Energy Transfer in Rigid Ru(II)/Os(II) Dinuclear Complexes with Biscyclometalating Bridging Ligands Containing a Variable Number of Phenylene Units

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Abstract
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 rMM = 11, 15.5, and 20 Å, 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 = H0 exp(−βrMM), with β ∼ 0.33 Å-1].

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