9.25\pm
0.02 eV, 9.14\pm
0.02 eV, 7.99\pm
0.02 eV und 8.12\pm
0.02 eV. Durch einen zweiten H-Verlust konnte das Cyanocyclopentadienyl-Radikal mit einer Ionisierungsenergie für die zwei niedrigsten Zustände im Kation mit 9.07\pm
0.02 eV (T0) und 9.21\pm
0.02 eV (S1) untersucht werden. Weitere Pyrolyseprodukte, deren Ionisierungsenergien bereits literaturbekannt sind und die bestätigt wurden, sind das Cyclopentadienyl-Radikal, das Cyclopenta-1,3-dien, das Propargyl-Radikal, das Penta-1,3-diin und das Cyanopropenyl.
• Das ortho-Benz-in wurde pyrolytisch aus dem selbst synthetisierten Benzocyclobutendion erzeugt und ein Schwellenphotoelektronenspektrum frei von Störsignalen konnte aufgenommen werden. Mit Hilfe von Rechnungen aufCASPT2(11,14) Niveau, die neben dem elektronischen Übergang in den kationischen Grundzustand noch die Übergänge in zwei weitere angeregte kationische Zustände beinhalten, wurde die Ionisierungsenergie im Vergleich zu früheren Experimenten auf 9.51 eV revidiert. Eine verdrillte Geometrie für den kationischen Grundzustand konnte erstmals nachgewiesen werden. Zusätzlich wurden die offenkettigen Isomere cis- und trans-Hexa-1,5-diin-3-en im Spektrum detektiert und zugeordnet.
• Die Auftrittsenergien aus der DPI des Vorläufermoleküls Benzocyclobutendion betragen für den ersten CO-Verlust 9.62\pm
0.05 eV und für den zweiten CO-Verlust 12.14\pm
0.10 eV. Damit konnte über einen thermochemischen Kreisprozess eine Bindungsdissoziationsenergie für die Ph-CO Bindung im Benzoylkation von 2.52 eV berechnet werden.
• Verschiedenen Pyrolyseprodukte des ortho-Benz-ins, wie Ethin, Buta-1,3-diin, Benzol, Biphenylen und 2-Ethinylnaphthalin, werden entweder in bimolekularen Reaktionen gebildet oder ortho-Benz-in fragmentiert unimolekular zu diesen. Die beiden kompetitiven Reaktionspfade tragen zur PAH-Bildung des ortho-Benz-ins bei.
• Die Triplett-Carbene Pentadiinyliden, Methylpentadiinyliden und Dimethylpentadiinyliden wurden als Pyrolyseprodukt aus ihren zugehörigen Diazovorläufern identifiziert und die Ionisierungsenergien mit 8.36\pm
0.03 eV, 7.77\pm
0.04 eV und 7.27\pm
0.06 eV bestimmt. Jede Methylierung stabilisiert folglich das Carben. Zusätzlich konnte ein weiteres C5H2 Isomer, das 3-(Didehydrovinyliden)cyclopropen, mit einer Ionisierungsenergie von 8.60\pm
0.03 eV charakterisiert werden.
• Zwei bismuthaltige, reaktive Spezies, das Dimethylbismut-Radikal\cdot
BiMe2 (IE = 7.27\pm
0.04 eV) und das Methylbismut-Carben :BiMe(IE = 7.88\pm
0.02 eV) wurden als Pyrolyseprodukte aus dem BiMe3 identifiziert. Beide Verbindungen zeigen eine ausgeprägte Schwingungsstruktur, die der Bi-C Streckschwingung zugeordnet wurde. Weiterhin wurden elementares Bismut Bi und das Bismut-Dimer Bi2 nachgewiesen.
• Die homolytische Dissoziation der ersten Me2Bi-CH3 Bindung im BiMe3 wurde untersucht und eine BDE von 210\pm
7 kJ/ mol bestimmt. Sie liegt um +15 % bzw. +28 kJ/ mol über dem aus der Literatur abgeschätzten Wert.
Photochemical reactions in solution often proceed via competing reaction pathways
comprising intermediates that capture a solvent molecule. A disclosure of the underlying
reaction mechanisms is challenging due to the rapid nature of these processes and the
intricate identification of how many solvent molecules are involved. Here combining
broadband femtosecond transient absorption and quantum mechanics/molecular mechanics
simulations, we show for one of the most reactive species, diphenylcarbene, that the
decision-maker is not the nearest solvent molecule but its neighbour. The hydrogen bonding
dynamics determine which reaction channels are accessible in binary solvent mixtures at
room temperature. In-depth analysis of the amount of nascent intermediates corroborates
the importance of a hydrogen-bonded complex with a protic solvent molecule, in striking
analogy to complexes found at cryogenic temperatures. Our results show that adjacent
solvent molecules take the role of key abettors rather than bystanders for the fate of the
reactive intermediate.
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.
We employ transient absorption from the deep-UV to the visible region and fluorescence upconversion to investigate the photoinduced excited-state intramolecular proton-transfer dynamics in a biologically relevant drug molecule, 2-acetylindan-1,3-dione. The molecule is a ß-diketone which in the electronic ground state exists as exocyclic enol with an intramolecular H-bond. Upon electronic excitation at 300 nm, the first excited state of the exocyclic enol is initially populated, followed by ultrafast proton transfer (≈160 fs) to form the vibrationally hot endocyclic enol. Subsequently, solvent-induced vibrational relaxation takes place (≈10 ps) followed by decay (≈390 ps) to the corresponding ground state.
Anhand der ersten Festkörperstrukturen von Dibortetraiodid (B\(_2\)I\(_4\)) wird gezeigt, dass dieses nicht, wie lange angenommen, analog zu den leichteren Dibortetrahalogeniden B\(_2\)F\(_4\), B\(_2\)Cl\(_4\) und B\(_2\)Br\(_4\) in allen Aggregatzuständen in Form diskreter Moleküle mit planaren, dreifach koordinierten Boratomen vorliegt. Röntgenstrukturanalysen, Festkörper‐NMR‐ und IR‐Messungen zeigen, dass B\(_2\)I\(_4\) im Festkörper in zwei polymeren Konformeren vorkommt, die tetraedrisch koordinierte Boratome enthalten. Anhand von DFT‐Rechnungen werden die IR‐Spektren in Lösung und im Festkörper simuliert und mit den experimentellen Daten verglichen.
