Energy transfer in rigid Ru(II)/Os(II) dinuclear complexes with biscyclometalating bridging ligands containing a variable number of phenylene units

We have prepared rodlike cyclometalated Ru(II)/Os(II) dinuclear complexes, (ttp)Ru(dpb-(ph)(n)-dpb)Os(ttp)(2+), where the biscyclometalating bridging ligands contain dipyridylbenzene fragments, dbp, separated by a variable number, n, of phenylene spacers, and the terminal ligand is a terpyridine derivative [dpbH is di-2-pyridyl-1,3-benzene, ttp is 4'-p-tolyl-2,2':6',2''-terpyridine, and n = 0-2]. The rigid bridging ligands keep the metal centers at a distance r(MM) 11, 15.5, and 20 Angstrom, depending on n. Photoinduced energy transfer has been investigated by luminescence spectroscopy in nitrile solvents at room temperature and at 77 K (i.e., in frozen medium). According to a classical description of the process, the energy transfer occurs in a nearly activationless regime, is governed by electronic factors, and can be described in terms of the Dexter-type mechanism. The obtained energy transfer rates roughly span 3 orders of magnitude and indicate (i) that the temperature (i.e., the state of the solvent) has a small influence on the process and (ii) that the interposed phenylene spacers are weak attenuators of intercenter electronic coupling, H [H = H-0 exp(-beta r(MM), with beta similar to 0.33 Angstrom(-1)].