New sky-blue and bluish-green emitting Ir(III) complexes containing an azoline ancillary ligand for highly efficient PhOLEDs

Article abstract of DOI:10.1016/j.dyepig.2016.03.053

Two Ir(III) complexes containing the chromophoric ancillary ligands 2-(4,5-dihydrooxazol-2-yl)phenol and 2-(1-ethyl-4,5-dihydro-1H-imidazol-2-yl)phenol, and a highly functionalized phenylpyridine derivative, 3-(4-(tert-butyl)pyridin-2-yl)-2,6-difluorobenzonitrile, as a cyclometalating ligand were designed and synthesized. The oxazoline/imidazoline heterocycle of the ancillary ligand has the effect of enhancing the metal to ligand charge transfer transition nature of the emitting excited state and the fluorine and cyano substituents on the ligand have enriched the intersystem crossings, as indicated by the experimental photoluminescence analysis. As a result, the oxazoline and imidazoline containing complexes exhibited high photoluminescence quantum yields of about >90% with bright sky-blue emission at 480 nm and bluish-green light at 495 nm, respectively, along with excellent thermal/morphological stability about 400 °C and good solubility, that make them suitable for both wet- and dry-processes. In particular, the phosphorescent OLEDs fabricated by a dry-process showed the maximum EQEs of 21.9% and 19.7% for the oxazoline and imidazoline containing complexes, respectively.

Full text of DOI:10.1016/j.dyepig.2016.03.053

Accepted Manuscript
New sky-blue and bluish-green emitting Ir(III) complexes containing an azoline
ancillary ligand for highly efficient PhOLEDs
Ganguri Sarada, Bomi Sim, Woosum Cho, Juho Yoon, Yeong-Soon Gal, Jang-Joo
Kim, Sung-Ho Jin
PII:
S0143-7208(16)30121-8
10.1016/j.dyepig.2016.03.053
DYPI 5178
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 23 February 2016
Revised Date: 30 March 2016
Accepted Date: 31 March 2016
Please cite this article as: Sarada G, Sim B, Cho W, Yoon J, Gal Y-S, Kim J-J, Jin S-H, New sky-blue
and bluish-green emitting Ir(III) complexes containing an azoline ancillary ligand for highly efficient
PhOLEDs, Dyes and Pigments (2016), doi: 10.1016/j.dyepig.2016.03.053.
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Graphical Abstract

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New Sky-blue and Bluish-green Emitting Ir(III) Complexes Containing an Azoline
Ancillary Ligand for Highly Efficient PhOLEDs
Ganguri Sarada,a Bomi Sim,c Woosum Cho,a Juho Yoon,a Yeong-Soon Gal,b Jang-Joo Kim,*c
and Sung-Ho Jin*a
a Department of Chemistry Education, Graduate Department of Chemical Materials, and Institute
for Plastic Information and Energy Materials, Pusan National University, Busan, 609-735,
Republic of Korea, E-mail: shjin@pusan.ac.kr
b Polymer Chemistry Laboratory, College of Engineering, Kyungil University, Gyeongsan 712-
701, Republic of Korea.
c
Department of Materials Science and Engineering, Seoul National University, Seoul 151-744,
Republic of Korea, E-mail: jjkim@snu.ac.kr
† Electronic supplementary information (ESI) available.

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Abstract: Two Ir(III) complexes containing the chromophoric ancillary ligands 2-(4,5-
dihydrooxazol-2-yl)phenol and 2-(1-ethyl-4,5-dihydro-1H-imidazol-2-yl)phenol, and a highly
functionalized phenylpyridine derivative, 3-(4-(tert-butyl)pyridin-2-yl)-2,6-difluorobenzonitrile,
as a cyclometalating ligand were designed and synthesized. The oxazoline/imidazoline
heterocycle of the ancillary ligand has the effect of enhancing the metal to ligand charge transfer
transition nature of the emitting excited state and the fluorine and cyano substituents on the
ligand have enriched the intersystem crossings, as indicated by the experimental
photoluminescence analysis. As a result, the oxazoline and imidazoline containing complexes
exhibited high photoluminescence quantum yields of about >90% with bright sky-blue emission
at 480 nm and bluish-green light at 495 nm, respectively, along with excellent
thermal/morphological stability about 400 °C and good solubility, that make them suitable for
both wet- and dry-processes. In particular, the phosphorescent OLEDs fabricated by a dry-
process showed the maximum EQEs of 21.9% and 19.7% for the oxazoline and imidazoline
containing complexes, respectively.
Keywords: phosphorescent organic light-emitting diode; iridium complex; rigidity; intersystem
crossings; photoluminescence quantum yields; external quantum efficiency

