Effect of substituents on properties of diphenylphosphoryl-substituted bis-cyclometalated Ir(III) complexes with a picolinic acid as ancillary ligand

Article abstract of DOI:10.1016/j.dyepig.2017.05.056

A series of diphenylphosphoryl-substituted bis-cyclometalated Ir(III) complexes (POFIrpic, 4Me-POFIrpic, 5Me-POFIrpic, 5F-POFIrpic and 5CF₃-POFIrpic) with a picolinic acid as the ancillary ligand have been synthesized and characterized. The influence of substituents (CH₃, F, CF₃) at the pyridyl moiety of the cyclometalating ligand POFdFppy (2-(2′,4′-difluoro-3′-diphenylphosphoryl)phenylpyridine) on the photophysical and electrochemical properties of these Ir(III) complexes has been investigated. These Ir(III) complexes exhibit intense emission bands at 455–492 nm and high phosphorescence quantum yields (Φ_PL) in a range of 54–64% in CH₂Cl₂. The phosphorescence oxygen sensing properties of these complexes were studied in solution and in polymer film, and a fast response time and excellent operational stability were observed. Two-site model fitting indicates that 5Me-POFIrpic is the most sensitive oxygen sensing material among the complexes, with a quenching constant K_sv^app of 0.00793 Torr^?1. Photodegradation tests demonstrate that introduction of a CF₃ group into the cyclometalating ligand improves the photostability of the corresponding Ir(III) complex.

Full text of DOI:10.1016/j.dyepig.2017.05.056

Dyes and Pigments 145 (2017) 136e143
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Effect of substituents on properties of diphenylphosphoryl-substituted
bis-cyclometalated Ir(III) complexes with a picolinic acid as ancillary
ligand
Hongcui Yu, Chun Liu*, Xin Lv, Jinghai Xiu, JianZhang Zhao
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Linggong Road 2, Dalian 116024, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A
series of diphenylphosphoryl-substituted bis-cyclometalated Ir(III) complexes (POFIrpic, 4Me-
Received 8 April 2017
Received in revised form
28 May 2017
Accepted 30 May 2017
Available online 1 June 2017
POFIrpic, 5Me-POFIrpic, 5F-POFIrpic and 5CF3-POFIrpic) with a picolinic acid as the ancillary ligand
have been synthesized and characterized. The influence of substituents (CH3, F, CF3) at the pyridyl moiety
of the cyclometalating ligand POFdFppy (2-(20,40-difluoro-30-diphenylphosphoryl)phenylpyridine) on the
photophysical and electrochemical properties of these Ir(III) complexes has been investigated. These
Ir(III) complexes exhibit intense emission bands at 455e492 nm and high phosphorescence quantum
yields (FPL) in a range of 54e64% in CH2Cl2. The phosphorescence oxygen sensing properties of these
complexes were studied in solution and in polymer film, and a fast response time and excellent oper-
Keywords:
Cyclometalated Ir(III) complex
Diphenylphosphoryl group
Phosphorescence
ational stability were observed. Two-site model fitting indicates that 5Me-POFIrpic is the most sensitive
oxygen sensing material among the complexes, with a quenching constant K
app
of 0.00793 Torrꢀ1
.
sv
Photodegradation tests demonstrate that introduction of a CF3 group into the cyclometalating ligand
improves the photostability of the corresponding Ir(III) complex.
© 2017 Elsevier Ltd. All rights reserved.
Oxygen sensing
Photostability
1. Introduction
green and red phosphorescent complexes, the OSPs based on blue
Ir(III) complexes are rarely reported [17]. It is attractive for devel-
Oxygen detection techniques are applied in various fields,
including chemical, clinical analysis, environmental monitoring,
oceanography and life science [1e4]. Optical methods with excel-
lent performances have attracted increasing interests for the
determination of oxygen in recent years. An optical oxygen sensor
relies on the quenching of the fluorescence or phosphorescence of
an oxygen-sensitive probe (OSP) by molecular oxygen [5,6]. The
essential demands for an efficient OSP, are high phosphorescence
quantum yield (FPL), long excited state lifetime and good photo-
stability [7]. Cyclometalated Ir(III) complexes, in particular, are
promising candidates as OSPs for oxygen sensing due to their life-
times on the microsecond time-scale, generally high FPL, and
unique color-tuning capability across the entire visible spectrum
[8e11]. So far, cyclometalated Ir(III) complexes as the OSPs for ox-
ygen sensing have been explored [12e16]. However, in contrast to
oping blue OSPs with high sensitivity and FPL, which will provide a
possibility for improving the efficiency of the sensors in the entire
visible region. In addition, excellent blue OSPs could offer a chance
to enhance the sensor performance when red emitters are present
in the media or in order to perform colorimetric sensing and multi-
functionalized optical measurements [17]. However, owing to the
large emission energy gap and relatively low FPL, the development
of blue emitting Ir(III) complexes as the OSPs with high perfor-
mances remains a challenge [18e20].
The most investigated sky-blue emitting bis-cyclometalated
Ir(III) complex is iridium(III) bis[(4,6-difluorophenyl)-pyridinato-
2
0
N,C ] picolinate (FIrpic) [21]. Some popular strategies aiming to
modify the structural frame of FIrpic are adopted to tune the
emission to blue, among which the diphenylphosphoryl (Ph2PO)
substituted Ir(III) complexes are good examples of the imple-
mentation of this strategy. In view of its electron-deficient feature,
the diphenylphosphoryl group (Ph2PO) has been widely used to
construct ambipolar host materials, for its ability to improve the
electron-injecting and electron-transporting performance [22].
* Corresponding author.
