Article pubs.acs.org/IC
Luminescent Dinuclear Ruthenium Terpyridine Complexes with a Bis-Phenylbenzimidazole Spacer Debiprasad Mondal, Sourav Biswas, Animesh Paul, and Sujoy Baitalik* Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700032, India S Supporting Information *
ABSTRACT: A conjugated bis-terpyridine bridging ligand, 2-(4-(2,6di(pyridin-2-yl)pyridin-4-yl)phenyl)-6-(2-(4-(2,6-di(pyridin-2-yl)pyridin4-yl)phenyl)-1H-benzo[d]imidazol-6-yl)-1H-benzo[d] imidazole (tpyBPhBzimH2-tpy), was designed in this work by covalent coupling of 3,3′-diaminobenzidine and two 4′-(p-formylphenyl)-2,2′:6′,2″-terpyridine units to synthesize a new series of bimetallic Ru(II)-terpyridine lightharvesting complexes. Photophysical and electrochemical properties were modulated by the variation of the terminal ligands in the complexes. The new compounds were thoroughly characterized by 1H NMR spectroscopy, high-resolution mass spectrometry, and elemental analysis. Absorption spectra of the complexes consist of very strong ligandcentered π−π* and n−π* transitions in the UV, metal-to-ligand, and intraligand charge transfer bands in the visible regions. Steady-state and time-resolved emission spectral measurements indicate that the complexes exhibit moderately intense luminescence at room temperature within the spectral domain of 653−687 nm having luminescence lifetimes in the range between 6.3 and 55.2 ns, depending upon terminal tridentate ligand and solvent. Variable-temperature luminescence measurements suggest substantial increase of the energy gap between luminescent 3metal-to-ligand charge transfer state and nonluminescent 3metal centered in the complexes compared to the parent [Ru(tpy)2]2+. Each of the three bimetallic complexes exhibits only one reversible couple in the positive potential window with almost no detectable splitting corresponding to simultaneous oxidation of the two remote Ru centers. All the complexes possess a number of imidazole NH protons, which became sufficiently acidic upon metal ion coordination. By utilizing these NH protons, we thoroughly studied anion recognition properties of the complexes in pure organic as well as predominantly aqueous media through multiple optical channels and spectroscopic methods. Finally computation investigations employing density functional theory (DFT) and time-dependent DFT were done to examine the electronic structures of the complexes and accurate assignment of experimentally observed optical spectral bands.
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MLCT state and nonemitting 3MC state is often responsible for the nonluminescent characteristics of the terpyridine complexes of Ru(II) at room temperature.19,20 Thus, to utilize as potential photosensitizers, room-temperature excited-state lifetimes of these complexes must be optimized. Several strategies such as substitution of the tpy ligands by electronwithdrawing groups,26−30 introducing coplanar hetero-aromatic moiety,31−35 incorporation of organic chromophore,36−39 or by using cyclometalated or anionic tridentate ligands,40−43 have been adopted to improve the room-temperature excited-state lifetimes, and in most of the strategies, the target is to increase the energy gap between the radiative 3MLCT and nonradiative 3 MC states by varying electronic nature of terpyridine ligands.44−50 In contrast to natural light-harvesting systems where the functional subunits are generally connected through supramolecular interactions (such as hydrogen bonding or van der Waals interaction), the artificial systems should possess many
INTRODUCTION Harnessing of light energy by transition metal-based photoactive units has been receiving increasing attention with regard to the construction of photochemical molecular devices1−5 such as sensitizers for photovoltaic6 and fuel cell devices,7 electroluminescent devices,8,9 and molecular sensors10−14 because of their favorable and versatile photophysical and redox properties. Among the numerous transition metal-based light harvesting building-blocks, ruthenium(II) complexes derived from polypyridine ligands have thoroughly been studied because of their relatively long-lived photoexcited state, which primarily arises due to metal-to-ligand charge transfer (MLCT) state and owing to their tunable photophysical and electrochemical properties.15−18 Among the polypyridines, bidentate bipyridine (bpy)type or tridentate terpyridine (tpy)-type chelating units have widely been used for the designing of photoactive ruthenium(II) complexes.15,16 The main advantages of tpy over bpy is their ability to construct linear rodlike architectures that are achiral in nature.19−24 But the excited-state lifetimes of these complexes are usually very short (τ < 1 ns for [Ru(tpy)2]2+) at room temperature.25 The small energy barrier between emitting © 2017 American Chemical Society
Received: December 7, 2016 Published: June 27, 2017 7624
DOI: 10.1021/acs.inorgchem.6b02937 Inorg. Chem. 2017, 56, 7624−7641
Article
Inorganic Chemistry Chart 1. Molecular Structures of the Ligand and Three Bimetallic Ru(II) Complexes
3
photoactive units preferably via covalent coupling, as several photons are needed for the occurrence of subsequent multielectron processes as well as photoinduced electron and energy transfer leading to the generation of charge-separated species.19−24 But luminescent bi- and multimetallic Ru(II)terpyridine complexes having long-lived excited states are relatively less in the literature compared with their monometallic counterparts. To this end and with regard to our continued efforts51−59 for developing terpyridine-based multimetallic Ru(II) complexes with enhanced excited-state lifetimes, we designed in this work a conjugated bis-terpyridine bridging ligand by covalent coupling of 3,3′-diaminobenzidine and two 4′-(p-formylphenyl)-2,2′:6′,2″-terpyridine (4′-tpyC6H4−CHO) units, which can be utilized to synthesize a new family of bimetallic Ru(II)-terpyridine complexes. 3,3′Diaminobenzidine and its derivatives are important precursors for the synthesis of polybenzimidazole fiber, which in turn can be utilized to construct materials that are resistant to chemicals and high temperature. But relatively little attention has been paid in connection with the development of suitable optoelectronic systems by covalent coupling of substituted benzidine moiety with Ru(II)-poylpyridyl unit. A dinucleating ligand 2,2′-bis(2-pyridyl)-6,6′-bibenzimidazole containing bidentate bpy-type chelating site has previously been designed by the condensation reaction of 3,3′-diaminobenzidine with 2 equiv of pyridine-2-carboxylic acid for the synthesis of bimetallic Ru(II) complexes based on [Ru(bpy)3]2+ unit.60 But to the best of our knowledge, there is no such report in connection with the Ru(II) complexes based on terpyridine ligands. The rigid coupling between the 3,3′-diaminobenzidine and Ru-tpy MLCT chromophores is expected to enlarge the energy gap between the emitting 3MLCT and nonemitting
