Platinum(II) Complexes with Pyridyl Azolate-Based Chelates: Synthesis,
Structural Characterization, and Tuning of Photo- and
Electrophosphorescence
Sheng-Yuan Chang,†Jakka Kavitha,† Shih-Wen Li,†Chan-Shou Hsu,†Yun Chi,*,†Yu-Shan Yeh,‡ Pi-Tai Chou,*,‡Gene-Hsiang Lee,‡Arthur J. Carty,*,§Yu-Tai Tao,|and Chin-Hsiung Chien|
Department of Chemistry, National Tsing Hua UniVersity, Hsinchu 300, Taiwan, Department of Chemistry and Instrumentation Center, National Taiwan UniVersity, Taipei 106, Taiwan, Steacie Institute for Molecular Sciences, National Research Council, Ontario K1A 0R6, Canada, and Institute of Chemistry, Academia Sinica, Taipei 115, Taiwan
Received August 16, 2005
A new series of luminescent platinum(II) azolate complexes with a formula of [Pt(N∧N)2], in which N∧N)mppz
(1), bppz (2a), bzpz (2b), bmpz (2c), bqpz (2d), fppz (3a), hppz (3b), bptz (4), hptz (5), were synthesized, and their photophyscial properties were examined. Single-crystal X-ray diffraction studies of 2c and 3b revealed a planar molecular geometry, in which the N∧N chelates adopt a trans configuration and show notable interligand C−H‚‚‚N hydrogen bonding within the complex. Interesting intermolecular interactions were observed in the solid state. Complex 2c formed a slipped-stack structure with a Pt‚‚‚Pt separation distance of 6.432 Å, while complex 3b showed a columnar stacking with the molecules oriented in an alternating order in relation to the chain axis, giving a much reduced Pt‚‚‚Pt distance of 3.442 Å. The lowest absorption band for all complexes revealed strong state mixings between the singlet and triplet (MLCT and intraligandππ*) manifolds. Complexes 1 and 2 showed mixed
3MLCT and3ππ phosphorescence in fluid solution. While radiationless deactivation was apparently dominant for
complxes 3−5 in solution, resulting in rather weak emission, strong phosphorescence was observed in the room-temperature solid state with the peak wavelength being significantly red shifted compared to that in solution. The emission nature has been tentatively assigned to be 3MMLCT in character. OLED devices with a multilayer
configuration of ITO/NPB/CBP:2a/BCP/Alq3/LiF/Al were successfully fabricated using a CBP layer doped with various
amount of 2a, ranging from 6 to 100%, as the emitting layer. A substantial red shift with increasing doping concentrations was observed in electroluminescence. With a neat film of 2a, the device showed a green emission withλmaxat 556 nm and an external QE of∼1.6% at a driving current of 20 mA. Similarly, for the device using a neat film of 3a, an electroluminescence centered at 616 nm with a slightly reduced external QE of∼2.1% was recorded. Aggregation of platinum(II) complexes in the solid state was proposed to account for the large red-shift in electroluminescence.
Introduction
Organometallic complexes with a third-row transition metal element are crucial for the fabrication of highly efficient organic light-emitting diodes (OLEDs).1The strong
spin-orbit coupling promotes efficient intersystem crossing from the singlet to the triplet excited-state manifold. As a result, numerous attempts have been made to exploit third-row transition metal complexes as phosphorescent dopant emitters,2-4 among which quite a few osmium(II)2 and iridium(III)3complexes have been reported to exhibit excel-* To whom correspondence should be addressed. E-mail:
[email protected] (Y.C.); [email protected] (P.-T.C.); [email protected] (A.J.C.).
†Department of Chemistry, National Tsing Hua University.
‡Department of Chemistry and Instrumentation Center, National Taiwan University.
§Steacie Institute for Molecular Sciences, National Research Council.
|Institute of Chemistry, Academia Sinica.
(1) (a) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M. E.; Forrest, S. R. Nature 1998, 395, 151. (b) Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Pure Appl. Chem. 1999, 71, 2095. (c) Gong, X.; Ostrowski, J. C.; Bazan, G. C.; Moses, D.; Heeger, A. J. Appl. Phys. Lett. 2002, 81, 3711.
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lent device performances. More recently, a breakthrough has been made with ruthenium(II)-based OLED devices,4 the results of which may attract attention from an economic viewpoint.
Recently, the efficiency of OLEDs was dramatically increased to, for example, g15% for external quantum efficiency by employing either green [Ir(ppy)3]5or red [Os-(fptz)2(PPh2Me)2]6emitters with a multilayer device archi-tecture. Despite the developments with iridium(III) and osmium(II), attempts to further expand the application of other analogues, such as platinum(II) emitting materials, for efficient OLED applications have still encountered many obstacles.7For example, the PtOEP (H
2OEP ) octaethylpor-phyrin) commonly exhibits a ligand-basedππ* phosphores-cence with a lifetime up to 30-50µs. Accordingly, saturation of the emission signals and a rapid drop in device efficiency at high driving currents are expected. A second factor contributing to the inferior efficiency is the planar geometry which could easily induce the formation of aggregates8 or excimers9via efficientπ-π stacking at the vacant coordina-tion sites on the Pt(II) atom. A design aimed at color tuning and suppressing the self-aggregation of Pt(II) complexes is needed to exploit the full potential of Pt(II) emitters; among these methods, the introduction of the acetylide ligand has shown its versatility in color tuning,10while employing rigid architecture, such as the camphor-based ligand, is an alterna-tive strategy for alleviating the aggregation.11One expects that this camphor-based substituent on the ligands would strictly inhibit the columnar alignment of complex molecules as well as the occurrence of the direct Pt‚‚‚Pt bonding in
the solid state. OLEDs with efficient saturated red emission have been successfully fabricated using this camphor-based Pt(II) dopant.11
In this study, we focused on the preparation, characteriza-tion, and photophysical properties of a series of readily sublimable and thermally robust Pt(II) pyridyl azolate complexes, 1-5. The azolate ligands employed in this investigation are superior to camphor-based ligand chro-mophores because of their greater structural tuning capability and flexibility at the ligand sites. Thus, the substituent fine-tuning photophysical properties are expected, especially in the solid state. Among complexes 1-5, several derivatives, such as 1, 2d, and 4, exhibit room-temperature phosphores-cence in the fluid and/or solid states, assigned to, depending on ancillary ligands, 3MLCT/3ππ*, 3ππ*, or 3MMLCT transitions. We expected that the various fundamental properties elaborated in this study would be beneficial to the future development of Pt(II) emitting materials, as complexes 2a and 3a are well suited for use as phospho-rescent OLEDs.
