Homo- and Heterobimetallic Ruthenium(II) and Osmium(II) Complexes

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Homo- and Heterobimetallic Ruthenium(II) and Osmium(II) Complexes Based on a Pyrene-Biimidazolate Spacer as Efficient DNA-Binding Probes in the Near-Infrared Domain Sourav Mardanya, Srikanta Karmakar, Debiprasad Mondal, and Sujoy Baitalik* Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700032, India S Supporting Information *

ABSTRACT: We report in this work a new family of homoand heterobimetallic complexes of the type [(bpy)2M(Py-Biimz)M′II(bpy)2]2+ (M = M′ = RuII or OsII; M = RuII and M′ = OsII) derived from a pyrenyl-biimidazole-based bridge, 2-imidazolylpyreno[4,5-d]imidazole (Py-BiimzH2). The homobimetallic Ru(II) and Os(II) complexes were found to crystallize in monoclinic form with space group P21/n. All the complexes exhibit strong absorptions throughout the entire UV−vis region and also exhibit luminescence at room temperature. For osmium-containing complexes (2 and 3) both the absorption and emission band stretched up to the NIR region and thus afford more biofriendly conditions for probable applications in infrared imaging and phototherapeutic studies. Detailed luminescence studies indicate that the emission originates from the respective 3MLCT excited state mainly centered in the [M(bpy)2]2+ moiety of the complexes and is only slightly affected by the pyrene moiety. The bimetallic complexes show two successive one-electron reversible metal-centered oxidations in the positive potential window and several reduction processes in the negative potential window. An efficient intramolecular electronic energy transfer is found to occur from the Ru center to the Os-based component in the heterometallic dyad. The binding studies of the complexes with DNA were thoroughly studied through different spectroscopic techniques such as UV−vis absorption, steady-state and time-resolved emission, circular dichroism, and relative DNA binding study using ethidium bromide. The intercalative mode of binding was suggested to be operative in all cases. Finally, computational studies employing DFT and TD-DFT were also carried out to interpret the experimentally observed absorption and emission bands of the complexes.

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INTRODUCTION Transition metal complexes that luminesce at room temperature and are capable of binding DNA have received considerable attention recently in view of their potential use as DNA structural probes and because of their possibility for developing useful therapeutic agents.1−6 In this regard, polypyridine complexes of Ru(II) and Os(II) are particularly promising because of their excellent photophysical and electrochemical properties, which can also be systematically modulated to a significant extent by changing the ligands, as well as due to their robust structure and water solubility.7,8 In addition, these complexes are capable of functioning as molecular “light-switches” for DNA.6,9−32 The complexes that are practically nonluminescent in aqueous medium but became strongly luminescent upon interaction with DNA can be considered as potential building blocks for biological imaging. It should be mentioned that a molecular species can bind to DNA through different types of modes such as through intercalation or groove binding or by means of electrostatic interactions.33−42 Among various modes of binding, intercalation is thought to be a very effective mode of interaction for many important applications. So far, DNA binding studies of © XXXX American Chemical Society

several polypyridine complexes of Ru(II) functionalized with other aromatic and heteroaromatic moieties have been carried out by different researchers, and several important issues related to their mode of binding, site-specificity light-switching behaviors, photocleavage activities, and photodynamic therapeutical behaviors have been thoroughly addressed.19−25,43−59 However, most of them suffer from short-wavelength absorption and emission, with the absorption maximum of the metal-to-ligand charge-transfer (MLCT) transition shorter than 500 nm, remarkably limiting their use in photodynamic therapy.1−6 An ideal photosensitizer should have strong absorption and emission within the phototherapeutic window of 600−900 nm, at which the tissue penetration of light is optimal. To this end, ligands with delocalized π systems can give rise to Ru(II) complexes with longer wavelength MLCT absorption and emission. However, the low-energy gap often leads to a short excited-state lifetime and thus limits their use as effective photosensitizers. In our endeavor to design suitable probes for DNA having absorption and emission windows Received: December 16, 2015

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DOI: 10.1021/acs.inorgchem.5b02912 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Synthesis of [(bpy)2Ru(Py-Biimz)Ru(bpy)2](ClO4)2 (1). To an ethanolic suspension of cis-[Ru(bpy)2Cl2]·2H2O (0.26 g, 0.5 mmol) was added solid AgClO4 (0.20 g, 1 mmol), and the mixture was refluxed for 1 h under a nitrogen atmosphere. After removal of the precipitated AgCl, the filtrate containing [Ru(bpy)2(EtOH)2]2+ was refluxed with an ethanolic solution of Py-BiimzH2 (0.07 g, 0.23 mmol) and a few drops of sodium ethoxide for 12 h under nitrogen protection. The resulting solution was cooled and filtered. The filtrate was concentrated by rotary evaporator to ∼10 mL, whereupon a redcolored compound was deposited. The compound was filtered and purified by silica gel column chromatography using acetonitrile as eluent. Finally, the compound was recrystallized from methanol, forming dark red crystals. Yield: 0.43 g (70%). Anal. Calcd for C60H42N12Cl2O8Ru2: C, 54.09; H, 3.17; N, 12.61. Found: C, 54.12; H, 3.13; N, 12.58. 1H NMR data (300 MHz, DMSO-d6, δ/ppm, see Scheme 1 for atom numbering): 8.63 (dd, 2H, J = 7.2 Hz 2H(3)),

stretched up to the NIR window, we recently reported the synthesis, structural characterization, and DNA-binding properties of a new family of monometallic Ru(II) and Os(II) complexes derived from a π-delocalized pyrene moiety (HImzPPy) functionalized with pyridyl-imidazole groups.60 With a view toward developing this area even further, we have set out to generate novel polypyridyl ligands that could be used to generate longwavelength absorbing and emitting complexes. To this end, we designed another pyrenyl-biimidazole-based bridging ligand capable of producing homo- and heterobimetallic Ru(II) and Os(II) complexes upon deprotonation of two imidazole NH protons. A pyrene core has been incorporated directly onto the biimidazole moiety in the hope of lengthening the 3MLCT lifetime of the Ru(II) complexes by the so-called “reservoir effect”.61−65 The underlying mechanism is the establishment of a fast equilibration between the 3MLCT state of the complex and the triplet excited state of the attached pyrene moiety on the ligand or just the pyrenyl-biimidazole ligand itself.61−70 We would expect that through the use of the delocalized π system the 1MLCT absorption and emission wavelength of the complexes may be shifted to longer wavelengths. Moreover, in an equivalent coordination environment, as Os(III) is less oxidizing than Ru(III) by 0.4−0.5 eV, we would expect a significant red-shift of both absorption and emission bands for Os(II)-containing complexes compared to its Ru(II) analogues. In addition, due to strong spin−orbit coupling induced by heavy Os(II), the spin-forbidden 3MLCT absorption bands may also be seen in the NIR region.71−77 In this work, we will report the synthesis, full characterization including single-crystal X-ray structures, photoredox properties, and DNA-binding aspects of a new class of homo- and heterobimetallic Ru(II) and Os(II) complexes based on a pyrenyl-biimidazolate bridging ligand. In addition, intermetallic communications between the two metal centers through the pyrenyl-biimidazolate spacer have also been investigated through electrochemical and spectroelectrochemical measurements. Finally, computational works employing DFT and TD-DFT will also be investigated for a better understanding of the electronic structures as well as for the interpretation of the experimentally observed optical properties of the complexes.78,79

