Institut für Physikalische und Theoretische Chemie
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In the present report, well-defined WO3 nanorods (NRs) and a rGO–WO\(_3\) composite were successfully synthesized using a one-pot hydrothermal method. The crystal phase, structural morphology, shape, and size of the as-synthesized samples were studied using X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements. The optical properties of the synthesized samples were investigated by Raman, ultraviolet-visible (UV-Vis) and photoluminescence (PL) spectroscopy. Raman spectroscopy and TEM results validate the formation of WO\(_3\) (NRs) on the rGO sheet. The value of the dielectric constant (ε′) of WO3 NRs and rGO–WO\(_3\) composite is decreased with an increase in frequency. At low frequency (2.5 to 3.5 Hz), the value of ε′ for the rGO–WO3 composite is greater than that of pure WO\(_3\) NRs. This could be due to the fact that the induced charges follow the ac signal. However, at higher frequency (3.4 to 6.0), the value of ε′ for the rGO–WO\(_3\) composite is less compared to that of the pure WO3 NRs. The overall decrease in the value of ε′ could be due to the occurrence of a polarization process at the interface of the rGO sheet and WO3 NRs. Enhanced interfacial polarization in the rGO–WO\(_3\) composite is observed, which may be attributed to the presence of polar functional groups on the rGO sheet. These functional groups trap charge carriers at the interface, resulting in an enhancement of the interfacial polarization. The value of the dielectric modulus is also calculated to further confirm this enhancement. The values of the ac conductivity of the WO\(_3\) NRs and rGO–WO\(_3\) composite were calculated as a function of the frequency. The greater value of the ac conductivity in the rGO–WO\(_3\) composite compared to that of the WO\(_3\) NRs confirms the restoration of the sp:\(^{++}\) network during the in situ synthesis of the rGO–WO\(_3\) composite, which is well supported by the results obtained by Raman spectroscopy.
Radiationless energy transfer is at the core of diverse phenomena, such as light harvesting in photosynthesis\(^1\), energy-transfer-based microspectroscopies\(^2\), nanoscale quantum entanglement\(^3\) and photonic-mode hybridization\(^4\). Typically, the transfer is efficient only for separations that are much shorter than the diffraction limit. This hampers its application in optical communication and quantum information processing, which require spatially selective addressing. Here, we demonstrate highly efficient radiationless coherent energy transfer over a distance of twice the excitation wavelength by combining localized and delocalized\(^5\) plasmonic modes. Analogous to the Tavis-Cummings model, two whispering-gallery-mode antennas\(^6\) placed in the foci of an elliptical plasmonic cavity\(^7\) fabricated from single-crystal gold plates act as a pair of oscillators coupled to a common cavity mode. Time-resolved two-photon photoemission electron microscopy (TR 2P-PEEM) reveals an ultrafast long-range periodic energy transfer in accordance with the simulations. Our observations open perspectives for the optimization and tailoring of mesoscopic energy transfer and long-range quantum emitter coupling.
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.
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.
Most proteins work in aqueous solution and the interaction with water strongly affects their structure and function. However, experimentally the motion of a specific single water molecule is difficult to trace by conventional methods, because they average over the heterogeneous solvation structure of bulk water surrounding the protein. Here, we provide a detailed atomistic picture of the water rearrangement dynamics around the –CONH– peptide linkage in the two model systems formanilide and acetanilide, which simply differ by the presence of a methyl group at the peptide linkage. The combination of picosecond pump–probe time-resolved infrared spectroscopy and molecular dynamics simulations demonstrates that the solvation dynamics at the molecular level is strongly influenced by this small structural difference. The effective timescales for solvent migration triggered by ionization are mainly controlled by the efficiency of the kinetic energy redistribution rather than the shape of the potential energy surface. This approach provides a fundamental understanding of protein hydration and may help to design functional molecules in solution with tailored properties.