E-mail address: cliu@dlut.edu.cn (C. Liu).
http://dx.doi.org/10.1016/j.dyepig.2017.05.056
0143-7208/© 2017 Elsevier Ltd. All rights reserved.

H. Yu et al. / Dyes and Pigments 145 (2017) 136e143
137
Furthermore, this strong electron-withdrawing group can tune the
emission of the corresponding Ir(III) complex toward a blue-shift.
For example, introduction of a Ph2PO group at the 3-position of
Emission intensity response of sensing films of the Ir(III) complexes
were recorded with a HITACHI F-7000 spectrofluorimeter. The
preparation of the ethylcellulose (EC) oxygen sensing films and
photostability test were performed following a protocol previously
developed by us [26]. The weight content of all Ir(III) complexes
immobilized in the EC film is 0.5 wt% and the photostability was
investigated upon continuous irradiation with a 16 W 254 nm UV
lamp (WFH-204B, the power density on the film is 23.4 W mꢀ2).
The structures of these Ir(III) complexes were optimized using
density functional theory (DFT) with the B3LYP functional and 6-
31G(d)/LanL2DZ basis set. The LanL2DZ basis set was used to treat
the iridium atom, whereas the 6-31G(d) basis set was used to treat
all other atoms. All the calculations were performed using Gaussian
09 software package [27]. CH2Cl2 was used as the solvent for the
calculations (CPCM model).
the phenyl unit for Ir(III) complexes POFIrpic [23] and Ir(dppfm)2
-
pic [24] tuned the emissions to the blue region, both of the com-
plexes exhibited excellent performances in organic light-emitting
diodes (OLEDs). Very recently, we reported several greenish-blue to
blue phosphorescent diphenylphosphoryl-substituted cationic
Ir(III) complexes with different N^N ancillary ligands [25], and it
was found that the ancillary ligand has profound effects on the
properties of the corresponding Ir(III) complexes. Similarly, effect of
the substituents at the pyridyl moiety of the diphenylphosphoryl-
substituted cyclometalating ligands on the Ir(III) complexes is
also of great interest for understanding the structure-property
relationship.
In this paper, a series of bis-cyclometalated Ir(III) complexes
with a diphenylphosphoryl group in the cyclometalating ligand and
a picolinic acid (pic) as the ancillary ligand have been synthesized
and characterized. The chemical structures of these Ir(III) com-
plexes (POFIrpic, 4Me-POFIrpic, 5Me-POFIrpic, 5F-POFIrpic and
5CF3-POFIrpic), are illustrated in Scheme 1. The influence of sub-
stituents (CH3, F, CF3) at the pyridyl moiety of the cyclometalating
ligand POFdFppy (2-(20,40-difluoro-30-diphenylphosphoryl)phe-
nylpyridine) on the photophysical and electrochemical properties
of the corresponding Ir(III) complexes has been investigated sys-
tematically. In addition, the oxygen sensitivity, operational stability
and photostability of these Ir(III) complexes were also studied.
2.2. Synthesis
Chemical structures and the detailed synthetic protocols of the
Ir(III) complexes are shown in Scheme 1. Cyclometalating ligands
and the Ir(III) complexes were synthesized according to the re-
ported method [23,24]. All the Ir(III) complexes were synthesized in
two steps from the cyclometalation of IrCl3$3H2O with the corre-
sponding ligand to form the chloride-bridged dimer initially, fol-
lowed by treatment with the picolinic acid in the presence of
Na2CO3.
2.2.1. Synthetic procedure of C^N cyclometalating ligand
2. Experimental
The different C^N cyclometalating ligands containing a diphe-
nylphosphoryl group were synthesized by the method developed
previously for analogous compounds (see Supporting Information).
5Me-POFdFppy: Yield 70.2%, white solid, Mp: 108e110 ꢁC; 1H
2.1. Materials and measurements
All starting materials were purchased from commercial sup-
pliers and used without further purification. 1H NMR and 13C NMR
spectra were recorded on a 400 MHz Varian Unity Inova spec-
trometer. Mass spectra were recorded with a MALDI micro MX
spectrometer. FT-IR spectra were recorded on a Nicolet-6700 IR
Spectrometer using KBr disks and wave numbers were given in
cmꢀ1. Melting points were determined using X-4 digital melting
point apparatus. UV-Vis absorption spectra were recorded on a
Lambda 750s spectrometer. Fluorescence spectra were recorded
NMR (400 MHz, CDCl3)
(m, 4H), 7.52e7.40 (m, 8H), 6.98 (td, J ¼ 8.9, 3.8 Hz, 1H), 2.28 (s, 3H).
13C NMR (101 MHz, CDCl3):
163.7, 161.4, 161.0, 158.9, 149.2, 147.7,
136.0, 135.7, 135.6, 132.6, 131.5, 131.4, 131.1, 130.2, 130.1, 129.7, 129.6,
127.9, 127.8, 127.6, 127.5, 124.1, 123.9, 123.0, 122.9, 112.1, 111.9, 111.8,
d 8.46 (s, 1H), 8.19e8.12 (m, 1H), 7.79e7.72
d
110.0, 109.8, 109.6, 109.1, 108.9, 108.7, 17.1. IR (KBr):
n 3060, 2972,
2925, 1607, 1583, 1464, 1439, 1402, 1198, 1117, 1096, 1050, 1002, 881,
824, 752, 723, 696, 587, 561, 521 cmꢀ1. HRMS (MALDI-TOF): calcd.
for C24H18NOF2P 405.1094, found 405.1089.
with
a
HITACHI F-7000 spectrofluorimeter. Phosphorescence
5F-POFdFppy: Yield 79.1%, white solid, Mp: 168e170 ꢁC; 1H
quantum yields were measured relative to [Ir(ppy)3] (FPL ¼ 0.4 in
CH2Cl2, under deoxygenated conditions). Phosphorescence life-
times were measured on an Edinburgh FLS920 Spectrometer. Cyclic
voltammograms of the Ir(III) complexes were recorded on an
electrochemical workstation (BAS100 B/W, USA) at room temper-
ature in a 0.1 M [Bu4N]PF6 solution under argon gas protection.
