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Photo- and electroluminescence from deep-red- and near-infrared-phosphorescent tris-cyclometalated iridium(III) complexes bearing largely π-extended ligands
Inorganic Chemistry Communications 38 (2013) 14–19
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Photo- and electroluminescence from deep-red- and near-infrared-phosphorescent tris-cyclometalated iridium(III) complexes bearing largely π-extended ligands Shigeru Ikawa a, Shigeyuki Yagi a,⁎, Takeshi Maeda a, Hiroyuki Nakazumi a, Hideki Fujiwara b, Shiro Koseki b, Yoshiaki Sakurai c a b c
Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1–1 Gakuen-cho, Naka-ku, Sakai, Osaka 599–8531, Japan Department of Chemistry, Graduate School of Science, Osaka Prefecture University, 1–1 Gakuen-cho, Naka-ku, Sakai, Osaka 599–8531, Japan Textile and Polymer Section, Technology Research Institute of Osaka Prefecture, 2-7-1 Ayumino, Izumi, Osaka 594–1157, Japan
a r t i c l e
i n f o
Article history: Received 8 August 2013 Accepted 30 September 2013 Available online 11 October 2013 Keywords: Iridium complex Cyclometalated ligand Phosphorescence Near-infrared emission Organic light-emitting diode
a b s t r a c t Deep-red- and near-infrared-phosphorescent tris-cyclometalated iridium(III) complexes bearing largely π-extended cyclometalated (C^N) ligands were newly synthesized, and their photo- and electroluminescence properties were investigated. When 2-(benzo[b]furan-2-yl)quinoline and 2-(benzo[b]thiophen-2-yl)quinoline were employed as C^N ligands, deep-red photoluminescence was obtained (Ir-1a and Ir-1b; λPL in CH2Cl2, 647 and 652 nm, respectively). In the case of the isoquinoline analogues of Ir-1a and Ir-1b, emission maxima were further red-shifted, ranging from deep-red to near-infrared regions (Ir-2a and Ir-2b; λPL in CH2Cl2, 696 and 690 nm, respectively). Especially, Ir-2b showed an excellent photoluminescence quantum yield (ΦPL = 0.15), and a polymer light-emitting diode doped with Ir-2b exhibited deep-red–near-infrared electroluminescence with a high external quantum efficiency (λEL = 694 nm, ηext max = 1.41%). © 2013 Elsevier B.V. All rights reserved.
Organic light-emitting diodes (OLEDs) have attracted much attention for the last two decades because their technological advantages, such as self-emission, fast response, simple device construction, and so on, make them applicable to flat panel displays and lighting apparatuses in next generations [1–6]. Recently OLEDs emitting in near-infrared (NIR) regions have been also receiving growing interest [7–10] because they are expected to be applied to night-vision readable displays and security sensors [11]. Therefore, NIR-emitting materials have been eagerly required for such purposes. In general, device efficiencies of OLEDs are markedly improved when fluorescent dopants are replaced with phosphorescent ones such as platinum(II) [12,13] and iridium(III) complexes [14–16] because they can effectively form triplet excitons upon electric excitation. With this respect, a number of bis- and triscyclometalated iridium(III) complexes (Ir(C^N)2LX and Ir(C^N)3, respectively [17]) emitting RGB phosphorescence have been developed [18–20]. Nevertheless, few examples of excellent NIR-phosphorescent iridium(III) complexes that allow us to fabricate high-efficiency NIROLEDs have so far been reported [11,21,22] since the photoluminescence (PL) quantum yield of an emitting material intrinsically tends to decrease as the emission wavelength is red-shifted (i.e., so-called energy gap law) [23–25]. Recently, Qiao and coworkers reported that an OLED doped with bis[(2-phenylbenzo[g]quinoline)-C2,N]iridium(III) acetylacetonate as a phosphorescent dopant gave NIR emission at 708 nm ⁎ Corresponding author. Tel.: +81 72 254 9324; fax: +81 72 254 9910. E-mail address: [email protected] (S. Yagi). 1387-7003/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.inoche.2013.09.075
[22]. Although this device exhibited relatively efficient device performance for NIR-OLED, the maximum external quantum efficiency was still low (ηext max = 1.07%). In the present paper, we report synthesis and PL properties of phosphorescent tris-cyclometalated iridium(III) complexes Ir-1 and Ir-2 emitting from deep-red to NIR regions. Also, we discuss about the electroluminescence (EL) properties of poly(Nvinylcarbazole)-based polymer light-emitting diodes (PLEDs) doped with those iridium(III) complexes. To obtain NIR phosphorescence, a quite low T1 level is required. So we designed tris-cyclometalated iridium(III) complexes Ir-1 and Ir-2 (Scheme 1), the C^N ligands of which are based on 2-(benzo[b]furan2-yl)- and 2-(benzo[b]thiophen-2-yl)quinolines and their isoquinoline analogues. Until now, theoretical studies about a variety of Ir(C^N)2LXand Ir(C^N)3-type complexes were reported, revealing that the excited states are mainly based on metal-to-ligand charge transfer (MLCT) transitions. That is, from density functional theory (DFT) calculations of the most basic complexes such as Ir(ppy)2(acac) and Ir(ppy)3 (ppy and acac; 2-phenylpyridinato-C2,N and acetylacetonato-O,O ligands, respectively), the HOMO consists of a mixture of phenyl-pπ and Ir-dπ orbitals, and the LUMO receives a considerable contribution from the C^N ligands, especially the pyridyl-π orbitals [26–29]. With this respect, replacement of the phenyl moieties of the C^N ligands by electronenriched benzo[b]furan and benzo[b]thiophene components gives rise to destabilization of the HOMO [30,31]. On the other hand, employment of the benzologues such as quinoline and isoquinoline in place of the pyridine moiety is effective on stabilization of the LUMO [32,33]. Thus,
S. Ikawa et al. / Inorganic Chemistry Communications 38 (2013) 14–19
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Scheme 1. Reagents and Conditions: (i) IrCl3 · 3H2O, 2-ethoxyethanol, H2O, 100 °C; (ii) acetylacetone, Na2CO3, 2-ethoxyethanol, 90 °C; (iii) C^N ligand, glycerol, 230 °C; (iv) C^N ligand, glycerol, 280 °C, microwave irradiation.
