Long-Lived Polypyridyl Based Mononuclear Ruthenium Complexes ...

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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)2RuIIL1/L2](ClO4)2 ([1]2+/[2]2+) (bpy-2,2′ bipyridine, L1 = 2,3-di(pyridin-2-yl)pyrazino[2,3-f ][1,10]phenanthroline) and L2 = 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 L1 and L2 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 RuII/RuIII oxidation couples at E1/2 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 τavg = 460 2+ 2+ em ns and λem max at 620 nm for [1] ; ϕ = 0.043 and τavg = 425 ns and λmax at 635 nm for [2] ). Further, the long luminescent lifetime of these complexes has been utilized to generate reactive oxygen species for efficient azo dye decomposition.

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nes.29,35,39,45−47 A systematic variation of donor and substituents 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 l i g a n d s 2, 3 - d i ( p y r i d i n - 2 - y l ) p y ra z i n o [ 2 , 3 - f ] [ 1 , 1 0 ] phenanthroline (L1) and 2,3-di(thiophen-2-yl)pyrazino[2,3f ][1,10]phenanthroline (L2) 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 L1

INTRODUCTION The coordination chemistry of ruthenium polypyridyl complexes has witnessed a rapid progress since the inception of [Ru(bpy)3]2+ in the scientific community. The longer excitedstate 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 photochemical 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. 2 9 − 3 3 , 3 6 − 3 8 , 4 3 − 4 5 A large variety of [(bpy)2RuII(N∧N)] complexes with the aforementioned dppz based ligand frameworks have been developed and studied.29−33,36−39,43−45 In most of the cases the dppz based ligands are flat and rigid with extended conjugation of are© 2017 American Chemical Society

Received: March 2, 2017 Published: May 16, 2017 6489

DOI: 10.1021/acs.inorgchem.7b00536 Inorg. Chem. 2017, 56, 6489−6498

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Inorganic Chemistry

The pure complexes [1]2+ and [2]2+ are 1:2 electrolytes in CH3CN and showed satisfactory microanalytical data. The presence of ClO4− counterions in both complexes was confirmed by observing the ν(ClO4−) stretching bands at 1100 cm−1 and 610 cm−1. Both the diamagnetic [1]2+ and [2]2+ have shown proton and carbon resonances corresponding to half of the molecules in (CD3)2SO due to internal symmetry and indicating their identities in solution (Figures 1 and S2). The molecular composition of the complexes [1]2+ and [2]2+ has further been evidenced by their positive ion ESI mass spectrometry. The molecular ion peaks centered at 900.1 (calculated molecular mass 899.12) for [1][(ClO4)2 − ClO4]+ and 908.4 (calculated molecular mass 909.04) for [2][(ClO4)2 − ClO4]+ in their positive ion ESI mass spectra indicate the entire molecular frameworks (Figure S3). The formation of complex [1] 2+ has been further authenticated by its single crystal X-ray structure (Figure 2). The important crystallographic and selected bond parameters along with the computed geometrical details are provided in Table S2 and Figure 2. The complex [1]2+ has crystallized out in monoclinic crystal system with C2/c space group. From the crystal structure it is observed that the ruthenium center is coordinated to the nitrogen donors of the phenanthroline end with distorted Oh geometry leaving the nitrogen atoms of pyridines at pyrazine end uncoordinated. This type of binding mode is also observed in earlier cases.56,57,59,62 The N1−Ru1−N2, N2−Ru1−N3, and N1−Ru1−N3 bond angles around the ruthenium center are 78.4(3)°, 86.6(3)°, and 175.2(2)°, respectively, indicating slightly distorted octahedral geometry. The Ru1−N1 [2.060 (7) Å], Ru1−N2 [2.064(7) Å], and Ru1−N3 [2.047(7) Å] bond distances are in good agreement with the structurally similar reported complexes.63−65 The bond parameters obtained from the DFT optimized structure for [1]2+ match fairly well with the X-ray crystallographic data of [1](ClO4)2 (Figure 2). The molecular structures of both complexes [1]2+ and [2]2+ were optimized by density functional theory (DFT) using B3LYP hybrid functional. The geometric parameters of the optimized structures of the complexes are listed in Tables S3− S4. From the structural parameters it is observed that the pyridyl/thiophene units of the ligands L1 and L2 are twisted significantly which is evident from the observed dihedral angles. The rest of the ligands are almost planar. The calculated Ru−N bond distances and N−Ru−N bond angles are in good agreement with the experimentally observed values (Figure 2). The frontier orbitals of the complexes obtained by time dependent density functional theory (TD DFT) calculation are depicted in Figures 3 and S4. The TD DFT calculations reveal that the HOMOs of both [1]2+ and [2]2+ are mainly composed of ligands L1 and L2 (100%) and LUMOs are contributed mainly by bipyridine unit (Tables S5 and S6). It is observed that the energy of HOMO and LUMO are −9.85 eV and −7.25 eV for [1]2+ and −9.45 and −7.13 eV for [2]2+, respectively, which suggests that HOMO of [1]2+ is slightly more stable than that of [2]2+ (the HOMO−LUMO energy gap is 2.60 eV for [1]2+ and 2.32 eV for [2]2+). The electrochemical properties of the complexes have been studied in CH3CN with reference to Ag/AgCl electrode (Figure 4). Both complexes have displayed a quasi-reversible one electron redox couple E1/2 at 1.26 (70 mV) V and 1.28 (62 mV) V for [1]2+ and [2]2+ vs Ag/AgCl, respectively, which could be assigned as RuII/RuIII oxidation process (Figure 4). The RuII/RuIII redox potentials of the complexes are consistent

