A hetero-bimetallic metal–organic framework (MOF) (Zn-Cd-MOF, Zn 2 Cd(DMF) 2 (L) 6 }n (L = 2,5-bis(phenylamino)-1,4-benzenedicarboxylic acid), featuring a dense topology assembled through synergistic Zn 2+ and Cd 2+ coordination, has been synthesized via a direct solvothermal approach. Compared to its monometallic analogues (Zn-MOF or Cd-MOF), Zn-Cd-MOF exhibits significantly improved stability in water and diverse organic solvents. Moreover, this bimetallic framework displays tunable fluescence properties, enabling effective distinction between protic and aprotic solvents. This unique fluorescence response is attributed to the excited-state intramolecular proton transfer (ESIPT) process of the ligand, in which competitive hydrogen bonding with water molecules modulates the enol-keto tautomer equilibrium. The material also demonstrates excellent reusability, maintaining stable performance over at least 5 consecutive cycles for detecting water in organic solvents, and retains structural integrity for over 2 years. This study offers a new strategy for constructing stable MOFs based on low-valence metal and carboxylate ligands, while also providing a practical design concept for identifying protic solvents in aprotic environments.
Xu et al. (Fri,) studied this question.