Transformation between 2D and 3D Covalent Organic Frameworks via Reversible [2 + 2] Cycloaddition.

We report the first transformation between crystalline vinylene-linked two-dimensional (2D) polymers and crystalline cyclobutane-linked three-dimensional (3D) polymers. Specifically, absorption-edge irradiation of the 2D poly(arylenevinylene) covalent organic frameworks (COFs) results in topological [2 + 2] cycloaddition cross-linking of the π-stacked layers in 3D COFs. The reaction is reversible, and heating to 200 °C leads to a cycloreversion while retaining the COF crystallinity. The resulting difference in connectivity is manifested in the change of mechanical and electronic properties, including exfoliation, blue-shifted UV-vis absorption, altered luminescence, modified band structure, and different acid-doping behavior. The Li-impregnated 2D and 3D COFs show a significant room-temperature ion conductivity of 1.8 × 10-4 S/cm and 3.5 × 10-5 S/cm, respectively. Even higher room-temperature proton conductivity of 1.7 × 10-2 S/cm and 2.2 × 10-3 S/cm was found for H2SO4-treated 2D and 3D COFs, respectively.