Modifying Emission Spectral Bandwidth of Phosphorescent Platinum(II

Article pubs.acs.org/IC

Modifying Emission Spectral Bandwidth of Phosphorescent Platinum(II) Complexes Through Synthetic Control Guijie Li,†,‡ Alicia Wolfe,† Jason Brooks,§ Zhi-Qiang Zhu,† and Jian Li*,† †

Department of Materials Science and Engineering, Arizona State University, Tempe, Arizona 85284, United States College of Chemical Engineering, Zhejiang University of Technology, Hangzhou, Zhejiang 310014, China § Universal Display Corporation, 375 Phillips Boulevard, Ewing, New Jersey 08618, United States ‡

S Supporting Information *

ABSTRACT: The design, synthesis, and characterization of a series of tetradentate cyclometalated Pt(II) complexes are reported. The platinum complexes have the general structure Pt(ppz-O-CbPy-R), where a tetradentate cyclometalating ligand is consisting of ppz (3,5-dimethyl-1-phenyl-pyrazole), CbPy (carbazolylpyridine) components, and an oxygen bridging group. Variations of the R group on the pyridyl ring with various electron withdrawing and donating substituents are shown to have profound effects on the photophysical properties of Pt complexes. Electrochemical analysis indicates that reduction process occurs mainly on the electron-accepting pyridyl group, and the irreversible oxidation process is primarily localized on the metal-phenyl portions. The studies of their photophysical properties indicate that the lowest excited state of the platinum complexes is a ligand-centered 3 π−π* state with minor to significant 1MLCT/3MLCT character and are strongly dependent on the nature of the electronaccepting pyridyl moiety. A systematic study of the substituent effects on the pyridyl ring demonstrates that the T1 state properties can be tuned by altering the functionality and positions of substituents. Importantly, it is revealed that how the emission spectra of the Pt(II) complexes can be significantly narrowed and why it can be achieved by incorporating an electrondonating group on the 4-position of the pyridyl ring. Most of the Pt(II) complexes reported here are highly emissive at room temperature in dichloromethane solutions (Φ = 1.1−95%) and in doped PMMA films (Φ = 29−88%) with luminescent lifetimes in the microsecond range (τ = 0.6−13.5 μs in solution and 0.9−11.3 μs in thin film respectively) and λmax = 442−568 nm and 440−544 nm in solution and thin film, respectively. Moreover, these complexes are neutral and thermally stable for sublimation, indicating that they can be useful for display and solid-state lighting applications.

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INTRODUCTION

0.1 (CIE: Commission International de L’Eclairge) will be crucial to achieve such goals.34,39 The study of structure−property relationships for tetradentate cyclometalated Pt(II) and Pd(II) complexes has emerged as an interesting research topic in recent years due to its flexibility provided for ligand design.39−54 Therefore, a variety of tetradentate Pt(II) complexes incorporating symmetric and asymmetric ligands have been developed, which, in some cases, have been demonstrated to be efficient green and red phosphorescent emitters, such as Pt(IndO∧N∧C∧N) with the ligand of 5,5-dibutyl-2-(3-(pyridine-2-yl)-4,6-difluoro-phenyl)5H-indeno[1,2-b]pyridine-9-olate,26,53 Pt(N2O2) using 2,9-bis(2′-hydroxyphenyl-4,7-di(tert-buyl)-1,10-phenanthroline40 or Schiff base N,N-phenylenediamine, 41 Pt(Prtmen) with (2E,3E)-N2,N3-bis((1H-pyrrol-2-yl)methylene)-2,3-dimethylbutane-2,3-diamine,42 Pt(C∧N*N∧C) with N,N-di(6-phenylpyridin-2-yl)aniline, Pt(N∧C*C∧N) with N,N-di(3-(pyridin-2yl)-phenyl)aniline and its derivatives43 and Pt-metalloporphyrin

Phosphorescent cyclometalated iridium and platinum complexes have attracted great attention in both academia and industry in the past two decades because of potential applications in the areas of photocatalysts for single-electrontransfer reactions,1−4 asymmetric catalysis,5,6 biological imaging agents,7 chemical sensors,8,9 and as phosphorescent emitters 1 0 − 1 8 for use in organic light-emitting diodes (OLEDs).19−36 These complexes emit from the triplet state with high efficiency because the strong spin−orbit coupling gives fast intersystem crossing from singlet to triplet state, and the triplet can emit with a high photoluminescent quantum efficiency and a high radiative decay rate.20,21,25,37,38 Because of the great endeavor and ongoing development of phosphorescent emitters, now they have demonstrated commercial viability in display and solid-state lighting applications. However, challenges remain for the development of efficient and stable deep blue phosphorescent emitters in OLED applications.39 Among all the challenges, the development of efficient and stable narrowband deep blue emitters with CIEy ≤ © 2017 American Chemical Society

Received: April 14, 2017 Published: June 26, 2017 8244

DOI: 10.1021/acs.inorgchem.7b00961 Inorg. Chem. 2017, 56, 8244−8256

Article

Inorganic Chemistry

Figure 1. Structures of the tetradentate Pt(ppz-O-CbPy-R) complexes.

complexes44−47 (see Figure S1). And several blue luminescent tetradentate Pt(II) complexes30,39,43,48−51 have been demonstrated in the past few years, which include the methylenebridged Pt(tetra-NHC) (NHC: N-heterocyclic carbine)48 and Pt(N∧C*C∧N) incorporated with the ligand of N,N-di(3-(3methyl-1H-pyrazol-1-yl)phenyl)aniline43 (see Figure S1). Recently, our group reported a series of highly emissive deep-blue tetradentate Pt(II) complexes, such as PtON1 (Figure 1), demonstrating a peak external quantum efficiency (EQE) of 25.2% and full-width at half-maximum (FWHM) value of 46 nm with CIE coordinates of (0.15, 0.13) in device settings.49 It is important to note that a saturated blue OLED employed PtON1-tBu as phosphorescent emitter, which exhibited a dramatically narrow emission spectrum with a FWHM value of 24 nm and CIE coordinates of (0.151, 0.098) was observed after an tert-butyl group (-tBu) was incorporated at the 4position of the pyridyl moiety in PtON1.39 This result demonstrated that a simple structure modification can significantly affect the emission spectra for this series of tetradentate Pt(II) complexes, but the substituent effect of -tBu was not clearly understood. Further study will be needed to fully understand the effects of substituent on emission properties. Here, we report a series of tetradentate cyclometalated Pt(ppz-O-CbPy-R) complexes with systematic substitution changes on the pyridyl ring (Figure 1). Most of these complexes are highly emissive in solution and in doped PMMA films with luminescent lifetimes in the range of microseconds. In addition, these complexes are neutral and thermally stable for sublimation. The electrochemical and photophysical properties of the Pt(ppz-O-CbPy-R) complexes are discussed here in detail. The electrochemical measurement

of all Pt(ppz-O-CbPy-R) complexes demonstrate that an irreversible oxidation process occurs on the metal-phenyl fragment, and the reduction process is predominantly localized on the neutrally coordinated electron-accepting pyridyl moiety. These assignments are supported by the HOMO/LUMO orbital pictures from density functional theory (DFT) calculations. We find that incorporating substituents on the pyridyl moieties can significantly affect the reduction potentials but only has a small influence on the oxidation potentials. The lowest excited state of this series of cyclometalated Pt(ppz-OCbPy-R) complexes is identified as a ligand-centered 3π−π* state with minor to significant 1MLCT/3MLCT(Pt → Cb-PyR) character. The lowest excited state properties are strongly dependent on the nature of the electron-accepting pyridyl moieties, and the 1MLCT/3MLCT(Pt → Cb-Py-R) state energy can be tuned by altering the substituent functionalities or substitution positions on the pyridyl ring. Notably, several efficient deep blue emitters demonstrated FWHM values of only 15−20 nm and quantum efficiencies of 80−95% in dichloromethane solution.

