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Cite this: Chem. Commun., 2014, 50, 530

Increased phosphorescent quantum yields of cationic iridium(III) complexes by wisely controlling the counter anions†

Received 26th September 2013, Accepted 1st November 2013

Dongxin Ma, Lian Duan,* Yongge Wei, Lei He, Liduo Wang and Yong Qiu*

DOI: 10.1039/c3cc47362g www.rsc.org/chemcomm

Phosphorescent quantum yields have been increased by 12 times by choosing bulky boracic anions as counterions for blue-emitting cationic iridium(III) complexes.

In the past several decades, tremendous efforts have been made to develop efficient light-emitting materials. Ionic transition metal complexes (iTMCs), including ionic copper, ruthenium, osmium, rhenium, and iridium complexes, are regarded as promising candidates for triplet emitters in organic light-emitting diodes (OLEDs).1 Ionic iridium(III) complexes with good photochemical stability and efficient emission of virtually all colors have attracted much attention.2,3 Many efforts have been made to develop various ionic iridium(III) complexes, but there are few reports on efficient phosphorescent materials, especially on the blue ones. In a previous study, L. He4 et al. reported a blue-emitting cationic iridium(III) complex with hexafluorophosphate (PF6 ) as the counterion, which emitted blue light at 475 nm. However, the photoluminescent quantum yield (PLQY) was only 0.03 in neat films, due to severe excited-state quenching caused by strong intermolecular interaction and molecular aggregation. To address this problem, we have introduced bulky groups in the counter anions of iridium(III) complexes to decrease the intermolecular interaction and molecular aggregation. Up to now, only PF6 (ref. 5–12) and trifluoromethanesulphate (CF3SO3 )13–15 have been widely used as charge-compensating anions for cationic iridium(III) complexes. In this communication, three novel bulky boracic anions were used to improve the PLQYs of cationic iridium(III) complexes. The properties of the newly designed complexes were compared with those of the original iridium(III) complex with PF6 (complex 1). Key Lab of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China. E-mail: [email protected], [email protected]; Fax: +86-10-62795137; Tel: +86-10-62788802 † Electronic supplementary information (ESI) available. CCDC 963326 and 963327. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3cc47362g

530 | Chem. Commun., 2014, 50, 530--532

For these compounds, the cation is the same, [Ir(ppy)2(pzpy)]+, where ppy is 2-phenylpyridine and pzpy is 2-(1H-pyrazol-1-yl)pyridine. And the boracic anions are tetrakis(phenyl)borate ([Bph4] , complex 2), tetrakis[4-(trifluoromethyl)phenyl]borate ([BArF12] , complex 3), and tetrakis[3,5-bis(trifluoromethyl)phenyl]borate ([BArF24] , complex 4), respectively (Scheme 1). Experiments show that PLQYs of complexes 2–4 are improved in the bulk. Especially, complex 4 with the largest counter anion shows the highest PLQY of 0.39 in neat films, 12 times higher than that of complex 1. Single crystals of complexes 1–3 were grown from solution and characterized by X-ray crystallography (Fig. 1). For complexes 1 and 2, the solvent was acetone–methanol, and for complex 3, acetone– deionized water was used.‡ The data for single crystals of complex 4 were not obtained yet. All the crystals exhibit distorted octahedral geometries around the iridium center with the two cyclometalated ligands adopting C,C-cis, N,N-trans configurations, which are in agreement with previous reports of cationic iridium complexes.16 Selected bond length and distances are summarized in Table 1. The structures of the cations are similar, while the distance between the neighboring two iridium atoms is increased with the size of the bulky boracic anions. Quantum chemical calculations also show that the molecular volume increases from complexes 1 to 4, which is 49.95, 288.44, 334.63 and 425.02 cm3 mol 1, respectively.

Scheme 1

Chemical structures of complexes 1–4.

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Fig. 2

Fig. 1

Single crystal structures of complexes 1–3.

Table 1 Selected bond lengths and distances in single crystals of complexes 1–3

a [Å]

b [Å]

c [Å]

d [Å]

Ib [Å] IIc [Å]

a

1 2.004(5), 2.011(5) 2.046(5), 2.047(5) 2.126(5) 2.137(5) 6.285 6.346 2 2.004(4), 2.015(4) 2.050(3), 2.051(3) 2.138(3) 2.168(3) 8.370 8.370 3 2.004(7), 2.051(6) 2.008(7), 2.048(5) 2.134(6) 2.165(6) 6.893 7.324 a The results of complex 1 originated from L. He.4 b I is the distance between the center iridium in the cation and the center atom in the anion (phosphorus for complex 1 and boron for complexes 2 and 3). c II is the distance between the neighboring two iridium atoms (Scheme 2).

