@unpublished{WohlgemuthMitric2020, author = {Wohlgemuth, Matthias and Mitric, Roland}, title = {Excitation energy transport in DNA modelled by multi-chromophoric field-induced surface hopping}, series = {Physical Chemistry Chemical Physics}, journal = {Physical Chemistry Chemical Physics}, edition = {submitted version}, doi = {10.1039/D0CP02255A}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-209467}, year = {2020}, abstract = {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.}, language = {en} } @phdthesis{Wirsing2023, author = {Wirsing, Sara}, title = {Computational Spectroscopic Studies with Focus on Organic Semiconductor Systems}, doi = {10.25972/OPUS-28655}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-286552}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2023}, abstract = {This work presents excited state investigations on several systems with respect to experimental spectroscopic work. The majority of projects covers the temporal evolution of excitations in thin films of organic semiconductor materials. In the first chapters, thinfilm and interface systems are build from diindeno[1,2,3-cd:1',2',3'-lm]perylene (DIP) and N,N'-bis-(2-ethylhexyl)-dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDIR-CN2) layers, in the third chapter bulk systems consist of 4,4',4"-tris[(3-methylphenyl)phenylamino] triphenylamine (m-MTDATA), 4,7-diphenyl-1,10-phenanthroline (BPhen) and tris-(2,4,6-trimethyl-3-(pyridin-3-yl)phenyl)borane (3TPYMB). These were investigated by aggregate-based calculations. Careful selection of methods and incorporation of geometrical relaxation and environmental effects allows for a precise energetical assignment of excitations. The biggest issue was a proper description of charge-transfer excitations, which was resolved by the application of ionization potential tuning on aggregates. Subsequent characterization of excitations and their interplay condenses the picture. Therefore, we could assign important features of the experimental spectroscopic data and explain differences between systems. The last chapter in this work covers the analysis of single molecule spectroscopy on methylbismut. This poses different challenges for computations, such as multi-reference character of low-lying excitations and an intrinsic need for a relativistic description. We resolved this by combining complete active space self-consistent field based methods with scalarrelativistic density-functional theory. Thus we were able to confidently assign the spectroscopic features and explain underlying processes.}, subject = {Theoretische Chemie}, language = {en} } @article{WeiserCuiDewhurstetal.2023, author = {Weiser, Jonas and Cui, Jingjing and Dewhurst, Rian D. and Braunschweig, Holger and Engels, Bernd and Fantuzzi, Felipe}, title = {Structure and bonding of proximity-enforced main-group dimers stabilized by a rigid naphthyridine diimine ligand}, series = {Journal of Computational Chemistry}, volume = {44}, journal = {Journal of Computational Chemistry}, number = {3}, doi = {10.1002/jcc.26994}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-312586}, pages = {456 -- 467}, year = {2023}, abstract = {The development of ligands capable of effectively stabilizing highly reactive main-group species has led to the experimental realization of a variety of systems with fascinating properties. In this work, we computationally investigate the electronic, structural, energetic, and bonding features of proximity-enforced group 13-15 homodimers stabilized by a rigid expanded pincer ligand based on the 1,8-naphthyridine (napy) core. We show that the redox-active naphthyridine diimine (NDI) ligand enables a wide variety of structural motifs and element-element interaction modes, the latter ranging from isolated, element-centered lone pairs (e.g., E = Si, Ge) to cases where through-space π bonds (E = Pb), element-element multiple bonds (E = P, As) and biradical ground states (E = N) are observed. Our results hint at the feasibility of NDI-E2 species as viable synthetic targets, highlighting the versatility and potential applications of napy-based ligands in main-group chemistry.}, language = {en} } @phdthesis{Walter2015, author = {Walter, Christof}, title = {Excitonic States and Optoelectronic Properties of Organic Semiconductors - A Quantum-Chemical Study Focusing on Merocyanines and Perylene-Based Dyes Including the Influence of the Environment}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-123494}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2015}, abstract = {The scope of computational chemistry can be broadened by developing new methods and more efficient algorithms. However, the evaluation of the applicability of the methods for the different fields of chemistry is equally important. In this thesis systems with an unusual and complex electronic structure, such as excitonic states in organic semiconductors, a boron-containing bipolaron and the excited states of pyracene were studied and the applicability of the toolkit of computational chemistry was investigated. Concerning the organic semiconductors the focus was laid on organic solar cells, which are one of the most promising technologies with regard to satisfying the world's need for cheap and environmentally sustainable energy. This is due to the low production and material costs and the possibility of using flexible and transparent devices. However, their efficiency does still not live up to the expectations. Especially the exciton diffusion lengths seem to be significantly too short. In order to arrive at improved modules, a fundamental understanding of the elementary processes occurring in the cell on the molecular and supramolecular level is needed. Computational chemistry can provide insight by separating the different effects and providing models for predictions and prescreenings. In this thesis, the focus was laid on the description of excitonic states in merocyanines and perylene-based dyes taking the influence of the environment into account. At first, the photochemical isomerization between two configurations of 6-nitro BIPS observed experimentally was studied by first benchmarking several functionals against SCS-ADC(2) in the gas phase and subsequently calculating the excited-state potential energy surface. The geometries obtained from a relaxed scan in the ground state as well as from a scan in the excited state were used. The environment was included using different polarizable continuum models. It was shown that the choice of the model and especially the question of the state specificity of the approach is of vital importance. Using the results of the calculations, a two-dimensional potential energy surface could be constructed that could be used to explain the experimental findings. Furthermore, the importance of the excited-state isomerization as a potential deactivation channel in the exciton transport was pointed out. Then the assessment of the suitability of different merocyanines for optoelectronic applications with quantum-chemical methods was discussed. At first, the effect of the environment on the geometry, especially on the bond length alternation pattern, was investigated. It was shown that the environment changes the character of the ground-state wave function of several merocyanines qualitatively, which means that the results of gas-phase calculations are meaningless - at least when a comparison with solution or device data is desired. It