Synthesis and properties of fluorinated cyclometalated Ir(III) complexes

Article abstract of DOI:10.1016/j.tet.2020.131390

Two new fluorinated cationic cyclometalated Ir(III) complexes have been prepared and fully characterized. The effects of the position of the fluorine on the photophysical, redox properties and oxygen sensing behaviors of the Ir(III) complexes were investi

Full text of DOI:10.1016/j.tet.2020.131390

Journal Pre-proof
Synthesis and properties of fluorinated cyclometalated Ir(III) complexes
Lei Wang, Zhanming Gao, Chun Liu, Yan Chen, Weitong Tao, Lu Lu
PII:
S0040-4020(20)30552-4
https://doi.org/10.1016/j.tet.2020.131390
TET 131390
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 5 June 2020
Revised Date: 3 July 2020
Accepted Date: 6 July 2020
Please cite this article as: Wang L, Gao Z, Liu C, Chen Y, Tao W, Lu L, Synthesis and properties
of fluorinated cyclometalated Ir(III) complexes, Tetrahedron (2020), doi: https://doi.org/10.1016/
j.tet.2020.131390.
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition
of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of
record. This version will undergo additional copyediting, typesetting and review before it is published
in its final form, but we are providing this version to give early visibility of the article. Please note that,
during the production process, errors may be discovered which could affect the content, and all legal
disclaimers that apply to the journal pertain.
© 2020 Published by Elsevier Ltd.

Synthesis and properties of fluorinated cyclometalated Ir(III)
complexes
Lei Wang,a,1 Zhanming Gao,a,1 Chun Liu,*,a,b Yan Chen,a Weitong Tao,a Lu Lua
a State Key Laboratory of Fine Chemicals, Dalian University of Technology,
Linggong Road 2, Dalian 116024, China; E-mail: cliu@dlut.edu.cn
b Eco-chemical Engineering Cooperative Innovation Center of Shandong, Qingdao
University of Science and Technology, Qingdao 266042, China.
1 These authors contributed equally to this work.
1

Abstract:
Two new fluorinated cationic cyclometalated Ir(III) complexes have been
prepared and fully characterized. The effects of the position of the fluorine on the
photophysical, redox properties and oxygen sensing behaviors of the Ir(III) complexes
were investigated. The results show that the introduction of a fluorine at the 6-position
on the pyridyl ring of the cyclometalating ligand results in a notable blue-shift (91
nm) of the corresponding Ir3 compared to the non-fluorinated Ir1. While the complex
Ir2 with a fluorine at the 5-position of the pyridyl ring shows a red-shift of 21 nm.
Moreover, the ΦPL and the phosphorescence lifetime of Ir3 increase effectively than
those of Ir1 and Ir2. The oxygen sensing properties of the complexes were studied in
CH2Cl2 and ethyl cellulose (EC) films, respectively. The fitting results indicate that
Ir3 demonstrates the highest oxygen sensitivity both in CH2Cl2 (I0/I100 = 235.7) and in
an EC film (I0/I100 = 35.0).
Keywords：Cyclometalated Ir(III) complexes, Fluorine, Oxygen sensing.
2

1. Introduction
Molecular oxygen is not only the first need for life activities in most organisms
but also closely related to many chemical, biochemical and physical processes [1-3].
Thus, the determination of oxygen concentration is of great significance in the fields
of biomedicine [4-8], industrial and agricultural production [9-11], and environmental
monitoring [12-15]. At present, oxygen detection technology is mainly divided into:
Winkler titration [16], electrical analysis [17,18] and luminescent oxygen sensing [19].
Compared with traditional analytical techniques, optical oxygen sensing has become
one of the research hotspots because of its high sensitivity, good reversibility, absence
of oxygen consumption and short response times [20, 21]. Most of the oxygen sensors
are based on the oxygen-sensitive probes (OSPs). The response process involves a
dynamic collision between molecular oxygen (triplet) and the OSP at an excited
electronic state, leading to a reduction of its intensity and decay time. Thus, the
development of high-performance phosphorescent materials as efficient OSPs has
become one of the most important research fields in luminescent oxygen sensing. So
far, transition metal complexes such as ruthenium complexes, iridium complexes,
porphyrin palladium/porphyrin platinum complexes and copper complexes, have been
used for luminescent oxygen sensing [22-28]. Due to their high phosphorescence
quantum yields, long excited-state lifetimes, and flexible tuning of photophysical and
electrochemical properties by the coordinating ligands, Ir(III) complexes are
particularly suitable for the use as OSPs [29-31].
The substituent effect has been recognized as an important factor for adjusting
3

