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Pt(II) Phosphors Featuring Both Dicarbene and Functional Biazolate Chelates: Synthesis, Luminescent Properties, and Applications in Organic Light-Emitting Diodes Jia-Ling Liao,† Yun Chi,*,† Jin-Yun Wang,‡ Zhong-Ning Chen,*,‡ Zheng-Hua Tsai,§ Wen-Yi Hung,*,§ Meu-Rurng Tseng,∥ and Gene-Hsiang Lee⊥ †

Department of Chemistry and Low Carbon Energy Research Center, National Tsing Hua University, Hsinchu 30013, Taiwan Fujian Institute of Research on the Structure of Matter, CAS, State Key Laboratory of Structural Chemistry, Fuzhou, 350002, China § Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung 202, Taiwan ∥ Material and Chemical Research Laboratories, Industrial Technology Research Institute, Hsinchu 31040, Taiwan ⊥ Instrumentation Center, National Taiwan University, Taipei 10617, Taiwan ‡

S Supporting Information *

ABSTRACT: Pt(II) metal complexes [Pt(C^C)(X^X)] comprising three functional dianionic azolate chelates (X^XH2: bipzH2 = 5,5′-di(trifluoromethyl)-3,3′-bipyrazole, bitzH2 = 5,5′-di(trifluoromethyl)-3,3′-bi-1,2,4-triazole, and phpzH2 = 3-(trifluoromethyl)-5-(4-(trifluoromethyl)phenyl)-1H-pyrazole), together with three different charge-neutral dicarbene chelates (i.e., C^C = 1,1′-methylene bis(3-methyl-imidazol-2ylidene), 1,1′-methylene bis(3-isopropyl-imidazol-2-ylidene), and 1,1′-(propane-1,3diyl) bis(3-isopropyl-imidazol-2-ylidene), were synthesized and found to show bright solid-state emission depending on the associated X^X and C^C chelates. Pt(II) complexes 1a, 2, and 6 were examined by X-ray diffraction studies, confirming the square-planar skeleton. These Pt(II) metal complexes are found to be nonemissive in degassed solution at RT. The photophysical measurements as neat powder reveals emission maxima ranging from purple to sky blue emission and with high quantum yields for the majority of them. (Time-dependent) density functional theory (DFT/TD-DFT) calculations were executed to elucidate the emission process that was predominated by the combined3LLCT/3LMCT/3IL character, where LLCT and LMCT and IL stand for ligand-to-ligand charge transfer, ligand-to-metal charge transfer, and intraligand ππ* transition processes. Organic light-emitting devices comprising complex 5a achieved high efficiency (8.9%, 19.4 cd·A−1, 22.5 lm·W−1) with a sky blue emission showing CIEx,y coordinates of (0.18, 0.32).

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chelates such as bipyridine31−35 and dicarbene chelate36−40

INTRODUCTION

During the past two decades, transition-metal phosphors have been extensively studied for their potential applications in fabricating optoelectronic devices such as organic light-emitting diodes (OLEDs) and others.1−10 In relevant studies, the Ir(III) metal phosphors were typically assembled using three monoanionic cyclometalates and relevant chelates to provide the charge-neutral coordination structure, and adequate emission hues spanning from blue, green to saturated red, as well as matched energy gaps required to achieve balanced carrier transport for the fabricated OLEDs.11−13 Similarly, neutral Pt(II) metal phosphors can also be assembled using a single dianionic tetradentate chelate,14−20 or two anionic bidentate chelates in either homoleptic or heteroleptic modes,21−26 to fulfill the square-planar coordination arrangement. The N-heterocyclic (NHC) carbene is known to be capable in producing Pt(II) metal complexes with distinctive structural and spectroscopic properties.27−30 In addition, heteroaromatic © XXXX American Chemical Society

shown below

are found to be useful in assembling emissive Pt(II) complexes with diversified electronic and optical properties, while inclusion of commensurate anions such as acetylide(s),40−44 dithiolate anion(s),45,46 and other chelating dianions47−50 is demanded for achieving both the charge neutrality and coordination saturation around the Pt(II) metal center. In this respect, the hydrocarbyl-based, dianionic bidentate chelate is of particular interest, due to the capability in imitating the coordination properties of other bidentate chelates, together Received: January 14, 2016

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DOI: 10.1021/acs.inorgchem.6b00097 Inorg. Chem. XXXX, XXX, XXX−XXX

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(trifluoromethyl)-5-(4-(trifluoromethyl)phenyl)-1H-pyrazole (phpzH2) was obtained by condensation of 1-(4-(trifluoromethyl)phenyl)ethanone with ethyl trifluoroacetate, followed by hydrazine cyclization. The synthetic protocol of phpzH2 is depicted in Scheme 1, while the associated spectral data are depicted in the Experimental Section.

with the potentials in realizing better chemical stability and structural tunability. However, reports on the design of hydrocarbyl-based dianions and associated metal complexes are sparse. The simplest example is the dianionic 2,2′-biphenyl, which is known to afford Ir(III) and Pt(II) metal complexes, albeit of lowered emission efficiency in ambient.51,52 The second example is the CF3 substituted biazolate chelates (i.e., both bipyrazolate bipz2− and bitriazolate bitz2−).53,54 It is believed that both the electrondeficient CF3 fragments and the excessive nitrogen atoms would stabilize the azolates, making them the ideal candidate for replacing two cis-arranged monodentate anions. Hence, new Ru(II) sensitizers for dye-sensitized solar cells were obtained by replacement of their thiocyanate ligands with the equivalent biazolate for tuning the spectral window, electrochemical properties, and longevity of solar cells.53,54

Scheme 1. Synthetic Protocol for PhpzH2a

a

Experimental conditions: (i) CF3CO2Et, NaOEt, THF, reflux; (ii) N2H4, EtOH, reflux.

