Photocontrollable Self-Assembly

Self-Assembly of Polymers and Their Applications in the Fields of Biomedicine and Materials

Polymer self-assembly can prepare various shapes and sizes of pores, making it widely used. The complexity and diversity of biomolecules make them a unique class of building blocks for precise assembly. They are particularly suitable for the new generation of biomaterials integrated with life systems as they possess inherent characteristics such as accurate identification, self-organization, and adaptability. Therefore, many excellent methods developed have led to various practical results. At the same time, the development of advanced science and technology has also expanded the application scope of self-assembly of synthetic polymers. By utilizing this technology, materials with unique shapes and properties can be prepared and applied in the field of tissue engineering. Nanomaterials with transparent and conductive properties can be prepared and applied in fields such as electronic displays and smart glass. Multi-dimensional, controllable, and multi-level self-assembly between nanostructures has been achieved through quantitative control of polymer dosage and combination, chemical modification, and composite methods. Here, we list the classic applications of natural- and artificially synthesized polymer self-assembly in the fields of biomedicine and materials, introduce the cutting-edge technologies involved in these applications, and discuss in-depth the advantages, disadvantages, and future development directions of each type of polymer self-assembly.

Photoresponsive Supramolecular Cages and Macrocycles

The utilisation of light to achieve precise manipulation and control over the structure and function of supramolecular assemblies has emerged as a highly promising approach in the development of complex, configurable, or multifunctional systems and nanoscopic machine-like entities. In this minireview, we highlight recent examples of self-assembled and covalently bound cages and macrocycles with a focus on the external and internal functionalisation of a structure with a photoswitchable unit or the embedment of a photoswitch into the framework of a structure. Functionalising the interior or exterior of a supramolecular cage or macrocycle with a photoresponsive group enables control over different properties, such as guest binding or assembly in the solid-state, while the overall shape of the assembly often undergoes no significant change. By directly integrating a photoswitchable unit into the framework of a supramolecular structure, the isomerisation can either induce a geometry change, the disassembly, or the disassembly and reassembly of the structure. Historical and recent examples covered in this review are based on azobenzene, diarylethene, stilbene photoswitches, or alkene motors that were incorporated into macrocycles and cages constructed by metal-organic, dynamic covalent, or covalent bonds.

Photo-Controlled Self-Assembly of Nanoparticles: A Promising Strategy for Development of Novel Structures

Photo-controlled self-assembly of nanoparticles (NPs) is an advanced and promising approach to address a series of material issues from the molecular level to the nano/micro scale, owing to the fact that light stimulus is typically precise and rapid, and can provide contactless spatial and temporal control. The traditional photo-controlled assembly of NPs is based on photochemical processes through NPs modified by photo-responsive molecules, which are realized through the change in chemical structure under irradiation. Moreover, photoexcitation-induced assembly of NPs is another promising physical strategy, and such a strategy aims to employ molecular conformational change in the excited state (rather than the chemical structure) to drive molecular motion and assembly. The exploration and control of NP assembly through such a photo-controlled strategy can open a new paradigm for scientists to deal with "bottom-up" behaviors and develop unprecedented optoelectronic functional materials.

Photoexcitation-Induced Assembly: A Bottom-Up Physical Strategy for Driving Molecular Motion and Phase Evolution

