LUMINESCENT AND REDOX-REACTIVE BUILDING-BLOCKS FOR THE DESIGN OF PHOTOCHEMICAL MOLECULAR DEVICES - MONONUCLEAR, DINUCLEAR, TRINUCLEAR, AND TETRANUCLEAR RUTHENIUM(II) POLYPYRIDINE COMPLEXES

The syntheses, characterization, absorption spectra, luminescence spectra, luminescence lifetimes, and electrochemical behavior of 16 mono-, di-, tri-, and tetrametallic ruthenium(II) polypyridine complexes have been investigated. The general formulas of the complexes studied are RuL2(BL), L2Ru(μ-BL)RuL24+, LRu[(μ-BL)RuL2]26+, and Ru[(μ-BL)RUL2]38+, where L = 2,2′-bipyridine (bpy) or 2,2′-biquinoline (biq) and BL = 2,3- or 2,5-bis(2-pyridyl)pyrazine (dpp). The absorption spectra of the complexes exhibit broad and intense (e up to ~ 50 000 M−1cm−1) metal-to-ligand charge-transfer (MLCT) bands, which in the oligonuclear complexes extend all over the visible region. All the complexes exhibit luminescence in the 600-850-nm region in a rigid matrix at 77 K (lifetimes of the order of 1 μs) and in fluid solution at room temperature (lifetimes of the order of 100 ns). Emissions can be assigned to specific metal-ligand chromophoric units, and the trends in the emission energies on changing ligands and/or nuclearity can be rationalized in the light of the trends observed for the potentials of first oxidation and first reduction processes. Corrected excitation spectra show that the luminescent excited state is populated with the same efficiency regardless of the excitation wavelength throughout the MLCT bands. In electrochemical experiments, the complexes show metal-centered oxidation and ligand-centered reduction processes. Most of the redox waves are reversible and can be assigned to specific metal(s) or ligand(s). The interaction between equivalent redox centers of the same complex is more or less weak, depending on the nature of BL and L. Each one of the mono-, di-, tri-, and tetranuclear complexes studied can be used as a building block for the design of luminescent and redox-reactive species containing a higher number of metal centers. Because of their broad and strong absorption bands in the visible region, relatively long luminescence lifetimes, and rich redox behaviors, complexes of this type can prove useful as antenna components for photosensitization purposes (including electron or hole injection on semiconductors), luminescence probes, and multielectron photocatalysts. © 1990, American Chemical Society. All rights reserved.