title: Marked differences in light-switch behavior of Ru(II) complexes possessing a tridentate DNA intercalating ligand.
abstract: 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.
authors.affiliation: Department of Chemistry, The Ohio State University, Columbus, Ohio 43210, USA.
authors.forename: Yao
authors.initials: Y
authors.lastname: Liu
authors.forename: Richard
authors.initials: R
authors.lastname: Hammitt
authors.forename: Daniel A
authors.initials: DA
authors.lastname: Lutterman
authors.forename: Randolph P
authors.initials: RP
authors.lastname: Thummel
authors.forename: Claudia
authors.initials: C
authors.lastname: Turro
mesh_terms: D004247:DNA / Q000737:chemistry*; D004249:DNA Damage; D004563:Electrochemistry / Q000379:methods*; D004583:Electrons; D007364:Intercalating Agents / Q000737:chemistry*; D008024:Ligands; D008027:Light; D049449:Luminescence; D009682:Magnetic Resonance Spectroscopy; D008956:Models, Chemical; D010777:Photochemistry / Q000379:methods*; D012428:Ruthenium / Q000737:chemistry*; D013053:Spectrophotometry; D013696:Temperature; D013997:Time Factors
chemical_list: D007364:Intercalating Agents; D008024:Ligands; D012428:Ruthenium; D004247:DNA