was demonstrated that using a polarizable continuum model with an effective epsilon, a qualitative agreement between the calculated geometry and the geometry in the crystal structure can be obtained. Therefore, by comparing the bond length alternation in solution and in the crystal, a rough estimate of the effect of the crystal environment can be made. It was further shown that the connection between the HOMO energy and the open-circuit voltage is not as simple as it is often implied in the literature. It was discussed that it is not clear whether the HOMO of a single molecule or a \$\pi\$-stack containing several monomers should be used and if the environmental charges of the bulk phase or the interface should be included. Investigating the dependence of the HOMO energy on the stack size yielded no definitive trend. Furthermore, it was discussed that the effect due the optimization of the modules (solvent, bulk heterojunction) during the production masks any potential correlation between the HOMO energy and measured open-circuit values. Therefore, a trend can only be expected for unoptimized bilayer cells. It was concluded that ultimately, the importance of the HOMO energy should not be overestimated. The correlation between the exciton reorganization energy and the so-called cyanine limit, which is predicted by a simple two-state model, was also discussed. By referring to the results of VB calculations, it was discussed that the correlation indeed exists and is non-negligible, although the effect is not as strong as one might have expected. In this context, a potential application of a VB/MM approach was covered briefly. The importance of the molecular reorganization energy and the device morphology was also discussed. It was concluded that the optimization of merocyanines for organic optoelectronic devices is inherently a multiparameter problem and one cannot expect to find one particular parameter, which solely controls the efficiency. The perylene-based dyes were studied with a focus on the description of a potential trapping mechanism involving an intermolecular motion in a dimer. The aim was to find methods which can be applied to larger model systems than a dimer and take the effect of the environment into account. As a test coordinate the longitudinal shift of two monomers against each other was used. At first, it was demonstrated how the character of an excited state in a dimer can be defined and how it can be extracted from a standard quantum-chemical calculation. Then several functionals were benchmarked and their applicability or failure was rationalized using the character analysis. Two recipes could be proposed, which were applied to a constraint optimization (only intermolecular degrees of freedom) in the excited states of the PBI dimer and to the description of the potential energy surfaces of ground and excited states along a longitudinal displacement in the perylene tetramer, respectively. It was further demonstrated that the semi-empirical OMx methods fail to give an accurate description of the excited-state potential energy surfaces as well as the ground-state surface along the test coordinate. This failure could be attributed to an underestimation of overlap-dependent terms. Consequently, it could be shown that the methods are applicable to large intermolecular distances, where the overlap is negligible. The results of DFT calculations with differently composed basis sets suggested that adding an additional single p-function for each atom should significantly improve the performance. QM/MM methods are ideally suited to take the effect of the environment on a a dimer model system into account. However, it was shown that standard force fields also give an incorrect description of the interaction between the monomers along the intermolecular coordinate. This failure was attributed to the isotropic atom-atom interaction in the repulsion term of the Lennard-Jones potential. This was corroborated using two simple proof-of-principle anisotropy models. Therefore, a novel force field called OPLS-AA_O was presented that is based on OPLS-AA, but uses an anisotropic model for the repulsion. The model involves the overlap integral between the molecular densities, which are modeled as a sum of atom-centered p-type Gaussian functions. It was shown that using this force field an excellent agreement with the DFT results can be obtained when the correct parameters are used. These parameters, however, are not very generalizable, which was attributed to the simplicity of the model in its current state (using the same exponential parameter for all atoms). As a short excursion, the applicability of an MO-based overlap model was discussed. It was demonstrated that the repulsion term based on the density overlap can be used to correct the failure of the OMx methods for the ground states. This is in accord with the assumption that an underestimation of the overlap terms is responsible for the failure. It was shown that OPLS-AA_O also gives an excellent description of the longitudinal shift in a PBI tetramer. Using the tetramer as a test system and applying the recipe obtained in the TDDFT benchmark for the QM-part and OPLS-AA_O for the MM-part in conjunction with an electrostatic embedding scheme, a QM/MM description of the excited states of the PBI dimer including the effect of the environment could be obtained. In the last chapter the theoretical description of the Bis(borolyl)thiophene dianion and the excited states of pyracene were discussed. The electronic structure of the Bis(borolyl)thiophene dianion - a negative bipolaron - was elucidated using DFT and CASPT2 methods. Furthermore, an estimation of the extent of triplet admixture to the ground state due to spin-orbit coupling was given. In the second project the S1 and S2 states of pyracene were computed using SCS-CC2 and SCS-ADC(2) and an estimation for the balance between aromaticity and ring strain was given. This also involved computing the vibrational frequencies in the excited states. In both studies the results of the computations were able to rationalize and complete experimental results.}, subject = {Exziton}, language = {en} } @phdthesis{Tarcea2004, author = {Tarcea, Nicolae}, title = {Light as a universal tool : Microcapsule sizing by elastic light scattering and mineral investigation by in situ Raman spectroscopy}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-9383}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2004}, abstract = {The present work consist of two major parts. The first part, extending over chapters 1, 2, 3 and 4, addresses the design and construction of a device capable of determining the shell thickness and the core size for monolayer spherical particles in a flow. The second part containing chapters 5, 6, 7, 8, 9 and 10, concentrate on the use of Raman spectroscopy as a space application, namely for use as a tool for in situ planetary investigations. This part directly addresses the MIRAS project, a study run under the auspices of Federal Ministry of Education and Research, BMBF and German Aerospace Center, DLR under national registration number 50OW0103. MIRAS stands for "Mineral Investigation by in situ Raman Spectroscopy". Microcapsule Sizing by Elastic Light Scattering The industrial development of processes based on microcapsules depends on the possibility to provide clear and complete information about the properties of these microcapsules. However, the tools for an easy and efficient determination of the microcapsule properties are lacking, several methods being often required to describe adequately the microcapsule behavior. Methods for evaluating the individual size and size distribution of both the core and the shell are required together with methods for measuring the mechanical strength, stability in appli-cation media, permeability of the shell, etc. Elastic light scattering measurements provide a possible way of determining properties such as core size, shell size and refractive index. The design and con-struction of a device capable of measuring the above mentioned parameters for a core-shell particle is the subject of the first part of this thesis. The basic principle of measurement for the device proposed here consists of an-alyzing one particle at a time by recording the elastic light scattering pattern at angles between approx. 