Tuning the Photophysics and Reverse Saturable Absorption of

University, Fargo, North Dakota 58108-6050, United States. Inorg. Chem. , 2016, 55 (22), pp 11908–11919. DOI: 10.1021/acs.inorgchem.6b02028. Pub...
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Tuning the Photophysics and Reverse Saturable Absorption of Heteroleptic Cationic Iridium(III) Complexes via Substituents on the 6,6′-Bis(fluoren-2-yl)-2,2′-biquinoline Ligand Xiaolin Zhu, Levi Lystrom, Svetlana Kilina, and Wenfang Sun* Department of Chemistry and Biochemistry, North Dakota State University, Fargo, North Dakota 58108-6050, United States S Supporting Information *

ABSTRACT: To understand the effects of the terminal substituent at the diimine ligand on the photophysics of heteroleptic cationic Ir(III) complexes and to obtain Ir(III) complexes with extended ground-state absorption to the near-IR region while retaining the long-lived and broadly absorbing triplet excited state, we synthesized three heteroleptic cationic iridium(III) complexes bearing cyclometalating 1-phenylisoquinoline (C^N) ligands and substituted 6,6′-bis(7-R-fluoren-2-yl)-2,2′-biquinoline (N^N) ligand (R = H, NO2, or NPh2). The photophysics of these complexes was systematically investigated via spectroscopic methods and time-dependent density functional theory. All complexes possess strong ligand-localized 1π,π* transitions mixed with ligandto-ligand charge transfer (1LLCT)/metal-to-ligand charge transfer (1MLCT) transitions below 400 nm, and a broad and featureless absorption band above 400 nm that arises from the N^N ligand-localized 1π,π*/1ILCT (intraligand charge transfer) transitions as well as the very weak 1,3LLCT/1,3MLCT transitions at longer wavelengths. The electronwithdrawing NO2 substituent on the N^N ligand leads to a blue-shift of the 1π,π*/1ILCT absorption band, while the electrondonating NPh2 substituent causes a pronounced red-shift of this band. The unsubstituted and NO2-substituted complexes (complexes 1 and 2, respectively) are moderately emissive at room temperature (RT) in solution as well as at 77 K in the glassy matrix, while the NPh2-substituted complex (3) is weakly emissive at RT, but the emission becomes much brighter at 77 K. Complexes 1 and 2 show very broad and strong triplet excited-state absorption from 460 to 800 nm with moderately long lifetimes, while complex 3 exhibits weak but broad absorption bands from 384 to 800 nm with a longer lifetime than those of 1 and 2. The nonlinear transmission experiment manifests that complexes 1 and 2 are strong reverse saturable absorbers (RSA) at 532 nm, while 3 shows weaker RSA at this wavelength. These results clearly demonstrate that it is feasible to tune the groundstate and excited-state properties of the Ir(III) complexes via the terminal substituents at the diimine ligand. By introducing the fluoren-2-yl groups to the 2,2′-biquinoline ligand to extend the diimine ligand π-conjugation, we can obtain Ir(III) complexes with reasonably long-lived and strongly absorbing triplet excited state while red-shifting their 1,3LLCT/1,3MLCT absorption band into the near-IR region. These features are critical in developing visible to near-IR broadband reverse saturable absorbers.

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INTRODUCTION Octahedral Ir(III) complexes have attracted intense interest in the past decade because of their versatile applications in organic light-emitting devices (OLEDs),1−3 light-emitting electrochemical cells,4−6 photocatalysis,7 biosensing,8−12 nonlinear optics,13−26 etc. These applications are intrinsically based on the efficient intersystem crossing induced by the heavy Ir(III) ion in these complexes that results in high phosphorescence efficiency, the presence of multiple charge transfer transitions that lead to broad charge-transfer absorption in the visible spectral region, their electrochemical characteristics, and/or high chemical and thermal stabilities. Recently, structural modifications of Ir(III) complexes to obtain desirable optical or electronic properties have been extensively developed, especially for OLEDs or light-emitting electrochemical cell applications. Among these Ir(III) complexes, cationic heteroleptic Ir(III) complexes have been the subject of interest because their ground-state and excited-state properties can be © XXXX American Chemical Society

readily tuned by utilizing different functionalized cyclometalating (C^N) ligands or diimine (N^N) ligands.27−30 In addition to the extensive investigations for OLEDs or light-emitting electrochemical cell applications, a growing interest in utilizing Ir(III) complexes as nonlinear optical materials has emerged. Roberto, Angelis, and co-workers studied the second-order nonlinear optical properties of some cationic Ir(III) complexes with substituted 1,10-phenanthroline ligand and revealed that the substituents on the phenanthroline ligand and the π-conjugation of the cyclometalating C^N ligand influenced the second-order nonlinearities of the Ir(III) complexes.13,14 Guerchais, Roberto, and co-workers also reported that substitution of neutral Ir(ppy)3 (ppy = 2phenylpyridine) complexes with vinyl-aryl substituents at the para-position of the pyridine rings could readily tune the Received: August 20, 2016

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DOI: 10.1021/acs.inorgchem.6b02028 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry second-order nonlinear optical response of these complexes.15 Schanze’s group discovered that a cationic Ir(III) complex with 4-ethynyl-N,N-dihexylaniline substituents on the N^N ligand exhibited “dual mechanism” nonlinear absorption (i.e., reverse saturable absorption (RSA) at 532 nm and two-photon initiated excited-state absorption at 1064 nm). 16 Zhao’s group demonstrated the two-photon and three-photon absorption of oligofluorene-substituted neutral Ir(ppy)3 complexes.17 In recent years, our group has systematically investigated the photophysics and reverse saturable absorption of cationic and neutral Ir(III) complexes with π-conjugated substituents at the N^N and/or C^N ligands.19−24 We also extended the πconjugation of the N^N or C^N ligands via benzannulation.24−26 We found that incorporation of the π-conjugated substituent to the N^N ligand via a single bond or triple bond linkage significantly prolonged the triplet excited-state lifetime,19,21 while extending the π-conjugation of the N^N ligands via benzannulation dramatically decreased the triplet lifetime, although such a change red-shifted the ground-state charge transfer absorption band(s) to the near-IR region.24,25 In contrast, attaching the π-conjugated substituent to the C^N ligand via a single or triple bond linkage induced more charge transfer characters to the lowest triplet excited state (T1) and thus decreased its lifetime,19,21 while benzannulation of the C^N ligands could switch the T1 state to the C^N ligand localized 3π,π* state and thus increased the T1 lifetime.26 Because long-lived triplet excited state and broadband ground-state absorption to the near-IR region are desirable features for ideal broadband reverse saturable absorbers and new generation photosensitizers for photodynamic therapy, it is important to maintain the long-lived triplet excited state while red-shifting the charge-transfer absorption bands. Our previous studies found that benzannulation of the N^N ligand redshifted the ground-state charge transfer absorption band(s) to the near-IR region but shortened the triplet lifetime,24,25 while the triplet lifetime was increased by attaching π-conjugated substituent to the N^N ligand.19,21 In addition, it has been reported that substitution of the bipyridine ligand at its 4, 5, or 6 positions all influenced their triplet energies and lifetimes.29,30 However, there has been no report on the effect of the terminal substituents at the π-conjugated substituents attached on the N^N ligand. Therefore, in this work, we synthesized three heteroleptic cationic Ir(III) complexes containing the substituted 6,6′bis(fluoren-2-yl)-2,2′-biquinoline ligand (1−3, structures shown in Chart 1) and systematically studied their photo-

