Visible Light-Harvesting Dinuclear Chromium(III) Complex with Anthracene-Based Phosphorescence for Photon Upconversion and Photocatalysis

Abstract

Luminescent 3d3 chromium(III) complexes are emerging as highly attractive, earth-abundant alternatives to precious 4d or 5d metal complexes in photophysical and photochemical applications. However, the Laporte-forbidden nature of their d–d transitions in the visible region severely hampers the light-harvesting efficiency of conventional photoactive CrIII complexes. To overcome this fundamental bottleneck, we anchored two anthracene-based antennas to the polypyridine ligands in a dinuclear CrIII complex, yielding the complex Cr2-An. The resulting energetically low-lying π–π* transition in the anthracene antenna enhances visible light absorption at 450 nm by a factor of 30 relative to the anthracene-free reference complex, Cr2-ref Both dimers exhibit ferromagnetic ground-state exchange interactions and display room temperature luminescence, albeit originating from different excited states. Specifically, Cr2-An exhibits anthracene-based phosphorescence with a lifetime of ∼150 ns in solution at room temperature, which is likely accessible via two pathways: direct intersystem crossing from the singlet excited state of the anthracene fragment, or intramolecular doublet-triplet energy transfer from the spin-flip excited states of the CrIII centers. These exceptional photophysical properties render Cr2-An an attractive photosensitizer for triplet–triplet annihilation upconversion (TTA-UC) and singlet oxygen-mediated oxidation reactions, delivering photocatalytic performance that surpasses the benchmark [Ru(bpy)3]Cl2 under visible-light irradiation. These findings offer a powerful molecular design strategy for bypassing the intrinsic optical limitations of first-row transition metals, opening new avenues for sustainable and efficient light energy conversion.