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Content archived on 2024-06-18

Photoinduced Catalysis in a Nanoparticle System

Final Report Summary - PHOTOCAT (Photoinduced Catalysis in a Nanoparticle System)

Executive Summary:

The initial goal of the project was to develop a new family of photoswitchable catalysts based on metallic nanoparticles co-functionalized with mixed monolayers comprising photoswitchable and catalytic ligands. This strategy failed because of synthetic difficulties described in detail separately. However, we envisioned two alternative approaches to light-controlled catalysis: we have successfully prepared and characterized these two systems, as well as demonstrated first examples of photocontrolled catalysis. The first of these systems is based on “nanobowls”: metallic nanoparticles with a unique shape of a bowl. These entities - likely the smallest metallic containers prepared to date - were synthesized by performing the so-called galvanic replacement reaction on heterodimeric nanoparticles, each comprising a reactive, silver, as well as a “dummy”, magnetite domain. We showed that it is possible to control both the cavity size as well as overall dimensions of the nanobowls. Furthermore, we demonstrated nanobowls’ ability to capture small objects inside their cavities, which is important in the context of studying the behavior - in particular, chemical reactivity - of these objects (e.g. small nanoparticles) inside the confinement of the nanobowl cavities. Most recently, we decorated the surfaces of nanobowls with photoresponsive ligands, which opens up the way to reversibly capture and release different guests using light.

The second of the light-controlled catalysis systems is based on spherical metallic nanoparticles functionalized with monolayers of molecular switches (azobenzenes or spiropyrans). In the “ground” state, these molecules exist in the form of relatively non-polar isomers, and the particles are soluble in hydrophobic solvents. Upon exposure to UV light, however, both types of molecules transform to give significantly more polar isomers, which entails nanoparticle aggregation. We modified the system such that the particles do not assemble into amorphous aggregates, but rather crystallize to give well-defined, three-dimensional colloidal crystals featuring nanopores between the densely packed spheres. Further, we demonstrated that small molecules intentionally added to the solution are captured within these nanopores - and their effective molarity increases by orders of magnitude. In cases where these small molecules can react with one another, we observe a significant increase in reaction kinetics, and the system shows a catalytic behavior: once a reaction has completed, the colloidal crystals can be disassembled upon exposure to visible light, and in the next UV-Vis irradiation cycle, additional “substrate” molecules can be converted into products.

To further expand the concept of control chemical reactions using external stimuli, we prepared “dual-responsive” nanoparticles comprising superparamagnetic cores and light-switchable shells. We showed that the assembly behavior of these particles - and therefore, in the long run - chemical reactivity can be controlled independently using light and magnetic field.

Overall, the Fellow believes he has successfully re-integrated within the FP7 associated country. His is now a faculty member at the Department of Organic Chemistry at the Weizmann Institute of Science.