physics and reverse saturable absorption. We aim to not only understand the effects of terminal substituent at the diimine ligand on the photophysics of heteroleptic cationic Ir(III) complexes but also obtain Ir(III) complexes with extended ground-state absorption to the near-IR region while retaining the long-lived and broadly absorbing triplet excited state. 2,2′Biquinoline (bqu) was chosen as the coordinating diimine ligand because our previous work demonstrated that the Ir(III) complex containing this ligand exhibited much red-shifted charge transfer ground-state absorption band extending to approximately 650 nm.25 1-Phenylisoquinoline (piq) was selected as the C^N ligand because we have shown that the Ir(III) complex bearing this ligand possessed relatively longlived triplet excited state and strong reverse saturable absorption.26 Fluoren-2-yl motif was introduced to the 6,6′position of biquinoline ligand based on its proven effect on increasing the triplet lifetime and broadening the triplet excitedstate absorption to the near-IR region due to the intraligand charge transfer transition induced by its π-donating ability.21 To explore the effect of the terminal substituent at the N^N ligands on the photophysics and RSA, an electron-withdrawing NO2 substituent or electron-donating NPh2 substituent was introduced to the 7-position of the fluoren-2-yl motif. The systematic photophysical studies were carried out for 1−3 to understand the structure−property correlations to develop effective broadband reverse saturable absorbing materials. Moreover, the UV−vis absorption and emission spectra of 1−3 were calculated by time-dependent density functional theory (TDDFT) to better understand the nature of the optical transitions of these complexes.

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EXPERIMENTAL SECTION

Synthesis and Characterization. All reagents were purchased from Sigma-Aldrich or Alfa Aesar Co. Ltd. Tetrahydrofuran was distilled under N2 over sodium benzophenone ketyl. All other reagents were used as received. Silica gels (230−400 mesh) used for chromatography were purchased from Sorbent Technology. The intermediate compounds were characterized by 1H NMR spectroscopy, while the ligands and Ir(III) complexes were characterized by 1H NMR, HRMS, and elemental analyses. 1H NMR spectra were obtained on a Bruker 400 MHz spectrometer using CDCl3 as the solvent with tetramethylsilane as internal standard. ESI-HRMS analyses were performed on a Bruker BioTOF III mass spectrometer. Elemental analyses were carried out by NuMega Resonance Laboratories, Inc. in San Diego, California. Scheme 1 illustrates the synthetic routes for ligands L1−L3 and complexes 1−3. The synthesis of the precursors 8a−8c and 10 followed the literature procedures.26,31 The synthetic procedures and characterization data for compounds 4−7, 9a−9c, L1−L3, and complexes 1−3 are reported herewith. 2-Amino-5-bromobenzaldehyde (4). To a stirred solution of 2aminobenzaldehyde (363 mg, 3.0 mmol) in DMF (60 mL) was added NBS (534 mg, 3.0 mmol). The solution was stirred at RT for 1 h. The yellow solution was poured into water (200 mL) and then extracted with CH2Cl2 (3 × 100 mL). The organic layer was washed with water and then dried over MgSO4. The solvent was then removed under reduced pressure to obtain yellow powder as the crude product, which was used for the synthesis of compound 7 immediately without further purification. 6-Bromo-N-methoxy-N-methylquinoline-2-carboxamide (5). A DMF (1 mL) solution of 6-bromoquinoline-2-carboxylic acid (2.0 g, 7.95 mmol), triethylamine (3.32 mL, 23.8 mmol), N,O-dimethylhydroxyamine hydrochloride (0.93 g, 9.50 mmol), and HBTU (3.31 g, 8.75 mmol) was stirred at RT for 2 h. The mixture was poured into water, and the precipitate was collected by filtration. The crude product was further purified by column chromatography (silica gel,

Chart 1. Molecular Structures of Complexes 1−3

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DOI: 10.1021/acs.inorgchem.6b02028 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Synthetic Routes for L1−L3 and 1−3