Chart 1

and L2 have shown emission properties with reasonably long lifetime with good quantum yield.56 Metal and metal complex mediated photocatalytic decomposition of aromatic azo compounds is a fascinating approach as this offers a cheaper and simpler operation procedure without generating any additional waste. The coloring agents are very toxic and cause severe problems to both aquatic and human life.60,61 Metal complexes having long excited state lifetimes exhibit efficient 1O2 generation which results in various photocatalytic reactions.52 Ruthenium polypyridyl complexes are known to act as photocatalysts for various organic transformations. Thus, visible-light-induced photodecomposition of azo dyes using ruthenium complexes would be an efficient strategy to get rid of the aforementioned problem. In our recent findings, we demonstrate that [(ppy)2IrIII] complexes with ligands L1 and L2 exhibit excellent anticancer activity with alternative mode of action, and DNA roadblock agent for various polymerase activity.56,59,62 To the best of our knowledge no ruthenium system is reported using these ligands. It is therefore considered worthwhile for the development of [(bpy)2RuII] complexes with the pyrazine based ligands (L1 and L2) and to study their physicochemical properties. Thus, the current contribution describes the synthesis, crystal structure, luminescent properties, and computational studies of [(bpy)2RuIIL1/L2]2+ ([1]2+/[2]2+) complexes. In addition, the visible-light photodegradation of structurally different azo dyes in the presence of the synthesized complexes and their mechanistic aspects has also been scrutinized.

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RESULTS AND DISCUSSION The semiflexible polypyridine pyrazine-based ligands (L1 and L2) were prepared by following the reported procedure.56,62 The ligands were reacted with the ruthenium precursor [(bpy)2RuIICl2] in 1:1 stoichiometric ratio in ethanol under air (Scheme 1). The pure air stable mononuclear complexes [(bpy)2RuIIL1/L2]2+ ([1]2+/[2]2+) were obtained by precipitation followed by chromatographic purification (see Experimental Section). Scheme 1. Synthetic Outline for the Preparation of Complexes [1]2+ and [2]2+

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Figure 1. 1H NMR spectra of the complexes [1]2+ and [2]2+ in (CD3)2SO.

Figure 2. ORTEP diagram of the complex [1](ClO4)2. Ellipsoids are drawn at 20% probability level. The hydrogen atoms and ClO4− counteranions have been eliminated for clarity. Table shows experimentally observed and DFT calculated selected bond distances and angles.

