Ab initio studies on photorelaxation

This work addresses relaxation mechanisms of photoexcited organic molecules of small and medium size, up to 62 atoms. For most systems it is investigated theoretically, how modifications, often in the form of substituents, influence the decay processes. The research in large parts is done in close collaboration with groups providing experimental data, which allows to formulate robust hypotheses and models. Four systems are discussed in this context. We find the formation of the dewar lesion in deoxyribonucleic acid (DNA) to only occur, when the nucleobase is embedded in the DNA backbone, which sterically hinders accessing alternative channels. Substituting hydroxy groups at certain points of thioindigo is shown to open up an efficient deactivation channel via excited state intramolecular proton transfer, and greatly enhance the photostability of the molecule. By substituting electron donating groups to the stilbene moiety of the hemithioindigo photoswitch and correlating their effect to their Hammett parameters, the isomerization speed of hemithioindigo is optimized. And lastly, when adding an aldehyde group to furan, an additional pathway is found for its derivatives furfural and β-furfural. Their relaxation is slowed down regardless. The effects on the excited state potential energy surfaces are described as general means, by which the surfaces can be influenced, and likely can be translated to other molecules as well. This eventually allows to predict properties and tailor molecules to yield desired behavior. In this context, for example for furan, furfural and β-furfural the structural implications of the aldehyde substituent on one conical intersection are deducted from the extended two-electron two-orbital model prior to any calculations or experiments. Alongside the system specific investigations, an interface for the on-the-fly dynamics package NewtonX to the quantum chemistry package Molpro was programmed. Non-adiabatic semiclassical on-the-fly dynamics are a powerful tool to simulate complete relaxation processes without constraints in the dimensionality. For the interface, which in its primary setup uses complete active space self consistent field theory calculations, a number of features has been implemented. Most notably, it enables non-adiabtatic dynamics on complete active space perturbation and ONIOM level of theory

Ab initio studies on photorelaxation

This work addresses relaxation mechanisms of photoexcited organic molecules of small and medium size, up to 62 atoms. For most systems it is investigated theoretically, how modifications, often in the form of substituents, influence the decay processes. The research in large parts is done in close collaboration with groups providing experimental data, which allows to formulate robust hypotheses and models. Four systems are discussed in this context. We find the formation of the dewar lesion in deoxyribonucleic acid (DNA) to only occur, when the nucleobase is embedded in the DNA backbone, which sterically hinders accessing alternative channels. Substituting hydroxy groups at certain points of thioindigo is shown to open up an efficient deactivation channel via excited state intramolecular proton transfer, and greatly enhance the photostability of the molecule. By substituting electron donating groups to the stilbene moiety of the hemithioindigo photoswitch and correlating their effect to their Hammett parameters, the isomerization speed of hemithioindigo is optimized. And lastly, when adding an aldehyde group to furan, an additional pathway is found for its derivatives furfural and β-furfural. Their relaxation is slowed down regardless. The effects on the excited state potential energy surfaces are described as general means, by which the surfaces can be influenced, and likely can be translated to other molecules as well. This eventually allows to predict properties and tailor molecules to yield desired behavior. In this context, for example for furan, furfural and β-furfural the structural implications of the aldehyde substituent on one conical intersection are deducted from the extended two-electron two-orbital model prior to any calculations or experiments. Alongside the system specific investigations, an interface for the on-the-fly dynamics package NewtonX to the quantum chemistry package Molpro was programmed. Non-adiabatic semiclassical on-the-fly dynamics are a powerful tool to simulate complete relaxation processes without constraints in the dimensionality. For the interface, which in its primary setup uses complete active space self consistent field theory calculations, a number of features has been implemented. Most notably, it enables non-adiabtatic dynamics on complete active space perturbation and ONIOM level of theory

Substituent effects on the relaxation dynamics of furan, furfural and β-furfural: a combined theoretical and experimental approach

For the series furan, furfural and β-furfural we investigated the effect of substituents and their positioning on the photoinduced relaxation dynamics in a combined theoretical and experimental approach. Using time resolved photoelectron spectroscopy with a high intensity probe pulse, we can, for the first time, follow the whole deactivation process of furan through a two photon probe signal. Using the extended 2-electron 2-orbital model [Nenov et al., J. Chem. Phys., 2011, 135, 034304] we explain the formation of one central conical intersection and predict the influence of the aldehyde group of the derivatives on its geometry. This, as well as the relaxation mechanisms from photoexcitation to the final outcome was investigated using a variety of theoretical methods. Complete active space self consistent field was used for on-the-fly calculations while complete active space perturbation theory and coupled cluster theory were used to accurately describe critical configurations. Experiment and theory show the relaxation dynamics of furfural and β-furfural to be slowed down, and together they disclose an additional deactivation pathway, which is attributed to the nO lonepair state introduced with the aldehyde group

Monitoring conical intersections in the ring opening of furan by attosecond stimulated X-ray Raman spectroscopy

Attosecond X-ray pulses are short enough to capture snapshots of molecules undergoing nonadiabatic electron and nuclear dynamics at conical intersections (CoIns). We show that a stimulated Raman probe induced by a combination of an attosecond and a femtosecond pulse has a unique temporal and spectral resolution for probing the nonadiabatic dynamics and detecting the ultrafast (similar to 4.5 fs) passage through a CoIn. This is demonstrated by a multiconfigurational self-consistent-field study of the dynamics and spectroscopy of the furan ring-opening reaction. Trajectories generated by surface hopping simulations were used to predict Attosecond Stimulated X-ray Raman Spectroscopy signals at reactant and product structures as well as representative snapshots along the conical intersection seam. The signals are highly sensitive to the changes in nonadiabatically coupled electronic structure and geometry. (C) 2015 Author(s)