60 and 120 grad. By comparing the experimentally recorded phase functions with the previously calculated phase functions stored in a database, the geometry of the scattering object can be identified. In our case the geometry is characterized by two parameters: the shell thickness and the core radius. In chapter 2 a short overview on the methods used for sizing microparticles is given. Different sizing methods are compared, and the advantages and disadvan-tages for the general problem of sizing are shortly discussed. It is observed that all sizing methods that are based on elastic light scattering theories are ensemble methods. Chapter 3 focusses on the theories used for calculating the theoretical scattering patterns with emphasize on the Mie theory. The generalization of Mie theory for layered particles is shortly presented and the far field intensity approximations are discussed. The last chapter (4) of this first part describes the experimental approach for building an automatic microcapsule sizer. The approach started by O. Sbanski [76] with the development of a software packet for calculating and storing theoret-ical phase functions for core-shell particles was continued with the designing and construction of a measuring device. The hardware construction and the software with all implemented corrections imposed by the individual setup components are described in detail. For the laser, the monochromaticity, the intensity profile of the beam as well as the planarity of the equi-phase fronts are taken into consid-eration. The flow cell with three different designs is described, and the influences of the employed design on the light scattering patterns are discussed together with the optical system used for recording the experimental phase functions. The detection system formed by two identical linear CCD arrays is presented together with the software approach used for data acquisition. Ways of improving the quality and the speed of the analyzing process are discussed. The final section presents measurements run on samples made of homogeneous spheres and also on samples containing industrial microcapsules. Mineral Investigation by in situ Raman Spectroscopy The envisaged future planetary missions require space-born instruments, which are highly miniaturized with respect to volume and mass and which have low needs of power. A micro Raman spectrometer as a stand alone device on a planetary surface (e.g. Mars) offers a wide spectrum of possibilities. It can assess the chemical analysis via determination of the mineral composition, detect organic molecules in the soil, identify the principal mineral phases, etc. The technical developments in the last years have introduced a new generation of small Raman systems suitable for robotic mineral characterization on planetary surfaces [20, 95]. Two different types of spectrometer were considered for the MIRAS study. As supporting laboratory experiments for the MIRAS study, the measure-ments on standard minerals and on SNC Mars meteorites are discussed in chapter 6. The following SNC meteorites have been investigated: Sayh al Uhaymir 060, Dar al Gani 735, Dar al Gani 476, Northwest Africa 856, Los Angeles, Northwest Africa 1068 and Zagami. Pyrite as a hitherto undescribed phase in the picritic (olivin-phyric) shergottite NWA 1068 as well as reduced carbon (e.g. graphite) and anatase in the shergottite Say al Uhaymir 060 are new findings for this class of meteorites. A detailed description of the proposed designs for MIRAS, with the compo-nents used for building the test version on a breadboard is covered in chapter 7. The scientific as well as the mission requirements imposed on the instrument are discussed. The basic design is presented and the main components that are brought together to build the device being the laser unit, the Raman head, the Rayleigh filtering box, and the spectral sensor (spectrometer with a matching de-tector) are described. The two proposed designs, one based on an acousto-optic tunable filter (AOTF) and the other based on a dispersive hadamard transform spectrometer are compared to each other. The actual breadboard setup with the detailed description of the components follows in Section 7.3. Further de-velopment of a Raman spectrometer for planetary investigations is proposed in combination with a microscope as part of the Extended-MIRAS project. The software developed for controlling the breadboard version of MIRAS is described in chapter 8 together with a short description of the structure of a relational database used for in house spectra management. The measuring pro-cedures and the data processing steps are presented. Spectra acquired with the MIRAS breadboard version based on the AOTF are shown in chapter 9. The final chapter addresses a rather different possibility of using Raman spectroscopy for planetary investigations. The chapter summarizes the content of four tech-nical notes that were established within the study contracted by the European Space Agency with firma Kayser-Threde in Munich concerning the possibility of applying Raman spectroscopy in the field of remote imaging.}, language = {en} } @phdthesis{Szeghalmi2005, author = {Szeghalmi, Adriana Viorica}, title = {The ground and excited state molecular structure of model systems undergoing photochemical processes and the characterization of active agents by means of vibrational spectroscopy and theoretical calculations}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-11961}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2005}, abstract = {The present thesis reports about vibrational and quantum chemical investigations on model systems undergoing photochemical processes and pharmaceutically active compounds, respectively. Infrared (IR) and Raman spectroscopy were applied for the characterization of the ground state molecular structure. Moreover, resonance Raman (RR) spectra contain additional information about the resonantly enhanced excited state molecular structure. A quantitative resonance Raman intensity analysis in conjunction with the simultaneous simulation of the absorption spectra by means of time-dependent propagation methods was accomplished in order to extract valuable information about the excited state molecular structures of the investigated systems. Surface enhanced Raman scattering (SERS) allows one to determine the interaction and adsorption site of active agents on a metal substrate. Furthermore, density functional theory (DFT) and potential energy distribution (PED) calculations