Cyclometalated Platinum(II) Complexes of Lepidine-Based Ligands as

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Organometallics 2010, 29, 3912–3921 DOI: 10.1021/om100573r

Cyclometalated Platinum(II) Complexes of Lepidine-Based Ligands as Highly Efficient Electrophosphors Marappan Velusamy,† Chih-Hsin Chen,† Yuh S. Wen,† Jiann T. Lin,*,† Chao-Chen Lin,‡ Chin-Hung Lai,‡ and Pi-Tai Chou*,‡ †

Institute of Chemistry, Academia Sinica, 128, Section 2, Academia Road, Taipei 115, Taiwan, Republic of China, and ‡Department of Chemistry, National Taiwan University, Section 4, Roosevelt Road, Taipei 106, Taiwan, Republic of China Received June 10, 2010

A new series of lepidine-containing-ligand-based cyclometalated platinum(II) complexes 1-7 with general formula [Pt(L)acac] (L = lepidine-based ligand, acac = acetylacetonate) have been synthesized. The complexes have been characterized using spectral and electrochemical techniques. The single-crystal X-ray structures of complexes 2 and 5 have been successfully determined. The effects of substituents on the electronic features and redox potentials of the cyclometalated phenyl ring of the complexes are discussed. Complexes 1-7 are highly phosphorescent in fluid as well as in solid state, covering a rather broad spectral range from yellow to saturated red. These complexes were successfully used as a dopant material to fabricate the electroluminescent device, ITO/NPB (40 nm)/ CBP-doped Pt complex (5 wt %) (20 nm)/BCP (10 nm)/Alq3 (20 nm)/LiF (1 nm)/Al (150 nm), which exhibits very promising performance. The device doped with 5 wt % 3 produced a maximum external quantum efficiency of 15.2% with a maximum brightness of 26847 cd m-2 and corresponds to a luminance efficiency of 29.83 cd A-1.

Introduction Recently, phosphorescent transition metal complexes, containing iridium(III),1 ruthenium,2 osmium(II),3 rhenium(I),4 gold(III),5 and platinum(II)6 centers, have received *To whom correspondence should be addressed. E-mail: jtlin@ chem.sinica.edu.tw; [email protected]. (1) (a) Evans, R. C.; Douglas, P.; Winscom, C. J. Coord. Chem. Rev. 2006, 250, 2093. (b) Lo, S.-C.; Burn, P. L. Chem. Rev. 2007, 107, 1097. (c) Tamayo, A. B.; Alleyne, B. D.; Djurovich, P. I.; Lamansky, S.; Tsyba, I.; Ho, N. N.; Bau, R.; Thompson, M. E. J. Am. Chem. Soc. 2003, 125, 7377. (d) Rayabarapu, D. K.; Paulose, B. M. J. S.; Duan., J.-P.; Cheng, C.-H. Adv. Mater. 2005, 17, 349. (e) Holder, E.; Langeveld, B. M. W.; Schubert, U. S. Adv. Mater. 2005, 17, 1109. (f) Cao, Y.; Liang, B.; Wang, L.; Xu, Y. H.; Shi, H. H. Adv. Funct. Mater. 2007, 17, 3580. (g) Lowry, M. S.; Bernhard, S. Chem.;Eur. J. 2006, 12, 7970. (h) Ulbricht, C.; Beyer, B.; Friebe, C.; Winter, A.; Schubert, U. S. Adv. Mater. 2009, 21, 4418. (i) Kim, J. J.; You, Y.; Park, Y.-S.; Kim, J.-J.; Park, S. Y. J. Mater. Chem. 2009, 19, 8347. (j) Kim, E.; Park, S. B. Chem. Asian. J. 2009, 4, 1646. (k) Wong, W.-Y.; Ho, C.-L. J. Mater. Chem. 2009, 19, 4457. (l) Wong, W.-Y.; Ho, C.-L. Coord. Chem. Rev. 2009, 253, 1709. (m) Williams, J. A. G. Chem. Soc. Rev. 2009, 38, 1783. (2) (a) Chou, P.-T.; Chi, Y. Eur. J. Inorg. Chem. 2006, 3319. (b) Bernhard, S.; Barron, J. A.; Houston, P. L.; Abruna, H. D.; Ruglovksy, J. L.; Gao, X.; Malliaras, G. G. J. Am. Chem. Soc. 2002, 124, 13624. (c) Buda, M.; Kalyuzhny, G.; Bard, A. J. J. Am. Chem. Soc. 2002, 124, 6090. (d) Tung, Y.-L.; Lee, S.-W.; Chi, Y.; Chen, L.-S.; Shu, C.-F.; Wu, F.-I.; Carty, A.-J.; Chou, P.-T.; Peng, S.-M.; Lee, G.-H. Adv. Mater. 2005, 17, 1059. (e) Chen, C.-Y.; Wu, S.-J.; Li, J.-Y.; Wu, C.-G.; Chen, J.-G.; Ho, K.-C. Adv. Mater. 2007, 19, 3888. (3) (a) Jiang, X.; Jen, A. K.-Y.; Carlson, B.; Dalton, L. R. Appl. Phys. Lett. 2002, 80, 713. (b) Carlson, B.; Phelan, G. D.; Kaminsky, W.; Dalton, L.; Jiang, X.; Liu, M. S.; Jen, A. K.-Y. J. Am. Chem. Soc. 2002, 124, 14162. (c) Kim, J. H.; Liu, M. S.; Jen, A. K.-Y.; Carlson, B.; Dalton, L. R.; Shu, C.-F.; Dodda, R. Appl. Phys. Lett. 2003, 83, 776. (d) Cheng, Y.-M.; Yeh, Y.-S.; Ho, M.-L.; Chou, P.-T. Inorg. Chem. 2005, 44, 4594. (e) Lee, T.-C.; Hung, J.-Y.; Chi, Y.; Cheng, Y.-M.; Lee, G.-H.; Chou, P.-T.; Chen, C.-C.; Chang, C.-H.; Wu, C.-C. Adv. Funct. Mater. 2009, 19, 2639. pubs.acs.org/Organometallics

