The Important Role of Coordination Geometry on Photophysical

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

The Important Role of Coordination Geometry on Photophysical Properties of Blue-Green Emitting Ruthenium(II) Diisocyano Complexes Bearing 2‑Benzoxazol-2-ylphenolate Pui-Yu Ho,† Shun-Cheung Cheng,‡ Shek-Man Yiu,‡ Vonika Ka-Man Au,† Jing Xiang,*,§ Chi-Fai Leung,*,† and Chi-Chiu Ko*,‡

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†

Department of Science and Environmental Studies, The Education University of Hong Kong, 10 Lo Ping Road, Tai Po, New Territories, Hong Kong, China ‡ Department of Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China § School of Chemical and Environmental Engineering, Yangtze University, JingZhou, China S Supporting Information *

ABSTRACT: A series of blue-green emitting RuII diisocyano complexes containing 2-benzoxazol-2-ylphenolate (PBO) have been prepared. The complexes were isolated under varied reaction conditions in two isomeric forms, i.e., trans,trans,trans- (1) and cis,trans,cis- (2), with varied ligand coordination geometry above the RuII center. The photoluminescence of the isomeric complexes has been compared and tuned by the systematic variation of the electronic properties of the isocyanides. The cis,trans,cisisomers exhibit structureless emission in the blue-green region (471−517 nm) upon excitation at λex > 400 nm in dichloromethane solution at room temperature. Both isomeric forms show similarly structured greenish emission at 499−523 nm on excitation at λex > 355 nm in a methanol/ ethanol (4:1) glassy medium at 77 K. On careful comparison with the corresponding absorption and electrochemical data, it is suggested that the solution emission of the cis,trans,cis- isomers (2) at room temperature is originated from the metal-to-ligand charge transfer (MLCT), while a ligand-centered (LC) parentage is assigned for the emission in a glassy state for both isomeric forms. In line with the above experimental results, DFT calculation demonstrates the change in the nature and relative energy of the HOMOs and LUMOs with respect to the varied ligand coordination geometry and π-accepting ability of the isocyanides.

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INTRODUCTION

In search of a blue emitter for the above electroluminescence devices, cyclometalated Ir complexes were in particular widely studied,48−53 but those of other transition metals remained much less explored.54−58 Luminescent complexes of group 3 elements and Zn-containing 2-benzoxazol-2-ylphenolate (PBO) and their photochemical applications have recently attracted much interest.59−69 However, the photophysical properties of their heavier transition-metal complexes, apart from related cyclometalated Ir complexes, are far less reported.70−72 We herein report the synthesis and photophysical properties of Ru(II) diisocyano complexes Ru(PBO)2(RNC)2. The complexes were obtained in the trans,trans,trans- (1) and cis,trans,cis- (2) isomeric forms, by the reaction of Ru(PBO)2(PPh3)2 with excess isocyanides (RNC; Scheme 1). Both isomeric forms exhibit blue-green photoluminescence between 499 and 523 nm in 77 K glass with emission lifetimes of 4.8−96 μs when excited (λex > 355 nm), whereas the cis,trans,cis- isomers also show blue to blue-

The development of metal-based photochemical molecular devices1 such as dye sensitized solar cells (DSSCs),2−5 organic light emitting diodes (OLEDs),6−14 light-emitting electrochemical cells (LECs),15−19 and photocatalysts17−25 has drawn much attention and fueled studies on the photophysical properties of heavier transition-metal complexes, particularly of Au,26−28 Ir,29−34 Os,35−37 Ru,37−39 and Pt40,41 modified with various ligands, which have theoretical internal EL quantum efficiencies (ηint) of nearly 100%.9,14,33 Extensive efforts were put into understanding the factors for the control and tuning of excited-state properties of transition metal emitters, and the effects of a more octahedral coordination environment in emitting metal complexes have particularly drawn much attention in recent years.42−44 However, the influence of ligand coordination geometry is still less considered and investigated, though the geometric arrangement of ligands above the metal centers should, in principle, play an important role in determining the relative energy of molecular orbitals and thus the properties of the excited states.45−47 © XXXX American Chemical Society

Received: February 26, 2019

A

DOI: 10.1021/acs.inorgchem.9b00560 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1. Synthesis of Compounds 1a−1e and 2a−2ea

a

(i) EtOH with 2 equiv 2,6-lutidine; (ii) EtOH with 2.2 equiv RNC; (iii) EtOH or toluene with 10 equiv RNC.

green emission peaking at 471−517 nm (τo < 10 μs) in dichloromethane solution at room temperature upon excitation (λex > 400 nm). Their photoexcited states and electrochemical properties were characterized by UV−vis spectroscopy and cyclic voltammetry, as well as by complementary DFT calculation. The change in ligand coordination geometry in the two isomeric forms is suggested to influence their respective excited state and thus emission properties by varying the nature and relative energy of LUMOs and HOMOs.