ConspectusIn the field of molecular assembly, photodriven self-assembly is a smart and crucial strategy to regulate the molecular orderliness, multiscale structure, and optoelectronic properties. Traditionally, photodriven self-assembly is based on photochemical processes, through molecular structural change induced by photoreactions. Despite great progress in the photochemical self-assembly, there still exists disadvantages (e.g., the photoconversion rate never reaches 100% with the possible side reactions). Therefore, the photoinduced nanostructure and morphology are often difficult to predict due to insufficient phase transition or defects. In contrast, the physical processes based on photoexcitation are straightforward and can fully utilize photons to avoid the drawbacks of photochemistry. The photoexcitation strategy excludes the change of molecular structure, only utilizing the molecular conformational change from the ground state to excited state. Then, the excited state conformation is employed to drive molecular movement and aggregation, further promoting the synergistic assembly or phase transition of the entire material system. The regulation and exploration of molecular assembly upon photoexcitation can open up a new paradigm to deal with the "bottom-up" behavior and develop unprecedented optoelectronic functional materials.This Account starts with a brief introduction to the problems faced by photocontrolled self-assembly and presents the photoexcitation-induced assembly (PEIA) strategy. Then, we focus on exploring PEIA strategy based on persulfurated arenes as the prototype. The molecular conformational transition of persulfurated arenes from the ground state to the excited state is conducive to the formation of intermolecular interactions, successively driving molecular motion, aggregation, and assembly. Next, we describe our progress in exploring PEIA of persulfurated arenes at the molecular level and then demonstrate that the PEIA of persulfurated arenes can synergistically drive molecular motion and phase transition in various block copolymer systems. Moreover, we provide the potential applications of PEIA in dynamic visual imaging, information encryption, and surface property regulation. Finally, an outlook on further development of PEIA is prospected.

Amplification of Molecular Asymmetry during the Hierarchical Self-Assembly of Foldable Azobenzene Dyads into Nanotoroids and Nanotubes

The amplification of molecular asymmetry through self-assembly is a phenomenon that not only comprehends the origin of homochirality in nature but also produces chiroptically active functional materials from molecules with minimal enantiomeric purity. Understanding how molecular asymmetry can be transferred and amplified into higher-order structures in a hierarchical self-assembly system is important but still unexplored. Herein, we present an intriguing example of the amplification of molecular asymmetry in hierarchically self-assembled nanotubes that feature discrete and isolatable toroidal intermediates. The hierarchical self-assembly is initiated via asymmetric intramolecular folding of scissor-shaped azobenzene dyads furnished with chiral side chains. When scalemic mixtures of the enantiomers are dissolved in a non-polar solvent and cooled to 20 °C, single-handed nanotoroids are formed, as confirmed using atomic force microscopy and circular dichroism analyses. A strong majority-rules effect at the nanotoroid level is observed and can be explained by a low mismatch penalty and a high helix-reversal penalty. The single-handed nanotoroids stack upon cooling to 0 °C to exclusively afford their respective single-handed nanotubes. Thus, the same degree of amplification of molecular asymmetry is realized at the nanotube level. The internal packing efficiency of molecules within nanotubes prepared from the pure enantiomers or their scalemic mixtures is likely different, as suggested by the absence of higher-order structure (supercoil) formation in the latter. X-ray diffraction analysis of the anisotropically oriented nanotube films revealed looser molecular packing within the scalemic nanotubes, which clearly reflects the lower enantiomeric purity of their internal chiral side chains.

Introduction of Ferrocene as a Facilitator for the Construction of Supramolecular Polymers

Proper monomer design is the key to enhancing the strength of noncovalent interactions between the molecules toward the efficient formation of supramolecular polymers (SPs). We have designed and synthesized 1,n'-disubstituted ferrocene-azobenzene-long alkyl chains, Fc(CONH-Azo-TDP)₂ , to afford SPs with a high probability. The design exploits the ''molecular ball-bearing'' property of the ferrocene core, which allows two azobenzene arms to rotate in the planes of cyclopentadienyl rings, generating the most suitable molecular conformation required for SP formation. This ferrocene monomer formed a supergel consisting of SPs supported by strong intermolecular (H-bonding and π-π stacking) interactions and higher enthalpy gain than the reference molecules, where the central ferrocene core was replaced by flexible aliphatic as well as rigid benzene linkers. The molecular conformation involved in SPs, the strength of noncovalent interactions, and the process of supramolecular polymerization were investigated through NMR, UV-Vis, XRD and TEM studies. The results demonstrate that ferrocene may act as a good modulator for constructing efficient SPs.

Photoresponsive Dissipative Macrocycles Using Visible-Light-Switchable Azobenzenes

Visible light can be used to shift dynamic covalent imine assemblies out of equilibrium. We studied a fluorinated azobenzene building block that reliably undergoes geometric isomerism upon irradiation. The building block was used in combination with two different amines, ethylenediamine and R,R-1,2-diaminocyclohexane, to create a library of imine macrocycles. Whereas the simple amine can be used to access a polymeric state and a defined bowl-shaped macrocycle, the chiral amine gives access to a rich network of macrocycles that undergo both isomerisation as well as interconversion between different macrocyclic species, thereby allowing for control over the number of monomers involved in the cyclo-oligomerization; ¹ H- and ¹⁹ F-DOSY NMR, MALDI-MS measurements, and UV/Vis spectroscopy were used to study the processes.

