Counterion Effects in [Ru(bpy) <sub>3</sub> ](X) <sub>2</sub> -Photocatalyzed Energy Transfer Reactions
Juliette Zanzi, Zachary Pastorel, Carine Duhayon, Elise Lognon, Christophe Coudret, Antonio Monari, Isabelle M. Dixon, Yves Canac, Michaël Smietana, Olivier Baslé
Abstract
High Resolution Image Download MS PowerPoint Slide Photocatalysis that uses the energy of light to promote chemical transformations by exploiting the reactivity of excited-state molecules is at the heart of a virtuous dynamic within the chemical community. Visible-light metal-based photosensitizers are most prominent in organic synthesis, thanks to their versatile ligand structure tunability allowing to adjust photocatalytic properties toward specific applications. Nevertheless, a large majority of these photocatalysts are cationic species whose counterion effects remain underestimated and overlooked. 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 (BAr F 4 – ) or a tightly bound (TsO – ) anion. In addition, the counterion identity also greatly affects the photophysical properties of the cationic ruthenium complex, with [Ru(bpy) 3 ](BAr F 4 ) 2 exhibiting the shortest 3 MLCT excited-state lifetime, highest excited state energy, and highest photostability, enabling remarkably enhanced performance (up to >1000 TON at a low 500 ppm catalyst loading) in TTEnT photocatalysis. These findings supported by density functional theory-based calculations demonstrate that counterions have a critical role in modulating cationic transition metal-based photocatalyst potency, a parameter that should be taken into consideration also when developing energy transfer-triggered processes.