MC states leading to improved room-temperature luminescence properties. Herein, we report the synthesis, characterization, and steady-state and time-resolved luminescence properties and electrochemical behaviors of a new family of bimetallic bis-tridentate Ru(II) complexes. The photophysical and electrochemical properties were fine-tuned by varying the coligand in the bimetallic Ru(II) complexes (Chart 1). Interestingly, all the complexes exhibit moderately strong luminescence at room temperature with excited-state lifetimes 2 orders of magnitude higher relative to parent [Ru(tpy)2]2+. The possibility of photoinduced intermolecular electron transfer was also be explored by reductive quenching of the excited states of the complexes by using known reductants such as triethanolamine (TEOA) or ascorbic acids as an electron donor. Apart from exhibiting room-temperature luminescence behaviors with reasonably long-lived excited states, the novelty of the present system over most of the previously reported systems is that significant modulation of the ground- and excited-state properties of the complexes can be done by exploiting the imidazole NH groups (varying between 2 and 6) present on both the bridging and terminal ligands in the complexes. Interestingly, the NH protons in the second coordination sphere of the complexes became sufficiently acidic upon coordination with two Ru(II)-tpy/H 2 pbbzim {2,6-bis(benzimidazole-2-yl)pyridine} units and capable to interact with the incoming anionic guest either through hydrogenbonding interactions or by anion-induced proton transfer reactions. In this work, we thoroughly investigated the anionsensing behaviors of three bimetallic complexes in both organic and aqueous medium through multiple optical spectroscopic techniques. Coordination complex-based receptors, by virtue of their versatile photophysical and electrochemical properties, 7625
DOI: 10.1021/acs.inorgchem.6b02937 Inorg. Chem. 2017, 56, 7624−7641
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Inorganic Chemistry
H(3‴)), 9.12−9.06 (m, 8H, H(6)+H(6′)), 8.78 (d, 2H, J = 7.9 Hz H(9)), 8.74 (d, 4H, J = 8.1 Hz H(7)), 8.61 (d, 4H, J = 8.1 Hz H(8)), 8.42 (d, 2H, J = 7.7 Hz, H(10)), 8.35−8.28 (m, 6H, H(8′+H(11)), 8.13−8.00 (m, 8H, H(4)+H(4′)), 7.63−7.52 (m, 12H, H(3)+H(3″)+ H(7′)), 7.33−6.27 (m, 8H, H(5)+H(5′)), 2.49 (s, 6H, (−CH3)). Electrospray ionization mass spectrometry (ESI-MS) (positive, CH 3 CN) m/z = 283.40(40%) [(tpy-C 6 H 4 −CH 3 )Ru(tpyBPhBzimH4-tpy)Ru(tpy-C6H4−CH3)]6+, m/z = 339.88 (100%) [(tpy-C 6 H 4 −CH 3 ) Ru(tpy-BPhBzimH 3 -tpy)Ru(tpy-C 6 H 4 − CH 3 )] 5+ and m/z = 424.60 (38%)[(tpy-C 6 H 4 −CH 3 )Ru(tpyBPhBzimH4-tpy)Ru(tpy-C6H4−CH3)]4+. Synthesis of [(tpy-py)Ru(tpy-BPhBzimH2-tpy)Ru(tpy-py)](ClO4)4· H2O (2). Synthesis of 2 was performed in the same way as 1 with the exception that [(tpy-py)Ru]Cl3 (0.294 g, 0.46 mmol) precursor was used instead of [(tpy-C6H4−CH3)Ru]Cl3. Yield: 0.311 mg, (56.9%). Anal. Calcd for C118H76N16O17Cl4Ru2: C, 60.64; H, 3.19; N, 9.59%. Found: C, 60.50; H, 3.12; N, 9.53. 1H NMR data (500 MHz, DMSOd6, TMS, δ, ppm, see Figure S2 (Supporting Information) for atom numbering): 14.58 (s, 2H, NH), 9.61(s,4H,(H3′)), 9.45 (s, 4H, H(3‴)), 9.19−9.00 (m, 4H, H(12)+H(20)), 8.92−8.80 (m, 8H, H(6)+H(6′)), 8.78 (d, 2H, J = 7.4 Hz H(15)), 8.76−8.68 (m, 6H, H(7)+H(13)), 8.66 (d, 2H, H(9)), 8.49 (t, 4H, J = 8.2 Hz H(8)), 8.42−8.10 (m, 6H, H(10)+H(11)+H(18)), 8.00−7.75 (m, 12H, H(14)+H(16)+H(17)+H(19)+H(4′)), 7.61 (t, 4H, J = 7.2 Hz H(4)), 7.50 (d, 4H, J = 7.2 Hz, H(3)), 7.39 (d, 4H, J = 7.6 Hz, H(3″)), 7.12 (t, 8H, J = 7.4 Hz, H(5)+H(5′)). ESI-MS (positive, CH3CN) m/z = 384.31 (40%) [(tpy-py)Ru(tpy-BPhBzimH3-tpy)Ru(tpy-py)]5+, m/z = 480.03 (100%) [(tpy-py)Ru(tpy-BPhBzimH2-tpy) Ru(tpy-py)]4+. Synthesis of [(H2pbbzim)Ru(tpy-BPhBzimH2-tpy)Ru(H2pbbzim)](ClO4)4 (3). Complex 3 was synthesized in same way as 1, except [(H2pbbzim)RuCl3] (0.238 g, 0.46 mmol) precursor was used instead of [(tpy-C6H4−CH3)Ru]Cl3. Yield: 0.321 mg, (65.7%). Anal. Calcd for C94H62N20O16Cl4Ru2: C, 54.44; H, 3.01; N, 13.51%. Found: C, 54.40; H, 3.08; N, 13.48. 1H NMR data (500 MHz, DMSO-d6, TMS, δ/ppm, see Figure S2 (Supporting Information) for atom numbering): 15.18 (s, 4H, NH(H2pbbzim)), 9.72(s, 4H, (H3′)), 9.07 (d, 4H, J = 8.2 Hz H(6)), 8.96 (d, 2H, J = 7.6 Hz H(9)), 8.89−8.72 (m, 8H, H(8)+H(25)), 8.70−8.52 (m, 8H, H(7)+H(11)+H(26)), 7.98−7.80 (m, 6H, H(4)+H(10)), 7.68 (d, 4H, J = 8.1 Hz, H(3)), 7.53 (d, 4H, J = 6.5 Hz, H(24)), 7.30−7.26 (m, 8H, H(5)+H(23)), 7.04−6.97 (m, 4H, H(22)), 6.09 (d, 4H, J = 8.2 Hz H(21)). ESI-MS (positive, CH3CN) m/z = 279.42 (34%) [(H2pbbzim)Ru(tpy-BPhBzimH4tpy)Ru(H2pbbzim)]6+, m/z = 335.11 (100%) [(H2pbbzim)Ru(tpyBPhBzimH 3 -tpy)Ru(H 2 pbbzim)] 5 + , m/z = 418.64(70%) [(H2pbbzim)Ru(tpy-BPhBzimH2-tpy)Ru (H2pbbzim)]4+, m/z = 557.85 (20%) [(Hpbbzim)Ru(tpy-BPhBzimH2-tpy)Ru(H2pbbzim)]3+. Caution! Perchlorate salts of the metal complexes used in this study are potentially explosive and therefore should be handled with care in small quantities. Physical Methods and Instrumentations. Instrumental specifications and the detailed experimental procedures adopted to perform UV−vis absorption, steady-state, and time-resolved luminescence measurements, electrochemical investigations, and computational methods employing DFT and TD-DFT were provided in the Supporting Information.