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1. Introduction
Organic light-emitting diodes (OLEDs) are one of the most successful devices in the field of
organic electronics with rapid commercialization in both display and lighting applications [1,2].
Phosphorescent materials play a key role as emitters for fabrication of highly efficient OLEDs as
they can harvest both singlet (S1) and triplet (T1) excitons through strong spin-orbit coupling
(SOC) assisted intersystem crossings (ISC) [3]. Among the heavy metal complexes, neutral
iridium(III) complexes are the most famous phosphors for application in phosphorescent OLEDs
(PhOLEDs) because of their high color tunability, short excited-state lifetime and thermal
stability [4-6]. In addition, Ir(III) complexes must possess high photoluminescence quantum
yields (PLQYs) for attaining high performance PhOLEDs.
Heteroleptic Ir(III) complexes are synthetically easier to access than the homoleptic
counterparts, though the latter are more stable due to their rigidity. Chromophoric ancillary
ligands are a new class of ligands that are anticipated to yield high PLQYs compared with the
non-chromophoric ones because the Ir(III) complexes with chromophoric ancillary ligands can
harvest more energy by excitation of both the ancillary ligand and the cyclometalating (C^N)
ligand [7]. Furthermore, Ir(III) complexes with these type of ancillary ligands are expected to
have high rigidity and thermal stability thus being comparable with the homoleptic analogs.
However, very limited research was carried out so far on this class of Ir(III) complexes [8-10].
An exemplary type of chromophoric ligand; 2-(2-hydroxyphenyl) benzazole has good electron
accepting nature, therefore, is widely used in Al(III), Be(II), and Zn(II) complexes to obtain
efficient electron transporting materials, host materials and emitters for OLED applications [11-
17]. A comprehensive experimental and theoretical study by S. Y. Park et al. on a series of Ir(III)
complexes containing 2-(2-hydroxyphenyl)oxazole based chromophoric ancillary ligands and a

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difluorophenylpyridine (dfppy) based C^N ligand revealed that emission originates from the
ancillary ligand centered 3LAC state by an interligand energy transfer (ILET) from C^N ligand to
the ancillary ligand exhibiting a weak interaction with MLCT [8]. Conversely, a large
contribution of MLCT in the emitting (lowest) excited state is very important to achieve high
radiative relaxation and PLQYs [18-24]. Z. Wu et al. have carried out computational studies on a
series of Ir(III) complexes containing 2-hydroxyphenyl attached oxazoline or imidazoline based
ancillary ligands and phenylpyridine (ppy) as C^N ligand [10]. Their study elucidates that these
Ir(III) complexes possess a large contribution of MLCT in the emitting excited state and a small
S1-T1 splitting energy (∆ES1-T1), unlike the previous study, and are thus, proposed to have higher
PLQYs than the other complexes with less MLCT contribution.
We anticipate that replacement of oxazole with oxazoline would enhance the MLCT
contribution in the Ir(III) complexes by raising 3LAC energy level close to MLCT level due to the
reduced conjugation in the heterocycle, yet maintains the advantages of the chromophoric
ancillary ligands. Based on this, we designed and synthesized two Ir(III) complexes with 2-(4,5-
dihydrooxazol-2-yl)phenol (oz) and 2-(1-ethyl-4,5-dihydro-1H-imidazol-2-yl)phenol (im) as
ancillary ligands and the ppy scaffold that was highly derivatized with electron-withdrawing (-F
and -CN) and electron-donating (tert-butyl) groups as C^N ligand, and well-studied their
photophysical process.
The new Ir(III) complexes possess high PLQYs, good solubility in common organic
solvents, and high thermal and morphological stabilities making them suitable for solution-
process as well as vapor deposition techniques for further commercialization purposes. We
fabricated monochromatic PhOLEDs by both solution-process and vapor deposition methods by
employing the new Ir(III) complexes as dopants and achieved the high external quantum

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efficiency (EQE) of 21.9% for the sky-blue emitting oz-based complex (tBuCN-FIroz) and
19.7% for the bluish-green emitting im-based complex (tBuCN-FIrim) in vapor deposition
method.
2. Experimental Section
Synthesis of 2-(4,5-dihydrooxazol-2-yl)phenol (oz). Methyl 2-hydroxybenzoate (1) (44 g, 0.29
mol) and ethanolamine (18.4 g, 0.29 mol, 18 mL) were reacted at 150 °C for 2 h. During the
reaction, methanol was evolved and which was collected in a Dean–Stark trap. A red-brown
viscous liquid was obtained and it was trituration in benzene to yield a pink solid. A white
crystals of N-(2-hydroxyethyl)-2-hydroxybenzamide (2) (27 g, 52%) were obtained after
recrystallization in ethyl acetate. To a solution of 2 (27 g, 0.15 mol) in chloroform (300 mL),
thionyl chloride (59 g, 0.5 mol) was added dropwise at 0 °C and the reaction was carried out
overnight at room temperature. The precipitated pink solid was filtered off, washed several times
with chloroform, and dried under vacuum. To the intermediate 2-(2-hydroxyphenyl)-2-
oxazolinium chloride (21 g, 71%) and water (200 mL), saturated aqueous NaHCO3 was added.
The precipitated solid was isolated (14 g). Color: Light pink solid. Yield: 81%. M.P.: 39 °C. IR
data (KBr) cm-1: 3074, 2990, 2953, 2885, 1641, 1493, 1371, 1252, 1231, 1152, 1127, 1067, 940,
1
800. H NMR (300 MHz, CD3OD, δ): 7.64-7.62 (d, J=7.8 Hz, 1H, Ar-H), 7.38-7.33 (t, J=15.3
Hz, 1H, Ar-H), 6.93-6.90 (d, J=8.4 Hz, 1H, Ar-H), 6.88-6.83 (t, J=14.7 Hz, 1H, Ar-H), 4.45-4.39
(t, J=18.6 Hz, 2H, Oz-CH2), 4.08-4.02 (t, J=18.6 Hz, 2H, Oz-CH2). 13C NMR (75 MHz, CD3OD,
δ): 165.91, 159.46, 132.90, 127.68, 118.30, 116.00, 110.506, 66.65, 52.94. Anal. Calcd for
C9H9NO2: C, 66.25; H, 5.56; N, 8.58. Found C, 65.89; H, 5.77; N, 8.82. UV-Vis: λmax (CHCl3):