Nanoelectronics is an essential technology for down-scaling beyond the limit of silicon-based electronics. Single-Wall Carbon Nanotubes (SWNT) are semiconducting components that exhibit a large variety of properties that make them usable for sensing, telecommunication, or computational tasks. Due to their high surface to volume ratio, carbon nanotubes are strongly affected by molecular adsorptions, and almost all properties depend on surface adsorption. SWNT with smaller diameters (0.7-0.9nm) show a stronger sensitivity to surface effects. An optimized synthesis route was developed to produce these nanotubes directly. They were produced with a clean surface, high quality, and large lengths of 2 μ m. The results complement previous studies on larger diameters (0.9-1.4nm). They allow performing statistically significant assumptions for a perfect nanotube, which is selected from a subset of nanotubes with good emission intensity, and high mechanical durability. The adsorption of molecules on the surface of carbon nanotubes influences the motion and binding strength of chargeseparated states in this system. To gain insight into the adsorption processes on the surface with a minimum of concurrent overlapping effects, a microscopic setup, and a measurement technique were developed. The system was estimated to exhibit excellent properties like long exciton diffusion lengths (>350nm), and big exciton sizes (8.5(5)nm), which was substantiated by a simulation. We studied the adsorption processes at the surface of Single-Wall Carbon Nanotubes for molecules in the gas phase, solvent molecules, and surfactant molecules. The experiments were all carried out on suspended individualized carbon nanotubes on a silicon wafer substrate. The experiments in the gas-phase showed that the excitonic emission energy and intensity experiences a rapid blue shift during observation. This shift was associated with the spontaneous desorption of large clusters of gaseous molecules caused by laser heat up. The measurement of this desorption was essential for creating a reference to an initially clean surface and allows us to perform a comparison with previous measurements on this topic. Furthermore, the adsorption of hydrogen on the nanotube surface at high temperatures was investigated. It was found that a new emission mode arises slightly red-shifted to the excitonic emission in these systems. The new signal is almost equally strong as the main excitonic peak and was associated with the brightening of dark excitons at sp3-defects through a K-phonon assisted pathway. The finding is useful for the direct synthesis of spintronic devices as these systems are known to act as single-photon emitters. The suspended nanotubes were further studied to estimate the effect of solvent adsorption on the excitonic states during nanotube dispersion for each nanotube individually. A significant quantum yield loss is observable for hexane and acetonitrile, while the emission intensity was found to be the strongest in toluene. The reference to a clean surface allowed us to estimate the exact influence of the dielectric environment of adsorbing solvents on the excitonic emission energy. Solvent adsorption was found to lead to an energy shift that is almost twice as high as suggested in previous studies. The amount of this energy shift, however, was comparably similar for all solvents, which suggests that the influence of the distinct dielectric constant in the outer environment less significantly influences the energy shift than previously thought. An interesting phenomenon was found when using acetonitrile as a solvent, which leads to greatly enhanced emission properties. The emission is more than twice as high as in the same air-suspended nanotubes, which suggests a process that depends on the laser intensity. In this study, it was reasonably explained how an energy down-conversion is possible through the coupling of the excitonic states with solvent vibrations. The strength of this coupling, however, also suggests adsorptions to the inside of the tubular nanotube structure leading to a coupled vibration of linear acetonitrile molecules that are adsorbed to the inner surface. The findings are important for the field of nanofluidics and provide an excellent system for efficient energy down-conversion in the transmission window of biological tissue. Having separated the pure effect of solvent adsorption allowed us to study the undisturbed molecular adsorption of polymers in these systems. The addition of polyfluorene polymer leads to a slow but stepwise intensity increase. The intensity increase is overlapping with a concurrent process that leads to an intensity decrease. Unfortunately, observing the stepwise process has a low spacial resolution of only 100-250nm, which is in the range of the exciton diffusion length in these systems and hinders detailed analysis. The two competing and overlapping processes processes are considered to originate from slow π-stacking and fast side-chain binding. Insights into this process are essential for selecting suitably formed polymers. However, the findings also emphasize the importance of solvent selection during nanotube dispersion since solvent effects were proven to be far more critical on the quantum yield in these systems. These measurements can shed light on the ongoing debate on polymers adsorption during nanotube individualization and allow us to direct the discussion more towards the selection of suitable solvents. This work provides fundamental insights into the adsorption of various molecules on the surface of individually observed suspended Single-Wall Carbon Nanotubes. It allows observing the adsorption of individual molecules below the optical limit in the solid, liquid, and gas phases. Nanotubes are able to act as sensing material for detecting changes in their direct surrounding. These fundamental findings are also crucial for increasing the quantum yield of solvent-dispersed nanotubes. They can provide better light-harvesting systems for microscopy in biological tissue and set the base for a more efficient telecommunication infrastructure with nano-scale spintronics devices and lasing components. The newly discovered solvent alignment in the nanotube surrounding can potentially also be used for supercapacitors that are needed for caching the calculation results in computational devices that use polymer wrapped nanotubes as transistors. Although fundamental, these studies develop a strategy to enlighten this room that is barely only visible at the bottom of the nano-scale.
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.
Nowadays, computational-aided investigations become an essential part in the chemical, biochemical or pharmaceutical research. With increasing computing power, the calculation of larger biological systems becomes feasible. In this work molecular mechanical (MM) and quantum mechanical approaches (QM) and the combination of both (QM/MM) have been applied to study several questions which arose from different working groups. Thus, this work comprises eight different subjects which deals with chemical reactions or proton transfer in enzymes, conformational changes of ligands or proteins and verification of experimental data.
This work firstly deals with reaction mechanisms of aromatic inhibitors of cysteine proteases which can be found in many organisms. These enzymes are responsible for various cancer or diseases as for example Human African Trypanosomiasis (HAT) or the Chagas disease. Aromatic SNAr-type electrophiles might offer a new possibility to covalently modify these proteases. Quantum mechanical calculations have been performed to gain insights into the energetics and possible mechanisms.
The next chapter also deals with Trypanosomiasis but the focus was set on a different enzyme. The particularity of Trypanosomiasis is the thiol metabolism which can also be modified by covalent inhibitors. In this context, the wild type and point mutations of the enzyme tryparedoxin have been investigated via molecular dynamic (MD) simulations to examine the influence of specific amino acids in regard to the inhibitor. Experimental data showed that a dimerization of the enzyme occurs if the inhibitor is present. Simulations revealed that the stability of the dimer decreases in absence of the inhibitor and thus confirms these experiments.
Further investigations concerning cysteine proteases such as cruzain and rhodesain have been conducted with respect to experimental kinetic data of covalent vinylsulfone inhibitors. Several approaches such as QM or QM/MM calculations and docking, MD or MMPBSA/MMGBSA simulations have been applied to reproduce these data. The utilization of force field approaches resulted in a qualitatively accurate prediction.
The kinase AKT is involved in a range of diseases and plays an important role in the formation of cancer. Novel covalent-allosteric inhibitors have been developed and crystallized in complex with AKT. It was shown that depending on the inhibitor a different cysteine residue is modified. To investigate these differences in covalent modification computational simulations have been applied.
Enoyl-(acyl carrier) (ENR) proteins are essential in the last step of the fatty acid biosynthesis II (FAS) and represent a good target for inhibition. The diphenylether inhibitor SKTS1 which was originally designed to target the ENR’s of Staphylococcus aureus was also crystallized in InhA, the ENR of Mycobacterium tuberculosis (TB). Crystal structures indicate a change of the inhibitor's tautomeric form. This subject was investigated via MD simulations. Results of these simulations confirmed the tautomerization of the inhibitor.
This work also deals with the development of a covalent inhibitor originating from a non-covalent ligand. The target FadA5 is an essential enzyme for the degradation of steroids in TB and is responsible for chronic tuberculosis. This enzyme was crystallized in complex with a non-covalent ligand which served as starting point for this study. Computations on QM or QM/MM level and docking and MD simulations have been applied to evaluate potential candidates.
The next chapter focuses on the modification of the product spectrum of Bacillus megaterium levansucrase, a polymerase which catalyzes the biosynthesis of fructans. The covalent modification of the wild type or mutants of the enzyme lead to an accumulation of oligosaccharides but also to polymers with higher polymerization degree. To understand these changes in product spectra MD simulations have been performed.
Finally, the proton transfer in catalytic cysteine histidine dyads was investigated. The focus was set on the influence of the relaxation of the protein environment to the reaction. Calculations of the enzymes FadA5 and rhodesain revealed that the preferred protonation state of the dyade depends on the protein environment and has an impact on the reaction barrier. Furthermore, the adaptation of the environment to a fixed protonation state was analyzed via MD simulations.
Ziel dieser Dissertation war es zu einem besseren Verständnis hinsichtlich folgender Themen beizutragen und Möglichkeiten aufzuzeigen, mit welchen die Voraussetzungen für Anwendungen von einzelnen, funktionalisierten Kohlenstoffnanoröhren, wie u.a. Einzelphotonenquellen, erfüllt werden können.
Eine wesentliche Voraussetzung für die Funktionalisierung von einzelnen Kohlenstoffnanoröhren ist zunächst eine Probenpräparation, welche SWNT-Suspensionen mit einem hohen Anteil an vereinzelten SWNTs hoher PL-Intensität bereitstellen kann. Um solche SWNT-Suspensionen herstellen zu können, wurden drei verschiedene Rohmaterialien und Dispergiermittel auf deren Entbündelungseffizienz- und relativer Photolumineszenzquantenausbeute untersucht. Anhand von photolumineszenzspektroskopischen Untersuchungen und Messungen der Extinktion stellte sich heraus, dass in Kombination des unaufbereiteten CVD-Kohlenstoffnanorohrrußes mit dem Copolymer PFO:BPy als Dispergiermittel und einem speziell in dieser Dissertation entwickelten Herstellungsverfahren für die Mikroskopieproben, stabile (6,5)-SWNT-Suspensionen mit einem großen Anteil an einzelnen SWNTs hoher PL-Intensität, hergestellt werden können.
