Homoleptic vs. heteroleptic orange light-emitting iridium complexes chelated with benzothiazole derivatives

Article abstract of DOI:10.1080/15421406.2013.849434

The homoleptic and heteroleptic iridium(III) complexes exhibiting orange phosphorescence were investigated to compare their emission colors, luminance efficiency, and stability. The homoleptic iridium complexes, Ir(pbt-R) ₃, were prepared from

Full text of DOI:10.1080/15421406.2013.849434

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Homoleptic vs. Heteroleptic Orange
Light-Emitting Iridium Complexes
Chelated with Benzothiazole Derivatives
Hye Joo Lee a & Yunkyoung Ha a
a Department of Information Display Engineering , Hongik
University , Seoul , 121-791 , Korea
Published online: 16 Dec 2013.
To cite this article: Hye Joo Lee & Yunkyoung Ha (2013) Homoleptic vs. Heteroleptic Orange Light-
Emitting Iridium Complexes Chelated with Benzothiazole Derivatives, Molecular Crystals and Liquid
Crystals, 584:1, 53-59, DOI: 10.1080/15421406.2013.849434
To link to this article: http://dx.doi.org/10.1080/15421406.2013.849434
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Mol. Cryst. Liq. Cryst., Vol. 584: pp. 53–59, 2013
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2013.849434
Homoleptic vs. Heteroleptic Orange Light-Emitting
Iridium Complexes Chelated with
Benzothiazole Derivatives
HYE JOO LEE AND YUNKYOUNG HA∗
Department of Information Display Engineering, Hongik University, Seoul
121-791, Korea
The homoleptic and heteroleptic iridium(III) complexes exhibiting orange phospho-
rescence were investigated to compare their emission colors, luminance efficiency,
and stability. The homoleptic iridium complexes, Ir(pbt-R)3, were prepared from the
reaction of the iridium(III) precursor and phenylbenzothiazole derivatives. The het-
eroleptic ones containing the anions of 4-methyl-2,3-diphenylquinoline (4-Me-dpq) and
2-phenylbenzothiazole (pbt) as ligands, Ir(pbt-OMe)2(4-Me-2,3-dpq) and Ir(4-Me-2,3-
dpq)2(pbt-OMe), were synthesized via the chloro-bridged iridium dimer. We investigated
photoabsorption and photoluminescence (PL) properties of the iridium complexes and
studied their bandgaps with cyclic voltammetry (CV). Orange phosphorescence with
the PL maxima of 530–620 nm was observed with these complexes, and the bandgaps
between their highest occupied molecular orbitals (HOMOs) and the lowest unoccupied
molecular orbitals (LUMOs) were correlated with CV data. The electro-luminescence
(EL) study of these complexes was not possible due to lack of sublimability. The light-
emitting mechanisms regarding the phosphorescence colors were discussed.
Keywords Heteroleptic iridium complexes; homoleptic iridium complexes; phenyl-
benzothiazole derivatives; phosphorescence; orange emission
Introduction
For the past decade, organic light-emitting diodes (OLEDs) based on phosphorescence
materials have drawn great interest due to their applications in low cost, more efficient
flat-panel displays and solid-state lighting [1–2]. In particular, cyclometalated iridium
complexes, which have relatively short excited-state lifetime, high phosphorescence effi-
ciency and flexible color tunability, are regarded as the most promising phosphors in organic
light-emitting devices [3–5]. Recently, the orange light-emitting iridium complexes beside
the three primary color emitting materials have drawing attention since they can be used
in combination with blue emitters to fabricate two-component white OLEDs, which has
definitely become one of the most important directions of OLED studies [6–8].
Herein, our interest is to design and synthesis orange phosphorescent iridium com-
plexes having high luminescence yields and excellent device performance for white OLED
applications. For such purpose, we introduced 2-phenylbenzothiazole (pbt) derivatives as
∗Address correspondence to Prof. Yunkyoung Ha, Department of Information Display Engi-
neering, Hongik University, Sangsu-dong, Mapo-gu, Seoul 121-791, Korea (ROK). Tel.: (+82)2-
320-1490; Fax: (+82)2-3142-0335. E-mail: ykha@hongik.ac.kr
[241]/53

54/[242]
H. J. Lee and Y. Ha
Scheme 1.
main ligands in the iridium complexes. The para-position of the phenyl moiety in the
ligands in the complexes was substituted with the functional groups having different elec-
tronegativities. Thus, we synthesized the homoleptic iridium complexes of pbt-R (R =
H, OMe, F), 1–3 (Scheme 1) and also prepared the heteroleptic complexes, 4-5, with the
anions of pbt-OMe and 4-methyl-2,3-diphenylquinoline (4-Me-dpq) as the ligands. The
photophysical and electrochemical properties of the homoleptic and heteropleptic iridium
complexes were investigated and compared.
Experimental
Synthesis of Ligands
2-(4-fluorophenyl)benzothiazole (pbt-F). 2-(4-fluorophenyl)-benzothiazole were synthe-
sized according to the literature method [9]. Yield: 76 %

