Controlling Spin-Correlated Radical Pairs with Donor-Acceptor Dyads: A New Concept to Generate Reduced Metal Complexes for More Efficient Photocatalysis

: One-electron reduced metal complexes derived from photoactive ruthenium or
iridium complexes are important intermediates for substrate activation steps in photoredox
catalysis and for the photocatalytic generation of solar fuels. However, owing to the heavy
atom effect, direct photochemical pathways to these key intermediates suffer from intrinsic
efficiency problems resulting from rapid geminate recombination of radical pairs within the
so-called solvent cage. In this study, we prepared and investigated molecular dyads capable
of producing reduced metal complexes via an indirect pathway relying on a sequence of
energy and electron transfer processes between a Ru complex and a covalently connected
anthracene moiety. Our test reaction to establish the proof of concept is the photochemical
reduction of ruthenium(tris)bipyridine by the ascorbate dianion as sacrificial donor in
aqueous solution. The photochemical key step in the Ru-anthracene dyads is the reduction of
a purely organic (anthracene) triplet excited state by the ascorbate dianion, yielding a spincorrelated radical pair whose (unproductive) recombination is strongly spin-forbidden. By
carrying out detailed laser flash photolysis investigations, we provide clear evidence for the
indirect reduced metal complex generation mechanism and show that this pathway can
outperform the conventional direct metal complex photoreduction. The further optimization
of our approach involving relatively simple molecular dyads might result in novel
photocatalysts that convert substrates with unprecedented quantum yields.