A quenching-triggered reversible single-crystalto-single-crystal (SC-SC) phase transition was discovered in a metal organic framework (MOP) PCN-526. uring 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 extraframework 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 MOP materials.