Photonic wires of nanometric dimensions. Electronic energy transfer in rigid rodlike Ru(bpy)(3)(2+)-(ph)(n)-Os(bpy)(3)(2+) compounds (ph=1,4-phenylene; n=3, 5, 7)

We have synthesized nine rodlike compounds of nanometric dimension with gen eral 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 solubi lity 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 complex es have been investigated in acetonitrile solution at 293 K and in butyroni trile rigid matrix at 77 K. The results obtained have been compared with th ose found for the separated chromophoric units ([RU(bpy)(3)](2+), [Os(bpy)( 3)](2+), and oligophenylene derivatives). The absorption spectrum of each d inuclear complex is essentially equal to the sum of the spectra of the comp onent species, showing that intercomponent electronic interactions are weak . In the homodinuclear compounds, the strong fluorescence of the oligopheny lene spacers is completely quenched by energy transfer to the metal-based u nits, which exhibit their characteristic metal-to-ligand charge-transfer (M LCT) phosphorescence. In the heterodinuclear compounds, besides complete qu enching of the fluorescence of the oligophenylene spacers, a quenching of t he 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 w ith increasing length of the oligophenylene spacer tin acetonitrile solutio n at 293 K, k(en) = 6.7 x 10(8) s(-1) for n = 3; k(en) = 1.0 x 10(7) s(-1) for n = 5; k(en) = 1.3 x 10(6) s(-1) for n = 7). It is shown that such an e nergy-transfer process takes place via a Dexter-type mechanism (superexchan ge interaction) with an attenuation coefficient of 0.32 per Angstrom, and 1 .5 per interposed phenylene unit.