54 Chemie
Refine
Year of publication
Document Type
Is part of the Bibliography
- no (21)
Keywords
- Helium nanodroplets (12)
- Electronic spectroscopy (4)
- Anthracene (2)
- Anthracene derivates (2)
- Isolated Molecules (2)
- Thermochemical cycles (2)
- Asymmetric-rotor molecule (1)
- Atomic and molecular spectra (1)
- BODIPY Chromophore (1)
- Bismut (1)
Institute
Microstructure and oxidation kinetics are closely intertwined factors that significantly influence the behavior of materials in oxidative environments. This relationship is of particular importance for redox materials such as
\(Ca_{1-x} Sr_x MnO_{3-\delta}\), where reversible oxygen ions exchange and oxidation state shifts are key to their functionality. In the first study, scanning electron microscope (SEM) was used to examine how varying Sr content affects the morphology and microstructure of \(Ca_{1-x} Sr_x MnO_{3-\delta}\) powder compositions. The results indicate that increasing Sr content leads to smaller particle sizes and improved particle size homogeneity. Granules with Sr concentrations ranging from 0 % to 40 % exhibit notable changes in morphology. However, the microporosity and d50 vary slightly across the samples in a non-monotonic manner, with no clear trend emerging with respect to Sr concentration. The second study investigates how macrostructural forms, such as foams and pellets, impact oxidation kinetics in \(Ca_{0.8} Sr_{0.2} MnO_{3-\delta}\). Parameters including particle size distribution of the raw material, overall microporosity, and structural characteristics of these macrostructures were analyzed for their effect on oxidation rates. Findings reveal that macrostructural configuration, alongside microstructural features like microporosity, significantly impacts oxidation kinetics. These studies collectively underscore the critical relationship between dopant concentration, microstructural characteristics, and structural morphology in determining the oxidative behavior of
\(Ca_{1-x} Sr_x MnO_{3-\delta}\), providing key insights into optimizing material performance in redox environments.
Green hydrogen is expected to play a major role in the energy mix of the future as it is not only an important feedstock for chemical production but also an energy carrier and fuel. For the transition from fossil routes to green production new, sustainable technologies are needed. Solar thermal energy can be directly utilized for hydrogen production without electrification via thermochemical cycles. Several thousands of cycles have already been investigated, the main types utilize redox materials with phase change properties, materials with non-stoichiometric behavior and sulfur-based cycles. This article gives an overview on the different classes of thermochemical cycles, their state of development, economics and their possibilities for a future industrial application.
Emission properties of [Os(phen)2(dppene)]2+ were investigated from 1.4 to 300 K in ethanol, PMMA, and PVK, a matrix which is frequently applied in OLEDs. These data provide the zero-field splitting values of the emitting T1 state and the individual decay times of its substates. The T1 state is assigned to be largely of MLCT character. The splittings change only moderately due to variation of the matrix, whereas the individual decay times are substantially affected. Further, energy transfer from the triplet state of PVK to [Os(phen)2(dppene)]2+ was studied by applying time-resolved emission spectroscopy and is assigned to be dominantly of Dexter type.
Strong oxygen quenching following a Stern−Volmer behavior of Ir(ppy-NPh2)3 demonstrates the high technological potential for oxygen sensing applications. This compound exhibits a phosphorescence quantum yield of 70% and an emission decay time of 4.3 μs in deaerated solution. The electronic structure of the emitting triplet, being responsible for these properties, is investigated.
A pulsed valve connected to a closed-cycle cryostat was optimized for producing helium droplets. The pulsed droplet beam appeared with a bimodal size distribution. The leading part of the pulse consists of droplets suitable for doping with molecules. The average size of this part can be varied between 104 and 106 helium atoms, and the width of the distribution is smaller as compared to a continuous-flow droplet source. The system has been tested in a single pulse mode and at repetition rates of up to 500 Hz with almost constant intensity. The droplet density was found to be increased by more than an order of magnitude as compared to a continuous-flow droplet source.
