Solvent-Triggered Reversible Phase Changes in Two Manganese-Based Metal-Organic Frameworks and Associated Sensing Events.

A flexible Mn-based MOF, Mn-sdc-1, has been successfully synthesized by using the ligand 4,4'-stilbenedicarboxylic acid (H2 sdc). Attributed to the flexibility of the framework, Mn-sdc-1 can transform into a new phase (Mn-sdc-2) with completely different structural geometry; this is induced by trace levels of H2 O at room temperature. Reversibly, the transformation from Mn-sdc-2 to Mn-sdc-1 can be triggered by DMF upon heating beyond 100 °C. These results inspired a study of the influences of temperature and H2 O volume in the solid-state transformation of two MOF phases and, for the first time, a phase diagram of MOFs has been depicted. This phase diagram reflects the gradual H2 O-/temperature-dependent changes between Mn-sdc-1 and Mn-sdc-2, which is very meaningful in achieving the controllable synthesis of these two MOFs and lead to targeting the desired water-stable structure. As a result, the obtained water-stable Mn-sdc-2 can be developed as an excellent Pb2+ sensor in aqueous solution through the luminescence quenching effect with a limit of detection of 31.4 nm.