"Phoenix", the molecule that is reborn

Photocatalysis uses light to promote chemical reactions under mild conditions, but traditional photocatalysts are generally limited to a single type of reactivity.
In the paper published in «Nature Catalysis» under the title "Bimodal multiphoton catalysis via structural regeneration", researchers from the universities of Padua, Ferrara, Parma and Perugia developed an organic photocatalyst capable of changing its structure and function during the reaction.

The molecule, called Phoenix, “Fenice”, is temporarily fragmented by light into different reactive species that perform opposite and complementary functions, before recombining and regenerating the original catalyst. Normally, the simultaneous creation of two such extreme processes would produce a chaotic mixture of waste products. In this case, however, the reaction is reversible and the molecule itself controls the equilibrium, allowing only the correct fragments to combine and regenerating the original catalyst.

The researchers developed a new chemical strategy capable of orchestrating, within the same process and at the same time, two opposite reactions: a reduction (gain of electrons) and an oxidation (loss of electrons) that are extremely difficult to achieve together.

«By harnessing light, the system generates extremely reactive electrons capable of breaking very strong chemical bonds. While these electrons activate the most challenging molecules, the catalyst in turn activates itself to oxidise other compounds," stresses Luca Dell'Amico, from the Department of Chemical Sciences of the University of Padua and coordinator of the published research. "The chemical fragments created by these two parallel actions combine with one another, giving rise to more than 50 new chemical products with yields of up to 78%. What is more, the catalyst is not consumed: at the end of the process it regenerates itself and is ready to start again without the need for external chemical substances to drive the reaction. We succeeded in exploiting the natural flexibility of the catalytic molecule to cover an exceptionally broad energy range, turning what would normally be a weakness of the molecule into its real strength. We managed to cover a record ‘energy window’ of 5.7 Volts in the same reaction vessel, something that until now had been considered almost impossible in organic chemistry».

The same molecule can use light to perform different tasks during a single reaction, making particularly difficult chemical transformations possible. It is as if a single catalyst could transform itself, use light energy in different ways and finally return to its initial form. Some of the extremely rapid events that occur after light absorption were also directly observed, clarifying how the catalyst transforms and is subsequently regenerated.

«The new principle was used to prepare, in a green and sustainable way, numerous molecules with potential biological activity, including structures that are difficult to obtain with conventional methods," concludes Luca Dell'Amico. "More than a single new reaction, the study therefore proposes a new way of using light to control chemical transformations, exploiting a catalyst’s ability to modify itself temporarily and then be ‘reborn’».