can offer multiple channels compared with their pure organic counterparts to visualize the anion recognition event.10,61−67 The metalloreceptors act as efficient multichannel sensors for F−, CN−, and AcO− in dimethyl sulfoxide (DMSO), whereas highly selective chromogenic and fluorogenic sensors for cyanide ion in predominantly aqueous medium. Importantly, the detection limits for cyanide in aqueous medium exhibited by complexes are very low and lie in the order from 1 × 10−7 to 1 × 10−8 M. It may be mentioned that there are only few metalloreceptors in the literature that can detect CN− either in organic or aqueous medium,68−70 but the majority of them suffer from lack of selectivity, and also the detection limit is also higher than the permissible level (0.2 ppm) for drinking water as recommended by Environment Protection Agency (EPA).71 Thus, the key point of the present work is to modulate the ground- and excited-state properties of a new class of luminescent linear rodlike bimetallic Ru(II)-terpyridine complexes via their NH groups through variation of solvent, amine, or anion coordination for the design of suitable functional materials. Finally, in conjunction with experimental investigations, computational studies employing density functional theory (DFT) and time-dependent (TD) DFT methods were also performed to corroborate the experimentally observed optical bands in the complexes.
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EXPERIMENTAL SECTION
Materials. Fine chemicals were purchased from Sigma-Aldrich Chemicals and were used as supplied. Solvents were procured from local vendors and dried by adopting standard literature protocols. The starting materials 4′-(p-formylphenyl)-2,2′:6′,2″-terpyridine (tpyC 6 H 4 −CHO), 72 [(tpy)RuCl 3 ], [(tpy-C 6 H 4 −CH 3 )RuCl 3 ], and [(H2pbbzim)RuCl3] were prepared and purified by literature methods.73,74 Synthesis of the Bridging Ligand. 2-(4-(2,6-Di(pyridin-2-yl)pyridin-4-yl)phenyl)-6-(2-(4-(2,6-di(pyridin-2-yl)pyridin-4-yl)phenyl)1H-benzo[d]imidazol-6-yl)-1H-benzo [d]imidazole (tpyBPhBzimH2-tpy). To a suspension of 3,3′-diaminobenzidine (0.10 g, 0.48 mmol) and sodium acetate (1.6 g) in acetic acid (25 mL), powdered tpy-C6H4−CHO (0.315 g, 0.96 mmol) was gradually added, and the mixture refluxed for 3 h under argon protection. After it cooled at room temperature, the resulting mixture was poured into icecooled water (∼100 mL), when a yellow colored precipitate appeared. The compound was collected by filtration, washed with water, and dried in air to yield 0.351 g (88.36%) of the product as a yellow powder. Anal. Calcd for C56H36N10: C, 79.24; H, 4.24; N, 16.50%. Found: C, 79.30; H, 4.14; N, 16.46. 1H NMR data (500 MHz, DMSOd6, tetramethylsilane (TMS), δ, ppm, see Figure S1 (Supporting Information) for atom numbering): 13.20 (s, 2H, NH), 8.77(s, 4H, (H3′)), 8.73 (s 4H, H(6)), 8.70−8.63 (m, 4H, H(3)), 8.42 (d, 2H, J = 7.30 Hz, H(9)), 8.19 (d, 4H, J = 7.12 Hz H(7)) 8.13−7.98 (m, 6H, H(4)+H(11)), 7.97−7.91(m, 6H, H(8)+H(10)), 7.70−7.50 (m, 4H, H(5)). Synthesis of [(tpy-C6H4−CH3)Ru(tpy-BPhBzimH2-tpy)Ru(tpyC6H4−CH3)] (ClO4)4·2H2O (1). tpy-BPhBzimH2-tpy (0.20 g, 0.23 mmol) and [(tpy-C6H4−CH3)Ru]Cl3 (0.244 g, 0.46 mmol) were suspended in 25 mL of ethylene glycol and refluxed for 12 h under argon protection. The resulting mixture was cooled to room temperature and poured into an aqueous solution of sodium perchlorate (1 g in 5 mL), when a red color compound precipitated. The compound was filtered and purified by silica gel column chromatography using MeCN as the eluent. The complex was again purified by recrystallizing from a mixture of MeCN−MeOH (1:1 v/v) in the presence of few drops of 1 × 10−5 M HClO4. Yield: 0.302 mg, (60.9%). Anal. Calcd for C100H74N16O18Cl4Ru2: C, 56.28; H, 3.49; N, 10.51%. Found: C, 56.12; H, 3.42; N, 10.61. 1H NMR data (500 MHz, DMSO-d6, TMS, δ, ppm, see Figure S2 (Supporting Information) for atom numbering: 14.58 (s, 2H, NH), 9.63(s, 4H, (H3′)), 9.43 (s, 4H,
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RESULTS AND DISCUSSION Synthesis and Characterization. The bridging bisterpyridine ligand (tpy-BPhImzH2-tpy) was prepared by refluxing a 1:2 mixture of 3,3′-diaminobenzidine and 4′-(pformylphenyl)-2,2′:6′,2″-terpyridine (tpy-C6H4−CHO) in acetic acid in the presence of excess sodium acetate. Symmetrical bimetallic Ru(II) complexes (1−3) were synthesized in a straightforward manner by refluxing the ligand (tpyBPhBzimH2-tpy) with appropriate metal precursor in 1:2 molar ratios in ethylene glycol solvent under argon protection. Silica gel column chromatography and recrystallization from appropriate solvent mixture in the presence of slightly acidic 7626
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Inorganic Chemistry
Figure 1. Energy-level diagrams depicting the dominant transitions arising out from S6 state for 1 (a), S3 state for 2 (b), and S7 state for 3 (c) in dimethyl sulfoxide.
