A series of dinuclear complexes of ruthenium(II) have been synthesized in which α-azodiimines {such as azobis(2-pyridine), apy, and azobis(4-methyl-2-pyridine), mapy} act as the bridge and 2,2‘-bipyridine (bpy) or 4,4‘-dimethyl-2,2‘-bipyridine (Me2bpy) as the terminal ligands. The diastereoisomeric forms of each species {ΔΛ (meso) and ΔΔ/ΛΛ (rac)} have been separated by cation-exchange chromatography and characterized by 1H-NMR spectroscopy. Electronic spectral and electrochemical studies show there to be differences in inter-metal communication between the diastereoisomeric forms in each case. Comparison of the spectroelectrochemical behavior of the range of complexes has allowed unequivocal assignment of the site of the successive reduction processes observed in dinuclear complexes of this type.

The effect of the presence of the anions PF 6 − , BF 4 − , ClO 4 − , and tosylate − on the potentials of the Ru III/II redox processes in a series of mono-, di-, and trinuclear complexes involving polypyridyl ligands are reported. The anions gives rise to a cathodic shift in the respective redox potentials in the sequence PF 6 − Ͻ BF 4 − ഠ ClO 4 − Ͻ tos − , with the magnitude of the shifts being largest for the tri-nuclear and smallest for the mononuclear species. In one dinuclear complex, a variation of anion was also observed to affect the difference in potentials between the [6+/5+] and [5+/4+] couples. These effects

Spectroelectrochemical investigations of two dinuclear complexes [RuIII(bpy)2(μ-BL)RuII(bpy)2]5+ (BL = 2,3-bis(2-pyridyl)-1,4-benzoquinoxaline (dpb); dipyrido(2,3-a;3‘,2‘-c)benzophenazine (dpb‘)) revealed that the intervalence charge transfer (IT) characteristics are sensitive to the stereochemistry. In particular, there appears to be a greater degree of metal−metal interaction in the meso compared with the rac diastereoisomeric form. Thermochromism, redox, and semiempirical computational studies suggest differential ion-paring between the diastereoisomeric forms.

Rigid alicyclic frameworks (often referred to as molracs, relating to the molecular rack nature of the frame) have been used to vary the separation between organic electron-acceptor (quinone) moieties and chromophoric polypyridylruthenium() centres, and between metal centres in Ru-Ru and Ru-Os dinuclear complexes. Photophysical studies have allowed a preliminary insight into the effectiveness of such alicyclic structures in mediating intramolecular photoinduced energy and electron transfer. In the chromophore-spacer-quinone dyads, solventdependent quenching of the ruthenium() MLCT emission was observed and attributed to electron transfer processes. Distance and stereochemical dependencies of the quenching suggested that through-bond coupling was a factor in these systems. In the heterodinuclear systems, the photo-excited ruthenium() chromophore underwent intramolecular energy transfer to the osmium() component. A through-space Förster dipole-dipole mechanism could adequately account for the rate of the energy transfer process observed.

There has been much activity in the field of intramolecular energy transfer and electron transfer in large molecular assemblies, and the next advances will clearly require sophisticated molecular design. Where metal complexes are involved as the light-harvesting chromophore['] the design and construction of ligands becomes an important component. Studies on intramolecular electron transfer have been reported in which metal complexes have been tethered to redox-active chromophores, but almost all of these systems contain flexible linkages.[21 Elegant syntheses have been reported recently for fully conjugated diad systems incorporating the 1 ,lo-phenanthroline moiety as the bidentate ligandJ3] and we have described the first molecular rack (molrac) in which two units of the same ligand are spatially separated.[41 We report here on the preparation of a new set of rigid bis(bidentate ligand) structures in which the individual bidentate ligands (1 ,lO-phenanthroline, 4,Sdiazafluorene, or 3,6-di(2-pyridyl)pyridazine) are fused to an alicyclic framework.The versatile spacers in molracs permit the development of bridging ligands in which the ligating groups are placed at a set distance and relative orientation in space. These compounds should allow assessment of the factors controlling intramolecular energy-and electron-transfer processes in polymetallic species, as well as lead to new reagents for the study of self-assembly processes.f51 Rigid alicyclic frameworks have been used in molrac systems since they have proven their worth in diad and triad systems devoid of metal centersf6' and are available in an increasing range of shapes and sizes.[4. '-lo] The typical olefinic spacers in the molracs have norbornene or cyclobutene diester end groups (or mixtures thereof), and these offer different dienophilic character. When the cycloaddition reagents 3, 2, and 1 (Scheme 1) are used to deliver the specific [' I Prof.