After the pioneering work of Forrest, Thompson, and coworkers ^[1] over the past decade, many efforts have been devoted toward the design, synthesis, and photophysical investigation of new phosphorescent cyclometalated iridium(Ⅲ) complexes ^{[2, 3, 4, 5, 6, 7, 8, 9, 10, 11]}. Substantial research has demonstrated that the chemical structures of cyclometalated ligands (C^N ligands) can strongly affect the photophysical properties of iridium(Ⅲ) complexes. On the other hand, the introduction of a strongly electron-withdrawing group in the C^N ligand could cause a great increase of the luminescence efficiency ^{[12, 13]}.

In pursuit of highly emissive iridium complexes, most recently, we have reported a series of cyclometalated phenylisoquinoline- based iridium(Ⅲ) complexes containing different electron-withdrawing substituents (-CF₃, -CHO, -CN and -F) ^[14]. Their photophysical and electrochemical properties have been investigated, showing that the introduction of the electronwithdrawing fluorine group in the C^N ligand could improve phosphorescence quantum yields effectively. These results prompted us to study how the fluorine group in different positions of C^N ligands affect the pertinent emission properties. Hence, two new fluorine-modificative phenylisoquinoline iridium( Ⅲ) complexes (3a and 3b) have been prepared in this paper (Scheme 1). The spectroscopic and luminescence properties of these complexes are studied, and the lowest-energy electronic transitions are calculated with density functional theory (DFT) and time-dependent DFT (TD-DFT).

Scheme 1

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Scheme 1.Synthetic routes of Ir(Ⅲ) complexes 3a-3b.

¹H NMR spectra were recorded on a Bruker AM 400 MHz instrument. Chemical shifts were reported in ppm relative to Me₄Si as the internal standard. ESI-MS spectra were recorded on an Esquire HCT-Agilent 1200 LC/MS spectrometer. The elemental analyses were performed on a Vario EL Cube Analyzer system. UV- vis spectra were recorded on a Hitachi U3900/3900H spectrophotometer. Fluorescence spectra were carried out on a Hitachi F-7000 spectrophotometer.

All air-sensitive and/or water-sensitive reactions were carried out under a dry nitrogen atmosphere. All commercial chemicals were used without further purification unless otherwise stated. Solvents were dried and degassed following standard procedures. Column chromatography was carried out using 200-300 μm mesh silica.

A mixture of 1-chloroisoquinoline (2.0 mmol), (2, 4-difluorophenyl) boronic acid (2.2 mmol), Pd(dppf)₂Cl₂ (5-10 mg) and potassium acetate (4.0 mmol) was dissolved in anhydrous 1, 4- dioxane (10 mL) and heated at 90 ℃ under nitrogen for 10 h. After cooling to room temperature, the mixture was poured into water (20 mL) and extracted with dichloromethane. The combined organic layers were dried over Na₂SO₄ and concentrated in vacuum. The crude product was purified by column chromatography to afford pure product 1a as a white solid. Yield: 93%. ¹H NMR (400 MHz, CDCl₃): δ 8.56 (d, 1H, J = 5.2 Hz), 7.83 (d, 1H, J = 8.0 Hz), 7.71 (d, 1H, J = 8.4 Hz), 7.64-7.66 (m, 2H), 7.46-7.51 (m, 2H), 7.00 (t, 1H, J = 8.4 Hz), 6.93 (t, 1H, J = 9.2 Hz). MS-ESI: m/z calculated: 241.1; found: 242.1 (M + 1), 264.1 (M + Na).

A procedure analogous to that for 1a was employed with (4- fluoro-2-methylphenyl)boronic acid instead of 2,4-difluorophenyl) boronic acid to afford 1b as a white solid. Yield: 90%. ¹H NMR (400 MHz, CDCl₃): δ 8.62 (d, 1H, J = 5.6 Hz), 7.89 (d, 1H, J = 8.0 Hz), 7.83 (d, 1H, J = 8.0 Hz), 7.68-7.72 (m, 2H), 7.54 (t, 1H, J = 8.0 Hz), 7.44 (t, 1H, J = 7.6 Hz), 7.13 (d, 1H, J = 8.0 Hz), 7.05 (d, 1H, J = 8.0 Hz), 2.47 (s, 3H). MS-ESI: m/z calculated: 237.1; found: 238.1 (M + 1), 260.1 (M + Na).