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305 nm. Fluorescence: excitation λmax: 305 nm, emission λmax (CHCl3): 460 nm, stokes shift: 155
nm.
Synthesis of 2-(1-ethyl-4,5-dihydro-1H-imidazol-2-yl)phenol (im). A mixture of Methyl 2-
hydroxybenzoate (1) (2 g, 13.1 mmol) and N-Ethyl ethylenediamine (3.48 g, 39.5 mmol) was
refluxed overnight. The unreacted N-Ethyl ethylenediamine was evaporated off under vacuum. A
yellow viscous gel was obtained after cooling the residue to room temperature. It was triturated
in a small amount of hexane to yield white solid, which was further purified by crystallization in
methanol to obtain pure product (1.15 g). Color: White solid. Yield: 46%. M.P.: 130 °C. IR data
(KBr) cm-1: 3239, 3049, 2980, 2938, 2696, 2480, 1630, 1538, 1456, 1440, 1302, 1238, 1190,
1
1139, 872, 765, 687. H NMR (300 MHz, CD3OD, δ): 7.78-7.75 (d, J=8.1 Hz, 1H, Ar-H), 7.30-
7.25 (t, J=13.5 Hz, 1H, Ar-H), 6.83-6.80 (d, J=8.1 Hz, 1H, Ar-H), 6.74-6.69 (t, J=14.7 Hz, 1H,
Ar-H), 3.61-3.57 (t, J=11.7 Hz, 2H, Im-CH2), 2.97-2.93 (t, J=11.7 Hz, 2H, Im-CH2), 2.85-2.78
13
(m, 2H, N-CH2), 1.21-1.16 (t, J=14.7 Hz, 3H, N-CH2CH3). C NMR (75 MHz, CD3OD, δ):
170.57, 163.52, 133.15, 128.14, 118.92, 116.53, 116.42, 47.03, 42.97, 37.30, 12.12. Anal. Calcd
for C11H14N2O: C, 69.45; H, 7.42; N, 14.73. Found C, 69.83; H, 7.68; N, 14.29. UV-Vis: λmax
(CHCl3): 306 nm. Fluorescence: excitation λmax: 305 nm, emission λmax (CHCl3): 439 nm, stokes
shift: 133 nm.
General procedure for synthesis of the Ir(III) complexes. To a solution of dimer (3) (0.9 g,
0.58 mmol) in 1,4-dioxane (30 mL), the ancillary ligand (oz) (0.95 g, 5.8 mmol) and anhydrous
K2CO3 (0.405 g, 2.9 mmol) were added and refluxed for 6 h at 105 °C under N2 atmosphere. HCl
(1N, 10 mL) was added to the reaction mixture and extracted with CH2Cl2. Organic layer was
dried over anhydrous Na2SO4 and concentrated under vacuum. The crude product was purified
by column chromatography on silica gel (EtOAc:Hexane, 3:7 v/v) to obtain the final compound.