DFT and TD-DFT Computational Studies. Molecular structures of selected complexes in both of their protonated as well as NH deprotonated forms were optimized in DMSO, and the optimized geometries are displayed in Figures S8 and S9 (Supporting Information). Calculated Ru−N bond distances were found to vary between 1.98 and 2.12 Å, while the N−Ru− N angles vary between 77.25° and 179.84° {Tables S1 and S2 (Supporting Information)}, which are consistent with related Ru(II) complexes.51−55 Note that two benzimidazole moieties get twisted among themselves around the single bond, and the extent of twist varies between 33.95° and 37.28°. The nonbonded Ru−Ru distances lie in the range between 30.99 and 31.13 Å and thus remain practically unaltered across the series as expected. Plots of selected frontier molecular orbitals (FMOs) are shown in Figures S10 and S11 (Supporting Information), while the energies and compositions of FMOs are summarized in Tables S3 and S4 (Supporting Information). On the one hand, highest occupied molecular orbitals (HOMOs) mainly localized on benzimidazole, pyrene, as well as on Ru(II), but the contribution of these moieties varies on a specific HOMO as well on the type of the complex (Figures S10 and S11, Supporting Information). Lowest unoccupied molecular orbitals (LUMOs), on the other hand, localized predominantly on the terpyridine moiety of either bridging or terminal terpyridine ligands. Interestingly, the compositions and energies of HOMOs and LUMOs change substantially upon removal of the imidazole NH protons in the complexes. Electronic charge density and their subsequent redistribution upon deprotonation could be observed through the electrostatic surface potential (ESP) plots (Figure S12, Supporting Information). Electronrich area in the ESP plots is indicated by red color, while the electron-deficient part is indicated by blue color. Removal of NH protons from the complexes leads to the accumulation of the negative charge on the bis-benzimidazole moieties. TD-DFT calculations were done on the ground-state optimized geometries of the complexes in DMSO to assign their absorption bands. Calculated spectral data together with the assignment of different bands were summarized in Table S5, (Supporting Information). Frontier orbitals that are
condition were employed for purification of the resulting complexes. The ligand as well as all the three metal complexes were thoroughly characterized by C, H, and N analyses, ESI mass spectrometry, and 1H NMR spectroscopic measurements, and relevant characterization data were already summarized in the Experimental Section. 1 H NMR spectra of tpy-BPhBzimH2-tpy and the complexes in DMSO-d6 are displayed in Figures S1 and S2 (Supporting Information). Many proton resonances in the aromatic region (6−10 ppm) are observed, and tentative assignments of the protons were done by taking advantage of their twodimensional {1H−1H} COSY NMR spectra as well as by comparing the spectra of structurally related Ru(II) complexes (Figures S3 and S4, Supporting Information). H3′, H7, H8, and H9 protons in the terpyridine site of the bridging ligand get significantly downfield-shifted upon coordination to two Ru(II) centers. By contrast, H3 proton of terpyridine moiety of the bridge gets remarkably upfield shifted (8.63 to ∼7.63 ppm), as this proton is under the influence of anisotropic ring current of an adjacent pyridine and/or phenyl moiety of the other tridentate terminal ligand. ESI mass spectrometry was also used for thorough characterization of the complexes. ESI mass spectra of 1−3 in conjunction with the assignment of the peaks are displayed in Figures S5−S7 (Supporting Information). The experimentally observed peaks as well as their isotopic distribution patterns correlate well with their corresponding simulated patterns. Many of the fragmented ions show sequential loss of imidazole NH protons along with the counterions keeping the bimetallic core intact, and thus the assignments of the different peaks in the complexes became simplified. For example, 3 exhibits four peaks with varied intensities at m/z = 279.42 (34%), m/z = 335.11 (100%), m/z = 418.64(70%), and m/z = 557.85 (20%) corresponding to the occurrence of [(H2pbbzim)Ru(tpyBPhBzimH 4 -tpy)Ru(H 2 pbbzim)] 6+ , [(H 2 pbbzim)Ru(tpyBPhBzimH 3 -tpy)Ru(H 2 pbbzim)] 5+ , [(H 2 pbbzim)Ru(tpyBPhBzimH2-tpy)Ru(H2pbbzim) ]4+, and [(Hpbbzim)Ru(tpyBPhBzimH2-tpy)Ru(H2pbbzim)]3+ (Figure S7, Supporting Information). 7627
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emission energy. The optimized geometry in the triplet state is displayed in Figure S17 (Supporting Information), while Ru−N bond distances and N−Ru−N bond angles are summarized in Tables S1 and S2 (Supporting Information). The geometrical parameters of the complex differ negligibly compared with the ground-state geometry. The calculated lowest energy maximum was found at 790 nm for 3 in dimethyl sulfoxide. The frontier MOs that are involved in single electron transitions for the emission process are shown in Figure S18 (Supporting Information), and the picture and compositions of the FMOs are summarized in Figure S19 and Table S4 (Supporting Information). By inspecting the contributions of MOs, it can be concluded that observed luminescence in the complex arises predominantly from its 3MLCT state. Calculation performed on the deprotonated form of 3 (3a) gives rise to luminescence maximum at much longer wavelength, specifically, 988 nm (Table S7, Supporting Information). Absorption and Luminescence Spectral Characteristics. Absorption and luminescence spectral signatures of the bimetallic Ru(II) complexes (1−3) were recorded in two solvents, specifically, acetonitrile and dimethyl sulfoxide, and the relevant photophysical data are given in Table 1. The absorption spectrum of the free ligand was recorded in DMSO only due to its solubility limitation in acetonitrile. Figure 3 and Figure S20 (Supporting Information) displayed the UV−vis absorption and luminescence spectra of the complexes together with the bridging ligand. All the complexes display more or less similar spectral feature containing a number of strong absorption bands in both UV and visible region. Assignment of different absorption bands was done by comparing the spectra of structurally related complexes. In addition, TD-DFT calculations also guided us for proper assignment of the experimentally observed bands in the complexes (Figures S21 and S22, Supporting Information). The strong absorption peak that is observed between 491 and 502 nm, depending on the type of the terminal ligand as well as the solvent, arises due to combined 1[RuII(d)6]→ 1 [RuII(d)5tpy-BPhBzimH2-tpy(π*)1] charge transfer (1MLCT) and bis-benzimidazole/pyrene to terpyridine charge transfer (ILCT) transitions. Another band in range of 357−380 nm mainly arises due to intraligand charge transfer from bisbenzimidazole moiety to the terpyridine moiety of either the bridging or the terminal ligand. In addition to ILCT, some amount of Ru(II) → tpy MLCT transition also contributes to this band. Three most intense bands that are noticed around 278 and 351 nm are due to π−π* transitions associated with the coordinated tridentate ligand. Weak and broad band at lower-energy region (566 and 606 nm) is also observed, which probably arise due to spin-forbidden 1 [Ru II (d) 6 ] → 3 [RuII(d)5tpy-BPhBzimH2-tpy(π*)1] transition. The 1MLCT band of the complexes was found to get red-shifted compared with their parent [Ru(tpy) 2 ]2+ (474 nm)17 and [Ru(H2pbbzim)2]2+ (475 nm)75 complexes probably because of the incorporation of bis-phenylbenzimidazole spacer between the 4′-position of terpyridine units in the bridge resulting in net stabilization of the LUMOs. It is of interest to note that 1 MLCT and ILCT bands in the complexes get red-shifted on passing from acetonitrile to more polar dimethyl sulfoxide. The solvent effect is particularly dramatic for 3 due to its greater push−pull characteristics compared with 1 and 2 as well as its ability to form strong intermolecular hydrogen bond with
involved in the lowest energy transition are displayed in Figure 1. Calculated lowest-energy absorption peak appeared at 484, 499, and 481 nm, for 1, 2, and 3, respectively. Inspection of Table S5 and Figure 1 suggests that the lowest excited singlet state is due to bis-benzimidazole bridge/Ru(II) → tpy(bridging/terminal) for 1, pyrene/bisbenzimidazole bridge/ Ru(II) → tpy(terminal/bridging) for 2, and bisbenzimidazole bridge/Ru(II) → tpy(bridging) for 3. Thus, by considering the participation of HOMOs and LUMOs, the calculated lowest energy band can be assigned as a mixture of both MLCT and ILCT. The next higher energy band at 422 nm for 1, at 421 nm for 2, and 440 nm for 3 mainly arise from S25, S33, and S16 excitations, respectively. These excitations mainly involve the transfer of charge from Ru(II) center to the tpy moiety of both the bridging and terminal ligands. The strong absorption bands observed in the UV regions are mainly due to either ILCT or π−π* electronic transitions within the π orbitals of the aromatic and heteroaromatic frameworks. TD-DFT calculations were also performed on the deprotonated form of the complexes and calculated bands along with their assignments are presented in Table S6 (Supporting Information). The red-shifted bands at 764 and 584 nm for 2a and 641 nm for 3a exclusively arise from charge transfer from bis-benzimidazole unit to the tpy moiety of both bridging and terminal tpy ligands. On the one hand, frontier orbitals that are involved in the lowest energy transition are displayed in Figure S13 (Supporting Information). The next higher energy band at 494 nm for 2a and 519 nm for 3a, on the other hand, consists of both benzimidazole to tpy and Ru(II) to tpy charge transfer characters. To verify the mode of assignment, electron density difference map (EDDM) and natural transition orbital (NTO) analysis plots were also sketched. Figure 2 displayed the EDDM plots of
Figure 2. Difference in electron density upon excitation from the ground S0 state to the singlet excited state (S6, S3, and S7 for 1, 2, and 3, respectively). Red and green color shows regions of decreasing and increasing electron density, respectively, with a surface value of 0.0004.
