Anyone who has ever been in the dentist's chair with a hole in their tooth is probably familiar with the procedure. After the hole in the tooth is drilled, a filling made of liquid plastic is inserted. This is then modelled in the mouth and hardened by UV light. This is made possible by photo­initiators. These are chemical compounds that are added to the filling paste. They decompose when exposed to light and form radicals that cause this paste to harden. For some years now, germanium-based photo­initiators have been used for this purpose. The advantage of these is that they absorb light with longer wavelength and therefore do not require UV light, which is hazardous to health, for curing.

This non-toxic photo­initiator has already established itself in the dental field, although it is expensive to produce. The production costs of one kilogram of this initiator are currently in the order of magnitude of a new small car. “Given the small quantities needed for dental fillings, the price of the photo­initiator is hardly a factor in the dental industry. For other appli­cations, however, the expensive production was a stumbling block – until now,” explains Michael Haas from Graz University of Technology in Austria.

Together with his team at the Institute of Inorganic Chemistry, Haas has developed a completely new method of synthesis for germanium-based photo­initiators. In contrast to conventional synthesis, this production method not only does not require sulphur, but is also signi­ficantly simpler, more efficient and cheaper. The researchers have succeeded in establishing an alternative approach to this class of compounds that is single-step and makes isolating the product absolutely simple. In the process, several silicon-based protective groups are cleaved off simul­taneously. The desired compound is then isolated by simple crystalli­zation. This opens up further biomedical appli­cations for this class of photo­initiators, for example in the production of contact lenses, prostheses, novel implants and artificial human tissue.

The researchers have now translated this alternative approach into application with the project partner Ivoclar Vivadent AG. The dental company already had a toxi­cologically safe germanium-based photo­initiator in its product portfolio. However, this also has serious disadvantages in production, as Haas explains: “In the case of Ivocerin, the synthesis is complex and multi-step process, and the removal of the reaction partners is also expensive and leads to enormous yield losses.” The foreseeable market launch of the new initiator will make dental fillings signi­ficantly cheaper in the future.

Michael Haas also sees potential for other biomedical appli­cations, such as contact lenses. For most of these appli­cations, toxico­logically questionable photo­initiators have been used up to now. The germanium-based initiators, which are harmless to health, have so far been too expensive for these appli­cations. The production of novel implants, prostheses or arti­ficial human tissue are also possible areas of application for the newly synthesized initiator. “It becomes interesting wherever the use of non-toxic materials is of central importance,” says Haas. At around twelve years old, research on photo­initiators is a relatively young field. Michael Haas and his research group have already successfully been granted two independent patents in the field of germanium-based photo­initiators in the past four years. “Since radical photo­initiators are used in many industrial processes, the absolute relevance of our results cannot yet be assessed,” says Haas.

For all its application orientation, Michael Haas' working group is also reaping a rich harvest in basic research. Haas, together with his doctoral student Manfred Drusgala and other colleagues describe a new method for the targeted synthesis of bisenolates, a special class of compounds from enolate chemistry. This class of compounds is charac­terized by the possibility of a double reaction at the central and active germanium atom – i.e. two reactions can be carried out simul­taneously. This allows the introduction of new functionalities, making this new class of compounds of great interest for further research in the field of photo­initiators. “This is also a milestone for the entire field of organo­metallic chemistry,” says Haas. He and his team are currently developing completely new types of water-soluble photo­initiators based on these molecules, something which represents previously untrodden ground in this field of research. (Source: TU Graz)

Reference: M. Drusgala et al.: Isolable Geminal Bisgermenolates: A New Synthon in Organometallic Chemistry, Ang. Chem. 60, 23646 (2021); DOI: 10.1002/anie.202111636

Link: Institute of Inorganic Chemistry, Graz University of Technology, Graz, Austria