Scheme 1

8.77−8.65 (m, 5H, 3H(3)+2H(10)), 8.45 (d, 1H, J = 5.1 Hz, 1H(3)), 8.38 (dd, 2H, J = 5.7 Hz, 2H(6)) 8.24−8.16 (m, 3H, 2H(9)+1H(3)), 8.11 (d, 1H, J = 5.4 Hz H(3)), 8.06−7.89 (m, 8H, 4H(4)+2H(11)+2H(6)), 7.86−7.83 (m, 4H, 2H(4)+2H(8)), 7.76 (d, J = 5.4 Hz, 1H, 1H(6)), 7.72−7.65 (m, 2H, 2H(4)), 7.48−7.45 (m, 3H, 3H(5)), 7.42−7.34 (m, 1H, 1H(5)), 7.27−7.24 (m, 1H, 1H(5)), 7.09−7.01 (m, 3H, 3H(5)), 6.87 (d, 1H, J = 7.8 Hz, 1H(6)), 6.72 (d, 1H, J = 7.8 Hz, 2H(6)). ESI-MS (positive, CH3CN): m/z = 567.12 (100%) [(bpy)2Ru(Py-Biimz)Ru(bpy)2]2+. Synthesis of [(bpy)2Os(Py-Biimz)Os(bpy)2](ClO4)2 (2). A mixture of Py-BiimzH2 (0.07 g, 0.23 mmol), cis-Os(bpy)2Cl2 (0.28 g, 0.5 mmol), and a few drops of sodium ethoxide was taken in 30 mL of an ethanol−water (2:1 v/v) mixture and refluxed for 48 h under a nitrogen atmosphere. After cooling to room temperature, the mixture was filtered, rotary evaporated to ∼20 mL, and poured into a saturated aqueous solution of NaClO4 (1 g NaClO4 in 5 mL of water), whereupon a black-colored microcrystalline product was deposited. The compound was collected by filtration and purified by silica gel column chromatography using acetonitrile as eluent. Further purification was done by recrystallization of the compound from a methanol−water (4:1) mixture. Yield: 0.49 g (65%). Anal. Calcd for C60H42N12Cl2O8Os2: C, 47.71; H, 2.80; N, 11.12. Found: C, 47.73; H, 2.77; N, 11.09. 1H NMR data (300 MHz, DMSO-d6, δ/ppm, see Scheme 1 for atom numbering): 8.83 (d, 1H, J = 8.1 Hz 1H(3)), 8.72 (d, 2H, J = 8.4 Hz 2H(3)), 8.65 (d, 3H, J = 7.8 Hz, 2H(3)+H(10)), 8.57 (d, 1H, J = 8.1 Hz, 1H(10)) 8.38 (d, 1H, J = 5.4 Hz, 1H(3)), 8.26 (dd, 2H, J = 5.1 Hz, 2H(6)), 8.00−7.96 (m, 4H, 2H(9)+2H(3)), 7.91−7.85 (m, 4H, 2H(4)+2H(11)), 7.81−7.75 (m, 3H, 2H(4)+1H(6)), 7.72−7.71 (m, 1H, 1H(6)), 7.64−7.60 (m, 3H, 3H(4)), 7.55−7.46 (m, 3H, 1H(4)+2H(8)), 7.22−7.05 (m, 4H, 3H(5)+1H(6)), 6.91 (d, 2H, J = 7.8 Hz, 2H(6)), 6.76 (d, 1H, J = 7.8 Hz, 1H(6)). ESI-MS (positive, CH3CN): m/z = 656.17 (100%) [(bpy)2Os(Py-Biimz)Os(bpy)2]2+. Synthesis of [(bpy)2Os(Py-Biimz)Ru(bpy)2](ClO4)2 (3). To an ethanol−water solution (1:1 v/v) of [(bpy)2Ru(Py-BiimzH2)](ClO4)2 (0.47 g, 0.5 mmol) was added solid [Os(bpy)2Cl2] (0.28 g, 0.5 mmol) in the presence of a few drops of sodium ethoxide under a nitrogen atmosphere, and the mixture was refluxed for ∼50 h. After completion of the reaction the volume of the resulting solution was reduced to ∼10 mL by rotary evaporation and poured into a saturated aqueous solution of NaClO4 (1 g NaClO4 in 5 mL of water), whereupon a dark red-black compound was deposited. The compound was collected by filtration and purified by silica gel column chromatography using acetonitrile as eluent. The compound was finally recrystallized from a

Chart 1

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EXPERIMENTAL SECTION

Materials. RuCl3·xH2O, OsCl3·xH2O, pyrene, 2,2′-bipyridine, and AgClO4 were purchased from Sigma-Aldrich and used as received. cis-[Ru(bpy)2Cl2]·2H2O80 and cis-[Os(bpy)2Cl2]81 were synthesized adopting literature procedures. We previously reported the synthesis and full characterization of the pyrenyl-biimidazole bridging ligand 10-(1-H-imidazole-2-yl)-9H-pyreno[4,5-d]imidazole (Py-BiimzH2)82 and its monometallic Ru(II) complex [(bpy)2Ru(Py-BiimzH2)](ClO4)2.83 B


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Inorganic Chemistry

Figure 1. ORTEP representations of Ru−Ru (1) (a) and Os−Os (2) (b) showing 30% probability ellipsoid plots. methanol−water (5:1) mixture to give a brown crystalline product. Yield: 0.42 g (60%). Anal. Calcd for C60H42N12Cl2O8OsRu: C, 50.70; H, 2.97; N, 11.82. Found: C, 50.72; H, 2.95; N, 11.79. ESI-MS (positive, CH3CN): m/z = 611.13 (100%) [(bpy)2Os(Py-Biimz)Ru(bpy)2]2+. Caution! AgClO4 and perchlorate salts of the metal complexes used in this study are potentially explosive and therefore should be handled with care in small quantities.