hexane/ethyl acetate = 4/1, v/v) to obtain 2.1 g of orange solid (yield: 90%). 1H NMR (400 MHz, CDCl3) δ 8.18 (d, J = 8.1 Hz, 1H), 8.07− 8.01 (m, 2H), 7.86−7.82 (m, 1H), 7.80−7.69 (br, 1H), 3.81 (s, 3H), 3.48 (s, 3H). 1-(6-Bromoquinolin-2-yl)ethanone (6). To a solution of 6-bromoN-methoxy-N-methylquinoline-2-carboxamide (2.14 g, 7.28 mmol) in THF (60 mL) was added methyl magnesium bromide (9.1 mL, 8.7 mmol, 1 M in THF solution) at 0 °C dropwise, and the mixture was stirred at 0 °C for 1 h. The mixture was then poured into water and extracted with ethyl acetate. The organic layer was washed with brine and dried over MgSO4. After removal of the solvent, the crude product was purified by column chromatography (silica gel, hexane/ethyl acetate = 4/1, v/v) to obtain a white solid (1.7 g, yield: 94%). 1H NMR (400 MHz, CDCl3) δ 8.21−8.02 (m, 4H), 7.89−7.83 (m, 1H), 2.67 (s, 3H). 6,6′-Dibromo-2,2′-biquinoline (7). Compounds 4 and 6 (700 mg, 2.8 mmol) were dissolved in ethanol (20 mL), and the solution was purged with N2. Saturated ethanolic KOH (20 mL) was then added dropwise, and the mixture was heated to reflux overnight. After being cooled to RT, the mixture was poured into water. The precipitate was collected by filtration and washed with CH2Cl2 (50 mL) to obtain the crude product. The crude product was further purified by column chromatography (silica gel, hexane/ethyl acetate = 8/1, v/v) to obtain a pale yellow powder (670 mg, yield: 64%). 1H NMR (400 MHz, CDCl3) δ 8.87 (d, J = 8.5 Hz, 2H), 8.26 (d, J = 8.5 Hz, 2H), 8.11 (d, J = 9.0 Hz, 2H), 8.08 (s, 2H), 7.85 (d, J = 8.5 Hz, 2H). General Synthetic Procedure for 9a−9c. Compounds 8a−8c (0.8 mmol), bis(pinacolato)diboron (304 mg, 1.2 mmol), Pd(dppf)Cl2 (32 mg, 0.04 mmol), and KOAc (196 mg, 2.0 mmol) were added to 15 mL of dry DMF. The mixture was heated to 90 °C under nitrogen overnight. Then, the mixture was poured into water and extracted with

diethyl ether. The organic layer was washed with brine and dried over MgSO4. After removal of the solvent, the crude product was purified by silica gel column chromatography to afford a pure product. 9a: Hexane/ethyl acetate (100/1, v/v) was used as the eluent to afford a colorless oil (330 mg, yield: 80%). 1H NMR (400 MHz, CDCl3) δ 7.80 (t, J = 7.2 Hz, 1H), 7.75 (d, J = 8 Hz, 1H), 7.68−7.63 (m, 2H), 7.35−7.33 (m, 1H), 7.30−7.23 (m, 2H), 2.02−1.91 (m, 4H), 1.34 (s, 12H), 0.88−0.63 (m, 22H), 0.49−0.41 (m, 8H). 9b: Hexane/ethyl acetate (100/1, v/v) was used as the eluent to afford a yellow oil (242 mg, yield: 54%). 1H NMR (400 MHz, CDCl3) δ 8.28 (d, J = 6.9 Hz, 2H), 7.90 (t, J = 8.3 Hz, 1H), 7.88−7.83 (m, 2H), 7.79 (d, J = 7.6 Hz, 1H), 2.14−2.01 (m, 4H), 1.39 (s, 12H), 0.91−0.67 (m, 22H), 0.57−0.40 (m, 8H). 9c: Hexanes/toluene (5/1, v/v) was used as the eluent to obtain a light yellow oil (366 mg, yield: 66%). 1H NMR (400 MHz, CDCl3) δ 7.79 (t, J = 8.5 Hz, 2H), 7.63−7.61 (m, 2H), 7.27−7.23 (m, 4H), 7.13−7.05 (m, 6H), 7.03−6.99 (m, 2H), 2.04−1.98 (m, 2H), 1.87− 1.77 (m, 2H), 1.37 (s, 12H), 0.94−0.65 (m, 22H), 0.60−0.48 (m, 8H). General Synthetic Procedure for L1−L3. Compounds 7 (124 mg, 0.30 mmol) and 9a−9c (0.66 mmol), Pd(PPh3)4 (35 mg, 0.03 mmol), and K2CO3 (248 mg, 1.80 mmol) were added to 18 mL of mixed solvent toluene/ethanol/water (4/1/1, v/v/v). The mixture was heated to reflux under nitrogen for 48 h. The reaction mixture was then cooled to RT and extracted with CH2Cl2. The organic layer was washed with brine and dried over Na2SO4. After removal of the solvent, the crude product was purified by column chromatography (silica gel) to afford pure product. L1: Hexane/ethyl acetate (200/1, v/v) was used as the eluent to obtain a light yellow solid (114 mg, yield: 46%). 1H NMR (400 MHz, CDCl3) δ 8.94 (d, J = 8.6 Hz, 2H), 8.44 (d, J = 8.6 Hz, 2H), 8.35 (d, J = 8.6 Hz, 2H), 8.13−8.10 (m, 4H), 7.85 (d, J = 7.8 Hz, 2H), 7.79− C