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RESULTS AND DISCUSSION Design, Synthesis, and Characterization. To investigate how structural modifications affect molecular rigidity, the ground and excited state properties of tetradentate Pt(II) complexes, PtON1 and a group of its derivatives were designed and synthesized. The platinum complexes have the general structure Pt(ppz-O-CbPy-R), where ppz is 3,5-dimethyl-1phenyl-pyrazole and CbPy is carbazolylpyridine. Both ppz and CbPy are C∧N cyclometalating ligands which are linked together by an oxygen atom. R can be a hydrogen atom or a substituent on the pyridyl ring. The structures and abbrevia8245


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Inorganic Chemistry Scheme 1. Synthesis of the Ligands and the Tetradentate Pt(ppz-O-CbPy-R) Complexes

no desirable product in the similar reaction condition, which can be attributed to the steric hindrance of methyl group on such position. Overall, the cyclometalated Pt(ppz-O-CbPy-R) complexes were prepared by direct metalation of the ligands with potassium tetrachloroplatinate (K2PtCl4) in acetic acid under reflux in nitrogen atmosphere.34 Yields ranged between 10 and 58% after purification with conventional column chromatography. It should be noted that all the metalation reactions were carried out a single time without further optimization. All new platinum complexes were characterized by 1H NMR, 13C NMR spectroscopy, and mass spectrometry. DFT Calculations. B3LYP DFT calculations were performed using the Titan software package25 (wave function, Inc.) at the LACVP**25,38 level. A similar theoretical approach has been used to investigate the optimized molecular geometry, ground and excited states properties for related cyclometalated Ir(III) and Pt(II) complexes.25,38,39,43,53 All Pt(ppz-O-CbPy-R) complexes exhibit significant distortion from planarity on the carbazolyl-pyridine (CbPy) section of the ligand, to accommodate square planar coordination to the metal (Figures S2− S10). Substituent effects on the 4-position (para position) and 5-position (meta position) of the pyridyl rings do not significantly affect the geometric structure of the inner Pt(N∧C∧C∧N) coordination, such as PtON1-Me and PtON1mMe. On the other hand, the steric hindrance of the methyl group on the 6-position (ortho position) of the pyridyl ring of PtON1-oMe significantly distorts the square planar configuration of metal complex (Table S1). The bond lengths of Pt− N1 (2.18 or 2.19 Å), Pt−C2 (1.98 Å), Pt−C3 (1.98 or 1.99 Å), and Pt−N4 (2.18 or 2.19 Å) are nearly identical for all the PtON1 analogues except the bond length of Pt−N4 (2.23 Å) of PtON1-oMe is significantly longer. These bond lengths are comparable to the corresponding bond lengths found in the crystal structure of PtON7-dtb, which has a similar Pt(II) core structure.39 The bond lengths and angles within the metal coordination are also similar to PtON6.39 However, as mentioned above, the N4−Pt-C1 bond angle (105.33°) in

tions of the cyclometalated Pt(II) complexes reported here are shown in Figure 1, and various substituents are incorporated on the pyridyl ring leaving the core structure intact. This group of substituents include the ligands with strong σ-electron-donating capability, such as tert-butyl (-tBu), N,N-dimethylamino (-NMe2), and methyl (-Me) groups, and the trifluoromethyl (−CF3) group, i.e., a strong σ-electron-withdrawing group, and selected aryl groups, including phenyl (-Ph) and 9-carbzolyl (-Cz) groups. In addition, PtON1 derivatives with methyl substitution at the 4, 5, and 6 (para, meta, ortho, respectively) positions of the pyridyl ring are also evaluated and included in this work. The synthetic route of the ligands and a representative complex, Pt(ppz-O-CbPy-R), is shown in Scheme 1. Ligand ON1 can be readily prepared through a direct C−O crosscoupling reaction55 of 3-(3,5-dimethyl-1H-pyrazol-1-yl)phenol 1 and 2-bromo-9-(pyridin-2-yl)-9H-carbazole 2 catalyzed by the copper(I) iodide (CuI) in the presence of picolinic acid and tripotassium phosphate (K3PO4) in 96% yield. However, it was difficult to introduce the substituent to the pyridyl moiety of the compounds 2 through the direct copper or palladium catalyzed C−N cross-coupling reaction of 2-bromocarbazole and various 2-bromo- or 2-chloropyridine derivatives because of nonselective coupling. Therefore, an alternate and more effective route was employed for the synthesis of the proposed ligands. Copper(I)-catalyzed coupling55 of phenol intermediate 1 and 4′-iodo-2-nitrobiphenyl 3 gave the nitro intermediate 4. Ring closure was accomplished through a reduction reaction promoted by excess PPh3 to get the critical precursor carbazole derivative 5.56 This intermediate was coupled with functionalized 2-bromo- or 2-chloropyridine smoothly catalyzed by Pd2(dba)2/JohnPhos in the presence of sodium t-butoxide (tBuONa) under nitrogen atmosphere57 to give the desired ligands in high yields of 88−97%, where Pd2(dba)2 is tris(dibenzylideneacetone)dipalladium and JohnPhos is 2-(ditert-butylphosphino)biphenyl. Unfortunately, the reaction between precursor 5 and 3-methyl-2-bromopyridine yielded 8246


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Figure 2. Density functional theory (DFT) calculation of HOMO (left) and LUMO (right) for PtON1.

Table 1. Photophysical and Electrochemical Properties of the Tetradentate Pt(ppz-O-CbPy-R) Complexesa absorption at RT complex PtON1

49

PtON1Me PtON1mMe PtON1oMe PtON1tBu PtON1NMe2 PtON1CF3 PtON1-Ph PtON1-Cz PtON749 PtON7dtb

λmax (nm) {ε, 103 cm−1 M−1} 265{35.7}, 299{20.3}, 317{19.5}, 366{8.5}, 437{0.10) 264{41.2}, 295{22.9}, 318{23.2}, 361{11.9}, 436{0.08) 265{46.4}, 297{26.5}, 317{25.1}, 366{11.7}, 438{0.20} 267{36.7}, 297{19.2}, 317{16.3}, 361{7.4}, 437{0.07} 264{36.8}, 294{20.1}, 318{20.2},363{10.1}, 436{0.06} 269{41.7}, 318{23.4}, 349{15.2}, 434{0.06} 265{106.0}, 306{71.1}, 399{13.7} 268{54.6},302{33.0}, 384{8.7} 265{68.6}, 322{47.2}, 335{45.5}, 376{16.7} 261{46.7}, 300{27.8}, 321{24.9}, 369{14.1}, 440{0.20} 261{36.8}, 289{22.7}, 323{18.8}, 363{12.1}, 395{5.4}, 438{0.13}

emission at RT

emission at 77 K

FWHM (nm/cm−1)

τ (μs)

454(sh), 478 444

85/3664

3.3

20/1001

450(sh), 476 450, 478(sh) 444

kr (104 s−1)

knr (104 s−1)

λmax (nm)

τ (μs)

Eox (V)

Ered (V)

71

21.5

8.8

440

9.2

0.49

−2.54

10.0

89

8.9

1.1

438

12.7

0.49

−2.66

79/3474

3.5

82

23.4

5.1

440

9.2

0.48

−2.56

121/5672

3.1

45

14.5

17.7

440

11.5

0.49

−2.61

20/1049

8.9

95

10.7

0.6

438

11.5

0.49

−2.65

442

15/815

13.5

80

5.9

1.5

436

16.4

0.42

−2.91

568

104/3171

0.6

1.1b

1.8

165

494

5.7

0.58

−2.04

546 496

95/3147 84/3304

0.8 1.8

19b 53b

23.8 29.4

101 26.1

470 441

6.7 4.9

0.51 0.52

−2.22 −2.42

452

64/2861

4.2

78

18.6

5.2

442

6.4

0.50

−2.58

446

20/945

5.4

85

15.7

2.8

439

7.4

0.46

−2.73

λmax (nm)

Φ (%)

a

The room temperature (RT) absorption and emission spectra were measured in a solution of dichloromethane and low temperature (77K) emission spectra were measured in a solution of 2-methyl-THF. Coumarin 47 was used as reference for quantum efficiency measurement unless noted. bCoumarin 6 was used as reference for quantum efficiency measurement.