Letztere Suspension diente als Ausgangsmaterial für die, in dieser Dissertation neuartige, entwickelte Methodik zur Differenzierung zwischen einzelnen SWNTs und Aggregaten mittels PL- und Ramanmessungen an einem PL-Mikroskopie-Aufbau, welche eine weitere Voraussetzung für Einzelpartikelstudien darstellt. Hierbei wurden im Rahmen einer statistischen Messreihe PL- und Ramanspektren von 150 SWNT-Objekten aufgenommen und hieraus resultierend die Parameter FWHM, Energie des S1-Emissions-Zustands und relative Photolumineszenzquantenausbeute ermittelt. Schließlich konnten die zwischen einer einzelnen SWNT und einem Aggregat charakteristischen Differenzen anhand der Korrelationen zwischen den drei Parametern dargestellt werden. Zudem erfolgte eine statistische Analyse zur Bestimmung der statistischen Signifikanz dieser Korrelationen. Hierbei wurde anhand der nicht-parametrischen Spearman-Korrelationskoeffizienten und der p-Werte gezeigt, dass in Kombination dieser drei Messparameter mit einer hohen Wahrscheinlichkeit zwischen einer einzelnen SWNT und einem Aggregat differenziert werden kann. Demnach konnte eine neuartige, im Vergleich zur Literatur, praktikable Methodik zur Differenzierung zwischen einzelnen SWNTs und Aggregaten, etabliert werden, welche die Voraussetzung für Einzelrohrstudien ist.
Der Fokus dieser Dissertation ist die Entschlüsselung der Reaktionsmechanismen der Arylierung und reduktiven Alkylierung von (6,5)-SWNTs im Ensemble und auf Einzelrohrbasis. Durch diese kovalenten Funktionalisierungsverfahren entstehen neue fluoreszierende Defekt-Zustände, deren zeitabhängiges Intensitätsverhalten in der vorliegenden Arbeit näher untersucht wurde. Hinsichtlich der Arylierung von SWNTs mit Diazoniumsalzen postulieren Studien einen zweistufigen Reaktionsmechanismus, welcher durch eine kombinatorische, spektroskopische Gesamtbetrachtung im Rahmen dieser Dissertation bestätigt werden konnte. Auch konnte erstmalig in der Literatur gezeigt werden, dass die Reaktion in hohem Maße reproduzierbar ist.
Reproduzierbarkeitsstudien wurden auch im Falle der reduktiven Alkylierung unternommen, wobei erstmalig festgestellt wurde, dass diese Reaktion lediglich im hohen Maße reproduzierbar ist, sofern die Reduktionslösung mindestens 17 Stunden vor Reaktionsstart angesetzt wird. Basierend auf diesem Resultat, wurden reproduzierbare Messreihen zur Untersuchung der Reaktionsbedingungen und des Reaktionsmechanismus unternommen, da diesbezüglich unzureichend Kenntnis in der Literatur vorhanden ist.
Zur Klärung des Reaktionsmechanismus, von welchem lediglich Annahmen existieren, wurde zum einen der Einfluss der Laseranregung auf die Reaktion untersucht. Da lediglich für den Falle des Ansetzens der Reduktionslösung unmittelbar vor Messbeginn, wobei die reaktiven SO2- -Radikale erzeugt werden, ein Einfluss der Laseranregung festgestellt werden konnte, nicht jedoch im weiteren Reaktionsverlauf, ist von keiner radikalischen Reaktion im Funktionalisierungsschritt auszugehen. Dies konnte durch den Einsatz von Konstitutionsisomeren des Iodbutans bestätigt werden, wobei das Iodbutanisomer, welches im Fall einer radikalischen Reaktion die höchste Reaktivität zeigen sollte, zu keiner Funktionalisierung der SWNTs führte. Im Gegensatz hierzu, konnte durch das 1-Iodbutan, mit dem primären C-Atom, eine hohe PL-Intensität der defekt-induzierten Zustände E11- und T- verzeichnet werden, was die weitere Annahme einer SN2-Reaktion stützt.
Im Rahmen dieser Dissertation konnte zudem erstmalig entdeckt werden, dass unter deren alkalischen, reduktiven Bedingungen, eine Funktionalisierung mit Acetonitril erfolgen kann, was durch die Durchstimmung der PL-Intensität des Defektzustands bei Variation des Volumenanteils von Acetonitril bestätigt werden konnte. Hierbei gilt es jedoch weiter zu analysieren, auf welche Art die Koordination bzw. Funktionalisierung von Acetonitril an den SWNTs erfolgt, was u.a. durch Ramanmessungen untersucht werden könnte.
Auch konnten neuartige Kenntnisse bezüglich der Reaktionskinetik basierend auf den Studien dieser Dissertation erhalten werden, wobei festgestellt wurde, dass das Reaktionsprofil mit dem einer komplexen Folgereaktion angenähert werden kann.
Zudem konnten neuartige Kenntnisse aus der Thermodynamik, wie die Ermittlung der Aktivierungsenergie der Adsorption von DOC-Molekülen auf der SWNT-Oberfläche, durch die Zugabe des Tensids DOC zum Reaktionsansatz und dem hieraus resultierenden Reaktionsabbruch, erhalten werden.
Schließlich fand eine Übertragung der Ergebnisse aus den Ensemblestudien der reduktiven Alkylierung auf Einzelpartikeluntersuchungen statt, wobei letztere erstmalig im Rahmen dieser Arbeit durchgeführt wurden. Aus der statistischen Analyse, welche von Martina Wederhake durchgeführt wurde, resultierte durch Erhöhung des Stoffmengenverhältnisses von 1-Iodbutan zu Kohlenstoff eine inhomogene Steigerung des Funktionalisierungsgrades. Ausblickend gilt es nun zu prüfen, ob die zeitlichen Reaktionsverläufe der photolumineszierenden Zustände, welche aus den Ensemble-Studien erhalten wurden, auf Einzelrohrbasis reproduziert werden können.
Es lässt sich demnach festhalten, dass mithilfe der Studien dieser Dissertation ein Probenherstellungsverfahren, welches stabile SWNT-Suspensionen mit einem großen Anteil an einzelnen Kohlenstoffnanoröhren, hoher PL-Intensität ermöglicht, etabliert werden konnte. Zudem wurde eine neuartige, praktikable und statistisch signifikante Methodik zur Differenzierung zwischen einzelnen Kohlenstoffnanoröhren und Aggregaten entwickelt. Schließlich konnten neue, essentielle Informationen bezüglich des Reaktionsmechanismus und den Reaktionsbedingungen der Arylierung und reduktiven Alkylierung von halbleitenden (6,5)-SWNTs erhalten werden. Wie in der Einleitung bereits erwähnt, sind sowohl der Erhalt einer stabilen SWNT-Suspension mit einem großen Anteil an einzelnen Nanoröhren hoher PL-Intensität, die Möglichkeit der Identifizierung einzelner SWNTs, als auch ein ausgiebiges Verständnis der Reaktionsmechanismen der Funktionalisierungsreaktionen, essentielle Voraussetzungen für die Verwirklichung von Einzelphotonenquellen auf Basis einzelner, funktionalisierter Kohlenstoffnanoröhren. Diese können aufgrund derer geeigneter Emissionseigenschaften als vielversprechende Kandidaten für das Ausgangsmaterial von Einzelphotonenquellen in der Quanteninformationstechnologie angesehen werden.
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.
Herein we report a broad series of new trinuclear supramolecular Ru(bda) macrocycles bearing different substituents at the axial or equatorial ligands which enabled investigation of substituent effects on the catalytic activities in chemical and photocatalytic water oxidation. Our detailed investigations revealed that the activities of these functionalized macrocycles in water oxidation are significantly affected by the position at which the substituents were introduced. Interestingly, this effect could not be explained based on the redox properties of the catalysts since these are not markedly influenced by the functionalization of the ligands. Instead, detailed investigations by X-ray crystal structure analysis and theoretical simulations showed that conformational changes imparted by the substituents are responsible for the variation of catalytic activities of the Ru macrocycles. For the first time, macrocyclic structure of this class of water oxidation catalysts is unequivocally confirmed and experimental indication for a hydrogen-bonded water network present in the cavity of the macrocycles is provided by crystal structure analysis. We ascribe the high catalytic efficiency of our Ru(bda) macrocycles to cooperative proton abstractions facilitated by such a network of preorganized water molecules in their cavity, which is reminiscent of catalytic activities of enzymes at active sites.