Next, preparation of the heteroleptic Pt(II) complexes bearing both functional dicarbene and biazolate follows a two-step process in one pot. Pt(II) complexes with the structural formula [Pt(C^C)Cl2] (C^C = dicarbene) were synthesized from reaction of Pt(DMSO)2Cl2 and imidazolium proligand in the presence of NaHCO3 in DMSO for 19 h at 120 °C and served as the synthetic intermediates. Without isolation of these intermediates, a biazolate class of chelates such as bipzH2 and bitzH2, together with 2 equiv of NaHCO3 in the case for the phpzH2 reaction, were added into the DMSO solution of the dicarbene Pt(II) intermediate and heated at 120 °C for another 12 h. Accordingly, heteroleptic Pt(II) complexes 1−6, together with 1a and 5a, were obtained as colorless or pale yellow solids with satisfactory yields. Their structural drawings are depicted in Scheme 2. The Pt(II) complexes 1a and 5a are methyl derivatives of the isopropylsubstituted dicarbene complexes 1 and 5, while Pt(II) complexes 2, 4, and 6 are derivatives of 1, 3, and 5, on which their methylene bridge is deliberately replaced by the propylene for studying the orientational effect imposed by the

In the present work, our aims are focused on the preparation of Pt(II) complexes with a series of functional dicarbene chelates together with the biazolate chelates, e.g., bipz2− and bitz2− and the closely related cyclometalating derivative phpz2−. The biazolates bipz2− and bitz2− are known to react with various Os(II), Ir(III), and Pt(II) metal reagents in the presence of bipyridine, phenylpyridine, and analogues to afford the corresponding emissive metal complexes.55−58 Upon substituting bipyridine with dicarbene, due to the destabilized empty π*-orbitals and the enhanced σ-bonding strength of the carbene moiety,59 the associated products would display a much blue-shifted emission peak maxima versus that of the bipyridine counterparts, while the greater ligand field strength of dicarbene would also destabilize the otherwise thermally accessible, yet nonemissive, metal-centered (MC) dd excited states,60−62 providing the suitable molecular design in retaining the emission efficiency. Details of experimental results and the discussion are elaborated in the following sections.

Scheme 2. Structural Drawings of the Studied Pt(II) Metal Complexes

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RESULTS AND DISCUSSION Syntheses and Characterizations. Three dicarbene proligands, i.e., 1,1′-methylene bis(3-methyl-imidazol-3-ium), 1,1′-methylene bis(3-isopropyl-imidazol-3-ium), and 1,1′-(propane-1,3-diyl) bis(3-isopropyl-imidazol-3-ium), with either a methylene or a propylene spacer, were prepared and isolated as hexafluorophosphate salts by the literature method.63,64 These dicarbene chelates are finding widespread use in the preparation of metal complexes suitable for activation of small molecules and as metal catalysts,65−67 and for making radiopharmaceuticals68 as well as luminescent materials.69 On the other hand, the dianionic chelates, namely, 5,5′-di(trifluoromethyl)-3,3′-bipyrazole (bipzH2), was best prepared using Claisen condensation employing ethyl trifluoroacetate and 2,3butanedione, and subsequent heating with hydrazine at reflux,53 while 5,5′-di(trifluoromethyl)-3,3′-di-1,2,4-triazole (bitzH2) was obtained from reaction of trifluoroacetamidine and oxalyl dihydrazide, dehydration, and sublimation. Syntheses of bipzH2 and bitzH2 have been documented in the literature,53 while 3B

DOI: 10.1021/acs.inorgchem.6b00097 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry dicarbene chelate. Furthermore, the coordination skeleton of these Pt(II) metal complexes is conceptually analogous to the derivatives bearing at least one carbene-containing functional chelate.70−74 These Pt(II) complexes 1−6 were purified by column chromatography and recrystallization, while mass spectrometry, 1 H and 19F NMR, and elemental analyses were executed for spectral characterizations. The key spectroscopic data were the two downfield 1H NMR signals assigned to the CH groups of the imidazol-2-ylidene fragments. These signals appeared as two doublets in the region of δ 7.72−7.40, and with a 3JHH coupling constant of ∼2.0 Hz due to the cis-C-H fragments of the Pt(II) complexes 1−4. In sharp contrast, Pt(II) complexes 5 and 6 possess the asymmetric phpz2− chelate rather than the symmetric bipz2− and bitz2− chelates; these imidazol-2-ylidene signals split to four distinctive doublets, for which their identifications were seriously masked with signals derived from the 4-trifluorophenyl fragment of the phpz2− chelate. The single-crystal X-ray diffraction studies on 1a, 2, and 6 were also executed, for which the perspective view, selected bond lengths, and angles are shown in Figures 1, 2, and 3. As

Figure 3. Structural drawing of complex 6 with thermal ellipsoids shown at 30% probability level. Selected bond distances: Pt−N(1) = 2.038(3), Pt−C(1) = 2.045(3), Pt−C(12) = 1.967(3), and Pt−C(18) = 2.047(3) Å. Selected bond angles: N(1)−Pt−C(1) = 79.85(13), C(12)−Pt−C(18) = 91.00(13), C(12)−Pt−N(1) = 173.76(12), and C(18)−Pt−C(1) = 174.21(13)°. H atoms are omitted for clarity.