the properties of the Ir(III) complexes. It has been demonstrated that the lifetimes and
quantum yields of Ir(III) complexes could be effectively influenced by different
substituents, especially the fluorine atom [32-34]. Unlike other halogens, fluorine has
the strongest electronegativity and atomic radius with the size near to hydrogen atom
due to its unique electronic structure, which makes it easier to modify the target
molecules and then modulate their performances by replacing hydrogen atoms. In
recent years, transition metal complexes modified by fluorine have been widely
applied in various fields [35-42]. For instance, perfluorinated platinum porphyrin
PtF28TPP showed better photostability and higher oxygen sensitivity than PtTFPP [43].
In another example, fluorinated cationic Ir(III) complexes served as highly efficient
specific agents for cell imaging [44]. In 2012, Baranoff et al. reported that neutral
2-(2,4-difluorophenyl)pyridine-derived
Ir(III)
complexes
immobilized
in
nanomaterials (AP200/19) exhibited a very high sensitivity to molecular oxygen [45].
Thus, it is clear that the fluorine substituent could improve the photophysical,
electrochemical and oxygen sensitivity properties of the complexes effectively. On the
other hand, the diphenylamino (DPA) group is also a useful and unique substituent for
tuning the properties of the metal complexes [46, 47]. Very recently, we found that a
DPA-substituted cationic cyclometalated Ir(III) complex showed a higher oxygen
sensitivity than that of a non-DPA substituted one [48].
Inspired by the results mentioned-above, we envision that there might be a more
effective combination between a DPA moiety and a fluorine atom for the construction
of Ir(III) complexes with high performances. In this paper, two new fluorinated
4

cationic cyclometalated Ir(III) complexes were synthesized (Fig. 1), which were used
to explore the effects of the position of fluorine on the photophysical and
electrochemical properties of the corresponding Ir(III) complexes. The oxygen
sensitivity and operational stability of these Ir(III) complexes were studied in detail.
Fig. 1 Chemical structures of the Ir(III) complexes for this study.
2. Experimental
2.1 Materials and methods
All starting materials were commercially available and used without further
purification. The solvents were treated as required prior to use. 1H NMR spectra were
recorded on a 400 MHz Varian Unity Inova spectrophotometer. 13C NMR spectra
were recorded on a 500 MHz Bruker Avance spectrophotometer. Mass spectra were
recorded with a MALDI micro MX spectrometer. UV/Vis absorption spectra were
recorded on a Lambda 750S UV/Vis spectrophotometer. Emission spectra were
recorded with F-7000 spectrofluorimeter. Photoluminescence quantum yields were
measured relative to [Ir(ppy)2(acac)] (ФP = 0.34 in CH2Cl2, under degassed
conditions). Phosphorescence lifetimes were measured on an Edinburgh FLS920
Spectrometer. Cyclic voltammograms of the Ir(III) complexes were recorded on an
5

electrochemical workstation (BAS100B/W, USA) at room temperature in a 0.1 M
[Bu4N]PF6 solution under argon. DFT calculations were carried out by B3LYP. The
LanL2DZ basis set was employed for iridium atom and the 6-31G* basis set was used
to treat all other atoms. All calculations were performed with the Gaussian 09
software package.
2.2 Synthesis
Chemical structures of the Ir(III) complexes for this study are shown in Fig. 1
and the detailed synthetic protocol is provided in Scheme S1. The cyclometalating
ligands were prepared via a palladium-catalyzed ligand-free and aerobic
Suzuki-Miyaura reaction in aqueous ethanol developed by our group [49]. The Ir(III)
complexes were prepared according to the literature method [50].
2.2.1 Synthesis of C^N ligands
The C^N ligands were synthesized by the palladium-catalyzed Suzuki-Miyaura
cross-coupling reaction [49].
2.2.2 Synthesis of the Ir(III) complexes
IrCl3·3H2O was reacted with 2.5 equiv. cyclometalating C^N ligand in a mixture
of 2-ethoxyethanol and water (9 mL/3 mL) at 110 °C under nitrogen for 24 h to afford
a cyclometalated iridium bridged-chloride dimer. Without further purification, the
dimer was mixed with 3.0 equiv. 1,10-phenanthroline in 2-ethoxyethanol at 120 °C
under nitrogen for 24 h. After cooling down to room temperature, a 10-fold excess of
KPF6 was added. The suspension was stirred for 3 h and washed with water and dried
6

