Design of Ru(II) Complexes Based on Anthraimidazoledione

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Design of Ru(II) Complexes Based on AnthraimidazoledioneFunctionalized Terpyridine Ligand for Improvement of RoomTemperature Luminescence Characteristics and Recognition of Selective Anions: Experimental and DFT/TD-DFT Study Debiprasad Mondal, Manoranjan Bar, Shruti Mukherjee, and Sujoy Baitalik* Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700032, India S Supporting Information *

ABSTRACT: In this work we report synthesis and characterization of three rigid and linear rodlike monometallic Ru(II) complexes based on a terpyridine ligand tightly connected to 9,10-anthraquinone electronacceptor unit through phenyl−imidazole spacer. The motivation of designing these complexes is to enhance their excited-state lifetimes at room temperature. Interestingly it is found that all three complexes exhibit luminescence at room temperature with excited-state lifetimes in the range of 1.6−52.8 ns, depending upon the coligand as well as the solvent. Temperature-dependent luminescence investigations indicate that the energy gap between the emitting 3MLCT state and nonemitting metal-centered state 3MC in the complexes increased enormously compared with parent [Ru(tpy)2]2+. In addition, by taking advantage of the imidazole NH proton(s), which became appreciably acidic upon combined effect of electron accepting anthraquinone moiety as well as metal ion coordination, we also examined anion recognition and sensing behaviors of the complexes in organic, mixed aqueous− organic as well as in solid medium through different optical channels such as absorption, steady-state and time-resolved emission, and 1H NMR spectroscopic techniques. In conjunction with the experiment, computational investigation was also employed to examine the electronic structures of the complexes and accurate assignment of experimentally observed spectral and redox behaviors.

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INTRODUCTION Ru(II) complexes derived from pyridine-based ligands play a key role for the development of suitable photosensitizers in light-harvesting devices (LHDs).1−10 The beauty of the polypyridine complexes of Ru(II) is that they absorb light in the visible region of the spectrum giving rise to metal-to-ligand charge-transfer (1MLCT) excited state, which undergoes very fast intersystem crossing to produce a potentially long-lived luminescent 3MLCT state capable of inducing transfer of electron or energy to a suitable acceptor unit in the complex system.2 Although we are still far away in duplicating the structure of PSII in a LHD, significant efforts have been devoted for mimicking its functions. To this end, several systems based on iconic [Ru(bpy)3]2+ unit with long roomtemperature lifetime (∼1 μs) have been designed.2−8 But designing of multinuclear systems based on [Ru(bpy)3]2+ unit gives rise to diasteromeric mixtures.11,12 Although several synthetic techniques have been developed for the synthesis of enantiopure complexes based on [Ru(bpy)3]2+,11,12 attention has been turned to synthesize linear and achiral complexes based on terpyridine (tpy)-type ligands. But the problem with the terpyridine complexes of Ru(II) is their inferior roomtemperature luminescence properties (e.g., [Ru(tpy)2]2+ is © XXXX American Chemical Society

nonluminescent at room temperature with excited-state lifetime (τ) of 0.25 ns),13 which restrict them for their use as suitable photosensitizers. Thus, the excited-state lifetimes of the terpyridine-based Ru(II) complexes must be optimized to utilize them to act as potential sensitizers. To this end, significant efforts have already been given for tailored designing of tridentate polypyridine ligands, which can give rise to Ru(II) complexes with enhanced excited-state lifetimes.14 In most of the strategies, emphasis was given to increase the energy gap between the radiative 3MLCT and nonradiative 3MC states by incorporating electron-accepting moiety on the tpy ligands, introducing aromatic or heteroaromatic moieties, by substituting pyridine units with different heterocycles to enlarge the bite angle of the tridentate ligand or by using cyclometalated ligands.15−28 With regard to our continued efforts for developing terpyridine-based Ru(II) complexes with enhanced excitedstate lifetimes,28 we recently designed a terpyridyl−phenylimidazole ligand (tpy-HPhImz-Anq) by angular annulation of diaminoanthraquinone and 4′-(p-formylphenyl)-2,2′:6′,2″-terReceived: June 21, 2016

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DOI: 10.1021/acs.inorgchem.6b01483 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry pyridine moieties.29 In this work we utilized this ligand to synthesize a new class of Ru(II)-terpyridine complexes. Despite their important roles as good electron acceptors with desirable thermal and electrochemical stability, relatively little attention has been paid in connection with the development of suitable system by covalent coupling of anthraquinone with Ru(II) poylpyridyl unit. Although a few donor−acceptor systems based on covalent coupling of Ru(bpy) 3 2+ and 9,10anthraquinone are reported in literature,30−38 to the best of our knowledge, there is no such report in combination Ru(tpy)22+ moiety. The rigid coupling between the 9,10anthraquinone and Ru-tpy MLCT chromophore is expected to enlarge the energy gap between the emitting 3MLCT and nonemitting 3MC states leading to improved room-temperature luminescence properties. Herein, we report the synthesis, characterization, steady-state, and time-resolved luminescence properties and electrochemical behaviors of a new family of bistridentate Ru(II) complexes. The photoredox properties of the complexes have been fine-tuned by changing the coligand in the complex (Chart 1). As will be seen, all the complexes exhibit

temperature luminescence properties, the design strategy also offers a number of imidazole NH proton(s), which became appreciably acidic upon combined effect of electron-accepting anthraquinone moiety as well as metal ion coordination. Taking advantage of these acidic NH protons, we exploit in this work detailed anion recognition and sensing behaviors of the complexes in solution as well as in the solid state through different spectroscopic techniques. It should be pointed out that the anion-sensing studies involving Ru(II) complexes derived from tridentate ligands are much less in the literature compared with their bidentate bipyridine or phenanthroline analogues.28,39 By virtue of their versatile geometry and excellent photophysical and redox properties, the metal complexes often offer multiple channels for recognition of selected anions.40−50 The present complexes are found to be highly efficient probes for F−, CN−, and AcO− in acetonitrile without selectivity and highly sensitive and selective probes for CN− in water. More importantly, the detection limits for CN− in water exhibited by complexes are very low and lie in the order of 1 × 10−8 M. A few metalloreceptors that recognize CN− in water often suffer from the lack of selectivity, and also the detection limit is higher than the permissible level (0.2 ppm) for drinking water as recommended by Environment Protection Agency (EPA).51 Finally, in conjunction with the experimental investigations, computation works using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) were also performed to investigate the electronic structures of the complexes as well as proper assignment of experimentally observed absorption and emission bands of the complexes. Emphasis was given to check the parity between the experimentally observed anion-induced changes in the optical properties of the complexes and those obtained by computation investigations.

