Polymerization-Induced Helicity Inversion Driven by Stacking Modes and Self-Assembly Pathway Differentiation

Polymerization-induced Chiral Self-assembly for the In situ Construction, Modulation, Amplification and Applications of Asymmetric Suprastructures

In the polymerization-induced chiral self-assembly (PICSA) process, chiral functional monomers undergo spontaneous supramolecular self-assembly and asymmetric stacking during living polymerization, leading to the in situ generation of chiroptical polymer assemblies characterized by diverse morphologies. The PICSA strategy facilitates precise control and manipulation of both non-covalent supramolecular helices and covalent macromolecular helices within aggregated cores, thereby driving significant advancements in fields such as chiral recognition materials, asymmetric catalysts, nonlinear optical materials, and ferroelectric liquid crystals (LC). This minireview summarizes recent developments in the in situ chiroptical construction and modulation associated with the PICSA methodology. Furthermore, it seeks to elucidate emerging PICSA systems founded on various living polymerization mechanisms, exploring hierarchical chirality transfer processes and the pathway complexities in both equilibrium and non-equilibrium conditions. This minireview also presents several illustrative examples that demonstrate the practical applications of chiral polymer materials synthesized through the PICSA approach, thereby illuminating future opportunities in this innovative field.

Dynamically Switchable Global Chirality in Racemic Polymer Systems

Any polymers composed of racemic segments are obviously optically inactive and lack any chiroptical applications. Here, we present an intriguing method for precisely generating global chirality in racemic copolymer assemblies without any external asymmetrical intervention via step-wise polymerization-induced chiral self-assembly (PICSA). Global supramolecular chirality of the nanoaggregates could be dynamically switched by the two diametrically opposed chiral conflict effects: "first come, first serve" effect and "late-comer lives above" effect, which can be controlled by the precisely specified the number and sequence of enantiomeric segments. Significantly, the supramolecular stacking manners of the racemic mesogenic building units as well as the liquid crystallinity of the solvophobic core play a crucial role for the chiral communication pathway of enantiomeric mesogens. Furthermore, such switchable global chirality in racemic polymers is broadly applicable and well regulable. We propose that this research may challenge the notion that racemic systems lack optical activity while highlighting their potential applications in functional racemic polymer materials and providing insights into the evolution of racemates towards homochirality on early Earth.

Activation and Deactivation of Chirality Transfer in the Superbundles of Sequence-defined Stereoisomers

Discrete oligomers can be used to precisely evaluate the structure-property relationship and enable unique chiroptical activities, however, the role of stereochemical sequences on chirality transfer is still unclear. Herein, we report the successful synthesis of a series of sequence-defined chiral azobenzene (Azo) oligomers via iterative stepwise chain growth strategy. Sequence-defined stereoisomers with one single chiral (L or D) stereocenter at the α-position, ω-position and middle- (m-) position have completely different self-assembly dynamics. ω-positional stereocenter can effectively command all Azo building blocks to adopt a tilted π-π stacking along the helical superbundles, exhibiting the activation of chirality transfer. However, discrete oligomers with the stereocenter at other positions can only self-assemble into non-helical nanowires, accompanied by the deactivation of chirality transfer.Cooperative supramolecular interactions, including the π-π interaction between Azo units, the intermolecular hydrogen bonding and steric hindrance, are intrinsic driving forces for these differentiations.

Mesogenic Ordering-Driven Self-Assembly of Liquid Crystalline Block Copolymers in Solution

With the development of nanotechnology, the preparation of polymeric nanoparticles with nicely defined structures has been well-developed, and the functionalization and subsequent applications of the resultant nanostructures are becoming increasingly important. Particularly, by introducing mesogenic ordering as the driving force for the solution-state self-assembly of liquid crystalline (LC) block copolymers (BCPs), micellar nanostructures with different morphologies, especially anisotropic morphologies, can be easily prepared. This review summarizes the recent progress in the solution-state self-assembly of LC BCPs and is mostly focused on four main related aspects, including an in-depth understanding of the mesogenic ordering-driven self-assembly, precise assembly methods, utilization of these methods to fabricate hierarchical structures, and the potential applications of these well-defined nanostructures. We hope not only to make a systematic summary of previous studies but also to provide some useful thinking for the future development of this field.

