Photocleavable Anionic Glues for Light-Responsive Nanoparticle Aggregates.

Integrating light-sensitive molecules within nanoparticle (NP) assemblies is an attractive approach to fabricate new photoresponsive nanomaterials. Here, we describe the concept of photocleavable anionic glue (PAG): small trianions capable of mediating interactions between (and inducing the aggregation of) cationic NPs by means of electrostatic interactions. Exposure to light converts PAGs into dianionic products incapable of maintaining the NPs in an assembled state, resulting in light-triggered disassembly of NP aggregates. To demonstrate the proof-of-concept, we work with an organic PAG incorporating the UV-cleavable o-nitrobenzyl moiety and an inorganic PAG, the photosensitive trioxalatocobaltate(III) complex, which absorbs light across the entire visible spectrum. Both PAGs were used to prepare either amorphous NP assemblies or regular superlattices with a long-range NP order. These NP aggregates disassembled rapidly upon light exposure for a specific time, which could be tuned by the incident light wavelength or the amount of PAG used. Selective excitation of the inorganic PAG in a system combining the two PAGs results in a photodecomposition product that deactivates the organic PAG, enabling nontrivial disassembly profiles under a single type of external stimulus.

Guest Encapsulation Alters the Thermodynamic Landscape of a Coordination Host.

The architecture of self-assembled host molecules can profoundly affect the properties of the encapsulated guests. For example, a rigid cage with small windows can efficiently protect its contents from the environment; in contrast, tube-shaped, flexible hosts with large openings and an easily accessible cavity are ideally suited for catalysis. Here, we report a "Janus" nature of a Pd6L4 coordination host previously reported to exist exclusively as a tube isomer (T). We show that upon encapsulating various tetrahedrally shaped guests, T can reconfigure into a cage-shaped host (C) in quantitative yield. Extracting the guest affords empty C, which is metastable and spontaneously relaxes to T, and the T⇄C interconversion can be repeated for multiple cycles. Reversible toggling between two vastly different isomers paves the way toward controlling functional properties of coordination hosts "on demand".

Altering the Properties of Spiropyran Switches Using Coordination Cages with Different Symmetries.

Molecular confinement effects can profoundly alter the physicochemical properties of the confined species. A plethora of organic molecules were encapsulated within the cavities of supramolecular hosts, and the impact of the cavity size and polarity was widely investigated. However, the extent to which the properties of the confined guests can be affected by the symmetry of the cage─which dictates the shape of the cavity─remains to be understood. Here we show that cage symmetry has a dramatic effect on the equilibrium between two isomers of the encapsulated spiropyran guests. Working with two Pd-based coordination cages featuring similarly sized but differently shaped hydrophobic cavities, we found a highly selective stabilization of the isomer whose shape matches that of the cavity of the cage. A Td-symmetric cage stabilized the spiropyrans' colorless form and rendered them photochemically inert. In contrast, a D2h-symmetric cage favored the colored isomer, while maintaining reversible photoswitching between the two states of the encapsulated spiropyrans. We also show that the switching kinetics strongly depend on the substitution pattern on the spiropyran scaffold. This finding was used to fabricate a time-sensitive information storage medium with tunable lifetimes of the encoded messages.

Molecular Photoswitching in Confined Spaces.