Published on Web 08/10/2010

immense attention due to their versatile applications in organic light-emitting diodes (OLEDs). Luminescent materials that can access the singlet and triplet emissive states are (4) (a) Lundin, N. J.; Blackman, A. G.; Gordon, K. C.; Officer, D. L. Angew. Chem., Int. Ed. 2006, 45, 2582. (b) Liu, C. B.; Li, J.; Li, B.; Hong, Z. R.; Zhao, F. F.; Liu, S. Y.; Li, W. L. Chem. Phys. Lett. 2007, 435, 54. (c) Wang, K.; Huang, L.; Gao, L.; Jin, L.; Huang, C. Inorg. Chem. 2002, 41, 3353. (d) Cleland, D. M.; Irwin, G.; Wagner, P.; Officier, D. L.; Gordon, K. C. Chem.;Eur. J. 2009, 15, 3682. (e) Mauro, M.; Procopio, E. Q.; Sun, Y.; Chien, C. H.; Donghi, D.; Panigati, M.; Mercandelli, P.; Mussini, P.; D'Alfonso, G.; Decola, L. Adv. Funct. Mater. 2009, 19, 2607. (5) (a) Wong, K.-C.; Zhu, X.; Hung, L.-L.; Zhu, N.; Yam, V. W.-W.; Kwok, H.-S. Chem. Commun. 2005, 2906. (b) Lee, Y.-A.; McGarrah, J. E.; Lachicotte, R. J.; Eisenberg, R. J. Am. Chem. Soc. 2002, 124, 10662. (c) Masahisa, O.; Mikio, H.; Daisuke, H. Chem. Phys. Lett. 2007, 436, 89. (6) (a) Lai, S.-W.; Che, C.-M. Top. Curr. Chem. 2004, 241, 27. (b) Cocchi, M.; Virgili, D.; Fattori, V.; Rochester, D. L.; Williams, J. A. G. Adv. Funct. Mater. 2007, 17, 285. (c) He, Z.; Wong, W.-Y.; Yu, X.; Kwok, H.-S.; Lin, Z. Inorg. Chem. 2006, 45, 10922. (d) Yang, C. L.; Zhang, X. W.; You, H.; Zhu, L. Y.; Chen, L. Q.; Zhu, L. N.; Tao, Y. T.; Ma, D. G.; Shuai, Z. G.; Qin, J. G. Adv. Funct. Mater. 2007, 17, 651. (e) Lin, Y.-Y.; Chan, S.-C.; Chan, M. C. W.; Hou, Y.-J.; Zhu, N.; Che, C.-M.; Liu, Y.; Wang, Y. Chem.;Eur. J. 2003, 9, 1264. (f) Cho, J.-Y.; Domercq, B.; Barlow, S.; Suponitsky, K. Y.; Li, J.; Timofeeva, T. V.; Jones, S. C.; Hayden, L. E.; Kimyonok, A.; South, C. R.; Weck, M.; Kippelen, B.; Marder, S. R. Organometallics 2007, 26, 4816. (g) Chang, S.-Y.; Kavitha, J.; Hung, J.-Y.; Chi, Y.; Cheng, Y.-M.; Li, E. Y.; Chou, P.-T.; Lee, G.-H.; Carty, A. J. Inorg. Chem. 2007, 46, 7064. (h) Lu, W.; Mi, B.-X.; Chan, M. C. W.; Hui, Z.; Che, C.-M.; Zhu, N.; Lee, S.-T. J. Am. Chem. Soc. 2004, 126, 4958. (i) Bhansali, U. S.; Polikarpov, E.; Swensen, J. S.; Chen, W.-H.; Jia, H.; Gaspar, D. J.; Gnade, B. E.; Padmaperuma, A. B.; Omary, M. A. Appl. Phys. Lett. 2009, 95, 233304. (j) Feng, K.; Zuniga, C.; Zhang, Y.-D.; Kim, D.; Barlow, S.; Marder, S. R.; Bredas, J. L.; Weck, M. Macromolecules 2009, 42, 6855. (k) Mitsumori, T.; Campos, L. M.; Garcia-Garibay, M. A.; Wudl, F.; Sato, H.; Sato, Y. J. Mater. Chem. 2009, 19, 5826. (l) Zhou, G. J.; Wang, Q.; Wong, W. Y.; Ma, D.; Wang, L.; Lin, Z. J. Mater. Chem. 2009, 19, 1872. (m) Rausch, A. F.; Yersin, H. Chem. Phys. Lett. 2010, 484, 261. (n) Fischer, T.; Czerwieniec, R.; Hofbeck, T.; Osminina, M. M.; Yersin, H. Chem. Phys. Lett. 2010, 486, 53. r 2010 American Chemical Society

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beneficial for electroluminescent devices. Strong spin-orbit coupling in the above-mentioned heavy metal complexes permits efficient intersystem crossing between singlet and triplet states and utilization of both states available for emission.7 Consequently, the device efficiencies based on phosphorescent metal complexes are far superior to those based on pure organic fluorescent compounds emitting only from the singlet excited state. Among these phosphorescent materials, iridium(III) complexes have been extensively studied because of their relatively short lifetime, high quantum efficiency, and easy tuning of the emission color8 via suitable ligand alternations. Cyclometalated complexes are also attractive due to their neutral nature, which allows high thermal stability and vapor deposition techniques for the creation of amorphous thin films suitable for the fabrication of light-emitting diodes. Even though many reports have concentrated on cyclometalated iridium(III) complexes, there were relatively fewer studies of analogous phosphorescent platinum(II) complexes and their exploitation in electroluminescent devices.6,9 A main obstacle in the development of highly emissive platinum(II) complexes in OLEDs is the great tendency of these square-planar species to aggregate at higher concentration, resulting in quenching of emission in the solid state. We previously reported a new series of lepidine-based cyclometalated iridium(III) emitters with rich photophysical properties.10 These lepidine-based complexes emit in the orange to red region and can be fabricated into devices of good performance. Therefore, we extended our studies to cyclometalated platinum complexes with an aim to unravel the structure versus luminescence property relationships. In this article, we present the synthesis and spectral and electroluminescent characteristics of platinum(II) cyclometalated complexes featuring aryl-substituted lepidine ligands. We observe that inhibition of stacking by incorporation of suitable substituents at the aryl moieties is beneficial to the emitting behavior of these complexes in doped matrixes. As a result, those ligands that are functionalized effectively to prevent intermolecular stacking interactions display better electroluminescent parameters. (7) (a) Baldo, M. A.; Lamansky, S.; Burrows, P. E.; Thompson, M. E.; Forrest, S. R. Appl. Phys. Lett. 1999, 75, 4. (b) Adachi, C.; Baldo, M. A.; Forrest, S. R.; Thompson, M. E. Appl. Phys. Lett. 2000, 77, 904. (c) Yersin, H. Top. Curr. Chem. 2004, 241, 1. (d) K€ohler, A.; B€assler, H. Mater. Sci. Eng. R 2009, 66, 71. (8) (a) Kwon, T.-H.; Cho, H. S.; Kim, M. K.; Kim, J.-W.; Kim, J.-J.; Lee, K. H.; Park, S. J.; Shin, I.-S.; Kim, H.; Shin, D. M.; Chung, Y. K.; Hong, J.-I. Organometallics 2005, 24, 1578. (b) Takizawa, S.; Nishida, J.; Tsuzuki, T.; Tokito, S.; Yamashita, Y. Inorg. Chem. 2007, 46, 4308. (c) Zhou, G.; Ho, C.-L.; Wong, W.-Y.; Wang, Q.; Ma, D.; Wang, L.; Lin, Z.; Marder, T. B.; Beeby, A. Adv. Funct. Mater. 2008, 18, 499. (d) You, Y.; Seo, J.; Kim, S. H.; Kim, K. S.; Ahn, T. K.; Kim, D.; Park, S. Y. Inorg. Chem. 2008, 47, 1476. (e) Tsuzuki, T.; Shirasawa, N.; Suzuki, T.; Tokito, S. Adv. Mater. 2003, 15, 1455. (9) (a) Laskar, I. R.; Hsu, S.-F.; Chen, T.-M. Polyhedron 2005, 24, 881. (b) Che, C.; Chan, S.; Xiang, H.; Chan, M. C. W.; Liu, Y.; Wang, Y. Chem. Commun. 2004, 1484. (c) Zhou, G.-J.; Wang, X.-Z.; Wong, W.-Y.; Yu, X.-M.; Kwok, H.-S.; Lin, Z. J. Organomet. Chem. 2007, 692, 3461. (d) Chan, S.-C.; Chan, M. C. W.; Wang, Y.; Che, C.-M.; Cheung, K.-K.; Zhu, N. Chem.;Eur. J. 2001, 7, 4180. (e) Yang, X.; Wang, Z.; Madakuni, S.; Li, J.; Jabbour, G. E. Adv. Mater. 2008, 20, 2405. (f) Yan, B.-P.; Cheung, C. C. C.; Kui, S. C. F.; Xiang, H.-F.; Roy, V. A. L.; Xu, S.-J.; Che, C.-M. Adv. Mater. 2007, 19, 3599. (g) Zhou, G.; Wang, Q.; Ho, C.-L.; Wong, W.-Y.; Ma, D. Chem. Commun. 2009, 3574. (h) Ho, C.-L.; Chiu, C.-H.; Wong, W.-Y.; Aly, S. M.; Fortin, D.; Harrey, P. D.; Yao, B.; Xie, Z. Y.; Wang, L. X. Macromol. Chem. Phys. 2009, 210, 1786. (i) Zhou, G.-J.; Wong, W.-Y.; Yao, B.; Xie, Z.; Wang, L. J. Mater. Chem. 2008, 18, 1799. (10) Justin Thomas, K. R.; Velusamy, M.; Lin, J. T.; Chien, C.-H.; Tao, Y.-T.; Wen, Y. S.; Hu, Y.-H.; Chou, P.-T. Inorg. Chem. 2005, 44, 5677.

