Blue Phosphorescence of Trifluoromethyl- and ... - ACS Publications

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Blue Phosphorescence of Trifluoromethyl- and TrifluoromethoxySubstituted Cationic Iridium(III) Isocyanide Complexes Nail M. Shavaleev,*,† Filippo Monti,‡ Rosario Scopelliti,† Nicola Armaroli,*,‡ Michael Graẗ zel,† and Mohammad K. Nazeeruddin*,† Laboratory of Photonics and Interfaces, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland Molecular Photoscience Group, Istituto per la Sintesi Organica e la Fotoreattivitá, Consiglio Nazionale delle Ricerche, Via P. Gobetti, 101, 40129, Bologna, Italy

† ‡

S Supporting Information *

ABSTRACT: We report the first comprehensive comparative synthetic, structural, electrochemical, and spectroscopic study of an extended series of fluorocarbon-modified iridium(III) complexes. We prepared seven new cationic Ir(III) complexes with tert-butyl isocyanide and trifluoromethylor trifluoromethoxy-substituted cyclometalating 2-phenylpyridines, [(C∧N)2Ir(CNtBu)2](CF3SO3), and characterized five of them by crystal structure analysis. The redox potentials and photophysical properties of Ir(III) complexes are determined by the type, position, and number of fluorocarbon groups in the cyclometalating ligand. The complexes exhibit pale blue to yellow-green phosphorescence at room temperature with quantum yields and excited-state lifetimes up to 73% and 84 μs in solution (under argon) and 7.5% and 4.3 μs in neat solid (under air). The structured and solvent-independent phosphorescence spectra, with 0−0 emission transition at 445−467 nm, and the long calculated radiative lifetimes, 43−160 μs, indicate that the complexes emit from a cyclometalatingligand-centered triplet excited state. Bulky fluorocarbon groups prevent intermolecular interaction (aggregation) of the complexes, thereby minimizing red-shift of phosphorescence color in going from solution to neat solid.

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exhibit green-blue/pale blue phosphorescence in solution.13 In neat solid, however, the phosphorescence of 9 was quenched and its color was red-shifted, because H/F substituents in 9 did not provide sufficient steric bulk to prevent aggregation.13,15 Bulky and electron-withdrawing fluorocarbon groups with inert C(sp3)−F bonds tune electronic properties, improve solubility, and reduce intermolecular interaction of metal complexes;10,16,17 however, fluorocarbons were rarely used in organometallic Ir(III) complexes.5−8,10,11,17 Here, we report a comparative synthetic, structural, electrochemical, and spectroscopic study of phosphorescent Ir(III) isocyanide complexes with trifluoromethyl- or trifluoromethoxy-substituted cyclometalating 2-phenylpyridines (Schemes 1 and 2). To our knowledge, this is the first comprehensive study of an extended series of fluorocarbon-modified organometallic complexes in general10,16 and Ir(III) complexes in particular.5−8,10,11,17 We demonstrate that the fluorocarbons change electrochemical and photophysical properties of organometallic Ir(III) complexes and that the fluorocarbon-modified Ir(III) complexes can exhibit blue phosphorescence in solution/neat solid at ambient conditions.

INTRODUCTION

High-energy emitters,1−3 in particular phosphorescent iridium(III) complexes,4−8 are required for generation of blue and white light in electroluminescent devices.9 One of the ways to blue-shift the phosphorescence color of a cyclometalated Ir(III) complex is to modify the electron-rich part of the ligand with an electron-withdrawing fluoro4−7,10−13 or trifluoromethyl5−8,10 group to stabilize the HOMO and to increase the HOMO− LUMO gap.14 An Ir(III) complex must exhibit blue phosphorescence in neat solid or in a solid matrix in order to be of use in blueelectroluminescent devices.9 Intermolecular interaction (aggregation) between Ir(III) complexes often causes the broadening and red-shift of the phosphorescence spectrum in going from solution to neat solid.13,15 For example, we recently reported that cationic bis-cyclometalated Ir(III) complexes 9 with strong-field electron-withdrawing isocyanide ligands (Chart 1) Chart 1. Reference Ir(III) Complexes13

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RESULTS AND DISCUSSION Synthesis. 2-Phenylpyridines L1−L8 were prepared by Suzuki−Miyaura coupling of arylboronic acids with 2-

Received: June 19, 2012

© XXXX American Chemical Society

A

dx.doi.org/10.1021/om300557d | Organometallics XXXX, XXX, XXX−XXX

Organometallics

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Scheme 1. Synthesis of 2-Phenylpyridinesa

cyclometalation of L4 occurred only at C6′.19 In contrast, cyclometalation of L8 occurred at C2′ and C6′ to give a mixture of [(C∧N)2Ir(μ-Cl)]2 complexes (the same outcome was reported for 3-methoxyphenyl-substituted C∧N ligands19); therefore, further experiments with L8 were not performed. Seven new iridium(III) complexes [(C∧N)2Ir(CNtBu)2](CF3SO3), 1−7, were prepared by reaction of [(C∧N)2Ir(μCl)]2 (C∧N = L1−L7) with silver triflate (to remove the chloride anion) and with tert-butyl isocyanide (Scheme 2).13,20,21 Purification by column chromatography on silica gave 1−7 as air- and moisture-stable yellow or pale yellow solids that are soluble in polar organic solvents. 1 H and 19F NMR spectra of 1−7 exhibit a single set of signals for the C∧N and CNtBu ligands and indicate that 1−7 have a C2-symmetry in solution. Integration of 1H peaks confirms the 1:1 ratio of C∧N to CNtBu. 19F NMR confirms the presence of the CF3/OCF3 groups and of the CF3SO3− anion. ESI+ TOF mass spectra contain a peak of the [(C∧N)2Ir(CNtBu)2]+ cation. Review of non-patent literature reveals that until now Ir(III) complexes with L2 and L6−L8 or a combination of L1−L8 and an isocyanide were not known, that charged Ir(III) complexes were described only for L1 (with neutral N∧N ligands),11 and that neutral Ir(III) complexes were reported only for L122 and L3−L5.5,6 Moreover, there was only one example of the use of a trifluoromethoxy/perfluoroalkoxy-modified cyclometalating ligand for Ir(III),5 while no such examples are known for precious metals Ru, Os, Rh, Pd, Pt, and Au. Structural Characterization. The following features are observed in the X-ray structures of 1 and 4−7 (Figures 1−3

a Reaction conditions: (a) K2CO3, Pd(PPh3)4, THF/water (3:1), under Ar, 85 °C.