Figure 3. Pictorial representations of important Kohn−Sham molecular orbitals of [1]2+. The hydrogen atoms have been eliminated for clarity.

with the structurally similar [(bpy)2RuII] complexes.66,67 Moreover, no noticeable shift of RuII/RuIII redox potential is observed for varying substituents at the pyrazine end, suggesting negligible or no influence of the pyridine/thoiphene moiety. In addition, the irreversible peak at 1.67 V for [2]2+ might be assigned as the further oxidation of thiol functionalities or RuIII/RuIV process.68 The complexes have also been displayed as ligand based reductions at the negative side of the cyclic voltammogram which might be due to the reduction of the ligands bpy and L1/L2.56,59 The absorption spectra of the complexes [1]2+ and [2]2+ have been recorded in various polar and nonpolar solvents at

room temperature (Figure 5, Table 1). Both complexes have shown high molar extinction coefficient in all measured solvents. In the high-energy ultraviolet region the absorption of the complexes is due to ligand-to-ligand charge transfer (LLCT; π−π*) transitions whereas the low energy visible region (>400 nm) is mainly originated by the metal to ligand charge transfer (MLCT; RuII → πbpy/L), which are characteristics of the polypyridyl ruthenium complexes. The DFT calculation suggests that the low energy transition is not purely the MLCT but ligand centered transitions are also involved. It 6491


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ground state (Figures 5a and S5a). However, there is slight change in molar extinction coefficient and peak position of the absorption maxima of [2]2+, suggesting there is a little interaction with ground state of complex (Figures 5d and S5d and Table 1) with solvents of varying polarity. The emission properties of the complexes have been also studied at room temperature in eight different air-saturated solvents (Figure 5b and Figure 5e). These solvents have been selected based on the solubility of the studied complexes and polarity. The emission maxima, lifetime, quantum yield, radiative rate, and nonradiative rate of the complexes are displayed in Table 1. The luminescence quantum yield of the complexes has been measured by comparing the reported quantum yield value of [(bpy)3Ru]2+ as a standard in water.71 The spectroscopic purity of the complexes has been tested by measuring excitation spectra in different wavelengths and by comparing with absorption spectra (Figure S5) in DMSO (dimethyl sulfoxide). It is observed that the luminescent properties, e.g., emission quantum yield (at least 2 times) and emission maxima (∼40 nm) of the complexes, get highly affected by the polarity of the solvents. This suggests a significantly strong interaction between the excited state of the complexes with the investigated solvents. Both complexes [1]2+/[2]2+ exhibit the highest quantum yield and emission maxima in DMSO. The emission lifetimes of the complexes have been measured in eight different solvents using time correlated single photon counting (TCSPC) instrument at room temperature. The complexes exhibit solvent dependent emission lifetimes (Figures 5c,f, S6c, and S6f) indicating the interaction of the solvents with the excited state of the complexes. The maximum emission lifetime in air-saturated condition is observed in DMSO (460 ns) and the minimum in CH3CN (178 ns). These measured lifetimes are similar to the related [(bpy)2Ru]2+ complexes and comparable to our previously reported mononuclear cyclometalated iridium

Figure 4. Cyclic voltammograms of the complexes [1]2+ and [2]2+ recorded in CH3CN against Ag/AgCl reference electrode.

is a combination of ILCT and MLCT (Tables S7 and S8). It is to be noted that the present DFT calculations were carried out in the gaseous state by taking an isolated molecule, whereas the experiments have been carried out in solution. Thus, direct correlation between the experimental observations with DFT results would be quite unlikely. Similar types of results are also observed in earlier cases.57,69,70 The solvent dependent UV−vis absorption experiment shows that the molar extinction coefficient and the absorption maximum for compound [1]2+ is little higher as compared to compound [2]2+. No significant changes in the absorption maximum and molar extinction coefficient are observed while varying the donors and substituents (distal pyridine/thiophene unit) (Figure 5a and Figure 5d). The TD-DFT analysis also reveals the same. The combined results indicate a very minor effect of pyridyl/ thiophene ring present on the pyrazine end of the ligands. Further, no solvent dependent shift of the absorption maxima is observed while varying the solvents of varying polarity for [1]2+, indicating a weak or no solvent−complex interaction at the