the present molecular design based on the HOMO–LUMO tuning should allow us to obtain NIR-emitting cyclometalated iridium(III) complexes. The synthesis of the iridium(III) complexes is shown in Scheme 1. The preparation of HC^N-2b was previously reported [34]. Similarly, HC^N-1a and HC^N-1b were also prepared by the Suzuki–Miyaura cross-coupling reaction of (benzo[b]furan-2-yl)- and (benzo[b] thiophen-2-yl)boronic acid with 2-chloroquinoline, respectively. HC^N-2a was newly synthesized by the Suzuki–Miyaura crosscoupling method in 92% yield (see the supplementary materials section). In general, Ir(C^N)3-type complexes are efficiently synthesized by the reaction of a μ-chloro-bridged iridium(III) dimer [(C^N)2Ir(μ-Cl)]2 or a bis-cyclometalated iridium(III) complex (C^N)2Ir(acac) with the corresponding C^N ligand [16]. These procedures yield a facial (fac) isomer exclusively when the reactions proceed at high temperatures (N 200 °C). In the present case, we examined the ligand replacement of (C^N-1a)2Ir(acac) and (C^N-1b)2Ir(acac) at 230 °C and yielded Ir-1a and Ir-1b in 11 and 14%, respectively (overall yields from HC^N-1a and HC^N-1b; 2.4 and 8.9%, respectively). For the synthesis of Ir-2, microwave-assisted reactions (2450 ± 30 MHz, 300 W, 280 °C, 45 min) brought about better results, where [(C^N-2a)2Ir(μ-Cl)]2 and [(C^N2b)2Ir(μ-Cl)]2 were reacted with the corresponding C^N ligands to yield Ir-2a and Ir-2b in 41 and 88%, respectively (overall yields from HC^N-2a and HC^N-2b; 31 and 86%, respectively). Unfortunately, in the case of the preparation of Ir-1a and Ir-1b, the microwave synthesis was unsuccessful, and trace amounts of the target complexes were obtained. According to 1H NMR spectral analysis, each complex of Ir-1 and Ir-2 was obtained as a single isomer. As shown in Fig. 1, the molecular structure of Ir-2b was determined by X-ray crystallographic analysis [35]. The molecule of Ir-2b adopts distorted octahedral coordination geometry around the metal center, and three C^N ligands are arranged in a fac manner, affording a racemic mixture of enantiomeric fac isomers. Although we attempted X-ray crystallographic analysis of Ir-1a, Ir-1b and Ir-2a, any suitable single crystals have not been obtained at this point. According to the previous reports [16,36], the fac isomer is thermodynamically more stable than the meridional one. Taking into
consideration that these three products were obtained at more than 230 °C, they should be fac isomers [37,38]. Fig. 2 shows the UV–vis absorption spectra of Ir-1 and Ir-2 in dichloromethane at rt. The data are also listed in Table 1. All complexes exhibit similar absorption spectral profiles. The absorption bands at b390nm are assigned to the spin-allowed π–π⁎ transitions at the C^N ligands, and the next absorption bands up to ca. 570 nm are assigned to the spin-allowed 1 MLCT transitions from the Ir(III) centers to the C^N ligands. As represented by the magnified spectra (Fig. 2, dashed lines), the lowerenergy absorption bands in the region of N 590 nm are assignable to the spin-forbidden 3MLCT transitions. In this region, the molar absorption coefficients of Ir-1b and Ir-2b are larger than those of the corresponding benzo[b]furan analogues (i.e., Ir-1a and Ir-2a, respectively),
Fig. 1. ORTEP drawing of Ir-2b. The hydrogen atoms are omitted for clarity.
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S. Ikawa et al. / Inorganic Chemistry Communications 38 (2013) 14–19
Fig. 2. UV–vis absorption spectra of Ir-1 and Ir-2 in dichloromethane at rt. The solid lines are the spectra at concentrations of 10 μM. The dashed lines are the ones 20 times expanded in intensity in the region from 540 to 750 nm.
indicating that the benzo[b]thiophene-derived complexes exhibit more effective spin–orbit coupling. Focusing on the N-heterocycle in the C^N ligand, the 3MLCT transition bands of Ir-2a and Ir-2b are bathochromically shifted in comparison with those of Ir-1a and Ir-1b. This indicates that the isoquinoline-based complexes exhibit more redshifted phosphorescence than the quinoline-based ones. Fig. 3a shows the PL spectra of Ir-1 and Ir-2 in dichloromethane at rt, and the spectral and photophysical profiles are summarized in Table 1. The PL lifetimes (τPLs) range from 1.13 to 2.09 μs, each of which is well fitted to single-exponential decay (χ2; 1.03–1.07), indicating that the obtained emission are exclusively phosphorescence. In the PL spectra of Ir-1a and Ir-1b, the emission maxima (λPL) are observed at 652 and 647 nm, respectively, and their emission colors are deep-red. The PL quantum yield (ΦPL) of Ir-1a is 0.12, whereas Ir-1b emits more efficiently in solution, affording ΦPL of 0.21. So, one can see that the improved ΦPL of Ir-1b rather than Ir-1a is attributed to the enhanced spin– orbit interaction as indicated in the UV–vis spectra. The isoquinolinebased C^N ligands gave rise to further red shifts of PL spectra: the λPLs of Ir-2a and Ir-2b are observed at 696 and 690 nm, respectively, and the shoulders of the spectra reached NIR regions. The ΦPLs of Ir-2a and Ir-2b are 0.068 and 0.15, respectively, and as discussed in Ir-1b, the stronger spin–orbit interaction of Ir-2b should bring about a larger ΦPL. The PL properties in polymer thin films were also investigated. Poly(methyl methacrylate) (PMMA) was chosen as a matrix polymer because the absence of absorption bands from near-UV to visible regions allows us to estimate the intrinsic PL properties of the iridium(III) complexes [39]. The PL spectra of Ir-1 and Ir-2 in PMMA films are almost identical to those in solutions (Fig. 3b). As summarized in Table 1, the ΦPLs in PMMA films are also similar to those in dichloromethane. It is worthy to note that Ir-2b is still emissive even in the solid matrix (ΦPL = 0.11) in spite of its low-lying triplet level. Such
Fig. 3. PL spectra of Ir-1 and Ir-2 (a) in dichloromethane ([Ir(C^N)3] = 10 μM) and (b) in PMMA thin films (PMMA/Ir(C^N)3 = 100/4, wt/wt).