imidazole is 5.54° for 1 and 3.98° for 2, indicating an almost planar structure of the pyrenyl-biimidazole moiety in the complexes. The distances between the nonbonding metal centers in both complexes are almost the same (5.567 Å for 1 and 5.567 Å for 2). CCDC reference numbers are 1439503 for 1 and 1439504 for 2. 1 H NMR and ESI-Mass Spectrometry. Characterization of the complexes in the solution state was done with the help of 1 H NMR and mass spectroscopy. 1H NMR spectra of the complexes recorded in DMSO-d6 are presented in Figures S1−S3 (Supporting Information). The spectra show the presence of many resonances with some overlapping features within the range 6.0−9.0 ppm, and all the protons were tentatively assigned with the help of their COSY NMR spectra. The 1H NMR spectra of the symmetrical homodinuclear complexes are similar and relatively simple, while the spectrum of the heterodinuclear Ru−Os complex is extremely complicated, with extensive overlapping among the peaks. In all the complexes, the signal at 6.0 ppm is confidently assigned as the H7 proton attached to the pyrene moiety. X-ray crystallographic analysis indicates that this proton is under the influence of the anisotropic ring current of the adjacent bipyridine rings, leading to a remarkable upfield shift. ESI-mass spectra of all three dimers are presented in Figures S4−S6 (Supporting Information). The experimental molecular ion peak for the bipositive ion for each of the three complexes is in good agreement with that of their calculated values. An expanded form of the molecular peak shows a separation of 0.5 D, and they match well with their corresponding simulated patterns. Redox Properties. Electrochemical behaviors of the complexes were studied through cyclic voltammetry (CV) and square wave voltammetry (SWV). The plot of the voltammograms is shown in Figure 2, and the related data along with those of the monometallic precursors are summarized in Table 1. Each of the three bimetallic complexes undergoes two successive one-electron reversible oxidation processes due to MII-MII/MIII-MII and MIII-MII/MIII-MIII processes, respectively. In the case of the heterometallic Ru−Os complex, the peak with a higher E1/2 value (1.03 V) corresponds to the oxidation of the Ru(II) center, while the peak with a lower E1/2 value

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RESULTS AND DISCUSSION Synthesis and Characterization. The pyrenyl-biimidazole l i g a n d 4- ( 1 H- i m i d a z o l e - 2-y l )- 3, 5-d i az a p e n t a cy c l o [9.6.2.02,6.07,19.014,18]nonadeca-1(17),2(6),3,7,9,11(19),12,14(18),15-nonaene (Py-BiimzH2) and its monometallic Ru(II) complex were obtained by following the procedure described previously.82,83 The ruthenium dimer was easily obtained by reacting [(bpy)2Ru(EtOH)2]2+ with the Py-BiimzH2 bridge, while the osmium dimer was prepared by treating a mixture of cis-[(bpy)2OsCl2] and Py-BiimzH2 in a 2:1 molar ratio in an ethanol−water (2:1) mixture for a longer time, as the Os(II)− Cl bonds are kinetically very inert. The heterodimer (Ru−Os) was obtained by reacting an equimolar amount of [(bpy)2Ru(Py-BiimzH2)]2+ with cis-[Os(bpy)2Cl2] in an ethanol−water mixture as described in the Experimental Section. All the complexes were purified by column chromatography followed by recrystallization from the appropriate solvent and thoroughly characterized by standard analytical tools. Description of the Crystal Structures of the Homobimetallic Complexes. ORTEP84 representations of the Ru−Ru (1) and Os−Os (2) complexes are shown in Figure 1, and the crystallographic parameters are given in Table S1 (Supporting Information). Selected bond lengths and bond angles are given in Table S2 and Table S3 (Supporting Information), respectively. Both complexes crystallized in monoclinic form with the P2(1)/n space group and adopt distorted octahedral geometry around the respective metal center. In general, the M−N(imidazole) distances are longer than the corresponding M−N(bpy) distances: the metal to imidazole nitrogen distances lie in the range 2.093(7)− 2.223(7) Å, while the metal to bipyridine nitrogen distances vary between 2.010(6) and 2.050(7) Å. The dihedral angle between the planes of pyrene-imidazole and the remote C


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Inorganic Chemistry

Figure 2. CVs (solid lines) and SWVs (dotted line) of complexes 1−3 in acetonitrile at room temperature showing both their oxidation and reduction.

Table 1. Electrochemical Dataa for Complexes 1−3 in Acetonitrile oxidationb E1/2(ox)/V compound

E1/2(1)

E1/2(2)

ΔE1/2 = E1/2(2) − E1/2(1)

Kc

Ru−Ru (1) Os−Os (2) Ru−Os (3) Ru (1a)c Os (2a)c

0.78 0.49 0.44 1.18 0.72

1.10 0.66 1.03

0.32 0.17 0.59

2.57 × 105 7.51 × 102

reduction E1/2(red)/V −1.40, −1.34, −1.40, −1.38, −1.44,

−1.70 −1.68 −1.68 −1.63, −1.77 −1.75

a

All the potentials are referenced against the Ag/AgCl electrode with E1/2 = 0.36 V for the Fc/Fc+ couple. bE1/2 values are obtained from square wave voltammetry (SWV) using a glassy carbon electrode. c[(bpy)2Ru(Py-BiimzH2)]2+ (1a) and [(bpy)2Os(Py-BiimzH2)]2+ (2a) are taken from ref 83.

window and reductions of ligands in the negative potential window is also verified by computational results. The extent of electronic interaction between the two metal centers in homobimetallic complexes can be estimated from the differences between the two oxidation potentials (ΔE1/2 = |E1/2(2) − E1/2(1)|). The ΔE1/2 value obtained in this way is found to be 0.32 V for Ru−Ru (1), whereas for the analogous Os−Os (2) complex, the corresponding value is 0.17 V. The extent of electronic interaction between the metal centers depends upon several factors such as the charge, rigidity, as well as π electron delocalization ability of the bridging ligand, and the mode of interaction is through either an electron transfer or a hole

(0.44 V) corresponds to the oxidation of the Os(II) center. In a similar environment, the Os(II) center oxidizes at relatively lower potential than the Ru(II) center as expected. In the negative potential window (0 to −2.0 V) all the complexes exhibit two successive reversible peaks with their E1/2 values more or less the same. Thus, these processes are clearly due to the reductions of the coordinated bpy as well as Py-Biimz units. As will be demonstrated later by DFT calculations, except for the HOMO, other higher occupied molecular orbitals predominantly reside on the metal centers, whereas the LUMOs are composed mainly of bpy moieties. Thus, the assignment of the peaks to the oxidation of the metals in the positive potential D


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Inorganic Chemistry

Figure 3. UV−vis absorption (a) and emission (b) spectra of the isomolar solutions (2.0 × 10−5 M) of 1−3. Inset of panel b shows the deconvoluted spectra of Ru−Os (3).

Table 2. Spectroscopic and Photophysical Data for Complex 1−3 in Acetonitrile luminescence compound Ru−Ru (1) Os−Os (2) Ru−Os (3) Ru (1a)a Os (2a)a a

absorption λmax/nm (ε/M−1 cm−1)

Φ/10−3

λmax/nm

501(16 510), 460(sh)(13030), 397(20 970), 375(24 750), 343(35 130), 719 290(10 1250), 243(81 200) 721(br)(5330), 505(18 900), 350 (36 300), 293(11 4800), 242(92 500) 826 720(br)(2540),504(17 430), 395(24 560), 375(28 570), 344(35 690), 722, 820 293(10 9000), 244(88 200)

10.0

470(br)(7250), 336(28 055), 288 (65 045), 238(57 075) 706(br)(2250), 490(13 800), 337 (41 300), 290(92 950), 237(86 950)

10.3 5.8

710 786

3.9 3.2 λex = 480 nm

τ/ns

E0−0/eV

τ1 = 6.9, τ2 = 42.0

2.00

τ1 = 3.6 Ru-centered τ1 = 6.1, τ2 = 44.2 Os-centered τ1 = 5.1, τ2 = 40.3 τ1 = 13.4, τ2 = 38.5 τ1 = 13.2