Modern Trends in Polymerization-Induced Self-Assembly

Polymerization-induced self-assembly (PISA) is a powerful and versatile technique for producing colloidal dispersions of block copolymer particles with desired morphologies. Currently, PISA can be carried out in various media, over a wide range of temperatures, and using different mechanisms. This method enables the production of biodegradable objects and particles with various functionalities and stimuli sensitivity. Consequently, PISA offers a broad spectrum of potential commercial applications. The aim of this review is to provide an overview of the current state of rational synthesis of block copolymer particles with diverse morphologies using various PISA techniques and mechanisms. The discussion begins with an examination of the main thermodynamic, kinetic, and structural aspects of block copolymer micellization, followed by an exploration of the key principles of PISA in the formation of gradient and block copolymers. The review also delves into the main mechanisms of PISA implementation and the principles governing particle morphology. Finally, the potential future developments in PISA are considered.

Regulating the Chiroptical Expression of Aggregated Solvophobic Core by Solvophilic Segments

The investigation of chiral supramolecular stacking is of essential significance for the understanding of the origin of homochirality in nature. Unlike structurally well-defined amphiphilic liposomes, it remains unclear whether the solvophilic segments of the amphiphilic block copolymer play a decisive role in the construction of asymmetric superstructures. Herein, insights are presented into the stacking patterns and morphological regulation in azobenzene-containing block copolymer assemblies solely by modulating the solvophilic chain length. The solvophilic poly(methacrylic acid) (PMAA) segments of different molecular weights could cause multi-mode chirality inversions involving stacking transitions between intra-chain π-π stacking, inter-chain H- and J-aggregation. Furthermore, the length of the solvophilic PMAA also affects the morphology of the chiral supramolecular assemblies; rice grain-like micelles, worms, nanofibers, floccules, and lamellae can be prepared at different solvophilic-solvophobic balance. The comprehensive mechanism is collectively revealed by utilizing various measurement methods, such as including circular dichroism (CD), small-angle X-ray scattering (SAXS), and wide-angle X-ray diffraction (WAXD). This study highlights the critical importance of fully dissolved solvophilic segments for the chiroptical regulation of the aggregated core, providing new insights into the arrangement of chiral supramolecular structures in polymer systems.

Metal-ion-triggered symmetry breaking of completely achiral azobenzene amphiphiles in water

Achieving control over symmetry breaking of completely achiral components in the aqueous phase is a significant challenge in supramolecular chemistry. Herein, we demonstrate that it is possible to construct chiral nanoassemblies by introducing metal ions (Zn2+, Fe3+, Al3+, Cu2+, and Ca2+) into completely achiral azobenzene amphiphiles with key structural factors in the pure aqueous phase. It is found that the coordination interactions, π-π stacking, hydrophilic and hydrophobic interactions, hydrogen bonding, and electrostatic interactions are crucial to the metal-ion-induced symmetry breaking of completely achiral building blocks. This study may provide an intriguing model system for constructing chiral assemblies based on completely achiral molecules.

Photoresponsive Viscoelasticity of the Granular Materials of Azobenzene-Bearing Molecular Nanoparticles

The large sizes of granular particles lead to their slow diffusive dynamics and significant interparticle friction, bringing enormous difficulty to tune the mechanical properties and processability of the granular materials (GMs). Herein, 1 nm polyhedral oligomeric silsesquioxane (POSS) particles functionalized with azobenzene are designed as structural units, and the obtained GMs show unique photoswitchable viscoelasticity. The azobenzene group can undergo a reversible trans-cis conformation switch while the π-π stacking among the azobenzene fragments is only favored by the trans-conformation due to molecular geometrical requirements. The POSS units from neighboring assemblies close pack to form microdomains, and the POSS is under confinement by both the supramolecular bonding and the other POSS in the microdomains. The simultaneous breaking of the two types of confinement is difficult and, therefore, the free diffusion of POSS is hindered, leading to the elasticity of the GMs of trans-POSS. For cis-POSS, the interparticle supramolecular interaction is weak and the POSS unit can undergo free diffusion, contributing to their high flowability at room temperature. The photoswitching viscoelasticity of GMs is further used for self-healing and photoswitchable adhesion. This work paves new pathways for the regulation of material viscoelasticity and the design of GM-based smart materials.