In nature, light is harvested by photoactive proteins to drive a range of biological processes, including photosynthesis, phototaxis, vision, and ultimately life. Bacteriorhodopsin, for example, is a protein embedded within archaeal cell membranes that binds the chromophore retinal within its hydrophobic pocket. Exposure to light triggers regioselective photoisomerization of the confined retinal, which in turn initiates a cascade of conformational changes within the protein, triggering proton flux against the concentration gradient, providing the microorganisms with the energy to live. We are inspired by these functions in nature to harness light energy using synthetic photoswitches under confinement. Like retinal, synthetic photoswitches require some degree of conformational flexibility to isomerize. In nature, the conformational change associated with retinal isomerization is accommodated by the structural flexibility of the opsin host, yet it results in steric communication between the chromophore and the protein. Similarly, we strive to design systems wherein isomerization of confined photoswitches results in steric communication between a photoswitch and its confining environment. To achieve this aim, a balance must be struck between molecular crowding and conformational freedom under confinement: too much crowding prevents switching, whereas too much freedom resembles switching of isolated molecules in solution, preventing communication.In this Account, we discuss five classes of synthetic light-switchable compounds-diarylethenes, anthracenes, azobenzenes, spiropyrans, and donor-acceptor Stenhouse adducts-comparing their behaviors under confinement and in solution. The environments employed to confine these photoswitches are diverse, ranging from planar surfaces to nanosized cavities within coordination cages, nanoporous frameworks, and nanoparticle aggregates. The trends that emerge are primarily dependent on the nature of the photoswitch and not on the material used for confinement. In general, we find that photoswitches requiring less conformational freedom for switching are, as expected, more straightforward to isomerize reversibly under confinement. Because these compounds undergo only small structural changes upon isomerization, however, switching does not propagate into communication with their environment. Conversely, photoswitches that require more conformational freedom are more challenging to switch under confinement but also can influence system-wide behavior.Although we are primarily interested in the effects of geometric constraints on photoswitching under confinement, additional effects inevitably emerge when a compound is removed from solution and placed within a new, more crowded environment. For instance, we have found that compounds that convert to zwitterionic isomers upon light irradiation often experience stabilization of these forms under confinement. This effect results from the mutual stabilization of zwitterions that are brought into close proximity on surfaces or within cavities. Furthermore, photoswitches can experience preorganization under confinement, influencing the selectivity and efficiency of their photoreactions. Because intermolecular interactions arising from confinement cannot be considered independently from the effects of geometric constraints, we describe all confinement effects concurrently throughout this Account.

Reversible photoswitching of encapsulated azobenzenes in water.

Efficient molecular switching in confined spaces is critical for the successful development of artificial molecular machines. However, molecular switching events often entail large structural changes and therefore require conformational freedom, which is typically limited under confinement conditions. Here, we investigated the behavior of azobenzene-the key building block of light-controlled molecular machines-in a confined environment that is flexible and can adapt its shape to that of the bound guest. To this end, we encapsulated several structurally diverse azobenzenes within the cavity of a flexible, water-soluble coordination cage, and investigated their light-responsive behavior. Using UV/Vis absorption spectroscopy and a combination of NMR methods, we showed that each of the encapsulated azobenzenes exhibited distinct switching properties. An azobenzene forming a 1:1 host-guest inclusion complex could be efficiently photoisomerized in a reversible fashion. In contrast, successful switching in inclusion complexes incorporating two azobenzene guests was dependent on the availability of free cages in the system, and it involved reversible trafficking of azobenzene between the cages. In the absence of extra cages, photoswitching was either suppressed or it involved expulsion of azobenzene from the cage and consequently its precipitation from the solution. This finding was utilized to develop an information storage medium in which messages could be written and erased in a reversible fashion using light.

Light-Responsive Dynamic DNA-Origami-Based Plasmonic Assemblies.

DNA nanotechnology offers a versatile toolbox for precise spatial and temporal manipulation of matter on the nanoscale. However, rendering DNA-based systems responsive to light has remained challenging. Herein, we describe the remote manipulation of native (non-photoresponsive) chiral plasmonic molecules (CPMs) using light. Our strategy is based on the use of a photoresponsive medium comprising a merocyanine-based photoacid. Upon exposure to visible light, the medium decreases its pH, inducing the formation of DNA triplex links, leading to a spatial reconfiguration of the CPMs. The process can be reversed simply by turning the light off and it can be repeated for multiple cycles. The degree of the overall chirality change in an ensemble of CPMs depends on the CPM fraction undergoing reconfiguration, which, remarkably, depends on and can be tuned by the intensity of incident light. Such a dynamic, remotely controlled system could aid in further advancing DNA-based devices and nanomaterials.

Modulating the Optical Properties of BODIPY Dyes by Noncovalent Dimerization within a Flexible Coordination Cage.

Aggregation of organic molecules can drastically affect their physicochemical properties. For instance, the optical properties of BODIPY dyes are inherently related to the degree of aggregation and the mutual orientation of BODIPY units within these aggregates. Whereas the noncovalent aggregation of various BODIPY dyes has been studied in diverse media, the ill-defined nature of these aggregates has made it difficult to elucidate the structure-property relationships. Here, we studied the encapsulation of three structurally simple BODIPY derivatives within the hydrophobic cavity of a water-soluble, flexible PdII6L4 coordination cage. The cavity size allowed for the selective encapsulation of two dye molecules, irrespective of the substitution pattern on the BODIPY core. Working with a model, a pentamethyl-substituted derivative, we found that the mutual orientation of two BODIPY units in the cage's cavity was remarkably similar to that in the crystalline state of the free dye, allowing us to isolate and characterize the smallest possible noncovalent H-type BODIPY aggregate, namely, an H-dimer. Interestingly, a CF3-substituted BODIPY, known for forming J-type aggregates, was also encapsulated as an H-dimer. Taking advantage of the dynamic nature of encapsulation, we developed a system in which reversible switching between H- and J-aggregates can be induced for multiple cycles simply by addition and subsequent destruction of the cage. We expect that the ability to rapidly and reversibly manipulate the optical properties of supramolecular inclusion complexes in aqueous media will open up avenues for developing detection systems that operate within biological environments.