Scheme 2

Bonds and distances in the complexes.

Effects of the counter anion on the intermolecular interaction, electrochemical properties, photoluminescence, and PLQYs have been studied. Electrochemical properties of complexes 1–4 are characterized (Table 2). The reduction potentials of complexes 1–4 are nearly equal, suggesting that influence of the counter anions is negligible. While the oxidation potentials are different, 0.12 V for complex 2, and 0.88 V for others. It is because in the counter anion

Table 2

Absorption and emission spectra of complexes 1–4.

of complex 2, the center boron is surrounded by four electrondonating phenyl groups, leading to a significant increase in electron density and strong chemical instability (see Fig. S1 in ESI†). The luminescence of complexes 1–4 is quenched by oxygen, so photophysical measurements in CH3CN solution were carried out under argon. The absorption and emission spectra obtained in solution are similar (Fig. 2a), where cations and anions are ionized. The emission peak is at 474 nm and the shoulder peak is around 502 nm (Table 2). The observed excited-state lifetimes are 1.56–1.61 ms. The PLQYs of complexes 2–4 are 0.31, 0.35 and 0.34, respectively, all higher than that of complex 1 (0.23). In neat films under air, the absorption and emission spectra of complexes 1–4 are quite different (Fig. 2b). It is recognized that for most iTMCs as emitters, there is always a red shift in the emission color from solution to neat films. The red shifts for complexes 1–3 are 4–9 nm, and the spectra are broadened. While for complex 4, neither red-shift nor spectra broadening occurs. The reason is that the large counter anion takes up more space and weakens intermolecular interaction, thus avoiding the red shift for neat films. It is interesting to note that the PLQYs of complexes 2–4 in neat films are increased obviously in contrast to that of complex 1. Especially, the PLQY of complex 4 reaches 0.39, 12 times higher than that of complex 1 (0.03).4 It is regarded that in complexes with bulky anions, molecular aggregation and concentration quenching are reduced, resulting in a considerable increase of PLQYs. However, previous work mainly focused on modifying the cations of cationic transition metal complexes,14,15 but the PLQYs of blue emitters were always below 0.10 and the wavelength of emission was always affected. In summary, three novel blue-emitting cationic iridium(III) complexes with bulky boracic anions have been designed and

Photophysical and electrochemical characteristics of complexes 1–4

Emission at room temperature jem [t [ms]] Absorptionb l [nm] 4 1 1 c d Neat filme (e [10 M cm ]) Solution l [nm] Thin film l [nm] CH3CN a

1 2 3 4

253 253 250 254

(5.51), (5.31), (5.88), (5.23),

380 380 379 379

(0.53) (0.58) (0.55) (0.54)

475, 474, 474, 474,

503 502 501 503

(sh) (sh) (sh) (sh)

483, 483, 478, 473,

504 508 506 502

(sh) (sh) (sh) (sh)

0.23 0.31 0.35 0.34

[1.56] [1.61] [1.57] [1.56]

0.03 0.14 0.25 0.39

[0.19 [0.30 [0.35 [0.32

(88%), (51%), (37%), (27%),

0.42] 3.00] 2.70] 1.20]

Emission at 77 Kf

Eg/V

l [nm]

Eox [V] Ered [V]

469, 469, 468, 467,

505 503 503 502

t [ms] (sh) (sh) (sh) (sh)

3.72 (90%), 1.01 0.88 4.50 0.12 4.60 0.87 5.00 0.87

2.19 2.18 2.18 2.17

a The results of complex 1 originated from L. He.4 b In CH3CN solution (2  10 5 M). c In degassed CH3CN solution, the symbol sh denotes the shoulder wavelength. d Neat films were made on round substrates by the solution process. e The percentage in parentheses denotes the percentage of each lifetime. f In CH3CN glass at 77 K. g Electrochemical data versus FcR/Fc (Fc is ferrocene) were collected in CH3CN solution (10 3 M).

