Mixed Valency in the Excited State

ALEJANDRO CADRANEL

Seminario; Early Career Researchers in Inorganic Chemistry; 2020

The field of mixed valency (MV) was born in 1969, with the first publication of the Creutz-Taube ion, [(NH3)5RuII(pz)RuIII(NH3)5]5+ and, since then, this bimetallic complex has become a hallmark of electron transfer research. In addition to the basic interest in electron transfer itself, applied research on MV deals with charge delocalization and charge transport along molecular junctions. The target is to achieve molecular wire behavior and to implement such molecular materials in molecular electronics. Studies have been traditionally focused on ground-state mixed valence (GS-MV) systems. This has been favored by the experimental accessibility of, on one hand, intervalence charge transfer (GS-IVCT) absorptions, whose characteristics can be related to the thermal pathways of electron transfer, and, on the other hand, ground state redox potentials, which allow to characterize electronic communication between the redox sites. However, the advent of robust, ultrafast transient absorption techniques with broad band detection enabled the observation of intervalence charge transfer bands in the excited state (ES-IVCT) and the study of MV systems in the excited state (ES-MV). The study of ES-MV is important in the context of solar energy conversion, where photoinduced electron transfer is one of the fundamental steps. Furthermore, ES-MV systems can be potentially incorporated into molecular electronics, where light inputs drive controlled changes in molecular conductivity, similar to what happens in photoswitchable mixed valence systems.We recognize three categories of ES-MV systems, that are naturally transient species prepared by light absorption. First, when GS-MV systems are, for example, irradiated into their GS-IVCT band, IVCT excited states are populated. The electronic configuration of the IVCT excited states is that one of the electronic isomer of the GS-MV state. Second, when GS-MV systems are prone to populate excited states exclusively centered in either the MV-donor or MV-acceptor, it is possible to prepare MV-D* or MV-A* excited sates. In this case, the MV-donor and the MV-acceptor retain in the ES those oxidation states they present in the GS. Finally, when the GS is not a GS-MV system, oxidation states need to be modified to create a ES-MV system. Therefore, a photoinduced charge transfer is required, that renders photoinduced mixed-valence (PIMV) systems. In these systems, a MV core is generated, together with a charge-transfer counterpart, that, in the absence of charge-shifts, will remain associated to the MV core.This presentation will discuss the three ES-MV categories with examples from our lab. They include *[Ru(tpm)(bpy)(NC)Ru(py)4(CN)]2+ and *[Ru(tpy)(bpy)(NC)Ru(CN)5]2- , where the influence of PIMV results in a prolongation of MLCT lifetimes,[1] on one hand, and the donor / acceptor roles of the MV interactions are reversed with respect to the ground state,[2] on the other hand. Additionally, *[(CN)5Cr(CN)Ru(py)4(NC)Cr(CN)5]4- will be presented as MV-A* systems, that populate Cr-centered excited states and consequently a splitting of the ES-IVCT band is observed.[3] Finally, the involvement of IVCT excited states in the deactivation of the GS-MV system *[Ru(tpy)(bpy)(NC)Os(CN)5]- will be shown,[4] illustrating the utilization of GS- and ES-IVCT bands as powerful tools to elucidate excited state decay mechanisms.[5]