Pyrrole-substituted tridentate complexes of Ru(II): Spectroscopy, electrochemistry, photosensitization and the role of orbital mixing

One-step syntheses are reported of 4′-(pyrrol-2-yl)-2,2′;6′,2″-terpyridine, 4-(pyrrol-2-yl)-2,6-di(pyrazol-2-yl)pyridine and of their homoleptic Ru(II) complexes, in good to very good yields. DFT calculations confirmed that the pyrrole rings lay coplanar with the tridentate cores and constituted effective π-donors, but also showed that the properties of the Ru(II) complexes defied classical analyses based on localized metal- or ligand-centred orbitals. The low-potential electrochemical oxidations led to electropolymerization but were not purely pyrrole-centred. The low-energy electronic spectral absorptions were not purely metal-to-ligand charge transfer (MLCT) in character, but resulted from mixed metal-to-ligand and intraligand transitions. The complexes’ photosensitization abilities showed that the pyrrole groups were beneficial to the survival of the photoexcited states, albeit not as much as p-tolyl groups.