Chemistry is making use of the trick that plants do with photosynthesis: driving chemical reactions that run poorly or do not occur spontaneously at all with light energy. This requires suitable photocatalysts that capture light energy and make it available for the reaction.
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. However, until now, photocatalytic quantum dots have been based almost exclusively on the highly toxic elements cadmium and lead.
Recently, a research team led by Prof. WU Kaifeng from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences (CAS) has now developed a novel photocatalyst based on low-toxicity quantum dots.
This study was published in Angewandte Chemie International Edition on Oct. 17.
The researchers introduced layered core/shell quantum dots that efficiently drive challenging organic transformations. The quantum dots are activated by commercially available blue LEDs, without the UV light that is usually required. The secret to their success lies in their core/shell structure and the variable coatings that can be used to "store" the light energy.
The quantum dots are only a few nanometers wide. Their core consists of zinc selenide (ZnSe) and is surrounded by a thin shell made of zinc sulfide (ZnS). Blue light raises the ZnSe to an excited state in which it can easily give up electrons. The shell prevents the electrons from immediately being captured by the so-called defects.
The team equipped the surface of the shell with special benzophenone ligands that absorb the electrons from the quantum dots, store them, and make them available for organic reactions.
"Coating the surface with biphenyl ligands can also directly absorb energy from excited quantum dots. This brings them into a long-lived, highly energetic triplet state," said Prof. WU.
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.