Experimental Section
General Procedures. All reactions were performed under
nitrogen. Solvents were distilled from appropriate drying agents prior to use. Commercially available reagents were used without further purification unless otherwise stated. All reactions were monitored by TLC with Merck precoated glass plates (0.20 mm with fluorescent indicator UV254). Compounds were visualized with UV light irradiation at 254 and 365 nm. Flash column chromatog-raphy was carried out using silica gel from Merck (230-400 mesh). Mass spectra were obtained on a JEOL SX-102A instrument operating in electron impact (EI) or fast atom bombardment (FAB) mode.1H and13C NMR spectra were recorded on a Bruker-400 or INOVA-500 instrument; chemical shifts are quoted with respect to the internal standard, tetramethylsilane, for1H and13C NMR data. Elemental analysis was carried out with a Heraeus CHN-O Rapid Elementary Analyzer. Pt(DMSO)2Cl2was prepared according to the literature method.123-Trifluoromethyl- and 3-heptafluoro-propyl-substituted 5-(2-pyridyl) pyrazoles (fppz)H and (hppz)H were prepared via the reaction of 2-acetylpyridine with either ethyl trifluoroacetate or ethyl perfluorobutanoate, followed by treatment with hydrazine hydrate in ethanol solution,13while 3-methyl-5-(2-pyridyl) pyrazole (mppzH), 3-tert-butyl-5-(2-3-methyl-5-(2-pyridyl) pyrazole (bppzH), 1-(5-tert-butyl-2H-pyrazol-3-yl) isoquinoline (bqpzH), (2-pyrazine) pyrazole (bzpzH), and 3-tert-butyl-5-(5-methyl-2-pyrazine) pyrazole (bmpzH) were prepared from reac-tions involving ethyl picolinate, ethyl isoquinaldate, ethyl pyrazi-necarboxylate, or ethyl 5-methylpyrazine-2-carboxylate with either acetone or pinacolone, respectively, followed by treatment of the condensation product with hydrazine hydrate under similar condi-tions. The respective 2-pyridyl triazole ligands, namely, 5-tert-butyl-3-(pyridin-2-yl)-1H-1,2,4-triazole (bptzH) and 5-heptafluoropropyl-3-(pyridin-2-yl)-1H-1,2,4-triazole (hptzH), were synthesized using previously reported procedures.14Their structures are depicted in Chart 1.
Spectroscopic and Dynamic Measurements. Steady-state
ab-sorption and emission spectra were recorded on a Hitachi (U-3310)
(2) (a) Carlson, B.; Phelan, G. D.; Kaminsky, W.; Dalton, L.; Jiang, X.; Liu, S.; Jen, A. K. Y. J. Am. Chem. Soc. 2002, 124, 14162. (b) Bernhard, S.; Gao, X.; Malliaras, G. G.; Abruna, H. D. AdV. Mater.
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(5) Adachi, C.; Baldo, M. A.; Forrest, S. R.; Lamansky, S.; Thompson, M. E.; Kwong, R. C. Appl. Phys. Lett. 2001, 78, 1622.
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spectrophotometer and an Edinburgh (FS920) fluorimeter, respec-tively. Both the wavelength-dependent excitation and the emission response of the fluorimeter were calibrated. A configuration of front-face excitation was used to measure the emission of the solid sample, in which the cell was made by assembling two edge-polished quartz plates with various Teflon spacers. Lifetime studies were performed with an Edinburgh FL 900 photon-counting system with a hydrogen-filled or nitrogen lamp as the excitation source. Data were analyzed using a nonlinear least squares procedure in combination with an iterative convolution method. The emission decays were analyzed by the sum of the exponential functions, which allowed partial removal of the instrument time broadening and consequently renderes a temporal resolution of∼200 ps.
To determine the photoluminescence quantum yield in solution, samples were degassed by three freeze-pump-thaw cycles under vigorous stirring conditions. 4-(Dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM,λem ) 615 nm, Exciton, Inc.) in methanol was used as a reference, assuming a quantum yield of 0.43 with a 430 nm excitation.15An integrating sphere (Labsphere) was applied to measure the quantum yield in the solid state, for which the solid sample film was prepared via either spin-coating or vapor-deposition methods and was excited by a 371 or 480 nm Ar+laser line; an intensified charge-coupled detector was used for subsequent quantum yield analyses of the resulting luminescence. To obtain the PL quantum yield in solid state, the emission was collected via an integrating sphere, and the quantum yield was calculated according to a reported method.16The values were taken to be the average of three separated measurements.
Synthesis of Pt(mppz)2 (1). 3-Methyl-5-(2-pyridyl) pyrazole
(mppzH, 83 mg, 0.52 mmol) was added slowly at room temperature to a suspension of NaH (15 mg, 0.63 mmol) in 15 mL of THF. After being stirred for 1.5 h, the solution was filtered and transferred into a reaction flask loaded with 100 mg of Pt(DMSO)2Cl2(0.24 mmol) and 10 mL of THF. The solution was brought to reflux for 12 h. After that, the solvent was evaporated under vacuum, and 60 mL of CH2Cl2was added to extract the product. The extract was then washed with water, dried over anhydrous Na2SO4, and concentrated to dryness, giving a yellowish green powder. Further purification was conducted using vacuum sublimation (160°C, 220 mTorr), yielding 80 mg of Pt(mppz)2as bright green needles (0.16 mmol, 65%). Other Pt(II) complexes, 2-4, were synthesized using Pt(DMSO)2Cl2and the corresponding neutral ligands under similar reaction conditions.
Data for 1. MS (EI, 70 eV), observed (actual) [assignment]:
m/z 511 (511) [M+].1H NMR (500 MHz, CDCl 3, 294 K): δ 10.64 (d,3J HH) 6.0 Hz, 2H), 7.86 (t,3JHH) 8.0 Hz,3JHH) 8.0 Hz, 2H), 7.55 (d,3J HH) 8.0 Hz, 2H), 7.28 (t,3JHH) 6.0 Hz,3JHH) 6.0 Hz, 2H), 6.43 (s, 2H), 2.45 (s, 6H).13C NMR (125 MHz, CDCl 3, 294K): δ 154.7 (Cpy), 152.5 (Cpz), 150.3 (CHpy), 147.6 (Cpz), 139.1 (CHpy), 121.6 (CHpy), 118.2 (CHpy), 103.0 (CHpz), 14.0 (CH3). Anal. Calcd for C18H16N6Pt: C, 42.27; N, 16.43; H, 3.15. Found: C, 42.08; N, 16.38; H, 3.49.
Data for 2a. MS (EI, 70 eV), observed (actual) [assignment]:
m/z 595 (595) [M+].1H NMR (400 MHz, CDCl 3, 294 K): δ 10.79 (d,3J HH) 6.0 Hz, 2H), 7.82 (ddd,3JHH) 7.9 Hz,3JHH) 7.6 Hz, 4J HH) 1.2 Hz, 2H), 7.55 (d,3JHH) 7.9 Hz, 2H), 7.19 (ddd,3JHH ) 7.6 Hz,3J HH) 6.0 Hz,4JHH) 1.2 Hz, 2H), 6.49 (s, 2H), 1.42 (s, 18H).13C NMR (125 MHz, CDCl 3, 294K): δ 161.4 (Cpy),155.4 (Cpz), 152.3 (CHpy), 149.4 (Cpz), 138.8 (CHpy), 120.7 (CHpy), 117.8 (CHpy), 99.4 (CHpz), 32.6 (Ct-butyl), 31.0 (CH3). Anal. Calcd for C24H28N6Pt: C, 48.40; N, 14.11; H, 4.74. Found: C, 48.31; N, 14.10; H, 4.88.
Data for 2b. MS (EI, 70 eV), observed (actual) [assignment]:
m/z 597 (597) [M+], 582 (582) [M+- CH3].1H NMR (500 MHz, CDCl3, 294 K): δ 10.54 (d,3JHH) 3.8 Hz, 2H), 8.83 (s, 2H), 8.38 (d,3J HH) 3.8 Hz, 2H), 6.50 (s, 2H), 1.43 (s, 18H).13C NMR (125 MHz, CDCl3, 294 K): δ 162.4 (Cpy), 149.7 (Cpz), 145.9 (Cpz), 144.2 (CHpy), 141.9 (CHpy), 140.6 (CHpy), 100.9 (CHpz), 32.7 (Ct-butyl),
30.9 (CH3). Anal. Calcd for C22H26N8Pt: C, 44.22; N, 18.75; H, 4.39. Found: C, 43.94; N, 19.21; H, 4.60.