the lowest energy band (S6 for 1, S3 for 2, and S7 for 3) and a close look at the plots concludes that the contribution of the MLCT character to the lowest energy band is greater than ILCT in 3, while the reverse situation holds for 1 and 2. NTO plots of lowest energy band of the complexes are shown in Figures S14−S16 (Supporting Information). NTO analysis also led to the same conclusion as EDDM plots, and mixed MLCT and ILCT character of the said band in the complexes was also verified. TD-DFT calculation using the triplet geometry of a representative complex (3) was also performed to obtain its 7628
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Inorganic Chemistry Table 1. Spectroscopic and Photophysical Data for Complexes 1−3 in Dimethylsulfoxide and Acetonitrile luminescence
at 77 Ka
at 298 K compounds 1
λabs, nm, (ε, M−1 cm−1)
DMSO
DMSO
573(5750) (br), 499(58500), 452(27920), 360(37780), 333(99430), 316(125500), 292(112500) 566 (5610) (br),492(56550), 357(40280), 328 (103250), 311 (140500), 284(130250) 568(7500) (br), 500(55500), 450(25980), 377(48250), 332(100500), 315(116250), 280(112200) 563(8400) (br), 493(56250), 363(51650), 328 (99680), 311 (117770), 278(115980) 606(4470) (br), 502(49400), 466(29830), 380(50750), 351(114925), 319 (122250) 288(96000) 577(4790) (br), 491(47500), 375(47750), 348(111750), 313(115250), 284(87000) 404(19100), 290(61730)
MeCN MeCN
490 (28 000) 475(17400)
MeCN 2
DMSO MeCN
3
DMSO MeCN
tpy-BPhBzimH2-tpy (L) [Ru(tpy-PhCH3)2]2+b [Ru(H2pbbzim)2]2+c a
λemi, nm
solvent
Φ × 10−3
τ, ns
kr × 105, s−1 knr × 107, s−1
660
0.24
τ = 6.3
0.38
15.86
655
0.32
τ = 9.3
0.34
10.7
658
0.20
τ = 9.1
0.23
10.90
653
0.21
τ = 8.8
0.19
11.36
687
1.10
τ2 = 55.2
0.19
1.80
684
1.24
τ2 = 50.8
0.24
1.96
485
335
τ = 2.3
640
≤0.03
77 K, and their values are obtained from room-temperature luminescence data. k2, on the other hand, is temperaturedependent rate constant, which takes into account the rate constant for accessing the 3MC state from 3MLCT state. The activation energy required for this process is designated as ΔE2. Nonlinear fitting of the experimental temperature versus lifetime data to eq 1 give rise to the values of k1, k2, and ΔE2 in the complexes (Figure 5). The ΔE2 value is 2587 ± 80, 2716 ± 44, and 3034 ± 40 cm−1 for 1, 2, and 3, respectively. k1 lies in the range from (1.17 ± 0.18)×107 to (6.47 ± 0.19)×107 s−1, while k2 varies between (2.37 ± 0.78)×1011 and (4.38 ± 0.55)×1011 s−1. The values of k1, k2, and ΔE2 were found to 7630
DOI: 10.1021/acs.inorgchem.6b02937 Inorg. Chem. 2017, 56, 7624−7641
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Inorganic Chemistry
using Pt working electrode and Ag/AgCl reference electrode at a scan rate of 100 mV/s. The voltammograms are shown in Figure 6, and relevant electrochemical data are presented in
Figure 6. Cyclic (solid line) and square-wave voltammograms (dotted line) of complexes 1−3 in acetonitrile acquired against Ag/AgCl reference electrode at a scan rate of 100 mV/s−1 at room temperature showing the oxidation of the complexes.
Table 2. Redox Dataa and E00 Value for Complexes 1−3 along with Redox Data Two Reference Complexes in Acetonitrile
Figure 5. Effect of temperature on excited-state decay profiles for 1 (a), 2 (b), and 3 (c) in acetonitrile. Increase of temperature from 268 to 333 K led to gradual decrease in lifetime of the complexes. Temperature-dependent lifetime data along with the values of different parameters and the corresponding nonlinear fit: (d) 1, (e) 2, and (f) 3.