over Na2SO4. The crude product was applied to a silica gel column and eluted with
CH2Cl2 to afford desired Ir(III) complexes.
Ir2, Yield 57%, orange solid; 1H NMR (400 MHz, DMSO-d6): δ 8.89 (d, J = 8.1
Hz, 2H), 8.36 (s, 2H), 8.32 (d, J = 4.8 Hz, 2H), 8.13 (dd, J = 8.1, 5.1 Hz, 2H), 7.88
(dd, J = 9.2, 5.5 Hz, 2H), 7.69 (d, J = 8.7 Hz, 2H), 7.56 – 7.49 (m, 2H), 7.26 (t, J =
7.7 Hz, 8H), 7.04 (dt, J = 23.8, 11.6 Hz, 14H), 6.57 (dd, J = 8.5, 1.9 Hz, 2H), 5.78 (d,
13
J = 8.1 Hz, 2H). C NMR (125 MHz, DMSO-d6): δ 164.24, 158.02, 155.56, 151.28,
150.38, 148.94, 146.85, 146.60, 139.33, 136.82, 136.51, 135.87, 131.71, 129.86,
128.74, 127.66, 126.46, 126.36, 126.20, 124.57, 121.84, 120.41, 120.35, 113.91.
HRMS (MALDI-TOF, m/z): calcd for C58H40N6F2Ir [M-PF6]+ 1051.2912, found:
1051.2933.
Ir3, Yield 68%, yellow solid; 1H NMR (400 MHz, DMSO-d6): δ 8.82 (d, J = 8.2
Hz, 2H), 8.29 (d, J = 3.7 Hz, 4H), 8.06 (dd, J = 8.2, 5.1 Hz, 2H), 7.77 (d, J = 8.8 Hz,
2H), 7.73 (d, J = 7.9 Hz, 2H), 7.63 (dd, J = 15.3, 7.7 Hz, 2H), 7.27 (t, J = 7.7 Hz, 8H),
7.09 (t, J = 7.3 Hz, 4H), 7.03 (d, J = 7.8 Hz, 8H), 6.59 (dd, J = 8.6, 2.1 Hz, 2H), 6.33
(dd, J = 7.8, 3.1 Hz, 2H), 5.84 (d, J = 2.0 Hz, 2H). 13C NMR (125 MHz, DMSO-d6): δ
167.26, 166.23, 163.70, 151.18, 149.90, 149.48, 147.24, 146.40, 145.47, 143.75,
138.77, 136.39, 130.80, 130.00, 128.32, 127.48, 126.94, 126.36, 124.88, 122.21,
115.53, 113.98, 105.88, 105.61, 99.99. HRMS (MALDI-TOF, m/z): calcd for
C58H40N6F2Ir [M-PF6]+ 1051.2912, found: 1051.2906.
7

