Jonas Kind

Evaporation of sessile multicomponent droplets is ubiquitous in many common processes, such as printing, cooling, and coating. The evaporation rate is not evenly distributed across the surface of the droplet. As a result, there are regions with high evaporation rates in which the more volatile part evaporates preferentially; this can induce local surface tension gradients, which, in combination with density differences, can lead to flows within the droplet originating from local concentration gradients. Current experimental methods are based on the addition of markers that can alter liquid properties. Thus, marker-free experimental evidence for the concentration fields proposed is needed. In this work, we use Raman imaging and MRI to quantify concentration gradients in 4.2 μL droplets of 90 mol% water and 10 mol% glycerol. MRI concentration maps with 33 μm resolution enable the investigation of local concentrations as close as 100 μm to the 3-phase contact line. Raman imaging allows even higher resolution and longer observation times. The results of both methods are in excellent agreement. In accordance with the simulations, an increasing glycerol content close to the 3-phase contact line was found. Close to the droplet apex, the glycerol content decreased. The horizontal and vertical concentration gradients were on the order of 1 × 10 ⁻² mol%μm ⁻¹ . These findings can be used for the development and optimization of inks, medical diagnostic devices, food processing procedures, 3D bioprinting, and many more; they might provide the experimental concentration fields sought for the optimization of simulations.

The investigation of reaction mechanisms is a complex task that usually requires the use of several techniques. To obtain as much information as possible on the reaction and any intermediates – possibly invisible to one technique – the combination of techniques is a solution. In this work we present a new setup for combined UV/Vis and NMR spectroscopy and compare it to an established alternative. The presented approach allows a versatile usage of different commercially‐available components like mirrors and fiber bundles as well as different fixed pathlengths according to double transmission or single transmission measurements. While a previous approach is based on a dip‐probe setup for conventional NMR probes, the new one is based on a micro‐Helmholtz coil array (LiquidVoxel™). This makes the use of rectangular cuvettes possible, which ensure well‐defined pathlengths allowing for quantification of species. Additionally, very low quantities of compound can be analyzed due to the microfabrication and small cuvette size used. As proof‐of‐principle this new setup for combined UV/Vis and NMR spectroscopy is used to examine a well‐studied photochromic system of the dithienylethene compound class. A thorough comparison of the pros and cons of the two setups for combined UV/Vis and NMR measurements is performed.

Obtaining understanding of a photochemical reaction relies on the observation, identification and quantification of the compounds involved. The photochemical properties of the individual components are of particular importance, and their determination, however, is not always trivial. This is also true for the quantitative measure on the ability to absorb light, the extinction coefficient ε_i if more than one species i is present and two or more species absorb light of the same wavelength. In this work, it is demonstrated how pure component spectra can be obtained with a simple combination of successive and repeated ex situ illumination, UV/Vis and NMR spectroscopy. From the complementary information accessible, the wavelength-dependent extinction coefficients of all species can be calculated yielding the pure component spectra. A comparison with published data shows excellent agreement and thus proves that this approach is highly reliable. Graphical abstract

The Pd-catalyzed allylic alkylation of 3,4-disubstituted, racemic cyclobutene electrophiles exhibits a highly unusual stereoselectivity that allows for controlling diastereo- and enantioselectivity only by the choice of ligand and independent of the configuration of the substrate. In order to shed light on the origin of stereoinduction, we performed a systematic mechanistic investigation, including preparation of various putative Pd-allyl intermediates, ¹H/³¹P NMR reaction monitoring, ²H-labeling studies, ESI-HRMS and ³¹P NMR analysis of reaction mixtures, and DFT structural computations. The mechanism disclosed exhibits several steps with stereospecificities deviating from the commonly accepted "double inversion rule": oxidative addition was found to follow a stereoconvergent course, giving anti-configured η¹-Pd-cyclobutene species as detectable on-cycle intermediates irrespective of the configuration of starting material, while the subsequent nucleophilic attack features a stereodivergent behavior. In stark contrast to their highly reactive anti-analogues, syn-Pd-cyclobutene complexes that can be formed as side products are rendered entirely unreactive by strong internal Pd-O chelation, preventing the formation of undesired product diastereomers.

Significance: Imagine you spill your drink and miss some spots when cleaning up. The next morning you notice that the stains look quite different on different surfaces. What has happened? In droplets of liquid mixtures, the components evaporate at different rates, which leads to gradients in concentration and surface tension. These gradients can cause, for example, so-called Marangoni flows, which in turn affect the evaporation process. To better understand evaporation-induced liquid flows, the concentration gradients have to be measured without disturbing the liquid. Marker molecules might be surface-active or even may affect the evaporation process. We report here on marker-free and contactless measurements of concentrations by spatially resolved Raman and NMR spectroscopy in evaporating binary droplets.

Photochromic compounds comprising small molecules and polymers have been shown to be of great interest in the field of stimuli-responsive materials for applications as sensors and (nano-) devices. In this regard dithienylethenes (DTEs) have been proven to be especially valuable because of their tolerance to chemicals and their good stability making them tunable and reversibly switchable between an open and a closed structure featuring different optical properties. By utilizing a potent in situ irradiation NMR spectroscopy approach, it is possible to determine the photochromic switching capability of two DTE derivatives in solution leading to crucial information about the structures, the equilibria of the chemical ring-opening and closing of the DTEs as well as the longtime stability of these molecular switches. In particular the perfluorinated thioether bearing compound shows remarkable stability with little fatigue related to the irreversibly cyclized byproduct known for DTEs. The thioether-bearing DTEs can be deprotected and used for immobilization on gold substrates which are characterized by water contact angle measurements prior to and after DTE functionalization. Finally, the electrochemically induced switching capability of the perhydro DTE is investigated by cyclic voltammetry proving its fast, quantitative and reversible cyclization. We envisage these materials for applications as sensors and optical switching devices.

NMR spectroscopy with in situ irradiation uncovered unique photoswitchable intramolecular hydrogen bonds (IMHBs) in 5‐phenylazopyrimidines with two hydrogen bond donors. These compounds form two stable rotamers, each with one IMHB, and the rotamer ratio changes reversibly upon UV or visible light irradiation. Strong substituent dependence of photoinduced structural changes was observed; using suitable substituents, orthogonal photoswitching can be achieved. For example, whereas UV irradiation caused switching between the two rotamers of the trans isomer of a compound with electron‐donating methoxy substituent, visible light enabled to obtain the cis photoisomer. No cis isomer was detected for compounds with electro‐neutral or electron‐accepting substituents, but photoswitching between the two trans isomers was observed. On the other hand, compounds without hydrogen‐bond donors or with one donor only formed stable cis isomers. A mechanism of the photoswitching was proposed by DFT computations.