Molecular Rubies in Photoredox Catalysis.

The molecular ruby and the tris(bipyridine) chromium(III) complex as well as the tris(bipyrazine)ruthenium(II) complex were employed in the visible light-induced radical cation [4+2] cycloaddition (tpe = 1,1,1-tris(pyrid-2-yl)ethane, dmcbpy = 4,4'-dimethoxycarbonyl-2,2'-bipyridine, bpz = 2,2'-bipyrazine), while serves as a control system (ddpd = ,'-dimethyl-,'-dipyridin-2-ylpyridine-2,6-diamine). Along with an updated mechanistic proposal for the Cr driven catalytic cycle based on redox chemistry, Stern-Volmer analyses, UV/Vis/NIR spectroscopic and nanosecond laser flash photolysis studies, we demonstrate that the very weakly absorbing photocatalyst outcompetes and even in particular at low catalyst loadings, which appears contradictory at first sight. The high photostability, the reversible redoxchemistry and the very long excited state lifetime account for the exceptional performance and even reusability of in this photoredox catalytic system.