New Tridentate Ligand Affords a Long-Lived 3 MLCT Excited State in a Ru(II) Complex: DNA Photocleavage and 1 O 2 Production.

Two new complexes, [Ru(tpy)(qdppz)](PF 6 ) 2 ( 1 ; qdppz = 2-(quinolin-8-yl)dipyrido[3,2-a:2',3'- c ]phenazine, tpy = 2,2':6',2″-terpyridine) and [Ru(qdppz) 2 ](PF 6 ) 2 ( 2 ), were investigated for their potential use as phototherapeutic agents through their ability to photosensitize the production of singlet oxygen, 1 O 2 , upon irradiation with visible light. The complexes exhibit strong Ru(dπ) → qdppz(π*) metal-to-ligand charge transfer (MLCT) absorption with maxima at 485 and 495 nm for 1 and 2 in acetone, respectively, red-shifted from the Ru(dπ) → tpy(π*) absorption at 470 nm observed for [Ru(tpy) 2 ] 2+ ( 3 ) in the same solvent. Complexes 1 and 3 are not luminescent at room temperature, but 3 MLCT emission is observed for 2 with maximum at 690 nm (λ exc = 480 nm) in acetone. The lifetimes of the 3 MLCT states of 1 and 2 were measured using transient absorption spectroscopy to be ∼9 and 310 ns in methanol, respectively, at room temperature (λ exc = 490 nm). The bite angle of the qdppz ligand is closer to octahedral geometry than that of tpy, resulting in the longer lifetime of 2 as compared to those of 1 and 3 . Arrhenius treatment of the temperature dependence of the luminescence results in similar activation energies, E a , from the 3 MLCT to the 3 LF (ligand-field) state for the two complexes, 2520 cm -1 in 1 and 2400 cm -1 in 2 . However, the pre-exponential factors differ by approximately two orders of magnitude, 2.3 × 10 13 s -1 for 1 and 1.4 × 10 11 s -1 for 2 , which, together with differences in the Huang-Rhys factors, lead to markedly different 3 MLCT lifetimes. Although both 1 and 2 intercalate between the DNA bases, only 2 is able to photocleave DNA owing to its 1 O 2 production upon irradiation with Φ Δ = 0.69. The present work highlights the profound effect of the ligand bite angle on the electronic structure, providing guidelines for extending the lifetime of 3 MLCT Ru(II) complexes with tridentate ligands, a desired property for a number of applications.