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

A 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.