Counterion Effects in [Ru(bpy)3](X)2-Photocatalyzed Energy Transfer Reactions

In this report, we show that modification of the X counterions constitutive of [Ru(bpy)3](X)2 photocatalysts modulates their catalytic activities in intermolecular [2+2] cycloaddition reactions operating through triplet-triplet energy transfer (TTEnT). Particularly noteworthy is the dramatic impact observed in low-dielectric constant solvent over the excited state quenching coefficient, which varies by two orders of magnitude depending on whether X is a large weakly bound (BArF-) or a tightly bound anion (TsO-). In addition, the counterion identity also greatly affects the photophysical properties of the cationic ruthenium complex, with [Ru(bpy)3](BArF4)2 exhibiting the shortest 3MLCT excited state lifetime, highest excited state energy and photostability, enabling remarkably enhanced performance (up to >1000 TON at low 500 ppm catalyst loading) in TTEnT photocatalysis.