CF3 has a quasi-reversible reduction process; however, PtON1tBu, PtON1-Cz, and PtON1-Ph exhibit well-defined reversible reduction processes (Figures 3, S14−S20). The irreversible oxidation process for all reported Pt(II) complexes can be attributed to the potential reactivity with the solvent on the square planar Pt(III) metal centers.58,59 It is found that structural modifications to the pyridyl ring strongly affect the first reduction potential of the complex without significantly affecting the oxidation potential. In line with this trend, functional groups with increasing electron donating strength (such as, -Me, -tBu, and -NMe2) on the 4-position of the pyridyl ring dramatically shift the reduction potential to more negative values, but keep the oxidation potentials nearly the same. This effect is particularly profound for PtON1-NMe2, which exhibits a 370 mV more negative than that of PtON1. A similar trend has also been observed for Pt(N∧C∧N)Cl analogues.59 On the other hand, the −CF3 group on the 4position of the pyridyl ring shifts the reduction potential to a less negative value (−2.04 V), which is 500 mV less negative than that of PtON1 (−2.54 V). Meanwhile, both PtON1-Ph

PtON1-oMe is larger than that in PtON1 (102.10°), and the N1−Pt-C3 bond angle (162.21°) is smaller than that in PtON1(165.66°) (Table S2). The HOMO and LUMO orbitals for the parent structure, PtON1, are shown in Figure 2. The HOMO consists of a mixture of localized phenyl-π, carbazolylπ, and Pt-d orbitals, while the LUMO predominantly occupies on the pyridyl-π orbitals with very little metal and pyrazolyl orbital character. Electrochemistry. Electrochemical properties were measured using cyclic voltammetry (CV) and differential pulsed voltammetry (DPV). All the electrochemical data reported here were measured relative to an internal ferrocenium/ferrocene (Cp2Fe+/Cp2Fe) reference in anhydrous dimethylformamide (DMF) solution under an nitrogen atmosphere. The redox potentials of PtON1 and its analogues were recorded in Table 1. All the Pt(ppz-O-CbPy-R) complexes exhibit an irreversible oxidation between 0.42 and 0.58 V, and a first reduction with a wide range between −2.04 and −2.91 V. PtON1-Me, PtON1mMe, PtON1-oMe, and PtON1-NMe2 show irreversible reduction processes in the CV measurements, and PtON18247


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electrochemical data demonstrated that the changes to the reduction potential due to the change of substituents on the pyridyl ring of PtON1 is consistent with the previous literature reports that the reduction process mainly occurs on the pyridyl ring, and the oxidation process is typically associated with the metal-phenyl portion.38,59 Photophysical Properties. The room temperature (RT) absorption and emission spectra and low temperature (77 K) emission spectra of all Pt(ppz-O-CbPy-R) were recorded in Table 1. For all Pt(ppz-O-CbPy-R) complexes, the high energy strong absorption bands below 300 nm (ε > 1 × 104 cm−1 M−1) are assigned to 1(π−π*) transitions on the cyclometalating ligands (LC).60−62 The relatively intense bands between 300 and 425 nm (ε > 4 × 103 cm−1 M−1) are assigned to metal-toligand charge-transfer (MLCT) transitions involving both the cyclometalating ligands and the platinum metal ions.63 The weaker, lowest energy absorption bands in the 425−500 nm region (ε < 300 cm−1 M−1) can be identified as direct absorption to the lowest triplet state (T1) on the basis of the small energy shift between absorption and emission at room temperature. Structural modifications to the pyridyl ring result in significant changes to the absorption bands associated with the 1MLCT and T1 transitions. Introducing strong electrondonating groups to the pyridyl ring (e.g., PtON1-NMe2) leads to an increase in 1MLCT and T1 transition energies with major changes in the molar absorptivity (Figure 4a); for example, molar absorptivity of the T1 absorption of PtON1-NMe2 at 434 nm (ε = 60 cm−1 M−1) is much smaller than that of PtON1 at 437 nm (ε = 100 cm−1 M−1) (Table 1). Moreover, modifying the Pt(ppz-O-CbPy-R) complex structures with the -Me group on the different positions of the pyridyl rings has a relatively

Figure 3. Cyclic voltammograms for PtON1 (dashed), PtON1-tBu (dash-dotted), and PtON1-Cz (solid).

and PtON1-Cz shift the oxidation potential to more positive values and shift the reduction potential to less negative values, which can be attributed to the existence of the conjugated group bonded to the pyridyl ring, making pyridyl group easier to be reduced and resulting in shifting their reduction potentials to less negative values. The effect of the methyl group at various substituent positions on the pyridyl ring was also investigated. It was found that methyl substitution on the para position shifts the reduction potential to a more negative value, while meta substitution of the methyl group had a marginal effect. Ortho substitution also results in a more negative shift in the reduction potential; however the geometrical distortion of Pt(II) complex may also play a significant role. Overall, the

Figure 4. (a) Comparison of room-temperature absorption spectra of PtON1 (green squares), PtON1-NMe2 (blue solid circles), and PtON1-Cz (red triangles) in CH2Cl2. The T1 absorption transitions are shown in the inset of (a). Luminescence spectra of (b) PtON1, (c) PtON1-NMe2, (d) PtON1-Cz at room temperature in CH2Cl2 (solid lines) and 77 K in 2-Me-THF (dash-dotted lines). The chemical structure and CIE coordinates (RT) of each emitter are shown in the inset. 8248


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Inorganic Chemistry

Figure 5. (a) Comparison of room-temperature absorption spectra of PtON1-Me (blue circles), PtON1-mMe (green solid squares), and PtON1oMe (red solid triangles) in CH2Cl2. The T1 absorption transitions are shown in the inset of (a). Luminescence spectra of (b) PtON1-Me, (c) PtON1-mMe, (d) PtON1-oMe at room temperature in CH2Cl2 (solid lines) and 77 K in 2-Me-THF (dash-dotted lines). The chemical structure and CIE coordinates (RT) of each emitter are shown in the inset.

Figure 6. (a) Comparison of room-temperature absorption spectra of PtON1 (green squares), PtON1-Ph (blue solid circles), and PtON1-CF3 (red solid triangles) in CH2Cl2. The T1 absorption transitions are shown in the inset of (a). Luminescence spectra of (b) PtON1, (c) PtON1-Ph, (d) PtON1-CF3 at room temperature in CH2Cl2 (solid lines) and 77 K in 2-Me-THF (dash-dotted lines). The chemical structure and CIE coordinates (RT) of each emitter are shown in the inset.

than these of PtON1-Me at 436 nm (ε = 80 cm−1 M−1) and PtON1-oMe at 437 nm (ε = 70 cm−1 M−1) (Table 1). Furthermore, the addition of electron-withdrawing groups (i.e., PtON1-CF3) and aryl groups (i.e., PtON1-Ph and PtON1-Cz)

small influence on the LC, 1MLCT, and T1 transition energies (Figure 5a). However, the molar absorptivity including the T1 absorption of three complexes are different; for example, PtON1-mMe at 438 nm (ε = 200 cm−1 M−1) is much bigger 8249


