Band-Gap Engineering of Isoreticular Hydrogen-Bonded Organic Frameworks for Boosting Photocatalytic Hydrogen Evolution

The rational modulation of band structures in organic semiconductors is central to advancing photocatalytic performance but remains challenging for hydrogen-bonded organic frameworks (HOFs) due to their structural sensitivity to the modification of organic building units (OBUs). Here, based on the stable mesoporous framework HOF-102, π-conjugation-extended and donor-acceptor-tuned OBUs were predesigned by substituting the steric naphthalene units with benzene-vinyl derivatives bearing ─H, ─CH₃, or ─CN groups. Through a shape-fitted π─π stacking strategy, three mesoporous HOFs isoreticular with HOF-102 were synthesized from the tailored OBUs, namely HOF-1022, HOF-1022(CH₃), and HOF-1022(CN). These as-synthesized HOFs exhibit pronounced variations in visible-light absorption, with band gaps adjustable from 2.46 to 1.86 eV. Among these HOFs, the D-A-optimized HOF-1022(CN) possesses the narrowest band gap and exhibits significantly enhanced intraframework electron transfer and suppressed charge recombination, yielding an impressive hydrogen evolution activity of 168.2 mmol g⁻¹ h⁻¹, which is 8.5 times higher than that of HOF-1022(CH₃), and an apparent quantum yield (AQY) of 7.3% at 420 nm. This study represents the first demonstration of band-gap engineering in HOFs materials and establishes a generalizable molecular-design principle for developing high-performance HOFs-based photocatalysts.