Effective Suppression of O2 Quenching of Photo-Excited Ruthenium Complex Using RNA Aptamer

Ruthenium polypyridyl complexes have been widely studied to convert light energy to chemical or electric energy for various applications. To practically use the complexes, it is necessary to suppress as much as possible the very effective deactivation of their triplet metal-to-ligand charge transfer ((MLCT)-M-3) excited states by O-2 (O-2 quenching). In a previous study, we reported that an RNA aptamer can almost completely suppress the O-2 quenching of [Ru(bpy)(3)](2+). Thus motivated, we here further investigate and clarify the generality of such O-2-insensitization of a ruthenium complex using aptamer. Specifically, we demonstrate the O-2 -insensitization using another aptamer for a ruthenium complex, (4,4'-methylphosphonic acid-2,2'-bipyridine) bis(4,4'-dimethyl-2,2'-bipyridine) ruthenium (RuCP). We selected an RNA aptamer that binds to RuCP using SELEX. The aptamer binding to RuCP elongated the lifetime of the (MLCT)-M-3 excited state of RuCP approximately 1.5 fold under aerobic conditions. The elongated lifetime of aptamer-bound RuCP was even longer than that of RuCP under anaerobic condition. We also found that a charge transfer can occur from a sacrificial electron donor to the aptamer-bound RuCP. Thus, we propose that RNA aptamer is one promising strategy to avoid the very effective deactivation of the (MLCT)-M-3 excited states of ruthenium complexes while retaining their effective photo-redox properties.