chelates, i.e., bipz2−, bitz2−, and phpz2−. Their packing diagrams are presented in Figure S1 (Supporting Information). As can be seen, among the Pt(II) molecules that are packed in slipped, and in antiparallel dimeric orientation, the shortest intermolecular Pt···Pt contacts are measured to be 7.821, 7.387, and 7.658 Å for the Pt(II) complexes 1a, 2, and 6, respectively. These Pt···Pt distances are found to be much longer than those of Pt(II) complexes with notable solid-state stacking interaction (e.g., 5 (0.77 V) in both the bipz2− and phpz2− substituted Pt(II) complexes, which reflected the influences of the coordination geometry of dicarbene

respectively. Finally, all of these carbene complexes 1−6 showed bright emission ranging from blue to violet in the solid state, which is in good agreement with the higher emission energy observed in many Pt(II) carbene complexes reported in the literature.36−40,82,83 Theoretical Calculation. (Time-dependent) density functional theory (DFT/TD-DFT) calculations were carried out to study the detailed electronic and photophysical properties of Pt(II) complexes 1a, 5a, and 1−6. The calculated absorption properties in DMSO are summarized in Tables S1−S8 (Supporting Information). The obtained absorption spectra agree well with their experimental results (Figure S4). The frontier molecular orbitals involved in the absorption processes are plotted in Figures S5−S12. The more intense absorptions of the S3 state (primarily coming from the transitions of (HOMO-2/HOMO-1 → LUMO)) for 1a and 1, the S1 state (HOMO-1 → LUMO) for 2, the S3 state (HOMO-1 → LUMO+2) for 3, the S4 state (HOMO-2 → LUMO) for 4, and the S3 states (HOMO-2 → LUMO) for complexes 5a/5−6 are calculated to be 292, 286, 286, 284, 269, 312, 310, and 298 nm, which are in good agreement with the experimental peak maxima at 294, 292, 268, 286, 260, 321, 320, and 291 nm, respectively. From the charge density difference (CDD) maps in the insets of Figure S4 and the plots of relative orbital distributions (Figures S5−S12), it can be concluded that these transitions are mainly ascribed as Pt(5d) → neutral π*(C^C)/ dianionic π*(X^X) chelates metal-to-ligand charge transfer (1MLCT), mixed with some intraligand (1IL) ππ* transitions that are associated with both C^C/X^X chelates. The higher energy absorption located at 278, 274, 262 (S4 states for 1a, 1, and 2), 254 (S6 state for 3), 296 and 281 (S4 and S6 states for 5a), 280 and 271 (S6 and S7 states for 5), and 265 nm (S7 for 6) are mainly assigned as 1IL ππ* transition residing on the C^C and X^X chelates, combined with some Pt(5d) → π*(C^C/X^X) charge-transfer (1MLCT) transitions. The energy levels of frontier molecular orbitals are depicted in Figure 6. By comparing 2 with 1, it can be seen that, with the

Figure 6. Plots of energy level of frontier orbitals for Pt(II) complexes 1a, 5a, and 1−6 in DMSO by the TD-DFT method at the B3LYP level.

change of the methylene spacer of dicarbene to propylene, the energy of LUMO increases from −1.04 to −0.66 eV, whereas that of HOMO keeps almost the same because of the minimal involvement of dicarbene. This explains the blue shift of the absorption of 2 versus that of 1. A similar situation was also observed for Pt(II) complexes 4 and 6 versus their reference complexes 3 and 5, respectively. Moreover, upon changing the dianionic N^N chelate from bipz2− to bitz2−, giving complexes 3 and 4, both of the HOMO and LUMO energy levels are E

DOI: 10.1021/acs.inorgchem.6b00097 Inorg. Chem. XXXX, XXX, XXX−XXX

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Table 2. Calculated Emission Energy of the T1 State in the Solid State and Orbital Transition Analyses of the Studied Pt(II) Complexes by the TD-DFT Method at the M06-2X Level λ, nm (eV) 1a

492 (2.52)

1

484 (2.56)

2

477 (2.60)

3

485 (2.56)

4

484 (2.56)

5a

532 (2.33)

5

531 (2.34)

6

526 (2.36)

transitions (%) HOMO HOMO HOMO HOMO HOMO HOMO HOMO HOMO HOMO HOMO HOMO HOMO HOMO HOMO HOMO HOMO HOMO HOMO

→ → → → → → → → → → → → → → → → → →

assignment (%)

LUMO (59%) LUMO+6 (19%) LUMO+4 (13%) LUMO (54%) LUMO+6 (38%) LUMO (46%) LUMO+2 (29%) LUMO+3 (15%) LUMO (55%) LUMO+4 (27%) LUMO+2 (11%) LUMO (58%) LUMO+2 (34%) LUMO (68%) LUMO+2 (27%) LUMO (71%) LUMO+2 (24%) LUMO (87%)

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LLCT(58%)/3LMCT(28%) 3 LLCT(53%)/3LMCT(23%)/3IL(20%) 3 LLCT(52%)/3LMCT(37%) 3 LLCT(58%)/3LMCT(27%)/3IL(10%) 3 IL(50%)/3LLCT(27%)/3LMCT(18%) 3 LMCT(46%)/3LLCT(40%) 3 LMCT(47%)/3LLCT(40%) 3 LMCT(48%)/3LLCT(35%)/3IL(12%) 3 LLCT(57%)/3LMCT(22%)/3IL(14%) 3 LLCT(41%)/3LMCT(30%)/3IL(22%) 3 LLCT(84%) 3 LLCT(42%)/3LMCT(33%)/3IL(19%) 3 LMCT(46%)/3LLCT(36%)/3IL(13%) 3 LLCT(40%)/3IL(34%)/3LMCT(17%) 3 IL(48%)/3LLCT(28%)/3LMCT(15%) 3 IL(39%)/3LLCT(39%)/3LMCT(14%) 3 IL(45%)/3LLCT(33%)/3LMCT(13%) 3 IL(58%)/3LLCT(21%)/3LMCT(12%)

