Piperazine-Linked Metalphthalocyanine Frameworks for Highly Efficient Visible-Light-Driven H₂O₂ Photosynthesis

Artificial photosynthesis of H₂O₂ from O₂ reduction provides an energy-saving, safe, and green approach. However, it is still critical to develop highly active and selective 2e^- oxygen reduction reaction photocatalysts for efficient H₂O₂ production owing to the unsatisfactory photosynthesis productivity. Herein, two new two-dimensional piperazine-linked CoPc-based covalent organic frameworks (COFs), namely, CoPc-BTM-COF and CoPc-DAB-COF, were afforded from the nucleophilic substitution reaction of hexadecafluorophthalocyaninato cobalt(II) (CoPcF₁₆) with 1,2,4,5-benzenetetramine (BTM) or 3,3'-diaminobenzidine (DAB). Powder X-ray diffraction analysis in combination with electron microscopy and a series of spectroscopic technologies reveals their crystalline porous framework with a fully conjugated structure and eclipsed π-stacking model. Ultraviolet-visible diffuse reflectance absorption spectra unveil their excellent light absorption capacity in a wide range of 400-1000 nm. This, together with their enhanced photo-induced charge separation and transport efficiency as disclosed by photocurrent response and photoluminescence measurements, endows the as-prepared piperazine-linked CoPc-based COFs with superior photocatalytic activity toward O₂-to-H₂O₂ conversion under visible-light irradiation (λ > 400 nm). In particular, CoPc-BTM-COF exhibits a record-high H₂O₂ yield of 2096 μmol h^-1 g^-1 among the COF-based photocatalysts and an impressive apparent quantum yield of 7.2% at 630 nm. The present result should be helpful for fabricating high-performance and low-cost photocatalysts for visible-light-driven H₂O₂ photosynthesis.