Homoleptic vs. Heteroleptic Orange Light-Emitting Complexes
[243]/55
4-Methyl-2,3-diphenylquinoline (4-Me-2,3-dpq). 4-methyl-2,3-diphenylquinoline was ob-
tained according to Friedlander reaction with the corresponding precursors, 2-
aminoacetophenone (1.35 g, 10.0 mmol) and deoxybenzoin (1.96 g, 10.0 mmol)] [10–11].
Yield: 75 %
Synthesis of Iridium Complexes
Ir(pbt-R)3(R=H, OMe, F). (1–3). Each complex was prepared from the reaction of Ir(acac)3
with the respective pbt-R ligand according to the reported procedure [12]. Ir(acac)3 (1.22 g,
2.5 mmol) and pbt-R (2.07 g, 10 mmol) were dissolved in 50 ml of ethylene glycol and the
mixture was refluxed for 24 hr. After cooling, 1N HCl solution was added and the resulting
precipitate was filtered off. The residue was purified by silica gel chromatography by using
CH2Cl2.
Ir(pbt)3 : An orange powder (Yield: 49 %). 1H NMR (400MHz, DMSO): δ 8.17–7.45
(m, 24H, aromatic C H).
1
Ir(pbt-OMe)3 : An orange powder (Yield: 49 %). H NMR (400MHz, DMSO):
δ 8.12–6.51 (m, 24H, aromatic C H) 3.51 (s, 6H).
Ir(pbt-F)3 : An orange powder (Yield: 49 %). 1H NMR (400MHz, DMSO): δ 8.26–6.04
(m, 21H, aromatic C H).
Ir(pbt-OMe)2(4-Me-2,3-dpq). (4). The cyclometalated Ir(III) μ-chloro-bridged dimer, (4-
Me-2,3-dpq)2Ir(μ-Cl)2Ir(4-Me-2,3-dpq)2 (1.9 mmol), was first prepared according to the
Nonoyama method. [13] In the second step, the resulting dimer and 4-Me-2,3-dpq ligands
(6.5 mmol) were mixed with Na2CO3 (500 mg) in 2-ethoxyethanol (30 mL). The mixture
was refluxed for 2 h and the red solid was filtered after cooling. Ir(4-Me-2,3-dpq)2(pbt-
OMe) were purified by chromatography on silica gel column with dichloromethane and
recrystallization. An orange powder was obtained. (Yield: 49 %) FAB-MS: calculated
1
967; found 673 (M+-4-Me-2,3-dpq). H NMR (400MHz, DMSO): δ 8.35–6.20 (m, 27H,
aromatic C-H) 3.52 (s, 6H) 2.50 (s, 3H).
Ir(4-Me-2,3-dpq)2(pbt-OMe). (5). The complexes were prepared from the reaction of 4-
Me-2,3-dpq with IrCl3·H2O and then treated with the pbt-OMe (2-phenylbeonzothiazole
derivative), similar to the procedure described above. A red powder was obtained. (Yield
1
65 %) FAB-MS: calculated 1021; found 781(M+-pbt-OMe). H NMR(DMSO) (δ, ppm):
8.35∼6.00 (m, aromatic C-H, 33H); 3.48 (s, 3H); 2.50 (s, 6H).
Measurements
UV-Vis absorption spectra were measured on a Hewlett Packard 8425A spectrometer. PL
spectra were measured on an Aminco-Bowman Series 2 luminescence. The UV-Vis and PL
spectra of the iridium complexes were measured in 10−4 M dilute CH2Cl2 solution. Cyclic
voltammograms were obtained at scan rate of 100 mV/s with Electrochemical Analyzer of
CH Instruments, and tetrabutylammonium hexafluorophosphate was added as an electrolyte
in CH2Cl2 solution.
Results and Discussion
The synthesis of the ligands, F-pbt and 4-Me-2,3-dpq, was straightforward, according to
the procedures reported previously [9–11]. The other ligands were purchased and used