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Inorganic Chemistry 7.75 (m, 6H), 7.45 (d, J = 7.1 Hz, 2H), 7.37 (t, J = 7.3 Hz, 2H), 7.33 (t, J = 7.3 Hz, 2H), 2.14−2.06 (m, 8H), 1.00−0.57 (m, 60H). Elemental Analysis Calcd (%) for C76H92N2: C, 88.32; H, 8.97; N, 2.71. Found: C, 88.64; H, 8.60; N, 2.92. L2: Hexane/ethyl acetate (100/1, v/v) was used as the eluent to obtain 97 mg of a yellow powder (yield: 40%). 1H NMR (400 MHz, CDCl3) δ 8.97 (d, J = 8.6 Hz, 2H), 8.45 (d, J = 8.6 Hz, 2H), 8.38−8.33 (m, 6H), 8.15 (s, 2H), 8.10 (d, J = 8.7 Hz, 2H), 7.95 (d, J = 7.8 Hz, 2H), 7.90−7.83 (m, 6H), 2.24−2.13 (m, 8H), 0.92−0.70 (m, 32H), 0.68−0.55 (m, 28H). Elemental Analysis Calcd (%) for C76H90N4O4· 0.6CH2Cl2: C, 78.33; H, 7.83; N, 4.77. Found: C, 78.01; H, 7.73; N, 4.94. L3: Hexane/ethyl acetate (98/2, v/v) was used as the eluent to afford a yellow oil that slowly became solidified into a yellow solid (48 mg, yield: 33%). 1H NMR (400 MHz, CDCl3) δ 8.93 (d, J = 8.6 Hz, 2H), 8.42 (d, J = 8.6 Hz, 2H), 8.33 (d, J = 8.6 Hz, 2H), 8.11−8.09 (m, 4H), 7.78−7.74 (m, 6H), 7.68−7.65 (m, 2H), 7.30−7.26 (m, 8H), 7.18−7.11 (m, 12H), 7.05−7.02 (m, 4H), 2.12−1.87 (m, 8H), 1.28− 0.82 (m, 40H), 0.80−0.63 (m, 20H). Elemental Analysis Calcd (%) for C100H110F6N4·3.5CH2Cl2: C, 74.65; H, 7.08; N, 3.36. Found: C, 74.95; H, 6.79; N, 3.62. General Synthetic Procedure for Complexes 1−3. To a stirred solution of L1−L3 (0.078 mmol) and 10 (50 mg, 0.039 mmol) in degassed CH2Cl2 (30 mL) and methanol (15 mL) was added AgSO3CF3 (20 mg, 0.078 mmol). The mixture was refluxed overnight under nitrogen. After being cooled to RT, 10-fold NH4PF6 was added. The suspension was stirred at RT for 2 h. After removal of the solvent, the crude product was purified by silica gel column chromatography. Complex 1. CH2Cl2/ethyl acetate (50/1, v/v) was used as the eluent to afford a red solid (65 mg, yield: 50%). 1H NMR (400 MHz, CDCl3) δ 8.90−8.84 (m, 4H), 8.73 (d, J = 8.8 Hz, 2H), 8.26 (d, J = 8.1 Hz, 2H), 8.07 (s, 2H), 7.92−7.73 (m, 14H), 7.60 (s, 2H), 7.55 (d, J = 7.9 Hz, 2H), 7.42−7.32 (m, 10H), 7.14−7.12 (m, 2H), 6.93 (t, J = 7.5 Hz, 2H), 6.52 (d, J = 7.7 Hz, 2H), 2.07−2.04 (m, 8H), 0.93−0.89 (m, 60H). ESI-HRMS (m/z): calcd for [C106H112IrN4]+, 1633.8529; found, 1633.8558. Elemental Analysis Calcd (%) for C106H112F6IrN4P: C, 71.56; H, 6.34; N, 3.15. Found: C, 71.26; H, 6.32; N, 3.01. Complex 2. CH2Cl2/ethyl acetate (50/1, v/v) was used as the eluent to afford a red solid (61 mg, yield: 45%). 1H NMR (400 MHz, CDCl3) δ 8.92−8.85 (m, 4H), 8.77 (dd, J = 8.6 and 3.5 Hz, 2H), 8.33−8.26 (m, 6H), 8.11 (s, 2H), 7.95−7.81 (m, 8H), 7.80−7.72 (m, 6H), 7.71−7.66 (m, 4H), 7.40 (d, J = 7.5 Hz, 2H), 7.34 (d, J = 6.5 Hz, 2H), 7.17−7.12 (m, 2H), 6.94 (t, J = 7.5 Hz, 2H), 6.51 (d, J = 7.7 Hz, 2H), 2.14−2.07 (m, 8H), 0.92−0.66 (m, 40H), 0.55−0.45 (m, 20H). ESI-HRMS (m/z): calcd for [C106H110IrN6O4]+, 1723.8230; found, 1723.8232. Elemental Analysis Calcd (%) for C106H110F6IrN6O4P: C, 68.11; H, 5.93; N, 4.50. Found: C, 67.95; H, 5.67; N, 4.58. Complex 3. Purified by column chromatography twice (CH2Cl2/ ethyl acetate = 20/1, v/v) to afford a red solid (23 mg, yield: 25%). 1H NMR (400 MHz, CDCl3) δ 8.86 (t, J = 8.9 Hz, 4H), 8.71 (d, J = 8.1 Hz, 2H), 8.25 (d, J = 7.2 Hz, 2H), 8.05 (s, 2H), 7.92−7.52 (m, 20H), 7.39 (d, J = 9.0 Hz, 2H), 7.32−7.25 (m, 10H), 7.13−6.91 (m, 18H), 6.51 (d, J = 7.7 Hz, 2H), 2.04−1.29 (m, 8H), 0.97−0.54 (m, 60H). ESI-HRMS (m/z): calcd for [C130H130IrN6]+, 1967.9986; found, 1967.9932. Elemental Analysis Calcd (%) for C130H130F6IrN6P· 0.5CH2Cl2: C, 72.70; H, 6.12; N, 3.90. Found: C, 72.61; H, 6.13; N, 3.81. Photophysical Measurements. All of the solvents used for photophysical studies were spectrophotometric grade and purchased from Alfa Aesar Co. Ltd. A Shimadzu UV-2501 spectrophotometer and an HORIBA Fluoro-Max 4 fluorometer/phosphorometer were used to measure the UV−vis absorption spectra and emission spectra, respectively, in different solvents. The emission quantum yields were determined by the relative actinometry method32 in degassed solutions in which a degassed CH3CN solution of [Ru(bpy)3]Cl2 (Φem = 0.097, λex = 436 nm)33 was used as the reference for 1−3 and a 1 N sulfuric acid solution of quinine bisulfate (Φem = 0.546, λex = 347.5 nm)34 was used as the reference for ligands L1−L3. The nanosecond transient difference absorption (TA) spectra and decays were measured in degassed toluene solutions on an Edinburgh LP920 laser flash