correlate reasonably well with structurally related complexes of Ru(II). Interestingly, substantial increase in the energy difference (ΔE2) between 3MLCT and 3MC state was observed in all the complexes with respect to the parents [Ru(tpy)2]2+ (ΔE2 = 1500 cm−1)76 and [Ru(tpy-PhCH3)2]2+ (ΔE2 = 1800 cm−1)30 complexes. We also calculated the 3MLCT−3MC barrier for 1 and 3 by DFT. The geometry in 3MLCT states was optimized by unrestricted Kohn−Sham (UKS) method by utilizing 1ground state (GS) optimized geometry.77,78 The geometry in 3MC states was obtained by elongating two opposite Ru−N bonds in the complexes to 2.40 Å. The calculated 3MLCT−3MC barrier was found to be 554 cm−1 for 1, while 1076 cm−1 for 3. The spin density plots of 3MLCT and 3 MC state of 1 and 3 are displayed in Figures S27 and S28, (Supporting Information). Although the trend is in line with experimental results but in terms of absolute values, the agreement between the calculated and experimental is not good. Increase in ΔE2 is expected to be due to extensive excitedstate delocalization induced by extended π-delocalized bisbiphenyl-benzimidazole spacer. In effect, substantial increase of room-temperature luminescence lifetimes of the present bimetallic Ru(II)-terpyridine complexes occur without too much lowering of the 3MLCT energy. Thus, the favorable absorption and emission spectral behaviors along with reasonably long lifetimes at room temperature highlights the viability of using this bimetallic Ru(II) complexes in lightharvesting applications. Electrochemical Behaviors. Cyclic and square wave voltammograms of the complexes were acquired in accetonitrile
compds
E1/2(ox), V
1 2 3 [Ru(tpy-C6H4-CH3)2]2+ c Ru(H2pbbzim)2]2+ d
1.30 1.33 1.12 1.25 0.76
Eb1/2(red), V −1.17, −1.20, −1.53, −1.24, −1.40,
−1.36, −1.96 −1.52, −2.12 −2.08 −1.46 −1.70
E00, eV 1.93 1.94 1.84
a
All the potentials are referenced against Ag/AgCl electrode with E1/2 = 0.36 V for Fc/Fc+ couple. bE1/2 values obtained from square wave voltammetric using glassy carbon electrode. cReference 17. dReference 75.
Table 2. Each complex exhibits one reversible wave in the positive potential window due to simultaneous oxidations of two Ru(II) centers and several quasi-reversible or irreversible waves in the negative potential window due to the reductions of coordinated bridging (tpy-BPhBzimH2-tpy) and terminal (tpyC6H4−CH3, tpy-Py, and H2pbbzim) ligands (Table 2). It is observed that E1/2 value for oxidation in both 1 and 2 are almost the same, while H2pbbzim terminal ligand results in a substantial lowering (∼180 mV) in the RuII/RuIII oxidation potential in 3. The reduction potentials of the complexes also vary according to the electronic nature of the terminal ligands. The excited-state energies (E00), estimated from luminescence maxima of the complexes in 4:1 EtOH−MeOH glass at 77 K, were found to lie between 1.84 and 1.94 eV. Taken into consideration of their high excited-state energies and higher anodically shifted redox potentials, present complexes appear to be particularly strong excited-state oxidants. E00 values and redox data (Table 2) allowed us to calculate the excited-state redox potential of the complexes with the help of eqs 2 and 3. The excited-state oxidation potential of the complexes were estimated to be −0.63, −0.61, and −0.72 V for 1, 2, and 3, respectively, while the excited-state reduction potential were 7631
DOI: 10.1021/acs.inorgchem.6b02937 Inorg. Chem. 2017, 56, 7624−7641
Article
Inorganic Chemistry
Figure 7. Luminescence quenching of 1 (a) and 2 (b) by TEOA in acetonitrile at room temperature. The excitation wavelength was 490 nm. (inset) Stern−Volmer plots for the emission quenching by using F0/F = 1 + Ksv × [TEOA].
spectral change in the presence of ascorbic acid also ruled out the possibility of ground-state deprotonation in the complexes. Thus, the present complexes can act as suitable photosensitizer in the visible region, where high oxidation power is required. Determination of pKa Values of the Complexes. The complexes in the present study contain up to six imidazole NH protons, which became acidic upon coordination to the metal centers and would be expected to be deprotonated on increasing the pH of the solution. To this end, we are interested to determine the pKa values of the complexes by performing absorption spectral titrations of the complexes in DMSO−H2O (3:2 v/v) solutions over the pH range of 3.0− 12.0 (Figures S32 and S33, Supporting Information). The absorption spectral profiles indicate one-step change for 1 and 2, while two-step changes for 3. The spectral changes can be represented by following acid−base equilibria (Scheme 1).
calculated to be +0.76 V for 1, +0.74 V for 2, and +0.31 V for 3 with the help of eqs 2 and 3.15 E *([Ru IIIRu III]5 + /[Ru IIRu II]4 + ) = E([Ru IIIRu III]5 + /[Ru IIRu II]4 + ) − E00
(2)
E *([Ru IIRu II(lig)]4 + /[Ru IIRu II(lig··−−)]2 + ) = E([Ru IIRu II(lig)]4 + /[Ru IIRu II(lig··−−)]2 + ) + E00 (3)
The values of excited-state redox potentials indicate that the complexes are capable to oxidize neighboring species in the excited state. To explore the possibility of occurrence of photoinduced electron-transfer process, triethanolamine (TEOA) and ascorbic acids were used as electron donors in the present study. Luminescence titration experiments of the complexes were performed in acetonitrile as a function of incremental addition of TEOA and ascorbic acid (Figure 7 and Figures S29 and S30, Supporting Information). Quenching of luminescence occurs in each case, although the extent and pattern of quenching depends upon the nature of the complex. For 1 and 2, the Stern−Volmer plots are linear and are consistent with Marcus theory.79 By contrast, the Stern− Volmer plot for complex 3 showed curved upward (Figure S29, Supporting Information). In addition, the emission energy of 3 decreases at higher TEOA concentrations suggesting even a chemical reaction (e.g., ground-state deprotonation). We performed UV−vis absorption titrations of the complexes upon incremental addition of TEOA to check whether any ground-state reaction occurs or not (Figure S31, Supporting Information). It is observed that 1 and 2 do not exhibit any noticeable spectral changes, but 3 exhibits two-step changes upon incremental addition of TEOA with resulting red-shift of the lower energy absorption bands indicating the occurrence of ground-state deprotonation of the NH groups in 3 in the presence of strong base like TEOA. From the values of the excited-state redox potentials of the complexes as well as [TEOA]/[TEOA]·+ (E = 0.82 V vs normal hydrogen electrode (NHE); E = 0.19 V vs Fc/Fc+),49 it appears that quenching of luminescence is due to the oxidation of TEOA to its radical cation [TEOA]·+ induced by the excited states of 1 and 2. Rate constants of the quenching processes (kq = Ksv/τ0) can be estimated from the lifetime data of the complexes, and the values were found to be 7.31 × 108 M−1 s−1 for 1 and 3.06 × 108 M−1 s−1 for 2. In contrast to TEOA, ascorbic acid quenches the excited state of both 1 and 3, and the Stern−Volmer plots are linear in both cases (Figure S30, Supporting Information). In addition, no detectable absorption
Scheme 1
In each step, successive absorption cures pass through one or more isosbestic points. The individual pKa values of the complexes were calculated from the spectrophotometric titration data in each segment with the help of eq 4.80 The calculated pKa value is 8.6 for 1 and 8.3 for 2. By contrast, two pKa values (5.5 for pKa1 and 8.5 for pKa2) are obtained for 3. It is of interest to note the pKa1 is much lower compared with pKa value of both 1 and 2 indicating that initial dissociation of imidazole NH protons occur from the terminal H2pbbzm moiety in 3. A − A0 pH = pK a − log Af − A0 (4) Multi-Channel Anion-Sensing Studies of the Metalloreceptors. Sensing behaviors of the complexes were thoroughly investigated in both neat DMSO and DMSO− H2O (1:100, v/v) media, and tetrabutylammonium (TBA) salts of F−, Cl−, Br−, I−, CN−, HSO4−, AcO−, NO3−, and H2PO4− ions were used as source of the anions in this study. On the one hand, the complexes under present investigation are very much 7632
DOI: 10.1021/acs.inorgchem.6b02937 Inorg. Chem. 2017, 56, 7624−7641
Article
Inorganic Chemistry
Figure 8. UV−vis absorption (a) 1, (b) 2, and (c) 3 and emission (d) 1, (e) 2, and (f) 3 spectral changes in dimethyl sulfoxide−water (1:100, v/v) medium upon addition of different anions as their TBA salts. (inset) Visual color changes for (a) 1, (b) 2, and (c) 3.
