Stereoselective Plasmonic Interaction in Peptide-tethered Photopolymerizable Diacetylenes Doped with Chiral Gold Nanoparticles

Stimuli-Responsive Control over Self-Assembled Nanostructures in Sequence-Specific Functional Block Copolymers

The precise sequence of a protein's primary structure is essential in determining its folding pathways. To emulate the complexity of these biomolecules, functional block copolymers consisting of segmented triblocks with distinct functionalities positioned in a sequence-specific manner are designed to control the polymer chain compaction. Triblock polymers P- b -C- b -F and P- b -F- b -C and random diblock copolymer P- b -C- r -F consist of a hydrophilic poly(ethylene oxide) (PEO) block and a hydrophobic block with coumarin (C) and ferrocene (F) moieties that are grafted in a sequence-specific or random manner onto the hydrophilic block. External stimuli such as UVB light, redox, and chemical cues influence the functional hydrophobic block to alter the packing parameters that are monitored with spectroscopic and scattering techniques. Interestingly, the positioning of the stimuli-responsive moiety within the hydrophobic block of P- b -C- b -F, P- b -F- b -C, and P- b -C- r -F affects the extent of the hydrophobic-hydrophilic balance in block copolymers that renders orthogonal control in stimuli-responsive transformation of self-assembled vesicles to micelles.

Divergent self-assembly propensity of enantiomeric phenylalanine amphiphiles that undergo pH-induced nanofiber-to-nanoglobule conversion

This study presents the pathway diversity in the self-assembly of enantiomeric single phenylalanine derived amphiphiles (single F-PDAs), viz.L-NapF-EDA and D-NapF-EDA, that form supramolecular hydrogels at varied concentrations (≥1 mg mL-1 and ≥3 mg mL-1, respectively). By fitting the variable temperature circular dichroism (VT-CD) data to the isodesmic model, various thermodynamic parameters associated with their self-assembly, such as association constant (K), changes in enthalpy (ΔH), entropy (ΔS), and Gibbs free energy (ΔG), were extracted. The self-assembly of these single F-PDAs was found to be enthalpy-driven but entropically-disfavored. Although self-assembly of the D-isomer was slow, it also exhibited greater free energy of association than the L-isomer. Consequently, thermally and mechanically more robust self-assemblies were formed by the D-isomer than the L-isomer. We term these results as the "butterfly effect in self-assembly" wherein the difference in the stereochemical orientation of the residues at a single chiral center present in these molecules resulted in strong differences in the self-assembly propensity as well as in their thermal and mechanical stability. These single F-PDAs form helical nanofibers of opposite chirality upon self-assembly at basic pH (≥8) that produce intense CD signals. However, upon decreasing the pH, a gradual nanofiber-to-nanoglobular transformation was noticed due to protonation-induced structural changes in the PDAs.

Plasticizer-Induced Enhancement of Mesoscale Dissymmetry in Thin Films of Chiral Polymers with Variable Chain Length

Conjugated polymers with chiral side chains are of interest in areas including chiral photonics, optoelectronics, and chemical and biological sensing. However, the low dissymmetry factors of most neat polymer thin films have limited their practical application. Here, a robust method to increase the absorption dissymmetry factor in a poly-fluorene-thiophene (PF8TS series) system is demonstrated by varying molecular weight and introducing an achiral plasticizer, polyethylene mono alcohol (PEM-OH). Extending chain length within the optimal range and adding this long-chain alcohol significantly enhance the chiroptical properties of spin-coated and annealed thin films. Mueller matrix spectroscopic ellipsometry (MMSE) analysis shows good agreement with the steady-state transmission measurements confirming a strong chiral response (circular dichroism (CD) and circular birefringence (CB)), ruling out linear dichroism, birefringence, and specific reflection effects. Solid-state NMR studies of annealed hybrid chiral polymer systems show enhancement of signals associated with aromatic π-stacked backbone and the ordered side-chain conformations. Further studies using Raman spectroscopy, X-ray diffraction (XRD), differential scanning calorimetry (DSC), atomic force microscopy (AFM), and polarized optical microscopy (POM) indicate that PEM-OH facilitates mesoscopic crystal domain ordering upon annealing. This provides new insights into routes for tuning optical activity in conjugated polymers.