Scheme 2. Synthesis of New Ir(III) Complexesa

Figure 1. Structure of 1 (50% probability ellipsoids; co-crystallized molecule of dichloromethane, H atoms, triflate anion, and the disorder of the tert-butyl group omitted; ORTEP). Heteroatom color codes: N, blue; F, green; Ir, black. a

Reaction conditions: (a) cyclometalating ligand, 2-ethoxyethanol/ water (3:1), under Ar, 120 °C; (b) under Ar, RT (i) AgCF3SO3, CH3OH/CH2Cl2; (ii) tert-butyl isocyanide, CH2Cl2. The numbering of the phenyl ring is indicated in the structures of 1 and 6.

and Table 1): (i) The Ir(III) ion is in a distorted octahedral [(C∧N)2Ir(C)2]+ coordination environment, and the two nitrogen atoms of the C∧N ligands are in trans-position to each other. (ii) The Ir−C bonds with the isocyanides are shorter than are those with the C∧N ligands. (iii) The Ir− (C∧N) bond lengths in 1, 4, 6, and 7 are 2.042(6)−2.078(14) Å for Ir−C and 2.048(12)−2.076(5) Å for Ir−N. Steric clash between the 5′-CF3 group and Ir in complex 5 causes elongation of the Ir−(C∧N) bonds to 2.12 Å for Ir−C and 2.069(3)−2.079(4) Å for Ir−N. (iv) The Ir−C (C∧N) bonds in 1, 4−7, and their analogues13,20,21 are longer than are those in

bromopyridine (Scheme 1). Reaction of L1−L8 with IrCl3·3H2O gave the cyclometalated complexes [(C∧N)2Ir(μCl)]2 (Scheme 2).18 The complexes with 2′-substituted ligands L2 and L7 are insoluble in noncoordinating solvents; the rest of the complexes are soluble in dichloromethane. The Ir−C bond with the 3′-substituted ligands L4 and L8 could form either at the 2′ or 6′ phenyl carbon; however, B


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Figure 3. Structures of 6 (top) and 7 (bottom) (50% probability ellipsoids; H atoms and triflate anion omitted; the disorder of the trifluoromethyl groups in 6 omitted; only one of the two independent molecules shown for 6 [Ir(1)]; ORTEP). Heteroatom color codes: O, red; N, blue; F, green; Ir, black.

Figure 2. Structures of 4 (top) and 5 (bottom) (50% probability ellipsoids; H atoms and triflate anion omitted; the disorder of the trifluoromethyl groups in 5 omitted; only one of the two independent molecules shown for 5 [Ir(2)]; ORTEP). Heteroatom color codes: N, blue; F, green; Ir, black.

Table 1. Structural Parameters of Ir(III) Complexesa bond lengths (Å)

Ir(III) complexes with weak-field N/O-ligand(s) [(C∧N)2Ir(N/O)n]0/+.20 We consider the elongation of Ir−C (C∧N) bonds in [(C∧N)2Ir(CNtBu)2]+ to be caused by the transinfluence of strong-field isocyanide ligands. (v) The Ir− isocyanide fragments are not linear, and the angles are 165.5(5)−178.4(2)° for Ir−CN or 158.9(9)−176.4(5)° for CN−C. (vi) The C∧N ligands are nearly planar, and the dihedral angles between the phenyl and pyridyl are 2.4−14.4°. The largest deviation from planarity is observed when a fluorocarbon is placed at the position 2′ (7) or 5′ (5), where it clashes with the 3-H (pyridyl) or Ir. (vii) Replacing a 4′-CF3 group (1) by a 4′-OCF3 one (6) does not alter the structure of the complex. (viii) Bulky CF3, OCF3, and tBu groups prevent face-to-face π−π stacking of the complexes. The shortest Ir···Ir distances are 8.8−9.5 Å. Structural parameters of 1 and 4−7 are similar to those of their analogues.13,20,21 Electrochemistry. Redox potentials of 1−7 were measured with cyclic voltammetry relative to Fc+/Fc23 in acetonitrile and DMF (Table 2 and Figure 4; Table S2 and Figures S1 and S2, Supporting Information). Trifluoromethyl-substituted complexes 1−5 undergo a reduction at −1.74 to −2.23 V, which is irreversible for 4 and 5 or quasi-reversible for 1 and 2. In contrast, 3 is the first Ir(III) isocyanide complex to exhibit a reversible reduc-

C∧ N

isocyanide complex 1 4 5 (Ir1)b 5 (Ir2)b 6 (Ir1)b 7

angles (deg)

Ir−C

Ir−C

Ir−N

Ph−Py

2.022(14) 2.031(15) 2.025(2) 2.020(2) 2.012(4) 2.015(4) 2.015(5) 2.021(4) 2.018(7) 2.028(7) 2.034(6) 2.023(6)

2.065(13) 2.078(14) 2.045(2) 2.050(2) 2.120(4) 2.124(4) 2.121(4) 2.122(5) 2.057(6) 2.066(6) 2.042(6) 2.049(6)

2.048(12) 2.070(12) 2.0604(18) 2.0678(19) 2.072(3) 2.069(3) 2.079(4) 2.076(4) 2.076(5) 2.068(5) 2.061(5) 2.060(4)

6.01 8.28 2.39 5.72 10.67 6.07 12.12 14.05 9.64 9.01 14.38 8.51

a

Each row corresponds to one ligand. tert-Butyl isocyanide in transposition to the carbon atom of the C∧N ligand in the same row. bTwo independent molecules, Ir(1) and Ir(2), are present in the unit cell of 5 and 6. The data for the disordered Ir(2) molecule of 6 are not shown.

