and Electroluminescence from 2-(Dibenzo[b,d

Article pubs.acs.org/JPCC

Photo- and Electroluminescence from 2‑(Dibenzo[b,d]furan-4yl)pyridine-Based Heteroleptic Cyclometalated Platinum(II) Complexes: Excimer Formation Drastically Facilitated by an Aromatic Diketonate Ancillary Ligand Tatsuya Shigehiro,† Shigeyuki Yagi,*,† Takeshi Maeda,† Hiroyuki Nakazumi,† Hideki Fujiwara,‡ and Yoshiaki Sakurai§ †

Department of Applied Chemistry, Graduate School of Engineering and ‡Department of Chemistry, Graduate School of Science, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan § Textile and Polymer Section, Technology Research Institute of Osaka Prefecture, 2-7-1 Ayumino, Izumi, Osaka 594-1157, Japan S Supporting Information *

ABSTRACT: To clarify the ancillary ligand effect on excimer formation of heteroleptic cyclometalated platinum(II) complexes, we investigated photo- and electroluminescence behavior for green-phosphorescent [(dibenzo[b,d]furan-4-yl)pyridinato-N,C3′]platinum(II) 1,3-diketonates Pt-1 and Pt-2 (1,3-diketonate ancillary ligands; 1,3-bis(3,4-dibutoxyphenyl)propane-1,3-dionate (bdbp) and dipivaloylmethanate (dpm) for Pt-1 and Pt-2, respectively). The X-ray crystallographic study reveals that both Pt-1 and Pt-2 form dimeric pairs in the solid states, indicating that they are likely to form the excimers. In PMMA films doped with Pt-1, red-shifted photoluminescence at >600 nm, assignable to the excimer emission, increasingly emerges along with the original monomer emission at 516 and 552 nm as the doping level of Pt-1 is increased to 44 wt %. Pt-2-doped films give quite modest excimer-based photoluminescence even when heavily doped to the same extent as Pt-1-doped films. This clearly indicates that the aromatic ancillary ligand (bdbp) effectively facilitates the excimer formation rather than the aliphatic (dpm). Similar efficient excimer formation is also observed for electroluminescent devices such as poly(9-vinylcarbazole)-based polymer light-emitting diodes (PLEDs) doped with Pt-1. The Pt-1-doped PLEDs afford electroluminescence from green to orange when the doping level of Pt-1 is varied from 7.1 to 24 wt %. Thus, referring to the Commission Internationale de L’Eclairage (CIE) chromaticity coordinate, the color tuning is available from CIE = (0.40, 0.57) to (0.56, 0.43) (@maximum luminance). We also found that the excimer-based luminescence of Pt-1 is enhanced in the PLEDs more than in polymer thin films upon photoexcitation. This result shows the difference in the mechanism of triplet excimer formation between photo- and electroluminescence and indicates that the efficient triplet excimer formation is caused by direct charge recombination of anion and cation radicals of Pt-1.

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INTRODUCTION For the last two decades, organic light-emitting diodes (OLEDs) have received considerable attention because they are applicable to thin and flexible flat-panel displays1−3 and solid-state illumination devices.4−6 In particular, solutionprocessed devices represented by polymer light-emitting diodes7,8 (PLEDs) attract increasing interest of researchers due to a lot of technical advantages; high utilization efficiency of constituent materials, availability of large-area devices, low cost of device fabrication, and so on. Electroluminescence of PLEDs is generally based on the emission of doped dyes. To obtain high-performance PLEDs, phosphorescent materials have been frequently used as emitting dopants9,10 because the triplet excitons are not only statistically generated upon electron−hole recombination but also obtained from the © 2012 American Chemical Society