Visible‐Light‐Responsive Self‐Assembled Complexes: Improved Photoswitching Properties by Metal Ion Coordination**

A photoswitchable ligand based on azobenzene is self‐assembled with palladium(II) ions to form a [Pd₂(E‐L)₄]⁴⁺ cage. Irradiation with 470 nm light results in the near‐quantitative switching to a monomeric species [Pd(Z‐L)₂]²⁺, which can be reversed by irradiation with 405 nm light, or heat. The photoswitching selectivity towards the metastable isomer is significantly improved upon self‐assembly, and the thermal half‐life is extended from 40 days to 850 days, a promising approach for tuning photoswitching properties.

Supramolecular Polymerization of a Photo-Fluttering Chiral Monomer: A Temporarily Suspendable Chain Growth by Light

Using a photochemically fluttering thiophene-fused cyclooctatetraene derivative (COT) as a nonplanar chiral monomer, we have succeeded in remotely suspending the supramolecular polymerization in a temporal manner by a completely new strategy. The COT monomer with an 8π electron core adopts a saddle shape in the ground state and flutters 5.8 × 10³ times faster upon photoirradiation than in the dark as a result of the stabilized planar conformation by the excited-state aromaticity (Baird aromaticity). Detailed investigation revealed that without photoirradiation the rate constant of the fluttering motion is 1/560 times smaller than that of the chain elongation, indicating that the fluttering of COT does not affect the chain elongation in the dark. In contrast, under photoirradiation (365 nm), the fluttering of COT is at least 11 times more rapid than the chain elongation, thereby suppressing the elongation event. The rapid fluttering of COT to suspend the chain elongation is not accompanied by a decrease in active monomer concentration, leading to depolymerization.

Photo-modulation of supramolecular polymorphism in the self-assembly of a scissor-shaped azobenzene dyad into nanotoroids and fibers

Recent advances in the research field of supramolecularly engineered dye aggregates have enabled the design of simple one-dimensional stacks such as fibers and of closed structures such as nanotoroids (nanorings). More complex and advanced supramolecular systems could potentially be designed using a molecule that is able to provide either of these distinct nanostructures under different conditions. In this study, we introduced bulky but strongly aggregating cholesterol units to a scissor-shaped azobenzene dyad framework, which affords either nanotoroids, nanotubes, or 1D fibers, depending on the substituents. This new dyad with two trans-azobenzene arms shows supramolecular polymorphism in its temperature-controlled self-assembly, leading to not only oligomeric nanotoroids as kinetic products, but also to one-dimensional fibers as thermodynamic products. This supramolecular polymorphism can also be achieved via photo-triggered self-assembly, i.e., irradiation of a monomeric solution of the dyad with two cis-azobenzene arms using strong visible light leads to the preferential formation of nanotoroids, whereas irradiation with weak visible light leads to the predominant formation of 1D fibers. This is the first example of a successful light-induced modulation of supramolecular polymorphism to produce distinctly nanostructured aggregates under isothermal conditions.

Biasing the Hierarchy Motifs of Nanotoroids: from 1D Nanotubes to 2D Porous Networks

Hierarchical organization of self-assembled structures into superstructures is omnipresent in Nature but has been rarely achieved in synthetic molecular assembly due to the absence of clear structural rules. We herein report on the self-assembly of scissor-shaped azobenzene dyads which form discrete nanotoroids that further organize into 2D porous networks. The steric demand of the peripheral aliphatic units diminishes the trend of the azobenzene dyad to constitute stackable nanotoroids in solution, thus affording isolated (unstackable) nanotoroids upon cooling. Upon drying, these nanotoroids organize at graphite surface to form well-defined 2D porous networks. The photoirradiation with UV and visible light enabled reversible dissociation and reconstruction of nanotoroids through the efficient trans↔cis isomerization of azobenzene moieties in solution.

