Minimalist Design for Solar Energy Conversion: Revamping the pi-Grid of an Organic Framework into Open-Shell Superabsorbers

We apply a versatile reaction to a versatile solid: theformerinvolves the electron-deficient alkene tetracyanoethylene (TCNE) asthe guest reactant; the latter consists of stacked 2D honeycomb covalentnetworks based on the electron-rich beta-ketoenamine hinges thatalso activate the conjugated, connecting alkyne units. The TCNE/alkynereaction is a [2 + 2] cycloaddition-retroelectrocyclization(CA-RE) that forms strong push-pull unitsdirectly into the backbone of the framework-i.e., using only the minimalist "bare-bones" scaffold,without the need for additional side groups of alkynes or other functions.The ability of the stacked alkyne units (i.e., aspart of the honeycomb mass) to undergo such extensive rearrangementhighlights the structural flexibility of these covalent organic framework(COF) hosts. The COF solids remain porous, crystalline, and air-/water-stableafter the CA-RE modification, while the resultingpush-pull units feature distinct open-shell/free-radical character,are strongly light-absorbing, and shift the absorption ends from 590nm to around 1900 nm (band gaps from 2.17-2.23 to 0.87-0.95eV), so as to better capture sunlight (especially the infrared regionwhich takes up 52% of the solar energy). As a result, the modifiedCOF materials achieve the highest photothermal conversion performances,holding promise in thermoelectric power generation and solar steamgeneration (e.g., with solar-vapor conversion efficiencies>96%).