Rapid multiple-quantum three-dimensional fluorescence spectroscopy disentangles quantum pathways
(2019)
Coherent two-dimensional spectroscopy is a powerful tool for probing ultrafast quantum dynamics in complex systems. Several variants offer different types of information but typically require distinct beam geometries. Here we introduce population-based three-dimensional (3D) electronic spectroscopy and demonstrate the extraction of all fourth- and multiple sixth-order nonlinear signal contributions by employing 125-fold (1⨯5⨯5⨯5) phase cycling of a four-pulse sequence. Utilizing fluorescence detection and shot-to-shot pulse shaping in single-beam geometry, we obtain various 3D spectra of the dianion of TIPS-tetraazapentacene, a fluorophore with limited stability at ambient conditions. From this, we recover previously unknown characteristics of its electronic two-photon state. Rephasing and nonrephasing sixth-order contributions are measured without additional phasing that hampered previous attempts using noncollinear geometries. We systematically resolve all nonlinear signals from the same dataset that can be acquired in 8 min. The approach is generalizable to other incoherent observables such as external photoelectrons, photocurrents, or photoions.
We present a theoretical study on exciton–exciton annihilation (EEA) in a molecular dimer. This process is monitored using a fifth-order coherent two-dimensional (2D) spectroscopy as was recently proposed by Dostál et al. [Nat. Commun. 9, 2466 (2018)]. Using an electronic three-level system for each monomer, we analyze the different paths which contribute to the 2D spectrum. The spectrum is determined by two entangled relaxation processes, namely, the EEA and the direct relaxation of higher lying excited states. It is shown that the change of the spectrum as a function of a pulse delay can be linked directly to the presence of the EEA process.
For the rational design of new fluorophores, reliable predictions of fluorescence quantum yields from first principles would be of great help. However, efficient computational approaches for predicting transition rates usually assume that the vibrational structure is harmonic. While the harmonic approximation has been used successfully to predict vibrationally resolved spectra and radiative rates, its reliability for non-radiative rates is much more questionable. Since non-adiabatic transitions convert large amounts of electronic energy into vibrational energy, the highly excited final vibrational states deviate greatly from harmonic oscillator eigenfunctions. We employ a time-dependent formalism to compute radiative and non-radiative rates for transitions and study the dependence on model parameters. For several coumarin dyes we compare different adiabatic and vertical harmonic models (AS, ASF, AH, VG, VGF, VH), in order to dissect the
importance of displacements, frequency changes and Duschinsky rotations. In addition we analyze the effect of different broadening functions (Gaussian, Lorentzian or Voigt). Moreover, to assess the qualitative influence of anharmonicity on the internal conversion rate, we develop a simplified anharmonic model. We adress the reliability of these models considering the potential errors introduced by the harmonic approximation and the phenomenological width of the broadening function.
The multistate metadynamics for automatic exploration of conical intersection seams and systematic location of minimum energy crossing points in molecular systems and its implementation into the software package metaFALCON is presented. Based on a locally modified energy gap between two Born–Oppenheimer electronic states as a collective variable, multistate metadynamics trajectories are driven toward an intersection point starting from an arbitrary ground state geometry and are subsequently forced to explore the conical intersection seam landscape. For this purpose, an additional collective variable capable of distinguishing structures within the seam needs to be defined and an additional bias is introduced into the off-diagonal elements of an extended (multistate) electronic Hamiltonian. We demonstrate the performance of the algorithm on the examples of the 1,3-butadiene, benzene, and 9H-adenine molecules, where multiple minimum energy crossing points could be systematically located using the Wiener number or Cremer–Pople parameters as collective variables. Finally, with the example of 9H-adenine, we show that the multistate metadynamics potential can be used to obtain a global picture of a conical intersection seam. Our method can be straightforwardly connected with any ab initio or semiempirical electronic structure theory that provides energies and gradients of the respective electronic states and can serve for systematic elucidation of the role of conical intersections in the photophysics and photochemistry of complex molecular systems, thus complementing nonadiabatic dynamics simulations.
Comparison of moving and fixed basis sets for nonadiabatic quantum dynamics at conical intersections
(2020)
We assess the performance of two different types of basis sets for nonadiabatic quantum dynamics at conical intersections. The basis sets of both types are generated using Ehrenfest trajectories of nuclear coherent states. These trajectories can either serve as a moving (time-dependent) basis or be employed to sample a fixed (time-independent) basis. We demonstrate on the example of two-state two-dimensional and three-state five-dimensional models that both basis set types can yield highly accurate results for population transfer at intersections, as compared with reference quantum dynamics. The details of wave packet evolutions are discussed for the case of the two-dimensional model. The fixed basis is found to be superior to the moving one in reproducing nonlocal spreading and maintaining correct shape of the wave packet upon time evolution. Moreover, for the models considered, the fixed basis set outperforms the moving one in terms of computational efficiency.
Comparison of moving and fixed basis sets for nonadiabatic quantum dynamics at conical intersections
(2020)
We assess the performance of two different types of basis sets for nonadiabatic quantum dynamics at conical intersections. The basis sets of both types are generated using Ehrenfest trajectories of nuclear coherent states. These trajectories can either serve as a moving (time-dependent) basis or be employed to sample a fixed (time-independent) basis. We demonstrate on the example of two-state two-dimensional and three-state five-dimensional models that both basis set types can yield highly accurate results for population transfer at intersections, as compared with reference quantum dynamics. The details of wave packet evolutions are discussed for the case of the two-dimensional model. The fixed basis is found to be superior to the moving one in reproducing true nonlocal spreading and maintaining correct shape of the wave packet upon time evolution. Moreover, for the models considered, the fixed basis set outperforms the moving one in terms of computational efficiency.
The origin of the solvent dependence of fluorescence quantum yields in dipolar merocyanine dyes
(2019)
Fluorophores with high quantum yields are desired for a variety of applications. Optimization of promising chromophores requires an understanding of the non-radiative decay channels that compete with the emission of photons. We synthesized a new derivative of the famous laser dye 4-dicyanomethylen-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM),i.e., merocyanine 4-(dicyanomethylene)-2-tert-butyl-6-[3-(3-butyl-benzothiazol-2-ylidene)1-propenyl]-4H-pyran (DCBT). We measured fluorescence lifetimes and quantum yields in a variety of solvents and found a trend opposite to the energy gap law.This motivated a theoretical investigation into the possible non-radiative decay channels. We propose that a barrier to a conical intersection exists that is very sensitive to the solvent polarity. The conical intersection is characterized by a twisted geometry which allows a subsequent photoisomerization. Transient absorption measurements confirmed the formation of a photoisomer in unpolar solvents, while the measurements of fluorescence quantum yields at low temperature demonstrated the existence of an activation energy barrier.
Collective Response in DNA-Stabilized Silver Cluster Assemblies from First-Principles Simulations
(2019)
We investigate fluorescence resonant energy transfer and concurrent electron dynamics in a pair of DNA-stabilized silver clusters. For this purpose we introduce a methodology for the simulation of collective optoelectronic properties of coupled molecular aggregates starting from first-principles quantum chemistry, which can be further applied to a broad range of coupled molecular systems to study their electro-optical response. Our simulations reveal the existence of low-energy coupled excitonic states, which enable ultrafast energy transport between subunits, and give insight into the origin of the fluorescence signal in coupled DNA-stabilized silver clusters, which have been recently experimentally detected. Hence, we demonstrate the possibility of constructing ultrasmall energy transmission lines and optical converters based on these hybrid molecular systems.
The photophysics of a molecular triad consisting of a BODIPY dye and two pyrene chromophores attached in 2-position are investigated by steady state and fs-time resolved transient absorption spectroscopy as well as by field induced surface hopping (FISH) simulations. While the steady state measurements indicate moderate chromophore interactions within the triad, the time resolved measurements show upon pyrene excitation a delocalised excited state which localises onto the BODIPY chromophore with a time constant of 0.12 ps. This could either be interpreted as an internal conversion process within the excitonically coupled chromophores or as an energy transfer from the pyrenes to the BODIPY dye. The analysis of FISH-trajectories reveals an oscillatory behaviour where the excitation hops between the pyrene units and the BODIPY dye several times until finally they become localised on the BODIPY chromophore within 100 fs. This is accompanied by an ultrafast nonradiative relaxation within the excitonic manifold mediated by the nonadiabatic coupling. Averaging over an ensemble of trajectories allowed us to simulate the electronic state population dynamics and determine the time constants for the nonradiative transitions that mediate the ultrafast energy transfer and exciton localisation on BODIPY.
