The Excited-State Creutz-Taube ion and other photoinduced mixed valence systems

CADRANEL, ALEJANDRO

Simposio; International Symposium on Photophysics and Photohcemistry of Coordination Compounds; 2023

In natural photosynthesis, photon absorption triggers energy transfer processes from the antenna complex to the reaction center. There, charge separation takes place at the special pair of chlorophylls, [PA·PB], which affords a charge separated state that contains an oxidized special pair [PA·PB]•+ and a reduced chlorophyll ChlA•–. In plants, [PA·PB]•+ transfers the hole to a tyrosine which subsequently accumulate in the calcium-manganese cluster and ultimately drive water oxidation. [PA·PB]•+ is a photoinduced mixed valence (PIMV) system, where, for example, hole transfer to the tyrosine crucially depends on the hole distributed between both chlorophylls, estimated in 80:20 between PA and PB.[1] The role of the special pair is key in natural photosynthesis, and its photoinduced mixed valence properties are determinant. Therefore, the study of PIMV systems is imperative for artificial solar energy conversion, including the establishment of prototypical models and critical comparisons with the much intensively explored ground-state analogs.In this talk, the excited-state Creutz-Taube ion [(NH3)5RuII+(pz•–)RuIII–(NH3)5]4+, prepared by MLCT irradiation of [(NH3)5RuII(pz)RuII(NH3)5]4+, will be presented as a reference model for PIMV systems and excited-state electron transfer.[2] The observation of photoinduced intervalence charge-transfer bands (PIIVCT), which allow to derive excited-state electronic couplings, will be highlighted. Additionally, other PIMV systems will be discussed, with a focus on the difference with ground-state analogs, partly arising due to the presence of other close-lying excited states and partly due to the simultaneous presence of an excited electron and an excited hole in charge-transfer excited states.[3]