Cyclic Photoisomerization of Azobenzene in Atomistic Simulations: Modeling the Effect of Light on Columnar Aggregates of Azo Stars

This computational study investigates the influence of light on supramolecular aggregates of three-arm azobenzene stars. Every star contains three azobenzene (azo) moieties, each able to undergo reversible photoisomerization. In solution, the azo stars build column-shaped supramolecular aggregates. Previous experimental works report severe morphological changes of these aggregates under UV-Vis light. However, the underlying molecular mechanisms are still debated. Here we aim to elucidate how light affects the structure and stability of the columnar stacks on the molecular scale. The system is investigated using fully atomistic molecular dynamics (MD) simulations. To implement the effects of light, we first developed a stochastic model of the cyclic photoisomerization of azobenzene. This model reproduces the collective photoisomerization kinetics of the azo stars in good agreement with theory and previous experiments. We then apply light of various intensities and wavelengths on an equilibrated columnar stack of azo stars in water. The simulations indicate that the aggregate does not break into separate fragments upon light irradiation. Instead, the stack develops defects in the form of molecular shifts and reorientations and, as a result, it eventually loses its columnar shape. The mechanism and driving forces behind this order-disorder structural transition are clarified based on the simulations. In the end, we provide a new interpretation of the experimentally observed morphological changes.

Columnar Aggregates of Azobenzene Stars: Exploring Intermolecular Interactions, Structure, and Stability in Atomistic Simulations

We present a simulation study of supramolecular aggregates formed by three-arm azobenzene (Azo) stars with a benzene-1,3,5-tricarboxamide (BTA) core in water. Previous experimental works by other research groups demonstrate that such Azo stars assemble into needle-like structures with light-responsive properties. Disregarding the response to light, we intend to characterize the equilibrium state of this system on the molecular scale. In particular, we aim to develop a thorough understanding of the binding mechanism between the molecules and analyze the structural properties of columnar stacks of Azo stars. Our study employs fully atomistic molecular dynamics (MD) simulations to model pre-assembled aggregates with various sizes and arrangements in water. In our detailed approach, we decompose the binding energies of the aggregates into the contributions due to the different types of non-covalent interactions and the contributions of the functional groups in the Azo stars. Initially, we investigate the origin and strength of the non-covalent interactions within a stacked dimer. Based on these findings, three arrangements of longer columnar stacks are prepared and equilibrated. We confirm that the binding energies of the stacks are mainly composed of π-π interactions between the conjugated parts of the molecules and hydrogen bonds formed between the stacked BTA cores. Our study quantifies the strength of these interactions and shows that the π-π interactions, especially between the Azo moieties, dominate the binding energies. We clarify that hydrogen bonds, which are predominant in BTA stacks, have only secondary energetic contributions in stacks of Azo stars but remain necessary stabilizers. Both types of interactions, π-π stacking and H-bonds, are required to maintain the columnar arrangement of the aggregates.

Light mediated drug delivery systems: a review

Achieving a novel drug delivery system needs site-specificity along with dosage control. Many physical, chemical, mechanical, and biological signals are used for developing these systems, out of which light has been used predominantly in the past decade. Light responsive drug delivery systems have tremendous potential, and their exploration is crucial in developing a precise and controlled delivery system. Spatio-temporal and intensity control of light allows better manipulation of drug delivery vehicles than mechanical, chemical, and biological signals. The use of ultraviolet (UV) and near-infrared (NIR) light has helped in upgrading therapeutic functionalities, while the use of up-conversion nanoparticles (UCNPs) has delivered an extension into theranostic tools. Biomaterials incorporated with photosensitizers can readily respond to changes in light and are vital in achieving clinical success via translational research. Further, the inclusion of biological macromolecules for the transportation of drugs, genes, and proteins has seen a broader application of light-controlled systems. The key objective of this review paper is to summarise the evolution of light-activated targeted drug delivery systems and the importance of biomaterials in developing one.

Photoreversible formation of nanotubes in water from an amphiphilic azobenzene derivative

An anionic azobenzene-appended derivative of L-ValylGlycine self-assembles into nanotubes in water. Irradiation with 365 nm light provokes trans-cis isomerization of the azobenzene unit and subsequent tube disassembly. Thermal or photoinduced (457 nm light) recovery of the trans isomer restores the nanotubes.

