Viscosity and Pore Accessibility Control of Type I Porous Liquids by Modifying Surface Functionality of Metal–Organic Polyhedra

Viscosity of porous liquids (PLs) is a critical factor for their processability and transportation, yet controlling their viscosity without compromising pore volume remains a significant challenge. In this work, we present a strategy to modulate the viscosity of type I PLs by varying surface functionality. A series of star-shaped PLs was synthesized by coordinating 12 equiv of soft polymer chains to isostructural rhodium metal-organic polyhedra (RhMOPs) as central cores with different surface groups (nonsubstituted, hydroxy, tert-butyl, or dodecyloxy groups). The functional groups on the RhMOP surface were found to significantly affect the conformation of polymer chains, thereby resulting in different rheological behavior and pore accessibility. The use of RhMOPs with dodecyloxy surface groups yielded a closely compact conformation of polymer chains on the MOP surface. The obtained PLs have a viscosity as low as 1.8 × 101 Pa·s and exhibit a temperature-dependent gating effect; CO2 uptake increases with increasing temperature. By contrast, the hydroxyl group afforded PLs with a loosely stretched polymer conformation, resulting in a dramatically high viscosity of 3.0 × 109 Pa·s and high gas accessibility to the inner RhMOP pores. This work provides a feasible way to adjust PLs' rheological properties without altering pore volume to meet the requirements of different practical applications.