Non-uniform Photoinduced Unfolding of Supramolecular Polymers Leading to Topological Block Nanofibers

Supramolecular polymerization and bulk properties relationship in ester-functionalized N-annulated perylenediimides

The synthesis of a series of N-annulated perylenediimides (NPDIs) 1-4 with an ester group and an alkyl spacer of different length in the peripheral chains was carried out, and the influence of the side chain architecture on the self-assembly, both in solution and in the solid state, was investigated. Solution studies evidenced that the carbonyl group plays a key role in the supramolecular organization of these derivatives, changing from an H-type isodesmic polymerization (4) to a J-type cooperative process as the spacer length decreases (1-3). On the other hand, bulk assays revealed an odd-even effect that correlates with the length of the alkyl spacer. Whereas the odd-spaced derivatives (2 and 4) organize in a disordered columnar hexagonal fashion, the even-spaced ones (1 and 3) show the formation of multiple crystalline (and liquid crystalline) structures. The results presented herein highlight the importance of side chain functionalization in the design of building blocks for in-solution and bulk purposes.

Molecular and supramolecular adaptation by coupled stimuli

Adaptation transcends scale in both natural and artificial systems, but delineating the causative factors of this phenomenon requires urgent clarification. Herein, we unravel the molecular requirements for adaptation and establish a link to rationalize adaptive behavior on a self-assembled level. These concepts are established by analyzing a model compound exhibiting both light- and pH-responsive units, which enable the combined or independent application of different stimuli. On a molecular level, adaptation arises from coupled stimuli, as the final outcome of the system depends on their sequence of application. However, in a self-assembled state, a single stimulus suffices to induce adaptation as a result of collective molecular behavior and the reversibility of non-covalent interactions. Our findings go beyond state-of-the-art (multi)stimuli-responsive systems and allow us to draw up design guidelines for adaptive behavior both at the molecular and supramolecular levels, which are fundamental criteria for the realization of intelligent matter.

Strategies for Synthesizing Supramolecular Block Copolymers

Over the past decade, controlled supramolecular polymerization has been extensively studied and gradually shifted to supramolecular block copolymerization. Supramolecular block copolymers (BCPs) are considered the holy grail for developing supramolecular materials with new functionalities due to their fascinating structures and ability to introduce diverse functions. From a thermodynamic view to kinetic aspects, great progress has been made in the synthetic strategies of BCPs in the past few years. This Concept summarizes various strategies to realize supramolecular block copolymerization. The focus is on providing researchers with a methodological basis for achieving heterogeneous nucleation-elongation.

Construction of Supramolecular Polymers and Covalent Polymers via the Same Monomers

Nature owns the ability to construct structurally different polymers from the same monomers. While polymers can be classified as covalent polymers (CPs) and supramolecular polymers (SPs), it is still difficult to synthesize CPs and SPs using same monomers like nature do. Herein, M1 with two diazo salts on both the ends was designed. Additionally, hydroquinone was chosen to be M2 for the existence of two hydroxyl groups. When mixing at room temperature, M1 and M2 self-assembled to SPs via N…H hydrogen bonds. In another way, upon the exposure to ultraviolet irradiation when blending M1 with M2, CPs were fabricated in the presence of covalent bonds. The excellent thermal stability of CPs was determined by TGA and DSC, while the great corrosion resistance of covalent polymers was detected by acid or alkali immersion. In this way, constructing two kinds of polymers using the same monomers was successfully achieved. This shows tremendous potential in fields of polymer science, supramolecular chemistry, which would boom the development of polymers.

