Triplet-triplet annihilation upconversion is a bimolecular process converting low-energy photons into high-energy photons. Here, we report a calcium-sensing system working via triplet-triplet... Show moreTriplet-triplet annihilation upconversion is a bimolecular process converting low-energy photons into high-energy photons. Here, we report a calcium-sensing system working via triplet-triplet annihilation (TTA) upconverted emission. The probe itself was obtained by covalent conjugation of a blue emitter, perylene, with a calcium-chelating moiety, and it was sensitized by the red-light-absorbing photosensitizer palladium(II) tetraphenyltetrabenzoporphyrin (PdTPTBP). Sensing was selective for Ca2+ and occurred in the micromolar domain. In deoxygenated conditions, the TTA upconverted luminescence gradually appeared upon adding an increasing concentration of calcium ions, to reach a maximum upconversion quantum yield of 0.0020. Show less
Cottom, J.P.; Hückmann, L.; Olsson, E.; Meyer, J. 2024
Most single-molecule studies derive the kinetic rates of native, intermediate, and unfolded states from equilibrium hopping experiments. Here, we apply the Kramers kinetic diffusive model to derive... Show moreMost single-molecule studies derive the kinetic rates of native, intermediate, and unfolded states from equilibrium hopping experiments. Here, we apply the Kramers kinetic diffusive model to derive the force-dependent kinetic rates of intermediate states from nonequilibrium pulling experiments. From the kinetic rates, we also extract the force-dependent kinetic barriers and the equilibrium folding energies. We apply our method to DNA hairpins with multiple folding pathways and intermediates. The experimental results agree with theoretical predictions. Furthermore, the proposed nonequilibrium single-molecule approach permits us to characterize kinetic and thermodynamic properties of native, unfolded, and intermediate states that cannot be derived from equilibrium hopping experiments. Show less
With its high stability and well-tuned binding strength for adsorbates, platinum is an excellent catalyst for a wide range of reactions. In applications like car exhaust purification, the oxidation... Show moreWith its high stability and well-tuned binding strength for adsorbates, platinum is an excellent catalyst for a wide range of reactions. In applications like car exhaust purification, the oxidation of hydrocarbons, and fuel cells, platinum is exposed to highly oxidizing conditions, which often leads to the formation of surface oxides. To reveal the structure of these surface oxides, the oxidation of Pt in O-2 has been widely studied. However, in most applications, H2O is also an important or even dominant part of the reaction mixture. Here, we investigate the interaction of H2O with Pt surface oxides using near-ambient-pressure X-ray photoelectron spectroscopy. We find that reversible hydroxylation readily occurs in H2O/O-2 mixtures. Using time-resolved measurements, we show that O-OH exchange occurs on a time scale of seconds. Show less
The understanding of photoinduced ligand exchange mechanisms in polypyridyl ruthenium(II) complexes operating in aqueous solution is of crucial importance to rationalize their photoreactivity.... Show moreThe understanding of photoinduced ligand exchange mechanisms in polypyridyl ruthenium(II) complexes operating in aqueous solution is of crucial importance to rationalize their photoreactivity. Herein, we demonstrate that a synergetic use of ab initio molecular dynamics simulations and static calculations, both conducted at the DFT level, can provide a full understanding of photosubstitution mechanisms of a monodentate ligand by a solvent water molecule in archetypal ruthenium complexes in explicit water. The simulations show that the photoinduced loss of a monodentate ligand generates an unreactive 16-electron species in a hitherto undescribed pentacoordinated triplet excited state that converts, via an easily accessible crossing point, to a reactive 16-electron singlet ground state, which combines with a solvent water molecule to yield the experimentally observed aqua complex in less than 10 ps. This work paves the way for the rational design of novel photoactive metal complexes relevant for biological applications. Show less
Recently, the bulk electrooxidation of CO on gold or platinum has been used to detect CO produced during CO2 reduction in neutral media. The CO bulk oxidation voltammetry may show two distinct... Show moreRecently, the bulk electrooxidation of CO on gold or platinum has been used to detect CO produced during CO2 reduction in neutral media. The CO bulk oxidation voltammetry may show two distinct peaks depending on the reaction conditions, which up to now have not been understood. We have used scanning electrochemical microscopy (SECM) to probe CO oxidation and pH in the diffusion layer during CO2 reduction. Our results show that the two different peaks are due to diffusion limitation by two different species, namely, CO and OH–. We find that between pH 7 and 11, CO oxidation by water and OH– gives rise to the first and second peak observed in the voltammetry, respectively. Additional rotating disc experiments showed that specifically in this pH range the current of the second peak is diffusion limited by the OH– concentration, since it is lower than the CO concentration. Show less