Figure 9. UV−vis absorption (a) 1 and (b) 2 and luminescence (c) 1 and (d) 2 spectral changes in dimethyl sulfoxide−water (1:100, v/v) medium upon incremental addition of CN−. (inset) The fit of the experimental absorbance (a) 1 and (b) 2 and luminescence (c) 1 and (d) 2 data to a 1:1 binding profile.
7633
DOI: 10.1021/acs.inorgchem.6b02937 Inorg. Chem. 2017, 56, 7624−7641
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Inorganic Chemistry
Figure 10. Changes in UV−vis absorption (a, b) and luminescence (c, d) spectra of 3 in dimethyl sulfoxide−water (1:100, v/v) medium upon incremental addition of CN−. (inset) The fit of the experimental absorbance (a, b) and luminescence (c, d) data to a 1:1 binding profile.
sensitive toward few anions such as F−, CN−, and AcO−, but they are unselective toward differentiating a particular one in the presence of others in neat DMSO. On the other hand, selectivity greatly increases in predominantly aqueous medium (1:100 DMSO−H2O, v/v). To this end, we will discuss the sensing behaviors of the complexes in 1:100 (DMSO−H2O, v/ v) in the main text from the viewpoint of their probable analytical application, while the behaviors of the complexes toward anions in neat DMSO will be described in Supporting Information (Figures S34−S57). Sensing Behaviors in Mixed Aqueous−Organic Medium. In contrast to DMSO, all the three complexes selectively recognize only CN− in 1:100 DMSO−H2O (v/v). Distinct selectivity of the complexes toward CN− was initially observed through their visual color changes as well as through their optical spectral responses (Figure 8). Absorption and luminescence titrations of the complexes were done on incremental addition of CN− ion to acquire quantitative information regarding receptor−anion interaction process (Figures 9 and 10). Upon gradual addition of CN−, the absorbance of the lowest energy band at 494 nm for 1 and at 496 nm for 2 decreased gradually with small red shift. In contrast to 1 and 2, the absorption spectral changes in 3 occurred through two successive steps, and the extent of change is found to be much higher than both 1 and 2. Similar to absorption spectra, quenching of luminescence of the complexes occurs only with CN− among the studied anions. Moreover, the extent of quenching is much more in 3 compared with both 1 and 2. Luminescence lifetime measurements of the complexes were also done, and the changes in decay profiles as a function of CN− ion are presented in Figure 11. As luminescence lifetime-based detection is much more advantageous over intensity-based method (as lifetimes of the species are independent of variation of light source, photodecaying of the probe material, and changes in the efficiency of the optical system), we are interested to measure the luminescence lifetimes of the complexes upon incremental addition of CN− (Figure 11). It is to be mentioned that
Figure 11. Changes in excited-state lifetime of 1 (a), 2 (b), and 3 (c) in dimethyl sulfoxide−water (1:100, v/v) medium upon incremental addition of CN−. (inset) The lifetime values for (a) 1, (b) 2, and (c) 3.
lifetimes of the complexes measured in (1:100) DMSO−H2O medium are found to be little less compared with those in DMSO due to the solvent effect. On the one hand, lifetime of complex 1 increases, while for 2 it decreases to a small extent upon addition of CN−. On the other hand, the extent of decrease of lifetime in 3 is much more with CN−. Absorption 7634
DOI: 10.1021/acs.inorgchem.6b02937 Inorg. Chem. 2017, 56, 7624−7641
Article
Inorganic Chemistry
Table 3. Equilibrium Constantsa,b (K, M−1) for 1−3 towards F−, CN−, AcO−, and H2PO4− Ions in Dimethyl Sulfoxide and CN− in Water−Dimethyl Sulfoxide (100:1 v/v) Medium at 298 K in dimethyl sulfoxide (1) from Abs
from Emi
−1
−
F CN− AcO− H2PO4−
(2) −1
−1
−1
−1
K1 (M )
K1 (M )
2.71 × 10 2.19 × 106 1.04 × 105
× × × ×
1.24 × 10 1.07 × 106
1.10 × 10 1.14 × 106
−1
CN
from Abs
K1 (M )
6
1.26 1.44 2.01 3.41
6
10 106 105 105
K1 (M )
6
from Emi −1
5.03 × 10 3.16 × 106 1.49 × 106 2.20 × 105 6.18 × 105 in water−dimethyl sulfoxide (100:1 v/v) medium 6
(1)
a
from Emi
K1 (M )
from Abs −
(3)
from Abs
6
K2 (M )
K1 (M )
K2 (M−1)
1.25 × 10 1.49 × 106 8.24 × 105
× × × ×
1.99 × 106 1.99 × 106 8.02 × 105
6
(2) from Emi
4.12 3.74 1.20 6.03
6
10 106 106 105
(3)
from Abs
from Emi
K1 (M )
K1 (M )
K1 (M )
K2 (M )
K1 (M )
K2 (M−1)
3.11 × 10
1.27 × 10
2.09 × 10
1.23 × 10
5.33 × 10
2.04 × 10
6.82 × 10
3.67 × 105
5
−1
5
−1
from Emi
K1 (M ) 5
−1
from Abs
K1 (M ) 5
−1
−1
5
−1
5
−1
5
b
t-Butyl salts of the respective anions were used for the studies. Estimated errors were less than 15%.