exciplex. The triplet state of this exciplex (ET = 2.64 eV, peak position at 470 nm) is higher than that of the Pt(II) complexes studied (5, 5a) such that the excitons can be efficiently confirmed within EML, while 8-hydroxyquinolinato lithium (Liq) and Al served as electron-injecting layer and cathode, respectively. Figure 9 depicts the current density−voltage−luminance (J− V−L) characteristics, device efficiencies, and EL spectra. Both devices displayed a low turn-on voltage of 2.2 V. The reduction of charge injection barrier by the exciplex forming cohost as well as HIL (4% ReO3: mCP) and low-lying LUMO level of PO-T2T (3.5 eV) also contributed to rapid charge injection from the electrode reducing the turn-on voltage (Figure 8). The blue emission device doped by 5 reveals a maximum brightness (Lmax) of 18230 cd·m−2 at 10.2 V (2280 mA·cm−2) and with CIE coordinates of (0.18, 0.31). The maximum external quantum (ηext), current (ηc), and power efficiencies (ηp) were recorded to be 7.9%, 17.1 cd·A−1, and 20.7 lm·W−1, respectively, which are lower than those for the 5a-based device (8.9%, 19.4 cd·A−1, and 22.5 lm·W−1) with CIE coordinates of (0.18, 0.32). The ηext of the OLED devices 5a and 5 showed small variation, which are consistent with the PLQY of 5a (59%) and 5 (55%) measured in a 10 wt % doping ratio in the mixed mCP:POT2T host. The devices displayed emission resembling that of the PL spectrum in the solid thin film, and there is no residual emission from the host and/or adjacent layers in Figure 9c, indicating that the electroluminescence solely originates from the Pt(II) dopant and with complete energy transfer taking place from host to dopant. The overall result showed that, although the device performance has not been fully optimized yet, the high performance of OLEDs offers 5 and 5a as promising candidates for use in light-emitting diode devices.

chelates, i.e., parallel or perpendicular. Finally, the reductive peak potentials of all Pt(II) complexes fall in the narrow region of −3.15 to −3.39 V in THF, for which the obtained data are close to the electrochemical widow of solvent, and should be used only with caution. Overall, the observed CV data showed that the bitz2− complexes 3 and 4 and phpz2− complexes 5 and 6 exhibited the greatest and least positive-shifted oxidation peak potential due to the highest and lowest electron deficiency versus the bipz2− counterparts 1 and 2. On the other hand, the Pt(II) complexes with propylene-bridged dicarbene, i.e., 2, 4, and 6, exhibited a more negatively shifted reduction peak versus those with the methylene-bridged dicarbene, i.e., 1, 3, and 5, due to the varied degree of π-conjugation at the dicarbene chelating site. Fabrication of OLEDs. Despite that complexes 1, 1a, and 3 have shown respectable emission Q.Y. in the solid state, they were not employed in the fabrication of OLEDs simply due to the lack of suitable carrier transport and host materials with a large triplet energy gap that can confine the excitons within the light-emitting layer. Instead, fabrication of OLEDs was attempted using complexes 5 and 5a, which possess a reduced HOMO/LUMO gap and are expected to be more suitable for making efficient OLED devices. To evaluate the EL properties of these Pt(II) complexes, OLEDs were fabricated using the mCP:PO-T2T (molar ratio 1:1) cohost with dopant 5 or 5a in the emissive layer (EML) with the structure: ITO/4% ReO3:mCP (45 nm)/mCP (15 nm)/mCP:PO-T2T:10% 5 or 5a (30 nm)/PO-T2T (50 nm)/Liq (0.5 nm)/Al (100 nm). Figure 8 shows the schematic diagram of the device structure and the molecular structures of the employed materials. To lower hole injection barrier from ITO to N,N′-dicarbazolyl-3,5benzene (mCP), we used rhenium oxides (ReO3) as dopant in mCP to produce Ohmic contact.84,85 mCP (ET: 2.94 eV, HOMO/LUMO: 6.1/2.4 eV) and ((1,3,5-triazine-2,4,6-triyl)tris(benzene-3,1-diyl))tris(diphenylphosphine oxide) PO-T2T (ET: 2.99 eV, HOMO/LUMO: 7.5/3.5 eV) were employed as the hole transporting layer and electron transporting layer (ETL).86,87 Simultaneously, mCP and PO-T2T were also used as the cohost of EML with the molar ratio of 1:1 to form an

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CONCLUSION

In conclusion, we reported the design of Pt(II) metal complexes 1−6, 1a, and 5a comprising both the neutral dicarbenes with distinctive alkyl appendages and the dianionic azolate chelates, cf. bipz2−, bitz2−, and phpz2−, respectively. F

DOI: 10.1021/acs.inorgchem.6b00097 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 9. (a) Current density−voltage−luminance (J−V−L) characteristics, (b) external quantum (ηext) and power efficiencies (ηP) as a function of brightness. (c) EL spectra for OLED devices incorporating Pt(II) phosphors 5a and 5; inset depicts the photo of an OLED device fabricated using 5a.

Figure 7. Charge density difference (CDD) plots between the lowestenergy triplet excited states T1 and ground states S0 based on the TDDFT method at the M06-2X level (isovalue = 0.0006). The electron and hole are displayed in purple and cyan colors, respectively. The H atoms are omitted for clarity.

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LLCT/3LMCT/3IL characters in the triplet excited states, which are in sharp contrast to the typical electronic transitions that involved the mixed 3IL ππ* and 3MLCT characters. The OLED device with a doped architecture afforded sky blue emission with high efficiencies (8.9%, 19.4 cd·A−1, and 22.5 lm· W−1). Therefore, this work provides guidelines for future development of emissive Pt(II) complexes with relevant design of ancillaries, extending the scope of third-row transition-metalbased phosphors for OLED applications

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EXPERIMENTAL SECTION

All reactions were conducted under nitrogen. Solvents were distilled from appropriate drying agents prior to use. Commercially available reagents were used without further purification. Mass spectra were obtained on a JEOL SX-102A instrument operating in electron impact (EI) or fast atom bombardment (FAB) mode. 1H, 13C, and 19F NMR spectra were recorded on an INOVA-500 instrument. Elemental analysis was carried out with a Heraeus CHN-O Rapid Elementary Analyzer. Preparation of phpzH2. A THF solution of 1-(4-(trifluoromethyl)phenyl)ethanone (5.01 g, 26.6 mmol) was added dropwise to a suspension of sodium ethoxide (2,73 g, 40.1 mmol) in anhydrous THF (100 mL) at 0 °C. After stirred for 10 min, ethyl trifluoroacetate (4.75 mL, 39.94 mmol) was slowly added at RT, and the mixture was refluxed overnight. For workup, the mixture was neutralized with 2 M HCl(aq) at 0 °C until pH 4−5. The solvent was removed in vacuo, and the residue was dissolved in ethyl acetate. It was next washed with

Figure 8. Molecular structures of materials used in this study and an energy level diagram of the device.

