Anti-Dissipative Strategies toward More Efficient Solar Energy Conversion

COTIC, AGUSTINA; CERFONTAINE, SIMON; SLEP, LEONARDO D.; ELIAS, BENJAMIN; TROIAN-GAUTIER, LUDOVIC; CADRANEL, ALEJANDRO

JOURNAL OF THE AMERICAN CHEMICAL SOCIETY

AMER CHEMICAL SOC

Año: 2023

0002-7863

In natural and artificial photosynthesis, lightabsorption and catalysis are separate processes linked together byexergonic electron transfer. This leads to free energy losses betweenthe initial excited state, formed after light absorption, and the activecatalyst formed after the electron transfer cascade. Additionaldeleterious processes, such as internal conversion (IC) andvibrational relaxation (VR), also dissipate as much as 20−30% ofthe absorbed photon energy. Minimization of these energy losses, aholy grail in solar energy conversion and solar fuel production, is achallenging task because excited states are usually strongly coupledwhich 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 IC by means of inter-ligand electron transfer of around 2000 cm−1 and effectively slowing downdissipation. Two types of excited states are populated upon visible light excitation, that is, a bridging-ligand centered metal-to-ligandcharge transfer [MLCT(Lm)], and a 2,2′-bipyridine-centered MLCT [MLCT(bpy)], which lies 800−1400 cm−1 higher in energy. Asa proof-of-concept, bimolecular electron transfer with tri-tolylamine (TTA) as electron donor was performed, which mimics catalystactivation by sacrificial electron donors in typical photocatalytic schemes. Both excited states were efficiently quenched by TTA.Hence, this novel strategy allows to trap higher energy excited states before IC and VR set in, saving between 100 and 170 meV.Furthermore, transient absorption spectroscopy suggests that electron transfer reactions with TTA produced the correspondingLm•−-centered and bpy•−-centered reduced photosensitizers, which involve different reducing abilities, that is, −0.79 and −0.93 Vversus NHE for Lm•− and bpy•−, respectively. Thus, this approach probably leads in fine to a 140 meV more potent reductant forenergy conversion schemes and solar fuel production. These results lay the first stone for anti-dissipative energy conversion schemeswhich, in bimolecular electron transfer reactions, harness the excess energy saved by controlling dissipative conversion pathways.