NMR (500 MHz, CDCl3)
d
8.53 (d, J ¼ 2.9 Hz, 1H), 8.28e8.11 (m, 1H),
7.86e7.74 (m, 4H), 7.70e7.64 (m, 1H), 7.59e7.53 (m, 2H), 7.53e7.45
(m, 4H), 7.44e7.36 (m, 1H), 7.09e6.97 (m, 1H). 13C NMR (126 MHz,
CDCl3)
d 164.9, 162.5, 162.2, 160.0, 159.8, 157.5, 147.6, 138.3, 138.0,
136.8, 136.6, 133.4, 132.4, 132.2, 131.2, 131.1, 128.7, 128.5, 125.5,
125.3, 124.1, 124.0, 123.9, 123.4, 123.2, 113.3, 113.1, 113.0, 111.3, 111.1,
Scheme 1. Chemical structures and synthetic routes to these Ir(III) complexes. (i) IrCl3$3H2O, EtOCH2CH2OH/H2O, 3:1 (v/v), 120 ꢁC, N2, 24 h (ii) Na2CO3, picolinic acid, CH2Cl2/EtOH,
4:1 (v/v), 40 ꢁC, N2, 12 h.

138
H. Yu et al. / Dyes and Pigments 145 (2017) 136e143
110.9, 110.4, 110.2, 110.0. IR (KBr):
n
3080, 3055, 1609, 1591, 1462,
148.5, 145.5, 144.6, 139.8, 136.2, 133.7, 133.5, 133.3, 132.6, 132.4,
132.2, 132.1, 132.0, 131.2, 131.1, 131.0, 129.4, 129.2, 128.9,128.8, 128.7,
128.6, 128.5, 126.5, 126.2, 125.9, 124.0, 123.7, 123.5, 123.3, 123.2,
1439, 1409, 1238, 1205, 1117, 1070, 1000, 824, 792, 753, 723, 696,
586, 559, 523 cmꢀ1. HRMS (MALDI-TOF): calcd. for C23H15NOF3P
409.0843, found 409.0841.
123.0, 120.4, 120.3, 116.3, 116.0, 105.0, 104.8, 104.0, 103.8. IR (KBr):
n
5CF3-POFdFppy: Yield 39.3%, white solid, Mp: 91e93 ꢁC; 1H
3059, 2965, 2927, 1667, 1619, 1597, 1523, 1331, 1286, 1183, 1139,
1094, 1055, 1020, 863, 847, 758, 723, 702, 695, 620, 589, 530,
NMR (400 MHz, CDCl3)
7.94e7.88 (m, 1H), 7.85e7.71 (m, 5H), 7.58e7.51 (m, 2H), 7.51e7.41
(m, 4H), 7.09e7.01 (m, 1H). 13C NMR (101 MHz, CDCl3):
165.4,
d
8.91 (d, J ¼ 2.2 Hz, 1H), 8.35e8.21 (m, 1H),
519 cmꢀ1
. HRMS (MALDI-TOF): calcd. for C54H32N3O4F10P2Ir
d
1231.1338, found 1232.1439 ([MþH]þ), 1270.1063 ([MþK]þ).
163.0, 162.9, 160.4, 154.8, 146.6, 137.1, 135.1, 133.8, 133.3, 132.1, 131.2,
131.1, 128.7, 128.6, 128.2, 128.1, 125.6, 125.3,124.7, 124.1, 124.0, 123.7,
123.5, 122.0, 119.3, 113.6, 113.5, 113.2, 111.7, 111.5, 111.3, 110.8, 110.6,
3. Results and discussion
110.4. IR (KBr):
n 3055, 2972, 2926, 1604, 1463, 1439, 1419, 1329,
3.1. Photophysical properties
1196, 1134, 1088, 1003, 854, 821, 777, 752, 720, 695, 591, 546, 525,
517 cmꢀ1. HRMS (MALDI-TOF): calcd. for C24H15NOF5P 459.0811,
found 459.0797.
UV-Vis absorption spectra and emission spectra of all Ir(III)
complexes in CH2Cl2 at room temperature are presented in Fig. 1
and the pertinent data are summarized in Table 1. The intense
short wavelength absorptions are primarily due to the spin-allowed
2.2.2. Synthetic procedure of Ir(III) complexes
A mixture of IrCl3$3H2O and 2.5 equiv. of C^N cyclometalating
ligand was heated at 120 ꢁC in 2-ethoxyethanol/water (3:1, v/v) for
24 h under a nitrogen atmosphere. Upon cooling to room temper-
ature, the mixture was poured into water and the formed precipi-
tate was filtered, washed by ethanol and n-hexane, to obtain the
chloro-bridged dimer. Without further purification, the dimer
was mixed with 3.0 equiv. picolinic acid and 5.0 equiv. Na2CO3 in
CH2Cl2/ethanol (4:1, v/v). The reaction mixture was stirred at 40 ꢁC
for 12 h under a nitrogen atmosphere, then the reaction mixture
was poured into brine, extracted with CH2Cl2. The organic layer was
dried over MgSO4 and the solvent was evaporated under reduced
pressure. The crude product was purified by column chromatog-
raphy on silica gel using CH2Cl2/CH3OH (10:1, v/v) as eluent to
obtain the desired Ir(III) complex.
intra-ligand (1
p-p*) transitions. The long wavelength (inset of
Fig. 1(a)) with lower extinction coefficient absorption can be
assigned to mixing among singlet and triplet metal-to-ligand
charge transfer (1MLCT and 3MLCT) transitions [28,29].