1.63 2.04 1.57 2.03 1.72

[(bpy)2Ru(Py-BiimzH2)]2+ (1a) and [(bpy)2Os(Py-BiimzH2)]2+ (2a) taken from ref 83.

transfer process, depending upon the π-acceptance or π-donating abilities of the bridging ligand, respectively. In the case with neutral ligands with extensive π delocalization, mixing of the higher metal dπ orbital with that of the lowest π* orbital of the ligand occurs and an electron transfer process takes place. By contrast, the anionic ligands induce the overlap between the highest occupied π orbitals of the bridge and dπ orbitals of the metal center via a hole transfer process. In the present case, the bridging ligand became dianionic (Py-Biimz2−) via deprotonation of the two imidazole N−H protons to form the bimetallic complexes. Hence, the intermetallic interaction occurs through the hole transfer process, which in turn justifies a larger separation (ΔE1/2) between the oxidation potentials in Ru−Ru (1) compared to its Os−Os (2) analogue.85,86 Absorption and Emission Spectral Studies. Absorption and emission spectral profiles of the complexes in acetonitrile are presented in Figure 3, and the related data are summarized in Table 2. In order to compare the relative intensity of the absorption and emission bands, isomolar solutions of the complexes were used. Literature data as well as the theoretical calculations (DFT and TD-DFT) help us properly assign the optical bands. The moderately intense bands in the range 500−505 nm (ε = 16 510−18 900 M−1 cm−1) arise due to spinallowed MII → bpy MLCT transitions with some ILCT character. Again, the highly intense absorption bands observed between 240 and 295 nm (ε = 81 200−114 800 M−1 cm−1) arise mainly due to π−π* and LLCT transitions within the bpy and Py-Biimz units. In the case of 2 and 3, relatively weak and broad absorption bands (ε = 2540−5330 M−1 cm−1) that stretched up to the NIR region (630−830 nm) are due to the spin-forbidden 1[OsII(dπ)6] → 3[OsII(dπ)5bpy/Py-Biimz(π*)1] transitions. Upon excitation at 480 nm, each complex exhibits luminescence at room temperature originating predominantly from

their respective 3MLCT excited states (Table 2 and Figure 3). The inset of Figure 3b shows two overlapping emission bands for the Ru−Os complex (3), deconvolution of which leads to two emission maxima at 722 and 820 nm, while single emission maxima are at 719 nm for Ru−Ru (1) and 826 nm for Os−Os (2). By comparing the spectra of symmetrical bimetallic compounds, the observed emission band at 722 nm for 3 arises from Ru-centered 3MLCT, while the band at 820 nm is due to Os-centered 3MLCT. Excited-state lifetimes of the complexes were measured, and the decay profiles along with the lifetimes are given in Figure S7 (Supporting Information). Both Ru−Ru (1) and Ru−Os (3) exhibit biexponential decay with a short component having a lifetime in the range 5−7 ns, while the lifetime of long component varies between 40 and 44 ns and the relative fractional intensities of both short and long components are given in Table S4 (Supporting Information). The Os−Os (2) complex, on the other hand, exhibits monoexponential decay with a much shorter lifetime (3.6 ns) compared with both 1 and 3. The single-exponential lifetime of 2 indicates that the emitting excited state is predominantly 3MLCT in nature. In the case of Ru−Ru (1), the short-lived component arises due to the deactivation of the pure 3MLCT state as in the case with 2, whereas the long-lived component most probably arises due to an excited-state equilibrium between the 3MLCT state and 3π−π* state of the coordinated pyrene moiety in the complex. The excited-state energy of the Os and Ru center in 3 can be calculated from the energy of the cutting point of the overlaid absorption and emission spectrum of 3 and the monometallic Ru(II) model complex, [(bpy)2Ru(Py-BiimzH2)]2+ (1a), respectively, and the calculated values are found to be 1.57 eV for the Os center and 2.04 eV for the Ru center (Figure S8, Supporting Information). Thus, the free energy changes (ΔG0) for intramolecular energy transfer from the Ru center to the Os center process acquired a E


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Inorganic Chemistry

Figure 4. Spectroelectrochemical changes during the oxidation of [(bpy)2Ru(Py-Biimz)Ru(bpy)2]2+ (1) in acetonitrile. The changes involve RuIIRuII → RuIIRuIII (a) and RuIIRuIII → RuIIIRuIII (b). The IVCT bands obtained from spectral deconvolution are shown in inset (a), and the LMCT band obtained from spectral deconvolution is shown in inset (b). Asterisks denote artifacts due to nonperfect background compensation.

Table 3. Absorption Spectral Data of Complexes 1 and 2 on Spectroelectrochemical Oxidation in Acetonitrile 1 2

Eop/cm−1

Δν1/2(exp)/cm−1

Δν1/2(Theo)/cm−1

λmax/nm (ε/M−1cm−1)

Hab/cm−1

λmax/nm (ε/M−1cm−1)

11 695 8326 8084

5896 4170 4048

5197 4385 4321

855 (3410) 1201 (1079) 1237 (3736)

1796 717 1513

678 (3147)

toward electrochemical oxidation is presented in Figure S9 (Supporting Information). On oxidation, both 1MLCT and 3 MLCT bands gradually decrease in intensity and ultimately almost completely disappeared when the oxidation of the metal centers is completed. During this process, a very broad band in the range 900−1500 nm developed and gradually intensified. Deconvolution of the broad band gives rise to the maximum of the IVCT band at 1237 nm (ν = 8084 cm−1, ε = 5096 M−1 cm−1). In contrast to homologous RuII−RuII species, only one IVCT band is observed for OsII−OsII in the longer wavelength region in the studied spectral range. The spectroelectrochemical behavior of the heterometallic Ru−Os (3) complex is presented in Figure S10 (Supporting Information). Due to its lower oxidation potential, the Os(II) center is oxidized first, and during oxidation of Os(II)−Ru(II) to Os(III)−Ru(II), the intensities of both the 1MLCT (640−810 nm) and 3MLCT band (440−530 nm) gradually diminished. Further oxidation of Os(III)−Ru(II) to Os(III)−Ru(III) leads to the augmentation of intensity of the LMCT band at ∼640 nm. Analysis of the IVCT band(s) of the complexes gives rise to the quantitative estimation about the degree of electronic coupling in the mixed-valence systems, and parameters related to the IVCT bands are given in Table 3.87−90 Eop, the energy of the optical transition, was calculated to be 11 695 and 8326 cm−1 for 1 and 8084 cm−1 for 2, respectively, while the bandwidth at half-height (Δν1/2) is found to be 5896 and 4170 cm−1 for 1 and 4048 cm−1 for 2. The Δν1/2 value for a mixed-valence (class II) system can be theoretically predicted from the Hush model with the help of eq 1.91 The agreement between the experimentally observed and theoretically calculated Δν1/2 values is found to be quite good.