excellent NIR-PL properties make us expect the possibility of a solidstate NIR electroluminescent device. The electrochemical behavior of Ir-1 and Ir-2 was examined using cyclic voltammetry. The cathodic and anodic scans were carried out in anhydrous DMF solutions at rt. The cyclic voltammograms of Ir-1 and Ir-2 are shown in Fig. 4, and the redox potentials determined in comparison with the ferrocene/ferrocenium redox (Fc/Fc+) are listed in Table 2. Reversible one-electron oxidation and reduction couples were obtained for Ir-1b and Ir-2a, whereas Ir-1a and Ir-2b showed reversible oneelectron oxidation potentials as well as one- and two-electron reduction ones. Oxidation potentials of Ir-1a and Ir-1b (Eox 1/2; 0.54 and 0.51 V, respectively) are slightly higher than those of Ir-2a and Ir-2b (Eox 1/2; 0.42 V), although the first reduction potentials of all complexes are almost similar (Ered 1/2; ‒2.07 to −2.11 V). It was found that the oxidation and reduction potentials are not so different between benzo[b]franand benzo[b]thiophene-derived complexes, i.e., between Ir-1a and Ir-1b and between Ir-2a and Ir-2b. The HOMO/LUMO energy levels (EHOMO/ELUMO) of Ir-1 and Ir-2 are also estimated in comparison with the energy level of ferrocene (EHOMO = −4.80 eV below vacuum
Table 1 UV–vis absorption and photoluminescence (PL) spectral data of Ir-1 and Ir-2a. Compd
Ir-1a Ir-1b Ir-2a Ir-2b a b c d
PMMA thin film
CH2Cl2 solution λabs/nm (10−4 ε/M−1 cm−1)b
λPL/nm (λex/nm)c
ΦPLc
τPL/μs (χ2)c
λPL/nm (λex/nm)d
ΦPLd
318 (4.6), 432 (2.4), 520 (0.83) [sh], 639 (0.012) [sh] 276 (4.7), 314 (4.6), 435 (2.1), 529 (0.57) [sh], 623 (0.022) 328 (3.8), 450 (2.6), 518 (1.1) [sh], 624 (0.031) [sh], 683 (0.015) [sh] 291 (4.2), 317 (4.3), 456 (3.0), 535 (1.1), 612 (0.046) [sh], 665 (0.024)
652 (454)
0.12
1.34 (1.07)
648 (454)
0.11
647 (453)
0.21
2.09 (1.03)
644 (453)
0.22
696 (468)
0.068
1.13 (1.05)
692 (467)
0.065
690 (468)
0.15
1.77 (1.05)
694 (467)
0.11
λabs, UV–vis absorption maximum; λPL, PL maximum; λex, excited wavelength for PL; ΦPL, PL quantum yield; τPL, PL lifetime. [sh]; shoulder peak. Obtained for deaerated samples at rt. Obtained under N2 atmosphere at rt. The ratio of PMMA/Ir(C^N)3 was adjusted to 100/4 (wt/wt).
S. Ikawa et al. / Inorganic Chemistry Communications 38 (2013) 14–19
Fig. 4. Cyclic voltammograms of (a) Ir-1 and (b) Ir-2 in anhydrous DMF at a scan rate of 100 mV s-1.
[40]) (Table 2). These levels are placed between HOMO and LUMO of poly(N-vinylcarbazole) (PVCz, ‒5.80 to −2.20 eV) and 2-(4-biphenylyl)5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD, ‒6.20 to −2.40 eV) [41], and thus Ir-1 and Ir-2 have the EHOMO and ELUMO values suitable for emitting dopants in PVCz-based PLEDs. DFT calculations were also carried out for Ir-1 and Ir-2. To obtain their optimized structures, the GAMESS suite of program codes was used [42]. In these calculations, the B3LYP functional [43] with the SBKJC effective core potentials and the associated basis sets [44,45] were used. Calculated EHOMOs and ELUMOs of Ir-1 and Ir-2 as well as the electron density distributions of the orbitals were listed in the supplementary materials section (Fig. S1 and Table S1). The HOMO distributions of electronic density primarily reside on the Ir-dπ orbital, and the LUMO distributions mainly reside on the quinoline (or isoquinoline) moieties of the C^N ligands, as is seen in typical cyclometalated iridium(III) complexes [26–29]. The EHOMOs of Ir-1a and Ir-1b are the same (−5.08 eV) as each other, and their ELUMOs are very close (−2.01 and −2.05 eV for Ir-1a and Ir-1b, respectively). In Ir-2a and Ir-2b, similar values are also found for the EHOMOs (−5.01 and −5.00 eV, respectively) and the ELUMOs (−2.06 and −2.11 eV, respectively). Therefore, the heteroatom substituent effects of the benzo[b]furan
Table 2 Electrochemical properties of Ir-1 and Ir-2. Complex
a Eox 1/2 (V)
a Ered 1/2 (V)
EHOMO (eV)b
ELUMO (eV)c
Eg (eV)d
Ir-1a Ir-1b Ir-2a Ir-2b
0.54 0.51 0.42 0.42
−2.36, ‒2.10 −2.11 −2.10 −2.28, ‒2.07
−5.34 −5.31 −5.22 −5.22
−2.70 −2.69 −2.70 −2.73
2.64 2.62 2.52 2.49
Fig. 5. (a) EL spectra and (b) external quantum efficiency (ηext)–current density curves of PLED-1 and PLED-2. The spectra were taken at the voltages where the maximum values of luminance were obtained.
and benzo[b]thiophene units on the HOMO/LUMO energy levels are hardly observed. Employing Ir-1 and Ir-2 as emitting dopants, PLEDs were fabricated; namely, PLED-1 and PLED-2, respectively. The device structure is as follows; ITO (anode, 150 nm)/PEDOT:PSS (40 nm)/emitting layer (100 nm)/CsF (1.0 nm)/Al (cathode, 250 nm). The emitting layer consists of PVCz doped with the iridium(III) complex and PBD (electron transporting material), where the weight ratio of PVCz:PBD:Ir-1 (or Ir-2) is adjusted to 1.0:0.30:0.040 (wt/wt/wt). The obtained EL spectra of the PLEDs are shown in Fig. 5a, each of which is comparable to the PL spectrum of the emitting dopant. Thus, PLED-1 exhibits deep-red EL (λEL; 654 and 648 nm for PLED-1a and PLED-1b, respectively), whereas EL spectra of PLED-2 range from deep-red to NIR regions (λEL; 697 and 694 nm for PLED-2a and PLED-2b, respectively). In Fig. 5b are shown the external quantum efficiency (ηext)–current density curves for the PLEDs. The device Table 3 Device performance of PLED-1 and PLED-2.
