The ability to guide the assembly of nanosized objects reversibly with external stimuli, in particular light, is of fundamental importance, and it contributes to the development of applications as ...diverse as nanofabrication and controlled drug delivery. However, all the systems described to date are based on nanoparticles (NPs) that are inherently photoresponsive, which makes their preparation cumbersome and can markedly hamper their performance. Here we describe a conceptually new methodology to assemble NPs reversibly using light that does not require the particles to be functionalized with light-responsive ligands. Our strategy is based on the use of a photoswitchable medium that responds to light in such a way that it modulates the interparticle interactions. NP assembly proceeds quantitatively and without apparent fatigue, both in solution and in gels. Exposing the gels to light in a spatially controlled manner allowed us to draw images that spontaneously disappeared after a specific period of time.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, SBMB, UL, UM, UPUK
The chemical behaviour of molecules can be significantly modified by confinement to volumes comparable to the dimensions of the molecules. Although such confined spaces can be found in various ...nanostructured materials, such as zeolites, nanoporous organic frameworks and colloidal nanocrystal assemblies, the slow diffusion of molecules in and out of these materials has greatly hampered studying the effect of confinement on their physicochemical properties. Here, we show that this diffusion limitation can be overcome by reversibly creating and destroying confined environments by means of ultraviolet and visible light irradiation. We use colloidal nanocrystals functionalized with light-responsive ligands that readily self-assemble and trap various molecules from the surrounding bulk solution. Once trapped, these molecules can undergo chemical reactions with increased rates and with stereoselectivities significantly different from those in bulk solution. Illumination with visible light disassembles these nanoflasks, releasing the product in solution and thereby establishes a catalytic cycle. These dynamic nanoflasks can be useful for studying chemical reactivities in confined environments and for synthesizing molecules that are otherwise hard to achieve in bulk solution.
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Nanoporous frameworks are polymeric materials built from rigid molecules, which give rise to their nanoporous structures with applications in gas sorption and storage, catalysis and others. ...Conceptually new applications could emerge, should these beneficial properties be manipulated by external stimuli in a reversible manner. One approach to render nanoporous frameworks responsive to external signals would be to immobilize molecular switches within their nanopores. Although the majority of molecular switches require conformational freedom to isomerize, and switching in the solid state is prohibited, the nanopores may provide enough room for the switches to efficiently isomerize. Here we describe two families of nanoporous materials incorporating the spiropyran molecular switch. These materials exhibit a variety of interesting properties, including reversible photochromism and acidochromism under solvent-free conditions, light-controlled capture and release of metal ions, as well reversible chromism induced by solvation/desolvation.
Novel light-responsive nanoparticles were synthesized by decorating the surfaces of gold and silver nanoparticles with a nitrospiropyran molecular photoswitch. Upon exposure to UV light in nonpolar ...solvents, these nanoparticles self-assembled to afford spherical aggregates, which disassembled rapidly when the UV stimulus was turned off. The sizes of these aggregates depended on the nanoparticle concentration, and their lifetimes could be controlled by adjusting the surface concentration of nitrospiropyran on the nanoparticles. The conformational flexibility of nitrospiropyran, which was altered by modifying the structure of the background ligand, had a profound impact on the self-assembly process. By coating the nanoparticles with a spiropyran lacking the nitro group, a conceptually different self-assembly system, relying on a reversible proton transfer, was realized. The resulting particles spontaneously (in the dark) assembled into aggregates that could be readily disassembled upon exposure to blue light.
Nature has long inspired scientists with its seemingly unlimited ability to harness solar energy and to utilize it to drive various physiological processes. With the help of man-made molecular ...photoswitches, we now have the potential to outperform natural systems in many ways, with the ultimate goal of fabricating multifunctional materials that operate at different light wavelengths. An important challenge in developing light-controlled artificial molecular machines lies in attaining a detailed understanding of the photoisomerization-coupled conformational changes that occur in macromolecules and molecular assemblies. In this issue of ACS Nano, Bléger, Rabe, and co-workers use force microscopy to provide interesting insights into the behavior of individual photoresponsive molecules and to identify contraction, extension, and crawling events accompanying light-induced isomerization.
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Azobenzene‐based protonated N‐heterocyclic carbenes (NHCs), N,N’‐bis(azobenzene)imidazolium chlorides (IAz‐X⋅HCl; X=OMe, Br, H) were successfully synthesized and switching abilities of the attached ...azobenzene units along with the concomitant photoinduced generation of geometric isomers were studied. Upon irradiation with 365 nm UV light, a p‐methoxy‐azobenzene derivative get transformed from all‐trans isomer to nearly all‐cis isomer at the photostationary state. The extent of photomodulation of electronic properties in the NHC ring of the p‐methoxy‐azobenzene derivative is determined through the Tolman Electronic Parameter (TEP). Iridium complex, (IAz‐OMe)IrCl(CO)2 was synthesized and infrared spectra were recorded in dichloromethane solution as film in NaCl crystals and by drop‐casting in an ATR crystal. Comparison in averaged carbonyl stretching frequency between all‐trans isomer (ν˜
ttav
) and all‐cis isomer (ν˜
ccav
) indicates a significant decrement of Δtt–ccν˜
av=2.7 cm−1 (film) and 3.8 cm−1 (ATR). Therefore, moderate to excellent enhancement of electron density (Δtt–cc TEP=2.3 cm−1 film and 3.2 cm−1 ATR) at the C‐2 position of the NHC is achieved through trans→cis isomerization of the remotely placed azobenzene units. This fine phototuning of electron‐donating ability at the catalytic center would pave the way to control NHC/NHC‐metal catalyzed organic transformations through external stimuli.
Photoswitches: The photoinduced trans→cis isomerization can alter the electron‐donating abilities of azobenzene‐based N‐heterocyclic carbene. Appropriate iridium‐complex (see Figure) was synthesized and utilized to calculate Tolman Electronic Parameter (TEP) to provide the extent of alteration.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
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.
Photoswitchable nanoclusters: A spiropyran molecular switch installed on the surfaces of Au25 nanoclusters by a ligand‐exchange reaction undergoes reversible photoisomerization when exposed to different wavelengths of light.
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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.
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A practical and convenient decarbonylation of a variety of aromatic, heteroaromatic, and alkenyl aldehydes by using palladium nanoparticles supported on novel, fibrous nanosilica, named KCC‐1‐PEI/Pd, ...has been developed. Complete conversion of aldehyde functionalities into deformylated products was achieved in all cases and in nearly all cycles tested by reusing the catalyst systems. This method eliminates further purification of products after their isolation. Syntheses of at least three different deformylated products have been shown in sequence with the same catalyst system, which neither requires use of any additives, such as oxidants and bases, nor CO scavengers.
More fiber in a catalyst! A practical and convenient decarbonylation of a variety of aromatic, heteroaromatic, and alkenyl aldehydes with palladium nanoparticles supported on novel, fibrous nanosilica, named KCC‐1‐PEI/Pd, has been developed (see figure). Complete conversion of aldehyde functionalities into deformylated products is achieved and this method eliminates further purification of products after isolation.
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A compound combining the features of a molecular rotor and a photoswitch was synthesized and was shown to exist as three diastereomers, which interconvert via a reversible cyclic reaction scheme. ...Each of the three diastereomers was isolated, and by following the equilibration kinetics, activation barriers for all reactions were calculated. The results indicate that the properties of molecular switches depend heavily on their immediate chemical environment. The conclusions are important in the context of designing new switchable molecules and materials.
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