negative value (−0.47 eV). In addition, the intensity as well as the quantum yield of the emission at 720 nm in 3 due to the Ru center is much less than that of the Ru−Ru (1) dimer. The negative free energy change as well as quenching of the emission intensity of the Ru center in 3 compared to 1 apparently indicates the possibility of the occurrence of intramolecular energy transfer from the Ru center to the Os center, although even if no Ru quenching occurs, the emission of the Ru−Os solution would be reduced to about half (assuming absorption is equally distributed between Ru and Os partners), since part of the light is absorbed by the Os, and this cannot contribute to Ru emission. So the quantitative aspects of the energy transfer process, if it occurs at all in 3, cannot be estimated properly. Spectroelectrochemistry. The extent of electronic communication between the two metal centers in the homobimetallic complexes can also be judged through their spectroelectrochemical measurements under controlled oxidation potential. Spectroelectrochemical changes of the complexes were measured in acetonitrile within the range 300−1500 nm (Figure 4 and Figures S9 and S10, Supporting Information). As the oxidation process progressed, the MLCT band of 1 decreased to some extent with simultaneous evolution of new bands in the range 800−1300 nm probably due to the RuII → RuIII IVCT transitions. On deconvolution of the broad band, we get two peak maxima for the IVCT bands at 855 nm (ν = 11695 cm−1, ε = 3410 M−1 cm−1) and at 1201 nm (ν = 8326 cm−1, ε = 1079 M−1 cm−1). Further electrolysis gives rise to the fully oxidized RuIII−RuIII species, wherein the IVCT band at 855 nm decreases sharply, while the band at 1201 nm decreases moderately and a new band is developed in the range between 500 and 700 nm with its maximum at 678 nm (ν = 14750 cm−1, ε = 3147 M−1 cm−1) on deconvolution due to LMCT transition. During spectroelectrochemical oxidations, the absorption spectra were found to pass through well-defined isosbestic points (585 and 705 nm). The assignment of the bands at 855 and 1201 nm as IVCT are confirmed by the decrease of their intensities when the complex gets fully oxidized. The spectral profile of the diosmium complex (2)

Δν1/2 = (2310Eop)1/2 cm−1

(1)

Moreover, values of the electronic coupling (Hab) were calculated by using eq 2.87,91,92 Hab = [2.06 × 10−2(εmax Δν1/2Eop)1/2 ]/rab

(2)

where rab is the effective electron transfer distance. In the absence of electroabsorption (Stark effect) spectroscopy, which F


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Inorganic Chemistry

Figure 5. Schematic drawings of the selective frontier molecular orbitals for [(bpy)2Ru(Py-Biimz)Ru(bpy)2]2+ (1) in acetonitrile.

the complexes are portrayed in Figure 7. The calculated bands are found to correlate well with the experimental bands. The spin-allowed lowest energy band lying between 497 and 520 nm in the complexes can be confidently assigned as due to the MLCT transition, while the next higher band with its maximum lying in the range 340−397 nm is predominantly due to ILCT/LLCT transition with some MLCT character mixed therein. Finally, the bands below 300 nm originated due to π−π* transitions within the bypyridine and pyrenyl-imidazole units. UKS calculations were also performed on the triplet state of complexes 1−3 to calculate the energy gap between the excited triplet and ground singlet state (Figure S14 and Table S6, Supporting Information). Selected frontier orbitals in the UKS optimized states are presented in Figures S15−S17 (Supporting Information), while the compositions of the different units are given in Table S7 (Supporting Information). It can be seen from the bond lengths and angles that the ground-state and excited-state structures are quite close to each other. The calculated emission band was obtained at 696, 790, and 799 nm for complexes 1, 2, and 3, respectively. The corresponding experimental values were at 719 nm for 1, 826 nm for 2, and 820 nm for 3. Thus, the correlation between the experimental and calculated result is quite good. Inspection of the MOs in the excited states indicates the 3MLCT nature of the emission in the complexes. Moreover, the occurrence of the Os-centered emission at lower energy compared with the analogous Ru-centered emission (according to the energy gap law) is also reflected in the calculated results.

can provide the effective electron transfer distance between the two metal centers through the measurements of the dipole moment change associated with the IVCT processes, the values of rab can be approximated with the nonbonding distance between the metal centers.71 The values of the nonbonded metal−metal distance, rab ≈ 5.567 Å for 1 and rab ≈ 5.562 Å for 2, were obtained from single-crystal X-ray structures. As the geometric distance is normally larger than the actual charge transfer distance due to electronic coupling across the bridging ligand, eq 2 thus gives rise to a lower limit for Hab. The calculated Hab value is found to be 1796 and 717 cm−1 for 1, while it is 1513 cm−1 for 2. Thus, the values of Δν1/2 and Hab of the IVCT bands suggest that the mixed-valence state (MIIMIII) in homobimetallic Ru(II) and Os(II) systems is on the borderline of class II and class III systems.87−92 DFT and TD-DFT Investigations. All three complexes were optimized in the solution state, and their optimized structures are shown in Figure S11 (Supporting Information); related bond lengths and bond angles are given in Tables S1 and S2 (Supporting Information). In general, the geometrical parameters obtained from the single-crystal X-ray structures are well matched with the optimized parameters in the solution state. Selected frontier molecular orbitals of the complexes are presented in Figure 5 and Figures S12 and S13 (Supporting Information), while their percent compositions and energy values are given in Table S5 (Supporting Information). With the exception of the HOMO, where the main contribution came from the pyrene and imidazole moieties, the other higher occupied molecular orbitals in all three complexes are predominantly metal-based. It is of interest to note that in Ru−Os (3) the contribution of the Os(II) center is significantly greater that of the Ru(II) center (Table S5, Supporting Information). The LUMOs, on the other hand, reside mainly on the bipyridines in all three dimers. With the optimized geometries of the complexes, we carried out TD-DFT calculations in acetonitrile medium in order to assign the experimentally observed absorption bands, and the relevant spectral data are presented in Table 4. For the sake of comparison, the calculated and experimental absorption spectra of the complexes are overlaid in Figure 6, and the contributions of different MOs to the spin-allowed lowest energy transition in

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DNA BINDING EXPERIMENTS Absorption Titration Experiments. DNA binding experiments were performed in Tris-HCl/NaCl buffer medium having an ionic strength of 50.0 × 10−3 m. All three complexes respond to CT-DNA, albeit to different extents, and the changes in their absorption spectral behaviors with increasing concentration of CT-DNA are shown in Figure 8. 1 exhibits the largest change among the three complexes, where all the spectra pass through several well-defined isosbestic points (503, 443, and 373 nm) along with red-shifts of both the MLCT band at 496 to 510 nm and mixed MLCT/LLCT band at 342 to 347 nm. G


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Inorganic Chemistry Table 4. Selected UV−Vis Absorption Energy Transitions at the TD-DFT/B3LYP Level for 1−3 in Acetonitrile excited state

λcal/nm/ εcal/ M−1 cm−1

oscillator strength ( f)

λexpt/nm/ εexpt/ M−1 cm−1

RuD S14

489 (12 100)

0.119

501 (16 510)

S31

396 (8700)

0.095

S60

354 (33 800)

0.201

397(sh) (20 970) 343 (35 130)

S144 S192 OsD S14

274 (87 300) 249 (22 200)

0.294 0.222

290 (101 250) 243 (81 200)

