Synthesis, Characterization, and DNA Binding Properties of Ruthenium(II) Complexes Containing the Redox Active Ligand Benzo[i]dipyrido[3,2-a:2′,3′-c]phenazine-11,16-quinone Simon P. Foxon, Charlotte Green, Michael G. Walker, Ashley Wragg, Harry Adams, Julia A. Weinstein, Simon C. Parker, Anthony J. H. M. Meijer,* and Jim A. Thomas* Department of Chemistry, University of Sheffield, Sheffield, U.K. S3 7HF * S Supporting Information ABSTRACT: Synthetic methods toward ruthenium(II) complexes incorporating the benzo[i]dipyrido[3,2-a:2′,3′-c]phenazine-11,16-quinone ligand, qdppn, are reported. In several cases, it was found that complexes containing coordinated benzo[i]dipyrido[3,2-a:2′,3′-c]phenazine, dppn, could be chemically or photochemically oxidized to their qdppn analogues. Since this method was not possible in all the cases, a new, higher yielding, convenient synthesis of qdppn was developed. The crystal structure of the complex [Ru(phen) 2 (qppn)](PF 6 ) 2 (phen = 1,10-phenanthroline) which was synthesized from free qdppn reveals that a combination of π-π stacking between coordinated phen and qdppn units, as well as anion- ligand hydrogen bonding, define large hexagonal channels which are occupied by anions and solvent molecules. Electrochemical and photo- physical studies reveal that the new qdppn-based complexes are not luminescent and, in contrast to their dppn analogues, they are also poor singlet oxygen sensitizers. Time-resolved studies and density functional theory (DFT) calculations indicate that optical properties of the new complexes are due to a short-lived charge separated state involving the quinone moiety of qdppn. The DNA binding properties of the new complexes have also been investigated. It was found that they are intercalators, displaying binding affinities which are comparable to their dppn analogues. ■ INTRODUCTION Polypyridyl d 6 transition metal complexes based on metal centers such as Ru II , Os II , Re I , and Ir III possess attractive photophysical properties: they typically absorb light in the visible wavelength region, possess long-lived metal-to-ligand charge transfer ( 3 MLCT) excited states, are luminescent, and have rich redox chemistry. 1 Such complexes have been widely used for a variety of functions including chromophores for energy conversion 2 and molecular devices. 3 In this context, ligands containing quinone moieties are of particular interest as components in chromophore quencher systems, as they possess good acceptor properties and can function as reversible 2e - redox couples. 4,5 In this context, the Loeb and Meyer groups reported on a Re I chromophore quencher system containing 12,17- dihydronaphtho[2,3-h]dipyrido[3,2-a:2′,3′-c]-phenazine-12,17- dione, aqphen, as the acceptor ligand, Figure 1. 6 They found that in the excited state, an electron is largely localized on the quinone portion of aqphen. Around the same time, Maiya and colleagues investigated the redox and DNA binding properties of [Ru(phen) 2 (aqphen)] 2+ (phen = 1,10-phenanthroline). 7,8 More recently, the Liu and Hauser groups have employed aqphen as an acceptor moiety in the construction of Ru II -based chromophore quencher systems that display long-lived charge separated states. 9 In related work, the Loeb group has also described the five step synthesis of an analogue of aqphen, benzo[ i ]dipyrido[3,2- a :2 ′ ,3 ′ - c ]phenazine-11,16-quinone (qdppn) and briefly outlined the syntheses and absorption spectra of two [Re(L)CO) 3 (qdppn)] complexes (L = Cl, CF 3 SO 3 ). 10 In a later study, Rao et al. revealed that photoirradiation of the complex [Ru(dtb-bpy) 2 (dppn)] 2+ (dtb-bpy = 4,4-di-tert-butyl-2,2-bipyridine; dppn = benzo[i]- dipyrido[3,2-a:2′,3′-c]phenazine) in aerobic conditions pro- duced [Ru(dtb-bpy) 2 (qdppn)] 2+ in essentially quantitative yields. 11 However, this brief report did not describe any photophysical studies on this system. Metal complexes containing ligands related to aqphen and qddpn have also been investigated as DNA binding substrates. The DNA light-switch complex, 12 [Ru(LL) 2 (dppz)] 2+ (LL = 2,2′-bipyridine, 1,10-phenanthroline) has attracted particular attention (dppz = dipyrido[3,2-a:2′,3′-c]phenazine). Its lumi- nescence properties offer a simple means of monitoring DNA binding: emission from aqueous solutions of the complex is quenched by water molecules, while binding to DNA through intercalation enhances luminescence by several orders of magnitude. 13,14 With the aim of identifying novel mono- and oligonuclear DNA binding substrates, we have investigated the Received: September 1, 2011 Published: December 6, 2011 Article pubs.acs.org/IC © 2011 American Chemical Society 463 dx.doi.org/10.1021/ic201914h | Inorg. Chem. 2012, 51, 463-471