Controlled Growth and Interconversion of Photoluminescent Metal–Organic Frameworks under High-Concentration Conditions

A major drawback to the implementation of metal–organic frameworks (MOFs) on scale is the vast quantity of organic solvents, typically N,N-dimethylformamide (DMF), required to synthesize even small quantities of MOF under traditional dilute (∼0.01 M) solvothermal conditions. High-concentration solvothermal methods offer the opportunity to synthesize MOFs with minimal solvent use but are currently limited by a lack of understanding of how dynamic self-assembly operates under these conditions. Herein, we systematically investigate the crystallization of a series of MOFs under variable concentration (0.01–0.2 M) and temperature (80–160 °C) conditions based on the dilute synthesis of the canonical framework Mg2(dobdc) (dobdc4– = 2,5-dioxido-1,4-terephthalate). Through this analysis, we identify controlling factors that lead to isolation of the highly photoluminescent phases Mg(DHT)(DMF)2 (DHT = dihydroxyterephthalate) and CORN-MOF-1 (Mg) (CORN = Cornell University) or Mg2(dobdc). Ultimately, we connect the preference for specific MOF phases to the extent of acid-catalyzed DMF hydrolysis and the competing influences of dimethylamine (Me2NH) and formate (HCO2–) at high concentrations, which is likewise affected by temperature, pH, and solvent composition. We use the insights gained to synthesize the Fe, Co, Ni, and Zn analogs of CORN-MOF-1 for the first time, as well as a second series of related MOFs, CORN-MOF-6 (M) (M = Mg, Mn, Fe, Co, Ni), based on the linker 2-hydroxyterephthalic acid (H3hbdc). Both series exhibit tunable luminescence properties based on the metal composition and crystal structure, making them potentially useful materials for optoelectronic applications. Overall, this work contributes to a clearer understanding of the factors that control MOF formation under high-concentration conditions.