A strategy towards photochromic SI-ROMP homopolymers based on spiropyran and spirooxazine in mesopores is presented and photochromism is systematically investigated using UV-VIS spectroscopy and NMR.
Lyotropic liquid crystalline phases of poly-β-phenethylaspartates are presented as new helically chiral enantiodifferentiating alignment media with thermoresponsive properties. In addition to displaying the highest enantiodifferentiation observed for homopolypeptides, the alignment media undergo a temperature induced helix reversal without perturbing the nematic phase. This offers the opportunity to measure residual dipolar couplings (RDCs) in high and low temperature helix conformation (P- and M-helix) in one polymer. Thus different mean orientations of a chiral analyte can be determined within the same sample. Furthermore, we investigated whether the resulting media are diastereomeric and whether we are able to obtain information about the alignment process.
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.
The photochromic and electrochemical switching capabilities of two dithienylethenes with different substitutions and their self-assembled monolayers on gold surfaces are characterized by in situ NMR-spectroscopy and cyclic voltammetry.
A supramolecular, lyotropic liquid crystalline alignment medium based on an azobenzene‐containing 1,3,5‐benzenetricarboxamide (BTA) building block is described and investigated. As we demonstrate, this water‐based system is suitable for the investigation of various water‐soluble analytes and allows for a scaling of alignment strength through variation of temperature. Additionally, alignment is shown to reversibly collapse above a certain temperature, yielding an isotropic solution. This collapse allows for isotropic reference measurements, which are typically needed in addition to those in an anisotropic environment, to be performed using the same sample just by varying the temperature. The medium described thus provides easy access to anisotropic NMR observables and simplifies structure elucidation techniques based thereon.
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