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,o wing 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,w ep repared and investigated molecular dyads capable of producing reduced metal complexes via an indirect pathway relyingo nas equence of energy and electron transfer processes between aR uc omplex and ac ovalently connected anthracene moiety.O ur test reaction to establish the proof-of-concept is the photochem-ical reduction of ruthenium(tris)bipyridine by the ascorbate dianion as sacrificial donor in aqueous solution. The photochemicalk ey step in the Ru-anthracene dyads is the reduction of ap urely organic (anthracene) triplet exciteds tate by the ascorbate dianion, yielding as pin-correlated 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 conventionald irect metal complex photoreduction. The furthero ptimization of our approach involving relativelys imple molecular dyads might result in novel photocatalysts that convert substrates with unprecedented quantum yields.

Quantitative laser flash photolysis experiments with several excitation wavelengths provided unprecedented insights into the charge-separated state photochemistry of molecular triads.

The distance dependences of electron transfer rates (k ET) in three homologous series of donor-bridge-acceptor compounds with reaction free energies (G ET 0) of ca.-1.2,-1.6, and-2.0 eV for thermal charge recombination after initial photoinduced charge-separation were studied by transient absorption spectroscopy. In the series with low driving-force, the distance dependence is normal and k ET decreases upon donoracceptor distance (r DA) elongation. In the two series with higher driving-forces, k ET increases with increasing distance over a certain range. This counter-intuitive behavior can be explained by a weakly distance dependent electronic donor-acceptor coupling (H DA) in combination with an increasing reorganization energy (). Our study shows that highly exergonic electron transfers can have distance dependences that differ drastically from those of the more commonly investigated weakly exergonic reactions.

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