Chart 1. Molecular Structures of the Complexes

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EXPERIMENTAL SECTION

Materials. 2,2′:6′,2″-Terpyridine (tpy), 1,2-diaminonanthraquinone, RuCl3·xH2O, and NaClO4 were procured from Sigma-Aldrich. Other reagent grade chemicals were purchased from local vendors and used as received. 4′-(4-Methylphenyl)-2,2′:6′,2″-terpyridine (tpyPhCH3), 4′-(p-formylphenyl)-2,2′:6′,2″-terpyridine (tpy-PhCHO),52 and 2,6-bis(benzimidazole-2-yl)pyridine (H2pbbzim)53,54 were prepared by adopting literature methods. [(tpy)RuCl3] and [(H2pbbzim)RuCl3] were synthesized by refluxing 1:1 molar ratio of RuCl3·3H2O and tpy or H2pbbzim in ethanol. Terpyridine−anthraimidazoledione ligand, 2-(4-(2,6-di(pyridine-4-yl)phenyl)-1H-anthra[1,2-d]imidazole6,11-dione (tpy-HPhImz-Anq) was prepared by following the recently reported procedure.29 Synthesis of the Metal Complexes. [(tpy)Ru(tpy-HPhImz-Anq)](ClO4)2 (1). A mixture of Ru(tpy)Cl3 (80 mg, 0.18 mmol) and tpyHPhImz-Anq (100 mg, 0.18 mmol) in ethylene glycol (25 mL) was subjected to stirring at 180 °C for 10 h under argon protection. The resulting solution was then cooled to room temperature and poured into a saturated aqueous solution of NaClO4·H2O (2.0 g in 5 mL of water). A red colored product that precipitated was filtered and purified via silica gel column chromatography eluting with acetonitrile. For further purification, recrystallization was done from MeOH/ MeCN (1:1) mixture in the presence of 5 μM of aqueous 1 × 10−4 M HClO4. Yield: 0.141 mg, (72%). Anal. Calcd for C51H32N8O10Cl2Ru: C, 39.67; H, 2.94; N, 10.20%. Found: C, 39.62; H, 2.91; N, 10.21. 1H NMR data (300 MHz, deuterated dimethyl sulfoxide (DMSO-d6), tetramethylsilane (TMS), δ (ppm; see Figure 1 for atom numbering): 13.8 (s, 1H, NH), 9.60 (s, 2H, (H3′)), 9.16 (d, 2H, J = 8.4 Hz H(6)), 9.11 (d, 2H, J = 8.4 Hz H(15)), 8.85 (d, 4H, J = 7.6 Hz 2H(6) + 2H(8)), 8.69 (d, 2H, J = 8.4 Hz H(7)), 8.56 (t, 1H, J = 8.1 Hz H(16)), 8.32−8.23(m, 3H, 1H(10) + 1H(11) + 1H(13)), 8.18 (d, 1H, J = 8.4

moderately strong luminescence at room temperature with excited-state lifetimes 2 orders of magnitude higher relative to parent [Ru(tpy)2]2+. The redox behaviors of the complexes were characterized by a single metal-based oxidation and several ligand-based reduction processes. UV−vis−NIR spectro-electrochemical measurements will also be performed to visualize the extent of charge delocalization in singly oxidized and singly reduced species. Apart from the enhancement of roomB


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Inorganic Chemistry

Figure 1. 1H NMR (300 MHz) spectra of 1−3 in DMSO-d6. Hz H(9)), 8.11−7.97 (m, 6H, 4H(4) + 1H(12) + 1H(14)), 7.57 (d, 2H, J = 5.1 Hz H(3)), 7.46 (d, 2H, J = 5.1 Hz H(3)), 7.32−7.26 (m, 4H, H(5)). Electrospray ionization mass spectrometry (ESI-MS; positive, CH3CN) m/z = 445.13 (100%) [(tpy)Ru(tpy-HPhImzAnq)]2+. [(H2pbbzim)Ru(tpy-HPhImz-Anq)](ClO4)2 (2). Complex 2 was synthesized in the same way as 1, except [(H2pbbzim)RuCl3] was used instead of Ru(tpy)Cl3. Yield 0.147 g (70%). Anal. Calcd for C55H34N10O10Cl2Ru: C, 56.56 ; H, 2.91; N, 10.28%. Found: C, 56.50; H, 2.89; N, 10.25. 1H NMR data (300 MHz, DMSO-d6, TMS, δ (ppm; see Figure 1 for atom numbering): 15.1 (s, 2H, NH(H2pbbzim)), 9.70 (s, 2H, (H3′)), 9.06 (d, 2H, J = 7.8 Hz H(6)), 8.89−8.77 (m, 6H, 2H(7) + 2H(8) + 2H(21)), 8.63 (t, 1H, J = 8.4 Hz H(22)), 8.32−8.25 (m, 3H, H(10) + H(11) + H(13)), 8.19 (d, 1H, J = 7.5 Hz H(9)), 7.99−7.94 (m, 4H, 2H(4) + 1H(12) + 1H(14)), 7.66 (d, 2H, J = 7.5 Hz H(3)), 7.50 (d, 2H, J = 5.1 Hz H(20)), 7.30−7.20 (m, 4H, 2H(5) + 2H(19)), 7.05−6.97 (m, 2H, H(18)), 6.09 (d, 2H, J = 8.7 Hz H(17)). ESI-MS (positive, CH3 CN) m/z = 323.04 (100%) [(H2pbbzim)Ru(tpy-H2PhImz-Anq)]3+ and m/z = 484.11 (28%) [(H2pbbzim)Ru(tpy-HPhImz-Anq)]2+. [Ru(tpy-HPhImz-Anq)2](ClO4)2 (3). A mixture of RuCl3·3H2O (47 mg, 0.18 mmol) and tpy-HPhImz-Anq (200 mg, 0.36 mmol) in 25 mL of ethylene glycol was stirred at 180 °C for 12 h under argon protection. After it cooled, the red solution was poured into a saturated aqueous solution of NaClO4·H2O, and on stirring for ∼5 min, a red microcrystalline compound deposited. The precipitate was collected by filtration, washed with cold water, and purified by column chromatography (silica gel) using acetonitrile as the eluent. On evaporation of the eluents to ∼10 mL, a red microcrystalline

compound deposited. The complex was further purified by recrystallizing from a mixture of MeCN and MeOH (1:5 v/v) in the presence of 5 μM of aqueous 1 × 10−4 M HClO4 (152 mg, Yield: (60%). Anal. Calcd for C72H42N10O12Cl2Ru: C, 61.22; H, 2.97; N, 8.50%. Found: C, 61.19; H, 2.93; N, 8.47. 1H NMR data (300 MHz, DMSO-d6, TMS, δ (ppm; see Figure 1 for atom numbering): 13.8 (s, 2H, NH), 9.61(s, 4H, (H3′)), 9.16 (d, 4H, J = 9.0 Hz H(6)), 8.84 (d, 4H, J = 9.0 Hz H(8)), 8.69 (d, 4H, J = 8.1 Hz H(7)), 8.31−8.23 (m, 6H, 2H(10) + 2H(11) + 2H(13)), 8.17 (d, 2H, J = 8.4 Hz H(9)), 8.10 (t, 4H, J = 6.3 Hz H(5)), 7.99−7.97 (m, 4H, 2H(12) + 2H(14)), 7.60 (d, 4H, J = 5.4 Hz H(3)), 7.31 (t, 4H, J = 7.6 Hz, H(4)). ESI-MS (positive, CH3CN) m/z = 606.12 (100%) [Ru(tpy-HPhImz-Anq)2]2+ and m/z = 404.42 (55%) [Ru(tpy-HPhImz-Anq) (tpy-H2PhImz-Anq)]3+. Caution! Perchlorate salts of the complexes in this work are explosive and should be handled in very small quantities with care. Physical Measurements. The instruments and detailed experimental procedures employed in UV−vis absorption, steady-state, and time-resolved luminescence measurements, electrochemical and spectro-electrochemical investigations and computational methods employing DFT and TD-DFT were provided in the Supporting Information.