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Results and Discussion Synthesis and Characterization. The structures of the new platinum complexes 1-7 (abbreviated as [PtC∧N(acac)]) (C∧N = cyclometalated ligands 1a-7a; acac = acetylacetonate) are shown in Figure 1. Some substituted lepidines (1a-5a) used in the present work for the assembly of luminescent platinum complexes were available from our earlier study.10 However, the fluorene- and bifluorene-containing ligands (6a and 7a) were strategically designed and synthesized for this work as illustrated in Scheme 1. Ligands 6a and 7a were obtained from 2-chlorolepidine and the corresponding boronic acid (9-diethyl-9H-fluoren-3-ylboronic acid or bifluorenylboronic acid) via Suzuki’s C-C coupling reaction.11 Cyclometalated complexes 6 and 7 were then obtained from the ligand 6a and 7a by a two-step protocol essentially the same as that used for the construction of iridium(III) cyclometalated complexes: (a) treatment of the ligands with K2PtCl4 produced the cholrobridged dimers 6b and 7b; (b) subsequent reaction of 6b and 7b with acetylacetone in the presence of sodium carbonate yielded the desired monomeric complexes 6 and 7. Complexes 1-5 were synthesized from 1a-5a by the same synthetic protocol. All the complexes were characterized by NMR, high-resolution mass spectra, and elemental analyses. Single-crystal X-ray structural characterization was also carried out on two selected complexes, 2 and 5. All of these complexes are soluble in common organic solvents including dichloromethane, chloroform, and tetrahydrofuran, while they are insoluble in hexane and protic solvents. They are intensely orange or red in the solid state; however, dissolution in solvent leads to dark yellow solutions and gives rise to intense orange or red color emission when exposed to visible light. Crystal Structures. Single crystals of the complexes 2 and 5 suitable for X-ray diffraction analysis were obtained by slow diffusion of hexane to the dichloromethane solutions of the corresponding complex. ORTEP plots of 2 and 5 are displayed in Figures 2 and 3, respectively. In both structures the platinum(II) center is located in a slightly distorted square-planar environment, as indicated by the bond angles C(21)-Pt-O(2) [2, 173.9(2)o; 5, 174.1(1)o] and O(1)-Pt-N(1) [2, 170.1(2)o; 5, 169.94(9)o]. The bond length of Pt-N [2, 2.070(4) A˚; 5, 2.038(3) A˚] is slightly longer compared to that of common Pt-N bonds (∼1.95-1.99 A˚)12 because this N atom is opposite the acetylacetonate oxygen atom having weak trans influence. Similarly, the larger Pt-O(2) [2, 2.082(3) A˚; 5, 2.107(2) A˚] bond length than Pt-O(1) [2, 2.010(3) A˚; 5, 2.003(2) A˚] can be attributed to the greater trans influence of the phenyl carbon C(21), which resides in the trans position to O(2). The bond distances and angles within the acac ligand are comparable with those found in other acetylacetonate Pt(II) complexes.13 In the crystal (11) (a) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457. (b) Suzuki, A. J. Organomet. Chem. 1999, 576, 147. (12) (a) Wong, W.-Y.; He, Z.; So, S.-K.; Tong, K.-L.; Lin, Z. Organometallics 2005, 24, 4079. (b) Yin, B.; Niemeyer, F.; Williams, J. A. G.; Jiang, J.; Boucekkine, A.; Toupet, L.; Bozec, H. L.; Guerchais, V. Inorg. Chem. 2006, 45, 8584. (c) Li, H.; Ding, J.; Xie, Z.; Cheng, Y.; Wang, L. J. Organomet. Chem. 2009, 694, 2777. (d) Ho, C.-L.; Wong, W.-Y.; Yao, B.; Xie, Z.; Wang, L.; Lin, Z. J. Organomet. Chem. 2009, 694, 2735. (e) Liu, J.; Yang, C.-J.; Cao, Q.-Y.; Xu., M.; Wang, J.; Peng, H.-N.; Tan, W.-F.; L€u, X.-X.; Gao, X.-C. Inorg. Chim. Acta 2009, 362, 575. (13) (a) Ghedini, M.; Pucci, D.; Crispini, A.; Barberio, G. Organometallics 1999, 18, 2116. (b) Brooks, J.; Babayan, Y.; Lamansky, S.; Djurovich, P. I.; Tsyba, I.; Bau, R.; Thompson, M. E. Inorg. Chem. 2002, 41, 3055. (c) Gong, J.-F.; Fan, X.-H.; Xu, C.; Li, J.-L.; Wu, Y.-J.; Song, M.-P. J. Organomet. Chem. 2007, 692, 2006.

