Switching From Negative to Positive Thermal Expansion of Porphyrin-Based Metal-Organic Frameworks Through Post-Metallization

Abstract

Achieving smart single-phase materials with switchable thermal expansion is challenging, as most materials undergo volumetric thermal expansion upon heating. Here, it is reported that the porphyrin-based metal-organic frameworks (MOFs) can show switchable thermal expansion, ranging from negative to positive, by controlling the central metal ions. The pristine compound, along with those functionalized with Ni2+ or FePz2+ (Pz stands for pyrazine) show pronounced negative thermal expansion (NTE), while the FeCl2+ functionalized analogue shows moderate positive thermal expansion (PTE). Detailed molecular dynamic simulations shed light on that the NTE originates from the collective low-frequency phonon modes, concerning the in-plane/out-of-plane rotations and trampoline-like vibrations of porphyrin groups. The introduction of FeCl2+ can disrupt the planarity of the porphyrin groups, stiffening the NTE related phonons, thus leading to the PTE. In contrast, the introduction of Ni2+ or FePz2+ groups can maintain the planar structure, thereby preserving the NTE. This study not only provides a molecular-level understanding of thermal expansion but also opens a pathway for tailoring thermal expansion through post-synthesis modification of molecular structures.