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Inorganic Chemistry results in a decrease in the energy of the 1MLCT and T1 transitions and a significant increase of the molar absorptivity for all LC 1MLCT and T1 transitions, although the T1 absorption of PtON1-CF3 , PtON1-Ph, and PtON1-Cz becomes less resolved (Figure 4a and Figure 6a). Structural modification significantly affects the emission spectra and luminescence lifetimes of Pt complexes (Table 1). In general, compared with the parent structure, PtON1, the introduction of electron-donating groups to the 4-position of pyridyl rings (i.e., PtON1-Me, PtON1-tBu, PtON1-NMe2) slightly increases the T1 emission energies and significantly increases the luminescence lifetimes; for example, the lifetimes of PtON1-Me, PtON1-tBu, PtON1-NMe2 are 10.0, 8.9, and 13.5 μs respectively at room temperature in dichloromethane solution, which are much longer than that of PtON1 with a τ of 3.3 μs. It is also found that the emission spectra become significantly narrower with the FWHM values of only 15−20 nm at room temperature in dichloromethane solution. On the other hand, the addition of electron-withdrawing group (i.e., PtON1-CF3) and aryl groups (e.g., PtON1-Ph and PtON1-Cz) decreases the emission energies and the luminescent lifetimes. Methyl substitutions on the 5- and 6-positions of the pyridyl group (e.g., PtON1-mMe and PtON1-oMe) are shown to have minimal effects on the T1 state energies and luminescent lifetimes. These results illustrate that the photophysical properties of the Pt(ppz-O-CbPy-R) complexes can be significantly modified by varying the substituents on the 4position of the pyridyl moieties without altering the core structure of Pt complexes. Most of these tetradentate Pt(ppz-O-CbPy-R) complexes are intensely emissive (Φ = 19−95%) and have relatively short luminescent lifetimes (τ = 0.6−13.5 μs) at room temperature in dichloromethane solution with the notable exception of PtON1-CF3 (Φ = 1.1%) (Table 1). The radiative decay (kr) and nonradiative decay (knr) rates can be readily calculated from Φ and τ values through the equations of kr = Φ/τ and knr = (1 − Φ)/τ, where Φ is the quantum efficiency and τ is the luminescent lifetime of the sample at room temperature.64 The knr values of the Pt(ppz-O-CbPy-R) complexes are in the range of 1.65 × 106 and 6 × 103 s−1, and the kr values span in a relatively narrower range from 2.94 × 105 to 1.8 × 104 s−1, which indicates that both knr and kr values are strongly structure-dependent. The low photoluminescent quantum yield (PLQY) of PtON1-CF3 can be attributed to a large nonradiative decay rate.65 All the tetradentate Pt(ppz-OCbPy-R) complexes are highly luminescent at 77 K and in doped poly(methyl methacrylate) (PMMA) films (Φ = 29− 88%) at the room temperature. Luminescent lifetimes were measured and reported to be in ranges of 4.9−16.4 μs at 77K (Table 1) and 0.9−11.3 μs in doped PMMA films at room temperature (Table 2). The kr values of the Pt(ppz-O-CbPy-R) complexes are in the range of 7.8 × 104 and 4.0 × 105 s−1 in doped PMMA films and tend to increase with the decrease of the emission energy (Table 2). And the knr values range between 1.1 × 104 and 7.89 × 105 s−1 (Table 2). And it is observed that both kr and knr values decrease if the electrondonating group is incorporated on the 4-position of the pyridyl ring; otherwise, kr and knr values will increase if an electronwithdrawing group or aryl group is introduced (Table 2). It is worth mentioning that the PL quantum yields of PtON1-Ph and PtON1-CF3 in doped PMMA films (88% and 29%) are significant higher than those reported in dichloromethane solutions (19% and 1.1%) at room temperature. Presumably

Table 2. Photophysical Properties of the Pt(ppz-O-CbPy-R) Complexes in Doped PMMA Films complex

λmax (nm)

Φ (%)

τ (μs)

kr (×104 s−1)

knr (× 104 s−1)

PtON149 PtON1-Me PtON1-mMe PtON1-oMe PtON1-tBu PtON1-NMe2 PtON1-CF3 PtON1-Ph PtON1-Cz PtON6-tBu PtON749 PtON7-dtb

449 445 445 449 445 440 544 503 480 447 452 447

85 84 84 78 88 88 29 88 64 81 89 91

4.5 7.6 4.3 4.8 8.8 11.3 0.9 2.2 2.0 7.4 4.1 4.7

18.9 11.1 19.5 16.2 10.0 7.8 32.2 40.0 32.0 10.9 21.7 19.4

3.3 2.1 3.7 4.6 1.4 1.1 78.9 5.5 18.0 2.6 2.7 1.9

this is due to the change of radiative decay rates; for example, the radiative decay rate of PtON1-Ph in doped PMMA films (4.0 × 105 s−1) is significantly higher than that in solution (2.38 × 105 s−1). And a similar phenomenon was also found in the previously reported tetradentate Pt(II) complex, i.e., PtOO1.25 Ground-State and Excited-State Properties. It is well documented that the lowest triplet excited state (T1) arises from a mixed ligand-centered charge-transfer state (3LC) with metal-to-ligand-charge-transfer character (1MLCT/3MLCT) due to strong spin−orbit coupling.37,38,66−68 However, it is not well understood why the addition of electron donating groups, such as −Me, −tBu, and −NMe2, at the 4-position of the pyridyl ring leads to the decrease of emission spectral bandwidth of their emission spectra. The comparison of absorption and emission spectra at room temperature and 77 K of PtON1,51 PtON1-NMe2 and PtON1Cz is shown in Figure 4. All three complexes have similar emission energies, small vibronic emission sideband with dominant emission peaks at λmax = 440, 436, and 441 nm respectively at 77 K. The FWHM of 77 K emission spectra is in the range of 8−12 nm, indicating that the emission arises mostly from a 3LC state mixed with small 1MLCT/3MLCT characters (Pt → CbPy-R). However, the room temperature emission spectra vary significantly among three compounds. PtON1 exhibits structured dual emission peaks at 454 and 478 nm, which is believed to originate from a mixed 3LC state of the phenyl-pyrazole (ppz) moiety and 1MLCT/3MLCT(Pt → CbPy) state of the carbazolyl-pyridine (CbPy) component respectively, and the latter one is thermally accessible at elevated temperature (Figure 7a).39 Moreover, the addition of the electron-donating group (−NMe2) to the 4-position of the pyridyl ring shifts the reduction potential to a more negative value and the 1MLCT/3MLCT(Pt → CbPy-NMe2) transition energies, resulting in a weaker mixing of the 1MLCT/3MLCT characters in the T1 state (Figure 7b) and less thermally accessible 1MLCT/3MLCT emission at the room temperature. Thus, PtON1-NMe2 exhibits a narrow emission spectrum with a FWHM value of 15 nm (815 cm−1) and a CIEy value of 0.064 (Figure 4c). To the best of our knowledge, such emission spectrum is among the narrowest for cyclometalated Ir(III) and Pt(II) complexes, and the color purity is higher than most reported quantum-dot emission spectra.69−71 This analysis can be further supported by the fact that PtON1-NMe2 has a longer luminescent lifetime, a lower kr value, and lower extinction coefficient of the triplet absorption while having more 8250


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Figure 7. (a−c) Schematic energy level diagram for state mixing in tetradentate Pt(ppz-O-CbPy-R) complexes at room temperature.