singlet excitons via intersystem crossing (ISC). Hence, phosphorescence-emitting dopants can achieve internal quantum efficiencies (ηint) as high as 100%.11,12 As phosphorescent materials, organometallic complexes with heavy-metal centers such as Ir(III),13−16 Pt(II),17−21 Ru(II),22 Os(II),23,24 and Au(III)25 have been studied because the strong spin−orbit coupling gives rise to efficient ISC to afford a lot of triplet excitons even at ambient temperature. Among them, cyclometalated platinum(II) complexes such as homoleptic (C∧N)Pt(C∧N) complexes (C∧N, a 2-arylpyridinate-type cyclometalated ligand),17 heteroleptic (C∧N)Pt(O∧O) complexes Received: August 7, 2012 Revised: November 8, 2012 Published: November 10, 2012 532

dx.doi.org/10.1021/jp307853t | J. Phys. Chem. C 2013, 117, 532−542

The Journal of Physical Chemistry C

Article

(O∧O, a diketonate-type ancillary ligand),18 and tridentate (N∧C∧N)PtX complexes (N∧C∧N, a tridentate cyclometalated ligand such as 1,3-di(pyridin-2-yl)benzenate; X, an anionic ligand)19 are as popular phosphorescent materials for OLEDs as homoleptic tris-cyclometalated14 and heteroleptic bis-cyclometalated iridium(III) complexes.13 This is because the strong ligand field effects caused by the covalent carbon−metal bonds facilitate radiative decay from the triplet ligand-centered and metal-to-ligand charge-transfer states (3LC and 3MLCT, respectively) to provide high emission quantum efficiencies. In general, the cyclometalated platinum(II) complexes have four-coordinated square-planar structures. This unique structural feature often brings about strong intermolecular stacking to give rise to formation of a dimeric excited state, that is, an excimer.19,26−28 The excimers of cyclometalated platinum(II) complexes are often emissive to afford red-shifted phosphorescence along with monomer emission. Utilizing excimer emission from cyclometalated platinum(II) complexes, a white-emitting OLED (WOLED) composed of only one emitting material was fabricated by Thompson and coworkers.29 More recently, Williams and coworkers also reported WOLEDs with excellent color-rendering properties that are composed of the monomer, excimer, and exciplex emission based on an N∧C∧N-coordinated platinum(II) complex.30 Thus, making good use of excimer-based emission allows us to tune emission colors of OLEDs without designing the combination of emitting materials. In this context, it is important to investigate how the excimer formation affects their electroluminescent properties upon fabrication of OLEDs doped with cyclometalated platinum(II) complexes. Few systematic studies, however, were reported on the effects of the ancillary ligands of cyclometalated platinum(II) complexes on excimer-based photo- and electroluminescence. We have so far developed a series of (C∧N)Pt(O∧O)-type complexes that have a 1,3-bis(3,4-dibutoxyphenyl)propane-1,3dionate (bdbp) ancillary ligand.20 We confirmed that the bdbp ligand is effective not only to increase the solubility in common organic solvents but also to enhance the photoluminescence (PL) quantum yields in comparison with the corresponding complexes bearing an aliphatic ancillary ligand such as dipivaloylmethanate (dpm). Thus, the (C∧N)Pt(bdbp) complexes are good candidates of phosphorescent dopants for solution-processed OLEDs. We have also demonstrated the fabrication of poly(9-vinylcarbazole)-based PLEDs containing these (C∧N)Pt(bdbp) complexes.21 In the present study, we report how the bdbp ancillary ligand affects excimer formation using the green-phosphorescent (C∧N)Pt(bdbp) complex Pt-1 (Figure 1) that has a high PL quantum yield (ΦPL = 0.59 in CHCl3 at 298 K).20 Although (C∧N)Pt(O∧O)-type complexes have been frequently reported, there is no example for excimer formation of a cyclometalated platinum(II) complex with an aromatic ancillary ligand. Excimer-based PL of Pt-1 in solutions and polymer films by contrast with a dpm analogue Pt-2 (Figure 1) is discussed along with excimer-based electroluminescence from poly(9-vinylcarbazole)-based PLEDs doped with Pt-1, especially focusing on emission color tuning.