are due to the presence of six imidazole NH protons in two different chemical environments. Determination of pKa values indicates that the NH protons associated with terminal H2pbbzim ligands in 3 are more acidic and therefore deprotonated first, while the NH groups associated with tpyBPhBzimH2-tpy bridge deprotonated in the second step. To check the possibility of anion-induced removal of the NH protons in the complexes, we also performed the absorption and luminescence titration measurements upon incremental addition of tetrabutylammonium hydroxide (TBAOH) (Figure S64, Supporting Information). Similarity of the spectral patterns of the complexes with OH− with those of F− and CN− indicates successive removal of the imidazole NH protons in the complexes. 1H NMR titration experiments were also performed to check the mode of interactions between the complexes (1 and 3) and the anions. DMSO-d6 solution of 1 and 3 was titrated as a function of F− ion (Figures S65 and S66, Supporting Information). For 1, the peak at δ = 14.58 ppm due to NH protons of the bridge gradually decreased in intensity and eventually disappeared upon addition of 12.0 equiv of F−. In case of 3, the peak at 15.18 ppm due to more acidic NH protons on H2pbbzim also decrease in intensity and finally disappeared upon addition of 5.0 equiv of F−, but at the same time, a new signal emerged at δ = 14.10 ppm due to the NH protons of the bridge. This new peak is again disappeared upon addition of 12.0 equiv of F−. In both cases, the integrated proportion of the NH peaks decreased gradually with the position of the peak remaining almost invariant, indicating a deprotonation of the N−H units in the complexes. C−H proton signals in H2pbbzim (H21, H22, H23, H24, H25, and H26) also get progressively upfield-shifted on gradual addition of F− to the solution of 3. Anion-induced deprotonation led to increase the electron density in the aromatic backbone adjacent to the imidazole moiety resulting in definite upfield shifts of selected C−H resonances. In contrast to 3, the CH protons adjacent to the bis-benzimidazole spacer are only slightly affected in the presence of F−. In contrast to neat DMSO medium, all the three complexes are very much selective toward CN− ion in DMSO−H2O (1:100 v/v) medium. The inability of the complexes to sense F− ion in aqueous medium is due to the higher free energy of hydration for F− (ΔGh° = −465 kJ/mol) compared with CN− (ΔGh° = −295 kJ/mol).81 Literature reports indicate that CN−
and luminescence titration data were used to evaluate the equilibrium constants (Ks) of the interaction process between the complexes and CN− ion, and the estimated values were found to be in the order of 1 × 105 M−1, which are 1 order of magnitude less compared with those in neat DMSO (Table 3). Detection limit of the complexes toward CN− in water is a very important parameter with regard to their practical applicability. Remarkably, the detection limit, calculated from luminescence titration data of the complexes toward CN−, was found to lie in range between 6.4 × 10−8 M and 2.41 × 10−8 M (Figures S58− S63, Supporting Information). Thus, the estimated values of detection limit offered by the complexes are even lower than 0.2 ppm, which is the permissible level as recommended by Environment Protection Agency (EPA) for drinking water.71 Selectivity of the complexes toward CN− was checked by recording their absorption and luminescence spectra in the presence of each of the studied competing anions. The color change as well as quenching of luminescence of the complexes was observed only after the addition of CN− to a solution containing a particular complex and the mixture of other studied anions. Taking into consideration the extreme selectivity and sensitivity as well as chromogenic and fluorogenic behavior toward CN− over other anions, the complexes might be useful and promising candidates for practical detection of CN− ion in water. Nature of Interaction between Metalloreceptors and Anions. The results of UV−vis absorption, steady-state, and time-resolved luminescence spectral investigations unequivocally indicate that the complexes strongly interact with selective anions, albeit in different extent, in both organic and mixed aqueous−organic medium. The complexes act as sensors for CN−, F−, AcO−, and to a lesser extent for H2PO4− in pure DMSO medium, but they are unselective for a particular anion in the presence of the others. By contrast, the complexes are highly selective toward CN− in DMSO−H2O (1:100 v/v) medium. 1 and 2 exhibit one-step change, while 3 shows twostep changes with F−, CN−, and AcO− in DMSO. In addition, subtle differences in the spectral patterns are observed among the complexes depending upon the number of imidazole NH protons as well as the electronic nature of the terminal ligand in the complexes. Moreover, for a particular complex, the extent of spectral change was found to vary depending on the nature and basicity of the anions. Two step changes that are observed in 3 7635
DOI: 10.1021/acs.inorgchem.6b02937 Inorg. Chem. 2017, 56, 7624−7641
Article
Inorganic Chemistry Table 4. Luminescence Spectral Data of the Representative Ru(II)-Terpyridine Complexes
complex 4+
[(tpy)Ru(tpp)Ru(tpy)] [(ttpy)Ru(tpy-tpy)Ru(ttpy)]4+ [(ttpy)Ru(tpy-ph-tpy)Ru(ttpy)]4+ [(tpy)Ru(tpy-E1-tpy)Ru(tpy)]4+ [(tpy)Ru(tpy-E2-tpy)Ru(tpy)]4+ RR1, RR2,RR3, RR4,RR5 [(tpy)Ru(L)Ru(tpy)]4+ RuTnRu (n = 1−5) [(tpy)Ru(4-tpy-2,2′-bpy-4-tpy)Ru(tpy)]4+ [tpy-Ru-L1-Ru-tpy]4+ [tpy-Ru-L2-Ru-tpy]4+ [{(mpt)Ru}2(tpvpt)]2+ [(mpt)2Ru(tpvpvpt)Ru(mpt)2]4+ [Ru(ttp) (tpbp)Ru(ttp)}]2+ ([(tpy)Ru(dpb-NHCO-dpb)Ru(tpy)]4+ [(EtOOCtpy)Ru(tpy-NHCO-tpy)Ru(tpy-NHCOCH3)]4+ [(tpy)Ru(tpy-H2PhImzPy-tpy)Ru(tpy)]4+ [(tpy-PhCH3)Ru(tpy-H2PhImzPy-tpy)Ru(tpy-PhCH3)]4+ [(H2pbbzim)Ru(tpy-H2PhImzPy-tpy)Ru-(H2pbbzim)]4+ [(tpy-NaPh)Ru(tpy-HImzPy)]2+ [(H2pbbzim)Ru(tpy-HImzPy)]2+ [Ru(tpy-HImzPy)2]2+ [(tpy-PhCH3)Ru(tpy-HImzphen)]2+ [(H2pbbzim)Ru(tpy-HImzphen)]2+ [Ru(tpy-HImzphen)2]2+
solvent
λmax em, nm
MeCN MeCN MeCN MeCN MeCN DCM MeCN MeCN MeCN MeCN MeCN MeCN MeCN MeCN MeCN MeCN DMSO DMSO DMSO MECN MECN MECN DMSO MeCN DMSO
826 720 656 722 735 ∼695 738 703−732 680 678 819 675 697 798 756 and 772 676 and 705 664 670 703 657 682 659 668 681 667
quantum yield (ΦRT) 4.7× 10−3 1.1× 10−4 1.4× 10−3 2.1× 10−3 2.5× 10−3 −3.0× 10−3 6.9× 104 0.8 × 105-23.8 × 105 2.2 × 10−4 0.0006 9.0 × 10−3
3.7 × 10−5 9 × 10−6 3.2 × 10−4 0.12 × 10−3 0.38 × 10−3 1.70 × 10−3 4.9 × 10−3 21.7 × 10−3 6.6 × 10−3 1.27 × 10−3 3.26 × 10−3 1.9 × 10−3
lifetime (τRT, ns)
ref
100 570 4 565 720 125−140 340 99−164 5 100 420