Figure 8. Photoluminescence spectra of (a) PtON1-tBu, (b) PtON6-tBu, (c) PtON7-tBu, and (d) PtON7-dtb at room temperature in CH2Cl2 (solid lines) and 77 K in 2-Me-THF (dash-dotted lines) with the molecular structure and CIE coordinates (RT) of each emitter inset.

temperature absorption and emission spectra of PtON1-Me, PtON1-mMe, and PtON1-oMe in dichloromethane and 77 K emission spectra in 2-methyl tetrahydrofuran are shown in Figure 5. All three complexes show very similar LC, 1MLCT, and T1 transition energies and narrow structured emission spectra at 77 K, similar to PtON1, indicating that the T1 states of all three complexes are localized mainly on the phenylpyrazole moiety. However, their room-temperature emission spectra are vastly different. PtON1-Me shows a narrow emission spectrum with a FWHM value of 20 nm, similar to previously described PtON1-NMe2. On the other hand, both PtON1-mMe and PtON1-oMe have less structured emission spectra with two dominant peaks at 450, 476 nm and 450, 478 nm respectively, similar to PtON1, which is consistent with the electrochemical analysis that the −Me group on the 4 (para) position provide a more negative value of reduction potential than those on the 5 (meta) or 6 (ortho) positions. It should be noted that the emission spectra of PtON1-mMe and PtON1oMe in doped PMMA films are very similar and have

pronounced vibronic features in the emission spectrum compared to PtON1. A similar change is also observed by comparing PtON1-Me or PtON1-tBu with PtON1. On the other hand, PtON1-Cz shows a very broad Gaussian shaped emission spectrum with a FWHM of 84 nm and a pronounced shift of the maximum emission wavelength to 496 nm (Figure 4d) at the room temperature, indicating a larger portion of 1 MLCT/3MLCT(Pt → CbPy-Cz) emission characteristics which are thermally accessible. PtON1-Cz also exhibits a lower MLCT transition energy and a less negative value of the reduction potential relative to PtON1, which indicates that PtON1-Cz has more 1MLCT/3MLCT(Pt → CbPy-Cz) characters mixed in the T1 state (Figure 7c). This assignment is also consistent with the fact that PtON1-Cz has a shorter luminescent lifetime, a higher kr value, and a much stronger MLCT absorption than PtON1. To further investigate the effect of the substituent position on the pyridyl ring, the methyl group is added to the 4 (para), 5 (meta), or 6 (ortho) position of the pyridyl ring. The room8251


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Solutions of coumarin 47 (coumarin 1) (Φ = 0.73, excited at 360 nm)72 in ethanol were used as references for PtON1-Me, PtON1mMe, PtON1-oMe, PtON1-tBu, and PtON1-NMe2. Solutions of coumarin 6 (Φ = 0.78, excited at 420 nm)73 in ethanol were used as references for PtON1-Cz, PtON1-CF3, and PtON1-Ph. The equation

comparable FWHM values to their room temperature solution emission spectra (Figure S31). In addition, the T1 state energy can be further decreased if the −Ph or −CF3 group is placed on the 4-position of the pyridyl ring. Thus, both PtON1-Ph and PtON1-CF3 have broad and less-resolved 77 K emission spectra with their emission peaking at 470 and 494 nm respectively, which are much different than other Pt(II) complexes discussed above. These indicated that the T1 states mainly localize on the CbPy-Ph or CbPy-CF3 moieties. In room temperature solution, they also show broad Gaussian-shaped emission spectra (Figure 6), which may be associated with the less negative values of the reduction potential. Thus, it is possible to tune the emission energy of Pt(ppz-O-CbPy-R) complexes over a wide spectral range by varying the substituents. The molecular engineering method developed here is critical to the design of high color purity blue phosphorescent emitters for display applications, which can be applied to other blue phosphorescent material classes with similar ligand scaffold. Our work demonstrated that the para alkyl substitution, such as the -tBu group, was also found to yield the narrow emission spectra for 4-phenylpyrazole and carbene based tetradentate complexes, such as PtON6-tBu,39 PtON7-tBu,34 and PtON7-dtb39 (Figure 8), whose FWHM values of the emission spectra are in the range of 19−20 nm (Table 1) and CIEy values of 0.1.

Φs = Φr

was used to calculate the quantum yields where Φs is the

quantum yield of the sample, Φr is the quantum yield of the reference, η is the refractive index of the solvent, As and Ar are the absorbance of the sample and the reference at the wavelength of excitation and Is and Ir are the integrated areas of emission bands.74 Phosphorescence lifetime measurements were performed on the same spectrometer with a time correlated single photon counting method using an lightemitting diodes (LED) excitation source. The absolute PL quantum efficiency measurements of doped thin film were carried out on a Hamamatsu C9920 system equipped with a xenon lamp, integrating sphere, and a model C10027 photonic multichannel analyzer. The radiative and nonradiative rate constants were calculated from the luminescent quantum yield (Φ) and lifetime (τ) of the phosphorΦ 1−Φ escent complexes according to κr = τ and κnr = τ respectively, where kr and knr are the radiative and nonradiative rate constants, respectively. Electrochemistry. Cyclic voltammetry and different pulsed voltammetry were performed using a CH Instrument 610B electrochemical analyzer, 0.1 M tetra-n-butylammonium hexafluorophosphate was used as the supporting electrolyte, and anhydrous dimethylformamide was used as the solvent under a nitrogen atmosphere. A silver wire was used as the pseudoreference electrode. A platinum wire was used as the counter electrode, and glassy carbon was used as the working electrode. The redox potentials are based on the values measured from different pulsed voltammetry and are reported relative to an internal reference ferrocenium/ferrocene (Cp2Fe/Cp2Fe+).75 The reversibility of reduction or oxidation was determined using CV.76 As defined, if the magnitudes of the peak anodic and the peak cathodic current have an equal magnitude as scan speeds of 100 mV/s or slower, then the process is considered reversible; if the magnitudes of the peak anodic and the peak cathodic currents are not equal, but the return sweeps are nonzero, the process is considered quasi-reversible; otherwise, the process is considered irreversible.76,77 General Procedures. 1H NMR spectra were recorded at 400 MHz, 13C NMR spectra were recorded at 100 MHz on Varian liquidstate NMR instruments in DMSO-d6 or CD2Cl2 solutions and chemical shifts were referenced to residual protiated solvent. 1H NMR spectra were recorded with residual H2O (δ = 3.33 ppm in DMSO-d6; δ = 1.52 ppm in CD2Cl2) as internal reference;78 13C NMR spectra were recorded with DMSO-d6 (δ = 39.52 ppm) or CD2Cl2 (δ = 53.84 ppm) as internal reference.78 19F NMR spectra was referenced to PhCF3 in C6D6 (δ = −65.00 ppm) as external standard. The following abbreviations (or combinations thereof) were used to explain 1H NMR ultiplicities: s = singlet, d = doublet, t = triplet, m = multiplet. Mass spectra were recorded on Shimadzu Biotech Axima Performance MALDI-TOF mass spectrometer. PtON1-tBu was prepared according our previously reported literature.39 Syntheses. General procedure for the synthesis of the platinum complexes: Ligand (1.0 equiv), K2PtCl4 (1.05 equiv), nBu4NBr (0.1 equiv), and solvent acetic acid (60 mL/mmol ligand) were added to a dry pressure tube equipped with a magnetic stir bar. The mixture was bubbled with nitrogen for 30 min, and then the tube was sealed. The mixture was stirred at room temperature for 19−48 h and then at 105−115 °C in an oil bath for another 3 days, cooled down to ambient temperature, and then water (120 mL/mmol ligand) was added. After being stirred at room temperature for 5 min, the precipitate was filtered off and washed with water three times, and dried in air under reduced pressure. The collected solid was purified through column chromatography on silica gel using dichloromethane or dichloromethane/hexane as eluent to obtain the desired product. Platinum(II)[6-(3,5-dimethyl-1H-pyrazol-1-yl-κN2)-1,2-phenyleneκC1]oxy[9-(pyridin-2-yl-κN)-9H-carbazole-1,2-diyl-κC1] (PtON1): 2(3-(3,5-dimethyl-1H-pyrazol-1-yl)phenoxy)-9-(pyridin-2-yl)-9H-carba-