tion.13,20,21 The 2′,4′-substituted complexes 1−3 have less negative reduction potentials than do the 3′,5′-analogues 4 and 5 (Table 2). Addition of a second CF3 group shifts the C


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Table 2. Redox Propertiesa Ered, Vc b

complex

R′

1 2 3 4 5 6 7 9Hh 9Fh

4′-CF3 2′-CF3 2′,4′-(CF3)2 3′-CF3 3′,5′-(CF3)2 4′-OCF3 2′-OCF3 H 4′-F

Eox, Vc,d f

f

ΔE, Ve

DMF

ACN

ACN

ACN

−2.10 (83)g −2.05 (117)g −1.74 (78)g −2.22 −2.10 −2.25 −2.21 −2.38 −2.34

−1.79 (83)g −2.23 −2.12 −2.28 −2.23 −2.38 −2.34

1.50 1.50 1.23 1.42

3.78 3.73 3.61 3.76

Relative to Fc+/Fc. On glassy carbon working electrode, in the presence of 0.1 M (NBu4)PF6, at scan rate 100 mV/s. Estimated error: ±50 mV. The anodic/cathodic peak separation for the standard, Fc+/Fc couple was 73−93 mV. bSubstituents on the phenyl ring of the cyclometalating ligand. c Irreversible process (unless stated otherwise); the oxidation or reduction peak potentials are reported. dOxidation of the complexes was outside of the electrochemical window of the solvent for 1−7 in DMF (>1.0 V relative to Fc+/Fc) and for 1−5 in acetonitrile (ACN) (>1.5 V relative to Fc+/ Fc). eRedox gap: ΔE = Eox − Ered. fCyclic voltammograms of 1 and 2 in ACN at 100 mV/s were not informative. gReversible (3) or quasi-reversible (1 and 2) process; E1/2red is reported; the anodic/cathodic peak separation is given in parentheses. hData from the literature.13 a

results suggest that the irreversibility of electrochemical processes in 1−7 arises from a chemical reaction that follows the electron transfer.24 The irreversible character of most of the observed electrochemical processes prevented us from determining the formal redox potentials of 1−7; nevertheless, we consider that the peak potentials and the redox gap correctly reproduce the qualitative trends in the energy of the HOMO, LUMO, and HOMO−LUMO gap of 1−7.13,25 The reduction and oxidation of 1−7 take place on the pyridyl and on the Ir−aryl fragment, respectively.13 We explain the highly positive oxidation potentials of 1−7 by the cationic charge of the complexes and by the presence of the electronwithdrawing fluorocarbons5−8 and the strong-field electronwithdrawing isocyanides.13,20,21,26,27 1−7 have less negative redox potentials than do the non-substituted/mono-fluorinated analogues 913 (Chart 1 and Table 2); therefore, fluorocarbons in 1−7 stabilize the HOMO and the LUMO of the complexes to a greater extent than do the H/F groups in 9. The redox gap in 6 and 7 is larger than that in 9H, and it is similar to that in 9F (Table 2). Absorption Spectroscopy. The infrared absorption spectra of 1−7 in neat films feature two peaks at 2217−2175 cm−1 separated by 20 cm−1 that are characteristic of stretching vibrations of the tert-butyl isocyanide CN bond (Table 3 and Figure S3 in the Supporting Information).13 The presence of two intense IR peaks confirms the cis-geometry of the

Figure 4. Cyclic voltammograms of 1−7 in DMF (glassy carbon working electrode, 0.1 M NBu4PF6, 100 mV/s). The unit on the vertical axis is 10 μA. CVs in acetonitrile are shown in the Supporting Information.

reduction potential to the less negative values: this shift is larger for the 2′,4′-bis-substitution (0.31−0.36 V from 1 or 2 to 3) than it is for the 3′,5′-bis-substitution (0.12 V from 4 to 5) (Table 2). The oxidation of 1−5 is outside of the electrochemical window of DMF and acetonitrile. Trifluoromethoxy-substituted complexes 6 and 7 undergo an irreversible reduction at −2.21 to −2.28 V and an irreversible oxidation at 1.50 V (Table 2). Complexes 6 and 7 exhibit more negative redox potentials than do the CF3 analogues 1 and 2 (Table 2); therefore, we conclude that the OCF3 group is a weaker electron-withdrawing substituent than is the CF3 group. We note that (i) the reduction potentials of 3−7 are solventindependent at a cyclic voltammetry scan rate of 100 mV/s in DMF and acetonitrile (Table 2), (ii) the reduction of 4, 6, and 7, which is irreversible at 100 mV/s, becomes quasi-reversible at scan rate 1 V/s in acetonitrile (Figure S2 and Table S2, Supporting Information), and (iii) the trends in redox potentials and redox gaps (ΔE = Eox − Ered) of 1−7 do not change when the scan rate is increased from 100 mV/s to 1 V/s (Table 2 and Table S2 in the Supporting Information). These

Table 3. UV−Vis and IR Absorption Maxima complex 1 2 3 4 5 6 7

IR, cm−1a 2195, 2201, 2209, 2201, 2217, 2195, 2202,

2175 2181 2190 2180 2198 2175 2181

λabs, nm (ε, 103 M−1 cm−1)b,c 252 348 290 297 304 253 253

(33), (8.8) (16), (13), (13), (34), (33),

298 (16), 309 (15), 346 (7.6) 339 307 312 297 300

(8.4) (13), 330 (8.2) (12, sh) (14), 308 (13, sh), 332 (11) (16), 309 (17), 335 (7.8)

Infrared absorption bands of solid films of the neat complex in the region of the CN stretching vibrations. bIn dichloromethane, at room temperature, at 250−600 nm. Errors: ±2 nm for λabs; ±5% for ε. c Lowest energy transition in 1−7 is a composite band with a broad main maximum and lower energy shoulder(s) (Figure 5). a

D


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isocyanide ligands in 1−7.28 The frequency (wavenumber) of the CN peaks in isocyanide complexes reflects the strength of both the σ bonding and the π back-bonding between the isocyanide ligand and the metal;29 therefore, variation of this frequency cannot be correlated to the change of only one of these two bonding interactions.29 Electronic spectra of 1−7 in dichloromethane exhibit the lowest energy band, which we assign to an (Ir-aryl)-to-pyridyl charge transfer (CT) transition,13 at 320−400 nm with molar absorption coefficients (ε) of about 10 × 103 M−1 cm−1 (Table 3 and Figure 5). The 4′-substituted complexes 1 and 6 have CT

Figure 6. Corrected and normalized luminescence spectra of 1−7 at room temperature in dichloromethane solution (DCM, blue trace) and in neat solid film (red trace).