were carried out for an exact assignment of the vibrational spectra. Complete active space self consistent field (CASSCF) and configuration interaction (CI) calculations for some model systems were also performed to assess the experimental results on the excited state potential surfaces. The fundamentals of resonance Raman spectroscopy are treated in detail, describing the physical processes and emphasizing the theoretical methodologies which allow one to obtain the information about the resonantly excited state via an RR intensity analysis. The Brownian oscillator model to determine the solvent reorganization energy is briefly presented. Furthermore, the SERS enhancement mechanisms and selection rules to determine the orientation of the molecules adsorbed on the metal substrate are discussed. The Hartree-Fock approach to calculate the ground state geometry is expatiated, and the basic characteristics of the CI and CASSCF calculations are specified. The chapter ends with a short description of the DFT calculations. Chapter 4 deals with the investigation of the excited state intramolecular proton transfer of the model system, 1-hydroxy-2-acetonaphthone (HAN). The vibrations showing the highest displacement parameters correspond to stretching and in-plane deformation modes of the naphthalene ring and the conjugated carbonyl group, while the OH stretching mode exhibits no observable enhancement. The cooperative effect of the skeletal vibrations reduces the distance between the carbonyl and hydroxyl oxygen atoms in accordance with a general electron density redistribution. Hence, the leading force in the proton transfer process is the increase in electron density on the carbonyl group and the decrease of the negative charge on the hydroxyl oxygen. In chapter 5 the structural and vibrational characteristics of the organic mixed valence system N,N,N',N'-tetraphenylphenylenediamine radical cation (1+) are discussed. The resonance Raman measurements showed that at least eight vibrational modes are strongly coupled to the optical charge transfer process in (1+). These Franck-Condon active modes were assigned to symmetric vibrations. The most enhanced band corresponds to the symmetric stretching mode along the N-phenylene-N unit and exhibits the largest vibrational reorganization energy. Nevertheless, symmetric stretching modes of the phenylene and phenyl units as well as deformation modes are also coupled to the electronic process. The total vibrational reorganization energy of these symmetrical modes is dominant, while the solvent induced broadening and reorganization energy are found to be small. Hence, (1+) adopts a symmetrical delocalized Robin-Day Class III structure in the ground state. Chapter 6 reports about a vibrational spectroscopic investigation of a model organic photorefractive thiophene derivative, 2-(N,N-diethylamino)-5-(2',2'-dicyanovinyl)-thiophene. The geometry of the first excited state were optimized and the FC parameters were calculated using the configuration interaction with single excitations method. These calculations show that the contribution of the zwitterionic structure to the excited state is significantly higher than in the ground state. The resonance Raman spectra indicate that several stretching modes along the bonds connecting the donor and acceptor moieties as well as the S-C stretching vibrations are enhanced. Chapter 7 presents the vibrational analysis of an aziridinyl tripeptide, a cysteine protease inhibitor active drug. The vibrational analysis reveals stronger H-bonding of the aziridine NH unit in the solid state of the aziridinyl tripeptide than in the liquid electrophilic building block, indicating medium strong intermolecular H-bond interactions in the crystal unit. The amide hydrogen atoms of the aziridinyl tripeptide are involved in weaker H-bonds than in an epoxide analogon. Furthermore, the characteristic vibrational modes of the peptide backbone were discussed. Chapter 8 reports on the adsorption mechanism of two related anti-leukemia active agents, 6-mercaptopurine (6MP) and 6-mercaptopurine-ribose (6MPR) on a silver colloid. Both molecules adsorb through the N1 and possibly S atom on the metal surface under basic conditions. The SERS spectra recorded for acidic pH values showed that the ribose derivative exhibits a different adsorption behavior compared to the free base. 6MP probably adsorbs on the silver sol through the N9 and N3 atoms, while 6MPR interacts with the surface via the N7 and probably S atoms. Around critical biological concentrations and pH values i.e. at low concentrations and almost neutral condition (pH 7-9), 6MPR interacts with the substrate through both N7 and N1 atoms, possibly forming two differently adsorbed species, while for 6MP only the species adsorbed via N1 was evidenced.}, subject = {Photochemie}, language = {en} } @article{SteinbacherBubackNuernbergeretal.2012, author = {Steinbacher, Andreas and Buback, Johannes and N{\"u}rnberger, Patrick and Brixner, Tobias}, title = {Precise and rapid detection of optical activity for accumulative femtosecond spectroscopy}, series = {Optics Express}, journal = {Optics Express}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-85913}, year = {2012}, abstract = {We present polarimetry, i.e. the detection of optical rotation of light polarization, in a configuration suitable for femtosecond spectroscopy. The polarimeter is based on common-path optical heterodyne interferometry and provides fast and highly sensitive detection of rotatory power. Femtosecond pump and polarimeter probe beams are integrated into a recently developed accumulative technique that further enhances sensitivity with respect to single-pulse methods. The high speed of the polarimeter affords optical rotation detection during the pump-pulse illumination period of a few seconds. We illustrate the concept on the photodissociation of the enantiomers of methyl p-tolyl sulfoxide. The sensitivity of rotatory detection, i.e. the minimum rotation angle that can be measured, is determined experimentally including all noise sources to be 0.10 milli-degrees for a measurement time of only one second and an interaction length of 250 μm. The suitability of the presented setup for femtosecond studies is demonstrated in a non-resonant two-photon photodissociation experiment.}, language = {en} } @phdthesis{Stehr2015, author = {Stehr, Vera}, title = {Prediction of charge and energy transport in organic crystals with quantum chemical protocols employing the hopping model}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-114940}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2015}, abstract = {As organic semiconductors gain more importance for application, research into their properties has become necessary. This work investigated the exciton and charge transport properties of organic semiconducting crystals. Based on a hopping approach, protocols have been developed for the calculation of Charge mobilities and singlet exciton diffusion coefficients. The protocols do not require any input from experimental data except for the x-ray crystal structure, since all needed quantities can be taken from high-level quantum chemical calculations. Hence, they allow to predict the transport properties of yet unknown compounds for given packings, which is important for a rational design of new materials. Different thermally activated hopping models based on time-dependent perturbation