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Bis{3-[4-(tert-butyl)pyridin-2-yl]-2,6-difluorobenzonitrile}(2-(4,5-dihydrooxazol-2-
yl)phenolate)iridium(III) [tBuCN-FIroz]. Color: Yellow powder. Yield: 50%. M.P.: not
detected up to 390 °C. IR data (KBr) cm-1: 2962, 2231, 1612, 1535, 1470, 1441, 1410, 1351,
1
1295, 1235, 1035, 851. H NMR (300 MHz, CDCl3, δ): 8.64-8.62 (d, J=6 Hz, 1H, Ar-H), 8.32-
8.29 (d, J=8.1 Hz, 2H, Ar-H), 8.20-8.18 (d, 1H, J=6 Hz, Ar-H), 7.64-7.62 (d, J=7.8 Hz, 1H, Ar-
H), 7.34-7.32 (d, J=4.2 Hz, 1H, Ar-H), 7.24-7.22 (d, J=6 Hz, 1H, Ar-H), 7.19-7.16 (d, J=8.7 Hz,
1H, Ar-H), 6.71-6.68 (d, J=8.1 Hz, 1H, Ar-H), 6.46-6.41 (t, J=14.1 Hz, 1H, Ar-H), 5.94-5.92 (d,
J=8.1 Hz, 1H, Ar-H), 5.74-5.71 (d, J=8.7 Hz, 1H, Ar-H), 4.42-4.27 (m, 2H, Oz-CH2), 3.62-3.52
13
(m, 1H, Oz-CH), 3.13-3.03 (q, 1H, Oz-CH), 1.45-1.42 (d, J=10.2 Hz, 18H, tBu). C NMR (75
MHz, CDCl3, δ): 168.45, 167.39, 164.56, 163.90, 163.67, 163.38, 162.42, 161.96, 161.64,
160.32, 157.96, 148.45, 147.82, 134.03, 130.04, 129.87, 129.56, 124.89, 121.21, 120.94, 120.31,
120.06, 116.14, 115.94, 115.31, 115.08, 113.77, 111.12, 110.96, 109.31, 66.57, 54.82, 35.38,
30.57. Anal. Calcd for C41H34F4IrN5O2: C, 54.90; H, 3.82; N, 7.91. Found C, 54.73; H, 3.91; N,
7.76. UV-Vis: λmax (CHCl3): 280 nm. Photoluminescence: excitation λmax: 373 nm, emission
λmax (CHCl3): 480 nm, stokes shift: 200 nm. X-ray crystallographic data: Triclinic, CCDC
1423073.
Bis{3-[4-(tert-butyl)pyridin-2-yl]-2,6-difluorobenzonitrile}(2-(1-ethyl-4,5-dihydro-1H-
imidazol-2-yl)phenolate)iridium(III) [tBuCN-FIrim]. Color: Yellow powder. Yield: 35%.
M.P.: 334 °C (decomp.). IR data (KBr) cm-1: 2968, 2231, 1605, 1571, 1538, 1483, 1470, 1435,
1423, 1410, 1333, 1290, 1251, 1033, 848. 1H NMR (300 MHz, CDCl3, δ): 9.02-9.0 (d, J=5.7 Hz,
1H, Ar-H), 8.34 (s, 1H, Ar-H), 8.08 (s, 1H, Ar-H), 8.05-8.03 (d, J=6.3 Hz, 1H, Ar-H), 7.37-7.35
(d, J=5.7 Hz, 1H, Ar-H), 7.07-7.04 (d, J=9 Hz, 1H, Ar-H), 6.82 (m, 2H, Ar-H), 6.42-6.37 (t,
J=14.7 Hz, 1H, Ar-H), 6.26-6.23 (d, J=8.7 Hz, 1H, Ar-H), 6.17-6.15 (d, J=8.7 Hz, 1H, Ar-H),

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5.66-5.63 (d, J=9 Hz, 1H, Ar-H), 3.76-3.66 (m, 2H, Im-CH2), 3.20-3.11 (m, 1H, Im-CH), 3.04-
2.92 (m, 2H, N-CH2), 2.76-2.65 (m, 1H, Im-CH), 1.49 (s, 9H, tBu), 1.32 (s, 9H, tBu), 1.24-1.17
13
(t, J=15 Hz, 3H, N-CH2CH3). C NMR (75 MHz, CDCl3, δ): 171.10, 168.50, 166.26, 163.98,
163.61, 162.83, 149.49, 148.52, 132.52, 130.02, 129.81, 128.41, 123.43, 120.48, 120.16, 119.82,
119.56, 119.34, 116.56, 116.34, 116.20, 115.48, 115.28, 114.12, 111.48, 50.79, 49.60, 46.12,
35.57, 35.22, 30.76, 30.45, 14.18. Anal. Calcd for C43H39F4IrN6O: C, 55.89; H, 4.25; N, 9.06.
Found C, 55.70; H, 4.19; N, 9.19. UV-Vis: λmax (CHCl3): 281 nm. Photoluminescence: excitation
λmax: 373 nm, emission λmax (CHCl3): 495 nm, stokes shift: 213 nm. X-ray crystallographic data:
Monoclinic, CCDC 1423074.
3. Results and Discussion
3.1 Synthesis and Characterization
The detailed synthetic route and the chemical structures of the new Ir(III) complexes: tBuCN-
FIroz and tBuCN-FIrim are given in Scheme 1. Ancillary ligand oz and the dimer (3) were
synthesized as per the reported procedures [25,26]. The ancillary ligand im was synthesized as
13
per the modified procedure [27]. The new Ir(III) complexes were well characterized by 1H, C
NMR (Fig. S1-S4, ESI†), IR spectra (Fig. S5, ESI†) and single crystal XRD for structural
confirmation (Table S1 & S2, ESI†).
3.2. Single Crystal XRD Characterizations
tBuCN-FIroz (CCDC 1423073) and tBuCN-FIrim (CCDC 1423074) possess distorted
octahedral geometry around the iridium center with the two C^N ligands positioned N,N-trans
and C,C-cis pattern (Fig. 1). As shown in Fig. S6 (ESI†), molecules of tBuCN-FIroz pack with
a few intermolecular contacts (2.93 and 2.84 A°) by C-H….π interactions, whereas those of

