Extended π-Conjugation in Ni-Phenanthroline Covalent Organic Cages Prolongs Charge Lifetime for Efficient Photocatalytic CO2 Reduction.

The local microenvironment of metal centers critically dictates the activity and selectivity in photocatalytic CO2 reduction. Herein, we report two isoreticular covalent organic cages, Ni-PHEN and Ni-BPY, featuring Ni(II) coordinated by phenanthroline and bipyridine motifs, respectively. The Ni-PHEN cage exhibits markedly enhanced CO2 photoreduction activity (9106 µmol g-1 h-1, 91.7% selectivity) compared to Ni-BPY (5181 µmol g-1 h-1, 86.9% selectivity) under visible light irradiation. Comprehensive spectroscopic characterizations reveal that the rigid and extended π -conjugated phenanthroline framework narrows the optical bandgap, broadens visible light absorption, and prolongs the fluorescence lifetime relative to the flexible bipyridine analogue. These distinct electronic properties promote efficient charge separation and facilitate CO2 activation at the Ni center, collectively contributing to the superior photocatalytic performance. This work establishes a ligand-engineering strategy for tuning the coordination microenvironment in covalent organic cages and underscores the pivotal role of extended π-conjugation in advancing efficient artificial photosynthesis.