Figure 5. Absorption (a, d), emission (b, e), and excited-state lifetime profile (c, f) of the complexes [1]2+ and [2]2+ in different solvents at 298 K. 6492


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Inorganic Chemistry Table 1. Spectroscopic and Photophysical Data of the Complexes [1]2+ and [2]2+ in Different Solvents at 298 Ka solvent CH3CN CHCl3 CH2Cl2 H(CO)NMe2 (CH3)2SO EtOAc MeOH THF a

compd 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2

ε × 104 (M−1 cm−1) (λAbs max (nm)) 3.08 2.46 2.97 2.41 3.09 2.61 3.33 2.52 2.99 2.45 2.71 2.14 3.20 2.43 3.12 2.34

(450) (425) (455) (425) (455) (425) (455) (425) (455) (430) (455) (425) (450) (425) (455) (425)

λEm max (nm)

quantum yield (relative)

τavg (ns)

kr × 106 (s−1)

knr × 106 (s−1)

608 615 605 610 590 600 615 630 620 635 620 635 605 610 610 610

0.020 0.020 0.030 0.020 0.040 0.017 0.030 0.041 0.050 0.043 0.010 0.018 0.030 0.017 0.030 0.022

178 186 270 324 250 376 248 261 460 425 195 225 220 214 279 268

0.11 0.11 0.11 0.06 0.16 0.05 0.12 0.15 0.11 0.10 0.51 0.08 0.13 0.08 0.11 0.08

5.47 5.26 3.59 3.03 3.84 2.61 3.91 3.68 2.06 2.25 5.07 4.36 4.41 4.59 3.47 3.65

Abbreviations: EtOAc, ethyl acetate; MeOH, methanol; THF, tetrahydrofuran.

Figure 6. Absorption spectra of azo dye Sudan black B in the absence (a) and presence (b) of complex [2]2+ upon visible light irradiation. Panel c represents the time-dependent normalized optical density at the absorption maxima of Sudan black B dye in different conditions. Panel d shows the catalytic properties of [2]2+: the change in optical density of six structurally different azo dyes with 1 μM [2]2+ and varying dye concentration (1−20 μM).

complexes (Table S1).71 Further, from the quantum yield and lifetime values, we have calculated the radiative and nonradiative rates of these complexes in all eight solvents. Nonradiative rate for both compounds in DMSO is lowest and highest in case of CH3CN. To further elucidate the emission properties of both [1]2+ and [2]2+, we conducted TD DFT calculation of lowest lying excited triplet states in gas phase. The energy of the first excited triplet state is obtained at 2.28 and 1.91 eV for [1]2+ and [2]2+, respectively, and are localized mainly on L1 and L2 (Tables S9 and S10). It is further observed that the first four excited triplet states are mainly ligand centered (Tables S9 and S10) and conventional 3MLCT state is in higher energy levels. Next, we have explored the dye degradation capability of our synthesized complexes as they have shown reasonably long

excited-state lifetime. The azo-based dyes are very toxic and cause severe problems in both aquatic and human life. Photodecomposition of colored dyes using a visible-light mediated approach could be compelling to get rid of this problem. To check whether our synthesized complexes can photobleach well-known azo dyes, e.g., Sudan black B has been selected. The degradation of dyes was conducted in DMSO as a solvent as the lifetime of the metal complexes is much longer compared to other studied solvents. The visible light mediated photodecomposition of six well-known dyes was monitored using UV−vis spectroscopy. It is observed that in the presence of [1]2+/[2]2+ the absorption band at 635 nm got diminished significantly within 2 h time, indicating decomposition of the dye (Figures 6a,b and S8). Similarly, the complexes have efficiently photodecomposed other structurally different azo 6493