theory were studied for the charge and exciton transport; i. e. the spectral overlap approach, the Marcus theory, and the Levich-Jortner theory. Their derivations were presented coherently in order to emphasize the different levels of approximations and their respective prerequisites. A short reference was made to the empirical Miller-Abrahams hopping rate. Rate equation approaches to calculate the stationary charge carrier mobilities and exciton diffusion coefficients have been developed, which are based on the master equation. The rate equation approach is faster and more efficient than the frequently used Monte Carlo method and, therefore, provides the possibility to study the anisotropy of the transport parameters and their three-dimensional representation in the crystal. The Marcus theory, originally derived for outer sphere electron transfer in solvents, had already been well established for charge transport in organic solids. It was shown that this theory fits even better for excitons than for charges compared with the experiment. The Levich-Jortner theory strongly overestimates the charge carrier mobilities and the results deviate even stronger from the experiment than those obtained with the Marcus theory. The latter contains larger approximations by treating all vibrational modes classically. The spectral overlap approach in combination with the developed rate equations leads to even quantitatively very good results for exciton diffusion lengths compared to experiment. This approach and the appendant rate equations have also been adapted to charge transport. The Einstein relation, which relates the diffusion coefficient with the mobility, is important for the rate equations, which have been developed here for transport in organic crystals. It has been argued that this relation does not hold in disordered organic materials. This was analyzed within the Framework of the Gaussian disorder model and the Miller-Abrahams hopping rate.}, subject = {Exziton}, language = {en} } @phdthesis{Siebert2002, author = {Siebert, Torsten Uwe}, title = {Four-Wave Mixing Techniques Applied to the Investigation of Non-Adiabatic Dynamics in Polyatomic Molecules}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-2456}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2002}, abstract = {In the experiments presented in this work, third-order, time-resolved spectroscopy was applied to the disentanglement of nuclear and electronic degrees of freedom in polyatomic molecules. The motivation for approaching this problem was given by the decisive role that the coupling of nuclear and electronic dynamics plays in the mechanism of photochemical reactions and photobiological processes. In order to approach this complex problem, different strategies within the framework of time-resolved, four-wave mixing spectroscopy were developed that allowed for the dynamic as well as the energetic aspects of vibronic coupling in non-radiative transitions of polyatomic molecules to be addressed. This was achieved by utilizing the influence of optical as well as Raman resonances on four-wave mixing processes. These resonance effects on third-order, optical processes allow for a high selectivity to be attained with respect to the interrogation of specific aspects of molecular dynamics. The development of different strategies within the framework of time-resolved, four-wave mixing spectroscopy for addressing the problem of vibronic coupling began with the experiments on gaseous iodine. This simple, well investigated molecular system was chosen in order to unambiguously characterize the effect of Raman resonances on four-wave mixing processes. A time-resolved degenerative four-wave mixing (DFWM) experiment was carried out on gaseous iodine that allowed for the dynamics of coherent Stokes Raman scattering (CSRS) as well as a coherent anti-Stokes Raman scattering (CARS) to be observed parallel to the dynamics of a DFWM process at different spectral positions of the FWM signal. Here, the state-selectivity of these different FWM processes manifests itself in the vibrational wave packet dynamics on different electronic potentials of iodine. It could be shown that Raman resonances determine the selectivity with which these FWM processes prepare and interrogate nuclear dynamics in different electronic states. With the insight gained into the relevance of Raman resonant processes in FWM spectroscopy, an experimental scheme was devised that utilizes this effect to selectively interrogate the dynamics of a specific vibrational mode within a polyatomic molecule during a radiationless electronic transition. Here, a CARS process was employed to selectively probe specific vibrational modes of a molecular system by variably tuning the energy difference between the lasers involved in the CARS process to be in Raman resonance with the vibrational energy spacing of a particular vibrational mode. Using this aspect of a tunable resonance enhancement within a CARS scheme, this optical process was incorporated in a time-resolved pump-probe experiment as a mode-selective probe mechanism. This type of experimental configuration, that employs four pulsed laser fields, was classified as a pump-CARS scheme. Here, a laser pulse independent of the CARS process initiates the molecular dynamics that are interrogated selectively with respect to the vibrational mode of the system through the simultaneous interaction of the three pulsed fields involved in the CARS process. Time-resolution on a femtosecond timescale is achieved by introducing a time delay between the independent pump laser and the laser pulses of the CARS process. The experimental configuration of a pump-CARS scheme was applied to the study of the nuclear dynamics involved in the radiationless electronic transition between the first excited singlet state (S1) and the electronic ground state (S0) of all-trans-b-carotene. The mode-selective CARS probe allowed for the characteristic timescale with which specific vibrational modes are repopulated in the S0 state to be determined. From the varying repopulation times of specific vibrational modes, a mechanism with which the full set of vibrational states of the S0 potential are repopulated subsequent to the internal conversion process could be postulated. Most importantly, the form of nuclear motion that primarily funnels the population between the two electronic states could be identified as the C=C symmetric symmetric stretch mode in the polyene backbone of b-carotene. With this, the reaction coordinate of this radiationless electronic transition could be identified. The experiment shows, that the CARS probe is capable of determining the nuclear motion coupled to a radiationless electronic transition in complex polyatomic systems. The S1/S0 internal conversion process in b-carotene was further investigated with time-resolved transient gratings. Here, the energetic aspects of a non-adiabatic transition was addressed by determining the influence of the vibrational energy on the rate of this internal conversion. In order to compare the rate of internal conversion taking place out of vibrational ground state modes versus this transition initiating out of vibrationally hot modes, the strategy of shifting the probe mechanism in the transient grating scheme to spectral positions within and out of the red flank of the S1 absorption profile was pursued. The interrogation of different vibrational states was verified by determining the degree of vibrational cooling, taking place parallel to the internal conversion process. With this strategy, it could be shown that vibrationally hot states contribute to the internal conversion with a higher rate than vibrational ground state modes. In summary, different third-order, optical processes in the framework of time-resolved FWM were applied to the study of non-adiabatic dynamics in polyatomic molecules. By utilizing the effect of optical as well as Raman resonances on different FWM processes, it could be shown that third-order, time-resolved spectroscopy is a powerful tool for gaining insight into complex molecular dynamics such as vibronic coupling. The experiments presented in this work showed that the CARS process, as a mode-selective probe in time-resolved experiments, is capable of disentangling nuclear and electronic dynamics.