Light-triggered Modulation of Supramolecular Chirality

Dynamically controlling the supramolecular chirality is of great significance in development of functional chiral materials, which is thus essential for the specific function implementation. As an external energy input, light is remote and accurate for modulating chiral assemblies. In non-polarized light control, some photochemically reactive units (e. g., azobenzene, ɑ-cyanostilbene, spiropyran, anthracene) or photo-induced directionally rotating molecular motors were designed to drive chiral transfer or amplification. Besides, photoexcitation induced assembly based physical approach was also explored recently to regulate supramolecular chirality beyond photochemical reactions. In addition, circularly polarized light was applied to induce asymmetric arrangement of organic molecules and asymmetric photochemical synthesis of inorganic metallic nanostructures, in which both wavelength and handedness of circularly polarized light have effects on the induced supramolecular chirality. Although light-triggered chiral assemblies have been widely applied in photoelectric materials, biomedical fields, soft actuator, chiral catalysis and chiral sensing, there is a lack of systematic review on this topic. In this review, we summarized the recent studies and perspectives in the constructions and applications of light-responsive chiral assembled systems, aiming to provide better knowledge for the development of multifunctional chiral nanomaterials.

Exploring the Circular Polarization Capacity from Chiral Cellulose Nanocrystal Films for a Photo-Controlled Chiral Helix of Supramolecular Polymers

Circularly polarized light (CPL) is key to asymmetric photochemistry as it could impart the chiral organization information into chemical products. Here, we demonstrate the circular polarization capacity of chiral cellulose nanocrystal (CNC) films to trigger photo-alignment of achiral supramolecular polymers into helical structures. Right-handed transmitted (T-) CPL was generated from self-assembled CNC films, which induced amorphous azobenzene (Azo) supramolecular polymers into chiral structures. The chiral induction effect of T-CPL is enhanced on Azo polymers with longer spacers. The absorptive dissymmetry factor (g_abs ) values of liquid-crystal supramolecular polymers can be amplified significantly (over 10 times) after T-CPL irradiation. Moreover, by integrating carbon dots into CNC films, CPL emission with a considerable luminescence dissymmetry factor (g_lum ) up to -0.66 was achieved, and it could be used for the photo-alignment of Azo polymers with high chiroptical properties. This work provides new insight for the photo modulation of supramolecular polymers by CPL-active materials.

Deciphering Internal and External π-Conjugation in C3 -Symmetric Multiple Azobenzene Connected Systems in Self-Assembly

Two tripodal C₃ -symmetric photoswitchable molecular systems T1 and T2 are reported that have extended conjugation at external and internal positions using an acryl group. The influence of the extended π-bonds in their absorption properties, thermal relaxation of the photoisomers and their propensities in forming supramolecular self-assemblies have been explored through spectroscopy, and microscopic studies. In particular, the investigations on the self-assembly have been carried out using scanning electron microscopy (SEM), transmission electron microscopy (TEM), polarized optical microscopy (POM), X-ray diffraction studies (XRD) and atomic force microscopy (AFM). Remarkably, the position of the acryl group influences the behaviour of the two target molecules in supramolecular assembly, and also in the formation of photoresponsive organic hydrogels or microcrystals.

Reversible supramolecular chiral structures induced in azopolymers by elliptically polarized light: influence of the irradiation wavelength and intensity

Photoinduced supramolecular chiral structures in azo materials have been extensively studied for the development of all-optical switches and because of their influence on the properties of certain types of polarization holographic gratings. Here, we investigate chiral structures induced by irradiation with elliptically polarized light in thin films of four azopolymers denoted as PAZO, P₁, P_1-2, which are amorphous, and P₂, which is liquid crystalline. Their formation is characterized in real time by the kinetics of azimuth rotation. The influence of the irradiation wavelength and intensity is also analyzed. The largest azimuth rotation per unit thickness is achieved in PAZO (33°/µm) and P_1-2 (25°/µm). Reversibility of the chiral structures is demonstrated by a tenfold change in the direction of rotation. Our results also indicate that chiral structures formation occurs significantly faster than the induction of linear birefringence.