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6.95 (d, J = 8.8 Hz, 4H, MeOPhNC), 6.89 (d, J = 8.2 Hz, 2H, PBO), 6.75 (d, J = 8.8 Hz, 4H, MeOPhNC), 6.57 (s, 2H, PBO), 3.76 (s, 6H, MeOPhNC). 13C NMR (101 MHz, CD2Cl2): δ 160.57, 159.36, 149.66, 149.46, 143.30, 132.52, 130.69, 128.58, 127.76, 124.42, 122.92, 122.82, 120.92, 119.70, 114.30, 113.29, 110.70, 109.95, 55.44. IR (KBr, cm−1): 2087 ν(CN). Anal. Calcd for C42H30N4O6Ru: C, 64.03; H, 3.84; N, 7.11. Found: C, 64.25; H, 3.59; N, 6.72. UV/vis (CH2Cl2) λmax, nm (ε, M−1 cm−1): 326 (31800), 445 (4300). ESIMS: m/z 787.6 [M]+. HRMS calcd for C42H30N4O6Ru [M]+: m/z 788.1209. Found: m/z 788.1204. trans,trans,trans-[Ru(PBO)2(PhNC)2] (1b). Yield: 18.8 mg (45.1%). 1 H NMR (400 MHz, (CD2Cl2): δ 8.57 (d, J = 7.1 Hz, 2H, PBO), 7.94 (dd, J = 8.0, 1.3 Hz, 2H, PBO), 7.61 (d, J = 7.7 Hz, 2H, PBO), 7.41 (dtd, J = 21.1, 7.5, 1.2 Hz, 4H, PBO), 7.33−7.19 (m, 8H, PBO + PhNC), 7.08−6.97 (m, 4H, PhNC), 6.91 (d, J = 8.5 Hz, 2H, PBO), 6.58 (t, J = 7.2 Hz, 2H, PBO). 13C NMR (101 MHz, CD2Cl2): δ 173.19, 160.66, 149.66, 143.22, 137.29, 132.66, 129.18, 128.64, 128.47, 128.03, 126.45, 124.49, 124.46, 122.84, 119.66, 113.38, 112.72, 110.00. IR (KBr, cm−1): 2080 ν(CN). Anal. Calcd for C40H26N4O4Ru: C, 66.02; H, 3.60; N, 7.70. Found: C, 66.33; H, 3.37; N, 7.37. UV/vis (CH2Cl2) λmax, nm (ε, M−1 cm−1): 331 (24400), 444 (4200). ESI-MS: m/z 728.2 [M]+. HRMS calcd for C40H26N4O4Ru [M]+: m/z 728.0998. Found: m/z 728.0994 trans,trans,trans-[Ru(PBO)2(4-ClPhNC)2] (1c). Yield: 29.6 mg (64.8%). 1H NMR (400 MHz, CD2Cl2): δ 8.51 (dd, J = 7.9, 0.8 Hz, 2H, PBO), 7.90 (d, J = 7.6 Hz, 2H, PBO), 7.63−7.54 (m, 2H, PBO), 7.39 (ddt, J = 14.0, 7.4, 3.9 Hz, 4H, PBO), 7.28−7.15 (m, 6H, PBO + ClPhNC), 6.97−6.81 (m, 6H, PBO + ClPhNC), 6.56 (s, 2H, PBO). 13C NMR (75 MHz, CD2Cl2): δ 168.06, 160.57, 149.59, 147.15, 143.08, 134.17, 132.79, 129.45, 128.70, 127.79, 126.48, 124.58, 124.55, 123.87, 122.78, 119.55, 113.52, 110.08. IR (KBr, cm−1): 2069 ν(CN). Anal. Calcd for C40H24Cl2N4O4Ru·H2O: C, 58.97; H, 3.21; N, 6.88. Found: C, 59.16; H, 3.25; N, 6.93. UV/vis (CH2Cl2) λmax, nm (ε, M−1 cm−1): 337 (25000), 449 (4600). ESIMS: m/z 797.2 [M + H]+. HRMS calcd for C40H24Cl2N4O4Ru [M + H]+: m/z 797.0295. Found: m/z 797.0250. trans,trans,trans-[Ru(PBO)2(2,6-Cl2PhNC)2] (1d). Yield: 27.1 mg (54.6%). 1H NMR (600 MHz, CD2Cl2): δ 8.76 (d, J = 7.8 Hz, 2H, PBO), 7.90 (d, J = 7.9 Hz, 2H, PBO), 7.63−7.57 (m, 2H, PBO), 7.40 (dt, J = 14.6, 7.0 Hz, 4H, BPO), 7.26 (d, J = 8.2 Hz, 4H, Cl2PhNC), 7.17−7.11 (m, 4H, PBO + Cl2PhNC), 6.85 (d, J = 7.9 Hz, 2H, PBO), 6.51 (t, J = 7.5 Hz, 2H, BPO). 13C NMR (151 MHz, CD2Cl2): δ 168.99, 157.30, 152.12, 149.51, 143.22, 132.31, 128.81, 128.65, 127.97, 125.70, 124.35, 124.31, 123.26, 120.50, 117.78, 113.57, 111.26, 109.73. IR (KBr, cm−1): 2065 ν(NC). Anal. Calcd for