Figure 5. Absorption spectra of 1−7 in dichloromethane solution. The unit on the vertical axis is 5 × 103 M−1 cm−1. The spectra (1−3; 4 and 5; 6 and 7) are shifted with respect to one another along the vertical axis by 10 × 103 M−1 cm−1. Additional absorption spectra are shown in the Supporting Information.

transition at higher energy than do the 2′-substituted analogues 2 and 7, respectively (Figure 5). Addition of a second CF3 group blue-shifts the CT transition from 4 (3′) to 5 (3′,5′); in contrast, a red-shift is observed from 1 (4′) to 3 (2′,4′), and no shift from 2 (2′) to 3 (2′,4′) (Figure 5). We expect the energy of the CT transition to exhibit direct correlation with the redox/HOMO−LUMO gap (ΔE, Table 2),13 and, indeed, this is the case for 6 and 7 (ΔE could be measured only for these two complexes). At higher energies, π−π* electronic transitions of 2phenylpyridines dominate the spectrum with ε = (13−34) × 103 M−1 cm−1 (Figure 5 and Table 3; Figures S4−S6 in the Supporting Information). Luminescence Spectroscopy. Complexes 1−7 exhibit pale blue to yellow-green phosphorescence at room temperature in solution and in neat solid (Figures 6 and 7, Table 4). The phosphorescence of 1−7 is quenched by air/oxygen, especially in liquid solutions. In solution, in 10−5 M dichloromethane and acetonitrile (under argon) and in 1 wt % poly(methyl methacrylate) (PMMA, drop-cast film, under air), (i) the emission spectra of 1−7 are solvent-independent and exhibit a vibronic structure that becomes better resolved at 77 K (Figures 6−8); (ii) the vibronic peaks are less resolved (even at 77 K) for complexes 2, 3, and 5, which have a CF3 group in the hindered 2′ or 5′ position (Figures 6−8); (iii) the luminescence decays are single-exponential functions suggesting the presence of only

Figure 7. Corrected and normalized luminescence spectra of 1−7 at room temperature in acetonitrile solution (ACN, blue trace) and in PMMA film (1 wt % of the complex, red trace).

one emissive center; (iv) the emission quantum yields (Φ) and the observed excited-state lifetimes (τ) are 27−73% and 24−84 μs (Table 4); (v) the calculated radiative lifetimes, τrad = τ/Φ, are 43−160 μs (Table 4). Complexes 1−7 are brightly emissive in solution and exhibit a narrow variation of quantum yields in a given solvent: 27− 47% in dichloromethane or 45−73% in acetonitrile under argon or 46−58% in PMMA under air (Table 4). The quantum yields and excited-state lifetimes of 1−7 do not show extreme dependence on the type or number of fluorocarbon(s); moreover, Φ and τ of 1−7 are better than or similar to those of the reported analogues;13,20,21 therefore, we conclude that modification with fluorocarbon(s) does not facilitate nonradiative processes in Ir(III) complexes. The photophysical properties of 1−7 in 1 wt % PMMA under air are similar to those in 10−5 M liquid solutions under argon; therefore, we consider that 1−7 in solid PMMA matrix do not form aggregates and are not quenched by oxygen. E


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Table 4. Photophysical Properties complex 1

2

3

4

5

6

7

9Hg 9Fg

mediuma DCM ACN PMMA solid DCM, 77 DCM ACN PMMA solid DCM, 77 DCM ACN PMMA solid DCM, 77 DCM ACN PMMA solid DCM, 77 DCM ACN PMMA solid DCM, 77 DCM ACN PMMA solid DCM, 77 DCM ACN PMMA solid DCM, 77 solid DCM, 77 solid DCM, 77

K

K

K

K

K

K

K K K

λ0−0, nmb

λmax, nmc

455 455 455 459 454

489 488 488 491 454 499 497 496 504 495 503 504 503 503 500 479 478 477 479 445 482 482 479 483 481 482 480 481 479 447 481 479 479 485 446 516 452 484, 509 447

464

467 448 446 447 450 445 455 454 449 451 452 449 450 448 447 449 449 450 457 446 456 452 456 447

Φ, %d τ, μse 44 67 46 5.1 42 57 49 5.8 37 45 49 7.5 45 70 55 3.8 27 53 47 5.0 40 68 55 6.3 47 73 58 5.3

τrad, μsf

24 41 25

55 61 54

33 26 41 27

62 72 55

38 25 33 24

68 73 49

32 27 49 32

60 70 58

48 42 84 45

160 160 96

84 30 58 34

75 85 62

53 24 42 25

51 58 43

Figure 8. Corrected and normalized luminescence spectra of 1−7 in dichloromethane solution at 77 K.