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Figure 1. Structures of the cyclometalated platinum(II) complexes featuring lepidine donors. Scheme 1. Synthetic Pathway Leading to the Formation of Complexes 6 and 7

packing, compound 2 forms head-to-tail dimers (Figure 4) in which the two molecules are related to each other by a center (14) (a) Ma, B.; Li, J.; Djurovich, P. I.; Yousufuddin, M.; Bau, R.; Thompson, M. E. J. Am. Chem. Soc. 2005, 127, 28. (b) Chassot, L.; Muller, E.; Von; Zelewsky, A. Inorg. Chem. 1984, 23, 4249.

of inversion. The two molecules have a plane-to-plane separation of 3.444 A˚, and the Pt-Pt distance (3.249 A˚) is considerably shorter than those calculated for other monocyclometalated π-π stacked platinum(II) complexes in the literature.6c,9c,14 Also, there exists a weak intermolecular π-π interaction between the lepdine units (Figure 4).

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Figure 2. ORTEP diagram of complex 2 with thermal ellipsoids shown at the 30% probability level. Bond distances (A˚): Pt-C(21)= 1.918(5), Pt-N(1) = 2.070(4), Pt-O(1) = 2.010(3), Pt-O(2) = 2.082(3). Bond angles (deg): C(21)-Pt-N(1) = 81.2(2), C(21)Pt-O(1) = 89.1(2), O(1)-Pt-O(2) = 88.4(1), N(1)-Pt-O(2) = 100.9(2), O(1)-Pt-N(1)=170.0(2), C(21)-Pt-O(2)=173.9(2).

Though close contact between the lepidine and naphthyl units from two different molecules is noticed for 5, i.e., the distance between C30 and C6 is 3.397 A˚ (Figure 5), the rather long Pt-Pt separation (8.603 A˚) precludes the formation of a Pt-Pt bond. Another prominent feature is the significant deviation of the naphthylene and the lepidine units from coplanarity due to the steric congestion between the C(3)-H and C(23)-H (Figure 3); the dihedral angle between the two units is calculated to be 25.49o. The torsional motion between lepdine and naphthyl moieties upon photoexcitation is believed to be the cause of low emission solution quantum yield and consequently inferior electroluminescent performance observed for complex 5 (vide infra), when compared with other complexes. Photophysical Properties. The absorption (Figure 6) and luminescence spectra of compounds 1-7 were measured in toluene, and pertinent data are collected in Table 1. The intense absorption peaks observed in the ultraviolet region (335-375 nm) originate from the localized lepidine ligand π f π* transitions. The lower energy absorption bands peaking in the 405-500 nm range can be assigned to the spinallowed singlet dπ(Pt) f π*(L) metal-to-ligand charge transfer (1MLCT) (1, 2, 5-7) or (1LMCT) (3, 4) in combination with the intraligand π(L) f π*(L) transition (vide infra).16 At the long-wavelength tail of each complex, the absorption, in part, should be attributed to triplet metal-to-ligand charge transfer (3MLCT) transition,15 for which the spin-forbidden nature in transition has become partially allowed due to the enhancement of spin-orbit coupling via the heavy metal center (Pt(II)). The position and intensity of the MLCT/ππ* (15) (a) Jolliet, P.; Gianini, M.; Zelewsky, A.; Bernardinelli, G.; Evans, H. S. Inorg. Chem. 1996, 35, 4883. (b) Chassot, L.; Muller, E.; Zelewsky, A. Inorg. Chem. 1984, 23, 4249. (c) Craig, C. A.; Garces, F. O.; Watts, R. J.; Palmans, R.; Frank, A. J. Coord. Chem. Rev. 1990, 97, 193. (d) Balashev, K. P.; Puzyk, M. V.; Kotlyar, V. S.; Kulikova, M. V. Coord. Chem. Rev. 1997, 159, 109. (16) Li, J.; Djurovich, P. I.; Alleyne, B. D.; Yousufuddin, M.; Ho, N. N.; Thomas, J. C.; Peters, J. C.; Bau, R.; Thompson, M. E. Inorg. Chem. 2005, 44, 1713.


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Figure 3. ORTEP diagram of complex 5 with thermal ellipsoids at the 30% probability level. Bond distances (A˚): Pt-C(21) = 1.955(3), Pt-N(1) = 2.038(3), Pt-O(1) = 2.003(2), Pt-O(2) = 2.107(2). Bond angles (deg): C(21)-Pt-N(1)=80.6(1), C(21)Pt-O(1)=89.4(1), O(1)-Pt-O(2)=87.93(9), N(1)-Pt-O(2)= 102.07(9), O(1)-Pt-N(1)=169.94(9), C(21)-Pt-O(2)=170.1(1).

transitions are also affected by the nature of the substituents present in the aryl moieties. For example, the electron-withdrawing CF3 results in a blue shift17 of the low-energy band in compound 2 (λmax = 417 nm), while the electron-donating amino group causes the opposite red shift in compounds 3 and 4 (λmax = 469 and 472 nm) when compared to the tertbutyl-substituted ligand complex 1 (λmax = 438 nm). Luminescence spectra of the complexes in toluene are displayed in Figure 7, while pertinent spectral data are listed in Table 1. Complexes 1-7 emit in the yellowish-orange to red region with moderate to high quantum yields (0.15-0.99 in the degassed solution). The emission spectral feature of 1-7 is nearly independent of concentration prepared in the range 10-6-10-4 M, and the excitation spectrum is effectively the same as the corresponding absorption spectrum. The results lead us to conclude that the emission originates from the monomeric species rather than ground-state dimer or excimer species. The solution quantum yields appear to be high among the known cyclometalated platinum complexes documented in the literature.18 Especially, to the best of our knowledge, cyclometalated platinum complexes that exhibit a quantum yield of greater than 0.5 at room temperature are extremely rare.13 The observed lifetimes, τobs, of the complexes were measured to be 2.1-23 μs (see Table 1), which fall in the range for cyclometalated platinum complexes reported in literatures.6,12 Together with the observed emission quantum yield (Φp), the radiative lifetimes τr (τr = τobs/ Φp) are deduced to be in the range 10-30 μs for 1-7. This, in (17) Babudri, F.; Farinola, G. M.; Naso, F.; Ragni, R. Chem. Commun. 2007, 1003. (18) (a) Farley, S. J.; Rochester, D. L.; Thompson, A. L.; Howard, J. A. K.; Williams, J. A. G. Inorg. Chem. 2005, 44, 9690. (b) Kavitha, J.; Chang, S.-Y.; Chi, Y.; Yu, J.-K.; Hu, Y.-H.; Chou, P.-T.; Peng, S.-M.; Lee, G.-H.; Tao, Y.-T.; Chien, C.-H.; Carty, A. J. Adv. Funct. Mater. 2005, 15, 223.

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Figure 4. Crystal packing diagrams of complex 2.