3-hydroxyflavone is a prototype system for excited state intramolecular proton transfer which is one step of a closed loop photocycle. It was intensively studied for the bare molecule and for the influence of solvents. In the present paper this photocycle is investigated for 3-hydroxyflavone and some hydrated complexes when doped into superfluid helium droplets by the combined measurement of fluorescence excitation spectra and dispersed emission spectra. Significant discrepancies in the proton transfer behavior to gas phase experiments provide evidence for the presence of different complex configurations of the hydrated complexes in helium droplets. Moreover, for bare 3-hydroxyflavone and its hydrated complexes the proton transfer appears to be promoted by the helium environment.
Electronic spectroscopy of molecules profits greatly from superfluid helium droplets serving as a gentle cryogenic matrix. Characteristic features of electronic spectra in helium droplets are a solvent shift, phonon wings, and in rare cases a splitting of zero phonon lines. For the majority of molecules investigated so far in helium droplets the vibrational fine structure in electronic spectra resembles what was observed in a supersonic jet. The electronic spectra of three methylated anthracene derivatives and one phenylated anthracene discussed in this paper reveal remarkable effects in the vibrational fine structure due to solvation in helium droplets.
For all four compounds the vibrational frequencies were almost not affected by the helium environment. However, if the electronic excitation is accompanied by nuclear rearrangement, the spectra showed remarkable line broadening in helium droplets. This is the case for 2-methylanthracene and 9-phenylanthracene. The corresponding line shape was of Lorentzian type and, thus, attributed to damping of the excited system by the helium environment. According to the linewidth the damping time constant was determined to be about 0.3 ps in the case of 2-methylanthracene and 0.1 ps for 9-phenylanthracene.
We present the fluorescence excitation and dispersed emission spectra of the parent compound of the boron dipyrromethene (BODIPY) dye class measured in a supersonic beam and isolated in superfluid helium nanodroplets. The gas‐phase spectrum of the isolated molecules displays many low‐frequency transitions that are assigned to a symmetry‐breaking mode with a strongly nonharmonic potential, presumably the out‐of‐plane wagging mode of the BF2 group. The data are in good agreement with transition energies and Franck–Condon factors calculated for a double minimum potential in the upper electronic state. The corresponding transitions do not appear in the helium droplet. This is explained with the quasi‐rigid first layer of helium atoms attached to the dopant molecule by van der Waals forces. The spectral characteristics are those of a cyanine dye rather than that of an aromatic chromophore.
Electronic spectra of molecules doped into superfluid 4He nanodroplets reveal important details of the microsolvation in superfluid helium. The vibrational fine structure in the electronic spectra of phthalocyanine derivatives and pyrromethene dye molecules doped into superfluid helium droplets have been investigated. Together with previous studies on anthracene derivatives [J. Chem. Phys.2010, 133, 114505] and 3-hydroxyflavone [J. Chem. Phys.2009, 131, 194307], the line shapes vary between two limiting cases, namely, sharp Lorentzians and nonresolved vibrational fine structure. All different spectral signatures are initiated by the same effect, namely, the change of the electron density distribution initiated by the electronic excitation. This change can be quantified by the difference of the electrostatic moments of the molecule in the electronic ground state and the corresponding Franck–Condon point in the excited state. According to the experimental data, electronic spectroscopy suffers from drastic line broadening when accompanied by significant changes of the charge distribution, in particular, changes of the dipole moment. Vice versa, the vibrational fine structure in electronic spectra of molecules doped into helium droplets is highly sensitive to changes of the electron density distribution.
This article reports on the microsolvation of anthracene in superfluid helium nanodroplets as revealed by electronic spectroscopy. Among the polyacene molecules benzene, naphthalene, anthracene, tetracene and pentacene only anthracene and tetracene have been found to exhibit multiplet splitting of the electronic and vibronic transitions which can not be explained by a rotational fine structure. The experimental approach for the investigation of the multiplet is the combined investigation of the fluorescence excitation spectrum and dispersed emission spectra. New experimental data are presented on the microsolvation of anthracene in helium droplets. A detailed analysis of the anthracene data will be contrasted to corresponding data on tetracene. These data together with those reported for benzene, naphthalene and pentacene might serve to test and develop theoretical models which are needed to understand microsolvation of single molecules in superfluid helium droplets.
