Photophysical properties of homometallic ruthenium(II) and osmium(II) complexes with a bis(dipyridophenazine) bridging ligand. From pico- to microsecond time resolution

The photophysical properties of [Ru-bidppz-Ru]4+, [Os-bidppz-Os]4+, and the bridging ligand bidppz (bidppz = 1,1′-dipyrido[3,2-a:2′,3′-c]phenazin-1,1′-yldipyrido[3,2 -a:2′,3′-c]phenazme) are investigated. Time-resolved spectroscopic studies were performed on (sub)pico- to microsecond time scales, offering insight into the excited states responsible for the photophysical behavior of the two complexes in different solvents. In particular, [Ru-bidppz-Ru]4+ shows a long excited-state lifetime, 9.7 μs, in deaerated dichloromethane solution, that becomes much shorter (360 ns) in deaerated butyronitrile. The solvent plays a crucial role for the spectroscopic properties of this complex, since the energy of the triplet metal-to-ligand charge-transfer (3MLCT) excited state is influenced by the polarity of the medium. (Sub)picosecond transient absorption spectra in dichloromethane indicate thermal population of the long-lived triplet intraligand (3IL) state of the bridging ligand from the lower lying 3MLCT state. For [Os-bidppz-Os]4+ in dichloromethane the 3MLCT state lies lower in energy and no interaction of this state with the 3IL state of the bridging ligand is observed.