These diphenylphosphoryl-substituted bis-cyclometalated
Ir(III) complexes exhibit intense emission bands at 455e492 nm
(Fig. 1(b)). Introducing a CH3 group at the 4-position of the pyridyl
ring results in a slight blue-shift of 3 nm for 4Me-POFIrpic (lmax at
5Me-POFIrpic: Yield 70.0%, yellow-green solid; 1H NMR
(400 MHz, CDCl3):
d
8.48 (s, 1H), 8.33 (d, J ¼ 7.6 Hz, 1H), 8.02e7.93
(m, 3H), 7.84e7.69 (m, 9H), 7.56e7.40 (m, 15H), 7.09 (s, 1H), 5.74
(dd, J ¼ 9.9, 3.9 Hz, 1H), 5.58 (dd, J ¼ 10.0, 4.0 Hz, 1H), 2.29 (s, 3H),
2.08 (s, 3H). 13C NMR (101 MHz, CDCl3):
d 172.6, 164.1, 163.8, 162.6,
162.2,161.9,161.5,161.2,160.6,160.2,160.0,159.6,158.7,151.0,148.4,
148.2, 147.7, 139.5, 138.8, 134.3, 134.1, 133.8, 133.3, 133.0, 132.7,
132.0, 131.9, 131.8, 131.7, 131.2, 131.1, 129.9, 129.6, 128.8, 128.6, 128.5,
128.4,128.3,123.6123.4,123.2,123.0,116.0,115.8,103.7,103.5,103.3,
103.1, 102.7, 102.5, 102.3, 102.1, 18.4, 18.3. IR (KBr):
n 3057, 2969,
2924, 1662, 1641, 1596, 1523, 1488, 1285, 1187, 1118, 1101, 1051, 1018,
885, 865, 839, 752, 735, 722, 698, 602, 524 cmꢀ1. HRMS (MALDI-
TOF): calcd. for C54H38N3O4F4P2Ir 1123.1903, found 1124.1915
([MþH]þ).
5F-POFIrpic: Yield 48.1%, yellow-green solid; 1H NMR
(400 MHz, CDCl3)
d
8.62 (t, J ¼ 2.4 Hz, 1H), 8.38 (d, J ¼ 7.7 Hz, 1H),
8.22e8.01 (m, 3H), 7.86e7.68 (m, 9H), 7.60e7.37 (m, 15H), 7.19 (t,
J ¼ 2.5 Hz, 1H), 5.76 (dd, J ¼ 9.7, 3.8 Hz, 1H), 5.59 (dd, J ¼ 9.7, 3.8 Hz,
1H). 13C NMR (101 MHz, CDCl3):
d 172.3, 164.5, 164.1, 162.8, 162.4,
161.8,161.6,161.5,160.2,159.8,158.9,158.7,157.4,156.4,150.8,148.4,
139.5,137.4,137.1,136.9,136.6,134.0,133.6,132.9,132.8,132.5,132.1,
132.0, 131.9, 131.2, 131.0, 129.3, 129.1, 128.8, 128.7, 128.6, 128.5,
128.4, 126.5, 126.3, 125.0, 124.8, 124.6, 124.4, 116.0, 115.7, 104.5,
104.3, 104.1, 103.5, 103.3, 103.1. IR (KBr):
n 3060, 2970, 1664, 1644,
1597, 1524, 1486, 1438, 1405, 1269, 1187, 1100, 1051, 1019, 855, 750,
732, 721, 698, 601, 540, 524 cmꢀ1. HRMS (MALDI-TOF): calcd. for
C
52H32N3O4F6P2Ir 1131.1401, found 1132.1443 ([MþH]þ), 1154.1273
([MþNa]þ).
5CF3-POFIrpic: Yield 44.6%, yellow solid; 1H NMR (400 MHz,
CDCl3)
d
8.98 (s, 1H), 8.39 (d, J ¼ 7.7 Hz, 1H), 8.28 (t, J ¼ 10.2 Hz, 2H),
8.09 (td, J ¼ 7.7, 1.2 Hz, 1H), 8.01e7.88 (m, 2H), 7.88e7.64 (m, 9H),
Fig. 1. Absorption (a) and emission spectra (b) of Ir(III) complexes (1.0 ꢂ 10ꢀ5 M in
CH2Cl2) at room temperature. The inset in (a) shows the magnified spectra from 300 to
450 nm. The inset in (b) shows the emission photographs in CH2Cl2 (excited at 365 nm
by a UV lamp).
7.65e7.37 (m, 14H), 5.75 (dd, J ¼ 9.5, 3.7 Hz, 1H), 5.57 (dd, J ¼ 9.6,
3.7 Hz, 1H). 13C NMR (101 MHz, CDCl3):
d 172.0, 168.1, 166.8, 165.3,
165.0, 164.5, 164.0, 162.7, 162.4, 161.9, 161.4, 160.4, 159.4, 150.7,

H. Yu et al. / Dyes and Pigments 145 (2017) 136e143
139
455 nm) in comparison with POFIrpic (lmax at 458 nm), while the
CH3 substituent at 5-position of pyridyl ring, a red-shift of 4 nm for
5Me-POFIrpic (lmax at 462 nm) is reached. Instead of a CH3 with a F
group at 5-position of pyridyl ring, 5F-POFIrpic (lmax at 464 nm)
gives a red-shift of approximately 6 nm versus the emission of the
complex POFIrpic. Introducing a CF3 group at the same position for
5CF3-POFIrpic, an almost featureless emission band was observed
(5CF3-POFIrpic) at the same position makes the oxidation process
shift to a more positive potential, indicating the relative difficulty of
oxidation. On the basis of the onset oxidation potentials and the
absorption edge data, the HOMO and LUMO energy levels can be
calculated and the results are presented in Table 2.