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tBuCN-FIrim pack with several intermolecular contacts and relatively stronger C-H….F
interactions (2.64, 2.87 A° and 2.7, 2.84 A°). The ancillary ligand of tBuCN-FIrim is
configurationally distorted with torsion angles of -31.71° [N5-C39-C38-C33] and -30.37° [N6-
C39-C38-C37] due to the presence of alkyl group. Such distortion is not observed in the case of
tBuCN-FIroz as from the torsion angles 4.72° [C9-C4-C3-N1] and 2.52° [C5-C4-C3-O1]
indication its rigid nature.
3.3. Thermal Properties
Thermal stabilities of the Ir(III) complexes were evaluated by measuring the decomposition (Td,
at 5% weight loss) and the glass transition (Tg) temperatures using thermogravimetric analysis
(TGA) and derivative thermogravimetric analysis (DTA), respectively. Unlike that of the already
reported heteroleptic Ir(III) complexes with conventional ancillary ligands such as
acetylacetonate and picolinate, Td of tBuCN-FIroz and tBuCN-FIrim is very high about 423
and 403 °C (Fig. 2a), respectively, and Tg was not observed within 300 °C (see Fig. 2b & c).
Thermal stability of tBuCN-FIroz is higher than tBuCN-FIrim due to the high rigidity of the
ancillary ligand, oz. The high thermal and morphological stabilities of tBuCN-FIroz and
tBuCN-FIrim render them suitable for both thermal evaporation and solution-process with
annealing treatments.
3.4. Photophysical Properties
UV-visible absorption and the photoluminescence (PL) spectra of tBuCN-FIroz and tBuCN-
FIrim measured in CHCl3 (10-5 M) at room temperature are given in Fig. 3a. tBuCN-FIroz
emits bright sky-blue light at the maximum PL (PLmax) intensity of 480 nm and tBuCN-FIrim
emits bluish-green light at PLmax of 495 nm in a structureless manner with a single peak
indicating MLCT contribution in the emitting excited state [28]. Furthermore, tBuCN-FIroz and

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tBuCN-FIrim demonstrated positive solvatochromism (Fig. S7, ESI†) with a variation in PLmax
of about 21-24 nm depending on the solvent polarity supporting the charge transfer nature of the
emission [29,30]. In contrast, the emission pattern of tBuCN-FIroz and tBuCN-FIrim
resembles the emission of their respective ancillary ligands as shown in Fig. 3b. This indicates
that emission from these Ir(III) complexes originates from the ancillary ligand centred excited
state. As shown in Fig. 3c & d, overlap of the PL spectrum of the free C^N ligand with the UV
spectra of the ancillary ligands supports the energy transfer from C^N ligand to the ancillary
ligand, while the reverse has not occurred. In order to understand the detailed mechanism of the
photophysical process, we calculated the excited state S1 & T1 energy levels of tBuCN-FIroz
and tBuCN-FIrim from the RT and low temperature PL analysis (Fig. 4).
The schematic energy level diagram, given in Fig. 5, illustrates the possibility of ILET
3
from the C^N ligand centred-3MLCCT or LCC state to the ancillary ligand centred-3LAC state
because of the higher energy of C^N ligand centred state. This indicates that emission of tBuCN-
3
FIroz and tBuCN-FIrim originates from LAC state. However, the lowest excited state (3LAC)
energy level of tBuCN-FIroz (2.66 eV) and tBuCN-FIrim (2.58 eV) is elevated relative to the
reported Ir(III) complex with oxazole-based ancillary ligand (complex 1, 2.48 eV) [8], this gets
1
3LAC level closer to the 3MLCCT/3LCC and MLCCT states (Fig. 5) indicating the possibility of
MLCT contribution in the lowest excited state, which is very important for achieving high
PLQYs. This MLCT contribution is further strengthened by the theoretical calculations of Z. Wu
et al. on Ir(III) complexes with the similar ancillary ligands [10]. In addition, the substituents (–F
and –CN) on the C^N ligand are expected to increase the ISC rates, which is very essential to
improve the internal quantum efficiency (IQE) of the Ir(III) complexes. As a matter of fact, the
C^N ligand showed significant phosphorescence even in dilute solutions at low temperature (see

