Organic radicals stabilization above 300 °C in Eu-based coordination polymers for solar steam generation

Organic radicals feature unpaired electrons, and these compounds may have applications in biomedical technology and as materials for solar energy conversion. However, unpaired electrons tend to pair up (to form chemical bonds), making radicals unstable and hampering their applications. Here we report an organic radical system that is stable even at 350 °C, surpassing the upper temperature limit (200 °C) observed for other organic radicals. The system reported herein features a sulfur-rich organic linker that facilitates the formation of the radical centers; on the solid-state level, the molecules are crystallized with Eu(III) ions to form a 3D framework featuring stacks of linker molecules. The stacking is, however, somewhat loose and allows the molecules to wiggle and transform into sulfur-stabilized radicals at higher temperatures. In addition, the resulting solid framework remains crystalline, and it is stable to water and air. Moreover, it is black and features strong broad absorption in the visible and near IR region, thereby enhancing both photothermal conversion and solar-driven water evaporation.

Organic radicals have potential applications in a variety of fields, including energy conversion. Here, the authors report Eu-based coordination polymers that enable the stabilization of organic radicals up to 350 °C; these systems can be used to enhance solar steam generation.

Post-Synthetic Modification of Metal-Organic Frameworks Bearing Phenazine Radical Cations for aza-Diels-Alder Reactions

Metal-organic frameworks (MOFs) consisting of organic radicals are of great interest because they have exhibited unique and intriguing optical, electronic, magnetic, and chemo-catalytic properties, and thus have demonstrated great potential applications in optical, electronic, and magnetic devices, and as catalysts. However, the preparation of MOFs bearing stable organic radicals is very challenging because most organic radicals are highly reactive and difficult to incorporate into the framework of MOFs. Herein we reported a post-synthetic modification strategy to prepare a novel MOF containing phenazine radical cations, which was used as heterogeneous catalyst for aza-Diels-Alder reaction. The zinc-based metal-organic framework Zn₂ (PHZ)₂ (dabco) (N) was successfully synthesized from 5,10-di(4-benzoic acid)-5,10-dihydrophenazine (PHZ), triethylene diamine (dabco) with Zn(NO₃ )₂  ⋅ 6H₂ O by solvothermal method. The as-synthesized MOF N was partially oxidized by AgSbF₆ to form MOF R containing ∼10% phenazine radical cation species. The resultant MOF R was found to keep the original crystal type of N and very persistent under ambient conditions. Consequently, MOF R was successfully employed in radical cation-catalyzed aza-Diels-Alder reactions with various imine substrates at room temperature with high reaction conversion. Moreover, heterogeneous catalyst MOF R was reusable up to five times without much loss of catalytic activity, demonstrating its excellent stability and recyclability. Therefore, the post-synthetic modification developed in this work is expected to become a versatile strategy to prepare radical-based MOFs for the application of heterogeneous catalysts in organic synthesis.