Tuning the Negative Thermal Expansion Behavior of the Metal-Organic Framework Cu₃BTC₂ by Retrofitting

The modular building principle of metal-organic frameworks (MOFs) presents an excellent platform to explore and establish structure-property relations that tie microscopic to macroscopic properties. Negative thermal expansion (NTE) is a common phenomenon in MOFs and is often ascribed to collective motions that can move through the structure at sufficiently low energies. Here, we show that the introduction of additional linkages in a parent framework, retrofitting, is an effective approach to access lattice dynamics experimentally, in turn providing researchers with a tool to alter the NTE behavior in MOFs. By introducing TCNQ (7,7,8,8-tetracyanoquinodimethane) into the prototypical MOF Cu₃BTC₂ (BTC = 1,3,5-benzenetricarboxylate; HKUST-1), NTE can be tuned between α_V = -15.3 × 10^-6 K^-1 (Cu₃BTC₂) and α_V = -8.4 × 10^-6 K^-1 (1.0TCNQ@Cu₃BTC₂). We ascribe this phenomenon to a general stiffening of the framework as a function of TCNQ loading due to additional network connectivity, which is confirmed by computational modeling and far-infrared spectroscopy. Our findings imply that retrofitting is generally applicable to MOFs with open metal sites, opening yet another way to fine-tune properties in this versatile class of materials.