photolysis spectrometer. The third harmonic output (355 nm) of a Nd:YAG laser (Quantel Brilliant, pulse width = 4.1 ns, repetition rate = 1 Hz) was used as the excitation source. Each sample was purged with argon for 45 min prior to measurement. The singlet depletion method35 was used to determine the triplet excited-state molar extinction coefficients (εT1−Tn) at the TA band maximum. After the εT1−Tn value was obtained, the triplet excited-state quantum yield was calculated by the relative actinometry36 in which SiNc in benzene was used as the reference (ε590 = 70 000 M−1 cm−1, ΦT = 0.20).37 Computational Methods. All calculations, including ground state geometry optimization and excited state calculations, were performed using Gaussian 09 software package.38 The ideal octahedral geometries of complexes 1−3 were optimized in their singlet and triplet spin configurations by DFT using hybrid Perdew, Burke, and Ernzerhof functional (PBE1PBE)39−41 and LANL2DZ basis set42−44 assigned for Ir(III) ion and 6-31g* basis set45−49 for all the remaining atoms. All calculations were done in dichloromethane solvent within the conductor-like polarizable continuous model (CPCM).50,51 To reduce the computational cost, the 2-ethylhexyl substituents on the fluorenes were reduced to methyl groups. This change does not affect the electronic levels contributing to absorption and emission in the visible spectral range. Absorption spectra of ligands L1−L3 and complexes 1−3 were obtained using TDDFT52,53 utilizing the same functional and basis set used in the ground state calculations. To obtain the spectrum, the lowest 60 optical transitions were calculated and broadened by a Gaussian distribution with the line width of 0.1 eV, corresponding to the thermal broadening of the experimental spectra. Although the calculated absorption spectra qualitatively agree with the experimental ones, they are systematically red-shifted (see Figure S7 in the Supporting Information). To better match the experimental data, the percentage of Hartree−Fock (HF) exchange in the PBE1PBE functional was increased from 25 to 40%. It is well-known that the portion of the orbital exchange used in the density functional changes the optical gap, i.e., the larger the HF exchange portion, the larger the energy gap. However, the spectral features remain nearly the same, independent of the hybrid functionals.54 Such a tunability of functionals allows for matching the calculated and experimental spectra better. The calculated spectra with 32% HF were found to match the experimental spectra the best. Calculations of the emission energies were carried out using analytical TDDFT gradients55,56 by optimizing either the lowest singlet (fluorescence) or triplet (phosphorescence) state as implemented in the Gaussian09 software. The same functional, basis set, and solvent model used for the ground state TDDFT calculations were used for the emission calculations as well. For optimization of the excited state, we started with several different initial guesses for the wave function, choosing either the very lowest triplet state (root = 1) or the second lowest triplet state (root = 2). Both guessing states were initially taken at the ground state geometry with the triplet spin configuration, where their electronic wave functions were calculated by regular TDDFT procedure. By starting with different initial wave functions, we were able to converge to the triplet states that are in a better agreement with experimental emission energies. The nature of optical transitions was classified based on analysis of natural transition orbitals (NTOs)57 implemented in Gaussian09 software. NTOs redistribute the electron density obtained from the transition density matrix for the hole (occupied) and electron (unoccupied) while presenting many-body excited states. To visualize these NTOs, the isosurface of 0.02 was used to obtain a better representation of excited orbitals. Nonlinear Transmission Experiment. The nonlinear transmission experiments at 532 nm for complexes 1−3 were carried out in toluene solutions in a 2 mm cuvette using 4.1 ns laser pulses. The linear transmission of 1−3 in toluene was adjusted to 80% in the 2 mm cuvette at 532 nm. A Quantel Brilliant ns laser with a repetition rate of 10 Hz was used as the light source. The experimental setup and details were described previously.58,59 The beam radius at the focal point was approximately 96 μm. D

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Inorganic Chemistry

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RESULTS AND DISCUSSION Geometry Optimization. The optimized geometries of 1− 3 in CH2Cl2 are shown in Figure S8 of the Supporting Information. After optimization, the fluorenyl substituents attached at the 6,6′-position of 2,2′-biquinoline are twisted from the quinoline rings by an angle of ca. 35° (which are similar to the angles in complexes with fluorenyl-substituted phenanthroline ligand21), while the angles between the two quinoline rings are around 24−25°. Upon geometry optimization, the propeller shaped diphenylamino substituents at the 7-position of fluorene twist from the fluorene plane at a dihedral angle of ca. 42° in 3. In contrast, the nitro groups in 2 are coplanar with the fluorene groups. For the C^N ligand, the isoquinoline rings twist from the phenyl rings at ca. 17° for all three complexes. Electronic Absorption. The UV−vis absorption of L1−L3 and 1−3 follows Beer’s law in CH2Cl2 in the concentration range studied (5 × 10−6 to 1 × 10−4 mol L−1), suggesting that no aggregation occurs within this concentration range. The experimental and calculated UV−vis absorption spectra of L1− L3 and 1−3 in CH2Cl2 are presented in Figure 1 and Figures

absorption spectra of L1−L3 in CH2Cl2 possess two intense absorption bands at 270−320 nm and 350−450 nm. On the basis of the large molar extinction coefficients of these absorption bands, the minor solvatochromic effect (Supporting Information, Figure S12), and the calculated natural transition orbitals (NTOs) (Supporting Information, Table S1), these bands are characterized as the predominant 1π,π* transitions. For L3, which contains the strong electron-donating NPh2 substituents, intramolecular charge transfer transition from the NPh2-fluorenyl components to the biquinoline component also makes some contributions (see NTO of S1 for L3 in Table S1). In comparison to L1, which contains no terminal substituent, both the electron-withdrawing substituent (NO2) in L2 and electron-donating substituent (NPh2) in L3 cause a red-shift of the 350−450 nm absorption band due to the delocalization of the electron density to these substituents, which extends the πconjugation in these molecules and thus induces the red-shift. As depicted in Figures 1b and 1c, both the experimental spectra and TDDFT calculations show that complexes 1−3 possess 2−3 intense absorption bands from 300 to 500 nm (the band extends to 600 nm for 3) as well as very weak and broad tails at the longer wavelengths. The overall trends revealed in the experimental data are well reproduced in the theoretical spectra. In comparison to the absorption spectra of L1−L3, the major absorption bands of 1−3 become broader and redshifted. Considering the large molar extinction coefficients and the NTOs representing the major transitions contributing to these bands (Tables 2 and 3), we attribute the intense highenergy absorption band(s) between 300 and 400 nm for 1−3 to the N^N and/or C^N ligand-localized 1π,π* transitions mixed with some ligand-to-ligand charge transfer (1LLCT)/ metal-to-ligand charge transfer (1MLCT) characters (see Table 3). For complex 3, intraligand charge transfer (1ILCT, π(diphenylaminofluorene) → π*(bqu)) transition also makes a major contribution to this band in addition to the 1 LLCT/1MLCT/1π,π* transitions. In contrast, the broad band at 400−500 nm for 1 and 2 mainly arises from the N^N ligand-based 1π,π* transitions mixed with 1ILCT (π(fluorene) → π*(bqu)) transitions (see NTOs for S2 state of 1 and S3 state of 2 in Table 2). For complex 3, the corresponding band at 450−600 nm is dominated by the 1 ILCT (π(diphenylaminofluorene) → π*(bqu)) transition with a minor contribution from the bqu 1π,π* transition (see NTOs for S1 state of 3 in Table 2).

Figure 1. Experimental UV−vis absorption spectra of L1−L3 (a) and 1−3 (b) and the calculated spectra of 1−3 with 32% HF (c) in CH2Cl2. The inset in panel b shows the expanded experimental spectra between 500 and 750 nm.