Figure 5. Crystal packing diagrams of complex 5.

combination with their drastically oxygen quenching dynamics, ensures the origin of emission to be the phosphorescence. As for the emission intensity, among 1-7 the naphthylcontaining complex 5 exhibits a relatively low quantum yield (Φp ∼0.15). Because the radiative lifetimes of all 1-7 are of the same magnitude (10-30 μs), 5 (knr ∼3.9 105 s-1, see Table 1) possesses the largest nonradiative decay rate constant among the title complexes. The radiationless deactivation pathways for 5 may be attributed to several factors. First of all, the π-stacking interactions present even in solutions via the planar naphthyl channels may serve as a deactivation channel. However, since both quantum yield and observed lifetime are concentration independent in solution (vide supra), this proposal may thus be discarded at least in the solution. In addition, the solvent quenching from the void axial position should apply for all title Pt(II) complexes and, hence, cannot be the sole deactivation factor for 5. Alternatively, the result may tentatively be rationalized by the

steric hindrance created between C(3)-H and C(23)-H (Figure 3), the result of which distorts the planarity of the C∧N ligand, as evidenced by the dihedral angle between the naphthylene and the lepidine units being 25.49o (vide supra). Upon excitation, the torsional motion between lepidine and naphthyl moieties may induce a radiationless deactivation channel. This, in combination with the rather small emission gap (see Figure 6) and thus effective operation of the energy gap law,19 leads to an increase of the nonradiative quenching rate substantially. MO Calculations. In order to gain more insight into the fundamental basis of the transition, time-dependent DFT has been performed. All pertinent data including energy gaps calculated and corresponding assignments of each transition are listed in Table S1 of the Supporting Information. Taking 1 as an example, the calculated energy gaps of the S0 f S1 (435 nm) (19) Siebrand, W. J. Chem. Phys. 1967, 47, 2411.

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Figure 6. Absorption spectra of complexes 1-7 recorded in toluene.

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Figure 7. Emission spectra of complexes 1-7 recorded at room temperature in toluene.

Table 1. Absorption, Emission, and Redox Properties of the Cyclometalated Platinum(II) Complexesa compd 1 2 3

4

5 6 7

a

λabs, nm, 10-3 M-1 cm-1 358 (11.502), 407 (8.296), 438 (5.313) 336 (6.907), 352 (6.633), 417 (5.732) 343 (9.977), 382 (5.213), 452 (6.828), 469 (6.932) 352 (8.423), 396 (7.247), 453 (9.221), 472 (9.496) 373 (11.797), 423 (7.537), 465 (6.045) 374 (12.481), 415 (6.832), 453 (7.352) 318 (40.176), 372 (18.655), 417 (8.246), 451 (9.162)

λem, nm

Φp

τobs, μs

kr 10-4

619

0.35

4.0

8.638

617

0.70

7.3

9.572

597, 639

0.87

23

593, 635

0.99

16

641, 680

0.15

600, 646

0.74

588, 633

0.60

2.1 12 7.0

knr 10-4

Ered, mV

HOMO/LUMO, eV

ΔE, eV

330

-2195

5.13/2.50

2.63

4.12

374

-2006

5.17/2.48

2.69

3.796

0.57

221, 663

-2197

5.02/2.55

2.48

6.079

0.608

323, 631

-2201

5.12/2.65

2.48

349

-2050

5.15/2.74

2.41

6.910

16.1

39.4

Eox, mV

6.045

2.12

335

-2119

5.14/2.63

2.51

8.590

5.76

690

-2027

5.49/3.02

2.47

Absorption and emission spectra were recorded in toluene. Electrochemical parameters were deduced for dichloromethane solutions.

and S0 f T1 (528 nm) transitions (see Figure 8) are close to the observed onsets of the absorption and phosphorescence spectra (see Figures 6 and 7). Similar results are obtained for the rest of the complexes (see Table S1). These results indicate that the TDDFT calculation, in a qualitative manner, can well predict the lowest Franck-Condon absorption in the singlet manifold as well as the phosphorescence energy gap based on the ground-state geometries of the studied complexes. As for the transition characteristics, frontier orbital analyses indicate that for 1, in view of emission, the S0 f T1 transition is primarily attributed to HOMO f LUMO and HOMO-1 f LUMO (see Figure 8), in which the HOMO of 1 is mainly localized at the central Pt(II) atom and phenyl moiety of the C∧N ligand, while the HOMO-1 is mainly localized at the Pt(II) atom and acac ligand. In comparison, the electron density of the LUMO is distributed at the lepidine moiety. Therefore, the lowest lying electronic transition of 1 possesses significant MLCT character. For complexes 3 and 4, the phenyl

moiety is attached by a more electron donating substituent, such that its π-electron energy should be lifted. Accordingly, the HOMO in 3 and 4 is mainly located at the phenyl moiety, as evidenced by the corresponding HOMO frontier orbitals shown in Table S1. The prominent π-π* character for 3 and 4 is consistent with the longer lifetime observed for both complexes (vide supra). It is also interesting to note that the lowest triplet state of 3 has certain LMCT character, whereas that of 4 has MLCT character. Careful analyses of frontier orbitals indicate that the S0-T1 transition for 3 is solely from the HOMO f LUMO, for which the contribution of metal d-character in the LUMO is non-negligible. Conversely, the S0-T1 transition for 4 has 7% of HOMO-2 f LUMO, where HOMO-2 possesses prominent metal d-character. Due to the electron-withdrawing CF3 group, which lowers the π-electron energy in the phenyl ring, the HOMO in 2 is then partly located at the acac site (see Table S1 of SI). Upon fusing the phenyl ring, forming the naphthalene-containing complex 5, due to the

3918


Velusamy et al.

Figure 8. Calculated lower lying transitions for complex 1 in both singlet and triplet manifolds and the corresponding frontier orbitals.

energy-lifting of the π-electron, the HOMO is then located at the naphthalene moiety. For all complexes the LUMO is located at the lepidine moiety. Although the complexes were used as the dopants instead of neat film in the OLEDs (vide infra), a fair comparison was still made between the calculated and the crystal structures, taking advantage of the available crystal structures for 2 and 5. Selected bond distances and angles for 2 and 5 are listed in Table S2 (see SI). The average deviations of metal-ligand distance (Pt-N1, Pt-C21, Pt-O1, Pt-O2) and ligand-metal-ligand angle (N1-Pt-C21, O1-Pt-O2) are 12.44% and 18.93% for 2 and 4.12 and 2.33% for 5, respectively. The larger discrepancy between the calculated and crystal structures for complex 2 can be attributed to its greater intermolecular interaction in the crystal (vide supra). Electrochemical Studies. To ensure the title complexes are suited for the application of OLEDs, the electrochemical properties of the complexes were studied by cyclic voltammetric method, and the redox potentials of the compounds are compiled in Table 1. The redox potentials were corrected relative to an internal ferrocene reference (Fc/Fcþ). All the compounds displayed an irreversible oxidation potential ranging from 0.221 to 0.690 mV, which is attributed to the oxidation of the platinum-phenyl center. This is in sharp contrast to that observed for the corresponding iridium(III)

