A Reversible Crystallinity-Preserving Phase Transition in Metal–Organic Frameworks: Discovery, Mechanistic Studies, and Potential Applications

A quenching-triggered
reversible single-crystal-to-single-crystal
(SC-SC) phase transition was discovered in a metal–organic
framework (MOF) PCN-526. During the phase transition,
the one-dimensional channel of PCN-526 distorts from
square to rectangular in shape while maintaining single crystallinity.
Although SC-SC transformations have been frequently observed in MOFs,
most reports have focused on describing the resulting structural alterations
without shedding light on the mechanism for the transformation. Interestingly,
modifying the occupancy or species of metal ions in the extra-framework
sites, which provides mechanistic insight into the causes for the
transformation, can forbid this phase transition. Moreover, as a host
scaffold, PCN-526 presents a platform for modulation
of the photoluminescence properties by encapsulation of luminescent
guest molecules. Through judicious choice of these guest molecules,
responsive luminescence caused by SC-SC transformations can be detected,
introducing a new strategy for the design of novel luminescent MOF
materials.