Ru(II) complexes possessing new tridentate ligands with extended pi systems, pydppx (3-(pyrid-2'-yl)-11,12-dimethyl-dipyrido[3,2-a:2',3'-c]phenazine) and pydppn (3-(pyrid-2'-yl)-4,5,9,16-tetraaza-dibenzo[a,c]naphthacene), were synthesized and characterized. The investigation of the photophysical properties of the series [Ru(tpy)(n)(L)(2-n)](2+) (L = pydppx, pydppn, n = 0-2) reveals markedly different excited state behavior among the complexes. The Ru(II) complexes possessing the pydppx ligand are similar to the pydppz (3-(pyrid-2'-yl)dipyrido[3,2-a:2',3'-c]phenazine) systems, with a lowest energy metal-to-ligand charge transfer excited state with lifetimes of 1-4 ns. In contrast, the lowest energy excited state in the [Ru(tpy)(n)(pydppn)(2-n)](2+) (n = 0, 1) complexes is a ligand-centered (3)pipi* localized on the pydppn ligand with lifetimes of approximately 20 mus. The [Ru(tpy)(n)(pydppn)(2-n)](2+) (n = 0, 1) complexes are able to generate (1)O(2) with approximately 100% efficiency. Both [Ru(tpy)(pydppn)](2+) and [Ru(pydppn)(2)](2+) bind to DNA, however, the former exhibits a approximately 10-fold greater DNA binding constant than the latter. Efficient DNA photocleavage is observed for [Ru(tpy)(pydppn)](2+), owing to its ability to photosensitize the production of (1)O(2), which can mediate the reactivity. Such high quantum yields of (1)O(2) photosensitization of transition metal complexes may be useful in the design of new systems with long-lived excited states for photodynamic therapy.

The tridentate ligand 3-(pyrid-2'-yl)dipyrido[3,2-a:2',3'-c]phenazine (pydppz) has been prepared in two steps by elaboration of 2-(pyrid-2'-yl)-1,10-phenanthroline. Both homoleptic [Ru(pydppz)(2)](2+) and heteroleptic [Ru(tpy)(pydppz)](2+) (tpy = 2,2';6',2' '-terpyridine) complexes have been prepared and characterized by (1)H NMR. The absorption and emission spectra are consistent with low-lying MLCT excited states, which are typical of Ru(II) complexes. Femtosecond transient absorption measurements show that that the (3)MLCT excited state of the heteroleptic complex [Ru(tpy)(pydppz)](2+) (tau approximately 5 ns) is longer-lived than that of the homoleptic complex [Ru(pydppz)(2)](2+) (tau = 2.4 ns) and that these lifetimes are significantly longer than that of the (3)MLCT state of the parent complex [Ru(tpy)(2)](2+) (tau = 120 ps). These differences are explained by the lower-energy (3)MLCT excited state present in [Ru(tpy)(pydppz)](2+) and [Ru(pydppz)(2)](2+) compared to [Ru(tpy)(2)](2+), resulting in less deactivation of the former through the ligand-field state(s). DFT and TDDFT calculations are consistent with this explanation. [Ru(tpy)(pydppz)](2+) and [Ru(pydppz)(2)](2+) bind to DNA through the intercalation of the pydppz ligand; however, only the heteroleptic complex exhibits luminescence enhancement in the presence of DNA. The difference in the photophysical behavior of the complexes is explained by the inability of [Ru(pydppz)(2)](2+) to intercalate both pydppz ligands, such that one pydppz always remains exposed to the solvent. DNA photocleavage is observed for [Ru(tpy)(pydppz)](2+) in air, but not for [Ru(pydppz)(2)](2+). The DNA damage likely proceeds through the production of small amounts of (1)O(2) by the longer-lived complex. Although both complexes possess the intercalating pydppz ligand, they exhibit different photophysical properties in the presence of DNA.

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 (3)ππ* 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 (3)MLCT and (3)ππ*. 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.