Photoinduced energy- and electron-transfer processes in dinuclear Ru II-OsII, RuII-OsIII, and Ru III-OsII trisbipyridine complexes containing a shape-persistent macrocyclic spacer

Abstract

The PF6 - salt of the dinuclear [(bpy) 2Ru(1)Os(bpy)2]4+ complex, where 1 is a phenylacetylene macrocycle which incorporates two 2,2′-bipyridine (bpy) chelating units in opposite sites of its shape-persistent structure, was prepared. In acetonitrile solution, the Ru- and Os-based units display their characteristic absorption spectra and electrochemical properties as in the parent homodinuclear compounds. The luminescence spectrum, however, shows that the emission band of the RuII unit is almost completely quenched with concomitant sensitization of the emission of the OsII unit. Electronic energy transfer from the RuII to the OsII unit takes place by two distinct processes (ken = 2.0 × 10 8 and 2.2 × 107 s-1 at 298 K). Oxidation of the OsII unit of [(bpy)2Ru(1)Os (bpy)2] 4+ by CeIV or nitric acid leads quantitatively to the [(bpy)2RII(1)OsIII(bpy)2] 5+ complex which exhibits a bpy-to-OsIII charge-transfer band at 720 nm (εmax = 250 M-1 cm-1). Light excitation of the RuII unit of [(bpy)2Ru II(1)OsIII(bpy)2]5+ is followed by electron transfer from the RuII to the OsIII unit (k el,f = 1.6 ε 108 and 2.7 × 107 s-1), resulting in the transient formation of the [(bpy) 2RuIII(1)OsII(bpy)2]5+ complex. The latter species relaxes to the [(bpy)2Ru II(1)OsIII(bpy)2]5+ one by back electron transfer (kel,b = 9.1 × 107 and 1.2 × 107 s-1). The biexponential decays of the [(bpy) 2*RuII(1)OsII(bpy)2] 4+, [(bpy)2*RuII(1)OsIII" (bpy)2]5+, and [(bpy)2RuIII(1) OsII(bpy)2]5+ species are related to the presence of two conformers, as expected because of the steric hindrance between hydrogen atoms of the pyridine and phenyl rings. Comparison of the results obtained with those previously reported for other Ru-Os polypyridine complexes shows that the macrocyclic ligand 1 is a relatively poor conducting bridge. © 2006 Wiley-VCH Verlag GmbH & Co. KGaA.