Vturn-on (V)a Lmax (cd m−2)b ηj max (cd A−1)b ηp max (lm W−1)b ηext max (%)b CIE (x, y)c
PLED-1a
PLED-1b
PLED-2a
PLED-2b
6.0 187 (13.5) 0.25 (7.0) 0.11 (7.0) 0.85 (7.0) (0.63, 0.28)
6.0 415 (14.5) 0.63 (7.5) 0.28 (7.0) 1.83 (7.0) (0.64, 0.27)
12.5 9.04 (17.5) 0.0077 (13.0) 0.0019 (13.0) 0.15 (12.5) ‒d
8.5 46.5 (16.5) 0.046 (11.0) 0.013 (11.0) 1.41 (8.5) (0.65, 0.26)e
Turn-on voltage is the one at which the luminance over 1 cd m−2 was obtained. Maximum values of the luminance (L), current efficiency (ηj), power efficiency (ηp), and external quantum efficiency (ηext). The values in the parentheses represent the voltages (V) at which they were obtained. c The Commission Internationale de L'Eclairage chromaticity coordinate obtained at the voltage where the Lmax was observed. d Visible EL is too weak to detect the correct value of CIE(x, y). e The value of CIE (x, y) is saturated because only deep-red EL was detected. a
a Potential values vs. Fc+/Fc were obtained in anhydrous DMF. b Calculated from the c red equation EHOMO = − 4.8 ‒ Eox 1/2. Calculated from the equation E LUMO = − 4.8 ‒ E 1/2 . d red Calculated from the relation Eg = Eox 1/2 ‒ E 1/2 .
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b
18
S. Ikawa et al. / Inorganic Chemistry Communications 38 (2013) 14–19
performance is also summarized in Table 3. Lmaxs, ηj maxs, and ηp maxs of the PLEDs are very low because the luminosity function remarkably decreases in the deep-red-to-NIR regions. The ηexts of all the PLEDs show gradual efficiency roll-off [46,47], and the maximum value of ηext (ηext max) of PLED-1b (or PLED-2b) is much higher than that of PLED-1a (or PLED-2a). This result reflects the magnitude of ΦPL, and it is indicated that the benzo[b]thiophene-derived complexes are more suitable for OLED application than the benzo[b]furan analogues. Especially, PLED-2b shows the highest ηext max of 1.41% at a current density of 15.8 mA cm−2. In conclusions, we demonstrated PL properties of phosphorescent tris-cyclometalated iridium(III) complexes Ir-1 and Ir-2 emitting from deep-red to NIR regions. The PLEDs doped with these complexes were also fabricated, and their device performance was investigated. Especially, adopting largely π-extended 1-(benzo[b]thiophen-2-yl) isoquinoline as a C^N ligand, relatively efficient PL ranging from deepred to NIR was obtained in dichloromethane as well as in PMMA film. Furthermore, the PVCz-based PLED doped with this complex afforded efficient EL in a similar region to PL, showing the excellent device performance for an NIR-OLED (ηext max = 1.41%). Acknowledgment This work was partially supported by JSPS Grant-in-Aid for Scientific Research (B) (no. 24350101). Appendix A. Supplementary material The supplementary crystallographic data for Ir-2b (except structure factors) have been deposited with the Cambridge Crystallographic Data Centre as supplementary publications CCDC 954294. Copies of the data can be obtained free of charge by application to CCDC, 12 Union Road, Cambridge CB21EZ, UK (e-mail: [email protected]). Supplementary data associated with this article (experimental details for the materials syntheses, X-ray crystallographic analysis, UV-vis and PL measurements, cyclic voltammetry analysis, DFT calculations, and PLED fabrication) can be found, in the online version, at http://dx. doi.org/10.1016/j.inoche.2013.09.075. References [1] M. Gross, D.C. Müller, H.G. Nothofer, U. Scherf, D. Neher, C. Bräuchle, K. Meerholz, Improving the performance of doped π-conjugated polymers for use in organic light-emitting diodes, Nature 405 (2000) 661–665. [2] Y. Shirota, Organic materials for electronic and optoelectronic devices, J. Mater. Chem. 10 (2000) 1–25. [3] S. Tokito, T. Iijima, T. Tsuzuki, F. Sato, High-efficiency white phosphorescent organic light-emitting devices with greenish-blue and red-emitting layers, Appl. Phys. Lett. 83 (2003) 2459–2461. [4] B.W. D'Andrade, S.R. Forrest, White organic light-emitting devices for solid state lighting, Adv. Mater. 16 (2004) 1585–1595. [5] J. Kido, H. Shionoya, K. Nagai, Single layer white light-emitting organic electroluminescent devices based on dye dispersed poly(Nvinylcarbazole), Appl. Phys. Lett. 67 (1995) 2281–2283. [6] V. Adamovich, J. Brooks, A. Tamayo, A.M. Alexander, P.I. Djurovich, B.W. D'Andrade, C. Adachi, S.R. Forrest, M.E. Thompson, High efficiency single dopant white electrophosphorescent light emitting diodes, New J. Chem. 26 (2002) 1171–1178. [7] Y. Sun, C. Borek, K. Hanson, P.I. Djurovich, M.E. Thompson, J. Brooks, J.J. Brown, S.R. Forrest, Photophysics of Pt-porphyrin electrophosphorescent devices emitting in the near infrared, Appl. Phys. Lett. 90 (2007) 213503 (3 pages). [8] C. Borek, K. hanson, P.I. Djurovich, M.E. Thompson, K. Aznavour, R. Bau, Y. Sun, S.R. Forrest, J. Brooks, L. Michalski, J. Brown, Highly efficient, near-infrared electrophosphorescence from a Pt-metalloporphyrin complex, Angew. Chem. Int. Ed. 46 (2007) 1109–1112. [9] K.R. Graham, Y. Yang, J.R. Sommer, A.H. Shelton, K.S. Schanze, J. Xue, J.R. Reynolds, Extended conjugation platinum(II) porphyrins for use in near-infrared emitting organic light emitting diodes, Chem. Mater. 23 (2011) 5305–5312. [10] O. Fenwick, J.K. Sprafke, J. Binas, D.V. Kondratuk, F.D. Stasio, H.L. Anderson, F. Cacialli, Linear and cyclic porphyrin hexamers as near-infrared emitters in organic lightemitting diodes, Nano Lett. 11 (2011) 2451–2456. [11] H. Xiang, J. Cheng, X. Ma, X. Zhou, J.J. Chruma, Near-infrared phosphorescence: materials and applications, Chem. Soc. Rev. 42 (2013) 6128–6185.