499 (18 200)

0.124

505 (18 900)

S56

360 (55 600)

0.216

350 (36 300)

S138

279 (131 500)

0.218

293 (114 800)

S197

248 (36 700)

0.108

242 (92 500)

Ru−Os 492 (15 550) S13

0.126

504 (17 430)

S59

354 (47 100)

0.147

344 (35 690)

S147

277 (145 800)

0.148

293 (109 000)

S197

238 (31 100)

0.146

244 (88 200)

key transitions

character

H-5→L+1 (10%), H-4→L (25%), H-3→L+1 (27%), H-1→L (10%), H-2→L (7%), H-2→L+3 (7%), H-1→L+3 (6%) H→L+4 (20%), H→L+6 (43%), H→L+7 (16%), H-1→L+5 (8%), H-7→L+3 (16%), H-2→L+6 (21%), H-1→L+12 (15%), H→L+13 (19%) H-2→L+4 (4%), H-1→L+11 (6%), H→L+9 (5%) H-12→L (19%), H-11→L+2 (28%), H-7→L+8 (20%), H-13→L+3 (7%) H-8→L+13 (58%), H-8→L+14 (13%), H-10→L+4 (4%), H-10→L+6 (6%) H-4→L (18%), H-3→L+1 (37%), H-2→L+3 (10%), H-1→L (10%), H-2→L (5%), H-1→L+3 (5%) H-2→L+6 (26%), H→L+13 (42%), H-2→L+10 (4%), H-1→L+12 (4%), H→L+11 (4%) H-11→L (10%), H-10→L+3 (38%), H-7→L+7 (12%), H-12→L+2 (9%), H-11→L+3 (9%) H-2→L+18 (19%), H-2→L+22 (10%), H→L+23 (40%), H-7→L+14 (4%), H-2→L+21 (5%), H-1→L+22 (6%) H-3→L (36%), H-3→L+1 (28%), H-2→L+3 (12%), H-4→L (6%), H-4→L+1 (6%) H-2→L+6 (12%), H-8→L (9%), H-8→L+1 (5%), H-7→L+3 (4%), H-6→L +15 (5%), H-2→L+4 (3%), H-2→L+7 (4%), H-2→L+8 (4%), H-2→L+9 (4%), H-2→L+12 (9%), H-1→L+12 (8%), H→L+12 (4%), H→L+13 (5%) H-7→L+9 (16%), H-7→L+11 (32%), H-13→L+2 (4%), H-12→L+1 (4%), H-11→L+3 (8%), H-8-→L+8 (3%) H-9→L+6 (34%), H-8→L+13 (31%), H-9→L+4 (9%), H-8→L+14 (5%), H-7→L+6 (3%)

MLCT, LLCT LLCT, ILCT, MLCT LLCT, MLCT π−π*, LLCT π−π* MLCT MLCT, ILCT, π−π* π−π*, LLCT π−π*, MLCT

MLCT, LLCT MLCT, LLCT LLCT, π−π* π−π*

Figure 6. Experimental and calculated absorption spectra of 1 (a and b, respectively), 2 (c and d, respectively), and 3 (e and f, respectively) in acetonitrile.

Taking the absorbance change of the mixed ILCT/LLCT bands as the reference, a hypochromism of 13%, 8%, and 9% is observed for complexes 1, 2, and 3, respectively. In contrast to the ILCT/LLCT bands, the hypochromism with regard to the MLCT bands is almost negligible. The greater changes in the ILCT/LLCT bands compared with their MLCT bands, as shown from the values of the hypochromism, indicate that the interactions of the present bimetallic complexes with DNA probably occur through the bridging pyrenyl-biimidazole ligand.

The isosbestic points also indicate the presence of two chemically distinct species (free and DNA-bound) and thus support the binding of the complexes with DNA. In contrast to 1, complexes 2 and 3 exhibit rather small changes, and the spectral changes are associated mainly with their ILCT/LLCT bands keeping the lowest energy MLCT bands almost unchanged. Hypochromism is a parameter that gives a quantitative idea about the strength of the interaction, and its value can be obtained by the formula H% = 100(Afree − Abound)/Afree. H


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Inorganic Chemistry

Figure 7. Energy level diagrams depicting the dominant transitions that comprise the lowest energy absorption band for 1 (a), 2 (b), and 3 (c) in acetonitrile.

turn reduces their mobility and decreases the vibrational mode of relaxation. It is of interest to note that addition of DNA to the heterometallic Ru−Os (3) complex induces significant augmentation of Os-centered emission, while the Ru-centered emission is almost unchanged. The observation can lead to the proposition, at least in a qualitative way, that DNA induces efficient energy transfer from the Ru center to the Os center compared with its free form. The extent of the binding affinity of the complexes toward DNA (Kb) was also measured from emission titration profiles and by utilizing eq 4. Regression analysis shows that Kb values are also on the order of 105 (Table 4).

Evaluation of intrinsic binding constants (Kb) leads to measuring the strength of the interaction between the complexes and DNA. From the absorption titration data the Kb values of the complexes were calculated with the help of eq 3, and the values are grossly 5 orders of magnitude (Table 5). Thus, the Kb values suggest that the complexes interact strongly with DNA. (εa − εf )/(εb − εf ) = (b − (b2 − 2Kb 2C t[DNA]/s)1/2 )/2KbC t

(3)

b = 1 + KbC t + Kb[DNA]/2s

(Ia − If )/(Ib − If )

where εa, εf, and εb represent the extinction coefficient corresponding to apparent, free, and fully bound forms of the complexes and s is the binding site size. Emission Titration Experiments. Addition of CT-DNA in Tris-buffer solution of the complexes leads to an increase of emission intensity at their respective positions with an enhancement factor of 2.20, 2.31, and 2.28 for 1, 2, and 3, respectively (Figure 8). Moreover, the enhancement of emission intensity of the present bimetallic complexes, in particular with Os−Os (2) and Os−Os (3), occurs in the near-infrared domain, justifying their usefulness as suitable biological probes for DNA. Augmentation of emission intensity is probably due to the insertion of the complexes into the DNA base pair, which in

= (b − (b2 − 2Kb 2C t[DNA]/s)1/2 )/2KbC t

(4)

where b = 1 + KbC t + Kb[DNA]/2s

Lifetime Measurements. The interactions of the complexes with CT-DNA were also monitored through the measurement of the excited-state lifetimes. It is observed that luminescence lifetimes of the complexes in Tris-buffer medium are much shorter relative to those measured in neat acetonitrile probably due to solvent effects. Moreover, in contrast to accetonitrile, the decay profile of diosmium complex 2 looks I


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Inorganic Chemistry

Figure 8. UV−vis absorption and luminescence spectral changes of 1 (1.6 × 10−5 M) (a and b, respectively), 2 (1.5 × 10−5 M) (c and d, respectively), and 3 (1.4 × 10−5 M) (e and f, respectively) in the presence of increasing amounts of CT-DNA (0−30 μM for 1, 0−38 μM for 2, and 0−32 μM for 3) in Tris-NaCl buffer medium (pH = 7.30). Insets show the binding profile with DNA.

