Photostable Polynuclear Ruthenium(II) Photosensitizers Competent for Dehalogenation Photoredox Catalysis at 590 nm

Higher nuclearity photosensitizers produced dehalogenation yields greater than 90% in the reported [Ru(bpy)(3)](2+)-mediated dehalogenation of 4-bromobenzyl-2-chloro-2-phenylacetate to 4-bromobenzyl-2-phenylacetate with orange light in 7 h, whereas after 72 h yields of 49% were obtained with [Ru(bpy)(3)](2+). Dinuclear (D1), trinuclear (T1), and quadrinuclear (Q1) ruthenium(II) 2,2'- bipyridine based photosensitizers were synthesized, characterized, and investigated for their photoreactivity. Three main factors were shown to lead to increased yields (i) the red-shifted absorbance of polynuclear photosensitizers, (ii) the more favorable driving force for electron transfer, characterized by more positive E-1/2(Ru2+*/+), and (iii) the smaller population of the (MC)-M-3 state (<0.5% for D1, T1 and Q1 vs 48% for [Ru(bpy)(3)](2+) at room temperature). Collectively, these results highlight the potential advantages of using polynuclear photosensitizers in phototriggered redox catalysis reactions.