EXPERIMENTAL SECTION

Reagents and Materials. 2-(2-Hydroxyphenyl)benzoxazole (HPBO) was obtained from Acros. Ruthenium(III) chloride hydrate and triphenylphosphine were purchased from Strem Chemical Company. [Ru(PPh3)3Cl2]73,74 and isocyanide ligands75 were synthesized according to literature procedures. All other reagents and organic solvents were of analytical grade and were used as received. Ru(PBO)2(PPh3)2. [Ru(PPh3)3Cl2] (480 mg, 0.5 mmol) was suspended in ethanol (30 mL) under an argon atmosphere. To the suspension, HPBO (212 mg, 1 mmol) and 2,6-lutidine (0.23 mL, 2 mmol) were added. The mixture was then heated to reflux for 1 h. The reaction mixture was cooled to room temperature. The dark reddish-purple crystalline solid was collected by filtration. The solid was washed with three portions of ethanol (2 mL) and then with diethyl ether (2 mL) and dried in the air. Yield: 0.348 g (66.4%). Anal. Calcd for C62H46N2O4P2Ru: C, 71.19; H, 4.43; N, 2.68. Found: C, 70.76; H, 4.61; N, 2.44. ESI-MS: m/z 1046 [M]+. trans,trans,trans-Ru(PBO)2(RNC)2 (1a−1e). The isocyanide ligand (0.126 mmol) was added to a suspension of Ru(PBO)2(PPh3)2 (60 mg, 0.057 mmol) in ethanol (15 mL) under an argon atmosphere. The resulting mixture was refluxed for 4 h, during which the solution gradually turned orange, and a reddish-orange solid was formed. The solid was collected by filtration and was purified by column chromatography on silica gel using dichloromethane as the eluent. The first major band was collected and dried in vacuo. Analytically pure complexes were obtained as reddish orange microcrystalline solids by recrystallization from dichloromethane/n-hexane. trans,trans,trans-Ru(PBO)2(4-MeOPhNC)2 (1a). Yield: 23.5 mg (52.0%). 1H NMR (400 MHz, (CD2Cl2): δ 8.56 (d, J = 7.8 Hz, 2H, PBO), 7.93 (d, J = 7.6 Hz, 2H, PBO), 7.60 (d, J = 7.8 Hz, 2H, PBO), 7.40 (dt, J = 15.1, 7.3 Hz, 4H, PBO), 7.22 (t, J = 7.2 Hz, 2H, PBO), B

DOI: 10.1021/acs.inorgchem.9b00560 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

cis,trans,cis-[Ru(PBO)2(2,6-Cl2PhNC)2] (2d). Yield: 18.9 mg (38.1%). 1H NMR (400 MHz, CD2Cl2): δ 8.18−8.11 (m, 2H, PBO), 7.84 (dd, J = 8.1, 1.7 Hz, 2H, PBO), 7.57−7.50 (m, 2H, PBO), 7.33−7.24 (m, 4H, PBO), 7.19 (d, J = 8.2, 4H, Cl2PhNC), 7.07−6.95 (m, 4H, PBO + Cl2PhNC), 6.52−6.40 (m, 4H, PBO). 13C NMR (101 MHz, CD2Cl2): δ 177.24, 171.35, 162.77, 149.87, 142.44, 134.14, 132.15, 128.81, 128.54, 128.37, 127.45, 125.36, 125.19, 123.26, 119.83, 114.19, 112.19, 110.95. Anal. Calcd for C40H22Cl4N4O4Ru: C, 55.51; H, 2.56; N, 6.47. Found: C, 55.13; H, 2.92; N, 6.10. IR (KBr, cm−1): 2011, 2110 ν(NC). UV/vis (CH2Cl2) λmax, nm (ε, M−1 cm−1): 320sh (16300), 344 (14400), 415 (8900). ESI-MS: m/z 866 [M]+. HRMS calcd for C40H22 N4O4Cl4Ru [M + H]+: m/z 864.9516. Found: 864.9481. cis,trans,cis-[Ru(PBO)2(2,4,6-Br3PhNC)2] (2e). Yield: 25.6 mg (37.2%). 1H NMR (300 MHz, CD2Cl2): δ 8.22 (d, J = 8.0 Hz, 2H, PBO), 7.96 (d, J = 7.4 Hz, 2H, PBO), 7.66 (d, J = 11.2 Hz, 6H, PBO + Br3PhNC), 7.50−7.33 (m, 4H, PBO), 7.18 (t, J = 7.2 Hz, 2H, PBO), 6.61 (t, J = 7.2 Hz, 4H, PBO). 13C NMR (75 MHz, CD2Cl2): δ 13C NMR (75 MHz, CD2Cl2): δ 176.53, 171.08, 162.33, 149.51, 141.82, 134.50, 133.92, 129.41, 128.52, 125.06, 124.92, 122.89, 120.95, 120.75, 119.47, 114.01, 111.84, 110.64. Anal. Calcd for C40H20Br6N4O4Ru·(C2H5)2O: C, 41.44; H, 2.37; N, 4.39. Found: C, 41.72; H, 2.62; N, 4.43. IR (KBr, cm−1): 1986, 2098 ν(NC). UV/ vis (CH2Cl2) λmax, nm (ε, M−1 cm−1): 320 (33900) 345 (32600), 415 (20000). ESI-MS: m/z 1202 [M] + . HRMS calcd for C40H20Br6N4O4Ru [M+Li]+: m/z 1202.5786. Found: m/z 1202.5557.

