INTERSYSTEM CROSSING TO BOTH LIGAND-LOCALIZED AND CHARGE-TRANSFER EXCITED-STATES IN MONONUCLEAR AND DINUCLEAR RUTHENIUM(II) DIIMINE COMPLEXES

The unsaturated bridging ligand 1,4-bis[2-(4′-methyl-2,2′-bipyrid-4-yl)ethenyl]benzene (dstyb) was prepared in a simple two-step sequence. The ruthenium complexes {[(dmb)2Ru](dstyb)}2n+ (n = 1,2; dmb = 4,4-dimethyl-2,2′-bipyridine) were prepared, and their redox and photophysical properties were examined. Both complexes have a single oxidation in cyclic voltammetry at 1.10 V vs SSCE, for the dinuclear complex n = 2. The first one-electron reductions are localized on the dstyb ligand and occur at -1.32 and -1.26 V for the mononuclear and dinuclear complexes, respectively. The emission maximum in room-temperature CH3CN is 680 nm for [(dmb)2Ru(dstyb)]2+ and 720 nm for {[(dmb)2Ru]2(dstyb)}4+. For both complexes emission quantum yields are <0.005, and luminescence lifetimes are 622 ns for the monomer and 2.02 μs for the dimer at room temperature. The very low radiative decay rates (Φem/τ) observed result from low intersystem crossing efficiencies for population of the emitting 3MLCT state. Transient absorption spectra of the two complexes provide evidence for the presence of a 3(π → π⋆) state. In the mononuclear complex the lifetime of the T1 → T2 absorbance of the 3(π → π⋆) state is 1.6 μs, much longer than the emission lifetime. The 3MLCT emission and the 3(π → π⋆) absorption lifetimes of the dinuclear complex are with in experimental error, indicating the states are equilibrated. Quenching of the transient absorbance with a series of triplet quenchers provides a measure of the triplet energy of the 3(π → π⋆) state of the complexes. © 1990, American Chemical Society. All rights reserved.