Excitons in the molecular aggregates of chromophores are key participants in important processes such as photosynthesis or the functioning of organic photovoltaic devices. Therefore, the exploration of exciton dynamics is crucial. Here we report on exciton localization during excited-state dynamics of the recently synthesized tetracene trimer [Liu et al., Org. Lett., 2017, 19, 580]. We employ the surface hopping approach to nonadiabatic molecular dynamics in conjunction with the long-range corrected time-dependent density functional tight binding (LC-TDDFTB) method [Humeniuk and Mitrić, Comput. Phys. Commun., 2017, 221, 174]. Utilizing a set of descriptors based on the transition density matrix, we perform comprehensive analysis of exciton dynamics. The obtained results reveal an ultrafast exciton localization to a single tetracene unit of the trimer during excited-state dynamics, along with exciton transfer between units.
Reactive hydrocarbon molecules like radicals, biradicals and carbenes are not only key players in combustion processes and interstellar and atmospheric chemistry, but some of them are also important intermediates in organic synthesis. These systems typically possess many low-lying, strongly coupled electronic states. After light absorption, this leads to rich photodynamics characterized by a complex interplay of nuclear and electronic motion, which is still not comprehensively understood and not easy to investigate both experimentally and theoretically. In order to elucidate trends and contribute to a more general understanding, we here review our recent work on excited-state dynamics of open-shell hydrocarbon species using time-resolved photoelectron spectroscopy and field-induced surface hopping simulations, and report new results on the excited-state dynamics of the tropyl and the 1-methylallyl radical. The different dynamics are compared, and the difficulties and future directions of time-resolved photoelectron spectroscopy and excited state dynamics simulations of open-shell hydrocarbon molecules are discussed.
In the last two decades, coherent multidimensional femtosecond spectroscopy has become a powerful and versatile tool to investigate chemical dynamics of a broad variety of quantum systems. The combination of transient information, equivalent to pumpprobe spectroscopy, with information about coupling between energetic states and the system environment allows an extensive insight into atomic and molecular properties. Many experimental 2D setups employ the coherence-detected approach, where nonlinear system responses are emitted as coherent electric _elds which are detected after spatial separation from the excitation pulses. As an alternative to this experimentally demanding approach, population-based 2D spectroscopy has been established. Here, the coherent information is encoded in the phases of a collinear excitation-pulse train and extracted from incoherent signals like uorescence via phase cycling. In principle, the use of uorescence as observable can boost the sensitivity down to the single-molecule level. The aim of this work was the realization of a pulse-shaper assisted fully collinear uorescence-detected 2D setup and the conducting of proof-of-principle experiments in the liquid phase. This inherently phase-stable and compact setup has been presented in chapter 3, with the utilized pulse shaper granting amplitude and phase modulation on a shot-to-shot basis. Two di_erent types of white-light sources have been applied and evaluated with regard to their respective advantages for 2D uorescence spectroscopy. A variety of artifact sources that can occur with the present setup have been discussed, and correction schemes and instructions for avoiding these artifacts have been provided. In chapter 4, the setup has been demonstrated by employing a four-pulse sequence on cresyl violet in ethanol. A detailed data-acquisition and data-analysis procedure has been presented, where phase cycling is used for extraction of the nonlinear contributions. Depending on the phase-cycling scheme, it is possible to recover all nonlinear contributions in a single measurement. Well-known quantum-beating behavior of cresyl violet during the population time could be reproduced. Due to measuring in a rotating-frame environment and 1 kHz shot-to-shot pulse incrementation, it was possible to obtain a 2D spectrum for one population time in 6 s. Via error evaluation it has been shown that 10_ averaging (1 min) is su_cient to obtain a root-mean-square error of < 0:05 compared to 400_ averaging, proving that the utilized acquisition scheme is well suited. The realization of the _rst experimental uorescence-detected 2Q 2D experiment and the _rst experimental access to the theoretically predicted 1Q-2Q contribution
An experimental setup for probing ultrafast dynamics at the diffraction limit was developed, characterized and demonstrated in the scope of the thesis, aiming for optical investigations while simultaneously approaching the physical limits on the length and timescale.
An overview of this experimental setup was given in Chapter 2, as well as the considerations that led to the selection of the individual components. Broadband laser pulses with a length of 9.3 fs, close to the transform limit of 7.6 fs, were focused in a NA = 1.4 immersion oil objective, to the diffraction limit of below 300 nm (FWHM).
The spatial focus shape was characterized with off-resonance gold nanorod scatterers scanned through the focal volume. For further insights into the functionality and limitations of the pulse shaper, its calibration procedure was reviewed. The deviations between designed and experimental pulse shapes were attributed to pulse-shaper artifacts, including voltage-dependent inter-layer as well as intra-layer LCD-pixel crosstalk, Fabry-Pérot-type reflections in the LCD layers, and space-time coupling. A pixel-dependent correction was experimentally carried out, which can be seen as an extension of the initial calibration to all possible voltage combinations of the two LCD layers.
The capabilities of the experimental setup were demonstrated in two types of experiments, targeting the nonlinearity of gold (Chapter 3) as well as two-dimensional spectroscopy at micro-structured surfaces (Chapter 4).
Investigating thin films, an upper bound for the absolute value for the imaginary part of the nonlinear refractive index of gold could be set to |n′′ 2 (Au)| < 0.6·10−16 m2/W, together with |n′ 2 (Au)| < 1.2·10−16 m2/W as an upper bound for the absolute value of the real part. Finite-difference time-domain simulations on y-shaped gold nanostructures indicated that a phase change of ∆Φ ≥ 0.07 rad between two plasmonic modes would induce a sufficient change in the spatial contrast of emission to the far-field to be visible in the experiment. As the latter could not be observed, this value of ∆Φ was determined as the upper bound for the experimentally induced phase change. An upper bound of 52 GW/cm2 was found for the damage threshold.
In Chapter 4, a novel method for nonlinear spectroscopy on surfaces was presented. Termed coherent two-dimensional fluorescence micro-spectroscopy, it is capable of exploring ultrafast dynamics in nanostructures and molecular systems at the diffraction limit. Two-dimensional spectra of spatially isolated hotspots in structured thin films of fluorinated zinc phthalocyanine (F16ZnPc) dye were taken with a 27-step phase-cycling scheme. Observed artifacts in the 2D maps were identified as a consequence from deviations between the desired and the experimental pulse shapes. The optimization procedures described in Chapter 2 successfully suppressed the deviations to a level where the separation from the nonlinear sample response was feasible.
The experimental setup and methods developed and presented in the scope of this thesis demonstrate its flexibility and capability to study microscopic systems on surfaces. The systems exemplarily shown are consisting of metal-organic dyes and metallic nanostructures, represent samples currently under research in the growing fields of organic semiconductors and plasmonics.
Space- and time-resolved UV-to-NIR surface spectroscopy and 2D nanoscopy at 1 MHz repetition rate
(2019)
We describe a setup for time-resolved photoemission electron microscopy (TRPEEM) with aberration correction enabling 3 nm spatial resolution and sub-20 fs temporal resolution. The latter is realized by our development of a widely tunable (215–970 nm) noncollinear optical parametric amplifier (NOPA) at 1 MHz repetition rate. We discuss several exemplary applications. Efficient photoemission from plasmonic Au nanoresonators is investigated with phase-coherent pulse pairs from an actively stabilized interferometer. More complex excitation fields are created with a liquid-crystal-based pulse shaper enabling amplitude and phase shaping of NOPA pulses with spectral components from 600 to 800 nm. With this system we demonstrate spectroscopy within a single plasmonic nanoslit resonator by spectral amplitude shaping and investigate the local field dynamics with coherent two-dimensional (2D) spectroscopy at the nanometer length scale (“2D nanoscopy”). We show that the local response varies across a distance as small as 33 nm in our sample. Further, we report two-color pump–probe experiments using two independent NOPA beamlines. We extract local variations of the excited-state dynamics of a monolayered 2D material (WSe2) that we correlate with low-energy electron microscopy (LEEM) and reflectivity (LEER) measurements. Finally, we demonstrate the in-situ sample preparation capabilities for organic thin films and their characterization via spatially resolved electron diffraction and dark-field LEEM.
