Enhanced Guest@Mof Interaction Via Stepwise Thermal Annealing: Tcnq@Cu-3(Btc)(2)

Confinement of guest molecules in porous materials such as metal-organic frameworks (MOFs) promises to deliver emergent properties separate from those of the individual components. Understanding the confinement mechanism is therefore important for the development of new synthesis routes that adjust MOF properties for specific applications. In this work, we developed a new synthetic method to confine guest molecules into MOF pores through a stepwise thermal annealing process, wherein the confinement of 7,7,8,8-tetracyanoquinodimethane (TCNQ) guest molecules into the Cu-3(BTC)(2) (BTC = benzene-1,3,5-tricarboxylic acid) MOF host is used as an example of how novel materials can be created with new physical properties. The stepwise thermal annealing process includes (1) an activation process of the pristine Cu-3(BTC)(2) MOF to maximize the TCNQ guest loading in the MOF host by effectively removing the residual solvents and (2) postannealing of the TCNQ-infiltrated MOF to enhance the interaction of the confined guest molecules with the MOF host. The obtained experimental results based on thermogravimetric analysis, N-2 gas adsorption, electron microscopy, X-ray diffraction, and infrared absorption, combined with density functional theory calculations provide evidence that the use of a stepwise thermal annealing process yields enhancements in the guest loading, packing, and interaction between the TCNQ guest and the MOF host. The new hybrid TCNQ@Cu-3(BTC)(2) system is stable and shows no significant signs of structural degradation even after submersion in water. This is due to the significantly stronger interactions of TCNQ with the framework metal ions compared to those of the water molecules competing for the same framework binding sites. It was also found that the TCNQ@Cu-3(BTC)(2) system maintains a significant CO2 and CH4 adsorption potential compared to the pristine MOF. The synthetic route developed in this work yields novel guest@MOF hybrid systems that will be useful for many MOF-based applications such as gas separations and chemical sensors working under humid conditions.