3. Results and discussion
3.1 Photophysical properties
UV-vis absorption spectra and emission spectra of Ir1-Ir3 (1.0 × 10-5 M in
CH2Cl2) at room temperature are displayed in Fig. 2 and the corresponding data are
summarized in Table 1. As shown in Fig. 2 (a), All of the Ir(III) complexes exhibit
two major absorption bands in their absorption spectra. The intense high-energy bands
1
below the 330 nm region are ascribed to the spin-allowed π-π* transitions from the
ligands [51, 52]. The low-energy absorption occurs in the range of 330-500 nm can be
assigned to both spin-allowed and spin-forbidden metal-to-ligand charge-transfer
3
transitions (1MLCT and MLCT) ligand-to-ligand charge transfer (1LLCT) character.
Different from the previously reported cyclometalated Ir(III) complexes [29, 32],
these three complexes have a larger molar absorption coefficient in the range of
350-450 nm owing to the electron-donating DPA group.
The normalized emission spectra of these complexes are shown in Fig. 2 (b).
The results reveal that introducing a fluorine at the 5-position of the pyridyl ring
results in a red-shift of 21 nm for Ir2 (λmax = 666 nm) in comparison with Ir1 (λmax
=
645 nm), while the complex Ir3 (λmax = 554 nm) with a fluorine at the 6-position of
the pyridyl ring shows a dramatic blue-shift of 91 nm.
8

Fig. 2 Absorption (a) and emission spectra (b) of Ir(III) complexes (1.0 × 10-5 M in CH2Cl2) at
room temperature.
The phosphorescence quantum yields (ΦPL) of Ir1-Ir3 are 0.65%, 0.35%, and
1.49%, respectively. The phosphorescence lifetimes (τ) of Ir1-Ir3 in CH2Cl2 are
0.10-1.88 μs at room temperature (Table 1, the phosphorescence decay profiles of the
Ir(III) complexes are provided in Fig. S2). The results show that introducing a
fluorine at the 6-position of the pyridyl ring leads to an increased ΦPL and a prolonged
phosphorescence lifetime for Ir3 compared with the non-fluorinated Ir1. However,
the introduction of a fluorine at the 5-position of the pyridyl ring has a slight effect on
the photophysical properties of the complex Ir2. The radiative decay rates (0.79-5.00
× 104 s-1) and nonradiative decay rates (5.24-99.7 × 105 s-1) of these complexes in
CH2Cl2 were calculated and listed in Table 1. It shows that the 6-fluorinated Ir3 has a
very smaller nonradiative decay rate (knr) in comparison with Ir1 and Ir2. Thus, the
position of the fluorine plays an important role in tuning the photophysical properties
of the Ir(III) complexes.
9

Table 1 Photophysical data of Ir(III) complexes Ir1-Ir3
c
ФPL
Complex
λabsa (nm)
λemb (nm)
τe (μs)
krf (104 s-1)
knrf (105 s-1)
(ФA)d
230(3.89),266(5.07),
389(0.66),413(0.32)
645(CH2Cl2)
559(EC film)
0.65%
6.10%
Ir1
0.13
5.00
76.4
232(5.55),267(5.17),
369(3.23),424(2.76)
666(CH2Cl2)
567(EC film)
0.35%
1.42%
Ir2
Ir3
0.10
1.88
3.50
0.79
99.7
5.24
229(4.42),269(4.39),
398(2.52),433(3.04)
554,574(CH2Cl2)
545,566(EC film)
1.49%
1.00%
a Measured in CH2Cl2 at a concentration of 10-5 mol/L and extinction coefficients (104 M-1 cm-1)
b
c
are shown in parentheses. The emission maxima are the values in bold style (λexc = 430 nm).
The quantum yields (Фsolution) in deoxygenated CH2Cl2 were measured with [Ir(ppy)2(acac)] (ФPL
= 0.34) as a standard. d The absolute phosphorescence quantum yields determined by employing
e
f
an integrating sphere. In deoxygenated CH2Cl2 solution. The radiative and nonradiative decay
rates of kr and knr were calculated from kr = ФPL × τ-1, knr = τ-1- kr. All measured at ambient
temperature.
3.2 Electrochemical properties
The electrochemical properties of the Ir(III) complexes were studied by cyclic
voltammetry (CV) and the results are listed in Table 2. The oxidation potentials of
Ir1-Ir3 are 0.95 V, 0.96 V and 1.09 V versus saturated calomel electrode (SCE),
respectively. The positive oxidative waves are assigned to the metal-centered
Ir(III)/Ir(IV) oxidation couple, in accordance with the reported cyclometalated Ir(III)
systems [53]. As shown in Table 2, slight differences in the oxidative potentials were
observed. It is found that introducing a fluorine at the 6-position of the pyridyl ring
makes the oxidation process shift to a more positive potential of the corresponding
complex Ir3. These complexes exhibit reduction waves at different potentials between
-1.45 V and -1.27 V, which can be assigned to the reduction of the phenanthroline
10