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RESULTS AND DISCUSSION Synthesis and Characterization. Anthraimidazoledionebased terpyridine ligand (tpy-HPhImz-Anq) was readily obtained by refluxing a 1:1 mixture of 1, 2-diaminonanthraquinone and 4′-(p-formylphenyl)-2,2′:6′,2″-terpyridine (tpyPhCHO) along with excess sodium acetate in acetic acid.

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Inorganic Chemistry

Figure 2. (left) DFT-optimized geometries for 1−3. (right) Molecular (ChemDraw) structures of the deprotonated form of the complexes (1a−3a).

Heteroleptic [(tpy)Ru(tpy-HPhImz-Anq)] 2+ (1) and [(H2pbbzim)Ru(tpy-HPhImz-Anq)]2+ (2) complexes were readily obtained by refluxing the (1:1) mixture of tpyHPhImz-Anq and appropriate metal precursor in ethylene glycol solvent. The homoleptic complex [Ru(tpy-HPhImzAnq)2]2+, however, was prepared by refluxing RuCl3·3H2O and tpy-HPhImz-Anq in 1:2 molar ratio in ethylene glycol. All the complexes were purified by column chromatography followed by recrystallization from appropriate solvent mixture in slightly acidic condition to retain the imidazole NH protons in the complexes intact. The complexes were thoroughly characterized by their elemental (C, H, and N) analyses, ESI-MS, and 1H NMR spectroscopic measurements, and the results are given in the Experimental Section. 1 H NMR spectra of the complexes in DMSO-d6 are presented in Figure 1. A large number of proton resonances with some overlapping peaks are observed throughout the entire aromatic region (6−10 ppm), and all the peaks were tentatively assigned with the aid of their two-dimensional {1H−1H} COSY NMR spectra (Figures S1−S3, Supporting Information). For the homoleptic complex (3), a single set of signals is observed for each proton giving rise to simplified spectra, while for the heteroleptic complexes (1 and 2) due to nonequivalence of the two ligands, the 1H NMR spectra look somewhat complicated. Both 1 and 3 exhibit a proton signal at ∼13.8 ppm due to the imidazole NH in tpy-HPhImz-Anq moiety, while complex 2 shows only one signal at more downfield position (∼15.1 ppm) in spite of the presence of two different types of NH protons, and this signal corresponds to two NH protons in H2pbbzim unit. The NH proton associated with tpy-HPhImz-Anq is in close proximity of one oxygen atom of the anthraquinone moiety. So there is a finite possibility of strong intramolecular hydrogen bonding between NH proton and O atom. Intermolecular hydrogen bonding between the O

of DMSO-d6 and NH is also a possibility. Thus, these hydrogen-bonding interactions are probably responsible for the appearance of only one broad NH signal in 2. ESI mass spectra of the complexes (1−3) along with the assignment of different peaks are shown in Figures S4−S6 (Supporting Information). The experimentally observed peaks along with their isotopic distribution patterns correspond very well to that of their corresponding simulated spectral patterns. In case of 1, the existence of the positive ion of the type [(tpy) Ru(tpy-HPhImz-Anq)]2+ is evident by the presence of a strong peak at m/z = 445.13. In contrast to 1, existence of doubly charged as well as triply charged species was evident for both 2 and 3. In case of 2, [(H2pbbzim)Ru(tpy-HPhImz-Anq)]2+ and [(H2pbbzim)Ru (tpy-H2PhImz-Anq)]3+ ion appeared at m/z = 484.11 and 323.04, respectively, while for complex 3, the doubly charged as well as triply charged species [Ru(tpyHPhImz-Anq)2]2+ and [Ru(tpy-HPhImz-Anq) (tpy-H2PhImzAnq)]3+ appeared at m/z = 606.12 and 404.42, respectively. Density Functional Theory and Time-Dependent Density Functional Theory Studies. The molecular structures of both protonated (1−3) as well as their NH deprotonated forms (1a−3a) were optimized in acetonitrile by DFT, and their optimized geometries are presented in Figures 2 and S7 (Supporting Information). Ru−N bond distances and N−Ru−N bond angles of the complexes (1−3) and their deprotonated forms (1a−3a) are presented in Table S1 and S2 (Supporting Information). Calculated Ru−N bond length in the complexes vary between 1.95 and 2.12 Å, and the values are found to be very close to those of related bis-tridentate complexes of Ru(II).28a−d The deviation from idealized octahedral geometry of the complexes around the Ru(II) center is evident from the optimized structures as well as from their geometrical parameters. D


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Table 1. Selected Ultraviolet−Visible 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)

key transitions

character

[(tpy-HPhImz-Anq)Ru(tpy)]2+ (1) H→L (56%), H→L+1 (24%), H-3→L+1 (4%), H-2→L + 2 (9%), H-1→L+3 (3%)

S5

467(38 459)

0.75

488(23 500)

S11

397(10 780)

0.12

411(14 300)

S25 S51

332(24 696) 300(61 971)

0.25 0.40

S5

462(61 551)

0.30

S16

400(16 684)

0.17

328(38 500) H-7→L (61%), H→L+5 (28%), H-7→L+1 (4%), H→L+8 (3%) 307 (55 000) H-8→L+2 (66%), H-1→L+7 (24%), H-6→L+2(6%) [(tpy-HPhImz-Anq)Ru(H2pbbzimd)]2+ (2) 490(24 650) H-1→L (30%), H-1→L+1 (20%), H-1→L+2 (12%), H→L+4 (19%), H-3→L+1 (12%), H-2→L+1 (3%), H→L+3 (7%) 398(14 500) H-3→L (79%), H-1→L (8%), H-1→L+1 (8%)

S26 S58

341(55 431) 306(91 097)

0.49 0.22

348(46 500) 315 (54 500)

S5

482(79 737)

1.53

496(39 100)

S21

398(21 672)

0.14

405(26 300)

S43 S92

331 (57 263) 298(79 464)

0.36 0.325

330(61 000) 311(69 000)

H-3→L (75%), H→L+1 (12%), H-4→L (3%), H→L (7%)

H-4→L+2 (96%) H-14→L (25%), H-13→L(47%), H-15→L (5%), H-9→L+3 (2%), H-8→L+3(4%), H-8→L+4 (5%), Ru(tpy-HPhImz-Anq)2]2+(3) H-1→L (44%), H-1→L+2 (9%), H→L+1 (20%), H→L+3 (12%)