Controlling the Multiple Chiroptical Inversion in Biphasic Liquid-Crystalline Polymers

While controlling the chirality and modulating the helicity is a challenging task, it attracts great research interest for gaining a better understanding of the origin of chirality in nature. Herein, structurally similar azobenzene (Azo) vinyl monomers were designed in which the alkyl chains comprised the chiral stereocenter with different achiral tail lengths. Combining the synchronous polymerization, supramolecular stacking and self-assembly, the multiple chiroptical inversion of the Azo-polymer supramolecular assemblies can be modulated by the tail length and DP of Azo blocks during in situ polymerization. The DP-, UV light-, temperature-, aging time-dependent chiroptical properties and liquid-crystalline (LC) characterization indicated that the amorphous-to-LC phase transition and biphasic LC interconversion allow the transcription of intra-chain π-π stacking, inter-chain H- and J-aggregation, thereby controlling the dynamic multiple reversal of supramolecular chirality.

Transfer, Amplification, Storage, and Complete Self-Recovery of Supramolecular Chirality in an Achiral Polymer System

Supramolecular chirality and its complete self-recovery ability are highly mystical in nature and biological systems, which remains a major challenge today. Herein, we demonstrate that partially cross-linked azobenzene (Azo) units can be employed as the potential chiral trigger to fully heal the destroyed helical superstructure in achiral nematic polymer system. Combining the self-assembly of Azo units and terminal hydroxyl groups in polymer side chains allows the vapor-induced chiral nematic phase and covalent fixation of the superstructure via acetal reaction. The induced helical structure of Azo units can be stored by inter-chain cross-linking, even after removal of the chiral source. Most interestingly, the stored chiral information can trigger perfect chiral self-recovery (CSR) behavior after being destroyed by UV light, heat, and solvents. The results pave a new way for producing novel chiroptical materials with reversible chirality from achiral sources.

Reversible Micrometer-Scale Spiral Self-Assembly in Liquid Crystalline Block Copolymer Film with Controllable Chiral Response

The spiral is a fundamental structure in nature and spiral structures with controllable handedness are of increasing interest in the design of new chiroptical materials. In this study, micrometer-scale spiral structures with reversible chirality were fabricated based on the assembly of a liquid crystalline block copolymer film assisted by enantiopure tartaric acid. Mechanistic insight revealed that the formation of the spiral structures was closely related to the liquid crystalline properties of the major phase of block copolymer under the action of chiral tartaric acid. The chiral spiral structures with controllable handedness were easily erased under ultraviolet light irradiation and restored via thermal annealing. This facile thermal treatment method provides guidance for fabrication of chiral micrometer-scale spiral structures with adjustable chiral properties.

Construction of Highly Ordered Glyco-Inside Nano-Assemblies through RAFT Dispersion Polymerization of Galactose-Decorated Monomer

Glyco-assemblies derived from amphiphilic sugar-decorated block copolymers (ASBCs) have emerged prominently due to their wide application, for example, in biomedicine and as drug carriers. However, to efficiently construct these glyco-assemblies is still a challenge. Herein, we report an efficient technology for the synthesis of glyco-inside nano-assemblies by utilizing RAFT polymerization of a galactose-decorated methacrylate for polymerization-induced self-assembly (PISA). Using this approach, a series of highly ordered glyco-inside nano-assemblies containing intermediate morphologies were fabricated by adjusting the length of the hydrophobic glycoblock and the polymerization solids content. A specific morphology of complex vesicles was captured during the PISA process and the formation mechanism is explained by the morphology of its precursor and intermediate. Thus, this method establishes a powerful route to fabricate glyco-assemblies with tunable morphologies and variable sizes, which is significant to enable the large-scale fabrication and wide application of glyco-assemblies.