ligands. The HOMO and LUMO energies can be calculated by following equations
) and ELUMO (eV) = -e(4.4 + ꢀꢇꢁꢅꢃꢈꢄꢅꢆ). The results are
ꢁꢃꢄꢅꢆ
EHOMO (eV) = -e(4.4 + ꢀ
ꢁꢂ
presented in Table 2.
Table 2 Electrochemical data of Ir1-Ir3
d
a (V)
ꢀ
ꢇꢅꢈ
a (V)
-1.35
EHOMOb (eV)
ELUMOc (eV)
Eg (eV)
ꢁꢃꢄꢅꢆ
ꢁꢃꢄꢅꢆ
Complex
ꢀ
ꢁꢂ
Ir1
Ir2
Ir3
0.95
-5.35
-5.36
-5.49
-3.05
-3.13
-2.95
2.30
2.23
2.54
0.96
1.09
-1.27
-1.45
a 0.1 M [Bu4N]PF6 in CH2Cl2, scan rate 100 mV s−1, measured using saturated calomel electrode
b
c
d
(SCE) as the standard. EHOMO (eV) = -e(4.4 + ꢀꢁꢁꢂꢃꢄꢅꢆ). ELUMO (eV) = -e(4.4 + ꢀꢇꢁꢅꢃꢈꢄꢅꢆ). Eg =
ELUMO - EHOMO. HOMO and LUMO denote highest-occupied molecular orbital and
lowest-unoccupied molecular orbital, respectively.
3.3 Theoretical calculations
The electronic structures of Ir1-Ir3 were investigated by density functional
theory (DFT) and time-dependent DFT (TD-DFT) calculations. As shown in Fig. 3
the HOMO distribution primarily resides on the DPA group and phenyl ring of the
cyclometalating ligands, the LUMO distribution is localized on the iridium center and
1,10-phenanthroline ligand. The HOMO-LUMO gaps calculated for Ir1-Ir3 are 2.65
eV, 2.62 eV, and 2.80 eV, respectively. The differences in the HOMO-LUMO gaps for
Ir1-Ir3 reveal that the HOMO and LUMO levels are strongly affected by the
cyclometalating ligands, which reasonably explains the red-shift from Ir1 to Ir2 and
blue-shift from Ir1 to Ir3, observed in the emission spectra (see Fig. 3).
11

Fig. 3 Calculated energy-level diagram and contour plots of the HOMO and LUMO for Ir(III)
complexes.
The vertical excitation energies and molecular orbitals involved in the excitations
for the lowest energy triplet state (T1) are summarized in Table S1 (Fig. S4 shows the
electron density maps of Ir1-Ir3 calculated by a TD-DFT approach). TD-DFT
calculations disclose that the excited states of the complexes Ir1-Ir3 involve much
more 3MLCT and 3LLCT characters and different degrees of 3LC character. Compared
3
with Ir1 and Ir2, the complex Ir3 has the predominant LC nature of the T1 state,
which can explain its fine structured emission band and long lifetime.
3.4 Oxygen sensing properties
The oxygen sensitivities of the Ir(III) complexes were investigated in CH2Cl2
firstly. As expected, the emission intensity of complexes decreased with the increase
of oxygen contents. In addition, the shape of the emission profile without distortions
under different oxygen concentrations guarantees that the decreased phosphorescence
can be attributed only to the oxygen quenching process. A completely different
quenching effect was observed for complexes Ir1-Ir3 as shown in Fig. 4.
12