complexes, as they exhibited reversible oxidation processes. The reduction of the lepidine ligand was observed as an irreversible wave from -2.01 to -2.20 V. The oxidation potential of the present complexes increases in the order 3 < 4 < 1 < 5 < 6 < 2 < 7 and is consistent with the nature of the substituent on the cyclometalated phenyl ring. For example, the strong electron donating substituent (dimethylamino) containing compound 3 exhibits negatively shifted (Eox = 0.22 V) oxidation potentials when compared with tert-butyl-substituted compound 1 (Eox = 0.33 V). On the contrary, compound 2, containing the electron-withdrawing CF3 substituent, shows a positively shifted oxidation potential (0.37 V) when compared with compound 1. An additional oxidation wave observed for compounds 3 (Eox = 0.66 V) and 4 (Eox = 0.63 V) is attributed to the oxidation of the amine groups.10,13 The first oxidation potentials were used to calculate the highest occupied molecular orbital (HOMO) energy level of the molecules and compared with ferrocene (4.8 eV). These HOMO values with the absorption spectra were then used to obtain the lowest unoccupied molecular orbital (LUMO) energy levels.20 (20) Thelakkat, M.; Schmidt, H.-W. Adv. Mater. 1998, 10, 219.

Article


3919

Table 2. Electroluminescent Parameters for the Devices with the Configuration ITO/NPB (40 nm)/CBP-Doped Pt Complex (5 wt %) (20 nm)/BCP (10 nm)/Alq3 (20 nm)/LiF (1 nm)/Al (150 nm)a Lmax, cd/m2 compd Von,b V (V at Lmax, V) λem, nm CIE (x,y) fwhm, nm ηext,,max, % ηp,max, lm/W ηc,max, cd/A L,c cd/m2 ηext,c % ηp,c lm/W ηc,c cd/A 1 2 3 4 5 6 7

5.0 4.5 6.0 4.5 5.0 5.0 6.5

56 727 (17.0) 23 892 (19.0) 26847 (25.5) 24 011 (17.5) 6130 (15.5) 28 988 (16.5) 19 523 (19.5)

576 578 598 594 632 590 588

0.50, 0.45 0.53, 0.46 0.60, 0.40 0.56, 0.40 0.55, 0.35 0.58, 0.40 0.59, 0.40

98 92 84 86 76 74 82

6.72 7.90 15.21 7.90 2.18 6.47 6.2

10.37 10.75 11.79 11.72 1.38 6.52 12.29

18.15 20.53 29.83 20.53 2.20 13.48 5.30

12484 9308 11265 8068 1360 7240 4528

4.63 3.60 5.77 4.11 1.35 3.50 2.86

3.56 2.08 1.57 2.20 0.39 7.30 0.823

12.50 9.34 11.31 8.10 1.36 2.04 4.63

a Lmax, maximum luminance; L, luminance; Von, turn-on voltage; V, voltage; ηext,max, maximum external quantum efficiency; ηp,max, maximum power efficiency; ηcmax, maximum current efficiency; ηext, external quantum efficiency; ηp, power efficiency; ηc, current efficiency; fwhm, full width at halfmaximum. b Von was obtained from the x-intercept of log(luminance) vs applied voltage plot. c At a current density of 100 mA/cm2.

Figure 9. EL spectra of the devices 1-7.

Electrophosphorescent Properties. OLEDs using these platinum complexes were fabricated by a high-vacuum thermal evaporation method onto glass substrates. The device structure used in these studies is similar to that developed by Thompson and Forrest:21 ITO/NPB (40 nm)/CBP-doped Pt complex (5 wt %) (20 nm)/BCP (10 nm)/Alq3 (2 nm)/LiF (1 nm)/Al (150 nm). Here, NPB (4,40 -bis[N-(1-naphthyl)-Nphenylamino]biphenyl) acts as the hole-transporting layer, BCP (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline) acts as the hole-blocking layer, CBP (4,40 -bis(N-carbazolyl)biphenyl) acts as the host for the platinum emitter, and Alq3 (tris(8-hydroxyquinoline)aluminum(III)) acts as the electrontransporting layer. Two different doping concentrations, 5 and 15 wt %, were tested for the device performance. Devices with 15% dopant had somewhat broadened and red-shifted EL spectra. This observation together with the lower device efficiencies at 15% doping level suggests that there is nonnegligible intermolecular interaction at higher doping concentration. Therefore, we will focus our discussion on the device with 5 wt % dopant. The turn-on voltages of the devices range from 4.5 to 6.0 V. The EL spectra of these devices are displayed in Figure 9, and the device performance characteristics are collected in Table 2. The devices for complexes 1 and 2 exhibit a strong yellow emission at 576 and 578 nm respectively. In contrast, the devices of the complexes 3, 4, and 5 have a bright orange emission at 598, 594, and 590 nm, respectively. No residual emission from the CBP host (∼400 nm) was discernible, indicating that energy transfer from the host (CBP) to the dopant (platinum complex) or exciton trapping in the host is very efficient in all the devices. However, the EL spectra of (21) Adachi, C.; Baldo, M. A.; Forrest, S. R.; Thompson, M. E. Appl. Phys. Lett. 2000, 77, 904.

Figure 10. I-V-L characteristics observed for the devices of the complexes (5% dopant).

devices 1 and 5 are slightly contaminated with those characteristic of NPB (∼450 nm) and Alq3 (∼510 nm), respectively. This outcome may be due to the different carrier mobility of 1 and 5 from others since the HOMO and LUMO levels of the two complexes do not significantly deviate from others. Low efficiency of the device from 5 may be due to the low solution quantum yield of 5, which in turn probably arises from the detrimental torsional deactivation (vide supra). The maximum external quantum efficiencies of the devices for other complexes fall in the range from 6.7 to 15.2% and maximum brightness from 26 700 to 56 700 cd m-2. Interestingly, compound 3 showed a remarkably high external quantum efficiency of 15.2% and brightness of 26800 cd m-2. To the best of our knowledge, these data are among the best performances of platinum(II) complex-based OLED

3920


Figure 11. Observed variation of the external quantum efficiency against current density in the electrophosphorescent devices.

devices.15 The device luminance and efficiencies of the present complexes are comparable to those obtained for iridium-based devices.6,12 Figure 10 presents the current density-voltage and luminance-voltage characteristics (I-V-L) of the devices fabricated, and Figure 11 shows the external quantum efficiency of the devices as a function of current density. The EL spectra of the compounds 1, 2, 5, and 6 were blue-shifted by 10-43 nm when compared with the PL spectra of the compounds in toluene solution. The blue shift in electroluminescence spectra may be attributed to the minimization of intermolecular interactions at the doping concentrations and absence of solvent-induced relaxation processes. However, the EL spectra observed for compounds 3 and 4 are similar to the PL spectra. This probably points to the fact that the incorporation of a trigonal amine functionality in compounds 3 and 4 efficiently prevents the approach of two molecules from stacking interactions. Lastly, the order of external quantum efficiency (5 < 7 < 6 < 1 < 2 < 4 < 3) is roughly opposite that of the magnitude of knr (5 > 1 > 7 > 2 > 6 > 4 > 3), while the values of kr remain much the same among the whole series, suggesting that the overall device performance is governed by the knr in this case.

