A Tetranuclear Ruthenium(II) Complex Containing both Electron-Rich and Electron-Poor Bridging Ligands. Absorption Spectrum, Luminescence, Redox Behavior, and Intercomponent Energy Transfer Scolastica Serroni, 1a Sebastiano Campagna,* ,1a Gianfranco Denti, 1b Tia E. Keyes, 1c and Johannes G. Vos* ,1c Dipartimento di Chimica Inorganica, Analitica e Struttura Molecolare dell’Universita `, via Sperone 31, I-98166 Vill. S. Agata, Messina, Italy, Laboratorio di Chimica Inorganica, Istituto di Chimica Agraria dell’Universita `, via S. Michele degli Scalzi 2, I-50124 Pisa, Italy, and School of Chemical Sciences, Dublin City University, Dublin 9, Ireland ReceiVed January 26, 1995 X The first luminescent and redox active multinuclear Ru(II) compound containing both electron-poor (2,3-bis(2- pyridyl)pyrazine, 2,3-dpp) and electron-rich (3,5-bis(pyridyn-2-yl)-1,2,4-triazole, Hbpt) polypyridine bridging ligands has been synthesized. The novel compound is [(bpy) 2 Ru(μ-bpt)Ru{(μ-2,3-dpp)Ru(bpy) 2 } 2 ] 7+ (1; bpy ) 2,2′-bipyridine). Its absorption spectrum, luminescence properties, and redox behavior have been studied and are compared with the properties of the parent complexes [Ru{(μ-2,3-dpp)Ru(bpy) 2 } 3 ] 8+ (2) and [(bpy) 2 Ru(μ- bpt)Ru(bpy) 2 ] 3+ (3). The absorption spectrum of 1 is dominated by ligand-centered bands in the UV region and by metal-to-ligand charge transfer bands in the visible region. Excited states and oxidation and reduction processes are localized in specific sites of the multicomponent structure. However, perturbations of each component on the redox and excited states of the others, as well as electronic interactions between the chromophores, can be observed. Intercomponent energy transfer from the upper-lying (μ-bpt)(bpy)Rufbpy CT excited state of the Ru(bpy) 2 (μ- bpt) + component to the lower-lying (bpy) 2 Rufμ-2,3-dpp CT excited state of the Ru(bpy) 2 (μ-2,3-dpp) 2+ subunit(s) is efficient in 1 in fluid solution at room temperature, whereas this process is not observed in a rigid matrix at 77 K. A two-step energy transfer mechanism is proposed to explain the photophysical properties of the new compound. Introduction Luminescent and redox-active multinuclear metal complexes are currently the object of great interest because of both theoretical reasons and potential practical applications. For example, this family of compounds holds a central position in the design of supramolecular systems capable of performing photoinduced energy migration and/or charge separation, with the ultimate goal of constructing devices for solar energy conversion and/or light-driven information processing. 2 A major class of luminescent and redox-active multinuclear metal complexes is based on Ru(II)-polypyridine building blocks connected by electron-poor (i.e., with low-lying π* orbitals) bridging ligands. One of the most used bridging ligand is the bis-chelating ligand 2,3-bis(2-pyridyl)pyrazine (2,3-dpp). 3,4 A second class of luminescent multinuclear metal complexes is based on the same building blocks connected by anionic electron-rich (with relatively high-lying π orbitals) bridging ligands. 5-8 An interesting difference between the two classes of multinuclear systems is that electron-poor bridging ligands can mediate metal-metal communication by a superexchange X Abstract published in AdVance ACS Abstracts, May 15, 1996. (1) (a) Universita ` di Messina. (b) Universita ` di Pisa. (c) Dublin City University. (2) (a) Meyer, T. J. Acc. Chem. Res. 1989, 22, 163. (b) Scandola, F.; Indelli, M. T.; Chiorboli, C.; Bignozzi, C. A. Top. Curr. Chem. 1990, 158, 73. (c) Balzani, V.; Scandola, F. Supramolecular Photochemistry; Horwood: Chichester, U.K., 1991. (d) Jones, W. E.; Baxter, S. M.; Mecklenburg, S. L.; Erickson, B. W.; Peek, B. M.; Meyer, T. J. 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