Photoresponsive Supramolecular Systems Research Articles

Supramolecular assemblies of amphiphilic donor-acceptor Stenhouse adducts as macroscopic soft scaffolds.

In the design of photoharvesting and photoresponsive supramolecular systems in aqueous medium, the fabrication of amphiphilic photoswitches enables a noninvasive functional response through photoirradiation. Although most aqueous supramolecular assemblies are driven by high-energy and biodamaging UV light, we have previously reported a design of amphiphilic donor-acceptor Stenhouse adducts (DASAs) controlled by white light. Herein, we present a series of DASA amphiphiles (DAs) with minor structural modifications on the alkyl linker chain length connecting the DASA motif with the hydrophilic moiety. The excellent photoswitchability in organic medium and the photoresponsiveness in aqueous medium, driven by visible light, were investigated by UV-vis absorption spectroscopy. The assembled supramolecular nanostructures were confirmed by electron microscopy, while the supramolecular packing was revealed by X-ray diffraction analysis. Upon visible-light irradiation, significant transformations of the DA geometry enabled transformations of the supramolecular assemblies on a microscopic scale, subsequently disassembling macroscopic soft scaffolds of DAs. The current work shows promising use for the fabrication of visible-light-controlled macroscopic scaffolds, offering the next generation of biomedical materials with visible-light-controlled microenvironments and future soft-robotic systems.

Photoisomerization, assembling and fluorescence photoswitching behaviors of a water-soluble stiff-stilbene with cucurbit[7]uril

Photoresponsive supramolecular systems have merited extensive attention for their applications in materials science and life science. Here, we synthesized a water-soluble stiff-stilbene molecular photoswitch, exhibiting outstanding photoisomerization reaction between its (E)- and (Z)-configurations upon irradiation at distinct light. The photoswitch can assemble with cucurbit[7]uril (CB[7]) to form a superior fluorescent supramolecular complex (compared to the only guest) with excellent water solubility, which manifested dramatic photoluminescence enhancement caused by restriction of intramolecular rotation and remained good photochromic characteristics. Furthermore, introduction of CB[7] influence photoreaction quantum yield (Φ) of the stiff-stilbene, leading to reduction of ΦE→Z and increase of ΦZ→E of the stiff-stilbene. Importantly, the photoadjustable supramolecular assembly can act as a fluorescence switch, and the phototunable guest further selectively modulate G-quadruplex structure of Tel22 upon light irradiation or with addition of CB[7]. The study provides a new simple way for accurately regulating photochromic properties of molecular switches and developing smart materials with potential applications in controlled modulation of G-quadruplex, targeted biological imaging and so on.

Multicolor-Luminescence Including White Light by Photomodulation of Supramolecular Assemblies in Aqueous Media.

Photo-responsive supramolecular systems offer intriguing functional aspects which have led to their applications in diverse fields such as optoelectronics and biomedicine. However, the modulation of the luminescence output in a spatiotemporal fashion by photo-controlled transformation still remains a challenging task. Herein, we report the controlled regulation of the emission color of supramolecular assemblies of amphiphilic cyanostilbenes (CSs) in water through in situ photomodulation employing UV and sunlight. Due to their aggregation-induced emission (AIE) features, the CS chromophores in the supramolecular assemblies exhibited bright greenish-yellow emission. Photoirradiation predominantly led to the formation of a cyclized product exhibiting aggregation-caused quenching (ACQ) features and having efficient cyan-blue emission in water but severely quenched emission in the solid state. Hence, starting from a unicomponent scaffold, photomodulation provided tunable emission ranging from greenish-yellow to cyan-blue including white light in water. Furthermore, using the contrasting AIE and ACQ behavior of the components in the photoirradiated mixtures, we were able to design rewritable fluorescent inks and encryption in solid films indicating the practical utility of these systems.

Advances in the Structural Strategies of the Self-Assembly of Photoresponsive Supramolecular Systems.

Photosensitive supramolecular systems have garnered attention due to their potential to catalyze highly specific tasks through structural changes triggered by a light stimulus. The tunability of their chemical structure and charge transfer properties provides opportunities for designing and developing smart materials for multidisciplinary applications. This review focuses on the approaches reported in the literature for tailoring properties of the photosensitive supramolecular systems, including MOFs, MOPs, and HOFs. We discuss relevant aspects regarding their chemical structure, action mechanisms, design principles, applications, and future perspectives.