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

MLCT, ILCT H-3→L+1 (63%), H→L+3 (15%), H-6→L (7%), H→L+1 (6%) ILCT, MLCT H-10→L + 1(26%), H→L+6 (51%), H-1→L+7 (8%), H→L+10 (6%) π−π* H-4→L + 7 (33%), H-1→L+9 (24%), H-21→L (9%), H-20→L (6%), H-11→L + 4 π−π* (2%), H-8→L+4 (6%), H-4→L+9 (6%)

Table 2. Selected Ultraviolet−Visible Absorption Energy Transitions at the TD-DFT/B3LYP Level for 1a, 2a, and 3a in Acetonitrile excited state

λcal, nm (εcal, M−1 cm−1)

oscillator strength ( f)

λexpt, nm (εexpt, M−1 cm−1)

key transitions

character

S1

529(63 271)

0.99

[(tpy-PhImz-Anq)Ru(tpy)]+ (1a) 500(34 216) H→L (95%)

S18

417(18 312)

0.15

440(16 859)

H-2→L+2 (76%), H-4→L+1 (9%), H-4→L+2 (4%), H-3→L (5%),

S31 S66

355(36 395) 306(112 932)

0.38 0.48

329(33 490) 307(62 774)

H→L+6 (91%), H-3→L+6 (3%), H→L+9 (2%) H-13→L+1 (64%), H-2→L+7 (26%), H-11→L+1 (3%)

S140

248(69 311)

0.31

S5

528(86 510)

0.90

271(41 097) H-17→L+1 (40%), H-11→L+5 (15%), H-4→L+11 (22%), H-16→L (5%) [(tpy-PhImz-Anq)Ru(pbbzimd)]− (2a) 520(21 176) H-1→L (85%), H→L+1 (5%)

S22

409(21 258)

0.10

S41

358(81 226)

0.31

S67

309(116 125)

0.29

S195

234(81 635)

0.51

S1

537(96 073)

2.0

S27

418(11 430)

0.15

S54 S106 S185

355(50 120) 305(109 606) 275(46 526)

0.82 0.38 0.57

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

MLCT, ILCT 410(21 381) H-4→L+2 (15%), H→L+5 (72%), H→L+7 (8%) MLCT, ILCT 355(40 551) H-3→L+3 (60%), H-2→L+6 (31%) π−π*, LLCT 319(56 034) H-15→L(15%), H-14→L (16%), H-11→L (20%), H-11→L+1 (21%), H- π−π* 13→L (9%), 242(55 256) H→L+21 (57%), H-6→L+10(10%), H-3→L+12 (17%) π−π* [Ru(tpy-PhImz-Anq)2]0 (3a) 512(53 866) H→L (49%), H-1→L+1 (44%) ILCT, MLCT 400(117 586) H-4→L+4(74%), H-6→L (6%), H-5→L+1(6%) ILCT, MLCT 333(42 894) H-1→L+7 (24%), H-1→L+8 (20%), H→L+7 (21%), H→L+8 (24%) LLCT 308(72 561) H-21→L+1(25%), H-20→L + 1(34%), H-4→L + 7 (8%) π−π* 275(56 676) H-19→L+5 (40%), H-21→L+4(10%), H-20→L+4 (17%), H-6→L π−π* +11(5%), H-5→L+10 (5%),

Selected frontier molecular orbital sketches are displayed in Figures S8−S12 (Supporting Information), and their compositions are presented in Table S3 (Supporting Information). It was observed that the highest occupied molecular orbitals (HOMOs) in their protonated forms mainly consist of Ru(II) with some finite contribution from phenyl−imidazole part of tpy-HPhImz-Anq. In case of HOMO−3, the contribution of

Ph-imidazole moiety is greater compared with the Ru(II) center. Lowest unoccupied molecular orbital (LUMO) mainly resides on the anthraquinone skeleton of tpy-HPhImz-Anq, while the other LUMOs resides on either the terpyridine moiety of tpy-HPhImz-Anq or on the coligand moiety. On going from protonated to their deprotonated forms, the compositions of HOMOs and LUMOs are found to change E


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

due to electronic transitions within the π orbitals of ph-tpy and anthraquinone moiety. On removal of imidazole NH protons, the calculated lowerenergy bands in the complexes were found to get red shifted, although the extent of shift is dependent on the type of the complex (Table 2). Calculated lowest-energy band for 1a is found at 529 nm, which corresponds to S1 excitation process involving the transition from HOMO mainly localized on phenyl−imidazole and to some extent on RuII to LUMO localized mainly on tpy unit of tpy-HPhImz-Anq. Thus, in contrast to the protonated form, the band at 529 nm has significant ILCT character and a very small amount of MLCT character. The next higher-energy band at 417 nm corresponding to S18 state consists of larger MLCT and smaller ILCT characters (Table 2). By considering the contributions of HOMOs and LUMOs, the calculated lowest-energy bands at 537 nm and next higher at 418 nm for 3a has more or less similar character to that of 1a. By contrast, the lowest-energy band at 528 nm for 2a is mainly MLCT character. Proper assignment of the bands can also be made through electron density difference map (EDDM) and natural transition orbital (NTO) analysis plots of the complexes. The EDDM plots of the lowest-energy band (S5 for 1−3) as well as their deprotonated forms (S1 for 1a and 3a, and S5 for 2a) are presented in Figure S15 (Supporting Information). From EDDM plots, we confidently say that the contribution of the MLCT character to the lowest-energy band is major than ILCT in the protonated forms, while in the deprotonated forms, the contribution of ILCT is much higher than MLCT in both 1a and 3a. Again, for the 2a, the lowest-energy band is of predominant MLCT character. NTO analysis plots are based on the calculated transition density matrices. The unoccupied NTOs are represented by “electron” transition orbital, while the occupied NTOs are represented by “hole” transition orbital.

substantially. In contrast to the protonated forms, the HOMOs of 1a and 3a are principally localized on phenyl−imidazole part of tpy-HPhImz-Anq along with finite contribution of Ru(II) center in some cases. The HOMOs in 2a, however, are principally localized on both pbbzim and Ru(II) center (Table S3, Supporting Information). In contrast to their protonated forms, the lower-energy LUMOs are mainly localized on the tpy unit of either tpy-HPhImz-Anq or the coligands. Distribution of electronic charge density among the complex moieties and their redistribution upon deprotonation of the imidazole NH protons can be visualized by mapping the electrostatic surface potential (ESP) over the total electron density (Figure S13, Supporting Information). Red color in the ESP plots denotes electron-rich area, whereas the blue color denotes the electropositive site of the complexes. Upon deprotonation, the additional negative charge was found to localize mainly over the phenyl−imidazole and anthraquinone units of tpy-HPhImz-Anq as expected. TD-DFT computations were performed on the ground-state optimized geometries of the complexes in acetonitrile medium. The theoretical UV−vis spectral data along with the assignment of different bands were presented in Tables 1 and 2. The involvement of the frontier orbitals in the lowest-energy transition process was shown in Figure 3 and Figure S14 (Supporting Information). The calculated lowest-energy absorption band in their protonated forms appeared at 467 nm for 1, at 462 nm for 2, and at 482 nm for 3. Considering the participation of the relevant molecular orbitals, it is apparent that the charge transfers occur from Ru(II) to antharquinone moiety as well as from ph-tpy part to the anthraquinone unit of same tpy-HPhImz-Anq. Thus, the calculated lowest-energy band can be assigned as a mixture of metal to ligand charge transfer (MLCT) and intra ligand charge transfer (ILCT). The strong absorption bands observed in the UV regions are mainly F


