Photonic wires of nanometric dimensions. Electronic energy transfer in rigid rodlike Ru(bpy)32+-(ph)(n)-Os(bpy)32+ compounds (ph = 1,4- phenylene; n = 3, 5, 7)

We have synthesized nine rodlike compounds of nanometric dimension with general formula [M(bpy)3-(ph)(n)-M'(bpy)3]4+ (M = M' = Ru(II); M = M' = Os(II); M = Ru(II), M' = Os(II); bpy = 2,2'-bipyridine; ph = 1,4-phenylene; n = 3, 5, 7; the central phenylene unit bears two alkyl chains for solubility reasons; the metal-to metal distance is 4.2 nm for the longest spacer). The absorption spectra and the luminescence properties (emission spectra, quantum yields, and excited-state lifetimes) of the nine dinuclear complexes have been investigated in acetonitrile solution at 293 K and in butyronitrile rigid matrix at 77 K. The results obtained have been compared with those found for the separated chromophoric units ([Ru(bpy)3]2+, [Os(bpy)3]2+, and oligophenylene derivatives). The absorption spectrum of each dinuclear complex is essentially equal to the sum of the spectra of the component species, showing that intercomponent electronic interactions are weak. In the homodinuclear compounds, the strong fluorescence of the oligophenylene spacers is completely quenched by energy transfer to the metal-based units, which exhibit their characteristic metal-to-ligand charge-transfer (MLCT) phosphorescence. In the heterodinuclear compounds, besides complete quenching of the fluorescence of the oligophenylene spacers, a quenching of the phosphorescence of the [Ru(bpy)3]2+ chromophoric unit and a parallel sensitization of the phosphorescence of the [Os(bpy)3]2+ chromophoric unit are observed, indicating the occurrence of electronic energy transfer. The rate of the energy-transfer process from the [Ru(bpy)3]2+ to the [Os(bpy)3]2+ unit is practically temperature independent and decreases with increasing length of the oligophenylene spacer (in acetonitrile solution at 293 K, k(en) = 6.7 x 108 s-1 for n = 3; k(en) = 1.0 x 107 s-1 for n = 5; k(en) = 1.3 x 106 s-1 for n = 7). It is shown that such an energy- transfer process takes place via a Dexter-type mechanism (superexchange interaction) with an attenuation coefficient of 0.32 per Å and 1.5 per interposed phenylene unit.