Molecular Factors Controlling the Isomerization of Azobenzenes in the Cavity of a Flexible Coordination Cage.

Photoswitchable molecules are employed for many applications, from the development of active materials to the design of stimuli-responsive molecular systems and light-powered molecular machines. To fully exploit their potential, we must learn ways to control the mechanism and kinetics of their photoinduced isomerization. One possible strategy involves confinement of photoresponsive switches such as azobenzenes or spiropyrans within crowded molecular environments, which may allow control over their light-induced conversion. However, the molecular factors that influence and control the switching process under realistic conditions and within dynamic molecular regimes often remain difficult to ascertain. As a case study, here we have employed molecular models to probe the isomerization of azobenzene guests within a Pd(II)-based coordination cage host in water. Atomistic molecular dynamics and metadynamics simulations allow us to characterize the flexibility of the cage in the solvent, the (rare) guest encapsulation and release events, and the relative probability/kinetics of light-induced isomerization of azobenzene analogues in these host-guest systems. In this way, we can reconstruct the mechanism of azobenzene switching inside the cage cavity and explore key molecular factors that may control this event. We obtain a molecular-level insight on the effects of crowding and host-guest interactions on azobenzene isomerization. The detailed picture elucidated by this study may enable the rational design of photoswitchable systems whose reactivity can be controlled via host-guest interactions.

Improving Fatigue Resistance of Dihydropyrene by Encapsulation within a Coordination Cage.

Photochromic molecules undergo reversible isomerization upon irradiation with light at different wavelengths, a process that can alter their physical and chemical properties. For instance, dihydropyrene (DHP) is a deep-colored compound that isomerizes to light-brown cyclophanediene (CPD) upon irradiation with visible light. CPD can then isomerize back to DHP upon irradiation with UV light or thermally in the dark. Conversion between DHP and CPD is thought to proceed via a biradical intermediate; bimolecular events involving this unstable intermediate thus result in rapid decomposition and poor cycling performance. Here, we show that the reversible isomerization of DHP can be stabilized upon confinement within a PdII6L4 coordination cage. By protecting this reactive intermediate using the cage, each isomerization reaction proceeds to higher yield, which significantly decreases the fatigue experienced by the system upon repeated photocycling. Although molecular confinement is known to help stabilize reactive species, this effect is not typically employed to protect reactive intermediates and thus improve reaction yields. We envisage that performing reactions under confinement will not only improve the cyclic performance of photochromic molecules, but may also increase the amount of product obtainable from traditionally low-yielding organic reactions.

Light-Driven Proton Transfer for Cyclic and Temporal Switching of Enzymatic Nanoreactors.

Temporal activation of biological processes by visible light and subsequent return to an inactive state in the absence of light is an essential characteristic of photoreceptor cells. Inspired by these phenomena, light-responsive materials are very attractive due to the high spatiotemporal control of light irradiation, with light being able to precisely orchestrate processes repeatedly over many cycles. Herein, it is reported that light-driven proton transfer triggered by a merocyanine-based photoacid can be used to modulate the permeability of pH-responsive polymersomes through cyclic, temporally controlled protonation and deprotonation of the polymersome membrane. The membranes can undergo repeated light-driven swelling-contraction cycles without losing functional effectiveness. When applied to enzyme loaded-nanoreactors, this membrane responsiveness is used for the reversible control of enzymatic reactions. This combination of the merocyanine-based photoacid and pH-switchable nanoreactors results in rapidly responding and versatile supramolecular systems successfully used to switch enzymatic reactions ON and OFF on demand.

Stimuli-responsive self-assembly of nanoparticles.