Article

Inorganic Chemistry

complexes are similar showing several strong peaks throughout the entire UV−vis region. The assignment of these bands was made possible by comparing the spectra of related complexes.2,14 Moreover, reasonably good correlation between the calculated and experimental spectra leads us to assign the experimentally observed bands in the complexes (Figure S26, Supporting Information). Each of the three complexes shows a strong peak varying between 488 and 505 nm, depending upon the solvent and the nature of the complex, with ε values in the range of 23 500−54 000 M−1 cm−1, which can be assigned as a combination of 1[RuII(dπ)6]→1[RuII(dπ)5tpy-HPhImz-Anq(π*)1] charge transfer (1MLCT) as well as Ph-tpy to anthraquinone charge transfer (ILCT) transitions. The next higher-energy band noticed in a very narrow range (398−429 nm) for all three complexes can be predominantly assigned as due to intraligand charge transfer from phenyl−imidazole moiety to the anthraquinone unit of tpy-HPhImz-Anq, although some Ru(II)→tpy MLCT character is also mixed therein. Three most intense bands that are noticed around 259 and 338 nm with their ε values lying in the range of 43 000− 80 000 M−1 cm−1 are due to π−π* transitions associated with the coordinated tridentate ligand. Complexes 2 and 3 also exhibit a broad shoulder at lower-energy region (∼580 nm) probably due to spin-forbidden 1[RuII(dπ)6]→3[RuII(dπ)5tpyHPhImz-Anq(π*)1] transition.2,55,56 Moreover, the energy difference between the lowest singlet peak at 490 nm (∼20 408 cm−1, ε = 24 650 M−1 cm−1) and the triplet band at ∼583 nm (∼17 152 cm−1, ε = 1950 M−1 cm−1) is 3256 cm−1 for 2, and the corresponding energy difference between the peak at 496 nm (∼20 161 cm−1, ε = 39 100 M−1 cm−1) and the triplet band at ∼575 nm (∼17 391 cm−1, ε = 4155 M−1 cm−1) is 2770 cm−1 for 3, which are of comparable magnitude to the singlet−triplet splitting energy for [Ru(tpy)2]2+ and the other monotpy complexes of Ru(II).2,55,56 The 1MLCT band of each complex (1−3) is shifted to lower energy compared to the parent [Ru(tpy)2]2+ (474 nm) and [Ru(H2pbbzim)2]2+ (475 nm) complexes. The differences in the MLCT bands between the parent and the complexes under investigation reflect the energy differences of the π* orbitals of the individual ligands. Interestingly, positions of 1MLCT and ILCT band in the complexes are found red-shifted on passing from MeCN to more polar DMSO. The solvent effect is particularly dramatic for 2 due to its greater push−pull characteristics compared with 1 and 3 (Table 3). Another point of interest is that present complexes exhibit very high molar extinction coefficient in the visible region compared to their parent, thus making them useful candidates for light absorption sensitizers in the visible region. Luminescence Spectral Behaviors. Upon excitation at their lowest-energy absorption maxima, all the three complexes exhibit a broad luminescence band at room temperature in both solvents with their maximum lying between 653 (1) and 695 nm (2), depending upon the coligands as well as the nature of the solvent (Figure 4b and Figure S25b, Supporting Information). It is to be mentioned that the luminescence studies of the complexes were performed in aerated conditions. The calculated luminescence maximum was obtained at 628 nm for 1, 642 nm for 2, and 597 nm for 3 in acetonitrile, and the corresponding experimental value at 653 nm for 1, 679 nm for 2, and 658 nm for 3 indicate a reasonably good correlation between them. For each complex, the emission maximum shifts to longer wavelength as the solvent polarity increases from CH3CN to (CH3)2SO. The stabilizing influence of a polar

The NTOs of lowest-energy band of both protonated as well as deprotonated form of the complexes are shown in Figures S16−S18 (Supporting Information). NTO analysis again gives rise to the same conclusion, and mixed MLCT and ILCT character of the band get verified. To get some insight about the luminescence characteristics of the complexes, we also performed unrestricted Kohn−Sham (UKS) calculations directly on the triplet state of the complexes (Figure S19 and Tables S1 and S2, Supporting Information). Selected molecular orbital sketches and their compositions in excited states were presented in Figures S20−S24 and Table S4 (Supporting Information). The variation of the geometrical parameters between the ground and excited states of the complexes is very small. UKS computations give rise to emission maximum at 628 nm for 1, 642 nm for 2, and 597 nm for 3, while for the deprotonated forms the value of emission maximum was found at 734, 750, and 735 nm for 1a, 2a, and 3a, respectively (Table S5). It is of interest to calculate the dipole moments of the complexes in both of their ground and excited states. The calculated dipole moment was 48.02, 49.83, and 9.30 D for 1, 2 and 3, respectively, in the ground state, while the excited-state moment was 56.29 D for 1, 60.65 D for 2, and 17.65 D for 3. The dipolar moment is found to be highest for 2 among the three complexes, which is quite expected due to its larger push−pull characteristics. Moreover, higher values of dipole moment in the excited state indicate greater charge transfer character of the complexes in their excited state. Photophysical Properties of the Complexes. Absorption Spectral Behaviors. The electronic absorption and luminescence spectral behaviors of the complexes were measured in both acetonitrile and dimethyl sulfoxide. UV−vis absorption spectra of 1−3 in acetonitrile are displayed in Figure 4a, while those in DMSO are presented in Figure S25a, (Supporting Information). The relevant spectral data of the complexes are summarized in Table 3, which also contains data for two model complexes. The main spectral features of the

Figure 4. UV−vis absorption (a) and normalized luminescence (b) spectra for 1−3, as well as the tpy-HPhImz-Anq in acetonitrile at room temperature. Excitation wavelength for recording luminescence spectra is 490 nm for 1−3 and 400 nm for tpy-HPhImz-Anq. (c, d) The excited-state decay profiles for 1−3 following pulsed excitation at 450 nm in acetonitrile and in dimethyl sulfoxide, respectively. G


Article

Inorganic Chemistry Table 3. Spectroscopic and Photophysical Data for Complexes 1−3 in Acetonitrile and Dimethylsulfoxide luminescence

at 77a K

at 298 K compounds 1

2

3

solvent

a

λemi, nm

Φ× 10−3

2.80

658

3.1

τ1 = 9.8

528 629

320 ≤0.05

τ = 2.3 τ = 0.25

496(31,400), 429(17,850), 335(48,650), 314(63,500), 277(53,000)

666

1.20

MeCN

488 (23,500),411(14,300), 328(38,500), 307 (55,000), 281 (42,000), 273(43,000)