C40H22Cl4N4O4Ru·3H2O: C, 52.25; H, 3.07; N, 6.09. Found: C, 52.12; H, 2.74; N, 5.92. UV/vis (CH2Cl2) λmax, nm (ε, M−1 cm−1): 323 (30200), 354 (22900), 420 (12400). ESI-MS: m/z 865.9 [M]+. HRMS calcd for C40H22 N4O4Cl4Ru [M + H]+: m/z 864.9516. Found: 866.9471. trans,trans,trans-[Ru(PBO)2(2,4,6-Br3PhCN)2] (1e). Yield: 25.9 mg (37.6%). 1H NMR (300 MHz, CD2Cl2): δ 8.75 (dd, J = 6.8, 2.3 Hz, 2H, BPO), 7.86 (dd, J = 8.1, 1.7 Hz, 2H, PBO), 7.63−7.53 (m, 6H, PBO + Br3PhNC), 7.43−7.29 (m, 4H, PBO), 7.10 (ddd, J = 8.6, 6.8, 1.8 Hz, 2H, PBO), 6.82 (d, J = 8.1 Hz, 2H, PBO), 6.52−6.42 (m, 2H, PBO). 13C NMR (75 MHz, CD2Cl2): δ 175.76, 173.32, 161.22, 149.81, 143.63, 134.74, 132.95, 129.20, 128.49, 124.86, 124.73, 123.87, 122.23, 121.74, 121.04, 113.70, 112.36, 110.19. IR (KBr, cm−1): 2041 ν(NC). Anal. Calcd for C40H20Br6N4O4Ru: C, 40.00; H, 1.68; N, 4.66. Found: C, 39.80; H, 1.91; N, 4.82. UV/vis (CH2Cl2) λmax, nm (ε, M−1 cm−1): 329 (28400), 375 (23000), 430 (14400). ESI-MS: m/z 1200 [M]+. HRMS calcd for C40H20Br6N4O4Ru [M +Li]+: m/z 1202.5786. Found: m/z 1202.5557. cis,trans,cis-[Ru(PBO)2(RNC)2] (2a−2e). Complex 2 was synthesized by a similar procedure to that of 1, except an excess of isocyanide (10 equiv) was added, and toluene was used as a solvent for analogues with 2,6-Cl2PhNC (2d) and 2,4,6-Br3PhNC (2e). The reaction mixture was refluxed overnight and resulted in a clear yellow solution. The product was purified by column chromatography on neutral alumina using dichloromethane/ethyl acetate (10:1) as the eluent. The last yellow band was collected and dried in vacuo. Analytically pure complexes were obtained as yellow microcrystalline solids by recrystallization from diethyl ether in the dark. cis,trans,cis-[Ru(PBO)2(4-MeOPhNC)2] (2a). Yield: 18.5 mg (41.0%). 1H NMR (400 MHz, CD2Cl2): δ 8.13−8.03 (m, 2H, PBO), 7.90 (dd, J = 8.0, 1.9 Hz, 2H, PBO), 7.70−7.60 (m, 2H, PBO), 7.49−7.37 (m, 4H, PBO), 7.11−6.98 (m, 6H, PBO + MeOPhNC), 6.85−6.75 (m, 4H, MeOPhNC), 6.51 (ddd, J = 8.0, 6.8, 1.1 Hz, 2H, PBO), 6.33 (dd, J = 8.5, 1.2 Hz, 2H, PBO), 3.77 (s, 6H, MeOPhNC). 13 C NMR (101 MHz, CD2Cl2): δ 170.89, 165.55, 162.37, 158.84, 149.66, 142.56, 133.39, 128.15, 127.06, 124.88, 124.81, 122.89, 122.76, 118.31, 114.46, 113.36, 112.37, 110.85, 55.53. Anal. Calcd for C42H30N4O6Ru·2H2O: C, 61.23; H, 4.16; N, 6.80. Found: C, 61.00; H, 3.82; N, 6.74. IR (KBr, cm−1): 2057, 2119 ν(NC). UV/vis (CH2Cl2) λmax, nm (ε, M−1 cm−1): 325 (41000), 422 (16500). ESIMS: m/z 788 [M]+. HRMS Calcd for C42H30N4O6Ru [M + H]+: m/z 789.1287. Found: m/z 789.1268. cis,trans,cis-[Ru(PBO)2(PhNC)2] (2b). Yield: 20.5 mg (49.1%). 1H NMR (400 MHz, CD2Cl2): δ 8.17−8.09 (m, 2H, PBO), 7.95 (dd, J = 8.0, 1.7 Hz, 2H, PBO), 7.69 (dd, J = 6.3, 2.6 Hz, 2H, PBO), 7.52− 7.42 (m, 4H, PBO), 7.40−7.26 (m, 6H, PhNC), 7.19−7.12 (m, 4H, PhNC), 7.08 (ddd, J = 8.6, 7.0, 1.8 Hz, 2H, PBO), 6.58 (dd, J = 10.9, 3.9 Hz, 2H, PBO), 6.40 (d, J = 8.5 Hz, 2H, PBO). 13C NMR (101 MHz, CD2Cl2): δ 170.87, 167.40, 162.41, 149.67, 142.43, 133.51, 129.72, 129.38, 128.18, 127.74, 125.83, 124.95, 124.91, 122.86, 118.24, 113.51, 112.35, 110.91. Anal. Calcd for C40H26N4O4Ru· 4H2O: C, 60.07; H, 4.28; N, 7.01. Found: C, 60.27; H, 4.00; N, 6.66. IR (KBr, cm−1): 2063, 2127 ν(NC). UV/vis (CH2Cl2) λmax, nm (ε, M−1 cm−1): 324 (31700), 354 (21800), 418 (15200). ESI-MS: m/z 728 [M]+. HRMS calcd for C40H26N4O4Ru [M + H]+: m/z 729.1075. Found: m/z 729.1060. cis,trans,cis-[Ru(PBO)2(4-ClPhNC)2] (2c). Yield: 16.9 mg (37.0%). 1 H NMR (400 MHz, CD2Cl2): δ 8.09−8.01 (m, 2H, PBO), 7.99− 7.91 (m, 2H, PBO), 7.73−7.64 (m, 2H, PBO), 7.51−7.41 (m, 4H, PBO), 7.32 (d, J = 8.6 Hz, 4H, ClPhNC), 7.09 (dd, J = 18.0, 4.9 Hz, 6H, PBO + ClPhNC), 6.59 (t, J = 7.4 Hz, 2H, PBO), 6.41 (d, J = 8.5 Hz, 2H, PBO). 13C NMR (101 MHz, CD2Cl2): δ 170.83, 169.11, 162.43, 149.68, 142.29, 133.66, 133.35, 129.59, 128.22, 127.37, 127.16, 125.07, 124.97, 122.81, 118.05, 113.70, 112.31, 111.02. Anal. Calcd for C40H24Cl2N4O4Ru·H2O: C, 58.97; H, 3.22; N, 6.88. Found: C, 59.16; H, 3.25; N, 6.93. IR (KBr, cm−1): 2047, 2112 ν(NC). UV/vis (CH2Cl2) λmax, nm (ε, M−1 cm−1): 320 (22700), 354 (15000), 418 (9000). ESI-MS: m/z 797 [M + H]+. HRMS calcd for C40H24Cl2N4O4Ru [M + H]+: m/z 797.0295. Found: m/z 797.0273.