Immature or semi-mature dendritic cells (DCs) represent tolerogenic maturation stages that can convert naive T cells into Foxp3\(^{+}\) induced regulatory T cells (iTreg). Here we found that murine bone marrow-derived DCs (BM-DCs) treated with cholera toxin (CT) matured by up-regulating MHC-II and costimulatory molecules using either high or low doses of CT (CT\(^{hi}\), CT\(^{lo}\)) or with cAMP, a known mediator CT signals. However, all three conditions also induced mRNA of both isoforms of the tolerogenic molecule cytotoxic T lymphocyte antigen 2 (CTLA-2α and CTLA-2β). Only DCs matured under CT\(^{hi}\) conditions secreted IL-1β, IL-6 and IL-23 leading to the instruction of Th17 cell polarization. In contrast, CT\(^{lo}\)- or cAMP-DCs resembled semi-mature DCs and enhanced TGF-β-dependent Foxp3\(^{+}\) iTreg conversion. iTreg conversion could be reduced using siRNA blocking of CTLA-2 and reversely, addition of recombinant CTLA-2α increased iTreg conversion in vitro. Injection of CT\(^{lo}\)- or cAMP-DCs exerted MOG peptide-specific protective effects in experimental autoimmune encephalomyelitis (EAE) by inducing Foxp3\(^{+}\) Tregs and reducing Th17 responses. Together, we identified CTLA-2 production by DCs as a novel tolerogenic mediator of TGF-β-mediated iTreg induction in vitro and in vivo. The CT-induced and cAMP-mediated up-regulation of CTLA-2 also may point to a novel immune evasion mechanism of Vibrio cholerae.
We present a joint experimental and computational study of the nonradiative deactivation of the benzyl radical, C\(_7\)H\(_7\) after UV excitation. Femtosecond time-resolved photoelectron imaging was applied to investigate the photodynamics of the radical. The experiments were accompanied by excited state dynamics simulations using surface hopping. Benzyl has been excited at 265 nm into the D-band (\(\pi\pi^*\)) and the dynamics was probed using probe wavelengths of 398 nm or 798 nm. With 398 nm probe a single time constant of around 70-80 fs was observed. When the dynamics was probed at 798 nm, a second time constant \(\tau_2\)=1.5 ps was visible. It is assigned to further non-radiative deactivation to the lower-lying D\(_1\)/D\(_2\) states.
Describing the light-to-energy conversion in OSCs requires a multiscale understanding of the involved optoelectronic processes, i.e., an understanding from the molecular, intermolecular, and aggregate perspective. This thesis presents such a multiscale description to provide insight into the processes in the vicinity of the organic::organic interface, which are crucial for the overall performance of OSCs. Light absorption, exciton diffusion, photoinduced charge transfer at the donor-acceptor interface, and charge separation are included. In order to establish structure-property relationships, a variety of different molecular p-type semiconductors are combined at the organic donor-acceptor heterojunction with fullerene C60, one of the most common acceptors in OSCs. Starting with a comprehensive analysis of the accuracy of diverse ab initio, DFT, and semiempiric methods for the properties of the individual molecules, the intermolecular, and aggregate/device stage are subsequently addressed. At all stages, both methodological concepts and physical aspects in OSCs are discussed to extend the microscopic understanding of the charge generation processes.
In der vorliegenden Arbeit wurde erfolgreich eine neue Gasphasen-Apparatur für
Photoelektronen-Imaging-Experimente simuliert, aufgebaut und in Verbindung mit einem ps-Lasersystem in Betrieb genommen.
Neben dem Aufbau der Apparatur stand die Aufklärung der Dynamik angeregter Zustände von aromatischen Heterocyclen und Pyrenen im Fokus dieser Arbeit. Die untersuchten Moleküle wurden durch Resonanzverstärkte Mehrphotonenionisation in einem Molekularstrahlexperiment sowohl zeit-, als auch frequenzaufgelöst untersucht.