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Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Photo- and electroluminescence from deep-red- and near-infrared-phosphorescent tris-cyclometalated iridium(III) complexes bearing largely π-extended ligands Shigeru Ikawa a, Shigeyuki Yagi a,⁎, Takeshi Maeda a, Hiroyuki Nakazumi a, Hideki Fujiwara b, Shiro Koseki b, Yoshiaki Sakurai c a b c
Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1–1 Gakuen-cho, Naka-ku, Sakai, Osaka 599–8531, Japan Department of Chemistry, Graduate School of Science, Osaka Prefecture University, 1–1 Gakuen-cho, Naka-ku, Sakai, Osaka 599–8531, Japan Textile and Polymer Section, Technology Research Institute of Osaka Prefecture, 2-7-1 Ayumino, Izumi, Osaka 594–1157, Japan
a r t i c l e
i n f o
Article history: Received 8 August 2013 Accepted 30 September 2013 Available online 11 October 2013 Keywords: Iridium complex Cyclometalated ligand Phosphorescence Near-infrared emission Organic light-emitting diode
a b s t r a c t Deep-red- and near-infrared-phosphorescent tris-cyclometalated iridium(III) complexes bearing largely π-extended cyclometalated (C^N) ligands were newly synthesized, and their photo- and electroluminescence properties were investigated. When 2-(benzo[b]furan-2-yl)quinoline and 2-(benzo[b]thiophen-2-yl)quinoline were employed as C^N ligands, deep-red photoluminescence was obtained (Ir-1a and Ir-1b; λPL in CH2Cl2, 647 and 652 nm, respectively). In the case of the isoquinoline analogues of Ir-1a and Ir-1b, emission maxima were further red-shifted, ranging from deep-red to near-infrared regions (Ir-2a and Ir-2b; λPL in CH2Cl2, 696 and 690 nm, respectively). Especially, Ir-2b showed an excellent photoluminescence quantum yield (ΦPL = 0.15), and a polymer light-emitting diode doped with Ir-2b exhibited deep-red–near-infrared electroluminescence with a high external quantum efficiency (λEL = 694 nm, ηext max = 1.41%). © 2013 Elsevier B.V. All rights reserved.
Organic light-emitting diodes (OLEDs) have attracted much attention for the last two decades because their technological advantages, such as self-emission, fast response, simple device construction, and so on, make them applicable to flat panel displays and lighting apparatuses in next generations [1–6]. Recently OLEDs emitting in near-infrared (NIR) regions have been also receiving growing interest [7–10] because they are expected to be applied to night-vision readable displays and security sensors [11]. Therefore, NIR-emitting materials have been eagerly required for such purposes. In general, device efficiencies of OLEDs are markedly improved when fluorescent dopants are replaced with phosphorescent ones such as platinum(II) [12,13] and iridium(III) complexes [14–16] because they can effectively form triplet excitons upon electric excitation. With this respect, a number of bis- and triscyclometalated iridium(III) complexes (Ir(C^N)2LX and Ir(C^N)3, respectively [17]) emitting RGB phosphorescence have been developed [18–20]. Nevertheless, few examples of excellent NIR-phosphorescent iridium(III) complexes that allow us to fabricate high-efficiency NIROLEDs have so far been reported [11,21,22] since the photoluminescence (PL) quantum yield of an emitting material intrinsically tends to decrease as the emission wavelength is red-shifted (i.e., so-called energy gap law) [23–25]. Recently, Qiao and coworkers reported that an OLED doped with bis[(2-phenylbenzo[g]quinoline)-C2,N]iridium(III) acetylacetonate as a phosphorescent dopant gave NIR emission at 708 nm ⁎ Corresponding author. Tel.: +81 72 254 9324; fax: +81 72 254 9910. E-mail address: [email protected] (S. Yagi). 1387-7003/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.inoche.2013.09.075
[22]. Although this device exhibited relatively efficient device performance for NIR-OLED, the maximum external quantum efficiency was still low (ηext max = 1.07%). In the present paper, we report synthesis and PL properties of phosphorescent tris-cyclometalated iridium(III) complexes Ir-1 and Ir-2 emitting from deep-red to NIR regions. Also, we discuss about the electroluminescence (EL) properties of poly(Nvinylcarbazole)-based polymer light-emitting diodes (PLEDs) doped with those iridium(III) complexes. To obtain NIR phosphorescence, a quite low T1 level is required. So we designed tris-cyclometalated iridium(III) complexes Ir-1 and Ir-2 (Scheme 1), the C^N ligands of which are based on 2-(benzo[b]furan2-yl)- and 2-(benzo[b]thiophen-2-yl)quinolines and their isoquinoline analogues. Until now, theoretical studies about a variety of Ir(C^N)2LXand Ir(C^N)3-type complexes were reported, revealing that the excited states are mainly based on metal-to-ligand charge transfer (MLCT) transitions. That is, from density functional theory (DFT) calculations of the most basic complexes such as Ir(ppy)2(acac) and Ir(ppy)3 (ppy and acac; 2-phenylpyridinato-C2,N and acetylacetonato-O,O ligands, respectively), the HOMO consists of a mixture of phenyl-pπ and Ir-dπ orbitals, and the LUMO receives a considerable contribution from the C^N ligands, especially the pyridyl-π orbitals [26–29]. With this respect, replacement of the phenyl moieties of the C^N ligands by electronenriched benzo[b]furan and benzo[b]thiophene components gives rise to destabilization of the HOMO [30,31]. On the other hand, employment of the benzologues such as quinoline and isoquinoline in place of the pyridine moiety is effective on stabilization of the LUMO [32,33]. Thus,
S. Ikawa et al. / Inorganic Chemistry Communications 38 (2013) 14–19
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Scheme 1. Reagents and Conditions: (i) IrCl3 · 3H2O, 2-ethoxyethanol, H2O, 100 °C; (ii) acetylacetone, Na2CO3, 2-ethoxyethanol, 90 °C; (iii) C^N ligand, glycerol, 230 °C; (iv) C^N ligand, glycerol, 280 °C, microwave irradiation.