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Fig. 4g) as a representation of strong SOC. In conclusion, this type of energy harvesting process
by both the C^N ligand and the chromophoric ancillary ligand along with the enhanced ISC and
MLCT nature are advantageous to obtain bright emissions with high PLQYs.
Emission from the neat films of tBuCN-FIroz and tBuCN-FIrim is red shifted by about
19-22 nm in comparison with the PLmax in solution, which is rational considering the strong
solid-state interactions described in XRD section. As anticipated, tBuCN-FIroz and tBuCN-
FIrim have exhibited high absolute PLQYs of 90±3% and 89±3%, respectively. The horizontal
dipole orientations (Θ) measured for tBuCN-FIroz and tBuCN-FIrim as a thin films of
pBCb2Cz:TSPO1:10% Ir(III) complex are 0.78 and 0.73, respectively (Fig. S8, ESI†). A high Θ
value of the emitter enhances the light out-coupling efficiency of the PhOLEDs [26,31].
3.5. Electrochemical and DFT Calculations
Highest occupied molecular orbital (HOMO) energy levels of tBuCN-FIroz and tBuCN-FIrim
were calculated from the oxidation peak potentials obtained from cyclic voltammetry (CV)
analysis (Fig. S9, ESI†). Optical band gaps (Eg) were calculated from the UV-absorption edge
and subsequently the lowest occupied molecular orbital (LUMO) energy levels were calculated
from the HOMO and Eg (see the data in Table 1). The HOMO energy levels calculated using
density functional theory (DFT) are -5.29 and -5.18 eV for tBuCN-FIroz and tBuCN-FIrim,
respectively, agree with the experimental values. From the DFT analysis (Fig. 6), HOMO is
distributed on phenolate part of the ancillary ligand and iridium metal centre; whereas LUMO is
located mostly on one of the C^N ligands as common for both the Ir(III) complexes, and slightly
on oxazoline ring for tBuCN-FIroz and on imidazoline ring for tBuCN-FIrim, which is well
matched with the reported analogous molecules [9]. Permanent dipole moment (µ) of tBuCN-
FIroz is 12.04D and tBuCN-FIrim is 13.23D, obtained from DFT analysis. The slight increase

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in µ for tBuCN-FIrim is the reason for its close packing in the solid-state with the shortest
intermolecular separation (Fig. S6) [31], consequently PLmax of tBuCN-FIrim is red shifted
relative to tBuCN-FIroz.
3.6. Sky-blue and Bluish-green Emitting PhOLEDs
3.6.1. PhOLEDs via Vapor Deposition
The optimized PhOLEDs, where tBuCN-FIroz (device A) and tBuCN-FIrim (device B) were
used as dopants, have the configuration of indium tin oxide (ITO) (70 nm)/ReO3 (1
nm)/pBCb2Cz (40 nm)/pBCb2Cz:TSPO1:10 wt% Ir(III) complex (30 nm)/TSPO1 (35 nm)/Al
(100 nm). Here, 9-(4-(9H-pyrido[2,3-b]indol-9-yl)phenyl)-9H-3,9’-bicarbazole (pBCb2Cz) was
used as hole transporting layer (HTL), diphenylphosphine oxide-4-(triphenylsilyl)phenyl
(TSPO1) as electron transporting layer (ETL) and pBCB2Cz:TSPO1:Ir(III) complex as emitting
layer (EML). The hosts were selected to have higher T1 energy levels (pBCb2Cz (T1=2.93 eV)
and TSPO1 (T1=3.4 eV)) than the two Ir(III) complexes (T1<2.7 eV).
The current density-voltage-luminance (J-V-L) and the EQE-L characteristics of the
PhOLEDs are given in Fig. 7a and b. As the HOMO-LUMO energy levels of both the Ir(III)
complexes are close to each other, J-V characteristics of the corresponding devices have similar
trends with the same turn-on voltage (Von) of 3.3 V and current densities. The maximum EQEs
of the PhOLEDs with tBuCN-FIroz and tBuCN-FIrim are 21.9% and 19.7%, and the EQEs at
1000 cd m-2 are 20.1% and 17.4%, respectively. This high efficiency of the tBuCN-FIroz based
device is mainly due to the high PLQY and Θ of the emitter in the thermally deposited films. The
electroluminescence (EL) spectrum of the tBuCN-FIroz based device is almost the same as the
PL spectrum in solution resulting in the sky-blue CIE coordinates (0.149, 0.349), whereas the
bluish-green coordinates (0.214, 0.504) were observed for the tBuCN-FIrim based device at 10

