COMPARISON OF PHYSICAL AND PHOTOPHYSICAL PROPERTIES OF MONOMETALLIC AND BIMETALLIC RUTHENIUM(II) COMPLEXES CONTAINING STRUCTURALLY ALTERED DIININE LIGANDS

The physical and photophysical properties of a series of monometallic, [Ru(bpy)(2)(dmb)](2+), [Ru(bpy)(2)(BPY)](2+), [Ru(bpy)(Obpy)](2+) and [Ru(bpy)(2)(Obpy)](2+), and bimetallic, [{Ru(bpy)(2)}(2)(BPY)](4+) an d [{Ru(bpy)(2)}(2)(Obpy)](4+), complexes are examined, where bpy is 2, 2'-bipyridine, dmb is 4,4'-dimethyl-2,2'-bipyridine, BPY is 1,2-bis(4- methyl-2,2'-bipyridin-4'-yl)ethane, and Obpy is 1,2-bis(2,2'-bipyridin -6-yl)ethane. The complexes display metal-to-ligand charge transfer tr ansitions in the 450 nm region, intraligand pi --> pi transitions at energies greater than 300 nm, a reversible oxidation of the ruthenium( II) center in the 1.25-1.40 V vs SSCE region, a series of three reduct ions associated with each coordinated ligand commencing at -1.3 V and ending at similar to-1.9 V, and emission from a (3)MLCT state having e nergy maxima between 598 and 610 nm. The Ru-III/Ru-II oxidation of the two bimetallic complexes is a single, two one-electron process. Relat ive to [Ru(bpy)(2)(BPY)](2+), the Ru-III/Ru-II potential for [Ru(bpy)( 2)(Obpy)](2+) increases from 1.24 to 1.35 V, the room temperature emis sion lifetime decreases from 740 to 3 ns, and the emission quantum yie ld decreases from 0.078 to 0.000 23. Similarly, relative to [{Ru(bpy)( 2)}(2)(BPY)](4+), the Ru-III/Ru-II potential for [{Ru(bpy)(2)}(2)(Obpy )](3+) increases from 1.28 to 1.32 V, the room temperature emission li fetime decreases from 770 to 3 ns, and the room temperature emission q uantum yield decreases from 0.079 to 0.000 26. Emission lifetimes meas ured in 4:1 ethanol:methanol were temperature dependent over 90-360 K. In the fluid environment, emission lifetimes display a biexponential energy dependence ranging from 100 to 241 cm(-1) for the first energy of activation and 2300-4300 cm(-1) for the second one. The smaller ene rgy is attributed to changes in the local matrix of the chromophores a nd the larger energy of activation to population of a higher energy dd state. Explanations for the variations in physical properties are bas ed on molecular mechanics calculations which reveal that the Ru-N bond distance increases from 2.05 Angstrom (from Ru-II to bpy and BPY) to 2.08 Angstrom (from Ru-II to Obpy) and that the metal-to-metal distanc e increases from similar to 7.5 Angstrom for [{Ru(bpy)(2)}(2)(Obpy)](4 +) to similar to 14 Angstrom for [{Ru(bpy)(2)}(2)(BPY)](4+).