Scissor-Shaped Photochromic Dyads: Hierarchical Self-Assembly and Photoresponsive Property

Unique relationships between hierarchically organized biological nanostructures and functions have motivated chemists to construct sophisticated artificial nanostructured systems from small and simple synthetic molecules through self-assembly. As one of such sophisticated systems, we have investigated scissor-shaped photochromic dyads that can hierarchically self-assemble into discrete nanostructures showing photoresponsive properties. We synthesized various azobenzene dyads and found that these dyads adopt intramolecularly folded conformation like a closed scissor, and then self-assemble into toroidal nanostructures by generating curvature. The toroids further organize into nanotubes and further into helical supramolecular fibers depending on the nature of alkyl substituents. All of these nanostructures can be dissociated and reorganized through the photoisomerization of azobenzene units. On the other hand, the introduction of stilbene chromophores instead of azobenzenes leads to one-dimensional supramolecular polymerization, which upon the intramolecular photocyclization of stilbene chromophores shifts to curved self-assembly leading to helicoidal fibers with distinct supramolecular chirality.

Photoresponsive supramolecular strategy for controlled assembly in light-inert double-chain surfactant system

HYPOTHESIS

One of the main advances in double-chain surfactant systems has been their progress from the construction of assemblies to the transformation application in medicine and material science, especially to the drug delivery systems. Thus, it is critical to develop stimuli-responsive assemblies based on double-chain surfactants. We predicted that reversible assembly switching can be achieved by manipulation of the ternary host-guest competitive complexation among β-cyclodextrin (β-CD), surfactants, and designed azobenzene (Azo).

EXPERIMENTS

In this work, Azo was introduced into vesicles using supramolecular assembly strategy. Vesicles are formed only when Azo moieties are in trans-form. UV switching of Azo groups led to fast disruption of the Azo@β-CD complexes and relatively slow disintegration of the vesicles. With the alterative irradiation of UV and Vis light, the photoisomerization of azo group provokes the reversible disassembly and reassembly of vesicles.

FINDINGS

This photo-responsive supramolecular strategy offered a controllable way to prepare responsive vesicles assembled from complex double-chain surfactants, such as phospholipids, which could be further used in drug delivery systems. This new perspective is instructive for the design and functional use of complex surfactants assembly. Importantly, the study results paved the way for the development of novel light-responsive assembly materials operating in aqueous media and biological field.

Gold Nanobeads with Enhanced Absorbance for Improved Sensitivity in Competitive Lateral Flow Immunoassays

BACKGROUND

Colloidal gold based lateral flow immunoassay (LFIA) commonly suffers from relatively low detection sensitivity due to the insufficient brightness of conventional gold nanoparticles (AuNPs) with the size of 20-40 nm.

METHODS

Herein, three kinds of gold nanobeads (GNBs) with the size of 94 nm, 129 nm, and 237 nm, were synthesized by encapsulating numerous hydrophobic AuNPs (10 nm) into polymer matrix. The synthesized GNBs exhibited the enhanced colorimetric signal intensity compared with 20-40 nm AuNPs. The effects of the size of GNBs on the sensitivity of LFIA with competitive format were assessed.

RESULTS

The results showed that the LFIA using 129 nm GNBs as amplified signal probes exhibits the best sensitivity for fumonisin B1 (FB1) detection with a cut-off limit (for visual qualitative detection) at 125 ng/mL, a half maximal inhibitory concentration at 11.27 ng/mL, and a detection limit at 1.76 ng/mL for detection of real corn samples, which are 8-, 3.82-, and 2.89-fold better than those of conventional AuNP40-based LFIA, respectively. The developed GNB-LFIA exhibited negligible cross-reactions with other common mycotoxins. In addition, the accuracy, precision, reliability, and practicability were demonstrated by determining real corn samples.

CONCLUSIONS

All in all, the proposed study provides a promising strategy to enhance the sensitivity of competitive LFIA via using the GNBs as amplified signal probes.