A mixture of IrCl₃-3H₂O (1.0 mmol) and the ligand 1a in 6 mL of ethoxyethanol and H₂O (v/v = 2: 1) was refluxed for 12 h under nitrogen. Upon cooling to room temperature, the orange precipitate was collected by filtration and washed with cooled MeOH. After drying, the crude product of cholorobridged dimer complex 2a was used directly in next step without further purification. A mixture of the above dimer complex 2a and 2,2'-bipyridine (N^N ligand, 2.0 equiv.) was dissolved in 6 mL of DCM and MeOH (v/v = 2: 1) and heated at 40 ℃ under nitrogen for 6 h. After cooling to room temperature, NH₄PF₆ (2.5 equiv.) was added to the above solution. The mixture was stirred at room temperature for 3 h, and then evaporated to dryness. The solid was purified by column chromatography to afford pure product 3a, Yield: 63%. ¹H NMR (400 MHz, CDCl₃): δ 8.70 (d, 2H, J = 8.0 Hz), 8.27 (t, 2H, J = 9.2 Hz), 8.16 (t, 2H, J = 8.0 Hz), 7.88 (d, 2H, J = 8.0 Hz), 7.67-7.79 (m, 8H), 7.54 (t, 2H, J = 6.4 Hz), 7.46 (d, 2H, J = 6.0 Hz), 7.20 (d, 2H, J = 6.0 Hz), 6.61 (t, 2H, J = 9.2 Hz). MS-ESI: m/z calculated: 829.2; found: 829.3 (M-PF₆). Anal. Calcd. for C₄₀H₂₄F₁₀IrN₄P: C, 49.33; H, 2.48, N, 5.75. Found: C, 49.56, H, 2.53, N, 5.91.

[Ir(fmpiq)₂(bipy)][PF₆] (3b) was obtained by a method similar to the preparation of 3a. Yield: 73%. ¹H NMR (400 MHz, CDCl₃): δ 8.74 (d, 2H, J = 7.2 Hz), 8.18 (t, 2H, J = 7.6 Hz), 8.16-8.20 (m, 3H), 8.16- 8.20 (m, 3H), 7.84-7.89 (m, 3H), 7.76 (t, 3H, J = 7.6 Hz), 7.66 (t, 3H, J = 7.6 Hz), 7.43-7.47 (m, 4H), 7.25-7.28 (m, 2H), 6.68 (t, 2H, J = 9.6 Hz), 2.31 (s, 6H). MS-ESI: m/z calculated: 821.2; found: 821.3 (M-PF₆). Anal. Calcd. for C₄₂H₃₀F₈IrN₄P: C, 52.23; H, 3.13, N, 5.80. Found: C, 52.48, H, 3.21, N, 5.82.

X-ray diffraction data were collected with an Agilent Technologies Gemini A Ultra diffractometer equipped with graphitemonochromated Mo Kα radiation (λ = 0.71073 Å) and Cu Kα radiation (λ = 1.5418 Å) at room temperature. Data collection and reduction were processed with CrysAlisPro software ^[15]. The structure was solved and refined using Full-matrix least-squares based on F² with program SHELXS-97 and SHELXL-97 ^[16] within Olex2 ^[17]. During the refinement, the SQUEEZE option in the program PLATON was used to exclude the contribution from the highly disordered PF₆ anion that cannot be modeled even with restraints ^{[18, 19]}. The 131 electrons voided by SQUEEZE closely corresponds to 1.87 PF₆ anion (131 electrons). All non-hydrogen atoms were found in alternating difference Fourier syntheses and least-squares refinement cycles and, during the final cycles, refined anisotropically. Hydrogen atoms were placed in calculated positions and refined as riding atoms with a uniform value of Uiso. Crystallographic data (excluding structure factors) for the structural analyses have been deposited with the Cambridge Crystallographic Data Centre, Nos. 1062895 and 1062896 for 3a and 3b, respectively.

All calculations were carried out with Gaussian 09 software package ^[20]. The density functional theory (DFT) and timedependent DFT (TD-DFT) were employed with no symmetry constraints to investigate the optimized geometries and electron configurations with the Becke three-parameter Lee-Yang-Parr (B3LYP) hybrid density functional theory ^{[21, 22, 23]}. The LANL2DZ basis set was used to treat the Ir atom, whereas the 6-31G^* basis set was used to treat C, H, N and F atoms. Solvent effects were considered within the SCRF (self-consistent reaction field) theory using the polarized continuum model (PCM) approach to model the interaction with the solvent ^{[24, 25]}.