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RESULTS AND DISCUSSION Synthesis and Characterization. The reaction of [Ru(PBO)2(PPh3)2] with 2.2 mol equivalents of the isocyanide (RNC) in refluxing ethanol under argon produces the respective trans,trans,trans-[Ru(PBO)2(RNC)2] (1a−1e). The resultant complexes are readily purified by column chromatography on silica gel using dichloromethane as an eluent. Recrystallization from dichloromthane/n-hexane yields the analytically pure compounds as an orange-red microcrystalline solid. The complexes are soluble in nonpolar organic solvents but only slightly soluble in polar organic solvents such as methanol, ethanol, and acetonitrile. By reacting [Ru(PBO)2(PPh3)2] with 10 molar equivalents of RNC overnight under the same conditions, cis,trans,cis[Ru(PBO)2(RNC)2] (2a−2c) were isolated (R = 4-MeOPh, Ph and 4-ClPh) (Scheme 1), while 2d and 2e (R = 2,6-Cl2Ph and 2,4,6-Br3Ph) could only be obtained when toluene was used as the solvent. After column chromatography on neutral alumina and recrystallization from diethyl ether, the analytically pure compounds are obtained as a yellow crystalline solid. The formation of isocyanide complexes was confirmed via IR spectroscopy. The trans,trans,trans-[Ru(PBO)2(RNC)2] (1) shows a single intense νNC in the range of 2087−2041 cm−1 (KBr) [1a (2087 cm−1) > 1b (2080 cm−1) > 1c (2069 cm−1) > 1d (2065 cm−1) > 1e (2041 cm−1)], which is in an order inverse to the π-accepting ability of the isocyanides, i.e., Br3PhNC > Cl2PhNC > ClPhNC ≥ PhNC > MeOPhNC. This is due to stronger π-back bonding between the ruthenium(II) center and the isocyanide ligands with better π-accepting ability. Consistent with the lower symmetry of the cis,trans,cisconformation, IR spectra of 2 show more than one νNC, ranging between 1986 and 2063 cm−1 as well as 2098 and 2127 cm−1, respectively, which exhibit similar decreasing trends of the νNC, as the substituent on the isocyanides becomes more electron-withdrawing. X-ray Crystallography. Single crystals of 1b, 1e, and 2b were obtained by recrystallization from of dichloromethane/nhexane in the dark at room temperature. The structures of the C