Self-Assembly of Photoresponsive Molecular Amphiphiles in Aqueous Media

Amphiphilic molecules, comprising hydrophobic and hydrophilic moieties and the intrinsic propensity to self-assemble in aqueous environment, sustain a fascinating spectrum of structures and functions ranging from biological membranes to ordinary soap. Facing the challenge to design responsive, adaptive, and out-of-equilibrium systems in water, the incorporation of photoresponsive motifs in amphiphilic molecular structures offers ample opportunity to design supramolecular systems that enables functional responses in water in a non-invasive way using light. Here, we discuss the design of photoresponsive molecular amphiphiles, their self-assembled structures in aqueous media and at air-water interfaces, and various approaches to arrive at adaptive and dynamic functions in isotropic and anisotropic systems, including motion at the air-water interface, foam formation, reversible nanoscale assembly, and artificial muscle function. Controlling the delicate interplay of structural design, self-assembling conditions and external stimuli, these responsive amphiphiles open several avenues towards application such as soft adaptive materials, controlled delivery or soft actuators, bridging a gap between artificial and natural dynamic systems.

Stimuli responsive dynamic transformations in supramolecular gels

Supramolecular gels are formed by the self-assembly of small molecules under the influence of various non-covalent interactions. As the interactions are individually weak and reversible, it is possible to perturb the gels easily, which in turn enables fine tuning of their properties. Synthetic supramolecular gels are kinetically trapped and usually do not show time variable changes in material properties after formation. However, such materials potentially become switchable when exposed to external stimuli like temperature, pH, light, enzyme, redox, and chemical analytes resulting in reconfiguration of gel matrix into a different type of network. Such transformations allow gel-to-gel transitions while the changes in the molecular aggregation result in alteration of physical and chemical properties of the gel with time. Here, we discuss various methods that have been used to achieve gel-to-gel transitions by modifying a pre-formed gel material through external perturbation. We also describe methods that allow time-dependent autonomous switching of gels into different networks enabling synthesis of next generation functional materials. Dynamic modification of gels allows construction of an array of supramolecular gels with various properties from a single material which eventually extend the limit of applications of the gels. In some cases, gel-to-gel transitions lead to materials that cannot be accessed directly. Finally, we point out the necessity and possibility of further exploration of the field.

Diarylethene-Powered Light-Induced Folding of Supramolecular Polymers

Helical folding of randomly coiled linear polymers is an essential organization process not only for biological polypeptides but also for synthetic functional polymers. Realization of this dynamic process in supramolecular polymers (SPs) is, however, a formidable challenge because of their inherent lability of main chains upon changing an external environment that can drive the folding process (e.g., solvent, concentration, and temperature). We herein report a photoinduced reversible folding/unfolding of rosette-based SPs driven by photoisomerization of a diarylethene (DAE). Temperature-controlled supramolecular polymerization of a barbiturate-functionalized DAE (open isomer) in nonpolar solvent results in the formation of intrinsically curved, but randomly coiled, SPs due to the presence of defects. Irradiation of the randomly coiled SPs with UV light causes efficient ring-closure reaction of the DAE moieties, which induces helical folding of the randomly coiled structures into helicoidal ones, as evidenced by atomic force microscopy and small-angle X-ray scattering. The helical folding is driven by internal structure ordering of the SP fiber that repairs the defects and interloop interaction occurring only for the resulting helicoidal structure. In contrast, direct supramolecular polymerization of the ring-closed DAE monomers by temperature control affords linearly extended ribbon-like SPs lacking intrinsic curvature that are thermodynamically less stable compared to the helicoidal SPs. The finding represents an important concept applicable to other SP systems; that is, postpolymerization (photo)reaction of preorganized kinetic structures can lead to more thermodynamically stable structures that are inaccessible directly through temperature-controlled protocols.

