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The results of the temperature dependence in fluid solution of the emission behavior of a series of [Re(L-L)-(CO)3py]+ complexes 4,7, 4, 2, 4', 5, 2, and phen) are presented along with an interpretation which assigns the lowest excited state as 3MLCT, with a contribution from a higher-energy, shorter-lived state of unknown character. The emission lifetime of [Re(3,4,7,8-Me4phen)(C0)3py]+ actually increases with temperature, reaches a maximum at approximately 230 K, and then decreases in the usual manner. This behavior is attributed to population of the lowest 3LC state, which in this complex lies just above the 3MLCT emitting state and below the upper state.

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Electrochemical measurements have been performed on a serie? of rhodium arene complexes of the formula [(MenC6H6_")RhCp*]2+ ( = 0-6; Cp* = r;-C5Me5). In general, two reduction waves are observed, corresponding to the Rh(III)/Rh(II) and Rh(II)/Rh(I) oxidation state changes. The Rh(I) complexes have been proposed to have a r/4-bonded arene, and this has been confirmed by X-ray crystallographic measurements on the hexamethylbenzene complex 6. (i?4-C6Me6)RhCp* crystallizes in the space group Pnma with a = 16.724 (4) A,b = 15.149 (4) Á, c = 7.963 (2) A, V = 2017.5 (9) A"3, and Z -4. The arene hinge angle is 41.8°. For nine of the complexes studied, both the Rh(II) monocation and the neutral Rh(I) complex have been detected. In all cases, the lifetimes of the Rh(II) complexes were less than the corresponding Rh(III) or Rh(I) complexes. Half-lives of the Rh(II) monocations steadily increased as the number of methyl groups on the arene increased, from about 5 ms for [(C6H6)RhCp*]+ to >1800 s for [(C6Me6)RhCp*]+. The standard reduction potential for the Rh(III)/Rh(II) couple decreased by about 28 mV for each Me group on the arene ring, and an analysis of previous literature data shows that this is the expected substituent effect when substituting Me for H in an arene complex. E°values for the Rh(II)/Rh(I) couples are, however, virtually independent of the degree of arene methylation. The implication of this observation is that the arene hapticity change is thermodynamically favored with an increasing degree of arene methylation. The "natural" lowering of the potential due to the inductive effects of added Me groups is thus counterbalanced by a raising of the potential due to increased stabilization of the 4 structure. The amount of stabilization of the 4 structure is about 33 mV/Me group or about 0.7 (kcal/mol)/Me. The favoring of the -4 structure with a higher degree of arene substitution may be due to decreased loss of resonance energies in the bending of the more highly substituted arene complexes.

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We demonstrate the feasibility of hexacoordinate silicon complexes with dianionic pincer ligands as electron transport and electroluminescent components of organic electronic devices.

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+) and [{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 transitions 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 reductions associated with each coordinated ligand commencing at -1.3 V and ending at approximately -1.9 V, and emission from a (3)MLCT state having energy maxima between 598 and 610 nm. The Ru(III)/Ru(II) oxidation of the two bimetallic complexes is a single, two one-electron process. Relative 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 emission lifetime decreases from 740 to 3 ns, and the emission quantum yield 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)](4+) increases from 1.28 to 1.32 V, the room temperature emission lifetime decreases from 770 to 3 ns, and the room temperature emission quantum yield decreases from 0.079 to 0.000 26. Emission lifetimes measured 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 energy is attributed to changes in the local matrix of the chromophores and the larger energy of activation to population of a higher energy dd state. Explanations for the variations in physical properties are based on molecular mechanics calculations which reveal that the Ru-N bond distance increases from 2.05 Å (from Ru(II) to bpy and BPY) to 2.08 Å (from Ru(II) to Obpy) and that the metal-to-metal distance increases from approximately 7.5 Å for [{Ru(bpy)(2)}(2)(Obpy)](4+) to approximately 14 Å for [{Ru(bpy)(2)}(2)(BPY)](4+).