OCF3 (7/6) group in 2-phenylpyridine. This blue-shift must arise from the stabilization of the HOMO by the fluorocarbon, because Eox (4, 6, 7) ≫ Eox (9H). Addition of a second CF3 group strongly stabilizes the LUMO (Ered becomes less negative), thereby red-shifting the phosphorescence color from 1 or 2 to 3 and from 4 to 5 (the ground-state absorption spectrum, however, is blue-shifted from 4 to 5). The shorter wavelength of the 0−0 transition at 77 K (Table 4) corresponds to the more negative reduction potential (higher LUMO energy) in 1−7 and to the larger redox/HOMO− LUMO gap in 6, 7, and 9 (Table 2, Figures S7 and S8, Supporting Information). The structured and solvent-independent phosphorescence spectra and the long radiative lifetimes indicate that 1−7 emit from a cyclometalating-ligand-centered (LC) excited state.13,14,30,31 The radiative lifetime of 1−7 is solvent-sensitive and varies, but by no more than a factor of 1.7, in the order acetonitrile ≥ dichloromethane ≥ PMMA (Table 4). This solvent dependence of τrad can arise from the change in the degree of mixing between the (solvatochromic) charge-transfer and the (non-solvatochromic) cyclometalating-ligand-centered excited states in 1−7 (the greater contribution of the CT excited state to the LC one is expected to shorten the radiative lifetime of the complex).30,31 We suggest that the elongation of Ir−(C∧N) bonds in complex 5 caused by the 5′-CF3 group (Table 1) decouples the Ir-to-ligand-charge-transfer excited state from the cyclometalating-ligand-centered one. We consider that this decoupling is reflected in the longer radiative lifetime of 5 (96−160 μs) when compared with the other complexes (43−85 μs) (Table 4). Long radiative lifetimes cause high sensitivity to the triplet−triplet interaction/oxygen quenching that can limit the use of 1−7 in electroluminescent devices, but can make 1−7 excellent oxygen sensors.21 In going from solution to neat solid (spin-coated film, under air) the quenching by oxygen and the aggregation/selfquenching13,15 of 1−7 cause the following changes: (i) the emission spectrum broadens (Figure 6); (ii) the 0−0 transition loses intensity and the long-wavelength transitions gain intensity (Figure 6); (iii) the luminescence decay becomes biexponential; (iv) the quantum yield and excited-state lifetime decrease to 3.8−7.5% and 0.8−4.3 μs (Table 4). Bulky

37 4.4 2.6

a

Dichloromethane (DCM) or acetonitrile (ACN) solution at room temperature under argon (10−5 M). PMMA solution (1 wt % of the complex, drop-cast film) or neat solid complex (spin-coated film) at room temperature under air. Frozen dichloromethane solution at 77 K. b Highest energy vibronic luminescence peak (0−0 transition). c Maximum of the luminescence spectrum. dλexc = 320 nm. eThe luminescence decay is a biexponential function in neat complex: the two components are 0.8−1.9 and 2.8−4.3 μs. fCalculated radiative lifetime: τrad = τ/Φ. gData from the literature.13

The phosphorescence color of 1−7 is determined by the type, position, and number of fluorocarbon groups in the 2phenylpyridine ligand. The wavelength of the 0−0 transition (λ0−0) at 77 K correctly characterizes the variation of phosphorescence color of 1−7 at room temperature (Table 4; λ0−0 peaks at room temperature are often not resolved). λ0−0 at 77 K is observed at 445−467 nm, and it increases in the order 3′-CF3 (4) ≤ 2′-OCF3 (7) ≤ 4′-OCF3 (6) = 4′-F (9F) < 3′,5′-(CF3)2 (5) ≤ H (9H) < 4′-CF3 (1) < 2′-CF3 (2) < 2′,4′(CF3)2 (3) (Table 4 and Figure 8). The strongest blue-shift of phosphorescence color in 1−7 (against a reference 9H13) is achieved by a single substitution with a 3′-CF3 (4) or 2′-/4′F


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fluorocarbons reduce intermolecular interaction of 1−7, thereby minimizing the red-shift of the emission color in going from solution to neat solid, especially for 6 (4′-OCF3) and the bis-CF3-substituted complexes 3 and 5 (Figure 6 and Table 4). Conclusions. Trifluoromethyl- and trifluoromethoxy-modified bis-cyclometalated cationic Ir(III) isocyanide complexes 1−7 exhibit bright phosphorescence at ambient conditions. Substitution with a single fluorocarbon group in the C∧N ligand can blue-shift the phosphorescence color of an Ir(III) complex; for example, we observe pale blue phosphorescence (λ0−0 = 446−452 nm) in solution from 4 (3′-CF3), 6 (4′-OCF3), and 7 (2′-OCF3) (Φ = 40−73%) and in neat solid from 6 (4′-OCF3) (Φ = 6.3%). The wavelength and efficiency of blue phosphorescence of 4, 6, and 7 are one of the best reported for cationic bis-cyclometalated Ir(III) complexes.13 In contrast, bis-CF 3 substitution in the C∧ N ligand red-shifts the phosphorescence color of the complex (3 and 5, Table 4 and Figures 6−8) and introduces steric strain (5, Table 1 and Figure 2). Photophysical properties of 1−7 in solution are better than or similar to those of the reported analogues.13,20,21 Bulky fluorocarbon groups (1−7) minimize intermolecular interaction and offer better control over the phosphorescence color and efficiency of an Ir(III) complex in neat solid than do the H/F groups (913). The trifluoromethoxy group gives better blue-phosphorescent Ir(III) complexes (6, 7) than does a fluoro (9F13) or trifluoromethyl (1, 2) group. We expect fluorocarbon-modified cyclometalating ligands to gain application in phosphorescent organometallic complexes32 of iridium(III),31 platinum(II),33 and gold(III).34

