Decoupled MOF Breathing: Pressure-Induced Reversal of Correlation Between Orthogonal Motions in a Diamondoid Framework.

Responsive porous materials can outperform more rigid analogues in applications requiring precise triggering of molecular uptake/release, switching or gradual change in properties. We have uncovered an unprecedented dynamic response in the diamondoid MOF SHF-62, (MeNH)[In(BDC-NHC(O)Me)] (BDC = 1,4-benzenedicarboxylate), by using pressure as a stimulus. SHF-62 exhibits two distinct framework "breathing" motions involving changes in 1) cross-section and 2) length of its 1D pores. Our study using synchrotron single-crystal X-ray diffraction in sapphire-capillary (p < 0.15 GPa) and diamond-anvil (0.15 < p < 5 GPa) cells reveals that different pressure regimes trigger positive and negative correlation between these two motions, requiring an unprecedented mechanical decoupling. Specifically, the DMF-solvated framework SHF-62-DMF, in DMF as pressure-transmitting medium, undergoes initial hyperexpansion of pore cross-section (p ≤ 0.9 GPa), due to DMF ingress, followed by reversal/reduction at p > 0.9 GPa while pore length contracts for all pressure increases, revealing decoupling of the two framework deformations. By contrast, nonpenetrating medium FC-70 imposes correlated compression (p < 1.4 GPa) of pore cross-section and length, resembling framework activation/desolvation motions but of greater magnitude. Similar behavior occurs for SHF-62-CHCl in CHCl (p < 0.14 GPa), suggesting minimal ingress of CHCl. These findings change our understanding of MOF dynamic responses and provide a platform for future responsive materials development.