Data for 2c. MS (EI, 70 eV), observed (actual) [assignment]:
m/z 626 (625) [M+], 611 (610) [M+- CH3].1H NMR (500 MHz, CDCl3, 294 K): δ 10.65 (s, 2H), 8.80 (s, 2H), 6.55 (s, 2H), 2.71 (s, 6H) 1.43 (s, 18H). 13C NMR (125 MHz, CDCl 3, 294 K): δ 162.1 (Cpy), 151.9 (Cpz), 146.9 (Cpz), 146.5 (CHpy), 143.9 (CHpy), 139.5 (CHpy), 99.9 (CHpz), 32.7 (Ct-butyl), 30.9 (CH3), 21.6 (CH3). Anal. Calcd for C24H30N8Pt: C, 46.07; N, 17.91; H, 4.83. Found: C, 46.15; N, 17.70; H, 5.02.
Data for 2d. MS (FAB), observed (actual) [assignment]: m/z
696 (696) [M+].1H NMR (400 MHz, CD
2Cl2, 294 K): δ 10.98 (d, J ) 5.6 Hz, 2H), 8.82 (d, J ) 7.9 Hz, 2H), 7.95 (d, J ) 7.9 Hz, 2H), 7.86 (t, J ) 7.0 Hz, 2H), 7.77 (t, J ) 7.4 Hz, 2H),7.66 (d, J
) 5.7 Hz, 2H), 7.06 (s, 2H), 1.55(s, 18H). Anal. Calcd for C32H32N6 -Pt: C, 55.24; H, 4.64; N, 12.08. Found: C, 54.88; H, 4.94; N, 11.98.
Data for 3a. MS (EI, 70 eV), observed (actual) [assignment]:
m/z 619 (619) [M+], 407 (407) [M+- fppz].1H NMR (400 MHz, acetone-d6, 294 K): δ 10.29 (d,3JHH) 6.0 Hz, 2H), 8.19 (ddd, 3J HH) 7.6 Hz,3JHH) 7.6 Hz,4JHH) 1.4 Hz, 2H), 7.94 (d,3JHH ) 7.6 Hz, 2H), 7.47 (ddd,3J HH) 7.6 Hz,3JHH) 6.0 Hz,4JHH) 1.4 Hz, 2H), 7.07 (s, 2H).19F (470 MHz, toluene-d 8, 294 K): δ
-60.70 (s, CF3). Anal. Calcd for C18H10F6N6Pt: C, 34.90; N, 13.57; H, 1.63. Found: C, 34.44; N, 13.12; H, 1.78.
Data for 3b. MS (EI, 70 eV), observed (actual) [assignment]:
m/z 820 (819) [M+].1H NMR (500 MHz, THF-d 8, 294 K): δ 10.53 (d,3J HH) 6.0 Hz, 2H), 8.14 (ddd,3JHH) 7.5 Hz,3JHH) 7.5 Hz, 4J HH) 1.0 Hz, 2H), 7.95 (d,3JHH) 7.5 Hz, 2H), 7.48 (ddd,3JHH ) 7.5 Hz,3J HH) 6.0 Hz,4JHH) 1.0 Hz, 2H), 7.13 (s, 2H).19F (470 MHz, THF-d8, 294 K): δ -80.83 (t,3JFF ) 8.5 Hz, CF3), -109.06 (dd,3J FF) 19.3 Hz, 3JFF) 8.5 Hz, CF2), -127.31 (s, CF2). Anal. Calcd for C22H10F14N6Pt: C, 32.25; N, 10.26; H, 1.23. Found: C, 32.23; N, 10.01; H, 1.51.
Data for 4. MS (EI, 70 eV), observed (actual) [assignment]:
m/z 597 (597) [M+].1H NMR (400 MHz, CDCl 3, 294 K): δ 10.49 (d,3J HH) 6.0 Hz, 2H), 8.07 (d,3JHH) 7.6 Hz, 2H), 8.00 (ddd, 3J HH ) 7.6 Hz,3JHH ) 7.6 Hz,4JHH ) 1.2 Hz, 2H), 7.40 (ddd, (13) (a) Thiel, W. R.; Eppinger, J. Chem.sEur. J. 1997, 3, 696. (b) Singh,
S. P.; Kumar, D.; Jones, B. G.; Threadgill, M. D. J. Fluorine Chem.
1999, 94, 199.
(14) (a) Kubota, S.; Uda, M.; Nakagawa, T. J. Heterocycl. Chem. 1975, 12, 855. (b) Funabiki, K.; Noma, N.; Kuzuya, G.; Matsui, M.; Shibata, K. J. Chem. Res. (M) 1999, 1301.
(15) Drake, J. M.; Lesiecki, M. L.; Camaioni, D. M. Chem. Phys. Lett.
1985, 113, 530.
Chart 1
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3J
HH) 7.6 Hz,3JHH) 6.0 Hz,4JHH) 1.2 Hz, 2H), 1.49 (s, 18H). Anal. Calcd for C22H26N8Pt: C, 44.22; N, 18.75; H, 4.39. Found: C, 44.12; N, 18.69; H, 4.48.
Synthesis of Pt(hptz)2(5). A solution of K2PtCl4(100 mg, 0.24 mmol) and hptzH (167 mg, 0.53 mmol) in a mixture of ethanol (15 mL) and water (5 mL) was heated at 80°C for about 16 h. After the mixture was cooled to room temperature, the precipitate was collected, washed with diethyl ether, and dried under vacuum to give Pt(hptz)2as an orange solid (78 mg, 0.096 mmol) in a 40% yield.
Data for 5. MS (EI, 70 eV), observed (actual) [assignment]:
m/z 822 (821) [M+].1H NMR (500 MHz, THF-d
8, 294 K): δ 10.31 (d,3J
HH) 6.0 Hz, 2H), 8.31 (t,3JHH) 7.0 Hz, 2H), 8.20 (d,3JHH
) 7.0 Hz, 2H), 7.74 (t, 3J
HH ) 6.0 Hz, 2H). Anal. Calcd for C22H8F14N8Pt: C, 29.24; N, 13.64; H, 0.98. Found: C, 28.74; N, 13.37; H, 1.14.
X-ray Diffraction Studies. Single-crystal X-ray diffraction data
were measured on a Bruker SMART Apex CCD diffractometer usingµ(Mo KR) 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. The structure was determined using the SHELXTL/PC program and refined using full-matrix least squares. Crystallographic refinement parameters of complexes 2c and 3b and their selective bond distances and angles are summarized in Tables 1 and 2, respectively.