the present molecular design based on the HOMO–LUMO tuning should allow us to obtain NIR-emitting cyclometalated iridium(III) complexes. The synthesis of the iridium(III) complexes is shown in Scheme 1. The preparation of HC^N-2b was previously reported [34]. Similarly, HC^N-1a and HC^N-1b were also prepared by the Suzuki–Miyaura cross-coupling reaction of (benzo[b]furan-2-yl)- and (benzo[b] thiophen-2-yl)boronic acid with 2-chloroquinoline, respectively. HC^N-2a was newly synthesized by the Suzuki–Miyaura crosscoupling method in 92% yield (see the supplementary materials section). In general, Ir(C^N)3-type complexes are efficiently synthesized by the reaction of a μ-chloro-bridged iridium(III) dimer [(C^N)2Ir(μ-Cl)]2 or a bis-cyclometalated iridium(III) complex (C^N)2Ir(acac) with the corresponding C^N ligand [16]. These procedures yield a facial (fac) isomer exclusively when the reactions proceed at high temperatures (N 200 °C). In the present case, we examined the ligand replacement of (C^N-1a)2Ir(acac) and (C^N-1b)2Ir(acac) at 230 °C and yielded Ir-1a and Ir-1b in 11 and 14%, respectively (overall yields from HC^N-1a and HC^N-1b; 2.4 and 8.9%, respectively). For the synthesis of Ir-2, microwave-assisted reactions (2450 ± 30 MHz, 300 W, 280 °C, 45 min) brought about better results, where [(C^N-2a)2Ir(μ-Cl)]2 and [(C^N2b)2Ir(μ-Cl)]2 were reacted with the corresponding C^N ligands to yield Ir-2a and Ir-2b in 41 and 88%, respectively (overall yields from HC^N-2a and HC^N-2b; 31 and 86%, respectively). Unfortunately, in the case of the preparation of Ir-1a and Ir-1b, the microwave synthesis was unsuccessful, and trace amounts of the target complexes were obtained. According to 1H NMR spectral analysis, each complex of Ir-1 and Ir-2 was obtained as a single isomer. As shown in Fig. 1, the molecular structure of Ir-2b was determined by X-ray crystallographic analysis [35]. The molecule of Ir-2b adopts distorted octahedral coordination geometry around the metal center, and three C^N ligands are arranged in a fac manner, affording a racemic mixture of enantiomeric fac isomers. Although we attempted X-ray crystallographic analysis of Ir-1a, Ir-1b and Ir-2a, any suitable single crystals have not been obtained at this point. According to the previous reports [16,36], the fac isomer is thermodynamically more stable than the meridional one. Taking into
consideration that these three products were obtained at more than 230 °C, they should be fac isomers [37,38]. Fig. 2 shows the UV–vis absorption spectra of Ir-1 and Ir-2 in dichloromethane at rt. The data are also listed in Table 1. All complexes exhibit similar absorption spectral profiles. The absorption bands at b390nm are assigned to the spin-allowed π–π⁎ transitions at the C^N ligands, and the next absorption bands up to ca. 570 nm are assigned to the spin-allowed 1 MLCT transitions from the Ir(III) centers to the C^N ligands. As represented by the magnified spectra (Fig. 2, dashed lines), the lowerenergy absorption bands in the region of N 590 nm are assignable to the spin-forbidden 3MLCT transitions. In this region, the molar absorption coefficients of Ir-1b and Ir-2b are larger than those of the corresponding benzo[b]furan analogues (i.e., Ir-1a and Ir-2a, respectively),
Fig. 1. ORTEP drawing of Ir-2b. The hydrogen atoms are omitted for clarity.
16
S. Ikawa et al. / Inorganic Chemistry Communications 38 (2013) 14–19
Fig. 2. UV–vis absorption spectra of Ir-1 and Ir-2 in dichloromethane at rt. The solid lines are the spectra at concentrations of 10 μM. The dashed lines are the ones 20 times expanded in intensity in the region from 540 to 750 nm.
indicating that the benzo[b]thiophene-derived complexes exhibit more effective spin–orbit coupling. Focusing on the N-heterocycle in the C^N ligand, the 3MLCT transition bands of Ir-2a and Ir-2b are bathochromically shifted in comparison with those of Ir-1a and Ir-1b. This indicates that the isoquinoline-based complexes exhibit more redshifted phosphorescence than the quinoline-based ones. Fig. 3a shows the PL spectra of Ir-1 and Ir-2 in dichloromethane at rt, and the spectral and photophysical profiles are summarized in Table 1. The PL lifetimes (τPLs) range from 1.13 to 2.09 μs, each of which is well fitted to single-exponential decay (χ2; 1.03–1.07), indicating that the obtained emission are exclusively phosphorescence. In the PL spectra of Ir-1a and Ir-1b, the emission maxima (λPL) are observed at 652 and 647 nm, respectively, and their emission colors are deep-red. The PL quantum yield (ΦPL) of Ir-1a is 0.12, whereas Ir-1b emits more efficiently in solution, affording ΦPL of 0.21. So, one can see that the improved ΦPL of Ir-1b rather than Ir-1a is attributed to the enhanced spin– orbit interaction as indicated in the UV–vis spectra. The isoquinolinebased C^N ligands gave rise to further red shifts of PL spectra: the λPLs of Ir-2a and Ir-2b are observed at 696 and 690 nm, respectively, and the shoulders of the spectra reached NIR regions. The ΦPLs of Ir-2a and Ir-2b are 0.068 and 0.15, respectively, and as discussed in Ir-1b, the stronger spin–orbit interaction of Ir-2b should bring about a larger ΦPL. The PL properties in polymer thin films were also investigated. Poly(methyl methacrylate) (PMMA) was chosen as a matrix polymer because the absence of absorption bands from near-UV to visible regions allows us to estimate the intrinsic PL properties of the iridium(III) complexes [39]. The PL spectra of Ir-1 and Ir-2 in PMMA films are almost identical to those in solutions (Fig. 3b). As summarized in Table 1, the ΦPLs in PMMA films are also similar to those in dichloromethane. It is worthy to note that Ir-2b is still emissive even in the solid matrix (ΦPL = 0.11) in spite of its low-lying triplet level. Such
Fig. 3. PL spectra of Ir-1 and Ir-2 (a) in dichloromethane ([Ir(C^N)3] = 10 μM) and (b) in PMMA thin films (PMMA/Ir(C^N)3 = 100/4, wt/wt).