653

1.20

DMSO

504(24,080),409(17450), 348(46,400), 335(51,000), 320(50,500),287(43,000)

695

6.1

MeCN

490(24,650),398(14,500), 348(46,500), 333 (52,800), 315 (54,500), 283(45,450)

679

11

DMSO

505(54,000), 414(29040), 338(75000), 319(80000), 289(78,000) 496(39,100),405(26,300), 330(61,000), 311 (69,000), 285 70,150), 259(53,300) 404(19108), 290(61728) 476 (17 700) 475(17400)

665

MeCN MeCN MeCN

τ, ns τ1 = 1.6 (77%) τ2 = 7.02 (23%) τ1 = 2.0 (85%) τ2 = 5.2 (15%) τ1 = 9.2 (10%) τ2 = 52.8 (90%) τ1 = 7.1 (53%) τ2 = 32.0 (47%) τ = 9.5

DMSO

MeCN tpy-HPhImz-Anqb [Ru(tpy)2]2+ c [Ru(H2pbbzim)2]2+ d

λabs, nm (ε, M−1 cm−1)

kr × 105 s−1 knr × 107 s−1 1.7

14.2

2.4

19.8

1.2

1.9

3.4

3.0

2.9

10.4

3.2

10.2

λemi, nm

Φ

642

0.19

646

0.25

642

0.18

598

MeOH−EtOH(1:4) glass. bReference 29. cReference 2c. dReference 57.

one NH in 1 and two NH in same chemical environment in 3. Moreover, 1H NMR spectra indicate that the NH protons associated with H2pbbzim in 2 are downfield-shifted compared with the NH proton associated with the imidazole ring of tpyHPhImz-Anq. Thus, the NH protons associated with H2pbbzim in 2 are supposed to be more acidic, and there is finite possibility of fractional dissociation of these NH protons in solvents like MeCN and DMSO. Interestingly, when the lifetime of 2 was recorded in the presence of HClO4, the decay profile was found to be monoexponential in nature with some enhancement of lifetime compared with that of neat MeCN. As will be demonstrated latter, incremental addition of strongly basic anions such as F− and CN− to the acetonitrile solution of 2 leads to lowering of lifetime along with increase of biexponential nature of the decay profiles. Thus, biexponential nature of the decay profile in 2 arises due to presence of both protonated as well as fractional deprotonated forms. Moreover, the contribution and lifetime of first component is found to be much less compared with second component indicating that the first component probably arises due to the fractional NH deprotonated form of 2. The most noteworthy feature of this family of complexes is that they exhibit room-temperature luminescence with excitedstate lifetimes almost 2 orders of magnitude greater than the parent [Ru(tpy)2]2+, which is practically nonluminescent at room temperature. Moreover, the superior luminescence properties of the complexes were achieved without enormous decrease of their excited-state energies. By comparing the luminescence characteristics of terpyridine complexes of Ru(II), the observed luminescence in the complexes are most probably thermally delayed emission and originate from their 3MLCT excited state(s). But the possibility of some small but finite contribution of lowest triplet (3π−π*) state of the coordinated

solvent is understandable in terms of enhanced charge displacement in the emissive excited states (higher excitedstate dipole moments). Similar to the absorption spectral profiles, the solvent effect is particularly dramatic for 2 with regard to the luminescence properties. At 77 K, the emission maxima of the complexes were found to blue-shifted to some extent with a significant enhancement of luminescence intensity and quantum yield (Table 3 and Figure S27, Supporting Information). Moreover, at 77 K, luminescence spectra of the complexes show vibronic progression in the lower-energy region with spacing of ∼1311 cm−1 for 1, ∼1513 cm−1 for 2, and ∼1452 cm−1 for 3, which are similar to those of [Ru(tpy)2]2+ and other related complexes of Ru(II), which arise due to aromatic stretching vibrations of the ligands.55 The zero−zero excitation energy (E00) values of the complexes were estimated from the energies at the intersection point of their absorption and emission bands (Figure S28, Supporting Information). The E00 values thus estimated are 2.32 eV for 1, 2.22 eV for 2, and 2.29 eV for 3. The excited lifetimes of the complexes were recorded via time correlated single photon counting (TCSPC) set up by exciting with 450 nm NanoLED, and the decay profiles are presented in Figure 4c,d. Table 3 summarizes useful luminescence data of 1−3 together with the data for two model complexes. Complex 3 exhibits strictly monoexponential decay with lifetime value of 9.8 ns in MeCN and 9.5 ns in DMSO. Although 1 exhibits biexponential decay, the contribution of second component is very small (∼15−23%), and lifetime of the predominant first component is 2.0 ns in MeCN and 1.6 ns in DMSO. In contrast to 1 and 3, 2 exhibits biexponential decay with lifetimes τ1 = 7.1 ns and τ2 = 32.0 ns in MeCN and τ1 = 9.2 ns and τ2 = 52.8 ns in DMSO. It is of interest to note that complex 2 possesses three imidazole NH protons in two different chemical environments compared to H


Article

Inorganic Chemistry

temperature in all cases, although the extent of change is different. The temperature-dependent luminescence lifetime data of 1−3 were fitted with eq 1 to know the involvement of different deactivation channels in the excited-state decay process.56

anthraquinone moiety to the emitting excited state in the complexes cannot be ruled out. Although the complexes exhibit phosphorescence behaviors, their lifetimes are rather short. The lifetimes of Ru(II) complexes derived from pyridine-based ligands are governed by the nonradiative decay rate constant knr and can be approximated by the equation: knr = knr0 + k′nr, where knr0 is regulated by direct energy transfer from the MLCT state to the ground state, while k′nr term is governed by thermally activated surface-crossing process from the lowest-lying 3MLCT state to a closely lying metal-centered (3MC) level and thus mainly depends on the energy gap ΔE between 3MLCT and 3MC states. The second term in the above equation generally dominates with Ru(II) complexes bearing tridentate ligands due to small ΔE between 3MLCT and 3MC states, which in turn arise due to the unfavorable bite angles of the terpyridine ligands compared with their bipyridine analogues around the octahedrally surrounded Ru(II) ion. As a result, Ru(tpy)2-type complexes exhibit very poor room-temperature luminescence properties along with very short lifetime. For the present complexes, the energy gap between the emitting 3MLCT and nonemitting 3MC state has been supposed to be increased by introducing strong electron-acceptor anthraquinone moiety to the 4′-position of terpyridine moiety through intervening phenyl−imidazole spacer, which in turn stabilizes the emitting excited state substantially. Temperature-dependent luminescence lifetime measurements, in the next section, will also provide support in favor of increased energy difference (ΔE) between the 3MLCT and 3MC in the complexes. Temperature-Dependent Deactivation of the Excited States. To better understand the deactivation dynamics of the energetically lowest luminescent 3MLCT state, both steadystate emission spectra as well as the excited-state lifetimes of the complexes 1−3 were measured as a function of temperature, in the range of 268−353 K. The respective experimental results are presented in Figure 5 and Figure S29 (Supporting Information). Luminescence quantum yield and excited-state lifetime is found to increase gradually with the decrease of