OLED Fabrication. Charge-transporting materials such as NPB {4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl}and Alq3 [tris-(8-hydroxyquinolinato) aluminum(III)], as well as the host material CBP (4,4′-N,N′-dicarbazolyl-1,1′-biphenyl), were synthesized ac-cording to literature procedures,17and were sublimed twice through a temperature-gradient sublimation system before use. BCP (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline) was obtained from Al-drich. Patterned ITO-coated glass substrates (sheet resistance e 30
Ω/0) with an effective individual device area of 3.14 mm2were cleaned by sonication in a detergent solution, water, and ethanol
and then dried by a flow of nitrogen. The substrates were further treated with oxygen plasma for 3 min before being loaded into the vacuum chamber. Various organic layers were deposited sequen-tially at a rate of 0.1-0.3 nm/s under a pressure of 2× 10-5Torr in an Ulvac Cryogenic deposition system. Phosphorescent dopants were coevaporated with CBP via two independent sources. A thin layer of LiF (1 nm) and a thick layer of Al (150 nm) followed as the cathode. The current-voltage-luminance of the devices was measured in ambient conditions with a Keithley 2400 Source meter and a Newport 1835C Optical meter equipped with an 818ST silicon photodiode. The EL spectrum was obtained using a HITACHI F4500 spectrofluorimeter. The active area of the device was 3.14 mm2and that of the silicon photodiode was 100 mm2. The device was placed close to the photodiode so that all the forward light went to the photodiode. The luminance was calibrated using a PR-650 Spectrascan Colorimeter (Photo Research, USA). The external quantum efficiency was calculated according to the method previously described.18The luminous flux (lm) is defined19by P
v
) Km∫λPe,λV(λ)dλ, where Kmis the maximum luminous efficiency (683 lm/W), Pe,λis the spectral concentration of radiant flux, and
V(λ) is the relative photonic luminous efficiency function; the
luminance (cd/m2) is defined by luminous flux/πa, where a is the device area. The luminous efficiency (cd/A) is defined by luminous flux/πI, where I is the current, and the power efficiency is defined
as luminous flux/IV, where V is the applied voltage.
Results and Discussion
Synthesis and Characterization. The chelating pyrazole
ligands are derived from a two-step synthetic reaction involving Claisen condensation, followed by treatment with hydrazine in a refluxing ethanol solution. Triazole ligands are prepared from 2-pyridyl carboximidamide or carbox-amidrazone using methods reported in the literature.14These ligands are represented as (N∧N)H, implying that the N∧N chelating anions can be easily produced by deprotonation of the neutral ligands with a suitable base.
The targeted Pt(II) emitting materials (Chart 2) were then obtained via two distinctive pathways. The first one incor-porates an in situ generation of the sodium salt of the (N∧N) -anion, followed by treatment with a suspension of [PtCl2 -(DMSO)2] in refluxing THF; while the second involves direct reaction of the neutral (N∧N)H ligands with K2PtCl4 in ethanol to yield Pt(II) complexes with the formula [Pt-(N∧N)2]. Both methods gave satisfactory yields, and detailed procedures are given in the Experimental Section. In addition, all Pt(II) complexes showed good stability in both the fluid and solid states, but their solubility in organic solvents varied (16) de Mello, J. C.; Wittmann, H. F.; Friend, R. H. AdV. Mater. 1997, 9,
230.
(17) (a) Sonsale, A. Y.; Gopinathan, S.; Gopinathan, C. Indian J. Chem.
1976, 14, 408. (b) Koene, B. E.; Loy, D. E.; Thompson, M. E. Chem.
Mater. 1998, 10, 2235.
Table 1. Crystal Data and Refinement Parameters for Complexes 2c and 3b
2c 3b
formula C24H30N8Pt C22H10F14N6Pt
mol wt 625.65 819.45
cryst syst monoclinic monoclinic
space group P21/c C2/c
cryst size (mm3) 0.25× 0.10 × 0.06 0.23× 0.05 × 0.03
morphology red slab orange needle
a (Å) 6.4321(2) 18.1823(3) b (Å) 13.3584(5) 20.0340(4) c (Å) 30.8313(11) 6.8846(1) β (deg) 91.8682(11) 92.5381(11) V (Å3) 2647.7(2) 2505.35(7) Z 4 4 Fcalcd(g/cm3) 1.570 2.173 temp (K) 295(2) 295(2) µ (mm-1) 5.326 5.731 reflns collected 25258 15234 independent reflns 6082 [R(int) ) 0.0706] 2887 [R(int) ) 0.0546] RF, Rw(F2) (all data)a 0.0835, 0.1313 0.0961, 0.1799 RF, Rw(F2) [I > 2σ(I)]a 0.0531, 0.1130 0.0563, 0.1507 GOF 1.095 1.124 aR F)∑||Fo- Fc||/∑|Fo|; Rw(F2) ) [∑w|Fo2- Fc2|2/∑w(Fo4)]1/2.
Table 2. Selected Bond Length (Å) and Angles (deg) for Complexes
2c and 3b 2c Pt(1)-N(1) 2.016(5) Pt(1)-N(2) 1.994(6) Pt(1)-N(5) 2.028(5) Pt(1)-N(6) 1.986(6) Pt(1)‚‚‚Pt(1) 6.432 H(1A)‚‚‚N(7) 2.256 H(13A)‚‚‚N(3) 2.296 N(1)-Pt(1)-N(2) 78.8(2) N(5)-Pt(1)-N(6) 79.2(2) N(1)-Pt(1)-N(6) 101.0(2) N(2)-Pt(1)-N(5) 101.0(2) 3b Pt-N(1) 2.056(8) Pt-N(2) 1.979(7) Pt‚‚‚Pt 3.442 H(1A)‚‚‚N(3) 2.319 N(1)-Pt(1)-N(2) 79.6(3) N(1)-Pt(1)-N(2′) 100.4(3) Chang et al.
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according to the nature of the ligands and the associated alkyl substituents. For example, pyridyl pyrazolate complexes 1 and 2a showed sparse solubility in organic solvents such as CHCl3, CH2Cl2, and THF at room temperature, while the pyrazine- and isoquinoline-substituted complexes 2b, 2c, and
2d were significantly more soluble under identical conditions.
For the fluorinated complexes, 3a, 3b, and 5, although the solubility in solvents improved with the increasing chain length of the fluoroalkyl substituent (i.e., CF3vs C3F7), they tend to exhibit relatively poor solubility at elevated temper-atures and rapidly form microcrystalline solids when the solution is cooled to room temperature. On the basis of these experimental facts, the Pt(II) complexes 2b, 2c, and 2d were best purified by routine recrystallization, while vacuum sublimation was employed for separation and purification of other Pt(II) products.
Complex 2c was examined by single-crystal X-ray dif-fraction analysis to establish its molecular structure. Figure 1 shows the ORTEP diagram of 2c, along with a stacking diagram showing the π-π stacking interactions between molecules. The Pt-N(pyrazolate) bond lengths observed in
2c vary between 1.986(6) and 1.994(6) Å, which are shorter
than those of the Pt-N(pyrazine) (2.016(5) and 2.028(5) Å). This difference reflects the stronger bonding to the anionic pyrazolate fragments versus that of the neutral pyrazine; the latter are within the range reported for those of diimine complexes [Pt(diimine)Cl2] (1.992-2.015Å) and others.20 Moreover, interesting intermolecular interactions were ob-served in the crystal lattice, for which the molecules showed a slipped packing arrangement with one bmpz ligand residing over the center of adjacent molecule. Rather short interplanar π-π stacking distances of∼3.36 Å were observed between each molecule, indicating the presence of a strong π-π noncovalent interaction. However, the Pt‚‚‚Pt separation was calculated to be 6.432 Å along the column, which is too long for any significant Pt-Pt contacts, such as those observed in several dimeric structures of cyclometalated Pt(II) com-plexes (3.15-3.76 Å)21to be expected.
