ARTICLES Photophysical Properties and Singlet Oxygen Production by Ruthenium(II) Complexes of Benzo[i]dipyrido[3,2-a:2′,3′-c]phenazine: Spectroscopic and TD-DFT Study Simon P. Foxon, Mohammed A. H. Alamiry, Mike G. Walker, Anthony J. H. M. Meijer,* Igor V. Sazanovich, Julia A. Weinstein,* and James A. Thomas* Department of Chemistry, UniVersity of Sheffield, Sheffield S3 7HF, United Kingdom ReceiVed: July 16, 2009; ReVised Manuscript ReceiVed: September 10, 2009 The photophysical properties of a series of Ru II complexes containing benzo[i]dipyrido[3,2-a:2′,3′-c]phenazine (dppn) as a ligand are reported. Transient absorption spectroscopy studies indicate that, in contrast to related Ru(dppz) complexes (dppz ) dipyrido[3,2-a:2′,3′-c]phenazine), the excited state of all the dppn systems is a long-lived ππ* triplet state. Computational studies (DFT and TD-DFT) confirm that the excited state is based on the dppn ligand. Near-infrared luminescence studies reveal that the complexes are efficient singlet oxygen sensitizers with yields of 70-83%. Introduction Over the past two decades, the number of studies on metal complexes that reversibly bind to nucleic acids has rapidly grown. These systems bind through a combination of interac- tions commonly observed for classical DNA binding substrates, such as electrostatics, groove binding, and intercalation between base pairs. 1,2 Complexes that bind with sequence 3 and structural 4 selectivity have been reported, and up to nanomolar binding affinities 5 have been achieved. Due to their rich redox and photochemical properties, the incorporation of transition-metal centers within these architectures can also enhance their functionality. Perhaps the best known class of metal-complex- based DNA intercalators are those based on [M(dppz)] moieties (where M ) ad 6 metal ion), with the prototypes being the much studied DNA lights switches [Ru(bpy) 2 (dppz)] 2+ and [Ru(phen) 2 (dppz)] 2+ (where bpy ) 2,2′-bipyridine and phen ) 1,10-phenanthroline). 6 These complexes only display the char- acteristic 3 MLCT emission of polypyridyl-Ru II systems in nonprotic solvents. In aqueous solutions, excited-state emission is quenched; however, on DNA intercalation, dppz inserts into the more hydrophobic environment of the base pair stack, causing Ru II f dppz-based 3 MLCT emission to reoccur. 7 This phenomenon is modulated when dppz is coordinated to other metal ions. For example, while [Os(bpy) 2 (dppz)] 2+ displays red shifting in its light-switch effect compared to the Ru II com- plexes, 8 research has revealed that the Re I complex [Re(CO) 3 (py)(dppz)] + (where py ) pyridine or picoline), 1 + , has very different excited-state properties. 9 Initial studies by the Schanze 9 and Yam 10 groups showed that this complex also displayed a light-switch effect, and although the emission enhancements were greatly diminished compared to those of the Ru II -based systems, the Re I complex was capable of cleaving DNA. These studies also suggested that photoexcitation of 1 + resulted in population and equilibration between the expected Re f dppz- based 3 MLCT and a longer lived dppz-centered π f π* excited state. Consequently, detailed time-resolved IR studies indicated that an initially populated dppz-based 1 π f π* state and a 1 MLCT state localized on the phen fragment of dppz relax into a 3 π f π* state centered on the phenazine section of dppz, which is in equilibrium with a close-lying Re f dppz 3 MLCT also located on the phenazine fragment. 11,12 When the mecha- nism of DNA cleavage was investigated, it was concluded that, whereas the related complex [Re(CO) 3 (py)(dppn)] + , 2 + (where dppn ) benzo[i]dipyrido[3,2-a:29,39-c]phenazine), which did not show a light-switch effect, indirectly cleaves DNA through singlet oxygen sensitization, complex 1 + directly cleaves DNA at guanine sites. 13 In fact, despite the huge amount of research into dppz-based systems, this latter study is one of the few to investigate the properties of complexes containing dppn. This is probably because, in the 1992 report that first described the synthesis of the ligand, Hartshorn and Barton revealed that [Ru(phen) 2 (dppn)] 2+ does not display a light-switch effect on binding with DNA. 14 In previous work, to facilitate the future construction of higher order structures with defined chirality, we have described a modular synthesis of oligomeric metallointercalators, in which achiral building blocks such as the [Ru(tpm)(dppz)(L)] 2+ unit (tpm ) tris-(1-pyrazolyl)methane, L ) N-donor ligands) were used to construct homo- and heterodinuclear systems. 15-17 In this work we also noted that [Ru(tpm)(dppn)(L)] 2+ complexes showed poor emission properties and no light-switch effect. These observationssand those by the Yam group on 1 + and 2 + ssuggest that the photoexcited states of M(dppn) complexes are generally different from those of M(dppz) systems. This prompted us to further investigate the differences between the excited states of the analogous [Ru II (dppz)] and [Ru II (dppn)] units, in the hope that their properties could be exploited in the assembly of multifunctional oligonuclear systems. Results and Discussion The complexes 1 + , 2 + , and [Ru(bpy) 2 (dppn)] 2+ , 3 2+ , and the previously reported [Ru(tpm)(MeCN)(dppn)] 2+ , 4 2+ , and [Ru(tp- * To whom correspondence should be addressed. E-mail: A.Meijer@ sheffield.ac.uk (A.J.H.M.M.); james.thomas@sheffield.ac.uk (J.A.T.); Julia.Weinstein@sheffield.ac.uk (J.A.W.). J. Phys. Chem. A 2009, 113, 12754–12762 12754 10.1021/jp906716g CCC: $40.75  2009 American Chemical Society Published on Web 09/30/2009