Evolution of a Combined UV/Vis and NMR Setup with Fixed Pathlengths for Mass-limited Samples.

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.

Extraction of pure component spectra from ex situ illumination UV/Vis and NMR spectroscopy.

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 εi 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.

Pd-Catalyzed Asymmetric Allylic Alkylation of Cyclobutenes: From Double Inversion to Double Retention.

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.

Concentration gradients in evaporating binary droplets probed by spatially resolved Raman and NMR spectroscopy.

Understanding the evaporation process of binary sessile droplets is essential for optimizing various technical processes, such as inkjet printing or heat transfer. Liquid mixtures whose evaporation and wetting properties may differ significantly from those of pure liquids are particularly interesting. Concentration gradients may occur in these binary droplets. The challenge is to measure concentration gradients without affecting the evaporation process. Here, spectroscopic methods with spatial resolution can discriminate between the components of a liquid mixture. We show that confocal Raman microscopy and spatially resolved NMR spectroscopy can be used as complementary methods to measure concentration gradients in evaporating 1-butanol/1-hexanol droplets on a hydrophobic surface. Deuterating one of the liquids allows analysis of the local composition through the comparison of the intensities of the C­H and C­D stretching bands in Raman spectra. Thus, a concentration gradient in the evaporating droplet was established. Spatially resolved NMR spectroscopy revealed the composition at different positions of a droplet evaporating in the NMR tube, an environment in which air exchange is less pronounced. While not being perfectly comparable, both methods­confocal Raman and spatially resolved NMR experiments­show the presence of a vertical concentration gradient as 1-butanol/1-hexanol droplets evaporate.

In Situ Switching of Site-Selectivity with Light in the Acetylation of Sugars with Azopeptide Catalysts.

We present a novel concept for the in situ control of site-selectivity of catalytic acetylations of partially protected sugars using light as external stimulus and oligopeptide catalysts equipped with an azobenzene moiety. The isomerizable azobenzene-peptide backbone defines the size and shape of the catalytic pocket, while the π-methyl-l-histidine (Pmh) moiety transfers the electrophile. Photoisomerization of the E- to the Z-azobenzene catalyst (monitored via NMR) with an LED (λ = 365 nm) drastically changes the chemical environment around the catalytically active Pmh moiety, so that the light-induced change in the catalyst shape alters site-selectivity. As a proof of principle, we employed (4,6-O-benzylidene)methyl-α-d-pyranosides, which provide a change in regioselectivity from 2:1 (E) to 1:5 (Z) for the monoacetylated products at room temperature. The validity of this new catalyst-design concept is further demonstrated with the regioselective acetylation of the natural product quercetin. In situ irradiation NMR spectroscopy was used to quantify photostationary states under continuous irradiation with UV light.

Photoswitching Behavior of 5-Phenylazopyrimidines: In Situ Irradiation NMR and Optical Spectroscopy Combined with Theoretical Methods.

The photoswitching behavior of 5-phenylazopyrimidines was investigated by optical methods and NMR spectroscopy with in situ irradiation sustained by mathematical modeling and DFT calculations. Irradiation of various compounds with electron-donating groups on the pyrimidine ring and substituents with electron-withdrawing as well as electron-donating substituent in the para-position of the phenyl ring were examined. All compounds could be successfully converted to the cis isomer; this isomerization and the subsequent thermal fading were studied. Switching cycles can be repeated without signs of photodegradation for most of the compounds, which makes them adept to molecular photoswitches. Interestingly, the chloro and cyano derivatives can be switched without UV light, which makes them vis(π → π*)-vis(n → π*) photoswitches. Surprisingly equal trans-to- cis photoisomerization quantum yields for π → π* and n → π* excitation indicate the blocking of the inversion pathway following π → π* excitation. In contrast to that, DFT computations suggest the inversion mechanism for the reverse thermal cis-to- trans isomerization of 5-phenylazopyrimidines.

Photoswitchable Intramolecular Hydrogen Bonds in 5-Phenylazopyrimidines Revealed By In Situ Irradiation NMR Spectroscopy.

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.

Distinction of trans-cis photoisomers with comparable optical properties in multiple-state photochromic systems - examining a molecule with three azobenzenes via in situ irradiation NMR spectroscopy.

Photochromic compounds like azobenzenes are widely used for the production of stimuli responsive materials. To analyse cascaded azobenzene switching inside a benzene-tricarboxamide (BTA) with three azobenzene moieties in a site-specific fashion, we used in situ irradiation NMR spectroscopy. Four photoisomers can be distinguished by their chemical shifts. Analysis of 1H, 13C and 15N shifts reveals that the configuration of one sidechain has an influence on the chemical shifts of both the other sidechains. Interconversion kinetics upon irradiation with ultraviolet (UV) light as well as molar fractions in photostationary states (PSS) were examined. Analysis of thermal fading of the different photoisomers into the ground state shows that thermal relaxation rates of all three azobenzene moieties behave as if they were independent of each other.

Studies of a photochromic model system using NMR with ex-situ and in-situ irradiation devices.

The switching behavior of a photochromic model system was investigated in detail via NMR spectroscopy in order to improve understanding of the compound itself and to provide ways to obtain insights into composition trends of a photo switchable (polymeric) material containing spiropyran/merocyanine units. In addition to the classical irradiation performed outside the magnet (ex-situ), a device for irradiation inside the NMR spectrometer (in-situ) was tested. Both setups are introduced, their advantages and disadvantages as well as their limits are described and the setup for future investigations of photochromic materials is suggested. The influence of different sample concentrations, irradiation procedures, and light intensities on the model system was examined as well as the dependence on solvent, temperature, and irradiation wavelengths. Using the recently published LED illumination device, it was even possible to record two-dimensional spectra on this model system with rather short half-life (7 min in DMSO). This way (13) C chemical shifts of the merocyanine form were obtained, which were unknown before. Copyright © 2016 John Wiley & Sons, Ltd.

Surface plasmon & visible light for polymer functionalization of mesopores and manipulation of ionic permselectivity.

The near-field of surface plasmons is locally confined to nanometer dimensions. Here, we use surface plasmons to initiate polymer-functionalization of mesoporous silica films allowing subsequent ionic permselectivity gating based on polymer charge. We expect this functionalization approach to open a new dimension of functional miniaturization e.g. in nanofluidics.