A Highly Ordered Quantum Dot Supramolecular Assembly Exhibiting Photoinduced Emission Enhancement

Multicomponent supramolecular assembly systems enable the generation of materials with outstanding properties, not obtained from single-component systems, via a synergetic effect. Herein, we demonstrate a novel supramolecular coassembly system rendering highly ordered quantum dot (QD) arrangement structures formed via the self-assembly of azobenzene derivatives, where the photocontrollable photoluminescence (PL) properties of the QDs are realized based on photoisomerization. Upon mixing the assembled azobenzene derivatives and QDs in apolar media, a time-evolution coaggregation into hierarchical nanosheets with a highly ordered QD arrangement structure occurs. Upon photoirradiation, the nanosheets transform into ill-defined aggregates without arranged QDs together with enhancing the PL intensity. In days, the photoirradiated coaggregates undergo recovery of the PL properties corresponding to the arranged QDs through thermal isomerization.

Circularly polarized luminescence of nanoassemblies via multi-dimensional chiral architecture control

Circularly polarized luminescence (CPL) materials are currently an important class of chiroptical materials that are attracting increasing interest. Nanoassemblies constructed from chiral or achiral building blocks show great potential for achieving CPL-active nanomaterials with high quantum yields and dissymmetry factors, which is crucial for further applications. In nanoassemblies, the dimensional morphology affects the chiroptical properties significantly since the microscopic packing modes will affect the luminescence processes and chirality transfer processes. In this review, we will show some examples for illustrating the relationship between multi-dimensional morphology and chiroptical properties. Furthermore, with dimensional morphology tuning, higher dissymmetry factors would be obtained. We hope to provide a useful and powerful insight into the design and control of CPL-active nanoassemblies via morphology control.

An Arylazopyrazole-Based N-Heterocyclic Carbene as a Photoswitch on Gold Surfaces: Light-Switchable Wettability, Work Function, and Conductance

A novel photoresponsive and fully conjugated N-heterocyclic carbene (NHC) has been synthesized that combines the excellent photophysical properties of arylazopyrazoles (AAPs) with an NHC that acts as a robust surface anchor (AAP-BIMe). The formation of self-assembled monolayers (SAMs) on gold was proven by ToF-SIMS and XPS, and the organic film displayed a very high stability at elevated temperatures. This stability was also reflected in a high desorption energy, which was determined by temperature-programmed SIMS measurements. E-/Z-AAP-BIMe@Au photoisomerization resulted in reversible alterations of the surface energy (i.e. wettability), the surface potential (i.e. work function), and the conductance (i.e. resistance). The effects could be explained by the difference in the dipole moment of the isomers. Furthermore, sequential application of a dummy ligand by microcontact printing and subsequent backfilling with AAP-BIMe allowed its patterning on gold. To the best of our knowledge, this is the first example of a photoswitchable NHC on a gold surface. These properties of AAP-BIMe@Au illustrate its suitability as a molecular switch for electronic devices.

Self-assembly of alkylated and perfluoroalkylated scissor-shaped azobenzene dyads into distinct structures

Two scissor-shaped azobenzene dyads bearing either linear alkyl or perfluoroalkyl side chains undergo distinct assembly pathways.

Do Columns of Azobenzene Stars Disassemble under Light Illumination?

The clustering properties of star-shaped molecules comprising three photochromic azobenzene-containing arms are investigated with specific focus on the influence of light on these structures. Previous experimental works report self-assembly of azobenzene stars in aqueous solution into long columnar clusters that are detectable using optical microscopy. These clusters appear to vanish under UV irradiation, which is known to induce trans-to-cis photoisomerization of the azobenzene groups. We have performed MD simulations, density functional theory, and density functional tight binding calculations to determine conformational properties and binding energies of these clusters. Our simulation data suggest that the binding strength of the clusters is large enough to prevent a breaking along their main axis. We conclude that very likely other mechanisms lead to the apparent disappearance of the clusters.

Photoresponsive Circular Supramolecular Polymers: A Topological Trap and Photoinduced Ring-Opening Elongation

Topological features of one-dimensional macromolecular chains govern the properties and functionality of natural and synthetic polymers. To address this issue in supramolecular polymers, we synthesized two topologically distinct supramolecular polymers with intrinsic curvature, circular and helically folded nanofibers, from azobenzene-functionalized supramolecular rosettes. When a mixture of circular and helically folded nanofibers was exposed to UV light, selective unfolding of the latter open-ended supramolecular polymers was observed as a result of the curvature-impairing internal force produced by the trans-to-cis photoisomerization of the azobenzene. This distinct sensitivity suggests that the topological features of supramolecular polymers define their mechanical stability. Furthermore, the exposure of circular supramolecular polymers in more polar media to UV irradiation resulted in ring opening followed by chain elongation, thus demonstrating that the circular supramolecular polymer can function as a topological kinetic trap.