Figure 1. Structures of cyclometalated platinum(II) complexes Pt-1 and Pt-2.

and poly(9-vinylcarbazole) (PVCz) were employed as host matrices. PMMA (Mw, not reported) and PVCz (Mw = 25 000−50 000) were purchased from Wako Pure Chemical Industries and Sigma-Aldrich, respectively. PMMA was used without further purification. PVCz was purified by reprecipitation from THF to methanol. For fabrication of PLEDs, poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS, P VP CH8000), 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD), CsF, and Al wires were purchased from Heraeus Clevios, Tokyo Chemical Industry, Wako Pure Chemical Industries, and the Nilaco Corporation, respectively. Indium−tin oxide (ITO) glass substrates with a sheet resistance (10 Ω/sq) were purchased from Sanyo Vacuum Industries. X-ray Crystallography. The single crystals suitable for the X-ray crystallography were grown by slow diffusion of a chloroform solution of Pt-1 (or Pt-2) to hexane. Diffraction data for Pt-1 were measured on a Rigaku RAXIS RAPID imaging plate area detector with graphite monochromated Cu− Kα radiation (λ = 1.54187 Å). The cell parameters for Pt-1 were collected at −150 ± 1 °C to a maximum 2θ value of 136.5°. The structure was solved by direct methods using the SHELX9731 program and expanded using the DIRDIF9932 program. All calculations were performed using the CrystalStructure 3.833 and SHELX97 packages of computer programs. Diffraction data for Pt-2 were measured on a Rigaku RAXIS RAPID imaging plate area detector with graphite monochromated Mo−Kα radiation (λ = 0.71075 Å). The cell parameters for the Pt-2 were collected at 23 ± 1 °C to a maximum 2θ value of 55.0°. The structure was solved by direct methods using the SIR9234 and expanded using the DIRDIF99 programs. All calculations were performed using the CrystalStructure 3.8 and CRYSTALS Issue 1135 packages of computer programs. The crystal data and refinement details of the crystal structure determination for Pt-1 and Pt-2 are given in Table S1 (Supporting Information). Analyses of Optical and Photophysical Properties. PMMA and PVCz films doped with varying concentrations of the platinum(II) complexes were prepared by spin-coating toluene solutions of a polymer−platinum(II) complex mixture onto quartz plates. Each prepared film was so thick as to obey the Lambert−Beer law (60−100 nm in PMMA films; 62−90 nm in PVCz films). The neat films of the platinum(II) complexes were also prepared by spin-coating toluene solutions of the platinum(II) complexes onto quartz plates. UV−vis spectra were recorded on a Shimadzu UV-3100 spectropho-

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EXPERIMENTAL SECTION Materials. The synthesis and characterization of the cyclometalated platinum(II) complexes Pt-1 and Pt-2 were previously reported.20 The PL data in solutions were also reported therein. To investigate the optical properties of these complexes in thin films, poly(methyl methacrylate) (PMMA) 533



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Figure 2. ORTEP drawings and crystal packings of (a) Pt-1 and (b) Pt-2. For the crystal packing of Pt-2, the layer structure of molecules A and B on the ab plane is illustrated. Key atoms are labeled in the top views, where C, N, O1, and O2 represent the atoms coordinated to the platinum(II) center. The interplane distances between the adjacent molecules are annotated in the side views. A2 and B2 represent dimeric pairs of molecules A and B, respectively.

tometer. PL spectra were recorded on a Jasco FP-6600 spectrofluorometer for solutions and polymer films. The phosphorescence spectra were also measured on the same spectrofluorometer in 2-methyltetrahydrofuran glass matrices at 77 K that was equipped with a Jasco PMU-183 phosphorescence measurement base unit. PL spectra of the neat films of the platinum(II) complexes were measured on a Horiba SPEX Fluorolog-3 spectrofluorometer. PL quantum yields were measured on a Hamamatsu Photonics C9920-12 absolute PL quantum yield measurement system. PL lifetimes were obtained

on a Horiba Jobin Yvon FluoroCube spectroanalyzer using a 390 nm nanosecond-order LED light source. The optical and photophysical measurements were carried out under inert gases such as argon and nitrogen, and the analyses were carried out immediately after sample preparation. Fabrication of PLEDs. The prepatterned ITO glass substrates were routinely cleaned by ultrasonic treatment in a detergent aqueous solution, distilled water, acetone, chloroform, and hexane. PEDOT:PSS (40 nm) was spin-coated onto the ITO layer pretreated with UV−O3 and then dried at 115 °C 534