Gas-phase IR–UV double-resonance laser spectroscopy is an IR absorption technique that bridges the gap between experimental IR spectroscopy and theory. The IR experiments are used to directly... Show moreGas-phase IR–UV double-resonance laser spectroscopy is an IR absorption technique that bridges the gap between experimental IR spectroscopy and theory. The IR experiments are used to directly evaluate predicted frequencies and potential energy surfaces as well as to probe the structure of isolated molecules. However, a detailed understanding of the underlying mechanisms is, especially in the far-IR regime, still far from complete, even though this is crucial for properly interpreting the recorded IR absorption spectra. Here, events occurring upon excitation to vibrational levels of polycyclic aromatic hydrocarbons by far-IR radiation from the FELIX free electron laser are followed using resonance-enhanced multiphoton ionization spectroscopy. These studies provide detailed insight into how ladder climbing and anharmonicity influence IR–UV spectroscopy and therefore the resulting IR signatures in the far-IR region. Moreover, the potential energy surfaces of these low-frequency delocalized modes are investigated and shown to have a strong harmonic character. Show less
Transition metal dichalcogenides (TMDCs) are a type of two-dimensional (2D) material that has been widely investigated by both experimentalists and theoreticians because of their unique properties.... Show moreTransition metal dichalcogenides (TMDCs) are a type of two-dimensional (2D) material that has been widely investigated by both experimentalists and theoreticians because of their unique properties. In the case of cobalt sulfide, density functional theory (DFT) calculations on free-standing S–Co–S sheets suggest there are no stable 2D cobalt sulfide polymorphs, whereas experimental observations clearly show TMDC-like structures on Au(111). In this study, we resolve this disagreement by using a combination of experimental techniques and DFT calculations, considering the substrate explicitly. We find a 2D CoS(0001)-like sheet on Au(111) that delivers excellent agreement between theory and experiment. Uniquely this sheet exhibits a metallic character, contrary to most TMDCs, and exists due to the stabilizing interactions with the Au(111) substrate. Show less
Calorimetric studies on ice II reveal a surprising H2O/D2O isotope effect. While the ice II to ice Ic transition is endothermic for H2O, it is exothermic for D2O samples. The transition enthalpies... Show moreCalorimetric studies on ice II reveal a surprising H2O/D2O isotope effect. While the ice II to ice Ic transition is endothermic for H2O, it is exothermic for D2O samples. The transition enthalpies are +40 and −140 J/mol, respectively, where such a sign change upon isotope substitution is unprecedented in ice research. To understand the observations we employ force field calculations using two water models known to perform well for H2O ice phases and their vibrational properties. These simulations reveal that the isotope effect can be traced back to zero-point energy. q-TIP4P/F fares better and is able to account for approximately three-fourths of the isotope effect, while MB-pol only catches approximately one-third. Phonon and configurational entropy contributions are necessary to predict reasonable transition enthalpies, but they do not have an impact on the isotope effect. We suggest to use these calorimetric isotope data as a benchmark for water models. Show less
Prabhu M.K., Boden D., Rost M.J., Meyer J., Groot I.M.N. 2020
The O–O bond formation process via water nucleophilic attack represents a thermodynamic and kinetic bottleneck in photocatalytic water oxidation because of the considerably high activation free... Show moreThe O–O bond formation process via water nucleophilic attack represents a thermodynamic and kinetic bottleneck in photocatalytic water oxidation because of the considerably high activation free energy barrier. It is therefore of fundamental significance and yet challenging to find strategies to facilitate this reaction. The microscopic details of the photocatalytic water oxidation step involving the O–O bond formation in a catalyst–dye supramolecular complex are here elucidated by density functional theory-based Car–Parrinello molecular dynamics simulations in the presence of an extra proton acceptor. Introducing a proton acceptor group (OH–) in the hydration shell near the catalytic active site accelerates the rate-limiting O–O bond formation by inducing a cooperative event proceeding via a concerted proton-coupled electron-transfer mechanism and thus significantly lowering the activation free energy barrier. The in-depth insight provides a strategy for facilitating the photocatalytic water oxidation and for improving the efficiency of dye-sensitized photoelectrochemical cells. Show less
Electrode surfaces may change their surface structure as a result of the adsorption of chemical species, impacting their catalytic activity. Using density functional theory, we find that the strong... Show moreElectrode surfaces may change their surface structure as a result of the adsorption of chemical species, impacting their catalytic activity. Using density functional theory, we find that the strong adsorption of hydrogen at low electrode potentials promotes the thermodynamics and kinetics of a unique type of roughening of 110-type Pt step edges. This change in surface structure causes the appearance of the so-called "third hydrogen peak" in voltammograms measured on Pt electrodes, an observation that has eluded explanation for over 50 years. Understanding this roughening process is important for structure-sensitive (electro)catalysis and the development of active and stable catalysts. Show less
