Anti-dissipative strategies towards more efficient solar energy conversion

Abstract In natural and artificial photosynthesis, light absorption and catalysis are separate processes linked together by exergonic electron transfer. This leads to free energy losses between the initial excited state, formed after light absorption, and the catalytic center formed after the electron transfer cascade. Additional deleterious processes, such as internal conversion and vibrational relaxation, also dissipate as much as 20-30 % of the absorbed photon energy. Minimization of these energy losses, a holy-grail in solar energy conversion and solar fuels production, is a challenging task, because excited states are usually strongly coupled which results in negligible kinetic barriers and very fast dissipation. Here we show that topological control of oligomeric {Ru(bpy)3} chromophores resulted in small excited-state electronic couplings, leading to activation barriers for internal conversion of around 2000 cm–1 and effectively slowing down dissipation. Two types of excited states are populated upon visible light excitation, i.e. a bridging-ligand centered metal-to-ligand charge transfer (MLCTLm), and a 2,2’-bipyridine-centered MLCT (MLCTbpy), which lies 800-1400 cm–1 higher in energy. As a proof-of-concept, bimolecular electron transfer with tri-tolylamine as electron donor was performed, which mimics catalyst activation by sacrificial electron donors in typical photocatalytic schemes. Both excited states were efficiently quenched by tri-tolylamine and produced the corresponding bpy•– and Lm•– centered reduced complexes, as confirmed by transient absorption spectroscopy. This efficiently generated two distinct reduced photosensitizers with different reducing abilities, i.e. –0.93 V and –0.79 V vs NHE for bpy•– and Lm•–, respectively. Hence, this novel strategy not only allows to trap higher energy excited states, before internal conversion and vibrational relaxation set in, saving between 110 and 170 meV and but also leads in fine to 140 meV more potent reductant for energy conversion schemes and solar fuels production. These results lay the first stone for anti-dissipative energy conversion schemes which, in bimolecular electron transfer reactions, harnesses the excess energy saved by controlling dissipative conversion pathways.