■

CONCLUSIONS In this article, a series of tetradentate Pt(ppz-O-CbPy-R) complexes were designed, synthesized, and characterized. The electrochemical and photophysical properties of the complexes are discussed in detail. The study of the electrochemical properties of these Pt(II) complexes demonstrates that the reduction process occurs on the pyridyl group, and the oxidation process mainly takes place on the metal-phenyl fragment. The addition of electron-donating groups on the pyridyl ring shifts reduction potentials to more negative values, while electron-withdrawing and aryl substituents shift reduction potentials to less negative values. Moreover, the excited state properties of these platinum complexes are strongly dependent on the functionality and position of substituents. As a result, the emission spectra of the Pt(ppz-O-CbPy-R) complexes can be significantly narrowed by incorporating electron-donating group on the 4-position of the pyridyl ring. Furthermore, several efficient deep blue emitters with narrow emission spectra are developed, such as PtON1-Me, PtON1-tBu, and PtON1-NMe2, which achieve FWHM values of 15−20 nm and quantum efficiencies of 80−95% in solution. Additionally, most of the Pt(II) complexes discussed here are highly emissive with reasonably short luminescent lifetimes. They are neutral and stable during the sublimation process, making them ideal candidates to study as phosphorescent materials. The molecular engineering method developed here is critical to the design of high color purity blue phosphorescent emitters for display and solid state lighting applications.

■

ηs2A r Is ηr2A sIr

EXPERIMENTAL SECTION

Photophysical Measurements. The UV−visible spectra were recorded on a Hewlett-Packard 4853 diode array spectrometer. Steady state emission experiments at room temperature were performed on a Horiba Jobin Yvon FluoroLog-3 spectrometer. Solution quantum efficiency measurements were measured at room temperature in dichloromethane solutions, which were thoroughly bubbled with nitrogen inside of a glovebox with less than 1 ppm of oxygen. 8252


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as the general procedure. Purification of the crude product by flash chromatography on silica gel (eluent: dichloromethane) afforded the title compound as a yellow solid 496 mg in 58% yield. 1H NMR (DMSO-d6, 400 MHz): δ 2.36 (s, 3H), 2.37 (s, 3H), 2.70 (s, 3H), 6.38 (s, 1H), 6.91 (d, J = 7.6 Hz, 1H), 7.10 (d, J = 7.0 Hz, 1H), 7.13 (d, J = 8.0 Hz, 1H), 7.17 (t, J = 8.0 Hz, 1H), 7.23 (d, J = 7.2 Hz, 1H), 7.36 (t, J = 7.6 Hz, 1H), 7.45 (t, J = 7.6 Hz, 1H), 7.80 (d, J = 8.0 Hz, 1H), 7.94 (s, 1H), 8.09 (t, J = 8.4 Hz, 2H), 9.03 (d, J = 6.0 Hz, 1H). 1H NMR (CD2Cl2, 400 MHz): δ 2.28 (s, 3H), 2.38 (s, 3H), 2.67 (s, 3H), 6.09 (s, 1H), 6.76 (d, J = 6.4 Hz, 1H), 7.05 (d, J = 8.0 Hz, 1H), 7.17 (d, J = 7.6 Hz, 1H), 7.22 (t, J = 7.6 Hz, 1H), 7.29 (d, J = 8.0 Hz, 1H), 7.38 (t, J = 7.2 Hz, 1H), 7.44 (d, J = 7.6 Hz, 1H), 7.81 (d, J = 8.0 Hz, 1H), 7.88 (s, 1H), 7.96 (d, J = 8.0 Hz, 1H), 8.06 (d, J = 7.6 Hz, 1H), 8.96 (d, J = 6.4 Hz, 1H). 13C NMR (CD2Cl2, 100 MHz): δ 14.85, 15.08, 21.59, 100.57, 107.22, 110.31, 111.81, 112.82, 113.15, 115.15, 115.35, 115.95, 116.37, 119.99, 120.19, 122.99, 124.32, 124.67, 129.27, 138.89, 141.95, 143.16, 148.40, 149.13, 149.89, 151.37, 152.76, 153.18, 153.21. MS (MALDI) for C29H22N4OPt [M]+: calcd 637.1, found 637.1. Platinum(II)[6-(3,5-dimethyl-1H-pyrazol-1-yl-κN2)-1,2-phenyleneκC 1 ]oxy[9-(5-methylpyridin-2-yl-κN)-9H-carbazole-1,2-diyl-κC 1 ] (PtON1-mMe): 2-(3-(3,5-Dimethyl-1H-pyrazol-1-yl)phenoxy)-9-(5methylpyridin-2-yl)-9H-carbazole ligand ON1-mMe (410 mg, 0.92 mmol, 1.0 equiv) reacted with K2PtCl4 (402 mg, 0.97 mmol, 1.05 equiv) in the present of nBu4NBr (30 mg, 0.092 mmol, 0.1 equiv) at room temperature for 19 h and then at 105−115 °C for another 3 days as the general procedure. Purification of the crude product by flash chromatography on silica gel (eluent: hexane/dichloromethane = 1:1) afforded the title compound as a yellow solid 111 mg in 19% yield. 1H NMR (CD2Cl2, 400 MHz): δ 2.34 (s, 3H), 2.47 (s, 3H), 2.74 (s, 3H), 6.20 (s, 1H), 7.00−7.03 (m, 1H), 7.20−7.26 (m, 3H), 7.35−7.39 (m, 1H), 7.41−7.46 (m, 1H), 7.74 (d, J = 9.2 Hz, 1H), 7.77 (dd, J = 8.0, 1.6 Hz, 1H), 7.94 (d, J = 8.4 Hz, 1H), 8.05 (d, J = 8.4 Hz, 2H), 9.09 (s, 1H). 1H NMR (DMSO-d6, 400 MHz): δ 2.40 (s, 3H), 2.43 (s, 3H), 2.77 (s, 3H), 6.47 (s, 1H), 6.96 (d, J = 8.0 Hz, 1H), 7.17 (d, J = 8.4 Hz, 1H), 7.23 (t, J = 8.0 Hz, 1H), 7.29 (d, J = 8.0 Hz, 1H), 7.40 (d, J = 8.4 Hz, 1H), 7.47−7.51 (m, 1H), 7.85 (d, J = 8.0 Hz, 1H), 8.02 (dd, J = 8.4, 2.0 Hz, 1H), 8.08 (d, J = 7.6 Hz, 1H), 8.13 (d, J = 8.4 Hz, 1H), 8.15 (d, J = 8.4 Hz, 1H), 9.09 (d, J = 1.6 Hz, 1H). 13C NMR (CD2Cl2, 100 MHz): δ 14.72, 15.16, 17.67, 100.21, 107.25, 110.41, 111.44, 112.78, 113.27, 114.93, 115.44, 115.58, 116.30, 120.23, 122.93, 124.33, 124.79, 128.05, 129.20, 138.97, 140.30, 142.11, 143.10, 147.68, 148.41, 149.93, 152.76, 153.20, 153.43. MS (MALDI) for C29H23N4OPt [M + H]+: calcd 637.9, found 638.0. Platinum(II)[6-(3,5-dimethyl-1H-pyrazol-1-yl-κN2)-1,2-phenyleneκC 1 ]oxy[9-(6-methylpyridin-2-yl-κN)-9H-carbazole-1,2-diyl-κC 1 ] (PtON1-oMe): 2-(3-(3,5-Dimethyl-1H-pyrazol-1-yl)phenoxy)-9-(6methylpyridin-2-yl)-9H-carbazole Ligand ON1-oMe (510 mg, 1.15 mmol, 1.0 equiv) reacted with K2PtCl4 (500 mg, 1.20 mmol, 1.05 equiv) in the present of nBu4NBr (37 mg, 0.115 mmol, 0.1 equiv) at room temperature for 23 h and then at 105−115 °C for another 3 days as the general procedure. Purification of the crude product by flash chromatography on silica gel (eluent: dichloromethane) afforded the title compound as a yellow solid 271 mg in 37% yield. 1H NMR (CD2Cl2, 400 MHz): δ 2.19 (s, 3H), 2.68 (s, 3H), 3.05 (s, 3H), 6.11 (s, 1H), 7.01 (dd, J = 7.6, 2.0 Hz, 1H), 7.08 (d, J = 7.6 Hz, 1H), 7.16− 7.22 (m, 3H), 7.34 (t, J = 8.4 Hz, 1H), 7.42 (t, J = 8.4 Hz, 1H), 7.64 (t, J = 8.4 Hz, 1H), 7.75 (t, J = 8.4 Hz, 1H), 7.96−7.99 (m, 2H), 8.09 (d, J = 8.4 Hz, 1H). 1H NMR (DMSO-d6, 400 MHz): δ 2.14 (s, 3H), 2.69 (s, 3H), 2.98 (s, 3H), 6.35 (s, 1H), 6.96 (dd, J = 8.0, 0.8 Hz, 1H), 7.12 (d, J = 8.0 Hz, 1H), 7.20 (t, J = 8.0 Hz, 1H), 7.25 (dd, J = 7.6, 0.8 Hz, 1H), 7.33 (d, J = 7.6 Hz, 1H), 7.37 (t, J = 7.6 Hz, 1H), 7.45−7.49 (m, 1H), 7.72 (d, J = 8.4 Hz, 1H), 8.00 (t, J = 8.0 Hz, 1H), 8.08−8.13 (m, 3H). 13C NMR (DMSO-d6, 100 MHz): δ 12.84, 13.99, 27.35, 102.71, 107.70, 109.58, 111.71, 112.80, 112.82, 113.41, 114.61, 115.98, 116.51, 120.09, 121.23, 122.89, 124.55, 124.60, 128.77, 138.72, 140.13, 141.31, 142.58, 147.18, 149.46, 149.53, 151.85, 153.31, 161.27. MS (MALDI) for C29H23N4OPt [M + H]+: calcd 638.2, found 638.0. Platinum(II)[6-(3,5-dimethyl-1H-pyrazol-1-yl-κN2)-1,2-phenyleneκC1]oxy[9-(4-(trifluoromethyl)pyridin-2-yl-κN)-9H-carbazole-1,2-diylκC1] (PtON1-CF3): 2-(3-(3,5-Dimethyl-1H-pyrazol-1-yl)phenoxy)-9-