The aim of this thesis was to develop new automatic enhanced sampling methods by extending the idea of Parrinello’s metadynamics to multistate problems and by introducing new quantum-mechanical electronic collective variables. These methods open up a rich perspective for applications to the photophysical processes in complex molecular systems, which play a major role in many natural processes such as vision and photosynthesis, but also in the development of new materials for organic electronics, whose function depends on specific electronic properties such as biradicalicity.
Doping plays a decisive role for the functionality of semiconductor-based (opto-)electronic
devices. Hence, the technological utilization of semiconductors necessitates control and a
fundamental understanding of the doping process. However, for low-dimensional systems like
carbon nanotubes, neither concentration nor distribution of charge carriers is currently well known.
The research presented in this thesis investigated the doping of semiconducting carbon nanotubes by spectroscopic methods. Samples of highly purified, intrinsic (6,5) single-wall carbon nanotubes were fabricated using polymer stabilization.
Chapter 4 showed that both electro- and redox chemical $p$-doping lead to identical bleaching,
blueshift, broadening and asymmetry of the S$_1$ exciton absorption band. The similar spectral changes induced by both doping schemes suggest that optical spectra can not be used to infer what process was used for doping. Perhaps more importantly, it also indicates that the distribution of charges and the character of the charge transfer states does not depend on the method by which doping was achieved.
The detailed analysis of the doping-induced spectral changes in chapter 5 suggests that surplus charges are distributed inhomogeneously. The hypothesis of carrier localization is consistent with the high sensitivity of the S$_1$ exciton photoluminescence to additional charge carriers and with the stretched-exponential decay of the exciton population following ultrafast excitation.
Both aspects are in good agreement with diffusion-limited contact quenching of excitons
at localized charges. Moreover, localized charges act – similar to structural defects – as
perturbations to the bandstructure as evidenced by a doping-induced increase of the D-band
antiresonance in the mid-infrared spectrum.
Quantum mechanical model calculations also suggest that counterions play a crucial role in
carrier localization. Counterion adsorption at the nanotube surface is thus believed to induce charge traps of more than 100 meV depth with a carrier localization length on the order of 3 - 4 nm. The doping-induced bleach of interband absorption is accompanied by an absorption increase in the IR region below 600 meV. The observed shift of the IR peak position indicates a continuous transition from localized to rather delocalized charge carriers. This transition is caused by the increase of the overlap of charge carrier wavefunctions at higher charge densities and was modeled by classical Monte-Carlo simulations of intraband absorption.
Chapter 6 discussed the spectroscopy of heavily (degenerately) doped nanotubes, which are
characterized by a Drude-response of free-carrier intraband absorption in the optical conductivity spectrum. In the NIR spectral region, the S$_1$ exciton and X$+^_1$ trion absorption is replaced by a nearly 1 eV broad and constant absorption signal, the so-called H-band. The linear and transient absorption spectra of heavily doped nanotubes suggest that the H-band can be attributed to free-carrier interband transitions.
Chapter 7 dealt with the quantification of charge carrier densities by linear absorption spectroscopy.
A particularly good measure of the carrier density is the S$_1$ exciton bleach. For a
bleach below about 50 %, the carrier density is proportional to the bleach. At higher doping
levels, deviations from the linear behavior were observed. For doping levels exceeding a
fully bleached S$_1$ band, the determination of the normalized oscillator strength f$\text{1st}$ over the
whole first subband region (trion, exciton, free e-h pairs) is recommended for quantification of carrier densities. Based on the nanotube density of states, the carrier density $n$ can be estimated using $n = 0.74\,\text{nm}^{−1} \cdot (1 − f_\text{1st})$.
In the last part of this thesis (chapter 8), the time-resolved spectroelectrochemistry was
extended to systems beyond photostable carbon nanotube films. The integration of a flowelectrolysis cell into the transient absorption spectrometer allows the investigation of in-situ electrochemically generated but photounstable molecules due to a continuous exchange of sample volume. First time-resolved experiments were successfully performed using the dye
methylene blue and its electrochemically reduced form leucomethylene blue.
Major advances in the chemistry of 5th and 6th row heavy p-block element compounds have recently uncovered intriguing reactivity patterns towards small molecules such as H\(_2\), CO\(_2\), and ethylene. However, well-defined, homogeneous insertion reactions with carbon monoxide, one of the benchmark substrates in this field, have not been reported to date. We demonstrate here, that a cationic bismuth amide undergoes facile insertion of CO into the Bi–N bond under mild conditions. This approach grants direct access to the first cationic bismuth carbamoyl species. Its characterization by NMR, IR, and UV/vis spectroscopy, elemental analysis, single-crystal X-ray analysis, cyclic voltammetry, and DFT calculations revealed intriguing properties, such as a reversible electron transfer at the bismuth center and an absorption feature at 353 nm ascribed to a transition involving σ- and π-type orbitals of the bismuth-carbamoyl functionality. A combined experimental and theoretical approach provided insight into the mechanism of CO insertion. The substrate scope could be extended to isonitriles.
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.
We present a theoretical study on exciton–exciton annihilation (EEA) in a molecular dimer. This process is monitored using a fifth-order coherent two-dimensional (2D) spectroscopy as was recently proposed by Dostál et al. [Nat. Commun. 9, 2466 (2018)]. Using an electronic three-level system for each monomer, we analyze the different paths which contribute to the 2D spectrum. The spectrum is determined by two entangled relaxation processes, namely, the EEA and the direct relaxation of higher lying excited states. It is shown that the change of the spectrum as a function of a pulse delay can be linked directly to the presence of the EEA process.
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.
We demonstrate two-quantum (2Q) coherent two-dimensional (2D)electronic spectroscopy using a shot-to-shot-modulated pulse shaper and fluorescence detection. Broadband collinear excitation is realized with the supercontinuum output of an argon-filled hollow-core fiber, enabling us to excite multiple transitions simultaneously in the visible range. The 2Q contribution is extracted via a three-pulse sequence with 16-fold phase cycling and simulated employing cresyl violet as a model system. Furthermore, we report the first experimental realization of one-quantum−two-quantum (1Q-2Q) 2D spectroscopy, offering less congested spectra as compared with the 2Q implementation. We avoid scattering artifacts and nonresonant solvent contributions by using fluorescence as the observable. This allows us to extract quantitative information about doubly excited states that agree with literature expectations. The high sensitivity and background-free nature of fluorescence detection allow for a general applicability of this method to many other systems.
Die vorliegende Dissertation widmete sich der Aufklärung der Photodissoziationsdynamik der drei Xylyl-Radikale ortho-, meta- und para-Xylyl sowie des Benzyl-Radikals mit Hilfe des Velocity-Map-Imagings. Diese reaktiven Intermediate sind insbesondere im Bereich der Verbrennungschemie von hoher Relevanz, da sie die primären Zerfallsprodukte der Xylole und des Toluols darstellen, welche als Antiklopfmittel in Ottokraftstoffen Verwendung finden.Dementsprechend ist eine Betrachtung des weiteren Zerfalls dieser resonanz-stabilisierten Radikale, insbesondere unter dem Gesichtspunkt der Rußbildung, von entscheidender Bedeutung.
Für alle drei Xylyl-Radikale konnte eine selektive pyrolytische Generierung aus den entsprechenden 2-(Methylphenyl)ethylnitriten realisiert werden. Die isomerspezifische Identifikation erfolgte mit Hilfe von REMPI-Spektroskopie der jeweiligen D0 -> D3-Übergänge. Nachfolgend wurde die Photodissoziation aller drei Xylyl-Isomere nach Anregung des D3-Zustandes bei ca. 310 nm und nach Anregung der D-Bande bei 250 nm untersucht. Das „einfachste” Experiment stellte in diesem Zusammenhang die Photodissoziation des para-Xylyl-Radikals dar. Es konnte die von Hemberger et al. in thermischen Zerfallsexperimenten beobachtete Reaktion p-Xylyl -> p-Xylylen + H verifiziert werden. Die VMI-Experimente lieferten die Kennwerte <fT>(309.6nm) = 33 % und <fT>(250nm) = 19 % unter Erhalt isotroper Images für beide Anregungswellenlängen. Die dazugehörigen Dissoziationsratenkonstanten wurden zu kH(309.6nm) ≈ 10^8 s-1 und kH(250nm) ≈ 5*10^7 s-1 bestimmt. Es ist verblüffend, dass die Photodissoziation scheinbar bei der höheren Anregungswellenlänge von 309.6 nm (und somit bei geringerer Anregungsenergie) schneller verläuft als bei 250 nm. Darüber hinaus ist es nicht möglich, die beobachteten Raten mittels des statistischen Modells der RRKM-Theorie zu beschreiben. Des Weiteren konnten auch die Translationsenergieverteilungen nicht mit dem „Quack-Fit” für statistische Dissoziationen angefittet werden. Bei der Photodissoziation des para-Xylyl-Radikals liegt eine Dissoziation nach Rückkehr in den rovibronisch hochangeregten elektronischen Grundzustand infolge der Photoanregung vor. Hierbei thermalisiert die innere Energie im elektronischen Grundzustand vor der Dissoziation scheinbar nur teilweise, sodass keine vollständige statistische Verteilung dieser innerhalb des para-Xylyls gegeben ist. Da dies eine Grundvoraussetzung der gängigen statistischen Modelle darstellt, ist es nicht verwunderlich, dass keine quantitative Reproduktion der experimentellen Ergebnisse durch Anwendung dieser Modelle ermöglicht wird.