Influence of metal coordination and light irradiation on hierarchical self-assembly processes

Smart light-responsive supramolecular materials have been extensively investigated in the past decade, but so far the impact of metal coordination on hierarchical supramolecular structures of light-responsive building blocks has remained nearly unexplored. Herein, we unravel the hierarchical self-assembly of a small π-conjugated azo-containing pyridyl ligand that is able to respond to UV-light and metal complexation. The ligand self-assembles in an antiparallel fashion into long twisted fibers, which are then disassembled upon photoisomerization of the azobenzene groups, resulting in shorter rigid rods with a different packing motif. Complexation of Pd(ii) ions enhances the cooperativity of the aggregation and induces a molecular rearrangement into slipped stacks with subsequent formation of long thin fibers. These are then transformed into thinner, shorter rods upon light irradiation. The observed different light-responsiveness, besides clearing up the influence of metal coordination and light irradiation in self-assembly processes, paves the way towards the design of novel supramolecular photochromic systems.

Optically-controlled supramolecular self-assembly of an antibiotic for antibacterial regulation

A new supramolecular photo-responsive antibiotic (azobenzene-norfloxacin/αCD) exhibited a higher “on–off” ratio of antibacterial ability than Azo-Nor alone.

The structural and electronic properties of 3,3′-azothiophene photo-switching systems

3,3′-Azothiophenes demonstrate effective photo-induced isomerization in solution. Activation energies and entropies confirm the thermal stability of cis conformations at 20 °C.

[2+2] Halogen-bonded boxes employing azobenzenes

Herein, we report the synthesis and crystal structures of three [2+2] supramolecular boxes assembled by halogen bonding.

The effect of low- and high-penetration light on localized cancer therapy

The design of a delivery system allowing targeted and controlled drug release has been considered one of the main strategies used to provide individualized cancer therapy, to improve survival statistics, and to enhance quality-of-life. External stimuli including low- and high-penetration light have been shown to have the ability to turn drug delivery on and off in a non-invasive remotely-controlled fashion. The success of this approach has been closely related to the development of a variety of drug delivery systems - from photosensitive liposomes to gold nanocages - and relies on multiple mechanisms of drug release activation. In this review, we make reference to the two extremes of the light spectrum and their potential as triggers for the delivery of antitumor drugs, along with the most recent achievements in preclinical trials and the challenges to an efficient translation of this technology to the clinical setting.

Hydrogen-bonded six-component assembly for capsule formation based on tetra(4-pyridyl)cavitand and isophthalic acid linker and its application to photoresponsive capsule

Two molecules of tetra(4-pyridyl)cavitand 1 and four molecules of isophthalic acid linker 2a with a triethylene glycol monomethyl ether (TEG) group self-assembled into a six-component capsule 1₂·2a₄ through eight pyNHO₂C hydrogen bonds, which encapsulates one molecule of guest G such as bis(4-acetoxyphenyl)acetylene and hexakis(4-iodophenyl)benzene to form G@(1₂·2a₄). Guest-encapsulation ability and selectivity of 1₂·2a₄ were revealed. trans-5-(p-Substituted-phenylazo)isophthalic acid with two dichotomous branching TEG groups trans-2b serves as a photoresponsive linker to form 1₂·(trans-2b)₄, which moderately reduced guest-encapsulation ability upon photoisomerization (at the photostationary state, 10% guest release upon subunit-trans-2b/subunit-cis-2b = 18 : 82).

Arylazopyrazole Photoswitches in Aqueous Solution: Substituent Effects, Photophysical Properties, and Host-Guest Chemistry

Getting the green light! Substituted arylazopyrazoles (AAPs) have been investigated as supramolecular photoswitches in aqueous solution. Selective photostationary states (PSSs) and improved binding affinities to β-cyclodextrin have been determined. The experimental findings are supported by results from DFT calculations.