Distant ultrafast energy transfer in a trimetallic {Ru–Ru–Cr} complex facilitated by hole delocalization

Multi-metallic complexes based on {Ru–Cr}, {Ru–Ru} and {Ru–Ru–Cr} fragments are investigated for their light-harvesting and long-range energy transfer properties. We report the synthesis and characterization of [Ru(tpy)(bpy)(μ-CN)Ru(py)4Cl]2+ and [Ru(tpy)(bpy)(μ-CN)Ru(py)4(μ-NC)Cr(CN)5]. The intercalation of {RuII(py)4} linked by cyanide bridges between {Ru(tpy)(bpy)} and {Cr(CN)5} results in efficient, distant energy transfer followed by emission from the Cr moiety. Characterization of the energy transfer process based on photophysical and ultrafast time-resolved absorption suggests the delocalization of holes in the excited state, providing a pathway for energy transfer between the end moieties. The proposed mechanism opens the door to utilize this family of complexes as an appealing platform for the design of antenna compounds as the properties of the fragments could be tuned independently.