S9−S11 of the Supporting Information, and their photophysical parameters are listed in Table 1. As depicted in Figure 1a, the Table 1. Experimental Photophysical Data for L1−L3 and 1−3 λ (nm)a [ε (104 L mol−1 cm−1)] L1 L2 L3 1

285 275 315 319

2

296 (7.95), 355 (7.79), 420 (7.45), 550 (0.165) 296 (7.71), 366 (7.52), 515 (2.19), 600 (0.390)

3

(7.97), (6.89), (7.96), (11.6),

370 385 383 439

(10.5) (13.1) (7.19) (5.07), 550 (0.167)

λem (nm)b [τem (μs)], Φem RT 437 501 560 657

λem (nm)c [τem (μs)] 77 K

λT1‑Tn (nm)d [εT1‑Tn (104 L mol−1 cm−1), τT (μs)], ΦT

(-), 0.94 (-), 0.05 (-), 0.16 (1.02), 0.13

633 (2.85), 693 (2.78)

658 (0.72), 0.10

629 (3.49), 688 (3.25)

697 (8.99, 730 (9.34, 520 (11.4, 349 (1.46, 0.44 555 (3.63,

62), 0.048 41), 0.24 12), 0.072 0.73), 514 (3.63, 0.88), 730 (3.00, 0.88),

608 (-), 0.002

644 (3.08), 701 (3.02)

585 (0.72, 3.96), 0.11

0.39), 755 (1.21, 0.39), 0.17

a

Room temperature electronic absorption band maxima and molar extinction coefficients in CH2Cl2. bRoom temperature emission energy, lifetime, and quantum yield in CH2Cl2. cEmission energy at 77 K measured in 2-methyltetrahydrofuran glassy matrix, c = 1 × 10−5 mol/L. dNanosecond TA band maximum, triplet extinction coefficient, triplet excited-state lifetime and quantum yield in toluene. The reference used was SiNc in C6H6 (ε590 = 70 000 L mol−1 cm−1, ΦT = 0.20). E

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Inorganic Chemistry Table 2. NTOs) Representing Transitions Contributing to the Low-Energy Absorption Bands of 1−3 in CH2Cl2

Table 3. NTOs Representing Transitions Contributing to the Main Absorption Bands at 300−400 nm for 1−3 in CH2Cl2

Photoluminescence. The emission characteristics of L1− L3 and 1−3 were investigated in different solvents at room temperature, and the normalized emission spectra in CH2Cl2 are shown in Figure 2. The emission data are summarized in Tables 1 and 4, and in Table S3 (Supporting Information). As we anticipated, incorporation of different functionalized fluorene motifs to the biquinoline core prominently tuned the emission of L1−L3 from the blue emission (437 nm in L1) to the yellow emission (560 nm in L3) in CH2Cl2. As depicted in Figure 3 for L1 and L3 and in Figure S13 for L2, the featureless emission and the positive solvatochromic effect suggest that the emitting state in CH2Cl2 can be assigned to the 1 π,π*/1ILCT state for L1 and L2 and predominantly the 1ILCT state for L3. This character of transition is confirmed by NTOs corresponding to the emitting S1 states (Table S4 of the Supporting Information). However, the emission of L1 and L2 in less polar solvents such as hexane and toluene exhibited salient vibronic structures (Figures 3 and S13), suggesting

In addition to these major absorption bands, all complexes exhibit weak absorption in the region of 500−650 nm for 1 and 2, extending to 700 nm for 3. With reference to the other Ir(III) complexes that exhibit such tails24,25,27,60,61 and in view of the NTOs of S1 states for 1 and 2 in Table 2, we ascribe these tails to 1,3LLCT/1,3MLCT transitions. It is worthy to note that introducing electron-withdrawing NO2 substituent to the fluorenyl motif at the N^N ligand leads to a blue-shift of the 1ILCT/1π,π* absorption band in 2 with respect to that in 1, whereas the electron-donating NPh2 substituent causes a pronounced red-shift of the 1ILCT band in 3. This trend clearly reflects the charge-transfer nature of this band. The NO2 group reduces the electron-donating ability of the fluorene component and thus induces the blue-shift of the 1 ILCT/1π,π* band, while NPh2 group increases the electron density on fluorene and thus its electron-donating ability, resulting in a stronger ILCT and a red-shift of the 1ILCT band in comparison to that in 1. F

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Figure 2. Normalized emission spectra of L1−L3 (λex = 347.5 nm) and 1−3 (λex = 436 nm) in CH2Cl2 solutions at RT.

Table 4. Emission Data for Complexes 1−3 in Different Solvents at RTa

1 2 c

3

hexane/ toluene (1/1)

toluene

628 (810, 0.11) 631 (170, 0.063) 603 (-, 0.002)

634 (930, 0.12) 636 (370, 0.083) 608 (-, 0.009)

λem (nm) [τem(ns), Φemb], CH2Cl2 657 (1000, 0.13) 658 (600, 0.10) 608 (-, 0.002)

acetone 658 (360, 0.076) 660 (360, 0.044) 611 (-, 0.001)

acetonitrile 658 (600, 0.065) 659 (480, 0.037) 624 (-, 0.001)

a The excitation wavelength was 436 nm. bThe reference used for the quantum yield measurement was [Ru(bpy)3]Cl2 in degassed CH3CN solution (Φem = 0.097, λex = 436 nm). cThe emission signals were too weak to allow for the lifetimes to be accurately measured.

Figure 3. Normalized emission spectra of L1 and L3 in different solvents at RT (λex = 347.5 nm).

