A photoredox catalytic cycle

Visible light lifts ground-state Ru(bpy)₃²⁺ to a long-lived excited state that is at once a stronger oxidant (−0.81 V) and a stronger reductant (+0.77 V). Follow the arrows: the excited state either donates an electron to an acceptor (oxidative quenching) or takes one from a donor (reductive quenching).