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 <i>N</i>,<i>N</i>-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) (dobdc^4- = 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 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.