Light-Controlled Lyotropic Liquid Crystallinity of Polyaspartates Exploited as Photo-Switchable Alignment Medium

Abstract

Two polyaspartates bearing ortho-fluorinated azobenzenes (pFAB) as photo-responsive groups in the side chain were synthesized: PpFABLA (1) and co-polyaspartate PpFABLA-co-PBLA [11, 75%(n/n) PpFABLA content]. As a consequence of the E/Z-isomerization of the side chain, PpFABLA (1) undergoes a visible-light-induced reversible coil–helix transition in solution: Green light (525 nm) affords the coil, and violet light (400 nm) affords the helix. pFAB significantly increases the thermal stability of the Z-isomer at 20 °C (t 1/2 = 66 d for the Z-isomer) and effectively counters the favored back formation of the helix. At 20%(w/w) polymer concentration, the helical polymer forms a lyotropic liquid crystal (LLC) that further orients unidirectionally inside a magnetic field, while the coil polymer results in an isotropic solution. The high viscosity of the polymer solution stabilizes the coexistence of liquid crystalline and isotropic domains, which were obtained with spatial control by partial light irradiation. When used as an alignment medium, PpFABLA (1) enables (i) the measurement of dipolar couplings without the need for a separate isotropic reference and (ii) the differentiation of enantiomers. PpFABLA-co-PBLA (11) preserves the helical structure, by intention, independently of the E/Z-isomerization of the side chain: Both photo-isomers of PpFABLA-co-PBLA (11) form a helix thatat a concentration of 16%(w/w)form an LLC. Despite the absence of a change in the secondary structure, the E/Z-isomerization of the side chain changes the morphology of the liquid crystal and leads to different sets of dipolar coupling for the same probe molecule.

Programming Peptide Liquid Crystal Media to Acquire Independent Sets of Residual Dipolar Couplings and Enantiodiscrimination in Multiple Solvent Systems

“…To quantitatively measure the difference in the orientation of artemether in these different media, β angle, an NMR parameter that can be exploited to evaluate the orientation difference in the alignment tensors of a certain solute in two different media systems − and quantify the enantiodiscriminating property of a certain alignment medium to enantiomers, , was introduced here. In general, a β angle of 0° means the same orientation of a targeted analyte in two kinds of media, whereas a β angle of 90° suggests orthogonal alignment tensors caused by two different media.…”

Photoswitches in Order: One-Pot Synthesis of Azobenzene Main-Chain and Segmented Copolymers

“…If the reaction to the light stimulus is further reversible (e.g., by applying light of another wavelength or by thermal relaxation), the corresponding light-responsive moiety is called photoswitch and can trigger a reversible response of the polymer when inserted in their structure . These reversible responses are again manifold, ranging from dissolved polymers that change their conformation and thereby the solution properties, , to the control of inter- and intramolecular forces or solid–liquid , and solid–liquid crystalline , phase transitions of the polymer systems, respectively.…”

Tandem “One-Shot” Measurement of Residual Chemical Shift Anisotropy and Residual Dipolar Coupling using Biphasic Supramolecular Peptide Liquid Crystals

“…In the past few decades, the continuous emergence of new alignment media has greatly advanced the utilization of anisotropic NMR spectroscopy in the stereochemical elucidation of organic molecules and proteins. − A representative and widely exploited anisotropic NMR measurement is residual dipolar coupling (RDC), which contains rich information on the relative orientation of interatomic vectors and therefore has been introduced to elucidate the configurations and conformations of analytes mainly through C–H/N–H vectors. − However, the RDC methodology is less sensitive for resolving proton-deficient organic molecules . Additionally, it is insufficient for situations in which analytes have very few C–H/N–H bonds or when C–H/N–H bonds are close to parallel .…”