DOI: 10.1021/acs.inorgchem.9b00560 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

isomer of complex 1b, the RuII in 2b bears two isocyanides and two PBO ligands dispositioned in a cis,trans,cis- configuration and exhibits a distorted octahedral geometry (Figure 3). The

complexes were determined by X-ray crystallography. The crystal data and experimental details are summarized in Table S1. Selected bond parameters are listed in Table S2. The perspective drawings of 1b and 1e (Figures 1 and 2) show that

Figure 3. Perspective drawing of 2b. Thermal ellipsoids are drawn at 50% probability (hydrogen atoms are omitted for clarity). Figure 1. Perspective drawing of 1b. Thermal ellipsoids are drawn at 50% probability (hydrogen atoms are omitted for clarity).

Ru−N (2.0669(18) and 2.0759(8) Å) and Ru−O (2.0695(16) and 2.0710(15) Å) distances and the O−Ru−N angles (88.35(7) and 88.43(7)°) are comparable with those in 1b. The isocyanides in 2b are positioned trans to the O(PBO). The shorter Ru−C (1.887(2) and 1.900(2) Å) and the longer CN (1.162(3)−1.175(3) Å) distances, as well as the more bent C−CN (161.7(2) and 174.1(2)°) are indicative of a stronger RuII−isocyanide π-back bonding in 2b with respect to the trans,trans,trans- isomer, in which the two isocyanides are competing for the same dπ(Ru) orbital.57,58,76 Electronic Absorption and Luminescent Properties. The absorption spectra for complexes 1 and 2 are shown in Figure 4, and their photophysical data are summarized in Table 1. The electronic absorption spectra of the trans,trans,transisomers (1a−1e) exhibit a high energy absorption band at approximately 330 nm (ε of 104 M−1 cm−1), which is tentatively assigned as the mixed ligand-centered π−π* transitions of the BPO and isocyanides. A tailed broad absorption (ε of 103 M−1 cm−1) is observed at lower energy region (with shoulder at approximately 450 nm) for 1a and 1c. When stronger π-accepting isocyanides (1d and 1e) were introduced, this lower-energy band (peaking at 420−430 nm) became more prominent (ε of 104 M−1 cm−1) and blue-shifted. The blue shift of this absorption band with better π-accepting isocyanide is consistent with the stabilization of dπ(Ru) as revealed by the trends of the metal-centered oxidation potentials (see below) and the assignment of a MLCT [dπ(Ru) → π* (BPO)] transition. Although the MLCT [dπ(Ru) → π*(CNR)] transition is expected to show an opposite trend with the π-accepting properties of the isocyanides, a mixing of the MLCT [dπ(Ru) → π*(CNR)] transition in the low-energy absorption is also suggested, particularly for complexes with better π-accepting ligands as the π*(CNR) orbital becomes more stabilized and closely lying to the π*(PBO). Thus, the lower-energy absorption is attributed to the mixing MLCT [dπ(Ru) → π* (BPO and CNR)] transitions. For the cis,trans,cis- complexes (2a−2e), major absorptions peaking at approximately 320 and 420 nm (ε of 104 M−1 cm−1) are observed in their electronic spectra.