In addition, unusually short N‚‚‚H contacts (2.256 and 2.296 Å) were detected between the ortho H atom of the
coordinated pyrazine and the uncoordinated N atom of the pyrazolate fragments, H(1A)‚‚‚N(7) ) 2.256 Å and H(13A)‚ ‚‚N(3) ) 2.296 Å. The tendency of the pyrazolate nitrogen to prefer to connect with the adjacent hydrogen atom is attributable to the greater basicity of the uncoordinated nitrogen atom after the Pt(II) metal coordination. For comparison, it has been reported that the solvated water molecule is held tightly between the pyrazolate ligands of [Ru(bpy)2(pz)2]‚H2O, and no anhydrous solid material could be isolated.22Note that exceedingly strong N-H‚‚‚N bonding was also observed between the adjacent pyrazolate ligands for the emissive iridium(III) complexes [(tpy)2Ir(pzH)2](CF3 -SO3) and [(tpy)2Ir(pz2H)].23
To offer a direct comparison with respect to 2c, we also conducted an X-ray structural analysis of the C3F7-substituted complex 3b because it showed very poor solubility and an enhanced tendency to form a needlelike solid during recrys-tallization. As illustrated in Figure 2, the Pt(II) cation is in the square planar arrangement, and the corresponding Pt-N
(18) Forrest, S. R.; Bradley, D. D. C.; Thompson, M. E. AdV. Mater. 2003, 15, 1043.
(19) Wyszecki, G.; Stiles, W. S. Color Science: Concepts and Methods, QuantitatiVe Data and Formulae; John Wiley & Sons: New York, 1982; p. 259.
(20) (a) Miskowski, V. M.; Houlding, V. H.; Che, C. M.; Wang, Y. Inorg. Chem. 1993, 32, 2518. (b) Wadas, T. J.; Lachicotte, R. J.; Eisenberg, R. Inorg. Chem. 2003, 42, 3772.
(21) (a) Chassot, L.; Mu¨ller, E.; Von Zelewsky, A. Inorg. Chem. 1984, 23, 4249. (b) Bailey, J. A.; Miskowski, V. M.; Gray, H. B. Inorg. Chem. 1993, 32, 369. (c) Ghedini, M.; Pucci, D.; Crispini, A.; Barberio, G. Organometallics 1999, 18, 2116.
(22) Sullivan, B. P.; Salmon, D. J.; Meyer, T. J.; Peedin, J. Inorg. Chem.
1979, 18, 3369.
(23) Li, J.; Djurovich, P. I.; Alleyne, B. D.; Tsyba, I.; Ho, N. N.; Bau, R.; Thompson, M. E. Polyhedron 2004, 23, 419.
Chart 2
Figure 1. (top) ORTEP diagram of 2c with thermal ellipsoids shown at the 50% probability level. (bottom) Diagram showing the molecular stacking and closest Pt‚‚‚Pt contact.
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distances are not so different from those observed for the previously discussed complex 2c. However, distinctive intermolecular interactions are observed in its packing diagram, for which the complex molecules are stacked into a column with an alternating staggered orientation along the chain axis. The overall structure is constructed so that each pyridyl fragment is sandwiched by two pyrazolate moieties, and likewise, each pyrazolate fragment is sandwiched by two pyridyl moieties of the neighboring molecules. The resulting noncovalent contact leads to the observed columnar arrange-ment with a shortened Pt‚‚‚Pt separation of 3.442 Å, which is almost identical to the average interplanar C-C distances of∼3.35 Å. The related linear Pt‚‚‚Pt‚‚‚Pt arrangement has been observed in several of the Pt(II) linear-chain complexes, such as [Pt(diimine)2(CN)2], diimine ) bipyridine and biisoquinoline.24
After the X-ray structural analyses were completed, these Pt(II) complexes were characterized by1H NMR to obtain their supplementary spectroscopic properties in solution. Of particular interest is the observation of downfield1H NMR signals in theδ 10.98-10.29 region among complexes 1-5, which are assigned to the interligand C-H‚‚‚N linkage between two trans-oriented chelating ligands. The proposed H-bonding interaction is reminiscent of those reported for the related pyridyl pyrazolate Os complexes2cas well as the cobaloxime complexes.25 It is believed that this unusual H-bonding, to a certain extent, provides a favorable driving
force stabilizing the trans geometry and hence a rigid molecular framework. Certainly, because of the nonlinear C-H‚‚‚N alignment (vide supra), we cannot rule out the possibility that the C-H‚‚‚N bonding is caused by the spatial constraint imposed by the ligand arrangement. In contrast to our discovery, the X-ray structural analyses of related Pt-(II) complexes such as Pt(thppy)2, thppy ) 2-(2′-thienyl) pyridine, revealed that ligated thiophene and pyridine frag-ments are oriented exclusively in a cis configuration, despite the large distortion and deformation between the ligands.26 This could be the result of the exceedingly strong trans effect exerted by the carbon donor atom of the ligated thiophene. The net result prevents the ligands from residing at the mutual trans disposition. In contrast, our ligands use weak electron-donating azolate N atoms to form dative bonds with respect to the Pt(II) metal ion. The reduction of the trans effect thus favors the azolate fragments to locate at the mutual trans dispositions.
Photophysical Properties. Table 3 summarizes the peak
wavelengths of the lowest-energy absorption and other important photophysical data. The room-temperature UV-vis and emission spectra of complexes 2a-d in THF solution are shown in Figure 3. These data allowed us to visualize the spectral variation associated with the derivatization of neutral pyridyl units without altering the anionic pyrazolate segment.
As revealed in Figure 3, it is obvious that the strong absorptions in the UV region (e350 nm) are derived from a typical ligand-centered ππ* transition because the corre-(24) (a) Kato, M.; Sasano, K.; Kosuge, C.; Yamazaki, M.; Yano, S.; Kimura,
M. Inorg. Chem. 1996, 35, 116. (b) Kato, M.; Kosuge, C.; Morii, K.; Ahn, J. S.; Kitagawa, H.; Mitani, T.; Matsushita, M.; Kato, T.; Yano, S.; Kimura, M. Inorg. Chem. 1999, 38, 1638.
(25) Gupta, B. D.; Singh, V.; Yamuna, R.; Barclay, T.; Cordes, W. Organometallics 2003, 22, 2670.
Figure 2. (top) ORTEP diagram of 3b with thermal ellipsoids shown at the 50% probability level. (bottom) Stacking diagram with an emphasis on the linear Pt‚‚‚Pt interaction.
Table 3. Photophysical Properties of Platinum(II) Complexes 1-5
λabsmax (, M-1cm-1) PLλmax (nm)a F τ (µs) 1 266 (20000), 320 (14300), 338 (10700), 418 (2500), 436 (2400) 495, 516b [490b, 512, 550b] 0.21 [2.4× 10-2] 0.4 [1.2] 2a 268 (18500), 320 (13100), 340 (10100), 416 (2200), 442 (2100) 494, 518b [489, 518, 550b] 0.19 [1.1× 10-2] 0.4 [0.4] 2b 271 (23200), 313 (11500), 340 (12100), 375 (12700), 473 (2200), 502 (2000) 576, 596b [626] 0.82 [6× 10-4] 2.4 [0.03] 2c 272 (25000), 314 (11100), 338 (11400), 374 (12100), 480 (2100), 500 (1700) 561, 592b [554b, 581, 623b, 658b] 0.86 [e10-4] 2.1 [0.09] 2d 283 (30900), 206 (26800), 351 (30400), 371 (20600), 463 (5200), 484 (4600) 584, 632, 685b [593, 634, 680b] 0.74 [4× 10-4] 13 [0.04] 3a 256 (24000), 310 (19700), 330 (12300), 377 (2900), 389 (2500) -c [595] -[0.24] -[0.3] 3b 256 (23400), 310 (19400), 328 (12300), 376 (2900), 388 (2500) -c [552] -[0.2] -[0.3] 4 280 (20300), 306 (16500), 326 (11000), 400 (3400), 420 (3155) 482, 512, 540b [564] 0.02 [0.16] 0.4 [0.4] 5 258 (21500), 296 (13300), 318 (12300), 360 (3500), -c [544] -[0.33] -[0.3] 376 (3200)
aThe absorption and emission spectra were recorded in degassed THF
at room temperature, while the solid-state spectra measured using thin film samples are listed in square brackets. Note that for 3b, no apparent difference in spectral profile between the thin film and single crystal measurements was observed.bShoulder.cToo weak to be observed.
