Long-Lived Polypyridyl Based Mononuclear Ruthenium Complexes: Synthesis, Structure, and Azo Dye Decomposition Koushik Singha, †,§ Paltan Laha, †,§ Falguni Chandra, ‡ Niranjan Dehury, † Apurba L. Koner,* ,‡ and Srikanta Patra* ,† † School of Basic Sciences, Indian Institute of Technology Bhubaneswar, Argul 752050, India ‡ Department of Chemistry, Indian Institute of Science Education and Research Bhopal, Bhopal By-pass Road, Bhauri, Bhopal 462 066, Madhya Pradesh, India * S Supporting Information ABSTRACT: Two mononuclear ruthenium complexes [(bpy) 2 Ru II L 1 /L 2 ](ClO 4 ) 2 ([1] 2+ /[2] 2+ ) (bpy-2,2′ bipyridine, L 1 = 2,3-di(pyridin-2-yl)pyrazino[2,3-f ][1,10]- phenanthroline) and L 2 = 2,3-di(thiophen-2-yl)pyrazino[2,3-f ][1,10]phenanthroline have been synthesized. The complexes have been characterized using various analytical techniques. The complex [1] 2+ has further been characterized by its single crystal X-ray structure suggesting ruthenium is coordinating through the N donors of phenanthroline end. Theoretical investigation suggests that the HOMOs of both complexes are composed of pyridine and pyrazine unit of ligands L 1 and L 2 whereas the LUMOs are formed by the contribution of bipyridine units. The low energy bands at ∼480 nm of the complexes can be assigned as MLCT with partial contribution from ligand transitions, whereas the rest are ligand centered. The complexes have shown Ru II /Ru III oxidation couples at E 1/2 at 1.26 (70 mV) V and 1.28 (62 mV) V for [1] 2+ and [2] 2+ vs Ag/AgCl, respectively, suggesting no significant role of distal thiophene or pyridine units of the ligands. The complexes are emissive and display solvent dependent emission properties. Both complexes have shown highest emission quantum yield and lifetime in DMSO (ϕ = 0.05 and τ avg = 460 ns and λ max em at 620 nm for [1] 2+ ; ϕ = 0.043 and τ avg = 425 ns and λ max em at 635 nm for [2] 2+ ). Further, the long luminescent lifetime of these complexes has been utilized to generate reactive oxygen species for efficient azo dye decomposition. ■ INTRODUCTION The coordination chemistry of ruthenium polypyridyl com- plexes has witnessed a rapid progress since the inception of [Ru(bpy) 3 ] 2+ in the scientific community. The longer excited- state lifetime of such system leading to exciting photophysical and photochemical properties makes them attractive for various applications in the diverse areas of chemistry and biology such as dye-sensitized solar cells (DSSCs), 1-4 light emitting electrochemical cells (LEECs), 5-10 sensors, 11-19 catalysis, 20-23 water oxidation/reduction catalysts, 24-28 molecular probe for DNA structure (light switch for DNA), 29-35 photodynamic therapy, 31,32,36 cellular imaging, 29,37-39 and fundamental studies of photoinduced electron and energy transfer processes. 40-42 Such properties of the metal complexes can be manipulated by proper design of ligand framework. In this context, polypyridyl dppz based ligands have become the most attractive considering their stability, tunable photophysical and photo- chemical properties with strong metal to ligand charge transfer (MLCT) transitions, and efficient intercalation with DNA base pair and allow the formation of adducts with functional groups of base pairs. 29 - 33 , 36 - 38 , 43 - 45 A large variety of [(bpy) 2 Ru II (N ∧ N)] complexes with the aforementioned dppz based ligand frameworks have been developed and stud- ied. 29-33,36-39,43-45 In most of the cases the dppz based ligands are flat and rigid with extended conjugation of are- nes. 29,35,39,45-47 A systematic variation of donor and sub- stituents on the dppz framework has been made in order to tailor the tunable photophysical properties such as excitation/ emission bands and emission lifetime (Table S1) with desired applications. 45,48-52 On the other hand, ligands having both rigid and flexible framework are relatively less explored. 53-59 Theoretical calculation reveals that incorporation of flexibility on the dppz-ligand framework alters energies of the frontier orbitals significantly which play an important role in controlling emission properties (Figure S1). 45,49 The pyrazine-based ligands 2,3-di(pyridin-2-yl)pyrazino[2,3- f ][1,10]- phenanthroline (L 1 ) and 2,3-di(thiophen-2-yl)pyrazino[2,3- f ][1,10]phenanthroline (L 2 ) offer a rigid framework at phenanthroline end and flexible donors site at the pyrazine end (Chart 1). Interestingly, it is observed that incorporation of flexibility to the dppz framework does not have any significant impact on the emission behavior (Table S1). However, these flexible pyridine/thiophene donors at pyrazine end can potentially coordinate with metals in various coordination modes to form multimetallic systems. 56 Moreover, the cyclometalated iridium complexes developed by us using L 1 Received: March 2, 2017 Article pubs.acs.org/IC © XXXX American Chemical Society A DOI: 10.1021/acs.inorgchem.7b00536 Inorg. Chem. XXXX, XXX, XXX-XXX