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synthesized, and the highest PLQY of 0.39 has been achieved, 12 times higher than that of the original complex with PF6 as the counter anion. In complexes with bulky anions, the intermolecular interaction and molecular aggregation are decreased, thus avoiding concentration quenching and leading to a substantial increase of PLQYs. It suggests that the choice of bulky counter anions is an effective and convenient way to improve the PLQYs of cationic transition metal complexes, without obvious influence on the emission wavelength. We would like to thank the National Natural Science Foundation of China (Grant No. 21161160447, 51173096 and 51073089) and the National Key Basic Research and Development Program of China (Grant No. 2011CB808403) for supporting foundation.

Notes and references ‡ Synthesis and characterization: the ancillary ligand pzpy was synthesized from 2-bromopyridine and pyrazole, catalyzed by Cu(I) according to the reported procedure.4 Complexes 1–4 were synthesized from a dichloro-bridged diiridium complex [Ir(ppy)2Cl]2 and the pzpy ligand, then Cl was replaced by PF6 , [Bph4] , [BArF12] and [BArF24] with an ion exchange reaction, respectively (Scheme S1 in ESI†). Complex 2: 1 H NMR (600 MHz, DMSO-d6) 9.30 (d, J = 2.8 Hz, 1H), 8.52 (d, J = 8.5 Hz, 1H), 8.35–8.30 (m, 1H), 8.26 (dd, J = 8.2, 4.1 Hz, 2H), 7.99–7.93 (m, 2H), 7.91 (d, J = 7.8 Hz, 1H), 7.88 (d, J = 7.8 Hz, 1H), 7.76 (d, J = 5.8 Hz, 1H), 7.71 (d, J = 5.8 Hz, 1H), 7.65 (d, J = 5.5 Hz, 1H), 7.55–7.51 (m, 1H), 7.28 (s, 1H), 7.19–7.15 (m, 9H), 7.02 (t, J = 7.6 Hz, 1H), 6.99 (t, J = 7.5 Hz, 1H), 6.92 (t, J = 7.3 Hz, 9H), 6.89 (d, J = 7.4 Hz, 1H), 6.85 (t, J = 7.4 Hz, 1H), 6.78 (t, J = 7.1 Hz, 4H), 6.20 (d, J = 8.9 Hz, 1H), 6.21–6.17 (m, 2H). MS (ESI) [m/z]: 646.25 (M–Bph4)+, 319.39 [M–Ir(ppy)2(pzpy)] . Anal. calcd for C54H43N5BIrH2O: C, 65.98; H, 4.61; N, 7.12. Found: C, 66.24; H, 4.62; N, 7.31%. Yields: 57%. Monoclinic, space group P21/c (no. 14), a = 9.5427 (19) Å, b = 13.247 (3) Å, c = 35.237 (7) Å, a = 90.001, b = 93.01 (3)1, g = 90.001. Complex 3: 1H NMR (600 MHz, DMSO-d6): 9.27 (d, J = 2.5 Hz, 1H), 8.49 (d, J = 8.4 Hz, 1H), 8.31–8.19 (m, 3H), 7.92 (s, 2H), 7.86 (dd, J = 11.3, 8.1 Hz, 2H), 7.73 (d, J = 5.6 Hz, 1H), 7.68 (d, J = 5.6 Hz, 1H), 7.62 (d, J = 5.2 Hz, 1H), 7.46 (t, J = 6.3 Hz, 1H), 7.31 (s, 16H), 7.24 (s, 1H), 7.18 (t, J = 6.4 Hz, 1H), 7.14 (t, J = 6.5 Hz, 1H), 7.00–6.92 (m, 2H), 6.85 (d, J = 7.6 Hz, 2H), 6.81 (t, J = 7.4 Hz, 1H), 6.19–6.14 (m, 2H). MS (ESI) [m/z]: 646.32 (M–BArF12)+, 591.32 [M–Ir(ppy)2(pzpy)] . Anal. calcd for C58H39N5BF12Ir: C, 56.32; H, 3.18; N, 5.66. Found: C, 56.54; H, 3.26; N, 5.51%. Yields: 81%. Triclinic, space group P1% , a = 12.880 (3) Å,