Chang et al.
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sponding transitions were also documented for the free ligands. The lower-lying absorption bands have relatively small extinction coefficients (2a, 2200 M-1cm-1at 416 nm;
2b, 2200 M-1 cm-1 at 473 nm; 2c, 2100 M-1cm-1at 480 nm) and are tentatively assigned to the transition incorporat-ing a state mixincorporat-ing among sincorporat-inglet and triplet metal-ligand charge transfer (1MLCT and3MLCT) and, to a certain extent, the intraligand3ππ transitions. Interpretation of these MLCT bands may be further complicated by the fact that these bands are clearly composed of two distinctive transitions; similar observations have been documented for the Pt(II) acetylide complexes.27Moreover, the close energetics and absorptivity between the 1MLCT and 3MLCT bands suggest that the 3MLCT transition, induced by spin-orbit coupling and the proximal energy levels with respect to 1MLCT, is greatly enhanced and becomes partially allowed. This salient spectral feature is in accordance with data for other recently published Pt(II) metal complexes with two N∧O bidentate chelates or bis(phenoxy)diimine auxiliaries capable of tetradentate bond-ing.28The strong singlet-triplet mixing is unambiguously confirmed by the significant overlap between the UV-vis absorption bands and the leading edge of the respective emission peak profile.
Also shown in Figure 3, complex 2a exhibits a fairly intensive emission maximized at 494 nm (Φ ) 0.19; τ ) 0.4µs) in degassed THF. The fast oxygen quenching rate of 1.78× 109M-1s-1, in combination with its spectral mirror image with respect to the lowest-absorption profile, leads us to conclude that the emission mainly originates from a triplet manifold. Similarly, complexes 2b and c also exhibit strong phosphorescence with peak wavelengths at 576 nm (Φ ) 0.82; τ ) 2.4 µs) and at 561 nm (Φ ) 0.86; τ ) 2.1 µs), which are comparable to those observed in the cyclo-metalated dipyridylbenzene Pt complexes.29The red shift of the peak wavelength and, hence the energy gap of 2b and c,
compared with that of 2a, is a result of pyrazine substitution, for which the extra nitrogen atom of pyrazine lowers theπ* energy level of Pt(II) metal complexes. This∼15 nm blue shift for 2c versus that of 2b is apparently caused by the methyl substituent, which destabilized the LUMO energy level by electron donation. Moreover, the lowest-energy absorption of 2d appears at 463 nm with 5200 M-1 cm-1, while the respective emission not only reveals a much larger Stokes shift but also possesses a distinctive vibronic-like progression with peak wavelengths at 584, 632, and 685 nm in THF solution (see Figure 3 and Table 3). The observed lifetime for 2d in THF (τ ) 13 µs, Φ ∼ 0.74) is also substantially longer than those of 2a-c, as well as those of complexes with a 3MLCT in the lowest-lying manifold incorporating Os(II), Ir(III), or Pt(II) heavy ions and with a 1MLCT state in proximity. Thus, for the case of 2d, a significant amount of state mixing with the ligand-centered 3ππ transitions is expected.30This can be rationalized by the decrease of theπ energy gap from the elongation of the π conjugation in 2d. The progression of g1220 cm-1for each successive peak of 2d cannot be rationalized by the much smaller d-level splitting in a typical square planar Pt(II) complex, but it is similar to that corresponding to the C-C vibrational modes (1270-1300 cm-1) for complexes involv-ing terpyridyl ligands.31
On the other hand, the solution absorption and solid-state emission spectra of complexes 2a, 3a and b, 4, and 5 are depicted in Figure 4. It is notable that all of the Pt(II) complexes shown in Figure 4 possess 2-pyridyl substituents; hence, the corresponding photophysical properties would reflect the influence of the anionic azolate segment (i.e., pyrazolate or triazolate) as well as the associated alkyl or fluoroalkyl substituents. For the UV-vis absorption spectral
(26) Gianini, M.; Forster, A.; Haag, P.; von Zelewsky, A.; Stoeckli-Evans, H. Inorg. Chem. 1996, 35, 4889.
(27) Whittle, C. E.; Weinstein, J. A.; George, M. W.; Schanze, K. S. Inorg. Chem. 2001, 40, 4053.
(28) Lin, Y. Y.; Chan, S. C.; Chan, M. C. W.; Hou, Y. J.; Zhu, N.; Che, C. M.; Liu, Y.; Wang, Y. Chem.sEur. J. 2003, 9, 1263.
(29) Williams, J. A. G.; Beeby, A.; Davies, E. S.; Weinstein, J. A.; Wilson, C. Inorg. Chem. 2003, 42, 8609.
(30) (a) Sacksteder, L.; Zipp, A. P.; Brown, E. A.; Streich, J.; Demas, J. N.; DeGraff, B. A. Inorg. Chem. 1990, 29, 4335. (b) Sacksteder, L.; Lee, M.; Demas, J. N.; DeGraff, B. A. J. Am. Chem. Soc. 1993, 115, 8230.
(31) (a) Yang, Q. Z.; Wu, L. Z.; Wu, Z. X.; Zhang, L. P.; Tung, C. H. Inorg. Chem. 2002, 41, 5653. (b) Yam, V. W. W.; Tang, R. P. L.; Wong, K. M. C.; Cheung, K. K. Organometallics 2001, 20, 4476. Figure 3. UV-vis absorption and emission spectra of complexes 2a-d
in THF at room temperature. Note that the normalized emission spectra were acquired in the degassed condition.
Figure 4. UV-vis absorption spectra of complexes 2a, 3a, 3b, 4, and 5 recorded in THF at room temperature, and the respective solid-state emission spectra obtained from a thin film sample at room temperature.
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measurements, a significant blue shift of the MLCT absorp-tion was noted by substituting trifluoromethyl or heptafluo-romethyl for a tert-butyl group. This is shown by observation of two lowest-energy MLCT transitions at 377 and 389 nm as well as at 376 and 388 nm for the fluorinated complexes
3a and b, respectively, for which both peaks show a
substantial blue shift compared to those (416 and 442 nm) of tert-butyl complex 2a. The decrease in the energy gap of the MLCT transitions is apparently caused by the electron-withdrawing effect of CF3 and C3F7 substituents, which increases theπ* acidity of the pyrazolate anion, accompanied by lowering the Pt(II) metal 5d energy level. Concomitantly, the higher energyππ* transitions of 3a and b, which are located at the UV region, also display a similar hypsochromic shift resulting from the increase of the intraligandππ* energy gap. For the triazolate complexes 4 and 5, the respective MLCT transitions are also shifted to the higher-energy region versus those of the tert-butyl- and CF3-substituted pyrazolate counterparts 2a and 3a, respectively. This delineation is consistent with a theoretical approach based on time-resolved density function theory calculation (not shown here), the results of which reveal that the HOMO is located at both the metal and azolate fragments, while the LUMO is located at the pyridyl fragment. Therefore, the substitution of, for example, the tert-butyl substituent with CF3(or C3F7) at the azolate fragment mainly lowers both the metal 5d and azolate π orbitals contributing to the HOMO resulting in an increase of the energy gap.
