Lowering the energetic landscape for negative thermal expansion in 3DL-MOFs

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 ex-pansion (NTE) properties by varying the metal ions and linkers used. We describe a new strategy to adjust NTE by using organic linkers that include additional rotational degrees of freedom. Specifically, we employ cubane-1,4-dicarboxylate and bicyclo[1.1.1]pentate-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 non-conjugated linkers is decou-pled 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.