}, subject = {Provitamin A}, language = {en} } @phdthesis{Settels2012, author = {Settels, Volker}, title = {Quantum chemical description of ultrafast exciton self-trapping in perylene based materials}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-69861}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2012}, abstract = {Im Rahmen dieser Dissertation wurden sehr lange Exzitonen-Diffusionsl{\"a}ngen (LD) unter idealen Bedingungen f{\"u}r Perylen-basierte Materialien simuliert. Dies ist ein Indiz daf{\"u}r, dass die sehr kurzen LD in realen Materialien aus einer extrinsischen sowie einer intrinsischen Immobilisierung resultieren. Letztere basiert auf einer Relaxation in sogenannten „Self-Trapping"-Zust{\"a}nde. Ein tieferes Verst{\"a}ndnis der dem Self-Trapping zugrunde liegenden atomistischen Prozesse ist notwendig, um zuk{\"u}nftig Materialien mit langen LD entwickeln zu k{\"o}nnen, bei denen eine intrinsische Exzitonen-Immobilisierung verhindert wird. F{\"u}r die Entwicklung eines solchen mechanistischen Verst{\"a}ndnisses ist das Vorliegen einer eindeutigen Korrelation zwischen der molekularen Anordnung und der LD unabdingbar. Diese weisen Einkristalle von Diindenoperylen (DIP) und α-Perylen-tetracarboxyl-anhydrid (α-PTCDA) auf. Bei ersteren wurde eine außergew{\"o}hnlich lange LD von 90 nm und bei letzteren nur 22 nm gemessen. Teil dieser Arbeit war es, Gr{\"u}nde f{\"u}r diesen Unterschied in der LD zu finden. Nur Self-Trapping kommt als Ursache in Frage. Aus diesem Grund eignen sich diese Materialien, um ein atomistisches Verst{\"a}ndnis des Self-Trappings exemplarisch an ihnen zu erarbeiten. Mutmaßlich k{\"o}nnten Differenzen in der elektronischen Struktur in DIP und α-PTCDA f{\"u}r das unterschiedliche Self-Trapping verantwortlich sein. Allerdings konnte gezeigt werden, dass es f{\"u}r viele Perylen-basierte Materialien keine signifikanten Unterschiede in der elektronischen Struktur gibt, wodurch diese f{\"u}r die Aufkl{\"a}rung von Immobilisierungsmechanismen zu vernachl{\"a}ssigen sind. Eine weitere m{\"o}gliche Begr{\"u}ndung w{\"a}re in Polarisationseffekten im Kristall zu suchen, welche die elektronische Struktur in Perylen-basierten Materialien unterschiedlich beeinflussen. Vor allem ihr Einfluss auf Ladungstrennungs-Zust{\"a}nde (CT), die oberhalb des optisch hellen Frenkel-Zustandes liegen, war fraglich, weil sie energetisch abgesenkt werden k{\"o}nnten. Ein signifikanter Einfluss von Polarisationseffekten konnte aber f{\"u}r alle Zust{\"a}nde mittels eines polarisierbaren Kontinuum-Modells ausgeschlossen werden. Die geringe LD im α-PTCDA ist folglich ein Indiz f{\"u}r ein Self-Trapping, das durch die Kristallstruktur aus π-Stapeln evoziert wird, welche in DIP fischgr{\"a}tenartig ist. Da Polarisationseffekte auszuschließen sind, {\"u}bt der Kristall lediglich durch sterische Restriktionen einen Einfluss auf das Dimer aus. Daher muss die Methode f{\"u}r die Beschreibung von Self-Trapping nur diese Effekte ber{\"u}cksichtigen, so dass sich f{\"u}r den Einsatz des mechanical embedding QM/MM-Ansatzes entschieden wurde. Nun konnten Potentialfl{\"a}chen berechnet werden, auf denen anschließend eine Wellenpaketdynamik durchgef{\"u}hrt wurde. Diese Methode erlaubt es erstmals, Mechanismen der Exzitonen-Immobilisierung in organischen Materialien auf einer atomistischen Ebene zu beschreiben. Als Erkl{\"a}rung f{\"u}r Self-Trapping in α-PTCDA dienten Potentialfl{\"a}chen, die eine intermolekulare Verschiebung des Dimers im Kristall abbilden. So wurde eine Exzitonen-Immobilisierung innerhalb von 500 fs gefunden, die aus einem irreversiblem Energieverlust und einer lokalen Verzerrung der Kristallstruktur resultiert und auf diese Weise den weiteren Transport des Exzitons verhindert. Im Fall von DIP kann diese Immobilisierung aufgrund hoher Energiebarrieren nicht stattfinden. Diese Barrieren resultieren aus der fischgr{\"a}tenartigen Kristallstruktur des DIP. Diese Diskrepanzen in der Dynamik erkl{\"a}ren die unterschiedlichen LD-Werte f{\"u}r DIP und α-PTCDA. In einem weiteren Fall wurde eine Exzitonen-Immobilisierung in helikalen π Aggregaten von Perylen-tetracarboxyl-bisimid (PBI) Molek{\"u}len festgestellt. Hier wird Self-Trapping durch einen Relaxationsmechanismus verursacht, in dem das Exziton durch geringe asymmetrische Schwingungen des Aggregats innerhalb von 200 fs von dem hellen Frenkel- in den dunklen Frenkel-Zustand transferiert wird, wobei dieser {\"U}bergang von einem CT-Zustand vermittelt wird. Der gesamte Vorgang ist nur bei helikalen Aggregaten m{\"o}glich, weil nur hier CT-Zust{\"a}nde sehr dicht bei dem hellen Frenkel-Zustand vorhanden sind. Im finalen Frenkel-Zustand tritt eine Torsionsbewegung um die π-Stapelachse ein, so dass ein Energieverlust und eine lokale {\"A}nderung der Aggregatstruktur erfolgt - also ein Self-Trapping des Exzitons. Dieser modellierte Mechanismus steht im Einklang zu allen vorliegenden experimentellen Daten. Diese Erkenntnisse lassen die Schlussfolgerung zu, dass in k{\"u}nftigen Materialen f{\"u}r organische Solarzellen eine irreversible und ultraschnelle Deformation des Aggregats nach der Photoanregung vermieden werden muss - will man lange LD erreichen. Nur so kann Self-Trapping von Exzitonen verhindert werden.}, subject = {Exziton}, language = {en} } @phdthesis{SeligParthey2012, author = {Selig-Parthey, Ulrike}, title = {Methods of Nonlinear Femtosecond Spectroscopy in the Visible and Ultraviolet Regime and their Application to Coupled Multichromophore Systems}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-74356}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2012}, abstract = {Time-resolved spectroscopic studies of energy transfer between molecules in solution form a basis for both, our understanding of fundamental natural processes like photosynthesis as well as directed synthetic approaches to optimize organic opto-electronic devices. Here, coherent two-dimensional (2D) spectroscopy opens up new possibilities, as it reveals the correlation between absorption and emission frequency and hence the full cause-and-effect chain. In this thesis two optical setups were developed and implemented, permitting the recording of electronic 2D spectra in the visible and in the hitherto unexplored ultraviolet spectral range. Both designs rely on the exclusive manipulation of beam pairs, which reduces the signal modulation to the difference between the transition frequency of the system and the laser frequency. Thus - as has been shown experimentally and theoretically - the timing precision as well as mechanical stability