excellent NIR-PL properties make us expect the possibility of a solidstate NIR electroluminescent device. The electrochemical behavior of Ir-1 and Ir-2 was examined using cyclic voltammetry. The cathodic and anodic scans were carried out in anhydrous DMF solutions at rt. The cyclic voltammograms of Ir-1 and Ir-2 are shown in Fig. 4, and the redox potentials determined in comparison with the ferrocene/ferrocenium redox (Fc/Fc+) are listed in Table 2. Reversible one-electron oxidation and reduction couples were obtained for Ir-1b and Ir-2a, whereas Ir-1a and Ir-2b showed reversible oneelectron oxidation potentials as well as one- and two-electron reduction ones. Oxidation potentials of Ir-1a and Ir-1b (Eox 1/2; 0.54 and 0.51 V, respectively) are slightly higher than those of Ir-2a and Ir-2b (Eox 1/2; 0.42 V), although the first reduction potentials of all complexes are almost similar (Ered 1/2; ‒2.07 to −2.11 V). It was found that the oxidation and reduction potentials are not so different between benzo[b]franand benzo[b]thiophene-derived complexes, i.e., between Ir-1a and Ir-1b and between Ir-2a and Ir-2b. The HOMO/LUMO energy levels (EHOMO/ELUMO) of Ir-1 and Ir-2 are also estimated in comparison with the energy level of ferrocene (EHOMO = −4.80 eV below vacuum
Table 1 UV–vis absorption and photoluminescence (PL) spectral data of Ir-1 and Ir-2a. Compd
Ir-1a Ir-1b Ir-2a Ir-2b a b c d
PMMA thin film
CH2Cl2 solution λabs/nm (10−4 ε/M−1 cm−1)b
λPL/nm (λex/nm)c
ΦPLc
τPL/μs (χ2)c
λPL/nm (λex/nm)d
ΦPLd
318 (4.6), 432 (2.4), 520 (0.83) [sh], 639 (0.012) [sh] 276 (4.7), 314 (4.6), 435 (2.1), 529 (0.57) [sh], 623 (0.022) 328 (3.8), 450 (2.6), 518 (1.1) [sh], 624 (0.031) [sh], 683 (0.015) [sh] 291 (4.2), 317 (4.3), 456 (3.0), 535 (1.1), 612 (0.046) [sh], 665 (0.024)
652 (454)
0.12
1.34 (1.07)
648 (454)
0.11
647 (453)
0.21
2.09 (1.03)
644 (453)
0.22
696 (468)
0.068
1.13 (1.05)
692 (467)
0.065
690 (468)
0.15
1.77 (1.05)
694 (467)
0.11
λabs, UV–vis absorption maximum; λPL, PL maximum; λex, excited wavelength for PL; ΦPL, PL quantum yield; τPL, PL lifetime. [sh]; shoulder peak. Obtained for deaerated samples at rt. Obtained under N2 atmosphere at rt. The ratio of PMMA/Ir(C^N)3 was adjusted to 100/4 (wt/wt).
S. Ikawa et al. / Inorganic Chemistry Communications 38 (2013) 14–19
Fig. 4. Cyclic voltammograms of (a) Ir-1 and (b) Ir-2 in anhydrous DMF at a scan rate of 100 mV s-1.
[40]) (Table 2). These levels are placed between HOMO and LUMO of poly(N-vinylcarbazole) (PVCz, ‒5.80 to −2.20 eV) and 2-(4-biphenylyl)5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD, ‒6.20 to −2.40 eV) [41], and thus Ir-1 and Ir-2 have the EHOMO and ELUMO values suitable for emitting dopants in PVCz-based PLEDs. DFT calculations were also carried out for Ir-1 and Ir-2. To obtain their optimized structures, the GAMESS suite of program codes was used [42]. In these calculations, the B3LYP functional [43] with the SBKJC effective core potentials and the associated basis sets [44,45] were used. Calculated EHOMOs and ELUMOs of Ir-1 and Ir-2 as well as the electron density distributions of the orbitals were listed in the supplementary materials section (Fig. S1 and Table S1). The HOMO distributions of electronic density primarily reside on the Ir-dπ orbital, and the LUMO distributions mainly reside on the quinoline (or isoquinoline) moieties of the C^N ligands, as is seen in typical cyclometalated iridium(III) complexes [26–29]. The EHOMOs of Ir-1a and Ir-1b are the same (−5.08 eV) as each other, and their ELUMOs are very close (−2.01 and −2.05 eV for Ir-1a and Ir-1b, respectively). In Ir-2a and Ir-2b, similar values are also found for the EHOMOs (−5.01 and −5.00 eV, respectively) and the ELUMOs (−2.06 and −2.11 eV, respectively). Therefore, the heteroatom substituent effects of the benzo[b]furan
Table 2 Electrochemical properties of Ir-1 and Ir-2. Complex
a Eox 1/2 (V)
a Ered 1/2 (V)
EHOMO (eV)b
ELUMO (eV)c
Eg (eV)d
Ir-1a Ir-1b Ir-2a Ir-2b
0.54 0.51 0.42 0.42
−2.36, ‒2.10 −2.11 −2.10 −2.28, ‒2.07
−5.34 −5.31 −5.22 −5.22
−2.70 −2.69 −2.70 −2.73
2.64 2.62 2.52 2.49
Fig. 5. (a) EL spectra and (b) external quantum efficiency (ηext)–current density curves of PLED-1 and PLED-2. The spectra were taken at the voltages where the maximum values of luminance were obtained.
and benzo[b]thiophene units on the HOMO/LUMO energy levels are hardly observed. Employing Ir-1 and Ir-2 as emitting dopants, PLEDs were fabricated; namely, PLED-1 and PLED-2, respectively. The device structure is as follows; ITO (anode, 150 nm)/PEDOT:PSS (40 nm)/emitting layer (100 nm)/CsF (1.0 nm)/Al (cathode, 250 nm). The emitting layer consists of PVCz doped with the iridium(III) complex and PBD (electron transporting material), where the weight ratio of PVCz:PBD:Ir-1 (or Ir-2) is adjusted to 1.0:0.30:0.040 (wt/wt/wt). The obtained EL spectra of the PLEDs are shown in Fig. 5a, each of which is comparable to the PL spectrum of the emitting dopant. Thus, PLED-1 exhibits deep-red EL (λEL; 654 and 648 nm for PLED-1a and PLED-1b, respectively), whereas EL spectra of PLED-2 range from deep-red to NIR regions (λEL; 697 and 694 nm for PLED-2a and PLED-2b, respectively). In Fig. 5b are shown the external quantum efficiency (ηext)–current density curves for the PLEDs. The device Table 3 Device performance of PLED-1 and PLED-2.