Table 5. Equilibrium Constants for 1−3 toward CT-DNA in Tris Buffer (pH = 7.30) Medium compound

absorption

emission

1 2 3

8.52 × 10 7.86 × 105 7.02 × 105

8.50 × 105 8.07 × 105 7.25 × 105

5

aqueous buffer medium, but in the presence of DNA its emission intensity increases dramatically due to intercalation. When a second DNA binder interacts with the DNA-EB adduct, it may replace EB and lead to quenching of the emission intensity significantly. Both DNA intercalators and groove binders reduce the intensity, but the reduction is moderate for groove binders, while intercalators can replace EB fully and reduce the intensity almost completely. Thus, we can differentiate between intercalators and groove binders with the help of this experiment. Now addition of all three dimers to the solution of the DNA-EB complex quenches the intensity completely with a factor of 99.2%, 98.1%, and 98.3% for 1, 2, and 3, respectively (Figure 10). Thus, all the complexes replace EB fully and support the DNA intercalation mode rather than groove binding. Apparent binding constants from the titration data were also calculated using eq 5:

biexponential in nature with a short component having a decay constant comparable to the lamp profile. Addition of CT-DNA leads to an increase of overall lifetimes of all three complexes, albeit to different extents (Figure 9 and Table S8, Supporting Information). The reason for the increase of lifetimes is again due to rigidity of the DNA-intercalated complexes and consequent inhibition of nonradiative relaxation channels to some extent. The changes of lifetimes of the complexes on incremental addition of DNA follow the same trend as those of their steady-state luminescence behaviors. Competitive DNA-Binding Study Using Ethidium Bromide. A competitive binding experiment using ethidium bromide (EB) gives us indirect evidence about the binding mode of the compounds with DNA. EB is nonfluorescent in

K app = KEB[EB]50% /[M]50%

(5)

where KEB is the binding constant of ethidium bromide to DNA and [EB]50% and [M]50% are the concentration of EB and metal at 50% displacement of EB from the DNA-EB conjugates. J


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Inorganic Chemistry

Figure 9. Changes in excited-state decay profiles of 1 (1.6 × 10−5 M) (a), 2 (1.5 × 10−5 M) (b), and 3 (1.4 × 10−5 M) (c) with increasing amounts of CT-DNA (0−40 μM for 1, 0−38 μM for 2, and 0−37 μM for 3) in Tris-NaCl buffer medium (pH = 7.30).

Figure 10. Emission spectra of the EB-DNA conjugate in the presence of incremental addition of 1 (a), 2 (b), and 3 (c) in Tris-NaCl buffer medium (pH = 7.30). Insets show the plot of the percent of free EB vs [Ru/Os]/[EB].

The Kapp value appeared as 3.20 × 106 M−1 for 1, 3.48 × 106 M−1 for 2, and 3.36 × 106 M−1 for 3 by taking KEB as 1.25× 106 M−1.55 It is to be mentioned here that the apparent binding constants calculated in this way vary considerably, and a 1:1 stoichiometry may not be followed all the time for the displacement of EB by the incoming complexes.93 A plot of [EB]/([EB] + [EB-DNA]) vs [M]/[EB] gives the value of the [M]/[EB] concentration ratio where 50% of the EB molecules were displaced. The values determined in this way are found to

be 0.39, 0.36, and 0.37 for 1, 2, and 3, respectively. Thus, from the experimental results, it can be said that the complexes bind to DNA probably through an intercalative mode. Circular Dichroism. The CD spectral changes of CT-DNA associated with the metal complexes are presented in Figure 11. As the complexes consist of statistical mixtures of both chiral and achiral diastereoisomers, no CD spectral bands are observed in their free state due to mutual cancellation induced by the diasteroisomers. Addition of each of the three complexes K


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Inorganic Chemistry

Figure 11. Changes in the CD spectra of CT-DNA (40 μM) with increasing concentration of 1 (0−30 μM) (a), 2 (0−35 μM) (b), and 3 (0−33 μM) (c) in Tris-NaCl buffer medium (pH = 7.30).

induces a remarkable change in their respective CD spectrum, which is indicative of a strong interaction between them. Free CT-DNA in its B-form exhibits two bands, at 273 and 242 nm, due to base stacking and helicity, respectively.94 On incremental addition of the metal complexes to DNA, the intensity of both bands increases gradually and significantly, although to different extents. All three dimers induce similar kinds of changes in their CD spectra with little variation of peak positions and intensity. An additional band at ∼288 nm with enormous increase in negative intensity is also observed for all complexes. Upon interaction with DNA, the apparent induced CD can arise from diastereomeric perturbations of the chromophores with similar binding modes (geometry, preferred sequence, etc.) as well as from different binding modes or different extents of binding for different enantiomers.95,96 Mode of Interaction between the Complexes and CT-DNA. The results derived from different spectroscopic techniques such as UV−vis absorption, steady-state and timeresolved emission, circular dichroism, and relative DNA binding study using ethidium bromide unequivocally indicate the bimetallic complexes interact strongly with CT-DNA in Trisbuffer medium. Now the interaction of the complexes with DNA can occur via different modes such as electrostatic, groove, and intercalation, and among the various modes, intercalation is thought to be the most effective mode for many important applications. Considering the extent of the spectral changes, it is evident that bimetallic complexes under the present investigation predominantly interact with DNA via an intercalation mode. The following evidence supports intercalation. (i) Hypochromism of the predominant ILCT/LLCT bands up to the extent of 12%, 8%, and 9% is observed for complexes 1, 2, and 3, respectively. Moreover, in contrast to the ILCT/LLCT bands, the hypochromism of the lowest energy MLCT band is almost

negligible, indicating that the interaction with DNA probably occurs through the extensively π-conjugated framework of the pyrenyl-imidazolate moiety in the complexes. (ii) The intrinsic binding constants are high (on the order of 105) as determined from both absorption and emission titration profiles, and there is a nice correlation between the two methods. (iii) There is a significant enhancement of emission intensity and lifetime of the complexes upon interaction with DNA. (iv) There are remarkable changes in the CD spectra of the complexes upon interaction with DNA. (v) Finally, the complexes fully replace EB from the DNA-EB adduct, which leads to almost complete quenching of the emission intensity. In a recent report, we demonstrated through several physicochemical techniques that monometallic Ru(II) and Os(II) complexes of composition [(bpy)2M(HImzPPy)]2+ derived from the related 2-pyridylpyrenoimidazole (HImzPPy) ligand efficiently intercalate into the DNA base-pairs with their Kb values on the order of 106 M−1. In this work, we expected that DNA binding affinities would be significantly enhanced on going from monometallic to bimetallic complexes with a more or less similar 2-imidazolyl-pyrenoimidazole (Py-BiimzH2) ligand. In practice, the DNA binding affinities of the present bimetallic complexes are found to be 1 order of magnitude less compared with the related monometallic complexes. The higher interacting affinity of the reported monometallic complexes is not only reflected in their values of binding constants but also evident from the extent of changes in their UV−vis absorption (Figure S18, Supporting Information), time-resolved emission, and circular dichroism spectroscopy (Figure S19, Supporting Information). Moreover, the extent of hypochromism H% (H% = 100(Afree − Abound)/Afree) was found to vary between 21% and 38% for the π−π* bands and between 20% and 29% for the MLCT peaks in the monometallic complexes, while for the present bimetallic L