532 | Chem. Commun., 2014, 50, 530--532

Communication b = 13.731 (3) Å, c = 17.199 (3) Å, a = 77.20 (3)1, b = 85.04 (3)1, g = 63.44 (3)1. Complex 4: 1H NMR (600 MHz, DMSO-d6): 9.30 (d, J = 3.0 Hz, 1H), 8.52 (d, J = 8.5 Hz, 1H), 8.35–8.29 (m, 1H), 8.25 (dd, J = 8.2, 4.0 Hz, 2H), 7.95 (ddd, J = 9.0, 4.8, 1.6 Hz, 2H), 7.88 (dd, J = 12.8, 7.5 Hz, 2H), 7.76 (d, J = 5.5 Hz, 1H), 7.72–7.67 (m, 5H), 7.66–7.64 (m, 1H), 7.61 (s, 8H), 7.54–7.50 (m, 1H), 7.28 (d, J = 2.0 Hz, 1H), 7.22 (t, J = 6.6 Hz, 1H), 7.18 (t, J = 6.6 Hz, 1H), 6.99 (dt, J = 15.1, 7.2 Hz, 2H), 6.92–6.87 (m, 2H), 6.84 (t, J = 7.4 Hz, 1H), 6.18 (dd, J = 9.3, 7.8 Hz, 2H). MS (ESI) [m/z]: 646.18 (M–BArF24)+, 863.13 [M–Ir(ppy)2(pzpy)] . Anal. calcd for C62H35N5BF24Ir: C, 49.35; H, 2.34; N, 4.64. Found: C, 49.15; H, 2.04; N, 4.71%. Yields: 51%. 1 C. W. Tang and S. A. Vanslyke, Appl. Phys. Lett., 1987, 51, 913. 2 R. D. Costa, E. Orti, H. J. Bolink, F. Monti, G. Accorsi and N. Armaroli, Angew. Chem., Int. Ed., 2012, 51, 8178. 3 T. Hu, L. He, L. Duan and Y. Qiu, J. Mater. Chem., 2012, 22, 4206. 4 L. He, L. Duan, J. Qiao, R. Wang, P. Wei, L. Wang and Y. Qiu, Adv. Funct. Mater., 2008, 18, 2123. 5 L. He, J. Qiao, L. Duan, G. Dong, D. Zhang, L. Wang and Y. Qiu, Adv. Funct. Mater., 2009, 19, 2950. 6 L. He, L. Duan, J. Qiao, G. Dong, L. Wang and Y. Qiu, Chem. Mater., 2010, 22, 3535. 7 E. A. Plummer, A. van Dijken, J. W. Hofstraat, L. De Cola and K. Brunner, Adv. Funct. Mater., 2005, 15, 281. 8 T.-H. Kwon, Y. H. Oh, I.-S. Shin and J.-I. Hong, Adv. Funct. Mater., 2009, 19, 711. 9 W.-Y. Wong, G.-J. Zhou, X.-M. Yu, H.-S. Kwok and Z. Liu, Adv. Funct. Mater., 2007, 17, 315. 10 E. A. Plummer, J. W. Hofstraat and L. De Cola, Dalton Trans., 2003, 2080–2084. 11 B. Park, Y. H. Huh, H. G. Jeon, C. H. Park and T. K. Kang, J. Appl. Phys., 2010, 108, 094506. 12 Y. Zhou, S. Han, G. Zhou, W.-Y. Wong and V. A. L. Roy, Appl. Phys. Lett., 2013, 102, 083301. 13 N. M. Shavaleev, F. Monti, R. Scopelliti, A. Baschieri, L. Sambri, N. Armaroli, M. Gratzel and M. K. Nazeeruddin, Organometallics, 2013, 32, 460. 14 N. M. Shavaleev, F. Monti, R. D. Costa, R. Scopelliti, H. J. Bolink, E. Orti, G. Accorsi, N. Armaroli, E. Baranoff, M. Gratzel and M. K. Nazeeruddin, Inorg. Chem., 2012, 51, 2263. 15 N. M. Shavaleev, F. Monti, R. Scopelliti, N. Armaroli, M. Gratzel and M. K. Nazeeruddin, Organometallics, 2012, 31, 6288. 16 (a) J. D. Slinker, A. A. Gorodetsky, M. S. Lowry, J. J. Wang, S. Parker, R. Rohl, S. Bernhard and G. G. Malliaras, J. Am. Chem. Soc., 2004, 126, 2763; (b) M. S. Lowry, W. R. Hudson, R. A. Pascal Jr and S. Bernhard, J. Am. Chem. Soc., 2004, 126, 14129.

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Increased phosphorescent quantum yields of cationic iridium(III) complexes by wisely controlling the counter anions.

Phosphorescent quantum yields have been increased by 12 times by choosing bulky boracic anions as counterions for blue-emitting cationic iridium(III) ...
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