These complexes are essentially nonemissive in solution; however, the corresponding neat powder showed improved emission efficiency, except for the bipz2− and bitz2− substituted complexes 2 and 4, for which their propylene spacer resulted in a perpendicularly arranged dicarbene that destabilized the emitting excited state and seriously quenched the emission. Both DFT and TD-DFT calculations revealed the combined G

DOI: 10.1021/acs.inorgchem.6b00097 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

1H), 1.88−1.80 (m, 1H), 1.38 (d, J = 6.5 Hz, 6H), 1.35 (d, J = 6.5 Hz, 6H). 19F NMR (470 MHz, d6-DMSO, 294 K): δ −58.87 (s, 6F). Anal. Calcd for C23H26F6N8Pt: N, 15.49; C, 38.18; H, 3.62. Found: N, 15.65; C, 37.81; H, 3.84. Selected Crystal Data of 2. C24.50H29Cl3F6N8Pt; M = 851.00; triclinic; space group = P1̅; a = 11.5196(6) Å, b = 12.2014(7) Å, c = 13.1408(7) Å; α = 67.4083(11)°, β = 65.8115(10)°, γ = 83.5424(12)°; V = 1553.35(15) Å3; Z = 2; ρcalcd = 1.819 Mg·m−3; F(000) = 830; crystal size = 0.50 × 0.10 × 0.06 mm3; λ(Mo-Kα) = 0.71073 Å; T = 200(2) K; μ = 4.841 mm−1; 20 170 reflections collected, 7117 independent reflections (Rint = 0.0421), GOF = 1.063, final R1 [I > 2σ(I)] = 0.0349 and wR2 (all data) = 0.0863. Preparation of 3. Treatment of 1,1′-methylene bis(3-isopropylimidazol-3-ium) dihexafluorophosphate (262 mg, 0.50 mmol) with Pt(DMSO)2Cl2 (200 mg, 0.48 mmol) and NaHCO3 (83 mg, 0.99 mmol), followed by addition of bitzH2 (136 mg, 0.50 mmol), gave complex 3 as a colorless powder. Yield: 65%. Spectra Data of 3. MS (FAB, 195Pt): m/z 705 [M+]; 1H NMR (500 MHz, d6-DMSO, 294 K): δ 7.72 (d, J = 2.0 Hz, 2H), 7.71 (d, J = 2.0 Hz, 2H), 6.23 (d, J = 13.0 Hz, 1H), 6.15 (d, J = 13.0 Hz, 1H), 5.13 (septet, J = 6.5 Hz, 2H), 1.63 (d, J = 6.5 Hz, 6H), 1.19 (d, J = 6.5 Hz, 6H). 13C NMR (125 MHz, d6-DMSO, 294 K): δ 158.11 (N-C-C-N of bitz), 154.69 (q, 2JCF = 35.6 Hz, C-CF3), 145.20 (NCN of dicarbene), 123.13 (NCHCHN), 121.15 (q, 1JCF = 269.9 Hz, CF3), 118.68 (NCHCHN), 62.82 (NCH2N), 52.90 (CH of iPr), 25.05 (CH3 of iPr), 21.45 (CH3 of iPr). 19F NMR (470 MHz, d6-DMSO, 294 K): δ −63.18 (s, 6F). Anal. Calcd for C19H20F6N10Pt: N, 20.08; C, 32.72; H, 2.89. Found: N, 19.89; C, 32.67; H, 3.05. Preparation of 4. Treatment of 1,1′-(propane-1,3-diyl) bis(3isopropyl-imidazol-3-ium) dihexafluorophosphate (129 mg, 0.25 mmol) with Pt(DMSO)2Cl2 (100 mg, 0.24 mmol) and NaHCO3 (42 mg, 0.50 mmol), followed by addition of bitzH2 (65 mg, 0.24 mmol), gave complex 4 as a colorless powder. Yield: 61%. Spectra Data of 4. MS (FAB, 195Pt): m/z 725 [M+]; 1H NMR (500 MHz, d6-DMSO, 294 K): δ 7.58 (d, J = 1.8 Hz, 2H), 7.46 (d, J = 1.8 Hz, 2H), 5.26 (septet, J = 6.6 Hz, 2H), 4.76−4.71 (m, 2H), 4.32 (dd, J = 14.5, 6.5 Hz, 2H), 2.32−2.26 (m, 1H), 1.90−1.82 (m, 1H), 1.36 (d, J = 6.6 Hz, 12H). 19F NMR (470 MHz, d6-DMSO, 294 K): δ −62.86 (s, 6F). Anal. Calcd for C21H24F6N10Pt: N, 19.30; C, 34.76; H, 3.33. Found: N, 19.15; C, 34.85; H, 3.55. Preparation of 5a. Pt(DMSO)2Cl2 (100 mg, 0.24 mmol), 1,1′dimethyl-3,3′-methylene-diimidazolium dihexafluorophosphate (117 mg, 0.25 mmol), and NaHCO3 (40 mg, 0.48 mmol) were dissolved in anhydrous DMSO (10 mL) and heated at 120 °C for 19 h. After then, 3-(trifluoromethyl)-5-(4-(trifluoromethyl)phenyl)-1H-pyrazole (phpzH2, 70 mg, 0.25 mmol) and NaHCO3 (40 mg, 0.48 mmol) were added. The mixture was heated at 120 °C for 12 h. After cooled to RT, DMSO was removed in vacuo and the residue was purified by silica gel column chromatography, eluting with ethyl acetate/acetone (20:1). The yellow crystals of 5a were obtained from a mixture of acetone and hexane (74 mg, 48%) at RT. Spectra Data of 5a. MS (FAB, 195Pt): m/z 649 [M+]; 1H NMR (500 MHz, d6-DMSO, 294 K): δ 7.62 (d, J = 2.0 Hz, 1H), 7.58 (d, J = 2.0 Hz, 1H), 7.48 (d, J = 2.5 Hz, 1H), 7.42 (d, J = 7.5 Hz, 1H), 7.37 (d, J = 2.0 Hz, 1H), 7.33 (s, 1H), 7.23 (d, J = 7.5 Hz, 1H), 6.60 (s, 1H), 6.15 (d, J = 13.0 Hz, 1H), 6.00 (d, J = 13.0 Hz, 1H), 3.99 (s, 3H), 3.70 (s, 3H). 19F NMR (470 MHz, d6-DMSO, 294 K): δ −59.00 (s, 3F), −60.77 (s, 3F). Anal. Calcd for C20H16F6N6Pt: N, 12.94; C, 36.99; H, 2.48. Found: N, 12.88; C, 36.78; H, 2.46. Preparation of 5. Heating of Pt(DMSO)2Cl2 (200 mg, 0.48 mmol) with 1,1′-methylenebis(3-isopropyl-imidazol-3-ium) dihexafluorophosphate (262 mg, 0.50 mmol) in the presence of NaHCO3 (82 mg, 0.98 mmol), followed by addition of phpzH2 (140 mg, 0.50 mmol) and NaHCO3 (82 mg, 0.98 mmol), gave complex 5 as a yellow powder. Yield: 68%. Spectra Data of 5. MS (FAB, 195Pt): m/z 705 [M+]; 1H NMR (500 MHz, d6-DMSO, 294 K): δ 7.62 (d, J = 2.0 Hz, 2H), 7.58 (d, J = 1.7 Hz, 1H), 7.55 (d, J = 1.7 Hz, 1H), 7.44 (d, J = 7.8 Hz, 1H), 7.32 (s, 1H), 7.24 (d, J = 7.8 Hz, 1H), 6.62 (s, 1H), 6.13 (d, J = 13.1 Hz, 1H), 6.00 (d, J = 13.1 Hz, 1H), 5.47 (septet, J = 6.6 Hz, 1H), 4.97 (septet, J