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and its color changed from yellow to nearly colorless. It was stirred at RT under argon for 96 h to give a pale yellow solution. It was directly loaded on the chromatography column (silica, 15 g) to avoid rotorevaporating foul-smelling tert-butyl isocyanide. Elution with 0.5−1.0% CH3OH in CH2Cl2 removed the impurities. Elution with 1.0−2.0% CH3OH in CH2Cl2 recovered the pure product as a pale yellow eluate. The complexes were often isolated as oils that solidified on standing. Further details are provided below. Complex 1. [(L1)2Ir(μ-Cl)]2 (104 mg, 0.079 mmol), AgCF3SO3 (46 mg, 0.179 mmol), and tert-butyl isocyanide (0.5 mL, 365 mg, 4.4 mmol) gave a pale yellow solid that exhibits bluish-green phosphorescence in solution and in powder: 133 mg (0.14 mmol, 88%). Anal. Calcd for C35H32F9IrN4O3S (MW 951.92): C, 44.16; H, 3.39; N, 5.89. Found: C, 44.03; H, 3.45; N, 5.69. 1H NMR (400 MHz, CD2Cl2): δ 9.02 (d, J = 5.6 Hz, 2H), 8.25−8.15 (m, 4H), 7.86 (d, J = 8.0 Hz, 2H), 7.57 (td, J = 6.4, 2.0 Hz, 2H), 7.38−7.31 (m, 2H), 6.36 (s, 2H), 1.40 (s, 18H, tert-butyl) ppm. 19F NMR (376 MHz, CD2Cl2): δ −63.37 (6F, Ph−CF3), −78.84 (3F, triflate) ppm. ESI+ TOF MS: m/ z 803.1 ({M − CF3SO3}+, 100%). Complex 2. [(L2)2Ir(μ-Cl)]2 (100 mg, 0.074 mmol), AgCF3SO3 (45 mg, 0.175 mmol), and tert-butyl isocyanide (0.5 mL, 365 mg, 4.4 mmol) gave a yellow glass that exhibits yellow-green phosphorescence in solution and in powder: 112 mg (0.118 mmol, 79%). Anal. Calcd for C35H32F9IrN4O3S (MW 952.17): C, 44.16; H, 3.39; N, 5.89. Found: C, 44.36; H, 3.23; N, 5.65. 1H NMR (400 MHz, CD2Cl2): δ 9.09 (dd, J = 5.2, 0.8 Hz, 2H), 8.48 (d, J = 8.4 Hz, 2H), 8.16 (td, J = 8.4, 1.2 Hz, 2H), 7.56−7.49 (m, 2H), 7.49 (d, J = 8.0 Hz, 2H), 7.07− 6.99 (m, 2H), 6.33 (d, J = 7.6 Hz, 2H), 1.36 (s, 18H, tert-butyl) ppm. 19 F NMR (376 MHz, CD2Cl2): δ −57.10 (6F, Ph−CF3), −78.84 (3F, triflate) ppm. ESI+ TOF MS: m/z 803.16 ({M − CF3SO3}+, 100%). Complex 3. [(L3)2Ir(μ-Cl)]2 (100 mg, 0.062 mmol), AgCF3SO3 (35 mg, 0.136 mmol), and tert-butyl isocyanide (0.5 mL, 365 mg, 4.4 mmol) gave a yellow glass that exhibits yellow-green phosphorescence in solution and in powder: 114 mg (0.105 mmol, 85%). Anal. Calcd for C37H30F15IrN4O3S (MW 1087.92): C, 40.85; H, 2.78; N, 5.15. Found: C, 41.15; H, 2.93; N, 4.91. 1H NMR (400 MHz, CD2Cl2): δ 9.20 (d, J = 5.2 Hz, 2H), 8.53 (d, J = 8.4 Hz, 2H), 8.30−8.22 (m, 2H), 7.76−7.67 (m, 4H), 6.47 (s, 2H), 1.42 (s, 18H, tert-butyl) ppm. 19F NMR (376 MHz, CD2Cl2): δ −57.56 (6F, Ph−CF3), −63.99 (6F, Ph−CF3), −78.78 (3F, triflate) ppm. ESI+ TOF MS: m/z 939.2 ({M − CF3SO3}+, 100%). Complex 4. [(L4)2Ir(μ-Cl)]2 (100 mg, 0.074 mmol), AgCF3SO3 (42 mg, 0.163 mmol), and tert-butyl isocyanide (0.5 mL, 365 mg, 4.4 mmol) gave a pale yellow solid that exhibits pale blue (in solution) or pale blue-green (in powder) phosphorescence: 119 mg (0.125 mmol, 84%). Anal. Calcd for C35H32F9IrN4O3S (MW 951.92): C, 44.16; H, 3.39; N, 5.89. Found: C, 44.26; H, 3.53; N, 5.46. 1H NMR (400 MHz, CD2Cl2): δ 9.01 (d, J = 6.0 Hz, 2H), 8.24−8.13 (m, 4H), 7.96 (s, 2H), 7.56 (td, J = 6.4, 2.0 Hz, 2H), 7.17 (dd, J = 8.0, 0.8 Hz, 2H), 6.33 (d, J = 8.0 Hz, 2H), 1.41 (s, 18H, tert-butyl) ppm. 19F NMR (376 MHz, CD2Cl2): δ −62.6 (6F, Ph−CF3), −78.8 (3F, triflate) ppm. ESI+ TOF MS: m/z 803.3 ({M − CF3SO3}+, 100%). Complex 5. [(L5)2Ir(μ-Cl)]2 (100 mg, 0.062 mmol), AgCF3SO3 (35 mg, 0.136 mmol), and tert-butyl isocyanide (0.5 mL, 365 mg, 4.4 mmol) gave a pale yellow glass that exhibits bluish-green phosphorescence in solution and in powder: 106 mg (0.097 mmol, 79%). Anal. Calcd for C37H30F15IrN4O3S (MW 1087.92): C, 40.85; H, 2.78; N, 5.15. Found: C, 40.63; H, 2.75; N, 5.19. 1H NMR (400 MHz, CD2Cl2): δ 9.05 (d, J = 5.6 Hz, 2H), 8.21−8.08 (m, 6H), 7.73 (s, 2H), 7.58−7.49 (m, 2H), 1.34 (s, 18H, tert-butyl) ppm. 19F NMR (376 MHz, CD2Cl2): δ −60.1 (6F, Ph−CF3), −63.1 (6F, Ph−CF3), −78.8 (3F, triflate) ppm. ESI+ TOF MS: m/z 939.1 ({M − CF3SO3}+, 100%). Complex 6. [(L6)2Ir(μ-Cl)]2 (100 mg, 0.071 mmol), AgCF3SO3 (40 mg, 0.156 mmol), and tert-butyl isocyanide (0.4 mL, 292 mg, 3.5 mmol) gave a pale yellow solid that exhibits pale blue phosphorescence in solution and in powder: 130 mg (0.132 mmol, 93%). Anal. Calcd for C35H32F9IrN4O5S (MW 983.92): C, 42.72; H, 3.28; N, 5.69. Found: C, 43.08; H, 3.16; N, 5.67. 1H NMR (400 MHz, CD2Cl2): δ 8.92 (d, J = 5.6 Hz, 2H), 8.13 (td, J = 7.6, 1.2 Hz, 2H), 8.06 (dd, J =