zole ligand ON1 (1340 mg, 3.11 mmol, 1.0 equiv) reacted with K2PtCl4 (1356 mg, 3.27 mmol, 1.05 equiv) in the present of nBu4NBr (100 mg, 0.31 mmol, 0.1 equiv) at room temperature for 48 h and then at 105−115 °C for another 3 days as the general procedure. Purification of the crude product by flash chromatography on silica gel (eluent: dichloromethane) afforded the title compound as a yellow solid 1057 mg in 55% yield. 1H NMR (DMSO-d6, 400 MHz): δ 2.40 (s, 3H), 2.73 (s, 3H), 6.42 (s, 1H), 6.96 (d, J = 8.0 Hz, 1H), 7.18 (d, J = 8.4 Hz, 1H), 7.22 (d, J = 8.0 Hz, 1H), 7.26−7.30 (m, 2H), 7.39 (t, J = 7.6 Hz, 1H), 7.47 (t, J = 7.6 Hz, 1H), 7.85 (d, J = 7.6 Hz, 1H), 8.08 (d, J = 8.0 Hz, 1H), 8.11−8.19 (m, 3H), 9.24 (d, J = 4.8 Hz, 1H). 13C NMR (DMSO-d6, 100 MHz): δ 14.31, 14.36, 100.17, 107.12, 110.29, 110.97, 112.18, 112.40, 115.07, 115.29, 115.68, 115.71, 119.02, 119.93, 122.85, 124.46, 124.70, 127.95, 137.84, 140.14, 141.87, 141.93, 147.40, 147.98, 149.66, 151.61, 151.99, 153.78. MS (MALDI) for C28H20N4OPt [M]+: calcd 623.1, found 623.0. The spectroscopic data is in agreement with that previously reported.49 Platinum(II)[6-(3,5-dimethyl-1H-pyrazol-1-yl-κN2)-1,2-phenyleneκC1]oxy[9-(4-N,N-dimethylpyridin-2-yl-κN)-9H-carbazole-1,2-diylκC 1 ] (PtON1-NMe 2 ): 2-(2-(3-(3,5-dimethyl-1H-pyrazol-1-yl)phenoxy)-9H-carbazol-9-yl)-N,N-dimethylpyridin-4-amine ligand ON1-NMe2 (560 mg, 1.18 mmol, 1.0 equiv) reacted with K2PtCl4 (514 mg, 1.24 mmol, 1.05 equiv) in the present of nBu4NBr (38 mg, 0.118 mmol, 0.1 equiv) at room temperature for 24 h and then at 105−115 °C for another 3 days as the general procedure. Purification of the crude product by flash chromatography on silica gel (eluent: hexane/dichloromethane = 1:2) afforded the title compound as a yellow solid 81 mg in 10% yield. 1H NMR (DMSO-d6, 400 MHz): δ 2.45 (s, 3H), 2.74 (s, 3H), 3.08 (s, 6H), 6.42 (s, 1H), 6.66 (dd, J = 7.2, 2.0 Hz, 1H), 6.92 (d, J = 8.0 Hz, 1H), 7.10 (d, J = 2.0 Hz, 1H), 7.12 (dd, J = 8.4, 1.2 Hz, 1H), 7.19 (t, J = 8.0 Hz, 1H), 7.25 (d, J = 8.0 Hz, 1H), 7.35 (t, J = 8.0 Hz, 1H), 7.45 (t, J = 8.4 Hz, 1H), 7.81 (dd, J = 8.4, 0.8 Hz, 1H), 8.12 (d, J = 8.0 Hz, 1H), 8.20 (d, J = 7.6 Hz, 1H), 8.67 (d, J = 6.4 Hz, 1H). 1H NMR (CD2Cl2, 400 MHz): δ 2.47 (s, 3H), 2.71 (s, 3H), 3.02 (s, 6H), 6.14 (s, 1H), 6.32 (dd, J = 7.2, 2.4 Hz, 1H), 6.97−6.99 (m, 1H), 7.13−7.21 (m, 4H), 7.32 (t, J = 7.6 Hz, 1H), 7.40 (t, J = 8.0 Hz, 1H), 7.75 (dd, J = 8.0, 2.0 Hz, 1H), 8.02 (d, J = 8.0 Hz, 2H), 8.69 (dd, J = 7.2, 2.0 Hz, 1H). 13C NMR (CD2Cl2, 100 MHz): δ 14.81, 15.14, 39.69, 96.01, 100.83, 103.58, 107.16, 110.14, 112.51, 112.94, 113.09, 115.00, 115.04, 116.24, 120.12, 122.39, 124.13, 124.25, 129.18, 139.41, 141.85, 143.99, 148.41, 149.53, 149.82, 152.65, 153.16, 153.32, 155.98. MS (MALDI) for C30H25N5OPt [M]+: calcd 666.2, found 666.0. Platinum(II)[6-(3,5-dimethyl-1H-pyrazol-1-yl-κN2)-1,2-phenyleneκC1]oxy[9-(4-(9H-carbazol-9-yl)pyridin-2-yl-κN)-9H-carbazole-1,2diyl-κC1] (PtON1-Cz): 9-(4-(9H-carbazol-9-yl)pyridin-2-yl)-2-(3(3,5-dimethyl-1H-pyrazol-1-yl)phenoxy)-9H-carbazole Ligand ON1Cz (910 mg, 1.53 mmol, 1.0 equiv) reacted with K2PtCl4 (672 mg, 1.60 mmol, 1.05 equiv) in the present of nBu4NBr (49 mg, 0.15 mmol, 0.1 equiv) at room temperature for 18 h and then at 105−115 °C for another 3 days as the general procedure. Purification of the crude product by flash chromatography on silica gel (eluent: dichloromethane/hexane = 2:1) afforded the title compound as an orange solid 617 mg in 51% yield. 1H NMR (DMSO-d6, 400 MHz): 2.56 (s, 3H), 2.77 (s, 3H), 6.48 (s, 1H), 6.98 (d, J = 8.0 Hz, 1H), 7.21 (d, J = 8.0 Hz, 1H), 7.25 (d, J = 8.0 Hz, 1H), 7.20−7.44 (m, 5H), 7.54 (d, J = 7.6 Hz, 2H), 7.70 (dd, J = 6.8, 2.0 Hz, 1H), 7.85 (d, J = 8.0 Hz, 3H), 8.13 (d, J = 7.6 Hz, 1H), 8.22−8.28 (m, 4H), 9.43 (d, J = 6.0 Hz, 1H). 13C NMR (DMSO-d6, 100 MHz): δ100.44, 107.19, 110.35, 110.45, 110.91, 111.15, 112.33, 112.48, 114.90, 115.39, 115.93, 120.08, 120.85, 121.73, 123.13, 123.97, 124.64, 124.75, 126.89, 128.16, 137.96, 138.56, 142.07, 142.22, 146.76, 147.40, 149.58, 149.78, 151.65, 151.99, 155.44. MS (MALDI) for C40H27N5OPt [M]+: calcd 688.2, found 688.1. Platinum(II)[6-(3,5-dimethyl-1H-pyrazol-1-yl-κN2)-1,2-phenyleneκC 1 ]oxy[9-(4-methylpyridin-2-yl-κN)-9H-carbazole-1,2-diyl-κC 1 ] (PtON1-Me): 2-(3-(3,5-Dimethyl-1H-pyrazol-1-yl)phenoxy)-9-(4methylpyridin-2-yl)-9H-carbazole Ligand ON1-Me (595 mg, 1.34 mmol, 1.0 equiv) reacted with K2PtCl4 (583 mg, 1.41 mmol, 1.05 equiv) in the present of nBu4NBr (42 mg, 0.13 mmol, 0.1 equiv) at room temperature for 19 h and then at 105−115 °C for another 3 days 8253


