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.

Confining molecules to volumes only slightly larger than the molecules themselves can profoundly alter their properties. Molecular switches—entities that can be toggled between two or more forms upon exposure to an external stimulus—often require conformational freedom to isomerize. Therefore, placing these switches in confined spaces can render them non-operational. To preserve the switchability of these species under confinement, we work with a water-soluble coordination cage that is flexible enough to adapt its shape to the conformation of the encapsulated guest. We show that owing to its flexibility, the cage is not only capable of accommodating—and solubilizing in water—several light-responsive spiropyran-based molecular switches, but, more importantly, it also provides an environment suitable for the efficient, reversible photoisomerization of the bound guests. Our findings pave the way towards studying various molecular switching processes in confined environments.

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.

The two-component self-assembly of a 90° Pd(II) acceptor and a triimidazole donor led to the formation of a water-soluble semi-cylindrical cage with a hydrophobic cavity, which was separately crystallized with hydrophilic- and hydrophobic guests. The parent cage was found to catalyze the Knoevenagel condensation reaction of a series of aromatic mono-aldehydes with active methylene compounds, such as Meldrum's acid or 1,3-dimethylbarbituric acid. The confined hydrophobic nanospace within this cage was also used in the catalytic Diels-Alder reactions of 9-hydroxymethylanthracene with N-phenylmaleimide or N-cyclohexylmaleimide.

COMMUNICATIONHere, we hypothesized that a water-soluble photoacid, indicated in Figure 1 as MCH + , could be used to reversibly tune the strength of interparticle interactions in aqueous environments. The design of our system is illustrated in Figure 1 , bottom panel. Initially, NPs functionalized with a ω -mercaptocarboxylic acid are readily soluble in an aqueous solution of MCH + owing to partial deprotonation of the COOH groups. Visible-lightinduced release [ 25 ] of H + causes protonation of the terminal COOmoieties, thereby inducing NP assembly. The resulting aggregates, however, are metastable-they disassemble when irradiation is discontinued, and the assembly-disassembly cycle could be repeated. This approach would enable us to not only switch to aqueous environments, but it would offer a system complementary to, and arguably more attractive than the one described before, [ 22 ] in that the metastable state of the system would be the assembled one.MCH + was fi rst described by Liao and co-workers in 2011. [ 26 ] Following the original report, several creative applications of this water-soluble "photoacid" have been reported. Aprahamian and co-workers demonstrated that MCH + could be used to reversibly operate a hydrazone-based switch, which otherwise relied on consecutive injection of an acid and a base to the system. [ 27 ] Eelkema, van Esch et al. achieved spatial control over gelation by using MCH + as an acid catalyst for the formation of hydrogels. [ 28 ] Liao and co-workers utilized the light-induced pH changes to inactivate multidrug-resistant bacteria. [ 29 ] Thanks to MCH + , the above and other diverse functions, [30][31][32] which previously relied on acid inputs, can now be realized by means of light. However, coupling of the light-induced proton release to nanoparticle assembly in water has yet to be demonstrated.In our initial attempt to address this defi ciency, we worked with 5.8 nm gold NPs functionalized with a monolayer of 11-mercaptoundecanoic acid (MUA) (5.8 nm refers to the diameter of the inorganic core). These NPs were readily soluble in water provided that a suffi cient fraction of the COOH groups was deprotonated (which we induced by adding ≈4 equivalents of tetramethylammonium hydroxide (TMA + OH -) for each surfacebound COOH; see the Experimental Section). The excellent solubility of these NPs can be attributed to i) the effi cient solvation of the terminal COOmoieties by water molecules, and ii) electrostatic repulsion between the negatively charged NPs. Consequently, acidifying the NP solution (induced by introducing a small amount of HCl) led to NP aggregation, which could be reversed by adding a base. However, we found that the amount of H + released during the ⎯ → ⎯ + + + MCH SP HVis reaction was insuffi cient to induce the MUA-coated particles to fl occulate (we tested a variety of initial pH values), even in the presence of a saturated solution of MCH + . Hence, we decided to work with NPs functionalized with another ω -mercaptocarboxylic acid, namely, MHA (6-mercaptohexa...