EXPERIMENTAL SECTION

Elemental analyses were performed by Dr. E. Solari, Service for Elemental Analysis, Institute of Chemical Sciences and Engineering (ISIC EPFL). 1H, 13C, and 19F NMR spectra were recorded with Bruker AV400 (400 MHz), AV200 (200 MHz), and AVIII-400 (400 MHz) spectrometers. Mass spectra were recorded with Q-TOF Ultima (Waters) and TSQ7000 (Thermo Fisher) spectrometers (MassSpectroscopy Service, ISIC EPFL). Purification, crystal growth, and handling of all compounds were carried out under air. All products were stored in the dark. Chemicals from commercial suppliers were used without purification. Chromatography was performed on a column with an i.d. of 30 mm on silica gel 60 (Fluka, No. 60752). The progress of reactions and the elution of products were followed on TLC plates (silica gel 60 F254 on aluminum sheets, Merck). Further experimental details are provided in the Supporting Information. Synthesis of Ir(III) Isocyanide Complexes 1−7. The structures of 1−7 are shown in Scheme 2. The syntheses of L1−L8 (Scheme 1) and [(C∧N)2Ir(μ-Cl)]2 are described in the Supporting Information. The reactions were performed under argon and in the absence of light. The solvents were deoxygenated by bubbling with Ar, but they were not dried. CAUTION: tert-butyl isocyanide is a foul-smelling volatile liquidensure adequate ventilation! The cyclometalated Ir(III) precursor [(C∧N)2Ir(μ-Cl)]2 was suspended (for C∧N = L2, L3, L7) or dissolved (for other C∧N) in CH2Cl2 (20 mL) at RT. A solution of AgCF3SO3 (excess, Aldrich) in CH3OH (10 mL) was added dropwise. The reaction mixture was stirred (24 h for C∧N = L2 and L7; 4 h for the other C∧N) to give a yellow solution containing the white precipitate of AgCl. It was filtered through a paper filter to remove AgCl and evaporated to dryness (these operations were done under air). The resulting Ir(III) solvento complex was suspended or dissolved in degassed CH2Cl2 (20 mL), and tert-butyl isocyanide (large excess, Aldrich) was added. The reaction mixture became a solution, G


Organometallics

Article

8.4, 1.2 Hz, 2H), 7.78 (d, J = 7.8 Hz, 2H), 7.50−7.44 (m, 2H), 6.97− 6.91 (m, 2H), 5.93 (dd, J = 2.0, 1.2 Hz, 2H), 1.38 (s, 18H, tert-butyl) ppm. 19F NMR (376 MHz, CD2Cl2): δ −57.8 (6F, OCF3), −78.9 (3F, triflate) ppm. ESI+ TOF MS: m/z 835.3 ({M − CF3SO3}+, 100%). Complex 7. [(L7)2Ir(μ-Cl)]2 (100 mg, 0.071 mmol), AgCF3SO3 (45 mg, 0.175 mmol), and tert-butyl isocyanide (0.5 mL, 365 mg, 4.4 mmol) gave a yellow solid that exhibits pale blue (in solution) or yellow-green (in powder) phosphorescence: 116 mg (0.118 mmol, 83%). Anal. Calcd for C35H32F9IrN4O5S (MW 983.92): C, 42.72; H, 3.28; N, 5.69. Found: C, 43.05; H, 3.03; N, 5.48. 1H NMR (400 MHz, CD2Cl2): δ 9.04 (dd, J = 6.0, 1.2 Hz, 2H), 8.64 (d, J = 8.0 Hz, 2H), 8.20−8.12 (m, 2H), 7.53−7.46 (m, 2H), 7.03−6.96 (m, 4H), 6.12− 6.05 (m, 2H), 1.37 (s, 18H, tert-butyl) ppm. 19F NMR (376 MHz, CD2Cl2): δ −57.08 (6F, OCF3), −78.86 (3F, triflate) ppm. ESI+ TOF MS: m/z 835.18 ({M − CF3SO3}+, 100%).

