Radicals Persistent above 300 ℃: from Framework Design to Solar Vapor Generation

Abstract The strong, stable radical signal is forged from two levels of structural design. On the molecular level, we fused terephthalic acid with two 1,4-dithiin (-S-CH = CH-S-) units to afford a polycyclic (3-ring), sulfur-rich linker (TTA) with versatile reactivity—e.g., for generating stable radicals. On the solid-state level, the linker was crystallized with Eu(III) to form EuTTA as a 3D metal-organic framework (MOF) featuring stacks of TTA molecules along the Eu-carboxyl rod domain. As revealed by single-crystal X-ray diffraction, the slightly loosened packing of the TTA molecules allows wiggle room for uniquely selective thermal reactions: among the repeat of three TTA molecules along the stack, one first transforms into benzodithiophene (by extruding one S atom from each of the dithiin wings) at 230°C, while the other two TTA molecules only react at higher temperatures (e.g., 300–350°C)—to form bis(dithiole) and other sulfur-stabilized radicals, as well as covalent crosslinks thereof. Aside from the highest thermal stability observed of its radicals (above 300°C), the black MOF product (i.e., EuTTA-350; obtained from heating at 350°C) remain crystalline, and water/air-stable, and features strong broad absorption in the visible and near IR region. As a result, EuTTA-350 achieves highest efficiencies in both photothermal conversion and solar-driven water evaporation among MOF materials, holding promise in solar steam generation.