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mA cm-2 (Fig. 7c & d). These color coordinates are unchanged throughout the measured current
densities.
3.6.2. PhOLEDs via Solution Process
We fabricated solution-processed PhOLEDs driven by the excellent solubility and morphological
stability of the two Ir(III) complexes. The two optimized PhOLEDs have the following
configuration; ITO/GraHIL (40 nm)/TCTA:DCzPPy:Ir(III) complex (10 wt%, 40 nm)/TPBi (30
nm)/LiF (1 nm)/Al (100 nm) [device C:tBuCN-FIroz, device D:tBuCN-FIrim]. Device
structure and the energy level diagram of vapor deposition and solution-process are given in Fig.
S10 (ESI†).
Here, we used gradient hole injection layer (GraHIL) instead of the conventional
PEDOT:PSS (AI 4083), as the former has merits of tuning the surface work function by self-
organization, therefore, can effectively reduce the energy barrier for injection of holes into the
EML. We incorporated a self-organized GraHIL composed of PEDOT:PSS and a perfluorinated
polymeric
acid,
tetrafluoroethyleneperfluoro-3,6-dioxa-4-methyl-7-octene-sulfonic
acid
copolymer (PFI), with a low surface energy in 4:1 weight ratio, respectively. This ratios of
GraHIL offers work function of ~5.6 eV, which is higher than PEDOT:PSS (~5.2 eV) [32,33].
Furthermore, PFI can shield the EML surface from the detrimental degradation by the acidic
PEDOT:PSS. A mixed host containing tris(4-carbazoyl-9-ylphenyl)amine (TCTA) as the hole
transporting and 2,6-bis(3-(carbazol-9-yl)phenyl)pyridine (DCzPPy) as the electron transporting
material was optimized to a 4:6 ratio, respectively, for better confinement of excitons in EML.
The optimal dopant ratio was 10 wt% similar to the dry process.
The J-V-L and the EQE-L characteristics of the PhOLEDs are given in Fig. 8a and b. The
maximum EQEs of the PhOLEDs with tBuCN-FIroz and tBuCN-FIrim are 15.5% and 19.8%,

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respectively. Here, EQE of tBuCN-FIrim based device is comparable to the EQE obtained in the
vapor deposition process. Even though the EQE of tBuCN-FIroz based solution-processed
device is a little lower than the corresponding vapor deposition result, the overall EQE is
maintained moderately high. The T1 energy level of tBuCN-FIroz (T1=2.66 eV) has a very small
energy difference with TCTA (T1=2.78 eV) and DCzPPy (T1=2.71 eV) compared to tBuCN-
FIrim (T1=2.58 eV). This might have caused poor energy transfer from the mixed host to the
guest in case of tBuCN-FIroz, even though the Von (4.5 eV) and current densities are similar for
both the PhOLEDs. Moreover, we analysed the surface morphology and film formation of the
EML of the two devices using the following configuration: ITO/GraHIL/EML by atomic force
microscopy (AFM) (Fig. S11, ESI†). The root mean square (RMS) roughness of the EML
surface for tBuCN-FIroz and tBuCN-FIrim are 1.12 and 0.93 nm, respectively. From this, it is
obvious that the N-ethyl group in the ancillary ligand of tBuCN-FIrim has enhanced the
solubility and film formation during spin-coating. As shown in Fig. 8c, the ELmax are slightly (4-
6 nm) red shifted in both the PhOLEDs as compared to the solution PL spectra resulting in the
CIE coordinates (0.152, 0.406) for tBuCN-FIroz and (0.215, 0.561) for tBuCN-FIrim at 10 mA
cm-2. These coordinates are almost unchanged even at 30 mA cm-2. Results of the PhOLEDs
fabricated in vapor deposition and solution process are given Table 2.
4. Conclusion
We designed and synthesized two Ir(III) complexes tBuCN-FIroz and tBuCN-FIrim with high
PLQYs, excellent thermal and morphological stabilities, and good solubility in common organic
solvents. Experimental observation of the photophysical process revealed that emission of
tBuCN-FIroz and tBuCN-FIrim originates from the ancillary ligand centered excited state
(3LAC) with substantial mixing of MLCT nature, by harvesting the excited state energy of both

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C^N and ancillary ligands via ILET from C^N ligand to the low energy chromophoric ancillary
ligand. The oxazoline and/or imidazoline heterocycle in the ancillary ligand not only enriched
the MLCT contribution but also improved the rigidity of tBuCN-FIroz and tBuCN-FIrim. The
substituents on C^N ligand (fluoro & cyano) have an effect of increasing intersystem crossings
evidently from its (free C^N ligand) strong phosphorescence even in dilute solutions at low
temperature as a representation of strong SOC. The monochromatic PhOLEDs fabricated with
tBuCN-FIroz and tBuCN-FIrim as dopants via both solution-process and vapor deposition
methods have resulted in the high efficiencies. Especially by vapor deposition process, the high
EQE of 21.9% for tBuCN-FIroz with sky-blue CIE coordinates (0.149, 0.349) and 19.7% for
tBuCN-FIrim with bluish-green coordinates (0.214, 0.504) were obtained. The PhOLED with
tBuCN-FIroz exhibited a very minute roll-off in efficiency with EQE of 20.1% at 1000 cd m-2.
Acknowledgments
This work was supported by grant fund from the National Research Foundation (NRF) (2011–
0028320) and the Pioneer Research Center Program through the NRF (2013M3C1A3065522) by
the Ministry of Science, ICT & Future Planning (MSIP) of Korea. This work
(2014R1A2A1A01002030) was supported by Mid-career Researcher Program through NRF
(National Research Foundation) grant funded by the MSIP (Ministry of Science, ICT and Future
Planning).
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Scheme 1. Synthetic route of tBuCN-FIroz and tBuCN-FIrim.

