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.