Reversible ON-OFF switching of FRET effect in the functionalized CB[6]-guest complex via photoisomerization

Host-guest complexation of amino-substituted styryl dye in basic (ASP) and protonated (ASPH+) forms with cucurbit[6]uril functionalized with naphthalene fluorophore (CB[6]-Nap) was used to obtained photoresponsive supramolecular systems with fluorescence resonance energy transfer (FRET) effect. Energy transfer from the naphthalene group of CB[6]-Nap (energy donor) to ASP or ASPH+ (energy acceptor) in inclusion complexes takes place with an efficiency of 75% and 92%, respectively. E-Z photoisomerization of ASPH+ in the presence and in the absence of CB[6]-Nap has been thoroughly investigated by UV/Vis absorption and fluorescence spectroscopy and NMR spectroscopy. The bending of ASPH+ structure upon E-to Z-isomerization leads to dissociation of the host-guest complex and, as a consequence, to the disappearance of energy transfer, while the back photoisomerization returns the system to its original state. Thus, light-controlled encapsulation and release of dye molecule in the ASPH+@CB[6]-Nap complex allow the FRET process to be reversibly switched. The results obtained demonstrated a novel example of the spatiotemporal control of complexation and fluorescence response in the supramolecular system by light.

Supramolecular Packing and Macroscopic Alignment Controls Actuation Speed in Macroscopic Strings of Molecular Motor Amphiphiles.

Three-dimensional organized unidirectionally aligned and responsive supramolecular structures have much potential in adaptive materials ranging from biomedical components to soft actuator systems. However, to control the supramolecular structure of these stimuli responsive, for example photoactive, materials and control their actuation remains a major challenge. Toward the design of “artificial muscles”, herein, we demonstrate an approach that allows hierarchical control of the supramolecular structure, and as a consequence its photoactuation function, by electrostatic interaction between motor amphiphiles (MA) and counterions. Detailed insight into the effect of various ions on structural parameters for self-assembly from nano- to micrometer scale in water including nanofiber formation and nanofiber aggregation as well as the packing structure, degree of alignment, and actuation speed of the macroscopic MA strings prepared from various metal chlorides solution, as determined by electronic microscopy, X-ray diffraction, and actuation speed measurements, is presented. Macroscopic MA strings prepared from calcium and magnesium ions provide a high degree of alignment and fast response photoactuation. By the selection of metal ions and chain length of MAs, the macroscopic MA string structure and function can be controlled, demonstrating the potential of generating multiple photoresponsive supramolecular systems from an identical molecular structure.

Poly(ionic liquid)-based nanogels and their reversible photo-mediated association and dissociation

Azo-incorporated PIL nanogels can undergo reversible photo-mediated association and dissociation, and they can also be used as the building blocks to fabricate photo-responsive supramolecular system.

An artificial muscle model unit based on inorganic nanosheet sliding by photochemical reaction

From the viewpoint of developing photoresponsive supramolecular systems in microenvironments to exhibit more sophisticated photo-functions even at the macroscopic level, inorganic/organic hybrid compounds based on clay or niobate nanosheets as the microenvironments were prepared, characterized, and examined for their photoreactions. We show here a novel type of artificial muscle model unit having much similarity with that in natural muscle fibrils. Upon photoirradiation, the organic/inorganic hybrid nanosheets reversibly slide horizontally on a giant scale, and the interlayer spaces in the layered hybrid structure shrink and expand vertically. In particular, our layered hybrid molecular system exhibits a macroscopic morphological change on a giant scale (~1500 nm) compared with the molecular size of ~1 nm, based on a reversible sliding mechanism.

Tunable nonlinear optical properties of PDA/Ag composite vesicles based on reversible host–guest interaction

We develop a novel photo-responsive supramolecular system in which the aggregation and disaggregation of polydiacetylene/silver (PDA/Ag) composite vesicles is mediated by a reversible photo-responsive molecular recognition process. The PDA/Ag composite vesicles provide necessary nonlinear optical (NLO) properties, the host–guest interaction between azobenzene and β-cyclodextrin get PDA/Ag composite vesicles sufficiently close to each other for providing the enhanced surface plasmon resonance and a corresponding NLO effect. NLO switching of PDA/Ag composite vesicles based on reversible host–guest interactions is realized by alternating visible and UV light irradiation, which may introduce a new way for preparation of smart NLO materials.