The capacity to respond or adapt to environmental changes is an intrinsic property of living systems that comprise highly-connected subcomponents communicating through chemical networks. The development of responsive synthetic systems is a relatively new research area that covers different disciplines, among which nanochemistry brings conceptually new demonstrations. Especially attractive are ligand-protected gold nanoparticles, which have been extensively used over the last decade as building blocks in constructing superlattices or dynamic aggregates, under the effect of an applied stimulus. To reflect the importance of surface chemistry and nanoparticle core composition in the dynamic self-assembly of nanoparticles, we provide here an overview of various available stimuli, as tools for synthetic chemists to exploit. Along with this task, the review starts with the use of chemical stimuli such as solvent, pH, gases, metal ions or biomolecules. It then focuses on physical stimuli: temperature, magnetic and electric fields, as well as light. To reflect on the increasing complexity of current architectures, we discuss systems that are responsive to more than one stimulus, to finally encourage further research by proposing future challenges.

Polysilsesquioxane Nanowire Networks as an "Artificial Solvent" for Reversible Operation of Photochromic Molecules.

Efficient isomerization of photochromic molecules often requires conformational freedom and is typically not available under solvent-free conditions. Here, we report a general methodology allowing for reversible switching of such molecules on the surfaces of solid materials. Our method is based on dispersing photochromic compounds within polysilsesquioxane nanowire networks (PNNs), which can be fabricated as transparent, highly porous, micrometer-thick layers on various substrates. We found that azobenzene switching within the PNNs proceeded unusually fast compared with the same molecules in liquid solvents. Efficient isomerization of another photochromic system, spiropyran, from a colorless to a colored form was used to create reversible images in PNN-coated glass. The coloration reaction could be induced with sunlight and is of interest for developing "smart" windows.

Supramolecular Control of Azobenzene Switching on Nanoparticles.

The reversible photoisomerization of azobenzene has been utilized to construct a plethora of systems in which optical, electronic, catalytic, and other properties can be controlled by light. However, owing to azobenzene's hydrophobic nature, most of these examples have been realized only in organic solvents, and systems operating in water are relatively scarce. Here, we show that by coadsorbing the inherently hydrophobic azobenzenes with water-solubilizing ligands on the same nanoparticulate platforms, it is possible to render them essentially water-soluble. To this end, we developed a modified nanoparticle functionalization procedure allowing us to precisely fine-tune the amount of azobenzene on the functionalized nanoparticles. Molecular dynamics simulations helped us to identify two distinct supramolecular architectures (depending on the length of the background ligand) on these nanoparticles, which can explain their excellent aqueous solubilities. Azobenzenes adsorbed on these water-soluble nanoparticles exhibit highly reversible photoisomerization upon exposure to UV and visible light. Importantly, the mixed-monolayer approach allowed us to systematically investigate how the background ligand affects the switching properties of azobenzene. We found that the nature of the background ligand has a profound effect on the kinetics of azobenzene switching. For example, a hydroxy-terminated background ligand is capable of accelerating the back-isomerization reaction by more than 6000-fold. These results pave the way toward the development of novel light-responsive nanomaterials operating in aqueous media and, in the long run, in biological environments.

Reversible switching of arylazopyrazole within a metal-organic cage.

Arylazopyrazoles represent a new family of molecular photoswitches characterized by a near-quantitative conversion between two states and long thermal half-lives of the metastable state. Here, we investigated the behavior of a model arylazopyrazole in the presence of a self-assembled cage based on Pd-imidazole coordination. Owing to its high water solubility, the cage can solubilize the E isomer of arylazopyrazole, which, by itself, is not soluble in water. NMR spectroscopy and X-ray crystallography have independently demonstrated that each cage can encapsulate two molecules of E-arylazopyrazole. UV-induced switching to the Z isomer was accompanied by the release of one of the two guests from the cage and the formation of a 1:1 cage/Z-arylazopyrazole inclusion complex. DFT calculations suggest that this process involves a dramatic change in the conformation of the cage. Back-isomerization was induced with green light and resulted in the initial 1:2 cage/E-arylazopyrazole complex. This back-isomerization reaction also proceeded in the dark, with a rate significantly higher than in the absence of the cage.

"Precipitation on Nanoparticles": Attractive Intermolecular Interactions Stabilize Specific Ligand Ratios on the Surfaces of Nanoparticles.

Confining organic molecules to the surfaces of inorganic nanoparticles can induce intermolecular interactions between them, which can affect the composition of the mixed self-assembled monolayers obtained by co-adsorption from solution of two different molecules. Two thiolated ligands (a dialkylviologen and a zwitterionic sulfobetaine) that can interact with each other electrostatically were coadsorbed onto gold nanoparticles. The nanoparticles favor a narrow range of ratios of these two molecules that is largely independent of the molar ratio in solution. Changing the solution molar ratio of the two ligands by a factor of 5 000 affects the on-nanoparticle ratio of these ligands by only threefold. This behavior is reminiscent of the formation of insoluble inorganic salts (such as AgCl), which similarly compensate positive and negative charges upon crystallizing. Our results pave the way towards developing well-defined hybrid organic-inorganic nanostructures.

