Photoinduced energy‐ and electron‐transfer processes in dinuclear ruthenium(II) and/or osmium(II) complexes connected by a linear rigid bis‐chelating bridge

Abstract

AbstractA rigid and linear bridging ligand containing two 4,5‐diazafluorene chelating units separated by an adamantane spacer (diazf‐a‐diazf) has been synthesized and its dinuclear complexes [(bpy)2Ru(diazf‐a‐diazf)Ru(bpy)2]4+ (RuII · FAF · RuII), [(bpy)2Os(diazf‐a‐diazf)Os(bpy)2]4+ (OsII · FAF · OsII), and [(bpy)2Ru(diazf‐a‐diazf)Os(bpy)2]4+ (RuII · FAF · OsII) have been prepared as PF−6 salts. In these novel compounds, each Ru‐based and Os‐based unit displays its own absorption spectrum and electrochemical properties, regardless of the presence of a second metal‐based unit. The luminescence properties have also been investigated and it has been shown that electronic energy transfer takes place in the mixed‐metal RuII · FAF · OsII species at 77K from the Ru‐based to the Os‐based unit with rate constant 2.6 · 108 s−1. At room temperature the intrinsic decay of the Ru‐based unit is too fast (3.3 · 109 s−1) to allow the occurrence of energy transfer. Partial oxidation of the binuclear compounds RuII · FAF · OsII and OsII · FAF · OsII by CeIV in acetonitrile‐water solutions leads to the mixed‐valence RuII · FAF · OsIII and OsII · FAF · OsIII species where the oxidized metal‐based unit quenches, by electron transfer, the luminescent excited state of the unit that is not oxidized. At room temperature, the rate constants for the excited state * RuII · FAF · OsIII · RuIII · FAF · OsII and * OsII FAF · OsIII · OsIII · FAF · OsII processes are 8.3 · 108 s−1 and 3.9 · 108 s−1, respectively, and the rate constant for the back‐electron‐transfer process RuIII · FAF · OsII · RuII · FAF · OsIII is 2.9 · 107 s−1.