The photoluminescence spectra of 3a and b, 4, and 5 were measured in a degassed THF solution at room temperature; however, the emission is extremely weak (Φ , 10-3) and is supported by the phosphorescence lifetime of <10 ns in the degassed solution (not shown here). Although the associated deactivation mechanism, at this stage, is still pending for resolution, because of a strong tendency of the Pt-Pt interaction, self-quenching via the aggregation effect cannot be eliminated. Unfortunately, the poor solubility makes concentration-dependent studies unfeasible. In sharp contrast, moderate to highly intense luminescence was obtained withλmaxlocated at 595, 552, 564, and 544 nm for
3a and b, 4, and 5, respectively, in the vacuum-deposited
thin films (Figure 4). The structured emission of 2a recorded in the solid state at room temperature, which shows three vibronic peaks at 489, 518, and 550 nm, is also depicted here as a reference. In comparison, the intensity ratio for these three vibronic peaks is appreciably different from that in fluid solution (see Figure 3 vs Figure 4). In addition, the highest-energy emission peak in 2a undergoes an ∼5 nm (207 cm-1) blue shift in the solid state, while the other vibronic maxima remain nearly unchanged. One possible interpretation, in which the thin film emission of 2a may consist of dual or multiple origins, has been discarded because of the similar spectral feature of 2a in a 77 K solid film (not shown here). Alternatively, this discrepancy may more plausibly be rationalized by different sites of 2a in the solid film so that the vibronic peaks, especially the 0-0 onset, are subject to environmental (inhomogeneous) per-turbation.
One remarkable feature is that the phosphorescence of complexes 3-5 exhibits a featureless line shape and a broader full width at half-maximum (fwhm), a phenomenon commonly encountered in the Pt(II) complexes, symbolizing the formation of either an excimer or an oligomer.32 Moreover, except for 2a, the emission showed a very large spectral shift with almost no spectral overlap between the lowest-energy MLCT absorptions (in fluid solution) and the onset of the phosphorescent emission signals (in solid film). This observation leads us to conclude that these phospho-rescences primarily originate from the distinctive metal-metal-to-ligand charge-transfer transition (3MMLCT or the [dσ*,π*] notation),33in line with that proposed by Miskowski
and co-workers.34The short Pt‚‚‚Pt contact observed in the columnar structure of 3b sheds light on the expected destabilization of the metal 5d orbitals, resulting in a drastic peak-to-peak shift between the mononuclear absorption (in solution) and the polynuclear emission (in solid film) (Scheme 1).
These intriguing solid-state photophysical properties are consistent with those of the Pt(II) complexes [Pt(diimine)-X2], X ) CN.24In the literature, it was reported that when two [Pt(diimine)X2] units are in close enough proximity that they allow direct Pt-Pt interaction, a low-energy photolu-minescence that is red-shifted from the 3MLCT or ππ* emission of mononuclear one becomes observable. The diagram illustrating the formation of columnarπ-π stacking and the Pt-Pt interaction in the solid state versus that of the respective MO energy levels is depicted in Scheme 1. We expected that, for complex 3a, after the formation of linear Pt-Pt-Pt crosstalk, notable perturbation would be introduced and the electronic transition associated with this binuclear species would best be represented as the MMLCT transition (or the [dσ*,π*] notation). However, in view of
the infinite columnar structure of 3b, we speculate that the resulting MO energy levels would become more closely spaced in the crystal lattice, forming a quasi-continuum of levels (or bands) of very closely spaced orbitals, as shown in the right-hand side of this simplified MO energy diagram.35 (32) (a) Lai, S. W.; Chan, M. C. W.; Cheung, T. C.; Peng, S. M.; Che, C. M. Inorg. Chem. 1999, 38, 4046. (b) Lai, S. W.; Chan, M. C. W.; Cheung, K. K.; Che, C. M. Inorg. Chem. 1999, 38, 4262. (33) (a) Lai, S. W.; Che, C. M. Top. Curr. Chem. 2004, 241, 27. (b) Lu,
W.; Chan, M. C. W.; Zhu, N.; Che, C. M.; Li, C.; Hui, Z. J. Am. Chem. Soc. 2004, 126, 7639.
Scheme 1
Chang et al.
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Moreover, complex 3a, which has the CF3 substituents, exhibits a 43 nm (or 1310 cm-1) bathochromic shift in emission peak λmaxversus that of complex 3b, which has the C3F7substituents. This result can be rationalized by an increase in the columnarπ-π stacking interaction because of the decrease of the steric hindrance for the CF3groups, as their electron-withdrawing character was somewhat comparable to or slightly less than that of C3F7. In contrast, for complex 5, an even more notable hypsochromic shift of 51 nm (1575 cm-1) was observed versus that of 3a. This is apparently caused by the blue shift of the lowest-energy MLCT absorption while a similar degree ofπ-π stacking was maintained. Empirically, we speculate that the degree ofπ-π stacking for 5 can be qualitatively estimated using the difference in the Stokes energy shift of∆E ) 8570 cm-1 versus that of 3a (∆E ) 8900 cm-1). Note that this calculation was done by subtracting the emission peak (in cm-1) of the solid sample from the lowest-energy absorption peak (in cm-1) in fluid solution. Finally, it is also noteworthy that the emission peak,λmax, of 4 occurs at 564 nm, and hence a∆E value of 6080 cm-1was deduced. The smaller∆E may be attributed to a combination effect incorporating a larger steric interaction and a greater electron-donating character for the tert-butyl substituents.
OLED Fabrication. The superior thermal stability renders
our Pt(II) complexes suitable for high performance OLED applications. Multilayer devices of the configuration ITO/ NPB(40 nm)/CBP:2a(30 nm)/BCP(10 nm)/Alq3(30 nm)/LiF-(1 nm)/Alnm)/LiF-(150 nm) were prepared with doping concentrations of 2a of 6, 20, 50, and 100% neat film composition. This configuration was adopted from those reported by Thompson and Forrest.36Very bright emissions were observed for all
doping concentrations, including the one with a pure layer of 2a. Table 4 summarizes the selected performance data for these OLED devices. The I-V-L curves, plotted in Figure 5, show a trend of increasing current density with increasing concentration of 2a. A similar dependence on concentration was observed previously,6,11 and this result implies that with the device mainly operated by charge trapping mechanism,37the more traps that are present, the higher current will be. Moreover, the EL spectra exhibit a large red shift ofλmaxwith increasing dopant concentration, 502, 514, 518, and 556 nm for devices with 6, 20, 50, and 100% dopant concentration (Figure 6). This may be attributed to the formation of excessive π-π stacking interactions between the Pt(II) emitting molecules, for which a reduced red-shift of the EL emission maxima (600-626 nm) has been reported in the Ir(III)-based OLEDs upon increasing dopant concentrations from 7 to 100%.38Concurrently, the fwhm of the EL signal increased from 60 to 102 nm. It is notable that the 6% doped device showed the highest external QE at a lower current density but decreased rapidly upon (34) (a) Houlding, V. H.; Miskowski, V. M. Coord. Chem. ReV. 1991, 111,
145. (b) Miskowski, V. M.; Houlding, V. H. Inorg. Chem. 1991, 30, 4446.