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and 3.51 Å (Pt-2; molecule B) apart. These distances almost correspond to the van der Waals contact between the πframeworks of the C∧N and O∧O ligands. Therefore, the paired molecules of Pt-1 and Pt-2 are in close proximity to each other, indicating that the adjacent molecules interact via strong π−π stacking in a solid state.36,37 The intermolecular Pt−Pt distances are 4.45 (Pt-1), 5.32 (Pt-2; molecule A), and 5.28 Å (Pt-2; molecule B) (see Figure S1 of the Supporting Information), and thus no Pt−Pt interaction is found for the dimeric pair, although Chen and coworkers found such an interaction in the dimeric pair of their cyclometalated platinum(II) complex (Pt−Pt distance; 3.49 Å).38 From these results, one can see that the present cyclometalated platinum(II) complexes are likely to form excimers and affect their luminescence properties in solid-state matrices such as polymers. UV−vis Absorption and Photoluminescence Properties. First, the PL spectra of Pt-1 and Pt-2 were obtained for deaerated chloroform and toluene solutions at room temperature (data not shown). Even when the concentration of Pt-1 (or Pt-2) was raised from 10 μM to 1 mM, no spectral changes were observed. In this study, therefore, we focused on the emission behavior of Pt-1 and Pt-2 in polymer thin films. PMMA was chosen as a matrix polymer because it has no absorption in the near-UV-to-visible region and allows for obtaining the intrinsic optical properties of the platinum(II) complexes. UV−vis absorption spectra of Pt-1 and Pt-2 in PMMA films at different concentrations from 9.1 to 44 wt % are shown in Figure 3. In the UV−vis spectra of Pt-1, the C∧N- and O∧Oligand centered (LCC∧N and LCO∧O) transition bands are

for 1 h. For fabrication of an emitting layer (EML), a mixture of PVCz, PBD, and the platinum(II) complex in dry toluene (PVCz; 10 mg/0.7 mL of toluene) was filtered through a 0.2 μm Millex-FG filter (Millipore). The obtained stock solution was then spin-coated onto the PEDOT:PSS layer under an argon atmosphere. Thereafter, CsF (1.0 nm) and Al (250 nm) layers were successively embedded on the EML by vacuum deposition with a base pressure of ca. 1 × 10−4 Pa. Finally, the device was covered with a glass cap and encapsulated with a UV-curing epoxy resin under a dry argon atmosphere to keep oxygen and moisture away from the device. The area of the emitting part was 10 mm2 (2 mm × 5 mm). The device fabrication was carried out in a glovebox filled with dry argon, except for the preparation of the PEDOT:PSS layer. The PLED performance was operated at room temperature using a Hamamatsu Photonics C-9920-11 organic EL device evaluating system. The reproducibility of the device performance of the PLEDs was confirmed by preparing more than two devices for each. The experimental errors of all device parameters (maximum luminance Lmax, maximum current efficiency ηj max, maximum power efficiency ηp max, and maximum external quantum efficiency ηext max) are 600 nm. Therefore, one can see that the employment of the bdbp ancillary ligand significantly facilitates excimer formation in a polymer matrix as well as in a solid state. Photoluminescence Decay Profiles. Next, we investigated the excimer-based PL behavior of Pt-1 and Pt-2 from the PL lifetime measurement of the PMMA films doped with different concentrations from 9.1 to 29 wt %. The PL decay profiles are shown in Figure 7, monitored at 631 and 600 nm for Pt-1 and Pt-2, respectively: the lifetime profile of Pt-2 could not be monitored at 631 nm due to low emission intensities even when added at high doping levels. The emission lifetime (τPL) and component at each concentration are listed in Table 1. The PL lifetimes of Pt-1 in PMMA films are best fitted to double-exponential decay (χ2 = 1.07−1.10). At the doping level of 9.1 wt %, two τPL values of 1.8 and 7.6 μs are observed for Pt-1, as shown in Figure 7a, where the latter is a main component (73%). As the doping level increases, both of these are significantly shorten to 0.20 and 0.74 μs at 29 wt % due to self-quenching or nonradiative decay from the excimer, and the relative amplitude of the short lifetime component increases to 48%. This result is comparable to the enhancement of the excimer emission, as observed in Figure 4a. Referring to the recent reports on excimer emission from phosphorescent organometallic complexes,19,28,43,44 it shows a shorter lifetime than the monomer emission. In addition, as previously

