Effect of Ligands with Extended π-System on the Photophysical Properties of Ru(II) Complexes†

Density functional theory calculations were performed on a series of six ruthenium complexes possessing tridentate ligands: [Ru(tpy)2]2+ (1; tpy) [2,2′;6′,2′′]-terpyridine), [Ru(tpy)(pydppx)]2+ (2; pydppx) 3-(pyrid-2′-yl)-11,12-dimethyldipyrido[3,2-a: 2′,3′-c]phenazine), [Ru(pydppx)2]2+ (3), [Ru(tpy)(pydppn)]2+ (4; pydppn) 3-(pyrid-2′-yl)-4,5,9,16-tetraazadibenzo[a,c]naphthacene), [Ru(pydppn)2]2+ (5), and [Ru(tpy)(pydbn)]+ (6; pyHdbn) 3-pyrid-2′-yl-4,9,16-triazadibenzo[a,c]naphthacene). The calculations were compared to experimental data, including electrochemistry and electronic absorption spectra. The theoretical results reveal that the lowest-lying singlet and triplet states in 4 and 5 are pydppn-based ππ * in character, which are remarkably different from the lowest-lying metal-to-ligand charge transfer (MLCT) states in 1-3. The calculated lowest triplet states in 4 and 5 are consistent with the 3ππ * states observed experimentally. However, although the extended π-system of pydbn- is similar to that of pydppn, the HOMO of 6 lies above those of 4 and 5, resulting in strikingly different spectroscopic properties. Calculations show that the lowest triplet excited state of 6 is a combination of 3MLCT and 3ππ*. This work demonstrates that the electronic structure of the tridentate ligand has a pronounced effect on the photophysical properties of ruthenium(II) complexes and that DFT and TD-DFT methods are a useful tool that can be used to predict photophysical and redox properties of transition metal complexes