(τ(T ))−1 = (k1 + k 2 exp[ −ΔE/ RT ])/(1 + exp[ −ΔE/ RT ]) (1)

where k1 is the temperature-independent rate constant that is the summation of the radiative (kr) and nonradiative (knr) decay constants from the 3MLCT state to the ground state at low temperature (77 K). Both of these rate constants are usually assumed to be temperature-independent at T > 77 K. The values of kr and knr were derived from the roomtemperature data. The temperature-dependent rate constant k2 represents the decay constant for accessing the 3MC state from 3 MLCT state, and ΔE is the energy difference between these two states.55 Fitting of the experimental kinetics data to the equation allowed us to calculate the values of k1, k2, and ΔE, and the calculated values of ΔE are 2716 ± 44, 3349 ± 59, and 2945 ± 85 cm−1 for 1, 2, and 3, respectively. The estimated k1 values are (4.02 ± 0.10) × 106, (5.61 ± 0.15) × 106, and (4.46 ± 0.20) × 106 s−1 for 1, 2, and 3, respectively, while the value of k2 is (3.64 ± 0.44) × 1011 for 1, (3.58 ± 0.57) × 1012 for 2, and (4.35 ± 0.10) × 1011 s−1 for 3. It is of interest to note that substantial increase in the energy difference (ΔE) between the 3 MLCT and 3MC occurs in all the three complexes compared to those of the reference complex, [Ru(tpy)2]2+ (ΔE = 1500 cm−1), and the increase is probably due to extensive excitedstate delocalization imparted by the anthraquinone moiety. Interestingly, the difference in energy between the emitting 3 MLCT and nonemitting 3MC states is a delicate function of the coligand in the complexes. Moreover, the extent of energy gap is nicely reflected in the measured lifetimes as well as the quantum yields of the complexes. The net outcome is the significant enhancement of the room-temperature lifetimes of a new family of bis-tridentate Ru(II) complexes without substantial lowering of the 3MLCT energy. Thus, favorable absorption and luminescence properties along with relatively long lifetimes at room temperature make these complexes suitable for light-harvesting applications. Electrochemical Behaviors. The redox properties of 1−3 were investigated through cyclic (CV) and square-wave (SWV) voltammetry in acetonitrile using Ag/AgCl reference electrode (Figure 6), and relevant electrochemical data are summarized in Table 4. Each complex shows a reversible couple due to RuII/ RuIII process in the positive potential window. Incorporation of H2pbbzim coligand facilitates the oxidation of Ru(II) center in 2 by ∼0.18 V compared with both heteroleptic tpy-complex (1) as well as homoleptic complex (3) because of its enhanced electron-donating properties. Each complex also exhibits several waves in the negative potential window (up to −2.1 V). Free tpy-HPhImz-Anq exhibits a reversible wave at E1/2 = −1.2 V due to reduction of the anthraquinone moiety and another one at E1/2 = −1.8 V due to reduction of its terpyridine moiety (Figure S30). Thus, the first reduction process that is observed around −1.00 V for each complex is confidently assigned as reduction of the anthraquinone moiety in the complex (Table 4), while the other waves that appeared beyond −1.00 V are due to the reductions of coordinated terpyridine (tpy coligand and/or tpy of tpy-HPhImz-Anq) and H2pbbzim units. Spectro-Electrochemistry. The changes of the electronic structures of 1−3 upon oxidation of the Ru(II) center and

Figure 5. Effect of temperature on steady-state emission (a, b) and excited-state decay profiles (c, d) for 1 and 2 in acetonitrile. The changes of the temperature-dependent lifetime data with the values of different parameters and the corresponding nonlinear fit are shown in the insets (c, d). I


Article

Inorganic Chemistry

successive absorption curves pass through several isosbestic points. Electrochemical reduction of the anthraquinone moiety in 2 at −1.1 V leads to evolution and gradual intensification of a new band at 530 nm at the expense of the original MLCT/ ILCT band at 490 nm. During electrochemical reduction, very weak and broad band in the region of 600−800 nm also evolved. Spectral changes of 1 upon oxidation as well as reduction are basically similar to that of 2 (Figure S31, Supporting Information), although the extent of change is slightly different. In case of complex 3, oxidation of the Ru (II) center leads to decrease of both mixed MLCT/ILCT as well as ILCT band, but no absorption band was found in the longer wavelength region as was observed with both 1 and 2. Reduction of 3 to 3−, however, leads to gradual evolution and augmentation of the intensity of the band in the longer wavelength region, which is similar to those of both 1 and 2, although the band position and the extent of intensity enhancement is different. Electrochemical reduction of free tpy-HPhImz-Anq at −1.2 V leads to red-shift of the lowest-energy band at 404 nm with gradual increase of absorption intensity and ultimately gives rise to an intense peak with its maximum at ∼465 nm (Figure S32, Supporting Information). Absorption titration experiment performed upon incremental addition of strongly basic anions (such as F− and CN−) also showed red shift of the band at 404 to 480 nm in acetonitrile. It is suggested that the imidazole N− H initially forms N−H···F− or N−H···CN− type of adduct, and in the presence of excess anions, the N−H bond finally split and formed deprotonated species, which in turn is responsible for the red shift of the band at 404 nm. Thus, spectroelectrochemical reduction of tpy-HPhImz-Anq gives rise to the deprotonation of tpy-HPhImz-Anq. Similar to the behavior of free tpy-HPhImz-Anq, spectroelectrochemical reduction at the anthraquinone center of the complexes also results in a red shift of their MLCT/ILCT bands (Figure 7 and Figure S31, Supporting Information). Moreover, UV−vis titration upon incremental addition of F− or CN− led to significant red shift of the mixed MLCT/ILCT band in all three complexes. Thus, similarity in the trend of their absorption spectral profiles in both experiments indicates that in the 1e-reduced form, the imidazole NH proton gets dissociated. Spin-density plots of the one-electron oxidized and oneelectron reduced forms of the complexes are displayed in Figure S33 and S34, respectively (Supporting Information), and the Mulliken spin density distributions among different moieties in the complexes are given in Table S6 (Supporting Information). The spin density is found to localize on the metal center upon oxidation in each case (0.86 for 1, 0.81 for 2, and 0.86 for 3). By contrast, in one-electron reduced forms, the spin density is exclusively localized in the anthraquinone moiety of tpy-HPhImz-Anq. Thus, the proposition that the oxidation is Ru(II)-centered while the first reduction is anthraquinonecentered is also verified by the computational study.58−60 TDDFT calculations showed that the lowest-energy band in the protonated form of the complexes arises principally from the charge transfer from Ru(II) to the anthraquinone moiety of tpy-HPhImz-Anq along with some contribution of phenyl− imidazole to anthraquinone CT. But in their deprotonated forms, the LUMO is switched from the anthraquinone to predominantly terpyridine moiety of tpy-HPhImz-Anq, and the lowest-energy band is originated from the charge transfer from

Figure 6. Cyclic and square-wave voltammograms of 1 (a), 2 (b), and 3 (c) in acetonitrile at a scan rate of 100 mV/s showing both oxidation and reduction processes. The reference electrode was Ag/AgCl.

