Photoredox-Active Colloids and Polymer Membranes of Titanium-Based MOF Nanoparticles

Nanosizing metal-organic frameworks (MOFs) facilitates their preparation in membrane- and thin-film-based technologies, while enabling solution-state techniques to probe size-dependent properties and molecular reactivity, but few MOFs have been prepared as nanoparticles (nanoMOFs). Here, we report a rapid reflux-based nanoparticle synthesis of the photoredox-active MOF Ti8O8(OH)4(terephthalate)6 (MIL-125) to achieve sizes below 30 nm in less than 2 hours, whereas previous routes generate particles larger than 100 nm in days. The resulting nanoMOFs display BET surface areas of ~1800 m2/g, rivaling the bulk counterparts, and the sizes can be reproducibly controlled by tuning the key reaction parameters identified here. Furthermore, the smallest nanoparticles exhibit colloidal stability for weeks, permitting analysis by solution-state techniques. Optical absorption and photoluminescence measurements of free-standing colloids offer the first direct evidence of Ti3+ and charge accumulation in a MOF during photoredox processes. These data also reveal for the first time that Ti centers in MIL-125 undergo a significant Jahn-Teller distortion upon photoreduction. Solution-state potentiometry measured in situ provides insight into energetic aspects of the photochemistry, while voltammetry of the colloids reports some of the only known redox potentials of a nanoMOF. Finally, we demonstrate that MIL-125 nanocrystals can be implemented into membranes by solution-processing techniques and demonstrate retention of photoactivity even when encased in polymer matrices. Taken together, these results demonstrate the feasibility of rapid nanoMOF synthesis and membrane fabrication and the deep fundamental insights they enable into photoredox chemistry.