Experimental and DFT characterization of metal-to-ligand charge-transfer excited states of (rutheniumammine)(monodentate aromatic ligand) chromophores.

The heretofore unknown emission properties of the metal-to-ligand charge-transfer (MLCT) excited states of several complexes with (ruthenium)(monodentate aromatic ligand, MDA) chromophores are given. Emission spectra and lifetimes in 77 K glasses are reported for several monometallic complexes of the type [Ru(NH3)(5-n)(L)(n)(MDA)](2+) and two bimetallic pyrazine (pz)-bridged [{Ru(NH3)(4-n)(L)(n)}2pz](4+) complexes (L = pz, pyridine, or a multipyridine ligand; MDA = pz or a substituted pyridine, Y-py). The emission maxima occur in the visible and near-IR spectral regions and have much more poorly resolved vibronic sidebands than do related complexes with Ru-bpy chromophores, and the excited-state lifetimes are characteristic of Ru-bpy MLCT excited states in this energy range. The emission yields of trans-[Ru(NH3)4(MDA)(pz)](2+) (MDA = py or pz) are less than 0.2%, and combined with the other observations, this implies that most of the excited-state quenching occurs in high-energy excited states whose population precedes that of the lowest-energy (3)MLCT excited state. The pz-bridged, bimetallic complexes have mixed-valence excited states, and they absorb and emit at lower energies than their monometallic analogues do.