Table 4. Redox Dataa for 1−3 Along with Two Reference Complexes and tpy-HPhImz-Anq in Acetonitrile compds

E1/2(ox), V

Eb1/2(red), V

1 2 3 tpy-HPhImz-Anq Ru(tpy)2]2+ c Ru(H2pbbzim)2]2+ d

1.30 1.13 1.28

−1.02, −1.31, −1.51 −1.04, −1.45, −1.83, −1.861 −1.01, −1.30, −1.49, −1.91 −1.2, −1.8 −1.29, −1.54 −1.40, −1.70

1.30 0.76

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 SWV using glassy carbon electrode. cReference 2c. dReference 57.

reduction of anthraquinone moiety were investigated by spectro-electrochemical measurements in the spectral range of 300−1200 nm, and the relevant spectral changes are displayed in Figure 7 for 2 and in Figure S31 (Supporting Information)

Figure 7. Spectro-electrochemical changes for 2 during oxidation (a) at 1.15 V and reduction (b) at −1.1 V in acetonitrile.

for 1 and 3. Upon electrochemical oxidation of 2 to 2+, the absorbance of the mixed MLCT/ILCT band at 490 nm gradually decreased, and at its expense a new broad absorption band evolved and gradually intensified in the wavelength range of 620−1050 nm probably due to LMCT (Figure 7a). The intensity of the ILCT band at ∼400 nm is also found to increase gradually, and during the electrochemical oxidation, J


Article

Inorganic Chemistry Ru(II) to the tpy moiety of tpy-HPhImz-Anq along with phenyl−imidazole to tpy CT. Anion-Sensing Studies of the Metalloreceptors through Different Spectroscopic Tools. Sensing Studies in Organic Medium. The recognition and sensing behaviors of the complexes were investigated in acetonitrile. On addition of 10 equiv of each of F−, Cl−, Br−, I−, CN−, HSO4−, AcO−, NO3−, and H2PO4− considerable color changes were observed with F− and CN−, while in the presence of AcO− and H2PO4− small color change was observed (Figure 8). Thus, the anion-

Figure 9. Changes in UV−vis absorption and luminescence spectra of 1 in acetonitrile upon incremental addition of CN− (a and b, respectively) and H2PO4− (c and d, respectively). (insets) The fit of the experimental absorbance and luminescence data to a 1:1 binding profile.

Figure 8. UV−vis absorption and emission spectral changes of 1 (a and c, respectively) and 2 (b and d, respectively) in acetonitrile upon addition of different anions as their tetrabutylammonium (TBA) salts. (a, b insets) The visual color changes.

specific color changes make the complexes suitable colorimetric anion sensors. The absorption maximum at 488 nm for 1 and 496 nm for 3 remains practically unaltered with Cl−, Br−, I−, NO3−, and HSO4−, while in the presence of AcO− and H2PO4− small changes are observed. F− and CN−, however, led to red shift of the band from 488 to 500 nm for 1 and from 496 to 512 nm for 3 (Figure 8a and Figure S35a, Supporting Information). The extent of red shift is much greater for 2 (490 to 520 nm) compared with both 1 and 3 nm in the presence of F− and CN− (Figure 8b). In addition, the sensing behavior of 2 is found to be remarkably different compared with both 1 and 3, as 2 contains three imidazole NH protons in two different chemical environments. Absorption titrations were performed to gather quantitative information about receptor−anion interaction event. UV−vis spectral changes upon gradual addition of selected anions are displayed in Figures 9a,c, 10a,b, S36a,c, S37a,b, S38a,b, and S39a−c (Supporting Information). One-step change occurs for both 1 and 3 with F−, CN−, and AcO−, although the extent of change depends on the nature of the anions. Incremental addition of CN−, F−, or AcO− leads to gradual increase of absorbance of the band at ∼500 nm along with red shift for 1 and 3 (Figures 9a, S36a,c, and S39a−c, Supporting Information). In contrast to both 1 and 3, occurrence of twostep changes with CN− and F− and one-step change with AcO− and H2PO4− are evident for 2 (Figure 10a,b and Figures S37a,b and S38a,b, Supporting Information). On gradual addition of F− and CN−, the absorption band at 490 nm gets red-shifted to 511 nm in the first step (up to 1 equiv), and further addition

Figure 10. Changes in UV−vis absorption (a, b) and luminescence (c) spectra as well as excited-state lifetime (d) of 2 in acetonitrile upon incremental addition of CN−. (a−c, insets) The 1:1 binding profile. (d, inset) The lifetime values.

(up to 3 equiv) leads to shifting of the band to 520 nm. For each complex, absorption lines are found to pass through several well-defined isosbestic points. Most probably two NH protons associated with H2pbbzim moiety in 2 dissociate initially in one step with F−, AcO−, CN−, and H2PO4−, whereas the NH proton attached to tpy-HPhImz-Anq dissociates in the second step in the presence of excess F− and CN−. It is to be noted that addition of excess H2PO4− leads to the precipitation of the resulting species for each complex. The extent of interaction can be judged by evaluating the equilibrium constants associated with the NH deprotonation from the titration data, and the values are given in Table 5. As already observed incorporation of anthraquinone moiety in the terpyridine ligand induces moderately strong roomtemperature luminescence in the present complexes, which in K


Article

Inorganic Chemistry

Table 5. Equilibrium Constantsa,b K for 1−3 towards F−, CN−, AcO−, and H2PO4− Ions in Acetonitrile and CN− in Water− Acetonitrile (100:1 v/v) Medium at 298 K in acetonitrile medium (1)

(2)

from Abs anion F− CN− AcO− H2PO4−

−1

K1, M 1.79 × 106 1.64 × 106 8.06 × 105 1.11 × 104

from Emi −1

K1, M 1.48 × 106 1.01 × 106 8.9 × 105 1.20 × 104

−1

CN a

−

K1, M 1.11 × 105

from Emi

−1

−1

K1, M K2, M 3.78 × 106 1.83 × 106 3.99 × 106 1.31 × 106 6 1.92 × 10 7.69 × 105 in water−acetonitrile (100:1 v/v) medium

(1) from Abs

(3)

from Abs

−1

K1, M 2.50 × 106 1.60 × 106 1.41 × 106 8.81 × 105

from Abs

−1

K1, M 1.15 × 105

(3)

from Abs −1

K1, M 1.77 × 105

from Emi −1

K2, M 1.53 × 105

K1, M−1 1.22 × 106 1.10 × 106 1.02 × 106

K1, M 1.72 × 106 1.07 × 106 1.01 × 106

(2) from Emi

from Emi

−1

−1

K1, M 1.66 × 105

from absorption −1

K2, M 1.29 × 105

−1

K1, M 1.22 × 105

from Emi K1, M−1 1.77 × 105

t-Butyl salts of the respective anions were used for the studies. bEstimated errors were

Design of Ru(II) Complexes Based on Anthraimidazoledione

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