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H NMR spectra were recorded on a Bruker Avance III 400 spectrometer using DMSO-d6 solvent. Elemental analyses were carried out with a EuroVector elemental analyzer. UV−vis spectra were recorded by using a PerkinElmer Lambda 35 spectrophotometer. FTIR spectra were obtained using Bruker Alpha FTIR spectrophotometer with samples prepared as KBr pellets. The emission spectra were recorded using Horiba Jobin Yvon Fluorolog spectrometer, and luminescence lifetimes were measured using a time correlated singlephoton counting instrument from (Delta Flex-01-DD/HORIBA). All the spectroscopic measurements were performed using a dilute solution (5 μM) of metal complexes. A diode laser 440 nm with fwhm 890 ps was used as the excitation source, and the target count was set to 5000. The lifetime and emission measurements were performed at magic angle polarization condition with respect to the excitation polarizer. The instrument response function (IRF) was recorded by using Ludox solution. [Ru(bpy)3](PF6)2 was used as reference for calculating quantum yield.72 Electrochemical measurements were carried out using a CHI 6205 electrochemical analyzer using [Et4N][ClO4] as the supporting electrolyte (0.1 M), and the solute concentration was kept at ∼10−3 M. For electrochemical measurements a glassy carbon working electrode, Pt wire counter electrode, and aqueous saturated Ag/AgCl electrode were used. The half-wave potential E298° was set equal to 0.5(Epa + Epc), where Epa and Epc are anodic and cathodic cyclic voltammetric peak potentials, respectively. In this cell, Fc/Fc+ couple had an E1/2 value of 0.39 V. Theoretical Calculation. Density functional theory calculations were performed on both polypyridyl based Ru complexes [1]2+ and [2]2+ using Gaussian 09 suite of quantum chemical programs.74 Ground-state geometry optimizations of [1]2+ and [2]2+ were performed with Becke three-parameter exchange functional in conjunction with Lee−Yang−Parr correlation functional (B3LYP).75−78 From frequency calculation it was confirmed that there was no imaginary frequency for any of the structures. The 631G(d) basis set was used for all atoms except heavy metal Ru.79,80 The LANL2DZ basis set with effective core potential was employed for Ru atom.81−83 SCF convergence criteria 10−8 and Gaussian’s pruned grid ultrafine for numerical calculation were maintained in all calculation. Vertical excitation of singlet and triplet state of optimized structure of both complexes was performed by using TD-DFT in gas phase.84−86 Evaluation of percentage of contribution of different groups to molecular orbital was done with the help of Gauss-Sum 3.0 software.87 Crystallography. Single crystal of complex [1](ClO4)2 was grown by slow evaporation of dimethyl sulfoxide at room temperature. Single crystal X-ray structural studies were performed on a Bruker D8 venture instrument. Data were collected at 120(2) K using Mo Kα radiation (λα = 0.710 73 Å). The data collection of [1](ClO4)2 was carried out by the APEX 10 software of the crystals. The data were collected by the standard φ−ω scan techniques and were scaled and reduced using SAINT and XPREP software. The direct method and XSHELL software have been used for solving the structures and refined by full matrix least-squares F2 with XSHELL software.88 All non-hydrogen atoms were refined anisotropically. The remaining hydrogen atoms were placed in geometrically constrained positions and refined with isotropic temperature factors, generally 1.2Ueq of their parent atoms. CCDC reference number for [1](ClO4)2 is 1535191. Syntheses of Metal Complexes. Synthesis of [(bpy)2Ru(L1)](ClO4)2 [1](ClO4)2. The ligand L1 (30.0 mg, 0.078 mmol) and [(bpy)2RuCl2] precursor (40.5 mg, 0.078 mmol) were taken in 30 mL of ethanol and refluxed in air for 16 h under dinitrogen atmosphere. After completion, the volume of the solvent was reduced under vacuum and saturated aqueous solution of NaClO4 was added to it, giving bright red precipitate. The precipitate was filtered, washed with cold distilled water, and dried in air. The crude product obtained was purified by neutral alumina column using CH2Cl2/CH3CN (10:1) as eluent. Yield: 68 mg (85%). Anal. Calcd for C44H30N10Cl2O8Ru ([1](ClO4)2): C, 52.91; H, 3.03; N, 14.02. Found: C, 52.40; H, 3.31; N, 14.3. Molar conductivity [ΛM/(Ω−1 cm2 M−1)] in acetonitrile: 260. A positive ion electrospray mass spectrum of [1](ClO4)2 in CH3OH exhibited signal at m/z = 900.1 corresponding to [[1](ClO4)2 −