Vturn-on (V)a Lmax (cd m−2)b ηj max (cd A−1)b ηp max (lm W−1)b ηext max (%)b CIE (x, y)c
PLED-1a
PLED-1b
PLED-2a
PLED-2b
6.0 187 (13.5) 0.25 (7.0) 0.11 (7.0) 0.85 (7.0) (0.63, 0.28)
6.0 415 (14.5) 0.63 (7.5) 0.28 (7.0) 1.83 (7.0) (0.64, 0.27)
12.5 9.04 (17.5) 0.0077 (13.0) 0.0019 (13.0) 0.15 (12.5) ‒d
8.5 46.5 (16.5) 0.046 (11.0) 0.013 (11.0) 1.41 (8.5) (0.65, 0.26)e
Turn-on voltage is the one at which the luminance over 1 cd m−2 was obtained. Maximum values of the luminance (L), current efficiency (ηj), power efficiency (ηp), and external quantum efficiency (ηext). The values in the parentheses represent the voltages (V) at which they were obtained. c The Commission Internationale de L'Eclairage chromaticity coordinate obtained at the voltage where the Lmax was observed. d Visible EL is too weak to detect the correct value of CIE(x, y). e The value of CIE (x, y) is saturated because only deep-red EL was detected. a
a Potential values vs. Fc+/Fc were obtained in anhydrous DMF. b Calculated from the c red equation EHOMO = − 4.8 ‒ Eox 1/2. Calculated from the equation E LUMO = − 4.8 ‒ E 1/2 . d red Calculated from the relation Eg = Eox 1/2 ‒ E 1/2 .
17
b
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performance is also summarized in Table 3. Lmaxs, ηj maxs, and ηp maxs of the PLEDs are very low because the luminosity function remarkably decreases in the deep-red-to-NIR regions. The ηexts of all the PLEDs show gradual efficiency roll-off [46,47], and the maximum value of ηext (ηext max) of PLED-1b (or PLED-2b) is much higher than that of PLED-1a (or PLED-2a). This result reflects the magnitude of ΦPL, and it is indicated that the benzo[b]thiophene-derived complexes are more suitable for OLED application than the benzo[b]furan analogues. Especially, PLED-2b shows the highest ηext max of 1.41% at a current density of 15.8 mA cm−2. In conclusions, we demonstrated PL properties of phosphorescent tris-cyclometalated iridium(III) complexes Ir-1 and Ir-2 emitting from deep-red to NIR regions. The PLEDs doped with these complexes were also fabricated, and their device performance was investigated. Especially, adopting largely π-extended 1-(benzo[b]thiophen-2-yl) isoquinoline as a C^N ligand, relatively efficient PL ranging from deepred to NIR was obtained in dichloromethane as well as in PMMA film. Furthermore, the PVCz-based PLED doped with this complex afforded efficient EL in a similar region to PL, showing the excellent device performance for an NIR-OLED (ηext max = 1.41%). Acknowledgment This work was partially supported by JSPS Grant-in-Aid for Scientific Research (B) (no. 24350101). Appendix A. Supplementary material The supplementary crystallographic data for Ir-2b (except structure factors) have been deposited with the Cambridge Crystallographic Data Centre as supplementary publications CCDC 954294. Copies of the data can be obtained free of charge by application to CCDC, 12 Union Road, Cambridge CB21EZ, UK (e-mail: [email protected]). Supplementary data associated with this article (experimental details for the materials syntheses, X-ray crystallographic analysis, UV-vis and PL measurements, cyclic voltammetry analysis, DFT calculations, and PLED fabrication) can be found, in the online version, at http://dx. doi.org/10.1016/j.inoche.2013.09.075. References [1] M. Gross, D.C. Müller, H.G. Nothofer, U. Scherf, D. Neher, C. Bräuchle, K. Meerholz, Improving the performance of doped π-conjugated polymers for use in organic light-emitting diodes, Nature 405 (2000) 661–665. [2] Y. Shirota, Organic materials for electronic and optoelectronic devices, J. Mater. Chem. 10 (2000) 1–25. [3] S. Tokito, T. Iijima, T. Tsuzuki, F. Sato, High-efficiency white phosphorescent organic light-emitting devices with greenish-blue and red-emitting layers, Appl. Phys. Lett. 83 (2003) 2459–2461. [4] B.W. D'Andrade, S.R. Forrest, White organic light-emitting devices for solid state lighting, Adv. Mater. 16 (2004) 1585–1595. [5] J. Kido, H. Shionoya, K. Nagai, Single layer white light-emitting organic electroluminescent devices based on dye dispersed poly(Nvinylcarbazole), Appl. Phys. Lett. 67 (1995) 2281–2283. [6] V. Adamovich, J. Brooks, A. Tamayo, A.M. Alexander, P.I. Djurovich, B.W. D'Andrade, C. Adachi, S.R. Forrest, M.E. Thompson, High efficiency single dopant white electrophosphorescent light emitting diodes, New J. Chem. 26 (2002) 1171–1178. [7] Y. Sun, C. Borek, K. Hanson, P.I. Djurovich, M.E. Thompson, J. Brooks, J.J. Brown, S.R. Forrest, Photophysics of Pt-porphyrin electrophosphorescent devices emitting in the near infrared, Appl. Phys. Lett. 90 (2007) 213503 (3 pages). [8] C. Borek, K. hanson, P.I. Djurovich, M.E. Thompson, K. Aznavour, R. Bau, Y. Sun, S.R. Forrest, J. Brooks, L. Michalski, J. Brown, Highly efficient, near-infrared electrophosphorescence from a Pt-metalloporphyrin complex, Angew. Chem. Int. Ed. 46 (2007) 1109–1112. [9] K.R. Graham, Y. Yang, J.R. Sommer, A.H. Shelton, K.S. Schanze, J. Xue, J.R. Reynolds, Extended conjugation platinum(II) porphyrins for use in near-infrared emitting organic light emitting diodes, Chem. Mater. 23 (2011) 5305–5312. [10] O. Fenwick, J.K. Sprafke, J. Binas, D.V. Kondratuk, F.D. Stasio, H.L. Anderson, F. Cacialli, Linear and cyclic porphyrin hexamers as near-infrared emitters in organic lightemitting diodes, Nano Lett. 11 (2011) 2451–2456. [11] H. Xiang, J. Cheng, X. Ma, X. Zhou, J.J. Chruma, Near-infrared phosphorescence: materials and applications, Chem. Soc. Rev. 42 (2013) 6128–6185.
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