56/[244]
H. J. Lee and Y. Ha
Figure 1. UV-Vis absorption spectra of iridium complexes in a 10−4 M CH2Cl2 solution.
directly. The homoleptic iridium complexes, Ir(pbt-R)3, were prepared from the one-pot
reaction of the corresponding main ligand with Ir(acac)3 [12]. The heteroleptic iridium
complexes were synthesized via two steps, as reported by Nonoyama [13]. The overall
synthetic schemes are illustrated in Scheme 1.
The UV-Vis absorption spectra of the complexes in CH2Cl2 are shown in Fig. 1. The
absorption spectra of iridium complexes have strong absorption bands appearing at the
ultraviolet region of the spectrum between 280 and 330 nm. These bands were assigned to
the spin-allowed 1π→π∗ transitions of the ligands. The 1π-π∗ bands are accompanied by
weaker and lower energy features extending into the visible region from 350 to 500 nm.
These absorption are assigned to a spin-allowed metal charge transfer (1MLCT) band,
and the weaker absorption bands at the longer wavelengths can be attributed to the spin-
forbidden 3MLCT and spin-orbit coupling enhanced 3π-π∗ transition. The intensities of the
3
MLCT and π-π∗ transition bands are varied, depending on the chelating ligands. These
intensities are generally attributed to mixing of the charge-transfer transitions with high
lying spin-allowed transitions on the chelating ligand [14].
The PL spectra of the Ir complexes in CH2Cl2 solution are shown in Fig. 2. The
homoleptic complexes, Ir(pbt)3, Ir(pbt-OMe)3 and Ir(pbt-F)3, showed the PL peaks at 550,
534, and 528 nm, respectively. Ir(pbt-OMe)2(4-Me-2,3-dpq) and Ir(4-Me-2,3-dpq)2(pbt-
OMe) exhibited the emission maxima at 554 and 627 nm, respectively. Unexpectedly, the
PL spectra of the homoleptic iridium complexes were not influenced by the substituent
characteristics of the chelating ligands since both electron-withdrawing F and electron-
donating OMe substituents led the emission of their complexes to shorter wavelengths
than those of the unsubstitued Ir(pbt-H)3. On the other hand, the heteroleptic iridium
complexes have drastic contrasts, depending on the species and number of the chelating

Homoleptic vs. Heteroleptic Orange Light-Emitting Complexes
[245]/57
Figure 2. PL spectra of the iridium complexes in a 10−4 M CH2Cl2 solution.
ligands. The anion of 4-Me-2,3-dpq was reported to lead its iridium complexes to red
phosphorescence around 600 nm [11]. Ir(4-Me-2,3-dpq)2(pbt-OMe) which contains two
4-Me-2,3-dpq ligands showed a bathochromic shift, resulting in the emission at 627 nm,
while Ir(pbt-OMe)2(4-Me-2,3-dpq) which has only one 4-Me-2,3-dpq ligand exhibited the
PL maxima at 554 nm near those of the homoleptic complexes. We attribute the red-shifted
emission by Ir(4-Me-2,3-dpq)2(pbt-OMe) to inter-ligand energy transfer (ILET) from pbt-
OMe ligand to 4-Me-2,3-dpq ligands. The energy which the pbt-OMe ligand absorbs might
be transferred and contributed to the emission by 4-Me-2,3-dpq ligands which have a
smaller energy gap, resulting in a bathochromic shift in PL of their complex. ILET has
been reported to occur between the ligands of the large and small energy gaps, according
to the literatures recently published [3, 15].
We investigated the electrochemical properties of the Ir complexes by the cyclic voltam-
metry (CV) as shown in Table 1, which reveal their positions of the HOMO/LUMO
[16–17]. The oxidation potentials which indicate the HOMOs of Ir(pbt)3, Ir(pbt-OMe)3,
Ir(pbt-F)3, Ir(pbt-OMe)2(4-Me-2,3-dpq) and Ir(4-Me-2,3-dpq)2(pbt-OMe) were reversible
at 0.50–0.75 V relative to an internal ferrocenium/ferrocene reference (Fc+/Fc), respec-
tively. The LUMOs of the complexes were not observed within the electrochemical win-
dows of the solvents and thus estimated from their respective absorption spectra. They were
calculated to be between −0.76 and −2.37 eV, according to optical-edge estimation [17].
The resulting ꢀEs (Eox − Ered) of the complexes were well correlated with their tendencies
of PL maxima.
The electroluminescence (EL) of these complexes prepared in this study was attempted,
but the vacuum sublimation of the complexes to fabricate the light-emitting device was not

