Nanocrystals store light energy and drive chemical reactions

Chemistry is increasingly using the trick that plants can do with photosynthesis: causing chemical reactions that work poorly or don’t happen spontaneously at all with light energy. This requires appropriate photocatalysts that capture light energy and make it available for reaction. In the review applied chemistry, a Chinese research team has just introduced layered core/shell quantum dots that efficiently drive complex organic transformations. Their low toxicity is a particular advantage.

Quantum dots are finely dispersed nanoscopic crystals of inorganic semiconductors. They absorb strongly in an adjustable range of the spectrum and are easy to recycle. Until now, photocatalytic quantum dots relied almost exclusively on the highly toxic elements cadmium and lead. This and their limited effectiveness have been major obstacles to their wider use.

A research team led by Kaifeng Wu (Chinese Academy of Sciences) has just introduced new quantum dots with very low toxicity and very high performance. They are activated by commercially available blue LEDs – the usually required UV light is not necessary. The secret of their success lies in their core/shell structure and the variable coatings which make it possible to “store” the light energy.

Quantum dots are only a few nanometers wide. Their core is made of zinc selenide (ZnSe) and is surrounded by a thin shell of zinc sulphide (ZnS). Blue light raises zinc selenide to an excited state in which it can easily give up electrons. The shell prevents electrons from being immediately captured by the so-called defects.

The team outfitted the surface of the shell with special benzophenone ligands that “suck up” electrons from the quantum dots, store them, and make them available for organic reactions. For example, the team was able to achieve reductive dehalogenations of aryl chlorides and additive-free polymerizations of acrylates, important reactions that work poorly or not at all with conventional photocatalysts.

A second version was made by coating the surface with biphenyl ligands capable of directly absorbing the energy of excited quantum dots. This brings them into a highly energetic, long-lived triplet state. The triplet energy “stored” in this way can be transferred to specific organic molecules, which then also enter a triplet state. In this state, they can undergo chemical reactions that are not possible in their ground state.

As a demonstration, the team carried out [2+2] homo-cycloadditions of styrene and cycloadditions of carbonyls with alkenes. These produce four-membered rings (cyclobutanes or oxetanes, respectively), which are substances that are important raw materials in areas such as pharmaceutical development.