Conclusions In conclusion, we have synthesized and characterized a series of new cyclometalated platinum(II) complexes. These complexes exhibit very high solution phosphorescence quantum yields up to ∼99%. Single-crystal X-ray structural determination on two complexes reveals the presence of π-π interaction and/or Pt-Pt interaction. OLEDs using these complexes as the dopants exhibit intense yellow to red electroluminescence, and high external quantum efficiency of up to 15.2% can be achieved.

Experimental Section Methods and Materials. All reactions and manipulations were carried out under N2 with the use of standard inert atmosphere and Schlenk techniques. Solvents were dried by standard procedures. The entire column chromatographic studies were performed under N2 with the use of silica gel (230-400 mesh, Macherey-Nagel GmbH & Co.) as the stationary phase. The 1H NMR spectra were obtained by using Bruker AMX400 spectrometers. Mass spectra were recorded on a JMS-700 double-

Velusamy et al. focusing mass spectrometer (JEOL, Tokyo, Japan). Cyclic voltammetry experiments were performed with a BAS-100 electrochemical analyzer at room temperature with a conventional three-electrode configuration consisting of a platinum working electrode, a platinum wire auxiliary electrode, and a nonaqueous Ag/AgNO3 reference electrode. The solvent in all experiments was dichloromethane, and the supporting electrolyte was 0.1 M tetrabutylammonium hexafluorophosphate. The E1/2 values were determined as 1/2(Epa þ Epc), where Epa and Epc are the anodic and cathodic peak potentials, respectively. The potentials are quoted against the ferrocene internal standard. Electronic absorption spectra were obtained on a Perkin-Elmer Lambda 900 UV-vis-NIR spectrophotometer for toluene solutions. Emission spectra were recorded in deoxygenated toluene solution at 298 K with a Jobin Yvon SPEX Fluorolog-3 spectrofluorometer. Luminescence quantum yields (Φem) were calculated using Ir(ppy)3 (Φem = 0.40 in toluene) as the reference. The lifetime was measured with the laser photolysis technique, in which the third harmonic of an Nd:YAG laser (8 ns, Continuum Surlite II) was used as the excitation source, coupled with a fast response photomultiplier (Hamamatsu model R5509-72) operated at -80 °C. Typically, an average of 512 shots was acquired for each measurement. The ligands, 2-(4-tert-butylphenyl)-4-methylquinoline (1a), 4-methyl-2-(4(trifluoromethyl)phenyl)quinoline (2a), N,N-dimethyl-4-(4-methylquinolin-2-yl)aniline (3a), 4-(4-methylquinolin-2-yl)-N,Ndiphenylaniline (4a), and 4-methyl-2-(naphthalen-1-yl)quinoline (5a), were prepared by adopting literature procedures. 2-(9,9-Diethyl-9H-fluoren-2-yl)-4-methylquinoline (6a) and 2-(20 ,70 -di-tert-butyl-9,90 -spirobi[fluorene]-2-yl)-4-methylquinoline (7a) were prepared by Suzuki coupling reactions according to the reported procedures, and only the preparation of 6a and 7a will be described in detail. 2-(9,9-Diethyl-9H-fluoren-2-yl)-4-methylquinoline (6a). A stirred mixture of 2-chlorolepidine (0.88 g, 5.0 mmol), 9-diethyl9H-fluoren-3-yl-boronic acid (1.46 g, 5.5 mmol), Pd(PPh3)4 (75 mg), Na2CO3 (0.75 g), toluene (10 mL), THF (10 mL), and H2O (2 mL) was heated at reflux for 24 h. When the reaction was completed, water was added to quench the reaction. The product was extracted with diethyl ether. The organic layer was collected, dried over anhydrous MgSO4, and evaporated under vacuum. The solid was adsorbed on silica gel and purified by column chromatography using CH2Cl2/hexane as the eluent. Yield: 75%. 1H NMR (δ, CDCl3): 0.37 (t, 6H, J = 7.24 Hz), 2.02-2.17 (m, 4H), 2.78 (s, 3H), 7.29-7.37 (m, 3H), 7.50-7.55 (m, 1H), 7.68-7.83 (m, 4H), 7.9-8.0 (m, 1H), 8.12-8.21 (m, 3H). FAB-MS: m/z 364.21 [M þ H]þ. 2-(20 ,70 -Di-tert-butyl-9,90 -spirobi[fluoren]-7-yl)-4-methylquinoline (7a). To a mixture of (20 ,70 -di-tert-butyl-9,90 -spirobi[fluoren]7-yl)boronic acid (2.83 g, 6.0 mmol), chlorolepidine (0.88 g, 5.0 mmol), Pd(PPh3)4 (75 mg), and Na2CO3 (1.6 g, 15 mmol) were added toluene (15 mL) and THF (15 mL), and the resulting mixture was stirred and heated to reflux under nitrogen atmosphere for 24 h. The reaction mixture was poured into water and extracted with dichloromethane, and the combined extracts were dried over MgSO4 and filtered. The crude product was purified by column chromatography on silica gel using dichloromethane/ hexane as the eluent to afford 7a as a white solid (1.7 g, 60% yield). 1H NMR (δ, CDCl3): 1.11 (s, 18H), 2.70 (s, 3H), 6.64 (d, 2H, J = 7.6 Hz), 6.69 (d, 2H, J = 1.5 Hz), 6.77 (s, 1H), 7.00-7.05 (m, 1H), 7.13-7.27 (m, 4H), 7.24-7.27 (m, 1H), 7.40-7.44 (m, 1H), 7.50 (d, 2H, J = 8.0 Hz), 7.58 (d, 1H, J = 7.4 Hz), 7.91 (d, 1H, J = 7.5 Hz), 8.18 (s, 1H), 8.30 (d, 1H, J = 8.7 Hz). FAB-MS: m/z 570.75 [M þ H]þ. Preparation of the Complexes. General procedure for the construction of orthometalated platinum(II) complexes: A mixture of the ligand (3 mmol, 1.09 g) and K2PtCl4 (1 mmol 0.415 g) was refluxed in 2-ethoxyethanol (6 mL) and water (3 mL) for 24 h. After cooling to room temperature, the mixture was added to water, and the precipitate collected was washed with water