3.3. Theoretical calculations
(lmax at 477 nm), which is red-shifted by about 19 nm relative to
POFIrpic. Compared with other Ir(III) complexes, a red-shifted
emission spectrum was achieved for 5CF3-POFIrpic, which may
result from the stabilization of the LUMO to a larger extent than the
HOMO due to the electron-withdrawing CF3, thus leading to a
smaller energy gap (see the computational results).
As shown in Table 1, these Ir(III) complexes exhibit high phos-
phorescence quantum yields (FPL) in the range of 54e64% in
CH2Cl2. These FPL values are comparable to some previously re-
ported bis-cyclometalated Ir(III) complexes [23,30e33], which
consisted of an electron-withdrawing substituent at the 30 position
of 2-(20,4'-difluorophenyl)pyridine as the cyclometalating ligand
Fig. 2 displays the energy levels and electron density distribu-
tions of the orbitals for all complexes by DFT calculations. It was
observed that the five complexes have very small differences in the
electron density distributions of the HOMOs, which are all mainly
located on the iridium center and the ppy moiety of the cyclo-
metalating ligands. However, unlike the case of the HOMOs, these
complexes show differences in the electron density distributions of
the LUMOs. For POFIrpic, the LUMO is primarily contributed from
the ppy of one of the cyclometalating ligands as well as the pic
ancillary ligand. Compared with POFIrpic, the introduction of an
electron-donating CH3 group in 4Me-POFIrpic and 5Me-POFIrpic
have no significant influence on the electron density distribution of
the LUMOs. The LUMO electron densities of 5F-POFIrpic are
located on the cyclometalating ligand and a relatively small
contribution from the pic ancillary ligand relative to those of the
POFIrpic, 4Me-POFIrpic, 5Me-POFIrpic. In contrast, the presence
of an electron-withdrawing group CF3 on the cyclometalating
ligand of 5CF3-POFIrpic redistributed the LUMO from the ancillary
ligand entirely to the cyclometalating ligand. Taking the energy
level of POFIrpic as a criterion, incorporation of a CH3 group into
the 4-position of the pyridyl ring in cyclometalating ligand,
increased slightly the HOMO-LUMO energy gap of the corre-
sponding complex, resulting in a blue-shift relative to POFIrpic.
The HOMO-LUMO energy gaps for the other complexes are reduced
and 5CF3-POFIrpic is the lowest one, which are consistent with
experimental results.
and a pic as the ancillary ligand. The phosphorescence lifetimes (t)
of the Ir(III) complexes were 0.85e2.82 ms in CH2Cl2 at room tem-
perature (Table 1). The radiative decay rate (kr z 1.95e7.53 ꢂ 105
s
ꢀ1) and nonradiative decay rate (knr z 1.56e4.23 ꢂ 105
s
ꢀ1) of
these complexes in CH2Cl2 were calculated and are listed in Table 1.
The relatively high radiative decay rate (kr ¼ 7.53 ꢂ 105
s
ꢀ1) and
short phosphorescence lifetime (
t
¼ 0.85
ms) indicate that 5CF3-
POFIrpic emits from a more pronounced 3MLCT excited state and is
more influenced by the spin-orbit coupling (SOC) effect than those
of other Ir(III) complexes [34,35].
3.2. Electrochemical properties
The electrochemical behavior of these Ir(III) complexes were
investigated by cyclic voltammetry (CV) in CH2Cl2 (the cyclic vol-
tamograms of these Ir(III) complexes see Fig. S2 in Supporting In-
formation) and the corresponding data are listed in Table 2. Several
groups reported that it was difficult in observing reversible
reduction potentials for blue phosphorescent Ir(III) complexes
[36e38]. Similarly, all investigated Ir(III) complexes except 5CF3-
POFIrpic exhibited irreversible reduction waves (see Fig. S2 in
Supporting Information). The reduction profile of 5CF3-POFIrpic
was found to be quasi-reversible, suggesting an electrochemical
stability of 5CF3-POFIrpic to some extent [39]. The onset oxidation
potentials of these Ir(III) complexes are 1.40 (POFIrpic), 1.41 (4Me-
POFIrpic), 1.34 (5Me-POFIrpic), 1.48 (5F-POFIrpic) and 1.51 (5CF3-
POFIrpic) V. It was found that introducing the electron-donating
group CH3 at the 4-position of pyridyl ring has no significant ef-
fect on the onset oxidation potential compared with that of
POFIrpic. However, a CH3 group at the 5-position of pyridyl ring
leads to a lower oxidation potential for 5Me-POFIrpic. By contrast,
introducing an electron-withdrawing group F (5F-POFIrpic) or CF3
To investigate the nature of the emissive excited state, the low-
lying triplet states of these Ir(III) complexes were calculated based
on their optimized geometry of the ground state (S0) using the
Table 2
Electrochemical data of these Ir(III) complexes.
d
Eꢃox
onset
ox
a
Complexes
Eg [eV] HOMOc [eV]
b
LUMOc [eV]
E
[V]
POFIrpic
1.40
1.41
1.34
1.48
1.51
2.86
2.91
2.85
2.79
2.72
ꢀ5.80
ꢀ5.81
ꢀ5.74
ꢀ5.88
ꢀ5.91
ꢀ2.94
ꢀ2.90
ꢀ2.89
ꢀ3.09
ꢀ3.19
ꢀ1.46
ꢀ1.50
ꢀ1.51
ꢀ1.31
ꢀ1.21
4Me-POFIrpic
5Me-POFIrpic
5F-POFIrpic
5CF3-POFIrpic
a
0.1 M [Bu4N]PF6 in CH2Cl2, scan rate 100 mV sꢀ1, measured using saturated
calomel electrode (SCE) as the standard.