requirements are greatly reduced, from fractions of the oscillation period of the exciting light wave to fractions of the pulse duration. Two-dimensional spectroscopy and femtosecond transient absorption (TA) as well as different theoretical approaches and simulation models were then applied to coupled multichromophore systems of increasing complexity. Perylene bisimide-perylene monoimide dyads were investigated in cooperation with Prof. Dr. Frank W{\"u}rthner and Prof. Dr. Bernd Engels at the University of W{\"u}rzburg. In these simplest systems studied, global analysis of six different TA experiments unequivocally revealed an ultrafast interchromophoric energy transfer in the 100 fs range. Comparison between the obtained transfer rates and the predictions of F{\"o}rster theory suggest a breakdown of this point-transition-dipole-based picture at the donor-acceptor distances realized in our compounds. Furthermore, a model including conformational changes and an interchromophoric charge transfer has been derived to consistently describe the observed pico- to nanosecond dynamics and fluorescence quantum yields. A second collaboration with Prof. Dr. Gregory Scholes (University of Toronto, Canada) and Prof. Dr. Paul Burn (University of Queensland, Australia) addressed the photophysics of a series of uorene-carbazole dendrimers. Here, a combination of 2D-UV spectroscopy and femtosecond ansiotropy decay experiments revealed the initial delocalization of the excited state wave function that saturates with the second generation. In room temperature solution, disorder-induced localization takes place on the time scales comparable to our instrument response, i.e. 100 fs, followed by energy transfer via incoherent hopping processes. Lastly, in tubular zinc chlorin aggregates, semi-synthetic analogues of natural lightharvesting antennae that had again been synthesized in the group of Prof. Dr. Frank W{\"u}rthner, the interchromophoric coupling is so strong that coherently coupled domains prevail even at room temperature. From an analysis of intensity-dependent TA measurements the dimensions of these domains, the exciton delocalization length, could be determined to span 5-20 monomers. In addition, 2D spectra uncovered efficient energy transfer between neighboring domains, i.e. ultrafast exciton diffusion.}, subject = {Femtosekundenspektroskopie}, language = {en} } @phdthesis{Seibt2009, author = {Seibt, Joachim}, title = {Theoretical investigations on the spectroscopy of molecular aggregates}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-37218}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2009}, abstract = {Die spektroskopischen Eigenschaften von Molek{\"u}laggregaten wurden mittels quantendynamischer Berechnungen untersucht. Hierbei wurden sowohl lineare als auch nichtlineare Spektroskopietechniken einbezogen. Zur Simulation von Absorptions- und CD-Spektroskopie wurden Kopplungseffekte sowie die relative Orientierung der Monomer-Einheiten in den Modellen ber{\"u}cksichtigt, um gemessene Spektren reproduzieren und so die entsprechenden Parameter zu bestimmen. Zur genaueren Beschreibung wurden auch Ergebnisse quantenchemischer Rechnungen verwendet. Dar{\"u}ber hinaus wurden Untersuchungen zur nichtlinearen optischen Spektroskopie an Dimeren durchgef{\"u}hrt.}, subject = {Theoretische Chemie}, language = {en} } @article{SchuergerEngel2023, author = {Sch{\"u}rger, Peter and Engel, Volker}, title = {Differential Shannon entropies characterizing electron-nuclear dynamics and correlation: momentum-space versus coordinate-space wave packet motion}, series = {Entropy}, volume = {25}, journal = {Entropy}, number = {7}, issn = {1099-4300}, doi = {10.3390/e25070970}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-362670}, year = {2023}, abstract = {We calculate differential Shannon entropies derived from time-dependent coordinate-space and momentum-space probability densities. This is performed for a prototype system of a coupled electron-nuclear motion. Two situations are considered, where one is a Born-Oppenheimer adiabatic dynamics, and the other is a diabatic motion involving strong non-adiabatic transitions. The information about coordinate- and momentum-space dynamics derived from the total and single-particle entropies is discussed and interpreted with the help of analytical models. From the entropies, we derive mutual information, which is a measure for the electron-nuclear correlation. In the adiabatic case, it is found that such correlations are manifested differently in coordinate- and momentum space. For the diabatic dynamics, we show that it is possible to decompose the entropies into state-specific contributions.}, language = {en} } @phdthesis{Schuerger2024, author = {Sch{\"u}rger, Peter}, title = {Information-Theoretical Studies on Time-Dependent Quantum Systems}, doi = {10.25972/OPUS-35221}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-352215}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2024}, abstract = {In this thesis, we apply the information-theoretic approach in the context of quantum dynamics and wave packet motion: Information-theoretic measures are calculated from position and momentum densities, which are obtained from time-dependent quantum wave functions. The aim of this thesis is to benchmark, analyze and interpret these quantities and relate their features to the wave packet dynamics. Firstly, this is done for the harmonic oscillator (HO) with and without static disorder. In the unperturbed HO, the analytical study of coherent and squeezed states reveals time-dependent entropy expressions related to the localization of the wave function. In the disordered HO, entropies from classical and quantum dynamics are compared for short and long times. In the quantum case, imprints of wave packet revivals are found in the entropy. Then, the energy dependence of the entropy for very long times is discussed. Secondly, this is donefor correlated electron-nuclear motion. Here, entropies derived from the total, electronic and nuclear density, respectively, are calculated in position and momentum space for weak and strong adiabatic electronic coupling. The correlation between electron and nucleus is investigated using different correlation measures, where some of these functions are sensitive to the nodal structure of the wave function. An analytic ansatz to interpret the information-theoretical quantities is applied as well.