Bei entsprechenden Experimenten zum ortho-Isomer konnten diese statistischen Modelle ebenfalls nicht zur quantitativen Beschreibung der Dissoziation verwendet werden. Abermals wurde mit kH(311.1nm) ≈ 10^8 s-1 und kH(250nm) ≈ 5*10^7 s-1 eine schnellere Dissoziation bei geringerer Anregungsenergie festgestellt. Dies erscheint demnach charakteristisch für die Xylyl-Radikale. Innerhalb der VMI-Experimente wurden isotrope Verteilungen erhalten, deren Fragmenttranslationsenergieverteilung nach Anregung des D3-Niveaus bei 311.1 nm jedoch nicht durch die von Hemberger et al. beschriebene Hauptreaktion o-Xylyl -> o-Xylylen + H erklärt werden konnte. Eine Fragmentation nach o-Xylyl -> Benzocyclobuten + H konnte auf diesem Weg als Hauptdissoziationspfad identifiziert werden. Innerhalb der Studien von Hemberger et al. ist eine Reaktion zu Benzocyclobuten bei Anregung mit 311.1 nm energetisch nicht zugänglich. Mittels quantenchemischer Rechnung konnte jedoch ein bislang unbekannter, energetisch zugänglicher Reaktionspfad zur Bildung von Benzocyclobuten unter simultaner Ringschlussreaktion und Wasserstofffragmentation identifiziert und charakterisiert werden. Die Kennwerte der Photodissoziationsreaktion des ortho-Xylyls konnten hierdurch zu <fT>(311.1nm) = 30 % und <fT>(250nm) = 16 % bestimmt werden. Wie bereits im Fall des para-Isomers liegt die Vermutung nahe, dass es sich um eine Dissoziation aus dem rovibronisch hoch-angeregten elektronischen Grundzustand handelt, welcher nicht vollständig vor der Fragmentation thermalisiert.
Im Rahmen der Experimente zum letzten der drei Xylyl-Isomere, dem meta-Xylyl-Radikal, konnte mit VMI eine Fragmentation nach m-Xylyl -> m-Xylylen + H als Hauptdissoziationpfad ausgeschlossen werden. Innerhalb der Experimente nach Anregung des D3-Niveaus um 310 nm konnten mit para-Xylylen und Benzocyclobuten zwei Reaktionsprodukte festgestellt werden, welche die erhaltene Translationsenergieverteilung erklären könnten, wobei die entsprechende maximale Überschussenergie einer Fragmentation zu para-Xylylen den Nullabfall der Verteilung geringfügig besser widerspiegelt. Die mittlere Fragmenttranslationsenergie liegt mit <fT>(p-Xylylen) = 29 % respektive <fT>(Bcb) = 25 % leicht unterhalb der entsprechenden Kennwerte der para- beziehungsweise ortho-Xylyl Experimente. Durch die nötige, der Dissoziation vorausgehende Isomerisierung scheint ein höherer Thermalisierungsgrad der Schwingungs- und Rotationsenergie innerhalb des elektronischen Grundzustands erreicht zu werden, aus welchem die geringen <fT>-Werte resultieren könnten. Der Effekt verminderter <fT>-Werte wurde in den Experimenten bei 250 nm nicht gefunden (<fT>(p-Xylylen) = 19 % respektive <fT>(Bcb) = 17 %). Vergleicht man an dieser Stelle die <ET>- anstelle der <fT>-Werte (<ET>(para) = 0.41 eV, <ET>(ortho) = 0.38 eV, <ET>(meta) = 0.41 eV), stellt man fest, dass <ET>(meta) = <ET>(para) gilt und somit ein weiteres Indiz dafür gefunden wurde, dass eine Umlagerung zu para-Xylyl mit anschließender Fragmentation zu para-Xylylen möglicherweise gegenüber jener zum ortho-Isomer mit nachfolgender Bcb-Bildung bevorzugt ist. Dies würde darüber hinaus im Einklang mit den Studien von Hemberger et al. stehen, in welchen beim thermischen Zerfall des meta-Xylyls para-Xylylen als alleiniges Fragmentationsprodukt gefunden wurde. Eine Betrachtung der Umlagerung mittels RRKM wies jedoch keinen bevorzugten Isomerisierungspfad aus. Schlussendlich lässt sich aufgrund der ermittelten Ratenkonstanten (kH(310nm) ≈ 10^8 s-1, kH(250nm) ≈ 4*10^7 s-1) sowie den <fT>-Werten vermuten, dass die Isomerisierung langsamer als die Dissoziation bei 310 nm verläuft, jedoch zumindest auf einer ähnlichen Zeitskala wie die entsprechende Dissoziation nach Anregung bei 250 nm. Eine zweifelsfreie Interpretation der meta-Xylyl Experimente gestaltet sich jedoch als schwierig.
Innerhalb der Studien zur Photodissoziation des Benzyl-Radikals konnten literaturbekannte Daten zur Fragmentation nach Anregung um 250 nm in guter Übereinstimmung reproduziert werden. Die experimentellen Daten zur Untersuchung der Photodissoziation nach Anregung des D3-Niveaus konnten jedoch nicht eindeutig interpretiert werden. Die literaturbekannte Lage des D3-Niveaus bei 305.3 nm konnte mittels REMPI-Spektroskopie reproduziert werden und anschließende 1H-Photofragmentspektren zeigten, dass eine Anregung des D3-Niveaus zur Bildung von Wasserstofffragmenten führt. Die beobachteten 1H-Fragmente zeigten jedoch eine deutlich zu hohe Überschussenergie für eine Einphotonenabsorption, sodass diese Mehrphotonenabsorptionen zugeordnet werden müssen. Es lässt sich vermuten, dass die Wasserstofffragmente aus einer Anregung eines „superexcited states” oberhalb des Ionisationspotentials, wahrscheinlich durch Zweiphotonenabsorption, stammen. Dieser „superexcited state” zeigt scheinbar keine (vollständige) Autoionisation und führt nachfolgend zumindest teilweise zur Fragmentation des Benzyl-Radikals. In der Folge liegt die Vermutung nahe, dass die Energien eines einzelnen 305 nm-Photons nicht zur Initiierung einer Photodissoziation des Benzyl-Radikals ausreichend ist oder aber, dass diese Photodissoziation zu langsam ist, um sie in einem VMI-Experiment zu beobachten. Potential für weitere Experimente zur Photodissoziation des Benzyl-Radikals nach Anregung des D3-Niveaus wird an dieser Stelle nicht gesehen.
Background: Cyclic aminals are core features of natural products, drug molecules and important synthetic intermediates. Despite their relevance, systematic investigations into their stability towards hydrolysis depending on the pH value are lacking.
Results: A set of cyclic aminals was synthesized and their stability quantified by kinetic measurements. Steric and electronic effects were investigated by choosing appropriate groups. Both molecular mechanics (MM) and density functional theory (DFT) based studies were applied to support and explain the results obtained. Rapid decomposition is observed in acidic aqueous media for all cyclic aminals which occurs as a reversible reaction. Electronic effects do not seem relevant with regard to stability, but the magnitude of the conformational energy of the ring system and pK a values of the N-3 nitrogen atom.
Conclusion: Cyclic aminals are stable compounds when not exposed to acidic media and their stability is mainly dependent on the conformational energy of the ring system. Therefore, for the preparation and work-up of these valuable synthetic intermediates and natural products, appropriate conditions have to be chosen and for application as drug molecules their sensitivity towards hydrolysis has to be taken into account.