(35) (a) Gliemann, G.; Yersin, H. Struct. Bonding 1985, 62, 87. (b) Yersin, H.; Riedl, U. Inorg. Chem. 1995, 34, 1642.
(36) Adachi, C.; Baldo, M. A.; Forrest, S. R.; Thompson, M. E. Appl. Phys. Lett. 2000, 77, 904.
(37) Gong, X.; Ostrowski, J. C.; Moses, D.; Bazan, G. C.; Heeger, A. J. AdV. Funct. Mater. 2003, 13, 439.
(38) Song, Y. H.; Yeh, S. J.; Chen, C. T.; Chi, Y.; Liu, C. S.; Yu, J. K.; Hu, Y. H.; Chou, P. T.; Peng, S. M.; Lee, G. H. AdV. Funct. Mater.
2004, 14, 1221.
Table 4. Performance Characteristics for the Electrophosphorescence Devices Based on Complexes 2a and 3a
concentration (%) max lum (V)a QE (%)b PE (lm/W)b LE (cd/A)b λmax (CIE)c 2a 6 27114 (15.0) 2.926 3.453 8.558 502 (0.21, 0.52) 20 25957 (15.0) 2.174 2.861 6.942 514 (0.26, 0.54) 50 24163 (12.4) 1.570 2.724 5.272 518 (0.28, 0.57) 100d 16084 (12.0) 1.609 3.463 5.099 556 (0.41, 0.53) 3a 7 23023 (15.0) 4.202 4.555 14.031 552 (0.38, 0.54) 20 40973 (15.0) 5.965 6.443 19.702 560 (0.42, 0.53) 50 31086 (15.0) 1.478 1.818 4.690 574 (0.46, 0.51) 100 19135 (15.0) 2.089 1.914 3.798 616 (0.56, 0.39)
aValues in the parentheses are the applied driving voltage.bData
collected at 20 mA/cm2.cMeasured at the driving voltage of 8 V.dA 14
nm thick pure layer was used.
Figure 5. I-V-L characteristics of OLED device ITO/NPB/CBP:2a/BCP/ Alq/LiF/Al as a function of dopant concentration.
Figure 6. EL spectra of OLED device ITO/NPB/CBP:2a/BCP/Alq/LiF/ Al as a function of dopant concentration.
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increasing of the driving current. Moreover, the higher doping levels resulted in a much lower initial QE, along with a reduced dependence on the driving current. The existence of bulky tert-butyl substituents may have the capability to suppress the self-quenching activities,39thus maintaining the amorphous property during fabrication.40
Complex 3a was also selected for use in fabricating similar multilayer OLED devices. As shown in Table 4, the doping levels were 7, 20, 50, and 100%, among which the best device performance with external QE≈ 6% was observed for an intermediate doping level of 20%. Moreover, a significant red shift in the EL maxima from 552 to 616 nm was observed with increasing dopant concentration (Figure 7a). It is believed that the large concentration-dependent red-shift of emission maxima is caused by the self-aggregation of the Pt(II) dopant in the thin film during device fabrication; such a tendency has been revealed by the solid-state X-ray structural study in the previous section. In addition, for the device employing 100% Pt(II) dopant 3a, a weak emission peak located at∼460 nm, probably originating from NPB, was clearly observed. This could be attributed to the enhanced electron conductivity of the CF3carrying 3a in a manner such that electron injection into the NPB layer occurred when a neat film of 3a was used.
For a further comparison, the observed red-shifted emis-sion with increasing dopant concentrations in EL spectra was also observed in the PL spectra of the same series of doped CBP films (Figure 7b), substantiating the similar origin of the red-shifted emission (i.e., self-aggregation of the dopant). This observation is related to the basic behaviors of the square planar Pt(II) complexes, for which self-aggregation is invoked in oligomerization and excimer formation for several mononuclear Pt(II) complexes in solution. Thus, the detected luminescence would display features involving both
3MLCT and 3MMLCT excited states which are generated from the monomer and the oligomeric counterpart, respec-tively.41Analogously, the related bathochromatic shift ob-served for the green-emitting complex 2a-based devices can be attributed to a similar effect.
Conclusion. We have demonstrated the synthesis of a
series of square planar light-emitting complexes by treatment of pyridyl azolate ligands with Pt(II) reagents such as K2 -PtCl4 or PtCl2(DMSO)2. The procedures are efficient and suited to a variety of ligand modifications, demonstrating the versatility of our rational design and the effectiveness of derivatization in this class of compounds. Photolumines-cence studies reveal that these complexes exhibit fairly high emission quantum yields in fluid solution or in the solid state, short phosphorescence radiative lifetimes in the range of several microseconds, and more importantly, a tuneable tendency to form aggregates, depending on the azolate ligands. The pyrazolate complexes 1 and 2 exhibited a slipped-stack arrangement with a negligible intermolecular Pt‚‚‚Pt contact. This was confirmed by the X-ray structural analysis on 2c and the relatively higher solubility in organic solvents. Their emissions, recorded in solution, may be attributed to3MLCT,3ππ, or mixing3MLCT/3ππ character, in view of their overall spectral profile and the associated radiation lifetimes. In sharp contrast, a strong intermolecular π-π stacking interaction is detected between the electron-rich azolate fragment and the electron deficient pyridyl group in complexes 3-5 giving an essentially linear columnar stacking arrangement in the solid state. The existence of notable intermolecular Pt-Pt contacts results in the formation of 3MMLCT excited states that account for the observed photophysical properties in complexes 3-5 in the solid state. Moreover, these Pt(II) complexes are suitable light-emitting materials, especially as phosphorescent dopants for OLED applications, because of their intense, readily tuneable emission hues and good thermal stability. For3MLCT/3ππ emitters such as 2a, the best device performance was realized using a 6% doping concentration. An external quantum efficiency of 2.9%, a luminous efficiency of 8.6 cd/A at 20 mA/cm2, and a luminescence of 27114 cd/m2at an applied voltage of 15 V were realized. With the3MMLCT emitters such as 3a, the best device performances are achieved at an ∼20% dopant concentration, along with an external quantum efficiency of 6.0%, and a luminous efficiency of 19.7 cd/A at 20 mA/cm2. These results are remarkable in that they show promise for OLED applications based on these Pt(II) emitting materials.
Acknowledgment. This work was funded by the National
Science Council of Taiwan, R.O.C., under Grants NSC 93-2113-M-007-012 and NSC 93-2752-M-002-002-PAE.
Supporting Information Available: X-ray crystallographic data
file (CIF) of complexes 2c and 3b. This material is available free of charge via the Internet at http://pubs.acs.org.
IC051393B
(39) Xie, H. Z.; Liu, M. W.; Wang, O. Y.; Zhang, X. H.; Lee, C. S.; Hung, L. S.; Lee, S. T.; Teng, P. F.; Kwong, H. L.; Zheng, H.; Che, C. M. AdV. Mater. 2001, 13, 1245.
(40) Ostrowski, J. C.; Robinson, M. R.; Heeger, A. J.; Bazan, G. C. Chem. Commun. 2002, 784.
(41) Lai, S. W.; Chan, M. C. W.; Cheung, T. C.; Peng, S. M.; Che, C. M. Inorg. Chem. 1999, 38, 4046.
Figure 7. (a) EL spectra of OLED device ITO/NPB/CBP:3a/BCP/Alq/ LiF/Al as a function of dopant concentration, and (b) PL spectra of 3a embedded in the CBP matrix with variable dopant concentration.
Chang et al.
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