}, subject = {St{\"o}rungstheorie}, language = {en} } @phdthesis{Schmidt2015, author = {Schmidt, Thomas Christian}, title = {Theoretical Investigations on the Interactions of Small Compounds with their Molecular Environments}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-127860}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2015}, abstract = {Im ersten Teil dieser Arbeit wird eine Kombination theoretischer Methoden f{\"u}r die strukturbasierte Entwicklung neuer Wirkstoffe pr{\"a}sentiert. Ausgehend von der Kristallstruktur eines kovalenten Komplexes einer Modellverbindung mit dem Zielprotein wurde mit Hilfe von quantenmechanischen und QM/MM Rechnungen die genaue Geometrie des vorausgehenden nicht-kovalenten Komplexes betimmt. Letztere ist der bestimmende Faktor f{\"u}r die Reaktivit{\"a}t des Inhibitors gegen{\"u}ber der katalytisch aktiven Aminos{\"a}ure und damit f{\"u}r die Ausbildung einer kovalenten Bindung. Aus diesem Grund wurde diese Geometrie auch f{\"u}r die Optimierung der Substitutionsmusters des Ihnibitors verwendet, um dessen Affinit{\"a}t zum Zielenzyme zu verbessern ohne dass dieser seine F{\"a}higkeit kovalent an das aktive Zentrum zu binden verliert. Die Optimierung des Substitutionsmuster wurde doch Methode des Molekularen Dockings unterst{\"u}tzt, das diese optimal dazu geeignet sind, Bindungsaffinit{\"a}ten vorherzusagen, die durch eine Modifikation der chemischen Struktur entstehen. Eine Auswahl der besten Strukturen wurde anschließend verwendet, um zu {\"u}berpr{\"u}fen, ob die ver{\"a}nderten Molek{\"u}le noch gen{\"u}gen Reaktivit{\"a}t gegen{\"u}ber dem Zielprotein aufweisen. Molek{\"u}ldynamik Simulationen der neuen Verbindungen haben jedoch gezeigt, dass die ver{\"a}nderten Verbindungen nur so and das Protein binden, dass die Bilung eine kovalenten Bindung zum Enzym nicht mehr m{\"o}glich ist. Daher wurden in einem weiteren Schritt die Modellverbindungen weiter modifiziert. Neben {\"A}nderungen im Substitutionsmuster wurde auch die chemische Struktur im Kern ver{\"a}ndert. Die Bindungsaffinit{\"a}ten wurde wieder mittels Docking {\"u}berpr{\"u}ft. F{\"u}r die besten Bindungsposen wurden wieder Simulationen zur Molek{\"u}ldynamik durchgef{\"u}hrt, wobei diesmal die Ausbildung einer kovalenten Bindung zum Enzyme m{\"o}glich erscheint. In einer abschließenden Serie von QM/MM Rechnungen unter Ber{\"u}cksichtigung verschiedener Protonierungszust{\"a}nde des Inhibitors und des Proteins konnten Reaktionspfade und zugeh{\"o}rige Reaktionsenergien bestimmt werden. Die Ergebnisse lassen darauf schließen, dass eines der neu entwickelten Molek{\"u}le sowohl eine stark verbesserte Bindungsaffinit{\"a}t wie auch die M{\"o}glichkeit der kovalenten Bindung an Enzyme aufweist. Der zweite Teil der Arbeit konzentriert sich auf die Umgebungseinfl{\"u}sse auf die Elektronenverteilung eines Inhibitormodells. Als Grundlage dient ein vinylsulfon-basiertes Moek{\"u}l, f{\"u}r das eine experimentell bestimmte Kristallstruktur sowie ein theoretisch berechneter Protein Komplex verf{\"u}gbar sind. Ein Referendatensatz f{\"u}r diese Systeme wurde erstellt, indem der Konformationsraum des Inhibitors nach m{\"o}glichen Minimumsstrukturen abgesucht wurde, welche sp{\"a}ter mit den Geometrien des Molek{\"u}ls im Kristall und im Protein verglichen werden konnten. The Geometrie in der Kristallumgebung konnte direkt aus den experimentellen Daten {\"u}bernommen werden. Rechnungen zum nicht-kovalenten Protein Komplex hingegen haben gezeigt, dass f{\"u}r das Modellsystem mehrere Geometrien des Inhibiors sowie zwei Protonierungszust{\"a}nde f{\"u}r die katalytisch aktiven Aminos{\"a}uren m{\"o}glich sind. F{\"u}r die Analyse wurden daher alle m{\"o}glichen Proteinkomplexe mit der Kristallstruktur verglichen. Ebenso wurden Vergleiche mit der Geometrie des isolierten Molek{\"u}ls im Vakuum sowie der Geometrie in w{\"a}ssriger L{\"o}sung angestellt. F{\"u}r die Geometrie des Molek{\"u}ls an sich ergab sich eine gute {\"U}bereinstimmung f{\"u}r alle Modellsysteme, f{\"u}r die Wechselwirkungen mit der Umgebung jedoch nicht. Die Ausbildung von Dimeren in der Kristallumgebung hat einen stark stablisierenden Effekt und ist einer der Gr{\"u}nde, warum dieser Kristall so gut wie keine Fehlordungen aufweist. In den Proteinkomplexen hingegen ergibt sich eine Abstoßung zwischen dem Inhibitor und einer der katalytisch aktiven Aminos{\"a}uren. Als Ursache f{\"u}r diese Abstoßung konnte die Einf{\"u}hrung der Methylaminfunktion ausgemacht werden. Vermutlicherweise f{\"u}hrt diese strukturelle {\"A}nderung auch dazu, dass der Modellinhibitor nicht in der Lage ist, so wie die Leitstruktur K11777 an das aktive Zentrum des Enzyms zu binden.}, subject = {Theoretische Chemie}, language = {en} } @article{SchmidtFantuzziKlopfetal.2021, author = {Schmidt, Paul and Fantuzzi, Felipe and Klopf, Jonas and Schr{\"o}der, Niklas B. and Dewhurst, Rian D. and Braunschweig, Holger and Engel, Volker and Engels, Bernd}, title = {Twisting versus delocalization in CAAC- and NHC-stabilized boron-based biradicals: the roles of sterics and electronics}, series = {Chemistry - A European Journal}, volume = {27}, journal = {Chemistry - A European Journal}, number = {16}, doi = {10.1002/chem.202004619}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-256636}, pages = {5160-5170}, year = {2021}, abstract = {Twisted boron-based biradicals featuring unsaturated C\(_2\)R\(_2\) (R=Et, Me) bridges and stabilization by cyclic (alkyl)(amino)carbenes (CAACs) were recently prepared. These species show remarkable geometrical and electronic differences with respect to their unbridged counterparts. Herein, a thorough computational investigation on the origin of their distinct electrostructural properties is performed. It is shown that steric effects are mostly responsible for the preference for twisted over planar structures. The ground-state multiplicity of the twisted structure is modulated by the σ framework of the bridge, and different R groups lead to distinct multiplicities. In line with the experimental data, a planar structure driven by delocalization effects is observed as global minimum for R=H. The synthetic elusiveness of C\(_2\)R\(_2\)-bridged systems featuring N-heterocyclic carbenes (NHCs) was also investigated. These results could contribute to the engineering of novel main group biradicals.}, language = {en} } @article{SchleierReuschLummeletal.2019, author = {Schleier, Domenik and Reusch, Engelbert and Lummel, Lisa and Hemberger, Patrick and Fischer, Ingo}, title = {Threshold photoelectron spectroscopy of IO and IOH}, series = {ChemPhysChem}, volume = {20}, journal = {ChemPhysChem}, number = {19}, doi = {10.1002/cphc.201900813}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-204751}, pages = {2413-2416}, year = {2019}, abstract = {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.}, language = {en} } @phdthesis{Schleier2021, author = {Schleier, Domenik}, title = {Using Photoionization to Investigate Reactive Boron Species and the Kinetics of Hydrocarbon Radicals}, doi = {10.25972/OPUS-24213}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-242137}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2021}, abstract = {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.}, subject = {Biradikal}, language = {en} } @phdthesis{Sauer2021, author = {Sauer, Susanne}, title = {Implementation and Application of QM/MM Hybrid Methods}, doi = {10.25972/OPUS-24321}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-243213}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2021}, abstract = {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.}, subject = {Quantenmechanik}, language = {en} } @article{SaalfrankFantuzziKupferetal.2020, author = {Saalfrank, Christian and Fantuzzi, Felipe and Kupfer, Thomas and Ritschel, Benedikt and Hammond, Kai and Krummenacher, Ivo and Bertermann, R{\"u}diger and Wirthensohn, Raphael and Finze, Maik and Schmid, Paul and Engel, Volker and Engels, Bernd and Braunschweig, Holger}, title = {cAAC-Stabilized 9,10-diboraanthracenes—Acenes with Open-Shell Singlet Biradical Ground States}, series = {Angewandte Chemie International Edition}, volume = {59}, journal = {Angewandte Chemie International Edition}, number = {43}, doi = {10.1002/anie.202008206}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-217795}, pages = {19338 -- 19343}, year = {2020}, abstract = {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.}, language = {en} }