Complexes 1−3 are emissive at 77 K. Comparison of the emission spectra in the 2-methyltetrahydrofuran (MTHF) matrix at 77 K and at RT is shown in Figures 4 and S14. At 77

dominant π,π* characters. The increased charge transfer character in more polar solvents is also reflected by the drastically decreased emission quantum efficiency, as being represented by L3 (see Table S3 of the Supporting Information), in which 1ILCT is the dominant feature in its emitting state. With the increased electron-donating ability of the substituent on fluorene motifs, the 1ILCT character obviously increases. Compared to that of the three ligands, the emission of the complexes is significantly red-shifted to ca. 660 nm for 1 and 2 and ca. 610 nm for 3 in degassed CH2Cl2. Considering the large red-shifts of the emission with respect to their corresponding excitation wavelengths, the relatively long emission lifetimes (Table 1), and that it is prone to oxygen quenching, we assign the emission of complexes 1−3 as phosphorescence. As shown in Figure 2 and Table 1, the emission energy of 1 and 2 are almost the same, along with similar emission lifetimes (ca. 1000 ns for 1 and 720 ns for 2). These features imply that the emission of 1 and 2 originates from the same excited state that is independent of the terminal substituent at the N^N ligand. Taking into account the minor negative solvatochromic effect (Figure S13) and the calculated NTOs (see Table S4 of the Supporting Information), we assign the emission of 1 and 2 as 3 π,π*/3MLCT/3ILCT states. In contrast, complex 3 containing the electron-donating NPh2 substituents exhibits a very weak emission with a nonmeasurable lifetime. These features suggest the predominant charge transfer nature of the emitting state for 3. With reference to the NTOs, such a state could be assigned as the 3ILCT/3π,π* state. 1

Figure 4. Emission spectra of 3 in deaerated MTHF at RT and 77 K. The inset includes the photographic images of 3 in aerated MTHF under UV light (365 nm) irradiation at (a) RT and (b) 77 K.

K, the emission spectra of these complexes become structured and narrower, and the emission is brighter and longer-lived (Figure 4 and Table 1). The thermally induced Stokes shifts for complexes 1−3 are 484, 537, and 865 cm−1, respectively, which is in line with their 3π,π*/3CT nature of the emitting states (for complex 3, the larger shift indicates a major 3CT contribution). In comparison to the reported Ir(III) complex that contains the unsubstituted bqu and piq ligands (i.e., complex 4 in ref 27), introduction of the π-conjugated fluorenyl substituents to G

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Figure 5. Time-resolved nanosecond TA spectra of L1−L3 and 1−3 in toluene solution. λex = 355 nm; A355 = 0.4 in a 1 cm cuvette.

much but only slightly decreases the emission lifetime and quantum yield. In contrast, electron-donating NPh2 substituent changes the nature of the emitting state and thus affects the emission energy, lifetime, and quantum yield distinctively. Transient Absorption (TA). To further understand the substituent effect on the triplet excited-state characteristics, the excited-state absorption of 1−3 and L1−L3 was investigated. The time-resolved nanosecond TA spectra recorded upon excitation at 355 nm at room temperature in degassed toluene solution are shown in Figure 5. As shown in Figure 5, all three ligands exhibit positive TA band at 420−800 nm and bleaching bands below 400 nm that

the bqu ligand causes a significant increase in the triplet lifetime (1000 ns for complex 1 in this work vs 490 ns for complex 4 in ref 27) and emission quantum yield (0.13 for complex 1 in this work vs 0.018 for complex 4 in ref 27) enhancement. Such changes are related to the alternation of the T1 state parentage upon fluorenyl substitution, i.e., changing the 3MLCT/3LLCT parentage for complex 4 in ref 27 to the 3π,π*/3CT parentage of the T1 state for complex 1 in this work. This is consistent with that observed in other Ir(III) complexes that contain πconjugated substituents at the bipyridine or phenanthroline ligand.19,21 It is worthy of noting that electron-withdrawing NO2 substituent does not influence the emission energy very H

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Inorganic Chemistry are in line with their 1π,π* absorption bands. The TA spectrum of L1 features broad absorption bands between 407 and 772 nm, while L2 possesses a more intense and red-shifted band from 414 to 800 nm. For L3, its TA band maximum appears at 516 nm, and its intensity is weaker and shape is different from those of the other two ligands. Considering the very long TA lifetime for L1 (62 μs) and L2 (41 μs) and their similar spectral feature, we attribute the observed TA of these two ligands to 3 π,π* state absorption. L3 has a much more blue-shifted TA and a much shorter lifetime (12 μs), suggesting the involvement of significant 3ILCT contribution. Resembling the similarity of the TA spectra of their corresponding ligands L1 and L2, the spectra of 1 and 2 are similar in that they both exhibit a bleaching band between 380 and 460 nm and two broad and strong absorption bands at 470−800 nm for 1 and 460−800 nm for 2. Especially for complex 1, its TA band extends beyond 800 nm. The lifetimes obtained from the decay of their TA spectra (Table 1) are consistent with those obtained from the decay of emission (Table 4), suggesting that the TA of 1 and 2 likely emanates from the same excited state that emits. Thus, the observed TA from 1 and 2 is tentatively ascribed to 3π,π*/3CT in nature. Complex 3 exhibits a bleaching band at ca. 366 nm and weak but broad absorption bands from 384 to 800 nm. Interestingly, the lifetime deduced from the decay of TA for 3 is much longer than those of 1 and 2. Although the emission lifetime of 3 was unable to be reliably measured due to the very weak signal, which prevents us from determining whether the transient absorbing excited state in 3 is the excited state that emits, the long-lived transient absorption suggests that the observed TA from 3 likely emanates from its 3π,π*/3CT state(s). It appears that both the terminal electron-withdrawing NO2 group and electron-donating NPh2 substituent on the diimine ligand reduce the TA intensity, and especially the effect of NPh2 is more dramatic. Reverse Saturable Absorption (RSA). RSA is a nonlinear absorption phenomenon in which the absorptivity of the material increases with the increased incident light fluence. RSA has important applications in optical switching,62 laser mode locking,63 optical pulse shaping,64 spatial light modulation,64,65 and laser beam compression and limiting,66,67 etc. To realize RSA, the material must have weak ground-state absorption at the interested wavelength to populate the excited state via onephoton absorption; meanwhile, the excited-state absorption at the same wavelength should be stronger than that of the ground state, i.e., the ratio of the excited-state absorption cross section (σex) vs ground-state absorption cross section (σ0) at the RSA wavelength must be larger than 1. The larger the σex/ σ0 ratio is, the stronger the RSA appears. For broadband RSA, it is expected that the σex/σ0 ratios should be larger than 1 in the spectral range of 400−900 nm. In addition, for RSA of nanosecond (ns) and longer pulse width laser beams, triplet excited-state absorption plays the dominant role. Thus, a high triplet excited-state quantum yield and long triplet lifetime would enhance the RSA. On the basis of the positive TA signals of 1−3 in the region of 470−800 nm and their triplet lifetimes, we envision that complexes 1−3 can exhibit RSA under ns laser irradiation. To demonstrate this, nonlinear transmission experiments were conducted at 532 nm using 4.1 ns laser pulses for 1−3 in toluene solutions. For convenience of comparison, the solutions of all complexes were prepared to obtain an identical 80% linear transmission in a 2 mm cuvette at 532 nm. Figure 6

