Design and facile synthesis of a photothermally active metal-organic framework bearing persistent radicals via post-synthetic thermal annealing

Materials bearing persistent radicals are ideal candidates for use in photothermal-related applications because of the broad absorption profiles, yet difficult to acquire stemming from rapid recombination of radicals. In this work, Eu(iii) and Yb(iii) were allowed to assemble with tritopic 4,4′,4′′-(triphenylene-1,5,9-triyl)tribenzoic acid, H₃TPTB into two isoreticular metal organic frameworks (MOFs), Eu-TPTB and Yb-TPTB, respectively. Gratifyingly, these MOFs could be thermally annealed at 450 °C for at most 24 hours to give Eu-TPTB-24 and Yb-TPTB-24 with retained framework integrity and persistent diradicals. The thermally annealed MOFs were investigated for photo-driven applications and Eu-TPTB-24 gave the best performance with a solar-to-vapor efficiency of 99.56% and a rate of evaporation of 1.45 kg m⁻² h⁻¹, better than most light-harvesting MOF materials. By a joint experimental-theoretical approach, Eu-TPTB-24 and Yb-TPTB-24 were probed to have diradicals on each linker and these radicals were found localized on one side of the linker pendants exhibiting facile temperature dependent reversible singlet-triplet interchange. This work highlights that non-planar linkers and Ln(iii) nodes of different electronic configurations can be considered as tuneable parameters for generating functional MOFs bearing persistent radicals, fueling the exploration of photothermal materials utilizing persistent radicals.