water, dried over Na2SO4, and concentrated to yield the crude 1,3dione. Without further separation, this 1,3-dione was dissolved in 150 mL of ethanol, together with 6 mL of 98% N2H4. After refluxing for 8 h, the solvent was removed in vacuo, and the residue was dissolved in ethyl acetate. The mixture was washed with water to remove the excess of N2H4 and then concentrated. Further purification was conducted by silica gel column chromatography using a 3:1 mixture of hexane and ethyl acetate, and recrystallized from ethyl acetate and hexane, giving a white solid of phpzH2 (5.06 g, 18.1 mmol, 68%). Spectra Data of phpzH2. MS (EI): m/z 280 [M+]; 1H NMR (500 MHz, d6-DMSO 294 K): δ 14.27 (s, NH, 1H), 7.98 (d, J = 8.0 Hz, 2H), 7.76 (d, J = 8.0 Hz, 2H), 7.22 (s, 1H). 19F NMR (470 MHz, d6DMSO, 294 K): δ −60.65 (s, 3F), −61.34 (s, 3F). 13C NMR (125 MHz, d6-DMSO, 294 K): δ 142.62 (C of pz), 142.55 (q, 2JCF = 36.8 Hz, C-CF3), 131.92 (C of ph), 129.21 (q, 2JCF = 31.9 Hz, C-CF3), 126.17 (CH of ph), 125.99 (CH of ph), 125.97 (CH of ph), 124.06 (q, 1 JCF = 270 Hz, CF3), 121.72 (q, 1JCF = 266 Hz, CF3), 102.12 (CH of pz). Anal. Calcd for C11H6F6N2: N, 10.00; C, 47.16; H, 2.16. Found: N, 10.31; C, 47.50; H, 2.29. Preparation of 1a. Pt(DMSO)2Cl2 (300 mg, 0.71 mmol), 1,1′methylene bis(3-methyl-imidazol-3-ium) dihexafluorophosphate (351 mg, 0.75 mmol), and NaHCO3 (120 mg, 1.43 mmol) were dissolved in anhydrous DMSO (10 mL) and stirred for 19 h at 120 °C. After the solution was cooled to RT, bipzH2 (203 mg, 0.75 mmol) was added. The mixture was then heated at 120 °C for another 12 h. After cooled to RT, DMSO was removed in vacuo and the residue was purified by silica gel column chromatography, eluting with ethyl acetate/acetone (20:1). The crystals of 1a were obtained from a layered mixture of acetone and hexane (297 mg, 65%) at RT. Single crystals of 1a were obtained from slow evaporation of a saturated CH2Cl2 solution of 1a at RT. Spectra Data of 1a. MS (FAB, 195Pt): m/z 639 [M+]; 1H NMR (500 MHz, d6-DMSO 294 K): δ 7.64 (d, J = 2.0 Hz, 2H), 7.46 (d, J = 2.0 Hz, 2H), 6.52 (s, 2H), 6.20 (d, J = 13.0 Hz, 1H), 6.10 (d, J = 13.0 Hz, 1H), 3.97 (s, 6H). 19F NMR (470 MHz, d6-DMSO, 294 K): δ −59.11 (s, 6F). Anal. Calcd for C17H14F6N8Pt: N, 17.52; C, 31.93; H, 2.21. Found: N, 17.71; C, 32.06; H, 2.27. Selected Crystal Data of 1a. C17H14F6N8Pt; M = 639.45; orthorhombic; space group = Pnma; a = 7.8209(5) Å, b = 21.6264(14) Å, c = 11.6972(7) Å; V = 1978.4(2) Å3; Z = 4; ρcalcd = 2.147 Mg·m−3; F(000) = 1216; crystal size = 0.20 × 0.15 × 0.08 mm3; λ(Mo-Kα) = 0.71073 Å; T = 200(2) K; μ = 7.171 mm−1; 12 163 reflections collected, 2329 independent reflections (Rint = 0.0341), GOF = 1.059, final R1 [I > 2σ(I)] = 0.0221 and wR2 (all data) = 0.0517. Preparation of 1. Treatment of 1,1′-methylene bis(3-isopropylimidazol-3-ium) dihexafluorophosphate (262 mg, 0.50 mmol) with Pt(DMSO)2Cl2 (200 mg, 0.48 mmol) and NaHCO3 (82 mg, 0.98 mmol), followed by addition of bipzH2 (136 mg, 0.50 mmol), gave complex 1 as a colorless powder. Yield: 70%. Spectra Data of 1. MS (FAB, 195Pt): m/z 695 [M+]; 1H NMR (500 MHz, d6-DMSO, 294 K): δ 7.65 (d, J = 2.0 Hz, 2H), 7.64 (d, J = 2.0 Hz, 2H), 6.53 (s, 2H), 6.18 (d, J = 13.5 Hz, 1H), 6.09 (d, J = 13.5 Hz, 1H), 5.32 (septet, J = 6.5 Hz, 2H), 1.60 (d, J = 6.5 Hz, 6H), 1.15 (d, J = 6.5 Hz, 6H). 13C NMR (125 MHz, d6-DMSO, 294 K): δ 148.77 (NCN), 147.80 (CH-C-C-CH), 141.59 (q, 2JCF = 35.6 Hz, C-CF3), 122.97 (q, 1JCF = 267.5 Hz, CF3), 121.85 (NCHCHN), 117.60 (NCHCHN), 96.59 (CH of pz), 62.14 (NCH2N), 51.84 (CH of iPr), 24.30 (CH3 of iPr), 20.76 (CH3 of iPr). 19F NMR (470 MHz, d6DMSO, 294 K): δ −59.25 (s, 6F). Anal. Calcd for C21H22F6N8Pt: N, 16.11; C, 36.26; H, 3.19. Found: N, 15.88; C, 36.10; H, 3.45. Preparation of 2. Treatment of 1,1′-(propane-1,3-diyl) bis(3isopropyl-imidazol-3-ium) dihexafluorophosphate (276 mg, 0.50 mmol) with Pt(DMSO)2Cl2 (200 mg, 0.48 mmol) and NaHCO3 (82 mg, 0.98 mmol), followed by addition of bipzH2 (136 mg, 0.50 mmol), gave complex 2 as colorless powder. Yield: 58%. Single crystals of 2 were obtained from a mixture of CH2Cl2 and hexane at RT. Spectra Data of 2. MS (FAB, 195Pt): m/z 723 [M+]; 1H NMR (500 MHz, d6-DMSO, 294 K): δ 7.50 (d, J = 1.7 Hz, 2H), 7.40 (d, J = 1.7 Hz, 2H), 6.48 (s, 2H), 5.26 (septet, J = 6.5 Hz, 2H), 4.72 (dd, J = 14.4, 11.0 Hz, 2H), 4.32 (dd, J = 14.4, 6.5 Hz, 2H), 2.33−2.28 (m, H

