Single-Atom Metal Oxide Sites as Traps for Charge Separation in the Zirconium-Based Metal–Organic Framework NDC–NU-1000

Solvothermal deposition in metal–organic frameworks (MOFs) can be used to mount single-metal-atom catalytic species at chemically reactive sites on hexa-Zr­(IV)–oxy­(oxo,hydroxo,aqua) nodes in nanoscale crystallites of the MOF NDC–NU-1000 in a self-limiting fashion. Upon photoexcitation of the 1,3,6,8-tetrakis­(p-benzoato)­pyrene chromophores of the parent framework, charge transfer may occur between the chromophores and the installed heterometal sites. Extended X-ray absorption fine structure studies revealed the single-atom nature of the installed species. A combination of steady-state and ultrafast optical spectroscopy was used to uncover evidence of a charge-separated (CS) state arising in the metalated samples. The relevant dynamics were characterized with transient photoluminescence and femtosecond transient absorption spectroscopy. We find that a titanium–oxy single-atom site gives rise to the longest lived CS species compared to cobalt and nickel in a similar arrangement. This study provides guidance in designing MOF-based catalytic systems for photocatalysis and solar fuel production.