Chemistry is increasingly mimicking the photosynthesis process to drive chemical reactions that tend to run poorly or do not occur spontaneously with light energy. This necessitates appropriate photocatalysts that can capture light energy that can subsequently be used for the required chemical reaction.
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In the journal Angewandte Chemie, a Chinese research team has outlined layered core/shell quantum dots that are able to effectively drive difficult organic transformations. A particular advantage of these layered core/shell quantum dots is their low toxicity.
Quantum dots are excellently distributed nanoscopic crystals of inorganic semiconductors. They are strongly absorbed in a modifiable range of the spectrum and are relatively easy to recycle. Until now, photocatalytic quantum dots have almost exclusively been based on cadmium and lead - two highly toxic elements. This and their limited efficiency is what has limited their wider application.
A team of researchers guided by Kaifeng Wu (Chinese Academy of Sciences) has currently presented unique quantum dots with extremely low toxicity levels and extremely high performance. They are triggered by commercially available blue LEDs — the UV light that is typically needed is not required. The secret to their success can be found in their core/shell structure and the adaptable coatings that can be used to “store” the light energy.
The quantum dots are only a few nanometers in width. Their core comprises zinc selenide (ZnSe) and is enclosed by a thin shell composed of zinc sulfide (ZnS). Blue light pushes the zinc selenide to an excited state wherein it can give up electrons without difficulty. The shell stops the electrons from instantly being trapped by so-called defects.
The team equipped the surface of the shell with special benzophenone ligands. These ligands “suck up” the electrons from the quantum dots, store them, and supply them when organic reactions take place. For instance, the researchers were able to perform reductive dehalogenations of aryl chlorides and additive-free polymerizations of acrylates — crucial reactions that function weakly or not at all by traditional photocatalysts.
A second variety was created by coating the surface with biphenyl ligands that can capture energy from the excited quantum dots straightaway. This puts them in a long-lived, extremely energetic triplet state. The triplet energy “stored” in this manner can be conveyed to particular organic molecules, which then also reach a triplet state. In this state, they can experience chemical reactions that are not feasible in their ground state.
To demonstrate the findings, the researchers performed [2+2] homo-cycloadditions of styrene and cycloadditions of carbonyls with alkenes. These create four-membered rings (cyclobutanes or oxetanes, respectively), which are materials that are crucial starting materials in domains such as pharmaceutical development.
Nie, C., et al. (2022) Low-Toxicity ZnSe/ZnS Quantum Dots as Potent Photoreductants and Triplet Sensitizers for Organic Transformations. Angewandte Chemie. doi.org/10.1002/anie.202213065.