Pathway Complexity in Supramolecular Copolymerization and Blocky Star Copolymers by a Hetero-Seeding Effect

This study unravels the intricate kinetic and thermodynamic pathways involved in the supramolecular copolymerization of the two chiral dipolar naphthalene monoimide (NMI) building blocks (O-NMI and S-NMI), differing merely by a single heteroatom (oxygen vs sulfur). O-NMI exhibits distinct supramolecular polymerization features as compared to S-NMI in terms of its pathway complexity, hierarchical organization, and chiroptical properties. Two distinct self-assembly pathways in O-NMI occur due to the interplay between the competing dipolar interactions among the NMI chromophores and amide-amide hydrogen (H)-bonding that engenders distinct nanotapes and helical fibers, from its antiparallel and parallel stacking modes, respectively. In contrast, the propensity of S-NMI to form only a stable spherical assembly is ascribed to its much stronger amide-amide H-bonding, which outperforms other competing interactions. Under the thermodynamic route, an equimolar mixture of the two monomers generates a temporally controlled chiral statistical supramolecular copolymer that autocatalytically evolves from an initially formed metastable spherical heterostructure. In contrast, the sequence-controlled addition of the two monomers leads to the kinetically driven hetero-seeded block copolymerization. The ability to trap O-NMI in a metastable state allows its secondary nucleation from the surface of the thermodynamically stable S-NMI spherical "seed", which leads to the core-multiarmed "star" copolymer with reversibly and temporally controllable length of the growing O-NMI "arms" from the S-NMI "core". Unlike the one-dimensional self-assembly of O-NMI and its random co-assembly with S-NMI, which are both chiral, unprecedentedly, the preferred helical bias of the nucleating O-NMI fibers is completely inhibited by the absence of stereoregularity of the S-NMI "seed" in the "star" topology.

Photoresponsive Supramolecular Polymers: From Light-Controlled Small Molecules to Smart Materials

Photoresponsive supramolecular polymers are well-organized assemblies based on highly oriented and reversible noncovalent interactions containing photosensitive molecules as (co-)monomers. They have attracted increasing interest in smart materials and dynamic systems with precisely controllable functions, such as light-driven soft actuators, photoresponsive fluorescent anticounterfeiting and light-triggered electronic devices. The present review discusses light-activated molecules used in photoresponsive supramolecular polymers with their main photo-induced changes, e.g., geometry, dipole moment, and chirality. Based on these distinct changes, supramolecular polymers formed by light-activated molecules exhibit photoresponsive disassembly and reassembly. As a consequence, photo-induced supramolecular polymerization, "depolymerization," and regulation of the lengths and topologies are observed. Moreover, the light-controlled functions of supramolecular polymers, such as actuation, emission, and chirality transfer along length scales, are highlighted. Furthermore, a perspective on challenges and future opportunities is presented. Besides the challenge of moving from harmful UV light to visible/near IR light avoiding fatigue, and enabling biomedical applications, future opportunities include light-controlled supramolecular actuators with helical motion, light-modulated information transmission, optically recyclable materials, and multi-stimuli-responsive supramolecular systems.

Acylhydrazone-based supramolecular assemblies undergoing a converse sol-to-gel transition on trans → cis photoisomerization

Photoisomeric supramolecular assemblies have drawn enormous attention in recent years. Although it is a general rule that photoisomerization from a less to a more distorted isomer causes the destruction of assemblies, this photoisomerization process inducing a converse transition from irregular aggregates to regular assemblies is still a great challenge. Here, we report a converse sol-to-gel transition derived from the planar to nonplanar photoisomer conversion, which is in sharp contrast to the conventional light-induced gel collapse. A well-designed acylhydrazone-linked monomer is exploited as a photoisomer to realize the above-mentioned phase transition. In the monomer, imine is responsible for trans-cis interconversion and amide generates intermolecular hydrogen bonds enabling the photoisomerization-driven self-assembly. The counterintuitive feature of the sol-to-gel transition is ascribed to the partial trans → cis photoisomerization of acylhydrazone causing changes in stacking mode of monomers. Furthermore, the reversible phase transition is applied in the valves formed in situ in microfluidic devices, providing fascinating potential for miniature materials.

Three-level hierarchical self-assembly of azobenzene conjugated phenylalanines into superhelical nanostructures with light-switchable helicity

Photosensitive azobenzene conjugated phenylalanines can self-assemble into a series of hierarchical superhelices, which can reversibly transform into achiral nanorods via light stimuli.