Figure 6. Nonlinear transmission curves for complexes 1−3 in toluene at 532 nm in a 2 mm cuvette. The pulse duration was 4.1 ns, and the linear transmission of all solutions was 80% at 532 nm in a 2 mm cuvette. The radius of the beam waist at the linear focal plane was approximately 96 μm.

displays the nonlinear transmission vs incident energy plots for complexes 1−3 at 532 nm. When the incident energy was increased, the transmission of the solutions decreased. This clearly indicates the occurrence of RSA with a trend of 1 ≈ 2 ≫ 3. The transmission of 1 and 2 decreased to ∼18% at the incident energy of ∼460 μJ, which is among the strongest organometallic reverse saturable absorbers for ns laser pulses at 532 nm.20−26,31,58,59,68,69 This trend clearly manifests that the strong electron-donating NPh2 substituent on the biquinoline ligand in complex 3 dramatically decreases the RSA at 532 nm. Such a decrease should be attributed to the significantly increased ground-state absorption cross section at 532 nm for 3 compared to that of the other two complexes. Because the strength of RSA is primarily determined by the σex /σ0 ratio, these key parameters at 532 nm were estimated based on the measured UV−vis absorption spectra, the nanosecond TA spectra at zero delay, and the procedure described by our group before.70 The values of the σex/σ0 ratio in Table 5 correlate with Table 5. Ground-State and Excited-State Absorption Cross Sections of Complexes 1−3 in Toluene complex

1

2

3

σ0 (10−18 cm2) σex (10−18 cm2) σex/σ0

7.53 174 23.1

6.86 195 28.4

70.1 221 3.15

the observed RSA trend very well. It appears that introducing an electron-withdrawing NO2 substituent on N^N ligands leads to a slightly decreased ground-state absorption and a slightly increased excited-state absorption at 532 nm compared to the that of the unsubstituted complex 1, while the electrondonating NPh2 substituent dramatically decreases the σex/σ0 ratio and results in a weaker RSA at 532 nm.

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CONCLUSIONS We synthesized and investigated the photophysics of three heteroleptic cationic iridium(III) complexes (1−3) bearing cyclometalating 1-phenylisoquinoline (C^N) ligands and terminally substituted 6,6′-bis(7-R-fluoren-2-yl)-2,2′-biquinoline (N^N) ligands (R = H, NO2, or NPh2) to understand the effects of terminal substituent at the diimine ligand. We also I

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obtained Ir(III) complexes with extended ground-state absorption to the near-IR region while retaining the longlived and broadly absorbing triplet excited state. We found that introducing the 7-R-fluoren-2-yl motifs to the 2,2′-biquinoline ligand caused a new 1π,π*/1ILCT band between 400 and 500 nm for 1 and 2 and between 450 and 600 nm for 3 in addition to the 1π,π*/1LLCT/1MLCT transitions below 400 nm and the very weak but broad 1,3LLCT/1,3MLCT band at 500−650 nm for 1 and 2 and 600−700 nm for 3. The electron-withdrawing terminal substituent NO2 induced a blue-shift of the 1 π,π*/ 1 ILCT absorption band without affecting the 1,3 LLCT/1,3MLCT band distinctively, while the electrondonating terminal substituent NPh2 enhanced the 1ILCT transition and caused a significant red-shift of the 1 π,π*/1ILCT and 1,3LLCT/1,3MLCT bands in comparison to those of complex 1 without the terminal substituents. Complexes 1−3 were weakly or moderately emissive at RT in deaerated solution. However, the emission became much stronger at 77 K in the glassy matrix. The NO2 substituent did not affect the emission energy but slightly decreased the emission lifetime, while the NPh2 substituent drastically quenched the emission at RT due to the increased charge transfer character in its emitting state. The nanosecond TA spectroscopic study demonstrated that 1 and 2 possessed very broad and strong absorption bands from the visible spectral region, extending into the near-IR region and originating from a moderately long-lived 3π,π*/3CT state. In contrast, the TA of complex 3 was much weaker, although the absorbing excited state was much longer lived compared to those of 1 and 2. Owing to the much stronger excited-state absorption with respect to that of the ground state, complexes 1 and 2 exhibited quite strong RSA at 532 nm for ns laser pulses, while the significantly increased ground-state absorption at 532 nm for complex 3 distinctively weakened the RSA of 3 at this wavelength. These results clearly demonstrate that the terminal substituents at the diimine ligand can significantly tune the ground-state and excited-state properties of the Ir(III) complexes, and it is feasible to obtain Ir(III) complexes with reasonably long-lived, strongly absorbing triplet excited state while red-shifting their ground-state charge transfer absorption band into the near-IR region. These features are critical in developing broadband reverse saturable absorbers that cover the visible to the near-IR region.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Telephone: 701-231-6254; Fax: 701-231-8831. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the Army Research Laboratory (Grant W911NF-10-2-0055) for the materials synthesis and characterization to W.S. and was also partially supported by the National Science Foundation (Grants CNS1229316 and DMR-1411086) for the computational part of the work to W.S. and S.K. S.K. acknowledges the U.S. Department of Energy Early-Career Grant DESC008446 and the Alfred P. Sloan Foundation (Research Fellowship BR2014-073) for partial financial support and the Center for Computationally Assisted Science and Technology (CCAST) at North Dakota State University for computer access and administrative support.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02028. 1 H NMR spectra of L1−L3 and 1−3, optimized groundstate geometries of 1−3, comparison of the experimental and theoretical UV−vis absorption spectra of L1−L3 and 1−3 in CH2Cl2, normalized UV−vis absorption spectra of L1−L3 and 1−3 in different solvents, normalized emission spectra of L2 and 1−3 at room temperature and of 1 and 2 at 77 K in MTHF glassy matrix, NTOs representing transitions contributing to the main absorption bands of L1−L3 and 1−3 in CH2Cl2, NTOs corresponding to the emitting excited states of L1−L3 and 1−3 in CH2Cl2, and the full author list for ref 38 (PDF) J

DOI: 10.1021/acs.inorgchem.6b02028 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.6b02028 Inorg. Chem. XXXX, XXX, XXX−XXX