DOI: 10.1021/acs.inorgchem.6b00097 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry = 6.6 Hz, 1H), 1.58−1.55 (m, 6H), 1.11 (d, J = 6.6 Hz, 3H), 1.07 (d, J = 6.6 Hz, 3H). 13C NMR (125 MHz, d6-DMSO, 294 K): δ 172.97 (NCN), 160.71 (C of pz), 159.04 (NCN), 157.81 (C-Pt of phpz), 148.37 (C-C-Pt), 140.87 (q, 2JCF = 34.3 Hz, C-CF3 of pz), 132.47 (CH-C-Pt), 125.29 (q, 1JCF = 272.4 Hz, CF3 of ph), 124.86 (q, 2JCF = 30.0 Hz, C-CF3 of ph), 123.12 (q, 1JCF = 267.7 Hz, CF3 of pz), 121.85 (NCHCHN), 121.37 (NCHCHN), 120.88 (CH of ph), 119.30 (CH of ph), 117.44 (NCHCHN), 117.16 (NCHCHN), 97.73 (CH of pz), 62.43 (NCH2N), 51.83 (CH of iPr), 51.30 (CH of iPr), 24.44 (CH3 of iPr), 24.36 (CH3 of iPr), 21.08 (CH3 of iPr), 19.93 (CH3 of iPr). 19F NMR (470 MHz, d6-DMSO, 294 K): δ −58.72 (s, 3F), −60.12 (s, 3F). Anal. Calcd for C24H24F6N6Pt: N, 11.91; C, 40.85; H, 3.43. Found: N, 11.91; C, 40.68; H, 3.44. Preparation of 6. Heating of Pt(DMSO)2Cl2 (199 mg, 0.47 mmol) with 1,1′-(propane-1,3-diyl)bis(3-isopropyl-imidazol-3-ium) dihexafluorophosphate (275 mg, 0.50 mmol) in the presence of NaHCO3 (80 mg, 0.95 mmol), followed by addition of phpzH2 (139 mg, 0.50 mmol) and NaHCO3 (80 mg, 0.95 mmol), gave complex 6 as a yellow powder. Yield: 63%. Single crystals of 6 were obtained from a mixture of acetone and hexane at RT. Spectra Data of 6. MS (FAB, 195Pt): m/z 733 [M+]; 1H NMR (500 MHz, d6-DMSO, 294 K): δ 7.50 (d, J = 2.0 Hz, 1H), 7.42−7.37 (m, 3H), 7.32 (d, J = 2.0 Hz, 1H), 7.19 (d, J = 7.5 Hz, 1H), 6.57 (s, 2H), 5.21 (septet, J = 6.5 Hz, 1H), 5.11 (septet, J = 6.5 Hz, 1H), 4.69 (dd, J = 14.0, 11.0 Hz, 1H), 4.57 (dd, J = 14.0, 11.0 Hz, 1H), 4.31− 4.21 (m, 2H), 2.32−2.25 (m, 1H), 1.84−1.76 (m, 1H), 1.44 (d, J = 6.5 Hz, 3H), 1.33 (d, J = 6.5 Hz, 3H), 1.29 (d, J = 6.5 Hz, 3H), 1.24 (d, J = 6.5 Hz, 3H). 19F NMR (470 MHz, d6-DMSO, 294 K): δ −58.72 (s, 3F), −60.83 (s, 3F). Anal. Calcd for C26H28F6N6Pt: N, 11.46; C, 42.57; H, 3.85. Found: N, 11.51; C, 42.48; H, 3.60. Selected Crystal Data of 6. C29H34F6N6OPt; M = 791.71; triclinic; space group = P1̅; a = 10.2995(6) Å, b = 10.7219(6) Å, c = 14.7831(8) Å; α = 96.1523(12)°, β = 103.3869(12)°, γ = 98.3918(12)°; V = 1554.20(15) Å3; Z = 2; ρcalcd = 1.692 Mg·m−3; F(000) = 780; crystal size = 0.30 × 0.15 × 0.15 mm3; λ(Mo-Kα) = 0.71073 Å; T = 200(2) K; μ = 4.583 mm−1; 20 254 reflections collected, 7124 independent reflections (Rint = 0.0404), GOF = 1.023, final R1 [I > 2σ(I)] = 0.0288 and wR2 (all data) = 0.0661. Cyclic Voltammetry. Cyclic voltammetry (CV) was performed using a 620A electrochemical analyzer, CH Instruments Inc. All electrochemical potentials were measured in a 0.1 M TBAPF6/CH2Cl2 and THF solution for oxidation and reduction reaction, and reported in volts using Fc/Fc+ as reference. Eap is defined as anodic peak potential, and Ecp is defined as cathodic peak potential. The Pt electrode and Au(Hg) alloy were selected as the working electrode of oxidation and reduction processes, respectively. Photophysical Measurement. Steady-state absorption and emission spectra were recorded by a Hitachi (U-3900) spectrophotometer and an Edinburgh (FLS920) fluorometer, respectively. The quantum yields of the titled complexes in the solid state were measured using an integrating sphere. Lifetimes were measured by a multichannel scaling (MCS) module with a μF900 microsecond flashlamp as the excitation source. Single-Crystal X-ray Diffraction Studies. Single-crystal X-ray diffraction studies were measured with a Bruker SMART Apex CCD diffractometer using (Mo-Kα) radiation (λ = 0.71073 Å). The data collection was executed using the SMART program. Cell refinement and data reduction were performed with the SAINT program. An empirical absorption was applied based on the symmetry-equivalent reflections and the SADABS program. The structures were solved using the SHELXS-97 program and refined using the SHELXL-97 program by full matrix least-squares on F2 values. The structural analysis and molecular graphics were obtained using the SHELXTL program on a personal computer.88 TD-DFT Calculation. All the calculations were implemented by using Gaussian 09 program package.89 The density functional theory (DFT) method, which combines the Becke’s three-parameter hybrid exchange theory and the Lee−Yang−Parr correlation theory (B3LYP),90,91 was used to optimize the structures in the ground state (S0) and lowest-energy triplet state (T1) for complexes 1a−6,