Photoresponsive Molecular Recognition and Adhesion of Vesicles in a Competitive Ternary Supramolecular System

A competitive photoresponsive supramolecular system is formed in a dilute aqueous solution of three components: vesicles of amphiphilic α-cyclodextrin host 1a, divalent p-methylphenyl guest 2 or divalent p-methylbenzamide guest 3, and photoresponsive azobenzene monovalent guest 5. Guests 2 and 3 form weak inclusion complexes with 1a (K(a)≈10(2) M(-1)), whereas azobenzene guest 5 forms a strong inclusion complex (K(a)≈10(4) M(-1)), provided it is in the trans state. The aggregation and adhesion of vesicles of host 1a is mediated by guest 2 (or 3) due to the formation of multiple intervesicular noncovalent links, as confirmed by using isothermal titration calorimetry (ITC), optical density measurements at 600 nm (OD600), dynamic light scattering (DLS), and cryogenic transmission electron microscopy (cryo-TEM). The addition of excess monovalent guest trans-5 to vesicles of 1a aggregated by divalent guest 2 (or 3) causes the dispersion of vesicles of 1a because trans-5 displaces 2 (as well as 3) from the vesicle surface. Upon UV irradiation of a dilute ternary mixture of vesicles of 1a, guest 2 (or 3), and competitor trans-5, compound trans-5 isomerizes to cis-5, and renewed aggregation of vesicles of 1a by guest 2 (or 3) occurs because 2 (as well as 3) displaces cis-5 from the vesicle surface. Subsequent visible irradiation causes the redispersion of vesicles of 1a because cis-5 reisomerizes into trans-5, which again displaces guest 2 (or 3) from the vesicle surface. In this way, the competitive photoresponsive aggregation and dispersion of vesicles can be repeated for several cycles.

High Binding Affinity of DABCO with Porphyrin in a Porphyrin-cis-Stilbene-Porphyrin Triad

Porphyrin-stilbene-porphyrin triad (mZnPst) where a central cis-stilbene unit is connected to zinc-tetraphenylporphyrin (ZnTPP) was synthesized and its binding ability with a selected guest ligand DABCO was investigated. The association constant was evaluated to be 2.47 x 10 8 M -1 , from the iterative least squares fitting to a 1:1 binding model, which is larger than that of monomer ZnTPP/DABCO complex (3.50 x 10 5 M -1 ). In addition, when a toluene solution of mZnPst is mixed with 1 equivalent of DABCO, the color of the solution was changed abruptly to light purple from bright reddish, providing further a visual evidence of forming strong complex. Such a high association constant suggests mZnPst/DABCO to be a promising photoresponsive supramolecular system.

Photoresponsive supramolecular systems: self-assembly of azodibenzoic acid linear tapes and cyclic tetramers.

A new strategy to effect photoinduced control over molecular self-assembly is reported. This strategy uses the reversible trans-cis photoisomerization of a novel azobenzene system, where the trans- and cis-forms self-assemble into dramatically different higher-order structures. The trans-azobenzene form of this molecule associates into infinite hydrogen-bonded linear tapes, while the cis-azobenzene form undergoes hydrogen-bonded self-assembly into cyclic tetramers. This results in a second level of association, where the cis-hydrogen-bonded supramolecular cycles ultimately form long, rod-like aggregates through stacking interactions.

Synthesis, Structural, Spectroscopic, and Alkali-Metal Cations Complexation Studies of a Bis-Anthracenediyl Macrotricyclic Ditopic Receptor

The macrotricyclic bis-anthracenediyl receptor (1) has been prepared in order to study the interplay between cation complexation and fluores- cence emission. The structure of this photoresponsive supramolecular system is based on two diazacrown ether moieties (N 2 O 4 ) linked through two anthracene chromophore-containing flexible spacers. (1) displays a dual (locally excited and excimer-type) fluorescence emission, and the relative contribution (Φ FE /Φ FM ) of these emissions is sensitive to metal cations. A spectroscopic study shows that the optical responses are dependent on the nature of the complexed cation, and the more pro- nounced effect is noticed with Rb +

Tunable photoresponsive supramolecular systems

Abstract