b
HOMO-LUMO gap (Eg) measured according to the onset of UV-Vis absorption
(Eg ¼ 1240/lonset eV).
c
EHOMO (eV) ¼ -e(4.4 þEooxnset), ELUMO (eV) ¼ EHOMO þ Eg.
d
onset
Eo*x ¼ E
- Eg.
ox
Table 1
Photophysical data of these Ir(III) complexes.
td
(m
s)
kre (105 sꢀ1
)
knr (105 sꢀ1
e
)
a
b
c
Complexes
labs (nm)
lem (nm)
ФPL
POFIrpic
230(0.47), 257(0.49), 287(0.35), 373(0.05)
230(0.42), 258(0.49), 283(0.35), 370(0.06)
230(0.46), 258(0.53), 288(0.36), 372(0.07)
230(0.34), 258(0.35), 289(0.23), 375(0.04)
230(0.41), 266(0.45), 290(0.37), 391(0.04)
458, 487 sh
455, 484 sh
462, 492 sh
464, 492 sh
477
0.57
0.54
0.55
0.60
0.64
2.02
2.33
2.82
2.57
0.85
2.82
2.32
1.95
2.33
7.53
2.13
1.97
1.59
1.56
4.23
4Me-POFIrpic
5Me-POFIrpic
5F-POFIrpic
5CF3-POFIrpic
a
Measured in CH2Cl2 at a concentration of 10ꢀ5 M and extinction coefficients (105 Lmolꢀ1cmꢀ1) are shown in parentheses.
In deoxygenated CH2Cl2; the symbol sh denotes the shoulder wavelength.
b
c
The quantum yields (
Ф
PL) in deoxygenated CH2Cl2 were measured with Ir(ppy)3
Measured in deoxygenated CH2Cl2.
Calculated from kr ФPL
ꢀ1, knr
(
ФPL ¼ 0.40) as a standard.
d
e
¼
ꢂ
t
¼
t
ꢀ1- kr in deoxygenated CH2Cl2.

140
H. Yu et al. / Dyes and Pigments 145 (2017) 136e143
Fig. 2. Calculated energy-level diagram and contour plots of the HOMO and LUMO for all investigated Ir(III) complexes.
Table 3
time-dependent DFT (TD-DFT) approach. The vertical excitation
energies and molecular orbitals involved in the excitations for the
lowest energy triplet state are summarized in Table 3 (see Fig. S1 in
the Supporting Information for the electron density maps of the
frontier molecular orbital that are mainly involved in the lowest-
lying transition). TD-DFT calculations show that T1 and T2 states
of these complexes are relatively close in energy (within 0.11 eV)
and, in principle, the emission might originate not only from the T1
state but also from the higher-lying T2 state [40,41]. For POFIrpic,
the orbitals involved are HOMO-4, HOMO-3, HOMO-1, HOMO,
LUMO, LUMOþ1 and LUMOþ2, which renders the T1 and T2 states
of POFIrpic with considerable nature of 3LC with some 3MLCT and
3LLCT character. The finding is in good agreement with the exper-
imental observation, where the vibronic fine structure is present in
emission spectrum. For 4Me-POFIrpic, 5Me-POFIrpic and 5F-
POFIrpic, distributions of the triplet states are similar to that of
POFIrpic. Their emissions also take place from the 3LC state ac-
cording to the calculated triplet states (Table 3) and the experi-
mental results of photophysical characterizations (section 3.1).
Based on the calculations, the 3MLCT character of the emissive state
is more pronounced in 5CF3-POFIrpic than in other Ir(III) com-
plexes, which may explain its almost featureless emission
spectrum.
Calculated triplet states of these Ir(III) complexes by a TD-DFT approach.
Complexes
States
T1
Ea (eV)
Compositionb
CIc
Character
POFIrpic
2.90
H / Lþ1
H / L
0.3941
0.3558
0.1858
0.4091
0.3427
0.2101
0.4613
0.2460
0.2287
0.4167
0.3493
0.1893
0.4636
0.2617
0.2125
0.4144
0.3356
0.2413
0.4037
0.3687
0.1839
0.4359
0.3153
0.2109
0.5532
0.2499
0.1616
0.4270
0.3518
0.2039
LC/LLCT
LC/MLCT/LLCT
LC/LLCT
LC/MLCT/LLCT
LC
LC/MLCT/LLCT
LC/LLCT
LC/MLCT/LLCT
LC/LLCT
LC/MLCT/LLCT
LC
LC/MLCT/LLCT
LC/LLCT
LC/MLCT/LLCT
LC/LLCT
LC/MLCT/LLCT
LC
LC/MLCT/LLCT
LC/LLCT
LC/MLCT/LLCT
LC/LLCT
LC/MLCT/LLCT
LC
LC/MLCT/LLCT
LC/MLCT/LLCT
LC/MLCT
LC/LLCT
LC/MLCT/LLCT
LC
H-4 / Lþ1
H / Lþ2
H-3 / Lþ2
H-1 / Lþ2
H / Lþ1
H / L
H-4 / Lþ1
H / Lþ2
H-3 / Lþ2
H-1 / Lþ2
H / Lþ1
H / L
H-4 / Lþ1
H / Lþ2
H-3 / Lþ2
H-1 / Lþ2
H / Lþ1
H / L
T2
T1
T2
T1
T2
T1
T2
T1
T2
3.00
2.92
3.01
2.87
2.98
2.84
2.94
2.79
2.90
4Me-POFIrpic
5Me-POFIrpic
5F-POFIrpic
H-4 / Lþ1
H-1 / L
H-3 / Lþ2
H / Lþ2
H / L
H-4 / L
H-3 / L
H / Lþ1
H-3 / Lþ1
H-1 / Lþ1
5CF3-POFIrpic
3.4. Oxygen sensitivity of the Ir(III) complexes in solution
MLCT/LLCT
The oxygen sensitivity of the Ir(III) complexes were investigated
in CH3CN firstly. It was found that the emission intensity of all the
Ir(III) complexes decreased with the increase of molecular oxygen
concentrations from 0 to 100%. For example, changing from an N2
atmosphere to 2.17% O2 (mixture with N2, v/v) will significantly
quench the emission intensity of the complex 5Me-POFIrpic by
over 80% (Fig. 3). In addition, the shape of the emission profile
without distortions under different oxygen concentrations gua-
rantees that the decreased phosphorescence can be attributed only
to the oxygen quenching process [42]. A similar quenching effect
was observed for other Ir(III) complexes (see Fig. S3 in Supporting
Information).