dyes (for their structure see Figure S7). It is known that photodecomposition of azo compounds happened through activation of dioxygen. To check the role of aerial oxygen, the photodecomposition of dyes was conducted in the presence and absence of oxygen. It is observed that the rate of decomposition is much faster in the presence of oxygen than in the absence (Figures 6c and S9). To validate whether radical species are involved in the decomposition process, we have employed radical scavenger Trolox (vitamin E analog). It is observed that the dye degradation has been completely quenched when Trolox is added. Further, addition of potassium superoxide did not result in any enhanced decomposition of the studied dye. Combining the experimental findings, we may posit that the complexes [1]2+/[2]2+ are photoexcited and the excited electron is transferred to oxygen which then form reactive oxygen species. Such reactive oxygen species reacts with water and forms hydroxyl or hydroperoxyl radical, and eventually, the in situ generated radical reacts with azo dyes and decomposed. Further, we have increased the amount of dyes (up to 20 times) with respect to fixed concentration of the metal complexes, and also it displays similar results (Figure 6d). The robustness of the metal complexes was tested via successive addition of dye 4 as a representative example for at least four times (Figure S10).

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CONCLUSION We have developed two mononuclear ruthenium complexes [1]2+ and [2]2+ incorporating bipyridine and pyrazine based ligands L1 and L2. The complexes have been characterized through various analytical techniques and theoretical calculations. The complex [1]2+ has further been characterized by single crystal X-ray crystallography which reveals the distorted octahedral geometry. The cyclic voltammetry studies reveal that the RuII/RuIII oxidation process appears at potentials 1.26 (70 mV) V and 1.28 (62 mV) V for [1]2+ and [2]2+ vs Ag/AgCl, respectively, and ligand based couples are observed in the negative potential of the voltammograms. The low energy bands for both complexes may be assigned as MLCT transitions, whereas the high-energy bands could be LLCT/ ILCT transition. The DFT studies of the complexes reveal that the low energy bands are not purely MLCT but admixture of MLCT/ILCT transition. Both complexes are emissive and display solvent dependent emission maxima at around (610 ± 20) nm upon excitation at 450 nm, respectively, for complexes [1]2+ and [2]2+. Ruthenium complexes with aforementioned emission properties and longer emission lifetime display efficient azo dye decomposition properties. Therefore, we believe our present understanding of these long luminescent lifetime ruthenium complexes would be very useful for the development of efficient visible-light photocatalyst and molecular probe for cellular imaging and therapeutic agents.

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EXPERIMENTAL SECTION

Materials. The starting compound [(bpy)2RuIICl2] 72 and the ligands 2,3-di(pyridin-2-yl)pyrazino[2,3-f ][1,10]phenanthroline (L1) and 2,3-di(thiophen-2-yl)pyrazino[2,3-f ][1,10]phenanthroline (L2) were prepared by following the reported procedures.56,59,73 All chemicals were purchased from commercial sources and used as received. Solvents were dried by conventional methods and distilled prior to use. Instrumentation. Electrospray ionization (ESI) MS spectra of the complexes were acquired using Thermo DSQ II mass spectrometer. 6494