The luminescence quantum efficiencies were calculated by comparison of the fluorescence intensities (integrated areas) of a standard sample fac-Ir(ppy)₃ and the unknown sample according to the equation ^{[26, 27, 28]}.

The crystals of 3a and 3b suitable for X-ray structural analysis were obtained by slow evaporation of CH₂Cl₂/MeOH solution. The crystallographic data and structure refinement details are given in Table 1; selected bond lengths and bond angles obtained by X-ray diffractions are collected in Table S1 in Supporting information.

Table 1

Table 1 Crystallographic data and structure refinement details for complexes 3a, 3b.

Table 1 Crystallographic data and structure refinement details for complexes 3a, 3b.

The perspective views of the cation of [Ir(f₂piq)₂(bipy)][PF₆] and [Ir(fmpiq)₂(bipy)][PF₆] are depicted in Fig. 1. In the two similar crystal structures, iridium(Ⅲ) adopts a distorted [(C^N)₂Ir(N^N)]⁺ octahedral geometry with the cis-C,C' and trans-N,N' configuration for phenylisoquinoline-based ligand. Simultaneously, the angles of atoms on the para positions of the octahedron range from 172.34(12)° to 173.61(15)° for 3a and 168.56(15)° to 171.91(17)° for 3b.

Fig. 1

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Fig. 1.ORTEP views of 3a and 3b with the atom-numbering scheme at the 50% probability level. Hydrogen atoms and PF6 anion are omitted for clarity.

The Ir-C bonds of these complexes (Ir-C_av = 2.018 Å) are shorter than the Ir-N bonds (Ir-N_av = 2.099 Å). In particular, the Ir-N bond lengths from C^N ligands are significantly shorter than that from N^N ligands, consistent with strong trans influence of the carbon donors. All other bond lengths and angles within the chelate ligands are typical for this type of complexes ^{[14, 29]}. The bite angles of the phenylisoquinoline-based ligands (80.26° for 3a, 79.51° for 3b) and the bipyridine ligands (77.03° for 3a, 76.12° for 3b) lie in the same range as that observed in the literature for similar complexes ^[30]. In addition, the X-ray data show the large torsion angles (C8-C9-C10-C11, -31.358 for 3a and -31.308 for 3b), which suggest that the -F group can cause larger molecular deformation.

The hydrogen bond interactions in the crystal structures are presented in Fig. 2 and the details are summarized in Table 2. This conformation of 3a allows the establishment of two intramolecular C-H…F and C-H…N hydrogen bonds, making the sevenmembered and five-membered rings, respectively. However, the crystal structure of 3b exhibit both the intramolecular (C-H…N) and intermolecular (C-H…F) hydrogen bond interactions. And the molecular skeleton of 3b is linked by the C-H…F intermolecular hydrogen bonds to form a one-dimensional chain structure.

Fig. 2

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Fig. 2.Part of the crystal structures of 3a and 3b, showing selected non-covalent contacts of the C-H…F and C-H…N types (dashed red lines). Atoms involved in hydrogen bonds are shown as balls of arbitrary radii. All other atoms and covalent bonds are represented as wires or sticks. Symmetry codes: (i)-x + 1, y, -z + 1/2 (3a); (i) x,y × 1, z (3b).

Table 2

Table 2 Hydrogen bonding arrangements for 3a and 3b ( Å , °).

Table 2 Hydrogen bonding arrangements for 3a and 3b ( Å , °).

The UV-vis absorption spectra of 3a and 3b in CH₂Cl₂ solution at room temperature are presented in Fig. 3, and the data are provided in Table 3. All these complexes exhibit intense bands at the higher energies (250-350 nm), which are assigned to the spinallowed ligand-centered (LC) π-π^* transitions. Somewhat weaker bands at lower energies (350-550 nm) are observed, which are attributed to both spin-allowed (singlet-to-singlet metal-to-ligand charge transfer, ¹MLCT) and spin-forbidden (singlet-to-triplet metal-to-ligand charge transfer, ³MLCT) transitions ^{[31, 32, 33, 34]}. In comparison with 3a, the lowest lying absorption band for complex 3b is significantly red-shifted. The findings indicate that the substitution of F by CH₃ could lower the HOMO-LUMO gap.

