Synthesis, Electrochemistry, and Excited-State Properties of Three Ru(II) Quaterpyridine Complexes

The complexes [Ru(qpy)LL'](2+) (qpy = 2,2':6',2″:6″,2‴-quaterpyridine), with 1: L = acetonitrile, L'= chloride; 2: L = L'= acetonitrile; and 3: L = L'= vinylpyridine, have been prepared from [Ru(qpy) (Cl)2]. Their absorption spectra in CH3CN exhibit broad metal-to-ligand charge transfer (MLCT) absorptions arising from overlapping (1)A1 → (1)MLCT transitions. Photoluminescence is not observed at room temperature, but all three are weakly emissive in 4:1 ethanol/methanol glasses at 77 K with broad, featureless emissions observed between 600 and 1000 nm consistent with MLCT phosphorescence. Cyclic voltammograms in CH3CN reveal the expected Ru(III/II) redox couples. In 0.1 M trifluoroacetic acid (TFA), 1 and 2 undergo aquation to give [Ru(II)(qpy)(OH2)2](2+), as evidenced by the appearance of waves for the couples [Ru(III)(qpy)(OH2)2](3+)/[Ru(II)(qpy)(OH2)2](2+), [Ru(IV)(qpy)(O)(OH2)](2+)/[Ru(III)(qpy)(OH2)2](3+), and [Ru(VI)(qpy)(O)2](2+)/[Ru(IV)(qpy)(O)(OH2)](2+) in cyclic voltammograms.