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Inorganic Chemistry ClO4]+ (calculated molecular mass 899.0). λmax/nm (ε/M−1 cm−1) in acetonitrile: 450 (15 903), 349 (18 227), 285 (85 174), 255 (43 994), 243 (41 817). 1H NMR: (400 MHz, (CD3)2SO at 298 K), δ ppm): 7.37 (t, 2H, J = 8 Hz), 7.47 (t, 2H, J = 6 Hz), 7.61 (t, 2H, J = 8 Hz), 7.74 (d, 2H, J = 4 Hz), 7.85 (d, 2H, J = 8 Hz), 8.04 (m, 4H), 8.14 (t, 2H, J = 6 Hz), 8.27 (m, 6H), 8.36 (d, 2H, 4 Hz), 8.88 (q, 4H, 8 Hz), 9.63 (d, 2H, 8 Hz). Synthesis of [(bpy)2Ru(L2)](ClO4)2 [2](ClO4)2. The ligand L2 (30 mg, 0.075) and [(bpy)2RuCl2] precursor (40.5 mg, 0.078) were taken in 30 mL of ethanol and refluxed under dinitrogen for 24 h. After completion, the volume of the solvent was reduced under vacuum and saturated aqueous solution of NaClO4 was added to it, giving bright red precipitate. The precipitate was filtered, washed with cold distilled water, and dried in air. The crude product obtained was purified by neutral alumina column using CH2Cl2/CH3CN (12:1) as eluent. Yield: 62 mg (76%). Anal. Calcd for C42H28N8S2Cl2O8Ru ([2](ClO4)2): C, 50.00; H, 2.79, N, 11.11, S, 6.34. Found: C, 49.73; H, 2.64; N, 10.81; S, 6.21. Molar conductivity [ΛM/(Ω−1 cm2 M−1)] in acetonitrile: 255. A positive ion electrospray mass spectrum of [2](ClO4)2 in CH3OH exhibited signal at m/z = 908.4 corresponding to [[2](ClO4)2 − ClO4]+ (calculated molecular mass 909.4), λmax/nm (ε/M−1 cm−1) in acetonitrile: 450 (22 779), 419 (24 118), 390 (25 006), 286 (85 829), 248 (51 222). 1H NMR: (400 MHz, (CD3)2SO at 298 K), δ ppm): 7.24 (dd, 2H, J = 4 Hz), 7.37 (t, 2H, J = 4 Hz), 7.55 (q, 2H, J = 4 Hz), 7.58 (t, 2H, J = 12 Hz), 7.70 (d, 2H, J = 4 Hz), 7.83 (d, 2H, 4 Hz), 7.96 (d, 2H, 4 Hz), 8.02 (m, 2H), 8.13 (t, 2H, J = 8 Hz), 8.24 (t, 4H, J = 8 Hz), 8.87 (q, 4H, 8 Hz), 9.43(d, 2H, 4 Hz). Degradation of Azo Based Dyes. The dye decomposition experiments were performed in air-saturated DMSO solution containing typically 1 μM dye using a 96 quartz plate (from Helma) in a plate reader instrument from BioTek, USA (Synergy H1). We used a 40 W CFL lamp as a light source with an intensity of ∼10.5 klx. The UV−vis spectra were measured in different times. The dye decomposition in absence of oxygen was performed in long-neck quartz cuvette fitted with a septum, and the dissolved oxygen was removed by purging nitrogen for 20 min.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by DST (Grant EMR/2015/ 002219), New Delhi, India. P.L. and N.D. and are grateful to IIT Bhubaneswar, Bhubaneswar, India, and University Grant Commission (UGC) New Delhi, India, respectively, for their research fellowship. A.L.K. and F.C. acknowledge financial and infrastructural support from IISERB and Department of Atomic Energy (DAE), India (Grant 37(2)/14/33/2014-BRNS/557).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00536. Comparison table for photophysical properties of related complexes, crystallographic data, 1H and 13NMR and ESI mass spectra, theoretically calculated data, and dye degradation of the complexes (PDF) Accession Codes

CCDC 1535191 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*A.L.K.: e-mail, [email protected]. *S.P.: e-mail, [email protected]. ORCID

Apurba L. Koner: 0000-0002-8891-416X Srikanta Patra: 0000-0002-0611-4047 Author Contributions §

K.S. and P.L. contributed equally. 6495


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