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FIGURE CAPTIONS
Fig. 1. ORTEP representation of tBuCN-FIroz (left) and tBuCN-FIrim (right). [Hydrogen
atoms are omitted for clarity]
Fig. 2. (a) TGA curves; (b, c) DTA curves of the Ir(III) complexes at a heating rate of 10 °C min-
1 under N2 atmosphere.
−5
Fig. 3. (a) UV-Visible absorption & PL spectra in CHCl3 (10 M), and neat film PL of the
Ir(III) complexes; (b) PL spectra of the free ligands along with the Ir(III) complexes; (c, d)
Representation of energy transfer from C^N ligand to the ancillary ligands. (RT=room
temperature, f=film).
Fig. 4. Measurements of the excited state S1 & T1 energy levels from the PL study at room
temperature (RT) and 77K (2-Me THF).
Fig. 5. Schematic representation of the photophysical process involved in tBuCN-FIroz and
tBuCN-FIrim.
Fig. 6. DFT calculated HOMO-LUMO orbital distribution of tBuCN-FIroz and tBuCN-FIrim.
[Hydrogen atoms were omitted for clarity]
Fig. 7. (a) J−V−L characteristics; (b) EQE–L characteristics; (c) EL versus PL; (d) CIE
coordinates of the optimized PhOLEDs in vapor deposition process.
Fig. 8. (a) J−V−L characteristics; (b) EQE–L characteristics; (c) EL versus PL of the optimized
PhOLEDs in solution-process.

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Fig. 1.

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120
100
80
tBuCN-FIroz
tBuCN-FIrim
T : 423 °C
d
403 °C
60
40
(a)
20
100 200 300 400 500 600 700 800
Temperature (°C)
tBuCN-FIroz
tBuCN-FIrim
(b)
(c)
0
100 200 300 400 500
Temperature (°C)
0
100 200 300 400 500
Temperature (°C)
Fig. 2.

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tBuCN-FIroz
tBuCN-FIroz
tBuCN-FIrim
oz ligand
im ligand
C^N ligand
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
tBuCN-FIroz(f)
tBuCN-FIrim
tBuCN-FIrim(f)
@RT
@RT
(a)
(b)
300 400 500 600 700 800
Wavelength (nm)
300 375 450 525 600 675
Wavelength (nm)
1.0
1.0
C^N ligand UV
oz ligand PL
im ligand PL
C^N ligand PL
oz ligand UV
im ligand UV
@RT
1.0
1.0
0.8
0.6
0.4
0.2
0.0
0.8
0.6
0.4
0.2
0.0
0.8
0.6
0.4
0.2
0.0
0.8
@RT
0.6
0.4
0.2
(d)
(c)
0.0
250
300
350
400
450
500
300
400
500
600
700
Wavelength (nm)
Wavelength (nm)
Fig. 3.

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1.0
0.8
0.6
0.4
0.2
0.0
1.0
tBuCN-FIrim
@77K
tBuCN-FIroz
@77K
T : 2.66 eV
1
(467 nm)
T : 2.58 eV
1
0.8 (480 nm)
0.6
0.4
0.2
0.0
(a)
(b)
440
460
480
500
520
440 460 480 500 520 540 560 580
Wavelength (nm)
Wavelength (nm)
1.0
0.8
0.6
0.4
0.2
0.0
1.0
im ligand
@77K
oz ligand
@77K
1
LC: 2.85 eV
(435 nm)
1LC: 2.96 eV
(419 nm)
0.8
0.6
0.4
0.2
0.0
(c)
(d)
400 420 440 460 480 500
Wavelength (nm)
400 425 450 475 500
Wavelength (nm)
Fluorescence
C^N ligand
@77 K
C^N ligand
@RT
C^N ligand
@77K
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
1
LC: 3.75 eV
(330 nm)
1
MLCT: 2.81 eV
(441 nm)
Phosphorescence
T : 2.74 eV
1
(452 nm)
(e)
(f)
(g)
300 350 400 450 500 550
Wavelength (nm)
400 425 450 475 500
Wavelength (nm)
300
400
500
600
700
Wavelength (nm)
Fig. 4.

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Fig. 5.

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HOMO
LUMO
tBuCN-FIroz
tBuCN-FIrim
Fig. 6.

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102
101
105
30
tBuCN-FIroz
tBuCN-FIrim
tBuCN-FIroz
tBuCN-FIrim
104
25
100
10-1
10-2
10-3
10-4
10-5
10-6
103
20
15
102
10
101
5
(a)
100
0
(b)
0
1
2
3
4
5
6
7
8
10
100
Luminance
1000
cd/m2
10000
Voltage (V)
(
)
tBuCN-FIroz (PL)
tBuCN-FIrim (PL)
tBuCN-FIroz (EL)
tBuCN-FIrim (EL)
1.0
0.8
0.6
0.4
0.2
0.0
(c)
(d)
400 450 500 550 600 650 700
Wavelength (nm)
Fig. 7.