No abstract

Porous materials are widely used in industry for applications that include chemical separations and gas scrubbing. These materials are typically porous solids, though the liquid state can be easier to manipulate in industrial settings. The idea of combining the size-and shape-selectivity of porous domains with the fluidity of liquids is a promising one and porous liquids composed of functionalized organic cages have recently attracted attention. Here, we describe an ionic-liquid, porous, tetrahedral coordination cage. Complementing the gas-binding observed in other porous liquids, this material also encapsulates non-gaseous guestsshape-and size-selectivity was observed for a series of alcohol isomers. Three gaseous guests, chlorofluorocarbons CFC-11, CFC-12, and CFC-13, were also shown to be taken up by the liquid coordination cage with an affinity increasing with their size. We hope that these findings will lead to the synthesis of other porous liquids whose guest-uptake properties may be tailored to fulfil specific functions. Recent work has shown that persistent cavities can be engineered into liquids, lending them permanent porosity. These new materials were initially proposed by James in 2007 1 , who recognised three distinct types of them. The simplest of these, Type I permanently porous liquids, consist of rigid hosts with empty cavities that are liquid in their neat state 2,3 , without requiring an additional solvent for fluidity 4-7. Metalorganic frameworks (MOFs) have also been observed to form liquid phases that are inferred to be porous 8,9 , although the high temperatures required preclude guest binding. Previously reported examples of porous liquids have included surface-modified hollow silica spheres 2 and hollow carbon spheres 3 , crown ether-functionalised organic cages 5 , and dispersions 4, 6 or slurries 7 of porous framework materials in ionic liquids. To date, applications of these materials have focussed on gas storage and separation 2,10,11. However, we are not aware of the binding of guest molecules larger than carbon dioxide or methane inside the cavities of porous liquids, restricting the potential application of these

Conspectus 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 with...

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 molecularlevel 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.

Subcomponent exchange transformed new high-spin FeII4L4 cage 1 into previously-reported low-spin FeII4L4 cage 2: 2-formyl-6-methylpyridine was ejected in favor of the less sterically hindered 2-formylpyridine, with concomitant high- to low-spin transition of the cage’s FeII centers. High-spin 1 also reacted more readily with electron-rich anilines than 2, enabling the design of a system consisting of two cages that could release their guests in response to combinations of different stimuli. The addition of p-anisidine to a mixture of high-spin 1 and previously-reported low-spin FeII4L6 cage 3 resulted in the destruction of 1 and the release of its guest. However, initial addition of 2-formylpyridine to an identical mixture of 1 and 3 resulted in the transformation of 1 into 2; added p-anisidine then reacted preferentially with 3 releasing its guest. The addition of 2-formylpyridine thus modulated the system’s behavior, fundamentally altering its response to the subsequent signal p-anisidine.