a
Calculated excitation energies for the triplet states.
b
Only the main configurations are presented. H and L denote HOMO and LUMO,
respectively.
c
The Configuration-interaction (CI) coefficients are in absolute values. Oscillator
strengths are zero because of the neglect of spin-orbit coupling in the TD-DFT
calculations.
I0
I
¼ 1 þ KSV$pO
(1)
2
where I is the emission intensity, the subscript 0 denotes the value
of the quantity in the absence of quencher, KSV is the Stern-Volmer
quenching constant and pO is the partial pressure of oxygen.
The I0/I100 value is used to evaluate the O2 sensing properties of
phosphorescent OSPs, where I0 and I100 represent the detected
In fluid solution, the relationship between the emission in-
tensity of OSP and the concentration of quencher (O2) is reflected
by the Stern-Volmer equation, which can be described as follows
(eqn (1)) [13,43].
2

H. Yu et al. / Dyes and Pigments 145 (2017) 136e143
141
I
f1
f2
¼
þ
(2)
I0 1 þ KSV1$p
1 þ KSV2$p
O2
O2
where I0 and I, are the emission intensities of a probe in the absence
and presence of oxygen, respectively. pO2 is the partial pressure of
oxygen.
The
weighted
quenching
con-
stant Ksavpp(Kapp ¼ f1$KSV1 þ f2$KSV2 ) is the guide of the sensitivity
SV
app
of an oxygen sensor, and higher values of K
indicate that the
sv
sensor is more sensitive to oxygen.
The Stern-Volmer plots for oxygen sensing films of these Ir(III)
complexes immobilized in EC are shown in Fig. 4. The fitting results
of the O2 sensing data of these Ir(III) complexes are summarized in
Table 4. The Ksavpp values of these Ir(III) complexes immobilized in EC
film are 0.00583 (POFIrpic), 0.00518 (4Me-POFIrpic), 0.00793
(5Me-POFIrpic), 0.00522 (5F-POFIrpic) and 0.00459 (5CF3-
Fig. 3. Emission spectra of complex 5Me-POFIrpic (1.0 ꢂ 10ꢀ5 M in CH3CN) under
different oxygen concentration at room temperature.
luminescence intensities in 100% N2 and 100% O2 atmosphere,
respectively [44]. Stern-Volmer plots of these Ir(III) complexes in
CH3CN were studied (see Fig. S4 in Supporting Information). The I0/
I100 value is 64.2 for complex 5CF3-POFIrpic. The I0/I100 values of
POFIrpic, 4Me-POFIrpic and 5F-POFIrpic are 105.8, 103.4 and
110.9, respectively. For 5Me-POFIrpic, the highest quenching effi-
ciency is increased to I0/I100 ¼ 150.3. Indeed, the incorporation of a
CH3 group at the 5-position of pyridyl ring in C^N cyclometalating
ligand is clearly advantageous in terms of increasing oxygen
sensitivity compared with other complexes.
POFIrpic) Torrꢀ1. It is clear that the 5Me-POFIrpic immobilized in
app
SV
EC exhibits the highest sensitivity to O2 at a
K
value up to
0.00793 Torrꢀ1
.
Generally, the complex 5Me-POFIrpic shows higher oxygen
sensitivity than the other Ir(III) complexes both in solution and
doped in EC film. The presence of CH3 in C^N cyclometalating ligand
improves oxygen sensitivity since its emission decay lifetime is
prolonged and thus increases sensing collision probability with O2
molecules. This indicates that rational chemical modification of the
C^N cyclometalating ligand can also pave the way to tune the ox-
ygen sensitivity.
3.5. Oxygen sensitivity of the Ir(III) complexes in polymer films
The oxygen sensors with ethylcellulose (EC) as an efficient
matrix have been reported [45,46]. Herein the oxygen sensitivity of
the Ir(III) complexes were also studied in EC film. The dynamic
response of the oxygen sensing film was tested against small steps
of O2 partial pressure variation (see Fig. S5 in Supporting Infor-
mation). As shown in Table 4, the I0/I100 values of these Ir(III)
complexes immobilized into EC films are 4.47 (POFIrpic), 4.02
(4Me-POFIrpic), 5.88 (5Me-POFIrpic), 4.06 (5F-POFIrpic) and 3.93
(5CF3-POFIrpic).