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.
In the context of quantum mechanical calculations, the properties of non-adiabatic coupling in a small system, the Shin-Metiu model, is investigated.
The transition from adiabatic to non-adiabatic dynamics is elucidated in modifying the electron-nuclear interaction. This allows the comparison of weakly correlated electron-nuclear motion with the case where the strong correlations determine the dynamics.
The studies of the model are extended to include spectroscopical transitions being present in two-dimensional and degenerate four-wave mixing spectroscopy.
Furthermore, the quantum and classical time-evolution of the coupled motion in the complete electron-nuclear phase space is compared for the two coupling cases.
Additionally, the numerically exact electron flux within the weak coupling case is compared to the Born-Oppenheimer treatment.
In the last part of the thesis, the model is extended to two dimensions.
The system then possesses potential energy surfaces which exhibit a typical 'Mexican hat'-like structure and a conical intersection in the adiabatic representation.
Thus, it is possible to map properties of the system onto a vibronic coupling (Jahn-Teller) hamiltonian. Exact wave-packet propagations as well as nuclear wave-packet dynamics in the adiabatic and diabatic representation are performed.
Molecules containing multiple bonds between atoms—most often in the form of olefins—are ubiquitous in nature, commerce, and science, and as such have a huge impact on everyday life. Given their prominence, over the last few decades, frequent attempts have been made to perturb the structure and reactivity of multiply-bound species through bending and twisting. However, only modest success has been achieved in the quest to completely twist double bonds in order to homolytically cleave the associated π bond. Here, we present the isolation of double-bond-containing species based on boron, as well as their fully twisted diradical congeners, by the incorporation of attached groups with different electronic properties. The compounds comprise a structurally authenticated set of diamagnetic multiply-bound and diradical singly-bound congeners of the same class of compound.
Molecules containing multiple bonds between atoms—most often in the form of olefins—are ubiquitous in nature, commerce, and science, and as such have a huge impact on everyday life. Given their prominence, over the last few decades, frequent attempts have been made to perturb the structure and reactivity of multiply-bound species through bending and twisting. However, only modest success has been achieved in the quest to completely twist double bonds in order to homolytically cleave the associated π bond. Here, we present the isolation of double-bond-containing species based on boron, as well as their fully twisted diradical congeners, by the incorporation of attached groups with different electronic properties. The compounds comprise a structurally authenticated set of diamagnetic multiply-bound and diradical singly-bound congeners of the same class of compound.
The invention of laser pulse shapers allowed for various quantum control experiments, where a chemical reaction is guided by specifically tailored laser pulses. However, despite of the prominent role of the liquid phase in chemistry, no successful attempt for controlling the selectivity of a bond-fission reaction has yet been reported in this state of matter. Promising candidates for such an experiment are C$_{\infty\mathrm{v}}$-symmetric trihalide anions with two different chemical bonds like $\ce{I2Cl-}$, because these molecules notionally offer the most simplest selectivity-control scenario of breaking either the one or the other bond and they are expected to dissociate under ultraviolet (UV) irradiation like it is known for the most-studied trihalide $\ce{I3-}$.
In order to investigate in this thesis the possibility that the dissociation reaction of such trihalides branches into two different photofragments, the ultrafast photodissociation dynamics of $\ce{I3-}$, $\ce{Br3-}$, $\ce{IBr2-}$ and $\ce{ICl2-}$ (point group D$_{\infty\mathrm{h}}$) as well as of $\ce{I2Br-}$ and $\ce{I2Cl-}$ (point group C$_{\infty\mathrm{v}}$) in dichloromethane solution were measured with broadband transient absorption spectroscopy in magic-angle configuration. The identification of the reaction pathway(s) relies on vibrational wavepacket oscillations, which survive the dissociation process and therefore carry not only informations about the reactant trihalides but also about the fragment dihalides.
These characteristic vibrational wavenumbers were extracted from the measured transient absorption spectra by globally fitting the population dynamics together with the wavepacket dynamics. Until recently, such a combined model function was not available in the well-established fitting tool Glotaran. This made it inevitable to develop a custom implementation of the underlying variable-projection fitting algorithm, for which the computer-algebra software Mathematica was chosen. Mathematica's sophisticated built-in functions allow not only for a high flexibility in constructing arbitrary model functions, but also offer the possibility to automatically calculate the derivative(s) of a model function. This allows the fitting procedure to use the exact Jacobian matrix instead of approximating it with the finite difference method.
Against the expectation, only one of the two thinkable photodissociation channels was found for each of the investigated C$_{\infty\mathrm{v}}$ trihalides. Since the photofragments recombine, their absorption signal as well as the reactant ground state bleach recover. This happens in a biexponential manner, which in the case of $\ce{I3-}$ was interpreted by Ruhman and coworkers with the direct formation of a neutral dihalogen fragment $\ce{I2}$ beside the negatively charged dihalide fragment $\ce{I2-}$. In this thesis, such a direct reaction channel was not found and instead the fast component of the biexponential decay is explained with vibrational excess energy mediating the recombination-preceding electron transfer process $\ce{I2- + I -> I2 + I-}$, while the slow component is attributed to cooled-down fragments.
In addition to the trihalide experiments, the possibility of a magic-angle configuration for polarization-shaping control experiments was theoretically investigated in this thesis by deriving magic-angle conditions for the third-order electric-dipole response signal of arbitrarily polarized laser pulses. Furthermore, the subtleties of anisotropy signals violating the well-known range of \numrange{-0.2}{0.4} were studied.
Energy Transfer Between Squaraine Polymer Sections: From helix to zig-zag and All the Way Back
(2015)
Joint experimental and theoretical study of the absorption spectra of squaraine polymers in solution provide evidence that two different conformations are present in solution: a helix and a zig-zag structure. This unique situation allows investigating ultrafast energy transfer processes between different structural segments within a single polymer chain in solution. The understanding of the underlying dynamics is of fundamental importance for the development of novel materials for light-harvesting and optoelectronic applications. We combine here femtosecond transient absorption spectroscopy with time-resolved 2D electronic spectroscopy showing that ultrafast energy transfer within the squaraine polymer chains proceeds from initially excited helix segments to zig-zag segments or vice versa, depending on the solvent as well as on the excitation wavenumber. These observations contrast other conjugated polymers such as MEH-PPV where much slower intrachain energy transfer was reported. The reason for the very fast energy transfer in squaraine polymers is most likely a close matching of the density of states between donor and acceptor polymer segments because of very small reorganization energy in these cyanine-like chromophores.
The mechanism of excimer formation: an experimental and theoretical study on the pyrene dimer
(2017)
The understanding of excimer formation in organic materials is of fundamental importance, since excimers profoundly influence their functional performance in applications such as light-harvesting, photovoltaics or organic electronics. We present a joint experimental and theoretical study of the ultrafast dynamics of excimer formation in the pyrene dimer in a supersonic jet, which is the archetype of an excimer forming system. We perform simulations of the nonadiabatic photodynamics in the frame of TDDFT that reveal two distinct excimer formation pathways in the gas-phase dimer. The first pathway involves local excited state relaxation close to the initial Franck–Condon geometry that is characterized by a strong excitation of the stacking coordinate exhibiting damped oscillations with a period of 350 fs that persist for several picoseconds. The second excimer forming pathway involves large amplitude oscillations along the parallel shift coordinate with a period of ≈900 fs that after intramolecular vibrational energy redistribution leads to the formation of a perfectly stacked dimer. The electronic relaxation within the excitonic manifold is mediated by the presence of intermolecular conical intersections formed between fully delocalized excitonic states. Such conical intersections may generally arise in stacked π-conjugated aggregates due to the interplay between the long-range and short-range electronic coupling. The simulations are supported by picosecond photoionization experiments in a supersonic jet that provide a time-constant for the excimer formation of around 6–7 ps, in good agreement with theory. Finally, in order to explore how the crystal environment influences the excimer formation dynamics we perform large scale QM/MM nonadiabatic dynamics simulations on a pyrene crystal in the framework of the long-range corrected tight-binding TDDFT. In contrast to the isolated dimer, the excimer formation in the crystal follows a single reaction pathway in which the initially excited parallel slip motion is strongly damped by the interaction with the surrounding molecules leading to the slow excimer stabilization on a picosecond time scale.