Figure 2. Perspective drawing of 1e. Thermal ellipsoids are drawn at 50% probability (hydrogen atoms are omitted for clarity).

the RuII centers exhibit a distorted octahedral geometry, where the two isocyanides and PBO ligands are arranged in a trans,trans,trans- configuration. The Ru−O (2.066(18), 2.066(4), and 2.079(4) Å) and Ru−N (2.089(2), 2.062(5), and 2.069(5) Å) distances, as well as the N−Ru−O bite angles (88.11(8)°, 87.13(19), and 87.9(18)°) are in agreement with those in other Ru(PBO) complexes, e.g., trans-[Ru(tolyterpy)(HPBO)(Cl)](PF6),70 [Ru(terpy)(HPBO)]Cl,71 and [Ru(bpy)2(PBO)](PF6)].72 The Ru−C(isocyanide) distances (1.994(3), 1.990(6), and 1.993(6) Å), the CN distances (1.153(4), 1.137(8), and 1.138(8) Å) and the CN−C angles (175.8(3), 175.2(7), and 179.4(6)°) are similar to those reported for trans,trans,trans-Ru(Q)2(RNC)2 (1.975(10) Å, 1.157(12) Å, and 174.17(85)°, respectively; Q = 8quinolinolate and R = 2,4,6-Br3Ph).46 As the geometrical D

DOI: 10.1021/acs.inorgchem.9b00560 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 4. Overlaid UV−vis absorption spectra of (a) 1a−1e and (b) 2a−2e in dichloromethane.

maximum is noted upon introduction of electron-withdrawing substituents. A comparison between isomers bearing identical ligand shows that the lowest-energy absorption is considerably blue-shifted in the cis,trans,cis-isomers (2a−2e). The differences of the lowest-energy absorption between the two isomeric forms can be ascribed to the variation of the RuII− isocyanide π-back bonding interactions. For the trans,trans,trans- isomer, the isocyanide ligands are trans to each other, which results in weaker π-back bonding interactions due to competition. While for the cis,trans,cis- isomer, the RuII− isocyanide π-back bonding interactions are significantly stronger because both isocyanide ligands are trans to an anionic phenolate ligand. The RuII−isocyanide π-back bonding interaction would lead to stabilization of the dπ(Ru) orbital, as reflected by a 0.5 V anodic shift of the metal-centered oxidation (see below) and the change of bonding parameters in the X-ray crystal structure (see above), and the destabilization of the π*(CNR) orbital. As a result of these two effects, it would greatly increase MLCT [dπ(Ru) → π*(CNR)] transition energy. As a consequence, the MLCT [dπ(Ru) → π*(CNR)] transition becomes higher lying than that of MLCT [dπ(Ru) → π*(BPO)] despite the fact that MLCT [dπ(Ru) → π*(BPO)] is also raised by the stabilization of the dπ(Ru) orbital. The normalized emission spectra of 1 and 2 in EtOH/ MeOH (4:1) glass at 77 K are shown in Figures 5a and S1. Similar structured emission bands peaking respectively at 499− 523 nm and 500−521 nm were observed for these two isomers. These emission bands exhibit consistently dependence on the π-accepting ability of the isocyanide, such that the

Table 1. Photophysical Properties of Complexes 1 and 2 absorption/nm (ε/M cm−1)a 1a 1b 1c 1d 1e 2a 2b 2c 2d 2e

−1

326 (31800), 445 sh (4300) 331 (24400), 444 sh (4200) 337 (25000), 449 sh (4600) 323 (30200), 354 sh (22900), 420 (12400) 329 (28400), 375 (23000), 430 (14400) 325sh (41000), 422 (16500) 324sh (31700), 354sh (21800), 418 (15200) 320sh (22700), 354sh (15000), 418 (9000) 320(16300), 344sh (14400), 415sh (8900) 320(33900), 345sh (32600),415sh (20000)

emission/nm at 77 Kb (τo/μs)

emission/nm at 298 Ka (τo/ns)

520, 559 sh (9.2) 523, 562 sh (4.8) 519, 554 sh (30.1) 499, 536, 578 (58.3) 499, 535, 577 (95.6) 521, 560, 610 (34.1) 517, 554, 606 (42.9) 516, 552, 604 (47.6) 501, 537, 584 (94.4) 500, 534, 581 (94.2)

sh sh sh

517 (2.8)

sh

507 (2.7)

sh

503 (3.0)

sh

475 (3.2)

sh

471 (2.5)

a

Dichloromethane at 298 K. bEtOH/MeOH (4:1, v/v) glass at 77 K.