Article and an ether/hexane mixture and vacuum-dried. The precipitate then reacted with acetylacetone (0.10 g, mmol) and Na2CO3 (0.16 g, mmol) in 2-methoxyethanol (10 mL) at 90 °C for 18 h. The solvent was removed under reduced pressure, and the residue was purified by column chromatography on silica gel using CH2Cl2/hexane as the eluent to afford a yellow product with an isolated yield of 70%. 1. 1H NMR (δ, CDCl3): 1.38 (s, 9H), 2.0 (s, 3H), 2.01 (s, 3H), 2.73 (s, 3H), 5.53 (s, 1H), 7.15-7.17 (m, 1H), 7.44-7.52 (m, 2H), 7.57 (s, 1H), 7.66-7.75 (m, 2H), 7.86-7.88 (m, 1H), 9.49 (d, 1H, J = 8.6 Hz). FAB-HRMS: calcd for C25H27NO2Pt 568.1690, found 568.1694. Anal. Calcd for C25H27NO2Pt: C, 52.81; H, 4.79; N, 2.46. Found: C, 52.78; H, 4.69; N, 2.35. 2. 1H NMR (δ, CDCl3): 2.01 (s, 3H), 2.02 (s, 3H), 2.75 (s, 3H), 5.55 (s, 1H), 7.31-7.33 (m, 1H), 7.54-7.58 (m, 2H), 7.61 (s, 1H), 7.70-7.72 (m, 1H), 7.89-7.96 (m, 2H), 9.54 (d, 1H, J = 8.6 Hz). FAB-HRMS: calcd for C22H18F3NO2Pt 580.0937, found 580.0939. Anal. Calcd for C22H18F3NO2Pt: C, 45.52; H, 3.13; N, 2.41. Found: C, 45.45; H, 3.10; N, 2.50. 3. 1H NMR (δ, CDCl3): 1.99 (s, 3H), 2.0 (s, 3H), 2.70 (s, 3H), 3.09 (s, 6H), 5.52 (s, 1H), 6.50-6.53 (m, 1H), 7.01 (d, 1H, J = 2.6 Hz), 7.39-7.45 (m, 3H), 7.62-7.66 (m, 1H), 7.80-7.83 (m, 1H), 9.41 (d, 1H, J = 8.6 Hz). FAB-HRMS: calcd for C23H24N2O2Pt 555.1486, found 555.1489. Anal. Calcd for C23H24N2O2Pt: C, 49.73; H, 4.35; N, 5.64. Found: C, 49.80; H, 4.40; N, 5.70. 4. 1H NMR (δ, CDCl3): 1.63 (s, 3H), 1.97 (s, 3H), 2.71 (s, 3H), 5.43 (s, 1H), 6.78 (dd, 1H, J = 2.4 Hz), 7.02-7.06 (m, 2H), 7.19-7.28 (m, 8H), 7.35 (d, 1H, J = 8.5 Hz), 7.45-7.48 (m, 2H), 7.63-7.68 (m, 1H), 7.83-7.85 (m, 1H), 9.45 (d, 1H, J = 8.6 Hz). FAB-HRMS: calcd for C33H28N2O2Pt 679.1799, found 679.1809. Anal. Calcd for C33H28N2O2Pt: C, 58.32; H, 4.15; N, 4.12. Found: C, 58.29; H, 4.20; N, 4.10. 5. 1H NMR (δ, CDCl3): 2.01 (s, 3H), 2.05 (s, 3H), 2.80 (s, 3H), 5.55 (s, 1H), 7.38-7.40 (m, 1H), 7.48-7.56 (m, 2H), 7.63 (d, 1H, J = 8.4 Hz), 7.69-7.72 (m, 1H), 7.83-7.84 (m, 1H), 7.90-7.97 (m, 2H), 8.18 (s, 1H), 8.45 (d, 1H, J = 8.6 Hz), 9.42 (d, 1H, J = 8.6 Hz). FAB-HRMS: calcd for C25H21NO2Pt 562.1220, found 562.1214. Anal. Calcd for C25H21NO2Pt: C, 53.38; H, 3.76; N, 2.49. Found: C, 53.45; H, 3.80; N, 2.55. 6. 1H NMR (δ, CDCl3): 0.36 (t, 6H, J = 7.24 Hz), 2.02-2.11 (m, 10H), 2.78 (s, 3H), 5.56 (s, 1H), 7.30-7.34 (m, 3H), 7.49-7.54 (m, 2H), 7.68-7.72 (m, 2H), 7.79 (d, 1H, J = 6.8 Hz), 7.90 (d, 1H, J = 8.2 Hz), 8.05 (s, 1H), 9.50 (d, 1H, J = 8.6 Hz). FAB-HRMS: calcd for C32H31NO2Pt 656.2003, found 656.2005. Anal. Calcd for C32H31NO2Pt: C, 58.53; H, 4.76; N, 2.13. Found: C, 58.45; H, 4.70; N, 2.0. 7. 1H NMR (δ, CDCl3): 1.10 (s, 18H), 2.02 (s, 3H), 2.15 (s, 3H), 2.60 (s, 3H), 5.59 (s, 1H), 6.65 (d, 1H, J = 7.5 Hz), 6.72 (d, 2H, J = 1.5 Hz), 6.94 (s, 1H), 7.04-7.08 (m, 1H), 7.32-7.39 (m, 4H), 7.44-7.48 (m, 1H), 7.63-7.67 (m, 1H), 7.73 (d, 2H, J = 8.1 Hz), 7.80 (d, 1H, J = 7.4 Hz), 7.93 (d, 1H, J = 7.5 Hz), 8.21 (s, 1H), 9.43 (d, 1H, J = 8.9 Hz). FAB-HRMS: calcd for C48H45NO2Pt 862.3098, found 862.3098. Anal. Calcd for C48H45NO2Pt: C, 66.81; H, 5.26; N, 1.62. Found: C, 66.75; H, 5.19; N, 1.59. Crystal Structure Analysis. The single-crystal X-ray diffraction experiments were carried out using graphite-monochromated Mo KR radiation (λ = 0.71073 A˚) on a Bruker SMART CCD diffractometer. The data collection was performed using the SMART program. Cell refinement and data reduction were performed with the SAINT program. The structure was determined using the SHELX software package. All non-hydrogen atoms were refined anisotropically, whereas hydrogen atoms


3921

were placed at calculated positions. Crystal data and structure refinement parameters are given in Table SX of the Supporting Information. LED Fabrication and Measurements. Prepatterned ITO substrates with an effective individual device area of 3.14 mm2 were cleaned as described in a previous report. Compound BCP (2,9dimethyl-4,7-diphenyl-1,10-phenanthroline) was purchased from Aldrich and used as received. Alq3 (tris(8-hydroxyquinolinato)aluminum), NPB (4,40 -bis[N-(1-naphthyl)-N-phenylamino]biphenyl), and CBP (4,40 -biscarbazolylbiphenyl) were synthesized according to literature procedures and were sublimed prior to use. In a vacuum chamber with

Cyclometalated Platinum(II) Complexes of Lepidine-Based Ligands as

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