Consecutive Photoinduced Electron Transfer in a Donor–Acceptor–Donor Chromophore and Corresponding Metal Organic Frameworks

Excited states of anion radicals function as potent "super-reductants" in photoreduction reactions that occur by consecutive photoinduced electron transfer (conPET). In this work, we study the debromination of 2-bromo-4-fluorobenzonitrile driven by radical anion doublet excited states of a π-conjugated donor-acceptor-donor (DAD) type photosensitizer, 4,7-di(4-pyridyl)-2,1,3-benzothiadiazole (DPBT), and two Zn-based metal organic frameworks that contain DPBT linkers, <b>BDC-MOF</b> and <b>BTC-MOF</b>. A thorough analysis of the photophysical properties of DPBT, the DPBT-linker based MOFs, and their anion radicals was performed, employing various spectroscopy techniques, including steady-state absorption, emission, and ultrafast transient absorption. The results indicate that the doublet excited states of DPBT^<b>-•</b>, <b>BDC-MOF</b>^<b>-•</b>, and <b>BTC-MOF</b>^<b>-•</b> exhibit lifetimes of 49, 40, and 38 ps, respectively. Using DPBT^-•, <b>BDC-MOF</b>^<b>-•</b>, and <b>BTC-MOF</b>^<b>-•</b> in the debromination reaction of 2-bromo-4-fluorobenzonitrile, we achieve promising yields of the debrominated product 4-fluorobenzonitrile in less than an hour with a violet-LED source (400 nm, 19 mW/cm²) irradiation. Photophysical mechanistic studies reveal that each step in the proposed conPET mechanism is active in the DPBT^-• systems. This research provides new directions toward the design of MOF-based photosensitizers for conPET photocatalysis.