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

(7) (a) Takizawa, S.-y.; Nishida, J.-i.; Tsuzuki, T.; Tokito, S.; Yamashita, Y. Inorg. Chem. 2007, 46, 4308. (b) Yang, C.-H.; Cheng, Y.M.; Chi, Y.; Hsu, C.-J.; Fang, F.-C.; Wong, K.-T.; Chou, P.-T.; Chang, C.-H.; Tsai, M.-H.; Wu, C.-C. Angew. Chem., Int. Ed. 2007, 46, 2418. (c) Chiu, Y.-C.; Hung, J.-Y.; Chi, Y.; Chen, C.-C.; Chang, C.-H.; Wu, C.-C.; Cheng, Y.-M.; Yu, Y.-C.; Lee, G.-H.; Chou, P.-T. Adv. Mater. 2009, 21, 2221. (8) (a) Ionkin, A. S.; Marshall, W. J.; Roe, D. C.; Wang, Y. Dalton Trans. 2006, 2468. (b) Ionkin, A. S.; Wang, Y.; Marshall, W. J.; Petrov, V. A. J. Organomet. Chem. 2007, 692, 4809. (c) Xu, M.; Zhou, R.; Wang, G.; Xiao, Q.; Du, W.; Che, G. Inorg. Chim. Acta 2008, 361, 2407. (d) Lee, S.-C.; Kim, Y. S. Mol. Cryst. Liq. Cryst. 2009, 505, 325. (9) (a) Slinker, J. D.; Rivnay, J.; Moskowitz, J. S.; Parker, J. B.; Bernhard, S.; Abruña, H. D.; Malliaras, G. G. J. Mater. Chem. 2007, 17, 2976. (b) Ulbricht, C.; Beyer, B.; Friebe, C.; Winter, A.; Schubert, U. S. Adv. Mater. 2009, 21, 4418. (c) Xiao, L.; Chen, Z.; Qu, B.; Luo, J.; Kong, S.; Gong, Q.; Kido, J. Adv. Mater. 2011, 23, 926. (d) Hu, T.; He, L.; Duan, L.; Qiu, Y. J. Mater. Chem. 2012, 22, 4206. (10) (a) Babudri, F.; Farinola, G. M.; Naso, F.; Ragni, R. Chem. Commun. 2007, 1003. (b) Chou, P.-T.; Chi, Y. Chem.Eur. J. 2007, 13, 380. (11) Sykes, D.; Tidmarsh, I. S.; Barbieri, A.; Sazanovich, I. V.; Weinstein, J. A.; Ward, M. D. Inorg. Chem. 2011, 50, 11323. (12) Ladouceur, S.; Fortin, D.; Zysman-Colman, E. Inorg. Chem. 2011, 50, 11514. (13) Shavaleev, N. M.; Monti, F.; Costa, R.; Scopelliti, R.; Bolink, H.; Ortí, E.; Accorsi, G.; Armaroli, N.; Baranoff, E.; Grätzel, M.; Nazeeruddin, Md. K. Inorg. Chem. 2012, 51, 2263. (14) Avilov, I.; Minoofar, P.; Cornil, J.; De Cola, L. J. Am. Chem. Soc. 2007, 129, 8247. (15) Kawamura, Y.; Brooks, J.; Brown, J. J.; Sasabe, H.; Adachi, C. Phys. Rev. Lett. 2006, 96, 017404. (16) (a) Springer, C. S., Jr.; Meek, D. W.; Sievers, R. E. Inorg. Chem. 1967, 6, 1105. (b) Miller, M. T.; Gantzel, P. K.; Karpishin, T. B. Angew. Chem., Int. Ed. 1998, 37, 1556. (c) Hasegawa, Y.; Ohkubo, T.; Sogabe, K.; Kawamura, Y.; Wada, Y.; Nakashima, N.; Yanagida, S. Angew. Chem., Int. Ed. 2000, 39, 357. (17) Leung, S.-K.; Liu, H.-W.; Lo, K. K.-W. Chem. Commun. 2011, 47, 10548. (18) (a) Nonoyama, M. Bull. Chem. Soc. Jpn. 1974, 47, 767. (b) Sprouse, S.; King, K. A.; Spellane, P. J.; Watts, R. J. J. Am. Chem. Soc. 1984, 106, 6647. (19) (a) Li, L.; Brennessel, W. W.; Jones, W. D. Organometallics 2009, 28, 3492. (b) Davies, D. L.; Lowe, M. P.; Ryder, K. S.; Singh, K.; Singh, S. Dalton Trans. 2011, 40, 1028. (20) Li, J.; Djurovich, P. I.; Alleyne, B. D.; Yousufuddin, M.; Ho, N. N.; Thomas, J. C.; Peters, J. C.; Bau, R.; Thompson, M. E. Inorg. Chem. 2005, 44, 1713. (21) Habibagahi, A.; Mebarki, Y.; Sultan, Y.; Yap, G. P. A.; Crutchley, R. J. ACS Appl. Mater. Interfaces 2009, 1, 1785. (22) Dedeian, K.; Djurovich, P. I.; Garces, F. O.; Carlson, G.; Watts, R. J. Inorg. Chem. 1991, 30, 1685. (23) (a) Willard, H. H.; Boldyreff, A. W. J. Am. Chem. Soc. 1929, 51, 471. (b) Gagné, R. R.; Koval, C. A.; Lisensky, G. C. Inorg. Chem. 1980, 19, 2854. (c) Gritzner, G.; Kůta, J. Pure Appl. Chem. 1984, 56, 461. (d) Tsierkezos, N. G. J. Solution Chem. 2007, 36, 289. (24) Nicholson, R. S.; Shain, I. Anal. Chem. 1964, 36, 706. (25) Cardona, C. M.; Li, W.; Kaifer, A. E.; Stockdale, D.; Bazan, G. C. Adv. Mater. 2011, 23, 2367. (26) (a) Connor, J. A.; Meyer, T. J.; Sullivan, B. P. Inorg. Chem. 1979, 18, 1388. (b) Indelli, M. T.; Bignozzi, C. A.; Marconi, A.; Scandola, F. J. Am. Chem. Soc. 1988, 110, 7381. (27) Stoyanov, S. R.; Villegas, J. M.; Cruz, A. J.; Lockyear, L. L.; Reibenspies, J. H.; Rillema, D. P. J. Chem. Theory Comput. 2005, 1, 95. (28) Leung, C.-F.; Ng, S.-M.; Xiang, J.; Wong, W.-Y.; Lam, M. H.-W.; Ko, C.-C.; Lau, T.-C. Organometallics 2009, 28, 5709. (29) Sarapu, A. C.; Fenske, R. F. Inorg. Chem. 1975, 14, 247. (30) Colombo, M. G.; Hauser, A.; Güdel, H. Inorg. Chem. 1993, 32, 3088.

S Supporting Information *

Synthesis and characterization of L1−L8 and [(C∧N)2Ir(μCl)] 2 (C ∧ N = L1−L8); NMR spectra; experimental techniques; crystallographic data (Table S1); cyclic voltammograms (Table S2 and Figures S1 and S2); absorption spectra (Figures S3−S6); λ0−0−Ered and λ0−0−ΔE plots (Figures S7 and S8); CIF of the crystal structures of 1 and 4−7, CCDC 892860−892864. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author

*Tel: +41 21 693 6124. Fax: +41 21 693 4111. E-mail: nail. shavaleev@epfl.ch; [email protected]; mdkhaja. nazeeruddin@epfl.ch. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the European Union (CELLO, STRP 248043; https://www.cello-project.eu/) and the CNR (MACOL, PM.P04.010) for financial support.

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