SYNTHESIS, ELECTROCHEMICAL-BEHAVIOR, AND SPECTROSCOPIC AND LUMINESCENCE PROPERTIES OF DINUCLEAR SPECIES CONTAINING [RU(DIIMINE)(3)](2+) AND[RE(DIIMINE)CL(CO)(3)] CHROMOPHORES BRIDGED BY A NONSYMMETRIC QUATERPYRIDINE LIGAND

The syntheses, electrochemical properties, ground state absorption spe ctra, luminescence spectra, and lifetimes of a series of homo- and het erodinuclear (and related mononuclear) complexes, containing a dicatio nic [Ru(bpy)(3)](2+)-type chromophore (bpy = 2,2'-bipyridine) and/or a neutral [ReCl(CO)(3)(bpy)]-type chromophore, are described. The conne cting ligand is the asymmetric 2,2':3',2 '':6 '',2'''-quaterpyridine, L, which contains two inequivalent bipyridyl binding sites A and B, of which the ''internal'' site (B) is more sterically hindered than the ''external'' site (A). The X-ray structure of [(CO)(3)ClReLRu(bpy)(2)] [PF6](2) . 2MeCN . 0.5Et(2)O (Re-I-AB-Ru-II has been determined: C49H4 1ClF12N10O3.5P(2)ReRu, triclinic, <P(1)over bar>; a = 10.449(7) Angstr om, b = 12.493(8) Angstrom, c = 22.906(11) Angstrom, alpha = 92.83(5)d egrees, beta = 101.11(5)degrees, gamma = 109.30(5)degrees, Z = 2; 9685 independent data (4 degrees less than or equal to 2 theta less than o r equal to 50 degrees) were refined on F-2 to final residuals of wR(2) = 0.129. (R(1) = 0.046). The Re(I) fragment is coordinated to the A s ite of L and the Ru(II) fragment is coordinated to the B site; the two bipyridine moieties of L are nearly perpendicular (86 degrees) to min imize the steric interaction between the halves of the complex, and wh ereas the {Re(CO)(3)Cl} fragment coordinated to the A site has typical structural parameters, the {RU(bpy)(2)}(2+) fragment at the more ster ically hindered B site displays noticeable lengthening of one of the R u-N bonds. From a comparison of the electrochemical and spectroscopic properties of the complexes, it is found that when one chromophore occ upies the A site (i) it is easier to oxidize and (ii) its lowest-lying metal-to-ligand charge-transfer excited state lies at lower energy, t han when the same chromophore is linked to the B site. Thus, for Ru-II -AB-Re-I the luminescent state is centered on the Ru-based moiety, whi le for the positional isomer [(CO)(3)ClReLRu(bpy)(2)](2+) (Re-I-AB-Ru- II) the luminescence is Re-centered. Thus intercomponent energy transf er (Dexter mechanism) takes place from a higher-lying Re-based level t o the lowest-lying Ru-based luminescent level in Ru-II-AB-Re-I and a r eversed Ru --> Re energy transfer operates in Re-I-AB-Ru-II.