Unraveling Dynamic Helicity Inversion and Chirality Transfer through the Synthesis of Discrete Azobenzene Oligomers by an Iterative Exponential Growth Strategy

Unraveling the chirality transfer mechanism of polymer assemblies and controlling their handedness is beneficial for exploring the origin of hierarchical chirality and developing smart materials with desired chiroptical activities. However, polydisperse polymers often lead to an ambiguous or statistical evaluation of the structure-property relationship, and it remains unclear how the iterative number of repeating units function in the helicity inversion of polymer assemblies. Herein, we report the macroscopic helicity and dynamic manipulation of the chiroptical activity of supramolecular assemblies from discrete azobenzene-containing oligomers (azooligomers), together with the helicity inversion and morphological transition achieved solely by changing the iterative chain lengths. The corresponding assemblies also differ from their polydisperse counterparts in terms of thermodynamic properties, chiroptical activities, and morphological control.

A Subtle Change in the Flexible Achiral Spacer Does Matter in Supramolecular Chirality: Two-Fold Odd-Even Effect in Polymer Assemblies

Precise control over the chirality and morphologies of polymer assemblies, a remaining challenge for both chemists and materials scientists, is receiving ever-increasing attention in the recent years. Herein, we report the subtle manipulation of the achiral spacers from the chiral stereocenter to the azobenzene (Azo) unit, of which the chiroptical consistency or chiroptical inversion of self-assemblies could be successfully controlled and present "two-fold" odd-even effect. Furthermore, morphological transitions from 0D spherical micelles, 1D worms, and nanowires to 3D vesicles, spindle- and dumbbell-shaped vesicles were also unexpectedly found to exhibit odd-even correlations. These observations were collectively elucidated by mesomorphic properties, stacking modes, chiroptical dynamics, and stimuli-responsive behaviors. Negligible modifications to the spacer structures can enable remarkable modulation of supramolecular chirality and anisotropic topologies in polymer assemblies, which is of great significance for the design of complex chiral functional polymers.

Sequence effect on the self-assembly of discrete amphiphilic co-oligomers with fluorene-azobenzene semirigid backbones

Sequences can have a dramatic impact on the unique properties and self-assembly in natural macromolecules, which has received increasing interest. Herein, we report a series of discrete amphiphilic co-oligomers with the same composition but different building blocks in a semirigid backbone. These sequence-defined oligomers possess two primary amine groups on the side chain of the azobenzene building block, and hence, they become amphipathic due to quaternization of the amine groups when protonated in acidic aqueous solution. These oligomer isomers assembled into different nanoparticles, including nanofibers, hollow vesicles and spherical micellar complexes, in a THF/water/HCl mixture under the same conditions. UV-vis absorption spectra, differential scanning calorimetry (DSC) and X-ray scattering (XRD) experiments combined with theoretical calculations reveal that the sequence-controlled co-oligomers induce different molecular packing conformations and arrangement modes of building blocks in self-assembly. Furthermore, these self-assembled nanoparticles demonstrate photoresponsive morphological transformation and fluorescence emission under UV light irradiation due to trans-to-cis photoisomerization of azobenzene. This work demonstrates that customizing functional nanoparticles can be achieved by controlling the sequence structure in synthetic co-oligomers.

Inverted and Amplified CP-EL Behavior Promoted by AIE-Active Chiral Co-Assembled Helical Nanofibers

It is well-known that high-performance circularly polarized organic light-emitting diodes (CP-OLEDs) remain a formidable challenge to the future application of circularly polarized luminescent (CPL)-active materials. Herein, the design of a pair of AIE-active chiral enantiomers (L/D-HP) is described to construct chiral co-assemblies with an achiral naphthalimide dye (NTi). The resulting co-assemblies emit an inverted CPL signal compared with that from the L/D-HP enantiomers. After thermal annealing at 120 °C, the inverted CPL signal of this kind of L/D-HP-NTi with a 1:1 molar ratio shows regular and ordered helical nanofibers arranged through intermolecularly ordered layered packing and is accompanied with a further amplified effect (|g_em | = 0.032, λ_em  = 535 nm). Significantly, non-doped CP-OLEDs based on a device emitting layer (EML) of L/D-HP-NTi exhibits a low turn-on voltage (V_on ) of 4.7 V, a high maximum brightness (L_max ) of 2001 cd m^-2 , and moderate maximum external quantum efficiency (EQE_max ) of 2.3%, as well as excellent circularly polarized electroluminescence (CP-EL) (|g_EL | = 0.023, λ_em  = 533 nm).

Building Permanently Optically Active Particles from Absolutely Achiral Polymer

Chirality in polymer particles represents one of the most dynamic areas of nanoscale materials today. The chirality of most chiral polymeric particles (CPPs) derived from achiral monomers/polymers has a strong...