Lowering the Energetic Landscape for Negative Thermal Expansion in 3D-Linker Metal-Organic Frameworks

Tuning the coefficient of thermal expansion (CTE) of functional materials is paramount for their practical implementation. The multicomponent nature of metal-organic frameworks (MOFs) offers an opportunity to finely adjust negative thermal expansion (NTE) properties by varying the metal ions and linkers used. We describe a new strategy to adjust the NTE by using organic linkers that include additional rotational degrees of freedom. Specifically, we employ cubane-1,4-dicarboxylate and bicyclo[1.1.1]pentane-1,3-dicarboxylate to form the MOFs CUB-5 and 3DL-MOF-1, respectively, where each linker has low torsional energy barriers. The core of these nonconjugated linkers is decoupled from the carboxylate functionalities, which frees the relative movement of these components. This results in enhanced NTE compared to the analogous, conjugated system; VT-PXRD results were used to calculate the CTE for 3DL-MOF-1 (αL = −13.9(2) × 10-6 K-1), and CUB-5 (αL = −14.7(3) × 10-6 K-1), which is greater than the NTE of MOF-5 (αL = −13.1(1) × 10-6 K-1). These results identify a new route to enhanced NTE behaviors in IRMOF materials influenced by low energy molecular torsion of the linker.