respectively, without symmetry constraint. The initial structures of 1a, 2, and 6 were extracted from the X-ray crystallographically determined geometries, while those of others were obtained from the modulation in structures of 1a, 2, and 6. To analyze the spectroscopic properties, 60 singlet excited states for complexes 1a−6 in DMSO were calculated to determine the vertical excitation energies by the time-dependent DFT (TD-DFT)92−94 method at the same functional used in the geometrical optimization progresses based on the optimized S0 ground-state geometries. The solvent effects were taken into account by performing the self-consistent reaction field (SCRF) calculations using the conductor-like polarizable continuum model method (CPCM).95,96 To explore the emission properties in the solid state, 6 triplet excited states were calculated based on the optimized lowestenergy triplet state structures using the hybrid functional M06-2X of Truhlar and Zhao.97 The B3LYP and M06-2X calculations were conducted using the Stuttgart−Dresden (SDD) basis set and the effective core potentials (ECPs) for the Pt atom98 and the 6-311G** basis set for all other nonmetal atoms, e.g., F, N, C, and H atoms. Visualization of the frontier molecular orbitals and charge density difference (CDD) map were performed by GaussView. The Ros & Schuit method (C-squared population analysis method, SCPA)99 was supported to analyze the partition orbital composition by using the Multiwfn 3.3 program.100 OLED Device Fabrications. All materials were purified by vacuum sublimation prior to use. The OLEDs were fabricated through direct vacuum deposition at 10−6 Torr on the ITO-coated glass substrates with a sheet resistance of 15 Ω·sq−1. The ITO surface was ultrasonically cleaned in acetone, methanol, and deionized water in sequence, followed by a final treatment with air plasma. The deposition rate was kept at ca. 1−2 Å·s−1. Subsequently, Liq was deposited at 0.1 Å·s−1 and then capped with Al metal (ca. 5 Å·s−1) through shadow masking without breaking the vacuum. The J−V−L characteristics of the devices were measured simultaneously in a glovebox using a Keithley 6430 source meter and a Keithley 6487 picoammeter equipped with a calibration Si-photodiode. EL spectra were measured using a photodiode array (Ocean Optics USB2000+).

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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.6b00097. Electrochemical data, results concerning the TD-DFT calculation of all Pt(II) complexes, and NMR spectra of selected Pt(II) metal complexes (PDF) X-ray crystallographic data of Pt(II) complexes 1a, 2, and 6 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.C.). *E-mail: [email protected] (Z.-N.C.). *E-mail: [email protected] (W.-Y.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Ministry of Science and Technology of Taiwan, under the grant numbers 104-2119-M007-001 and 103-2112-M-019-004-MY3. We also thank Prof. K.-T. W. of NTU for providing the host material PO-T2T.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.6b00097 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.6b00097 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.6b00097 Inorg. Chem. XXXX, XXX, XXX−XXX