Two-stage framework for a topology-based projection and visualization of classified document collections

Figure 1: Island-like visualization of a document point cloud’s topological structure. By sharing similar dimensions, documents accumulate in subspaces of the high dimensional information space. Considering dimensions as words, clusters are assumed to describe topics, i.e., islands, in the final visualization. During the last decades, electronic textual information has become the world’s largest and most important information source available. People have added a variety of daily newspapers, books, scientific and governmental publications, blogs and private messages to this wellspring of endless information and knowledge. Since neither the existing nor the new information can be read in its entirety, computers are used to extract and visualize meaningful or interesting topics and documents from this huge information clutter. In this paper, we extend, improve and combine existing individual approaches into an overall framework that supports topological analysis of high dimensional document point clouds given by the well-known tf-idf document-term weighting method. We show that traditional distance-based approaches fail in very high dimensional spaces, and we describe an improved two-stage method for topology-based projections from the original high dimensional information space to both two dimensional (2-D) and three dimensional (3-D) visualizations. To show the accuracy and usability of this framework, we compare it to methods introduced recently and apply it to complex document and patent collections

Photoinduced Heterocyclic Ring Opening of Furfural: Distinct Open-Chain Product Identification by Ultrafast X-ray Transient Absorption Spectroscopy.

The ultraviolet-induced photochemistry of five-membered heterocyclic rings often involves ring opening as a prominent excited-state relaxation pathway. The identification of this particular photoinduced mechanism, however, presents a challenge for many experimental methods. We show that femtosecond X-ray transient absorption spectroscopy at the carbon K-edge (∼284 eV) provides core-to-valence spectral fingerprints that enable the unambiguous identification of ring-opened isomers of organic heterocycles. The unique differences in the electronic structure between a carbon atom bonded to the oxygen in the ring versus a carbon atom set free of the oxygen in the ring-opened product are readily apparent in the X-ray spectra. Ultrafast ring opening via C-O bond fission occurs within ∼350 fs in 266-nm photoexcited furfural, as evidenced by fingerprint core (carbon 1s) electronic transitions into a nonbonding orbital of the open-chain carbene intermediate at 283.3 eV. The lack of recovery of the 1sπ* ground-state depletion in furfural at 286.4 eV indicates that internal conversion to the ground state is a minor channel. These experimental results, augmented by recent advances in the generation of isolated attosecond pulses at the carbon K-edge, will pave the way for probing ring-opened conical intersection dynamics in the future

Making fast photoswitches faster - Using Hammett analysis to understand the limit of donor-acceptor approaches for faster Hemithioindigo photoswitches

Hemithioindigo (HTI) photoswitches have a tremendous potential for biological and supramolecular applications due to their absorptions in the visible-light region in conjunction with ultrafast photoisomerization and high thermal bistability. Rational tailoring of the photophysical properties for a specific application is the key to exploit the full potential of HTIs as photoswitching tools. Herein we use time-resolved absorption spectroscopy and Hammett analysis to discover an unexpected principal limit to the photoisomerization rate for donor-substituted HTIs. By using stationary absorption and fluorescence measurements in combination with theoretical investigations, we offer a detailed mechanistic explanation for the observed rate limit. An alternative way of approaching and possibly even exceeding the maximum rate by multiple donor substitution is demonstrated, which give access to the fastest HTI photoswitch reported to date. An unexpected principal limit to the photoisomerization rate for donor-substituted hemithioindigos (HTIs; see figure) has been discovered; this provides a quantitative estimate for the highest possible photoisomerization rate. A mechanistic explanation for the observed limit is offered together with an alternative way of approaching the maximum rate by multiple donor substitution. This approach gave access to the fastest HTI photoswitch reported to date

Monitoring conical intersections in the ring opening of furan by attosecond stimulated X-ray Raman spectroscopy

Attosecond X-ray pulses are short enough to capture snapshots of molecules undergoing nonadiabatic electron and nuclear dynamics at conical intersections (CoIns). We show that a stimulated Raman probe induced by a combination of an attosecond and a femtosecond pulse has a unique temporal and spectral resolution for probing the nonadiabatic dynamics and detecting the ultrafast (∼4.5 fs) passage through a CoIn. This is demonstrated by a multiconfigurational self-consistent-field study of the dynamics and spectroscopy of the furan ring-opening reaction. Trajectories generated by surface hopping simulations were used to predict Attosecond Stimulated X-ray Raman Spectroscopy signals at reactant and product structures as well as representative snapshots along the conical intersection seam. The signals are highly sensitive to the changes in nonadiabatically coupled electronic structure and geometry

Monitoring conical intersections in the ring opening of furan by attosecond stimulated X-ray Raman spectroscopy

Attosecond X-ray pulses are short enough to capture snapshots of molecules undergoing nonadiabatic electron and nuclear dynamics at conical intersections (CoIns). We show that a stimulated Raman probe induced by a combination of an attosecond and a femtosecond pulse has a unique temporal and spectral resolution for probing the nonadiabatic dynamics and detecting the ultrafast (∼4.5 fs) passage through a CoIn. This is demonstrated by a multiconfigurational self-consistent-field study of the dynamics and spectroscopy of the furan ring-opening reaction. Trajectories generated by surface hopping simulations were used to predict Attosecond Stimulated X-ray Raman Spectroscopy signals at reactant and product structures as well as representative snapshots along the conical intersection seam. The signals are highly sensitive to the changes in nonadiabatically coupled electronic structure and geometry