Competitive Host-Guest and Guest-Guest Interaction Dependent Flexibility of Crystal-Size-Engineered ZIF-8: An Inelastic Neutron Scattering Study.

Metal-organic frameworks (MOFs) exhibit remarkable structural flexibility, influenced by factors such as crystal size and guest molecule interactions. Here, we investigate the crystal-size-dependent flexibility of ZIF-8 by using inelastic neutron scattering (INS). Our study provides direct experimental evidence of reduced flexibility in small ZIF-8 crystals, underscoring the importance of crystal-size-engineering in modulating MOF properties. The enhanced stiffness observed in smaller ZIF-8 crystals can be attributed to the reduction in the unit cell size, which leads to increased rotational hindrance of the framework. While gas-adsorption-induced flexibility in MOFs is widely reported, the influence of competitive host-guest and guest-guest interactions on framework flexibility remains poorly understood. Through INS experiments, we uncovered a nonmonotonic flexibility behavior in nanosized ZIF-8 upon CO2 exposure, indicating intricate molecular dynamics. We observed a hindrance of methyl rotation (40 cm-1) at low CO2 dosing, attributed to host-guest interactions, followed by a resurgence at higher dosing levels, suggesting enhanced guest-guest interactions. Density functional theory calculations and Grand Canonical Monte Carlo simulations provided mechanistic insights, revealing an enhancement in guest-guest interaction energy and a reduction in host-guest interaction energy with dosing. Phonon calculations for both pure and CO2-adsorbed ZIF-8 were performed by using density functional perturbation theory. This approach allowed for a detailed analysis of vibrational properties and the impact of CO2 adsorption on the framework dynamics. In the minimum energy structure, CO2 was found to adsorb near the cavity opening, where it can interact with three methyl groups through its oxygen atom. The INS and DFT data confirmed that the suppressed mode at ∼40 cm-1 in INS spectra at low CO2 dosing in nanosized ZIF-8-s was caused by blocking of methyl rotation due to its strong affinity for the CO2 molecule. Our findings shed light on the complex interplay between crystal size, guest molecule interactions, and framework flexibility in MOFs, offering valuable insights in designing tailored porous materials for different applications.