Out-of-Equilibrium Aggregates and Coatings during Seeded Growth of Metallic Nanoparticles.

Although dissipative self-assembly is ubiquitous in nature, where it gives rise to structures and functions critical to life, examples of artificial systems featuring this mode of self-assembly are rare. Here, we identify the presence of ephemeral assemblies during seeded growth of gold nanoparticles. In this process, hydrazine reduces Au(III) ions, which attach to the existing nanoparticles "seeds". The attachment is accompanied by a local increase in the concentration of a surfactant, which therefore forms a bilayer on nanoparticle surfaces, inducing their assembly. The resulting aggregates gradually disassemble as the surfactant concentration throughout the solution equilibrates. The lifetimes of the out-of-equilibrium aggregates depend on and can be controlled by the size of the constituent nanoparticles. We demonstrate the utility of our out-of-equilibrium aggregates to form transient reflective coatings on polar surfaces.

Reversible Photoisomerization of Spiropyran on the Surfaces of Au25 Nanoclusters.

Au25 nanoclusters functionalized with a spiropyran molecular switch are synthesized via a ligand-exchange reaction at low temperature. The resulting nanoclusters are characterized by optical and NMR spectroscopies as well as by mass spectrometry. Spiropyran bound to nanoclusters isomerizes in a reversible fashion when exposed to UV and visible light, and its properties are similar to those of free spiropyran molecules in solution. The reversible photoisomerization entails the modulation of fluorescence as well as the light-controlled self-assembly of nanoclusters.

Differing Isomerization Kinetics of Azobenzene-Functionalized Self-Assembled Monolayers in Ambient Air and in Vacuum.

Azobenzenealkanethiols in self-assembled monolayers (SAMs) on Au(111) exhibit reversible trans-cis photoisomerization when diluted with alkanethiol spacers. Using these mixed SAMs, we show switching of the linear optical and second-harmonic response. The effective switching of these surface optical properties relies on a reasonably large cross section and a high photoisomerization yield as well as a long lifetime of the metastable cis isomer. We quantified the switching process by X-ray absorption spectroscopy. The cross sections for the trans-cis and cis-trans photoisomerization with 365 and 455 nm light, respectively, are 1 order of magnitude smaller than in solution. In vacuum, the 365 nm photostationary state comprises 50-74% of the molecules in the cis form, limited by their rapid thermal isomerization back to the trans state. In contrast, the 455 nm photostationary state contains nearly 100% trans-azobenzene. We determined time constants for the thermal cis-trans isomerization of only a few minutes in vacuum and in a dry nitrogen atmosphere but of more than 1 day in ambient air. Our results suggest that adventitious water adsorbed on the surface of the SAM stabilizes the polar cis configuration of azobenzene under ambient conditions. The back reaction rate constants differing by 2 orders of magnitude underline the huge influence of the environment and, accordingly, its importance when comparing various experiments.

Tailoring the properties of surface-immobilized azobenzenes by monolayer dilution and surface curvature.

Photoswitching in densely packed azobenzene self-assembled monolayers (SAMs) is strongly affected by steric constraints and excitonic coupling between neighboring chromophores. Therefore, control of the chromophore density is essential for enhancing and manipulating the photoisomerization yield. We systematically compare two methods to achieve this goal: First, we assemble monocomponent azobenzene-alkanethiolate SAMs on gold nanoparticles of varying size. Second, we form mixed SAMs of azobenzene-alkanethiolates and "dummy" alkanethiolates on planar substrates. Both methods lead to a gradual decrease of the chromophore density and enable efficient photoswitching with low-power light sources. X-ray spectroscopy reveals that coadsorption from solution yields mixtures with tunable composition. The orientation of the chromophores with respect to the surface normal changes from a tilted to an upright position with increasing azobenzene density. For both systems, optical spectroscopy reveals a pronounced excitonic shift that increases with the chromophore density. In spite of exciting the optical transition of the monomer, the main spectral change in mixed SAMs occurs in the excitonic band. In addition, the photoisomerization yield decreases only slightly by increasing the azobenzene-alkanethiolate density, and we observed photoswitching even with minor dilutions. Unlike in solution, azobenzene in the planar SAM can be switched back almost completely by optical excitation from the cis to the original trans state within a short time scale. These observations indicate cooperativity in the photoswitching process of mixed SAMs.