Which bridge to cross, which mountain to climb – Supramolecular Photocatalysis Outpacing Conventional Catalysis

Unequivocal assignment of rate limiting steps in supramolecular photocatalysts is of utmost importance to rationally optimize photocatalytic activity. By spectroscopic and catalytic analysis of a series of three structurally similar [(tbbpy) 2 Ru-BL-Rh(Cp*)Cl] 3+ photocatalysts just differing in the central part (alkynyl, triazole or phenazine) of the bridging ligand (BL) we were able to derive design strategies for improved photocatalytic activity of this class of compounds (tbbpy = 4,4´-tert-butyl- 2,2´-bipyridine, Cp* = pentamethylcyclopentadienyl). Most importantly, not the rate of the transfer of the first electron towards the Rh III center but rather the rate at which a two-fold reduced Rh I species is generated can directly be correlated with the observed photocatalytic formation of NADH from NAD + . Interestingly, the complex which exhibited the fastest intramolecular electron transfer kinetics for the first electron is not the one that allowed the fastest photocatalysis. With the photocatalytically most efficient alkynyl linked system, it was even possible to overcome the rate of thermal NADH formation. Moreover, for this photocatalyst loss of the alkynyl functionality under photocatalytic conditions was identified as an important deactivation pathway.