58/[246]
H. J. Lee and Y. Ha
Table 1. Physical parameters for the complexes
Ir complex
λem /nma Eox /Vb HOMO /eVc LUMO / eVd ꢀE /eVd
Ir(pbt)3
Ir(pbt-OMe)3
Ir(pbt-F)3
Ir(pbt-OMe)2(4-Me-2,3-dpq)
Ir(4-Me-2,3-dpq)2(pbt-OMe)
550
534
528
554
627
0.50
0.52
0.74
0.75
0.60
−4.30
−4.28
−4.06
−5.55
−4.20
−1.11
−1.01
−0.76
−2.37
−1.26
3.19
3.27
3.30
3.18
2.94
aMeasured in CH2Cl2 solution.
bscan rate: 100 mV/s, Electrolyte: tetrabutylammonium hexafluorophosphate. The potentials are
quoted against the internal ferrocene standard.
cDeduced from the equation HOMO = − 4.8 − Eox.
dCalculated from the optical edge ꢀE = LUMO - HOMO.
successful so far. We now try to fabricate devices of these orange phosphorescent complexes
by the solution process to investigate the EL performance of the complexes.
Conclusions
We have synthesized and characterized the homoleptic and heteroleptic iridium complexes
chelated with the anions of phenylbenzothiazoles having the substituents of different elec-
tronegativities. Surprisingly, the substituent, R, was found to have little influence on pho-
toluminescence of the orange phosphorescent homoleptic complexes, Ir(pbt-R)3. On the
other hand, the heteroleptic iridium complexes, Ir(OMe-pbt)2(4-Me-2,3-dpq) and Ir(4-Me-
2,3-dpq)2(OMe-pbt), exhibited quite contrasting luminescence properties. While Ir(OMe-
pbt)2(4-Me-2,3-dpq) showed the emission maxima at 554 nm, similar to the homoleptic
ones, Ir(4-Me-2,3-dpq)2(OMe-pbt) which contain two red-emitting 4-Me-2,3-dpq ligands
underwent ILET to display red phosphorescence at 627 nm. The electrochemical mea-
surement of these complexes supported their luminescence characteristics. The orange
light-emitting iridium complexes developed in this study are now under investigation as
candidates for the possible application to orange phosphors for white OLEDs.
Acknowledgment
This research was supported by the Korea Research Foundation (NO. 2011- 0003765).
References
[1] Tang, C. W., & Vanslyke, S. A. (1987). Appl. Phys. Lett., 16, 913.
[2] D’Andrade, B. W., & Forrest, S. R. (2004). Adv. Mater., 16, 1585.
[3] You, Y., & Park, S. Y. (2009). Dalton Trans., 1267.
[4] Chen, K., Yang, C.-H., Chi, Y., Liu, C.-S., Chang, C.-H., Chen, C.-C., Wu, C.-C., Chung, M.-W.,
Cheng, Y.-M., Lee, G.-H., & Chou, P.-T. (2010). Chem. Eur. J., 16, 4315.
[5] Li, J., & Liu, D. (2009). J. Mater. Chem., 17, 7584.
[6] Reneke, S., Lindner, F., Schwartz, G., Seidler, N., Walzer, K., Lu¨ssem, B., & Leo, K. (2009)
Nature, 459, 234.
[7] Sasabe, H., Takamatsu, J., Watanabe, S., & Kido, J. (2010). Adv. Mater., 22, 5003.
[8] Seo, J. H., Park, H.-R., Kim, Y. K., & Ha, Y. (2011). J. Nanosci. Nanotechnol., 11, 7198.
[9] Zhang, L., Li, B., Shi, L., & Li, W. (2009). Optical Mater., 31, 905.

Homoleptic vs. Heteroleptic Orange Light-Emitting Complexes
[247]/59
[10] Wu, F. I., Su, H. J., Shu, C. F., Luo, L., Diau, W. F., Cheng, C. H., Duan, J. P., & Lee, G. H.
(2005). Mater. J. Chem., 15, 1035.
[11] Park, G. Y., & Ha, Y. (2008). Syn. Metals., 158, 120.
[12] Lee, S. J., Park, K.-M., Yang, K., & Kang, Y. (2009). Inorg. Chem., 48, 1030.
[13] Nonoyama, M. (1975). Organomet. Chem., 86, 263.
[14] Holmes, R. J., D’Andrade, B. W., Forrest, S. R., Ren, X., Li, J., & Thompson, M. E. (2003).
Appl. Phys. Lett., 83, 3813.
[15] Park, H.-R., & Ha, Y. (2012). J. Nanosci. Nanotechnol., 12, 1365.
[16] Thomas, K. R. J., Velusamy, M., Lin, J. T., Chien, C., Tao, Y., Wen, Y. S., Hu, Y., & Chou, P.
(2005). Inorg. Chem., 44, 5677.
[17] Zhao, Q., Jiang, C., Shi, M., Li, F., Yi, T., Cao Y., & Huang, C. (2006). Organometallics, 25,
3631.