Visible Light Harvesting Cr(III) Photocatalysts

Pseudo-octahedral Cr(III) photosensitizers offer a promising alternative to Ru(II)/Ir(III) MLCT chromophores due to their long-lived, strongly photo-oxidizing excited states, yet suffer from poor visible-light absorption. To address this challenge, we prepared a series of homoleptic Cr(III) trissdphenanthroline) complexes bearing arylacetylene units on the 4- and 7-positions at each phenanthroline core. This modification produced a >1 eV bathochromic shift in intraligand (IL) π-π* transitions and enhanced absorption across the UVA and blue-to-green regions (ε ∼25,000-100,000 M-1 cm-1) compared to [Cr(phen)3]3+, eclipsing traditional MLCT photosensitizers while leaving 2E ligand-field-emission energies largely unaffected. Nanosecond transient absorption and photoluminescence measurements confirmed long excited-state lifetimes (τ = 33-119 μs in CH3) while featuring strongly oxidizing excited-state potentials: E°'([Cr]3+*/2+) = 0.97-1.09 V vs. Fc+/0. High quantum yields for 1O2 production (ΦΔ ≤ 0.85) resulted in efficient photosensitized oxidation of 1,5-dihydroxynaphthalene to juglone under blue or green light irradiation within 30 min (0.1-0.2 mol % catalyst loading). Photocatalytic [4 + 2] cycloadditions under blue or green light (yields up to 100%) were also demonstrated at low catalyst loadings (0.1-0.3 mol %). The combined results demonstrate that large visible absorption cross sections are readily achievable in Cr(III) diimines by leveraging IL transitions, without compromising excited-state energetics or lifetimes.