The higher energy absorptions are also assigned as the mixed ligand-centered π−π* transitions of the BPO and isocyanides. Similar to the series of 1a−1e, the lower-energy band is also attributed to the mixture of MLCT [dπ(Ru) → π*(BPO) and π*(CNR)] transitions. This lowest-energy component is attributed to a predominant MLCT [dπ(Ru) → π*(BPO)] transition as a clear progressive blue-shift of the absorption

Figure 5. Overlaid emission spectra of (a) 1e and 2a−2e and (b) 2e, HPBO and 2,4,6-Br3PhNC in ethanol/methanol (4:1) glass at 77 K. λex= 355 nm. E

DOI: 10.1021/acs.inorgchem.9b00560 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 6. Normalized emission spectra of (a) 2a−2e in dichloromethane (λex= 400 nm) at room temperature and (b) 2e in photopolymerizable EGDA (λex= 400 nm) in the presence of wt 1% photoinitiator as a function of irradiation time at room temperature.

the varied spatial arrangement of the ligands in 2, the much stronger RuII−isocyanide π-back bonding interaction would also increase d−d splitting (Δ), and thus the crossing of the ligand-field state with the MLCT emissive state is less efficient. It is noted that the emission in the solution state shows a slight blue shift compared to that in the 77 K glassy state, which is an unexpected rigidochromic effect for luminescent MLCT transition metal complexes. To further examine the rigidity effect on the emission properties, the luminescent properties of 2e, which shows the largest blue-shift in the solution state compared to the glassy state, in polymerizable medium have been examined. Figure 6b shows the emission spectral change of 2e in ethylene glycol dimethacrylate (EDGA) containing a photoinitiator during the course of photopolymerization. Upon excitation, the initial emission spectrum of 2e (λem ≈ 469 nm) in EGDA solution is similar to that in dichloromethane. Photopolymerization is then initiated by irradiating the solution with UV light (320−380 nm). During the course of polymerization, as the rigidity increases, the emission intensity increases with a gradual red-shift until λem = 518 nm after complete polymerization of the EGDA to a poly-EGDA rigid matrix (∼1 h of irradiation). To confirm the stability of the complex during the course of photopolymerization, the Ru complex in the poly-EGDA matrix was finely ground and extracted with dichloromethane. The ESI-MS analysis of the extract shows that the parent peak of 2e ([M]+, m/z = 1202 amu) is the major peak and the only signal showing the Ruisotopic pattern. Moreover, the dichloromethane extract also displays identical UV−vis absorption and emission characteristics. Although the rationale for such a rigidochromic effect cannot be elucidated with this study, we also attribute the slight red shift of these complexes in the 77 K glassy medium to the rigidity effect. Similar rigidity effects have not been reported in MLCT emitters but a dimeric ReI carbonyl complex bearing the same auxiliary PBO ligand, [Re2(CO)6(PBO)2], was also reported to exhibit emission of very similar energy in both room-temperature solutions and 77 K glassy media of butylonitrile and toluene.77 Electrochemical Properties. The electrochemical properties of 1 and 2 have been studied by cyclic voltammetry (CV) in 0.1 M [nBu4N]PF6 dichloromethane solution at room temperature. Typical CVs of the complexes are shown in Figures 7 and S2 and S3 and detailed electrochemical data are summarized in Table 2. The trans,trans,trans- complexes (1) show reversible and quasi-reversible/irreversible waves between 0.12 and 0.34 and berween 1.34 and 1.42 V vs SCE,

peak shifts to the blue when more electron-withdrawing substituents are introduced. In view of the close resemblance of the emission maxima for the trans,trans,trans- and cis,trans,cis- isomers and their dependence on the electronic nature of the isocyanide ligands, they are assigned as ligandcentered (LC) excited state origins mixed with some MLCT [dπ(Ru) → π(BPO)] character. The much shorter emission lifetimes of 1a and 1b can be explained by their lower-lying MLCT state and thus increase of the 3MLCT excited state component, which is well-documented to show a much shorter emission lifetime than the 3LC excited state, in the emissive state. To gain insight into the ligand-centered emission, the emission properties of the free tribromophenyl isocyanide and the free protonated ligand 2-(2-benzoxazolyl)phenol (HPBO) in a 77 K glassy medium have been studied (Figure 5b). The structured emission profile of complex 2e resembles closely that of free HPBO except a red shift of 30 nm, probably due to the perturbation of the metal center.77−80 This suggests that the ligand-centered emissive excited state is derived from a PBO ligand. The normalized emission spectra of 2a−2e in degassed dichloromethane at room temperature are shown in Figure 6a. Upon excitation with λex > 400 nm, complexes 2a−2e exhibit a structureless blue to blue-green emission at 471−517 nm with τo < 10 ns and emission quantum yields