For the heterogeneous oxygen sensing films, usually, a modified
Stern-Volmer is required to quantify the quenching effect. A two-
site model was proposed by Demas and co-workers [47,48],
which has been generally accepted and widely used ever since
[7,49,50]. In the two-site model, the OPS is taken (at least) two
distinctly different environments into account, one (f1) being
quenchable, the other (f2) either not being quenched at all, or being
quenched at a very different rate (f1 þ f2 ¼ 1). Each component
shows different quenching constants (KSV1 and KSV2), it reads as
follows (eqn (2)).
Fig. 4. Stern-Volmer plots for oxygen sensing films of the Ir(III) complexes immobi-
lized in EC (intensity ratios I0/I versus O2 partial pressure).
Table 4
Parameters for the O2-sensing film of the Ir(III) complexes with EC as the supporting matrix (fitting of the result to the two-site model).
r2c
tres (s)e
trec (s)e
a
a
b
b
app
SV
d
Complexes
lem (nm)
I0/I100
G1
G2
KSV1
KSV2
K
POFIrpic
461, 488
458, 485
465, 494
467, 492
480
4.47
4.02
5.88
4.06
3.93
0.94684
0.92998
0.95764
0.91921
0.95550
0.05316
0.07002
0.04236
0.08079
0.04450
0.00616
0.00557
0.00828
0.00568
0.00480
0.0000
0.0000
0.0002
0.0000
0.0001
0.99676
0.99659
0.99390
0.99597
0.99619
0.00583
0.00518
0.00793
0.00522
0.00459
4
4
4
3
5
5
6
5
5
6
4Me-POFIrpic
5Me-POFIrpic
5F-POFIrpic
5CF3-POFIrpic
a
Ratio of the two portions of the Ir(III) complexes.
Quenching constant of the two portions.
b
c
Determination coefficients.
Weighted quenching constant, K
app
SV
d
e
¼ f1KSV1 þ f2KSV2
.
The response and recovery time for optical oxygen sensors are defined as the time for emission intensity changes to reach 95% of the whole variation when switching from
100% N2 to 100% O2 or vice versa.

142
H. Yu et al. / Dyes and Pigments 145 (2017) 136e143
Fig. 5. Reversibility and emission intensity response of sensing film of 5Me-POFIrPic immobilized in EC when cycling from 100% N2 to 100% O2 atmosphere.
excited state Eo*x potentials correlates well with higher photo-
stability of 5CF3-POFIrPic compared to the others (Table 2). It is
reasoned that the Ir(III) complex with a CF3 substituent on the
pyridyl ring may be more resistant to photobleaching processes and
this has increased photostability [51].
4. Conclusions
In summary, a series of diphenylphosphoryl-substituted bis-
cyclometalated Ir(III) complexes were synthesized and their pho-
tophysical, electrochemical and oxygen sensing properties were
studied in detail. These Ir(III) complexes exhibit intense emission
bands at 455e492 nm and high photoluminescence quantum
yields in the range of 54e64% in CH2Cl2. Introducing a CH3 group at
4-position of pyridyl ring, resulting in the emission band of 4Me-
POFIrpic being blue-shifted relative to that of the un-substituted
POFIrpic. Whilst introducing a CH3, F or CF3 group at 5-position
of pyridyl ring gives emission bands of the corresponding Ir(III)
complexes (5Me-POFIrpic, 5F-POFIrpic, 5CF3-POFIrpic) red-
shifted in comparison with that of POFIrpic. 5Me-POFIrpic per-
forms the best oxygen sensitivity and 5CF3-POFIrpic exhibits the
highest photostability.
Fig. 6. Photo-degradation histograms for the Ir(III) complexes/EC films under contin-
uous illumination for 2.0 h at ambient atmospheric condition.
3.6. Operational stability and photostability of oxygen sensing films
Acknowledgments
Reversibility and stability are important factors to influence the
overall performance of a sensor. Therefore, the operational stability
and photostability tests of oxygen sensing films were conducted.
The emission intensity is monitored when the oxygen sensing film
is exposed to an atmosphere which periodically varies between
100% N2 and 100% O2 within 4000 s. All the oxygen sensing films
demonstrated a stable optical signal during the quenching and
recovering cycles (Fig. 5 and Fig. S6 in Supporting Information),
suggesting that they have good operational stability. Fast response
times of the Ir(III) complexes immobilized in EC films were ob-
tained at around 3e5 s on going from N2 to O2 and recovery times
were at around 5e6 s on going from O2 to N2 (Table 4). Additionally,
no photobleaching phenomenon was observed with increasing
exposure time, which indicates that they have very good
photostability.
The photostability of the sensing films were further studied
upon continuous irradiation by a 16 W UV lamp. As shown in Fig. 6,
the emission intensity of the Ir(III) complexes immobilized in EC
reduced by different degree after 2.0 h of irradiation. In fact, 46.7%
(POFIrpic), 44.9% (4Me-POFIrpic), 49.4% (5Me-POFIrpic), 45.5%
(5F-POFIrpic), and 29.9% (5CF3-POFIrpic) decreases in the in-
tensity are observed. It was observed that introduction of a CH3
group into the pyridyl ring has negative effects on the photo-
stability of the corresponding complexes 4Me-POFIrPic and 5Me-
POFIrPic. It is clear that introduction of an electron-withdrawing F
or CF3 group on the 5-position of the pyridyl ring has a positive
effect on the photostability. Especially, the complex 5CF3-POFIrPic
demonstrated the highest photostability. The increase in the
The authors thank the financial support from the National
Natural Science Foundation of China (21276043, 21421005), and
Open Fund of Key Laboratory of Biotechnology and Bioresources
Utilization (Dalian Minzu University, KF2015002), State Ethnic Af-
fairs Commission & Ministry of Education.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2017.05.056.
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