Metal-based photocatalysts such as ruthenium and iridium polypyridyl complexes have dominated the molecular photocatalysis field for the past century on account of their favorable photophysical properties that promote electron-transfer catalysis. Recently, the development of metal-free, organic photocatalysts has started to provide sustainable alternatives to these compounds. Research in this area is mainly devoted to enhancing the photosensitization properties of these new organic photosensitizers by populating their triplet excited states, a phenomenon that is key to increasing photocatalytic efficiencies.

In the current research published in Nature Catalysis, we have sought to fine-tune the photosensitization properties of pyrene by its incorporation into the skeleton of a homo[2]catenane, thereby increasing its photocatalytic efficiency.

 The research has entailed two fundamental parts: (i) the design and synthesis of the three constitutionally isomeric pyrene-based homo[2]catenanes and (ii) the investigation of their photosensitization properties and photocatalytic activities.

• The strategy for preparing the pyrene-based homo[2]catenane was inspired by our recent discovery¹ of a good synthetic approach to making the anthracene-based homo[2]catenane—namely, the incorporation of anthracene between two pyridinium units leads to favorable [p··p] interactions, thereby promoting homo[2]catenane self-templation.

In our attempts to generalize this approach, we anticipated that pyrene could be used as precursor for the formation of the homo[2]catenane on account of its strong [p···p] stacking compared to the situation with anthracene.

In addition, we have selected two pyrene-based constitutional isomers, 1,6-bis-4-pyridyl-pyrene and 2,7-bis-4-pyridyl-pyrene, as starting precursors in the preparation of the homo[2]catenanes. The 2,7-constitution gave rise to a homo[2]catenane in 60% yield, in contrast with the poor yield (<1%) obtained for the 1,6-based constitution. The strong [p···p] stacking between the two rings in the 2,7-pyrene-based constitution encouraged us to attempt the synthesis of an asymmetrical homo[2]catenane. Subsequently, the combination of the two different pyrene-based constitutions (1,6 and 2,7) led to the synthesis of a third homo[2]catenane.

• The idea of using the pyrene-based catenanes for photocatalysis was inspired by our recent investigation² of a pyrene-based host-guest system which was embedded into a polymer and used for heterogenous photocatalysis of a sulfur mustard simulant. We anticipated that the mechanically interlocked compounds could overcome some challenges with this system, especially when compared with the dynamic nature of host-guest systems.

Three routes of investigation were launched simultaneously—namely, theoretical calculations, a transient absorption study for triplet-state dynamics, and photocatalysis experiments. Transient absorption (TA) data (recorded in collaboration with the Wasielewski group at Northwestern) revealed that all the compounds generate significant triplet populations. Additionally, based on the TD-DFT data, we have devised a general mechanism for enhancing the efficiency of the triplet-state population through the formation of a mechanical bond. These promising photosensitization properties were reflected in the observed photocatalytic activities of this class of mechanically interlocked molecules, demonstrated by the model of detoxification of the sulfur mustard simulant CEES. In addition, by incorporating pyrene units into the pyridinium-based cyclophanes and homo[2]catenanes, we were able to improve (i) the rate of the reaction from 2 hours (using unfunctionalized pyrene) to less than 5 minutes and (ii) the conversion yield of CEES to 2-chloroethyl sulfoxide (CEESO) from 75% to >95%.

This finding highlights the role of the mechanical bond in bringing the photosensitizers into close contact, enhancing the singlet oxygen generation and leading to better photocatalysts.

Overall, this investigation highlights the benefits of using the mechanical bond to fine-tune the energy landscape of common aromatic chromophores and the development of new classes of chromophores based on mechanically interlocked molecules for use as photosensitizers and photocatalysts.

If you would like to know more details, please take a look at our article published in Nature Catalysis: https://doi.org/10.1038/s41929-022-00799-y. I hope you enjoy it!

References

1. Garci, A. et al. Mechanical-bond-induced exciplex fluorescence in an anthracene-based homo[2]catenane. Am. Chem. Soc. 142, 7956–7967 (2020).

2. Beldjoudi, Y. et al. Supramolecular porous organic nanocomposites for heterogeneous photocatalysis of a sulfur mustard simulant. Adv. Mater. 32, 2001592 (2020).