In this thesis the excited-state dynamics of radicals and biradicals were characterized with femtosecond pump-probe spectroscopy.
These open-shell molecules play important roles as combustion intermediates, in the formation of soot and polycyclic aromatic hydrocarbons, in atmospheric chemistry and in the formation of complex molecules in the interstellar medium and galactic clouds. In these processes molecules frequently occur in some excited state, excited either by thermal energy or radiation. Knowledge of the reactivity and dynamics of these excited states completes our understanding of these complex processes.
These highly reactive molecules were produced via pyrolysis from suitable precursors and examined in a molecular beam under collision-free conditions. A first laser now excites the molecule, and a second laser ionizes it. Time-of-flight mass spectrometry allowed a first identification of the molecule, photoelectron spectroscopy a complete characterization of the molecule - under the condition that the mass spectrum was dominated by only one mass. The photoelectron spectrum was obtained via velocity-map imaging, providing an insight in the electronic states involved. Ion velocity map imaging allowed separation of signal from direct ionization of the radical in the molecular beam and dissociative photoionization of the precursor. During this thesis a modified pBasex algorithm was developed and implemented in python, providing an image inversion tool without interpolation of data points. Especially for noisy photoelectron images this new algorithm delivers better results.
Some highlighted results:
• The 2-methylallyl radical was excited in the ππ*-state with different internal energies using three different pump wavelengths (240.6 , 238.0 and 236.0 nm). Ionized with 800 nm multi-photon probe, the photoelectron spectra shows a s-Rydberg fingerprint spectrum, a highly positive photoelectron anisotropy of 1.5 and a bi-exponential decay ( τ1= 141\pm43 fs, τ2= 4.0\pm0.2 ps for 240.6 nm pump), where the second time-constant shortens for lower wavelengths. Field-induced surface hopping dynamics calculations confirm that the initially excited ππ*-state relaxes very fast to an s-Rydberg state (first experimentally observed time-constant), and then more slowly to the first excited state/ground state (second time-constant). With higher excitation energies the conical intersection between the s-Rydberg-state and the first excited state is reached faster, resulting in shorter life-times.
• The benzyl radical was excited yith 265 nm and probed with two wavelengths, 798 nm and 398 nm. Probed with 798 nm it shows a bi-exponential decay (\tau_{1}=84\pm5 fs, \tau_{2}=1.55\pm0.12 ps), whereas with 398 nm probe only the first time-constant is observed (\tau_{1}=89\pm5 fs). The photoelectron spectra with 798 nm probe is comparable to the spectrum with 398 nm probe during the first 60 fs, at longer times an additional band appears. This band is due to a [1+3']-process, whereas with 398 nm only signal from a [1+1']-process can be observed. Non-adiabatic dynamic on the fly calculations show that the initially excited, nearly degenerate ππ/p-Rydberg-states relax very fast (first time-constant) to an s-Rydberg state. This s-Rydberg state can no longer be ionized with 398 nm, but with 798 nm ionization via intermediate resonances is still possible. The s-Rydberg state then decays to the first excited state (second time-constant), which is long-lived.
• Para-xylylene, excited with 266 nm into the S2-state and probed with 800 nm, shows a bi-exponential decay (\tau_{1}=38\pm7 fs, \tau_{2}=407\pm9 fs). The initially excited S2-state decays quickly to S1-state, which shows dissociative photoionization. The population of the S1-state is directly visible in the masses of the dissociative photoionization products, benzene and the para-xylylene -H.
• Ortho-benzyne, produced via pyrolysis from benzocyclobutendione, was excited with 266 nm in the S2 state and probed with 800 nm. In its time-resolved mass spectra the dynamic of the ortho-benzyne signal was superposed with the dynamics from dissociative photoionization of the precursor and of the ortho-benzyne-dimer. With time-resolved ion imaging gated on the ortho-benzyne these processes could be seperated, showing that the S2-state of ortho-benzyne relaxes within 50 fs to the S1-state.