Fig. 3

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Fig. 3.Electronic absorption spectra of 3a and 3b in CH2Cl2 at room temperature.

Table 3

Table 3 Photophysical properties of complexes 3a and 3b.

Table 3 Photophysical properties of complexes 3a and 3b.

The normalized emission spectra of 3a and 3b in degassed CH₂Cl₂ solution at room temperature are given in Fig. 4, and the corresponding data are also listed in Table 3. The two iridium complexes exhibit broad and structureless emission bands centered at 584 and 600 nm, respectively, indicating that they are yellow and orange light-emitting materials. It is interesting to note the substituent changes in C^N ligand of 3b have a marked bathochromic effect on the phosphorescence spectra. It is also evident from Table S2 (Supporting information) that the substitution of F by CH₃ on the C^N ligand increases the electron density around the iridium center (38.83→39.76%) and raises the highest occupied molecular orbital (HOMO) level (-6.083→-5.848 eV), thereby resulting in a reduced HOMO-LUMO energy gap (3.358→3.177 eV).

Fig. 4

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Fig. 4.Normalized emission spectra of 3a and 3b in CH2Cl2 at room temperature.

Phosphorescence relative quantum yields (Φ) of 3a and 3b in dichloromethane solution were measured to be 0.074-0.11 (Table 3) at room temperature by using typical phosphorescent fac-Ir(ppy)₃ as a standard (Φ = 0.40) ^[28]. The comparatively low phosphorescence yield observed for CH₃-substituted complex 3b may be attributed to the intramolecular electron-transfer quenching of the excited state by the methyl group ^[35]. A comparison of the quantum yields observed for these complexes with the previously known related complex [Ir(C^N)₂(bipy)]PF₆ (C^N = 1- (3-fluoro-4-methylphenyl)isoquinoline) (Φ = 0.16) ^[14] is noteworthy. The results show the substitution at 3,4-positions is more effective than 2,4-positions.

Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations were performed for complexes 3a and 3b to investigate the lowest-energy electronic transition of the ultraviolet absorption spectra. The most representative molecular frontier orbital diagrams are presented in Fig. 5. The calculated spin-allowed electronic transitions and electron density distributions are summarized in Table 4 and Table S2.

Fig. 5

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Fig. 5.The frontier molecular orbital diagrams of complexes 3a and 3b from DFT calculations.

Table 4

Table 4 Main experimental and calculated optical transitions for 3a and 3b.

Table 4 Main experimental and calculated optical transitions for 3a and 3b.

For all of the species investigated, the electron density of the highest occupiedmolecular orbital(HOMO) is centeredonthemetal with a substantial contribution from the cyclometalating ligands, as is also reported for similar iridium cyclometalated complexes ^{[36, 37]}. Conversely, the electron density in the lowest unoccupied molecular orbital (LUMO) locates mainly on the ancillary ligands. The successive unoccupied molecular orbital LUMO + 1 primarily originates fromthe cyclometalating ligands. The theory calculations of DFT reveal that the lowest-energy electronic transitions (438 nm for 3a, 459 nm for 3b) both arise from HOMO→LUMO + 1 orbital electronic transitions (Table 4). In comparison with 3a (3.583 eV), the reduction of energy gap for 3b (3.463 eV) is consistent with the red shift observed in absorption spectra.

In conclusion, two new fluorinated phenylisoquinoline-based iridium(Ⅲ) complexes [Ir(C^N)₂(bipy)][PF₆] (3a and 3b) have been synthesized and thoroughly characterized by studying their crystallographic, spectroscopic and luminescence properties. X-ray diffraction studies of complexes 3a and 3b reveal that they all possess pseudo-octahedral coordination geometry. The theoretical calculations have also been performed to rationalize the lowest-energy electronic transitions of ultraviolet absorption spectra. Photoluminescence is observed in the yellow (3a) and orange regions (3b) at room temperature with quantum yield of 0.074-0.11. Upon a change of substituent on cyclometalated ligand, the absorption and emission spectra exhibit a strong dependence on the chemical structures of the ligands. The results will facilitate the design of new phenylisoquinoline-based ligands for light-emitting iridium complexes.

This work was supported by the National Natural Science Foundation of China (No. 21501037), the Natural Science Foundation of Hainan Province (No. 20152017) and the Science and Research Project of Education Department of Hainan Province (Nos. Hjkj2013-25 and Hnky2015-27).

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cclet.2015.12.007.