Photocatalytic CO₂-to-CO conversion is a promising route toward solar fuel production and carbon-neutral chemical manufacturing. One strategy to improve the efficiency of molecular photocatalytic systems is the integration of a selective CO₂ reduction catalyst with a photosensitiser capable of harvesting a broad range of the solar spectrum. Conjugated covalent linkage of the catalyst and photosensitiser into a molecular assembly can facilitate intramolecular electron transfer and enhance overall photocatalytic performance.

Here, we combine the well-established catalyst Re(CO)₃(bpy)Cl with BODIPY-based photosensitizers. Meso-substitution of BODIPY with phenyl (Ph), anthracene (An), or phenyl-anthracene (Ph-An) groups induces spin-orbit charge-transfer intersystem crossing, resulting in efficient triplet-state formation with triplet quantum yields following the trend Ph-An > An > Ph. [1,2] These long-lived triplet states can be reduced by a sacrificial electron donor to generate BODIPY radical anions, which subsequently transfer electrons to the catalytic center. In bimolecular photocatalytic systems, this enhances CO₂ reduction activity, with photocatalytic performance following the same trend. [2] However, the kinetics of the bimolecular photocatalytic reaction depends on stochastic encounters and charge transfer between the two components, which limits the overall activity.

To force the photosensitiser and catalytic center into close contact, we designed and synthesized a series of novel rhenium–BODIPY supramolecular assemblies in which a bipyridine ligand is bound to a meso-substituted BODIPY through an ethylene linker at the α-position. This design provides close contact and conjugation between the photosensitizer and catalytic center. In addition to a comparison of the conjugated monomolecular system to the established bimolecular ones, the meso-substituents were systematically varied to study the influence of sterically hindered groups on the photophysical properties and photocatalytic activity of the supramolecular systems.

Compared with the best-performing bimolecular system, consisting of An-BODIPY and Re(CO)₃(bpy)Cl, the respective tethered assembly exhibited more than a sevenfold increase in CO production. Surprisingly, the supramolecular catalysts displayed a reversed activity trend relative to the corresponding intermolecular systems, with Ph-BODIPY-bpy-Re(CO)₃Cl emerging as the most active catalyst and achieving a turnover number (TON) exceeding 500 after 7 h of irradiation. These results indicate that Ph- and An/Ph-An-substituted assemblies operate through distinct photocatalytic pathways, revealing a new strategy for utilizing short-lived excited states in photocatalytic CO₂ reduction.