Rigid Rod-Like Dinuclear Ru(II)/Os(II) Terpyridine-Type Complexes. Electrochemical Behavior, Absorption Spectra, Luminescence Properties, and Electronic Energy Transfer through Phenylene Bridges

The absorption spectra, the luminescence properties (at 293 and 77 K), and the electrochemical behavior of six dinuclear heterometallic compounds have been investigated. The compounds are made of Ru(tpy){sub 2}{sup 2+}- and Os(tpy){sub 2}{sup 2+}-type components (tpy = 2,2{prime}:6{prime},2 inches-terpyridine, which in some cases carries p-tolyl (Meph) or methylsulphone (MeO{sub 2}S) substituents in the 4{prime} position), connected by n phenylene (ph) spacers (n=0,1, and 2). In the resulting rigid rod-like structures of general formula (X{sub 1}tpy)Ru(tpy(ph){sub n}tpy)Os(tpyX{sub 2}){sup 4+} the metal-to-metal distance varies form 11 to 20 {Angstrom}. The absorption spectra of the two components are slightly perturbed in the dinuclear compounds, and metal-metal and ligand-ligand interactions are evidenced by the trends of the oxidation and reduction potentials. The luminescence of the Ru-based unit is quenched by the connected Os-based unit with practically unitary efficiency, regardless of the number of interposed phenylene spacers. Quenching is accompanied by quantitative sensitization of the Os-based luminescence. The rate of energy transfer at 293 K is larger than 10{sup 10} s{sup -1} in all cases. The Foerster (Coulombic) mechanism does not satisfactorily account for such a fast rate, particularly for the species with n=2. It is concluded that the observed energy-transfer processes take place most likely via a Dexter (electron exchange) mechanism. This is consistent with the strong electronic coupling of the Ru-based units in the compound with n=0, and with the relatively small insulating effect expected for the phenylene spacers. 37 refs., 7 figs., 3 tabs.