Photophysics and Photoinduced Electron-Transfer Reactivity of Ruthenium(II) Complexes with Oligo(thiophene-bipyridine) Ligands,1

A novel series of mono-, di-, and trimeric ruthenium(II) complexes has been synthesized in which the quasi-linear polytopic ligand is constituted by 2,2‘-bipyridine (bpy) subunits linked in the 5,5‘ positions by 2,5-diethynyl-3,4-dibutylthiophene spacers. Each Ru center is capped by two unsubstituted bpy ligands, and the final complexes are soluble and photostable. Cyclic voltammetry was used to assign the first oxidation and the sequential reduction potentials. Oxidation is metal based while the first reduction is based on the thiophene-substituted ligand. The number of ethynyl-thiophene “modules” and the way these modules are substituted strongly influence the photophysical properties of the complexes. In the case of the mononuclear derivatives ( RuT and TRuT ), luminescence arises from the MLCT manifold and the thiophene-substituted bpy ligands act as the “acceptor” ligands. The photophysical properties of the di- and trimetallic complexes ( RuTRu and RuTRuTRu ) are consistent with an “intraligand” 3 π,π* excited state, where the excitation is localized on the thiophene−bipyridine oligomeric ligands. The dimeric complex ( RuTRu ) has an especially long lifetime (7.3 μs), presumably because the 3 π,π* state is more than 0.1 eV below the lowest MLCT configuration. The excited states of all complexes are efficiently quenched by MV 2+, with a rate constant ranging from 4 × 10 7 to 4 × 10 8 M -1 s -1 . Analysis of the rate-free energy correlations for the photoinduced electron-transfer reactions indicate that the nature of the lowest excited state (i.e. 3 π,π* or MLCT) has little or no influence on the dynamics of electron transfer.