observed at 298 and 384 nm, respectively, at the doping level of 9.1 wt %. The singlet−triplet-mixed metal-to-ligand charge transfer (1,3MLCT) bands, which are likely to lie in the lower energy regions, are masked by the LCO∧O band. The absorption bands increase proportionally to the doping level from 9.1 to 44 wt % without any remarkable changes of the band shapes. In the case of Pt-2, LCC∧N and 1,3MLCT transition bands are observed at 288 and >370 nm, respectively, and the band shapes are almost similar at any doping levels. These results show that the present platinum(II) complexes do not form any charge-transfer complexes39,40 and aggregates.41,42 In general, when molecules form aggregates in polymer matrices with lowpermittivity such as PMMA and PVCz, attractive electrostatic interactions should be main driving factors, and thus highly polarized molecules are likely to form aggregates. The present cyclometalated platinum(II) complexes, however, are less polarized so that they are expected to be molecularly dispersed even in high doping levels. PL spectra of Pt-1 and Pt-2 in PMMA films are shown in Figure 4. Table 1 summarizes the selected PL data of Pt-1 and

Figure 4. PL spectra of (a) Pt-1 and (b) Pt-2 in PMMA films upon varying the concentration up to 44 wt %. Each set of the spectra is normalized by the spectral intensity at the high-energy emission peak of 515−520 nm.

Pt-2 in PMMA films, along with their photophysical profiles. Both Pt-1 and Pt-2 exhibit PL spectra similar to those in solutions at the low concentration (0.99 wt %): the emission maxima (λPL) are observed at 516 and 552 nm. Thus, these spectra are assigned to their monomer emission. As the doping level of Pt-1 is raised, new emission bands are generated at >600 nm at the concentrations over 9.1 wt % (Figure 4a). 536



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Table 1. Photoluminescence Properties of Pt-1 and Pt-2 in PMMA Films at Different Dopant Concentrations Pt-1 λPL/nma

doping level 9.1 wt % 17 wt % 23 wt % 29 wt %

516, 517, 517, 517,

552, 553, 553, 553,

600[sh] 602 602 614

Pt-2

τPL/μs (%)b

ΦPLc,d

λPL/nma

1.8 (27), 7.6 (73) 0.65 (29), 4.6 (71) 0.47 (32), 3.8 (68) 0.20 (48), 0.74 (52)

0.086 0.055 0.036 0.031

516, 518, 518, 519,

552 553 554 554

τPL/μs (%)b 7.4 5.8 5.0 4.1

(13), (16), (21), (26),

15 (87) 13 (84) 11 (79) 9.8 (74)

ΦPLc,d 0.43 0.22 0.19 0.16

a Pt-1 and Pt-2 in PMMA films were excited at 383 and 342 nm, respectively. bEmission decay profiles of Pt-1 and Pt-2 in PMMA films were monitored at 631 and 600 nm, respectively. The values in parentheses are the relative amplitudes. The values of χ2 are

and Electroluminescence from 2-(Dibenzo[b,d

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