Article

Inorganic Chemistry Notes

(4-(trifluoromethyl)pyridin-2-yl)-9H-carbazole Ligand ON1-CF3 (900 mg, 1.81 mmol, 1.0 equiv) reacted with K2PtCl4 (787 mg, 1.90 mmol, 1.05 equiv) in the present of nBu4NBr (58 mg, 0.18 mmol, 0.1 equiv) at room temperature for 22 h and then at 105−115 °C for another 3 days as the general procedure. Purification of the crude product by flash chromatography on silica gel (eluent: hexane/dichloromethane = 1:1) afforded the title compound as a yellow solid 720 mg in 58% yield. 1H NMR (CD2Cl2, 400 MHz): δ 2.41 (s, 3H), 2.72 (s, 3H), 6.17 (s, 1H), 7.03−7.06 (m, 1H), 7.11 (d, J = 5.6 Hz, 1H), 7.18−7.25 (m, 2H), 7.31 (dd, J = 8.0, 1.6 Hz, 1H), 7.41−7.50 (m, 2H), 7.80 (dd, J = 8.4, 1.6 Hz, 1H), 7.93 (d, J = 8.4 Hz, 1H), 8.07 (d, J = 7.6 Hz, 1H), 8.36 (s, 1H), 9.39 (d, J = 6.4 Hz, 1H). 13C NMR (CD2Cl2, 100 MHz): δ 14.97, 15.14, 100.19, 107.38, 110.48, 110.49, 112.74 (q, J = 3.7 Hz), 113.36, 113.43, 113.49 (q, J = 2.6 Hz), 114.76, 115.79, 116.57, 120.57, 122.90 (q, J = 272.4 Hz), 124.03, 124.92, 125.19, 129.77, 138.56, 140.11 (q, J = 35.5 Hz), 142.31, 142.56, 148.34, 149.87, 150.00, 152.96, 153.01, 155.49. 19F NMR (DMSO-d6, 376 MHz): δ −64.10. MS (MALDI) for C29H19F3N4OPt [M]+: calcd 691.1, found 690.9. Platinum(II)[6-(3,5-dimethyl-1H-pyrazol-1-yl-κN2)-1,2-phenyleneκC 1 ]oxy[9-(4-phenylpyridin-2-yl-κN)-9H-carbazole-1,2-diyl-κC 1 ] (PtON1-Ph): 2-(3-(3,5-Dimethyl-1H-pyrazol-1-yl)phenoxy)-9-(4phenylpyridin-2-yl)-9H-carbazole ligand ON1-Ph (679 mg, 1.34 mmol, 1.0 equiv) reacted with K2PtCl4 (584 mg, 1.41 mmol, 1.05 equiv) in the present of nBu4NBr (43 mg, 0.134 mmol, 0.1 equiv) at room temperature for 19 h and then at 105−115 °C for another 3 days as the general procedure. Purification of the crude product by flash chromatography on silica gel (eluent: hexane/dichloromethane = 1:1) afforded the title compound as a yellow solid 517 mg in 55% yield. 1H NMR (DMSO-d6, 400 MHz): δ 2.35 (s, 3H), 2.70 (s, 3H), 6.35 (s, 1H), 6.96 (d, J = 8.0 Hz, 1H), 7.18−7.26 (m, 3H), 7.35−7.37 (m, 2H), 7.45−7.48 (m, 4H), 7.71−7.72 (m, 2H), 7.84 (d, J = 8.4 Hz, 1H), 8.12 (d, J = 7.2 Hz, 1H), 8.18 (s, 1H), 8.22 (d, J = 8.4 Hz, 1H), 9.14 (d, J = 6.4 Hz, 1H). 13C NMR (DMSO-d6, 100 MHz): δ 14.32, 14.35, 100.17, 107.15, 110.29, 111.18, 112.23, 112.38, 112.42, 115.04, 115.31, 115.69, 116.63, 120.02, 122.90, 124.72, 126.96, 128.01, 129.37, 130.11, 136.08, 137.99, 141.88, 142.09, 147.41, 148.26, 149.61, 150.15, 151.64, 152.01, 153.99. MS (MALDI) for C34H24N4OPt [M]+: calcd 699.2, found 699.0.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Department of Energy (contract no. EE0007090), Universal Display Corporation, Advanced Photovoltaics Center, National Natural Science Foundation of China (Grant Nos. 21602198) and “Qianjiang Talents Plan” (Grant No. QJD1602017) for partial support of this work.

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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.7b00961. Synthesis experimental details; 1H NMR, 13NMR characterization; spectra of new ligands; tetradentate Pt(II) complexes discussed in this report (Figure S1); optimized molecular structure of PtON1 analogues based on the DFT calculation (Figures S2−S11); chemical structures of PtON1, PtON6, PtON7, and their derivatives (Figure S12); selected bond lengths for PtON1 and its analogues based on the DFT calculation and X-ray crystallographic analysis (Table S1); selected bond angles for PtON1 and its analogues based on the DFT calculation and X-ray crystallographic analysis (Table S1); cyclic voltammogram for PtON1 analogues (Figures S13−S21); absorption spectra and emission spectra (Figures S25−S34) (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Guijie Li: 0000-0002-0740-2235 Jian Li: 0000-0003-3300-0990 8254


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Modifying Emission Spectral Bandwidth of Phosphorescent Platinum(II

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