Frontier orbitals of photosubstitutionally active ruthenium complexes: an experimental study of the spectator ligands' electronic properties influence on photoreactivity.

The synthesis and characterization of Ru(tpy)(Rbpy)(L) complexes (tpy = 2,2':6',2''-terpyridine, Rbpy = 4,4'-dimethyl-2,2'-bipyridine (dmbpy), or 4,4'-bis(trifluoromethyl)-2,2'-bipyridine (tfmbpy), X = Cl or PF, and n = 1 or 2) are described. The dmbpy and tfmbpy bidentate ligands allow for investigating the effects of electron-donating and electron-withdrawing ligands, respectively, on the frontier orbital energetics as well as the photoreactivity of these ruthenium polypyridyl complexes for five prototypical monodentate ligands L = Cl, HO, CHCN, 2-(methylthio)ethanol (Hmte), or pyridine. According to spectroscopic and electrochemical studies, the dmbpy analogues displayed a singlet metal-to-ligand charge transfer (MLCT) transition at higher energy than the tfmbpy analogues. The shift of the MLCT to higher energy results from the lowest unoccupied molecular orbital (LUMO) for the dmbpy analogues being tpy-based, whereas for the tfmbpy analogues orbital inversion occurs resulting in a tfmbpy-based LUMO. The energy level of the highest occupied molecular orbital (HOMO) was considerably affected by the nature of the monodentate ligand. Visible light irradiation of the complexes demonstrated that the tfmbpy analogue increased the rate and quantum yields of photosubstitution reactions, compared to the dmbpy analogue, suggesting that the electron-withdrawing substituents allowed better thermal accessibility of the triplet metal-centered (MC) state from the photochemically generated triplet metal-to-ligand charge transfer (MLCT) excited state. A correlation between the photolability of the monodentate ligands and the electrochemical reversibility of the metal-based oxidation is also reported.