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Inorganic Chemistry

heteobimetallic Ru(II) and Os(II) complexes under present investigation exhibit significant augmentation of luminescence intensity and excited-state lifetimes, justifying their usefulness as suitable biological probes for DNA in the NIR domain.

complexes, the extent of hypochromism is much less (varying between 8% and 12%) for the ILCT bands and almost negligible for the MLCT bands. The addition of all three dimers to the solution of the DNA-EB complex, on the other hand, quenches the emission intensity completely by 99.2%, 98.1%, and 98.3% for 1, 2, and 3, respectively (Figure 10), with the values of the apparent binding constants lying on the order of 106 M−1, which are comparable to their related HImzPPybased monometallic complexes, suggesting the complexes replace EB fully and supporting the intercalative mode of binding.55 Although the EB displacement experiments indicate that bimetallic complexes are almost as efficient as monometallic complexes in replacing EB from the EB-DNA adduct, but the results of other spectroscopic measurements indicate that DNA binding affinities of the bimetallic complexes are a little less compared to the related monomers. The reason for this observation is not very clear to us. One probable reason is that the two metal centers in a particular bimetallic complex may not bind to the same double strands of the DNA simultaneously. The strong intercalative binding of the monometallic complexes with DNA was attributed to the synergistic contributions of the extensive π-delocalized planar pyrene moiety and increased hydrophobicity imparted by the imidazole ring. In the formation of the bimetallic complexes, both the imidazole protons in Py-BiimzH2 become deprotonated, and introduction of an additional [M(bpy)2]2+ from the opposite face of the monometallic complex may provide some obstacle to the incoming DNA to interact with the pyrene moiety, since bpy was found to be inefficient for intercalative binding with DNA, which in turn may provide some explanation for the lack of hypochromism and negligible bathochromic shifts of the MLCT bands of the present bimetallic complexes compared with the related monomers. Recently, Wang and co-workers reported DNA binding properties of a series of bimetallic Ru(II) complexes derived from polypyridyl-imidazole-based bridging ligands such as N-ethyl-4,7-bis([1,10]-phenanthroline[5,6-f ]imidazol-2-yl)carbazole (ebipcH2),50 1,10-bis(3-(1Himidazo[4,5-f ][1,10]phenanthrolin-2-yl)-9H-carbazol-9-yl)decane, 9 7 and 1,3-bis(3-(1H-imidazo[4,5-f ][1,10]phenanthrolin-2-yl)-9H-carbazol-9-yl) propane97 with different aromatic or heteroaromatic moieties as the rigid spacers or long-chain methylene groups as the flexible bridging linkers. These complexes exhibit red-shift and hypochromisms of their visible MLCT and π−π* UV bands to some extent on interaction with DNA, and their Kb values were found to lie on the order of 106 M−1, which are comparable with the Kb values of some of the proven classical intercalators such as Δ,Δ-[μ(11,11′-bidppz)(phen) 4 Ru 2 ] 4+ , 98 [Ru(phen) 2 dppz] 2+ , 9 [(bpy)2Ru(ebipcH2)Ru(bpy)2]4+,50 and EB.99 In contrast to the emission enhancements that were observed for DNA molecular light-switch Ru(II) complexes, such as the dipyrido[3,2-a:2,3′-c]phenazine (dppz)-based complexes of [Ru(bpy)2(dppz)]2+ 6 and [Ru(bpy)2(hdppz)]2+,100 dipyrido[2,2d:2′,3′-f ]quinoxaline (dpq)-based complexes of [Ru(phen)2(Hcdpq)]2+,54 [Ru(phen)2dpqa]2+,101 [Ru(bpy)2(bipp)]2+,102 [Ru(bpy)2(bopp)]2+,102 and [Ru(bpy)2(btpp)]2+,102 and analogous complexes [Ru(bipy)2 (pip)]2+ 49 and [(bpy) 2Ru(HImzPPy)]2+,60 the bimetallic Ru(II) complexes reported by Wang’s group show significant emission quenching, thus restricting them for their probable use as DNA molecular light-switches. Although the extent of intercalation is slightly less than that of the classical intercalators, the homo- and

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CONCLUSION We reported in this work a new series of homo- and heterometallic Ru(II) and Os(II) complexes derived from a rigid and π-conjugated pyrenyl-biimidazolate bridging ligand, which can act as an efficient DNA intercalator in aqueous Trisbuffer solution. Characterizations of the complexes were done by NMR and high-resolution mass spectroscopic techniques and by single-crystal X-ray crystallography. The absorption spectra, electrochemical properties, and both steady-state and time-resolved emission behaviors of the complexes were thoroughly examined. Although we expected the bimetallic complexes, in particular the Ru−Ru (1) and Ru−Os (3), should exhibit longer lifetimes corresponding to their 3LC and/ or 3ILCT state localized on a pyrenyl-imidazole chromophore, measurements of the excited-state lifetimes indicate that the luminescence predominantly arises from their respective 3 MLCT state and is only slightly affected by the presence of the pyrene unit in Ru−Ru (1) and Ru−Os (3). The interaction between the bimetallic complexes and CT-DNA was investigated in detail via different techniques such as absorption, emission, excited-state lifetime, circular dichroism, and a relative DNA binding study using ethidium bromide. The outcomes of physicochemical measurements unequivocally suggest that the complexes interact with CT-DNA via an intercalative binding mode with the values of the intrinsic binding constant on the order of 105 M−1. The affinity of the complexes toward DNA arises due to the presence of the pyrenyl-imidazolate moiety in the complexes, since bipyridines were found to be ineffective for intercalative binding with DNA. Moreover, significant enhancement of the luminescence intensity and lifetimes of the complexes in the presence of CT-DNA justifies their usefulness for the development of biological probes or photodynamic therapy applications. It is of interest to note that the enhancement of Os-centered emission in Ru−Os (3) is much greater than that of the Ru-centered emission in the presence of DNA, indicating that DNA induces efficient energy transfer from the Ru center to the Os center. Additionally, DFT and TDDFT calculations on the complexes were also carried out to get a better insight into their electronic structures and excited-state properties. Good correlations between the experimental and theoretical results help us to assign the main absorption and emission behaviors of the complexes.

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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.5b02912. 1 H−1H COSY NMR, ESI mass, UV−vis absorption, steady-state and time-resolved luminescence spectra, and molecular orbital spectra related to DFT and TD-DFT calculations (Figures S1−S19 and Tables S1−S8) (PDF) Crystallographic data (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. M


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Inorganic Chemistry Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support received from DST and CSIR New Delhi, India, through project grant nos. [EMR/2015/001163 (SERB)] and [01(2766)/13/EMR-II], respectively, is gratefully acknowledged. The single-crystal XRD facility under DST-FIST and TCSPC facility under the DST-PURSE program of the Department of Chemistry (JU) is gratefully acknowledged. S.M. thanks UGC, while S.K. and D.M. acknowledge CSIR for their research fellowships.

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