Fig. 4 Emission spectra of Ir1-Ir3 (1.0 × 10-5 M in CH2Cl2) under different oxygen concentrations
at room temperature.
The oxygen-sensing data were fitted using the Stern-Volmer (SV) equation
shown below.
ꢉ
ꢋ
ꢊ
ꢊ
=
= 1 + ꢌꢍꢎ ꢏꢁ
ꢋ
(eqn 1)
ꢐ
ꢉ
Stern-Volmer plots are shown in Fig. 5. The I0/I100 value are used to evaluate the
oxygen sensing properties of phosphorescent OSPs, where I0 and I100 represent the
detected luminescence intensities in 100% N2 and 100% O2 atmosphere, respectively.
The I0/I100 values of complexes in CH2Cl2 are 7.9 (Ir1), 5.5 (Ir2) and 235.7 (Ir3).
Compared with 5-fluorinated Ir2, the 6-fluorinated Ir3 exhibits a much higher
sensitivity to O2 in CH2Cl2, which illustrates the significant effect of the fluorine
position on the oxygen sensitivity. The oxygen sensitivities are consistent with
13

lifetimes of the complexes, the same as literature reports [54, 55].
Fig. 5 Stern-Volmer plots (intensity ratios I0/I versus O2 partial pressure) of Ir1-Ir3 in CH2Cl2 (1.0
× 10-5 M). The inset is the enlarged view between 0 to 160 Torr of the O2 partial pressure.
Inspired by the high oxygen sensitivity of Ir3 in solution, we further examined
the oxygen sensitivities of the complexes at solid state. These three compounds were
doped in ethyl cellulose (EC) films at a 0.5 wt% content. The dynamic responses of
the films toward oxygen were tested by varying the oxygen partial pressures. As
shown in Fig. S5, the emission intensities of Ir1-Ir3 immobilized EC films were
effectively quenched by the stepwise increase of the partial pressure of oxygen.
14

Fig. 6 Stern-Volmer plots for oxygen sensing films of the Ir(III) complexes immobilized in EC
(intensity ratios I0/I versus O2 partial pressures).
The oxygen-sensing data were fitted with the Demas two-site model [56] (see
equation 2, Stern-Volmer plots are shown in Fig. 6) and the results are summarized in
Table 3.
ꢉ
ꢋ
ƒ
ƒ
ꢐ
ꢊ
ꢊ
ꢑ
=
= 1 +
+
(eqn 2)
ꢉ
ꢋ
ꢒꢓꢔ
ꢗ
ꢒꢓꢔ
ꢗ
ꢕꢖꢐ ꢘ
ꢐ
ꢕꢖꢑ
ꢘ
ꢐ
As shown by the results in Table 3, the I0/I100 values of EC film sensors
immobilized with Ir1-Ir3 are 8.2, 11.4 and 35.0, respectively. The Weighted
ꢙꢗꢗ
ꢍꢎ
quenching constant ꢌ
values of sensing films are 0.0135 (Ir1), 0.0216 (Ir2) and
0.0730 (Ir3) Torr-1. It is clear that the fluorinated complexes Ir2 and Ir3 exhibit
higher oxygen sensitivity than non-fluorinated Ir1. In addition, the position of the
fluorine affects the sensitivity of the sensors strongly. It is observed that the sensing
film immobilized with the 6-fluorinated Ir3 exhibits the highest O2 sensitivity at a
ꢙꢗꢗ
ꢌ
value up to 0.07300 Torr-1. The reason for this might be that Ir3 has the longest
ꢍꢎ
phosphorescence lifetime (1.88 µs) among these three complexes.
Table 3 Parameters for the O2-sensing films immobilized with Ir1-Ir3
ꢙꢗꢗ
e
a
b
b
c
c
Complex
I0/I100
ƒ1
ƒ2
KSV1
KSV2
r2 d
ꢌ
ꢚꢛ
Ir1
Ir2
8.2
0.9605
0.9658
0.0395
0.0342
0.0140
0.0105
0.0001
0.0001
0.9887
0.9947
0.0135
0.0216
11.4
Ir3
35.0
0.9888
0.0113
0.0738
0.0001
0.9957
0.0730
a
I0 and I100 represent the detected emission intensities in 100% N2 and 100% O2 atmosphere,
b
c
respectively. Ratio of the two portions of the Ir(III) complexes. Quenching constant of the two
portions. d Determination coefficients. e Weighted quenching constant, ꢌꢚꢙꢛꢗꢗ = ꢜ ꢌꢚꢛꢒ + ꢜ ꢌꢚꢛꢝ
.
ꢒ
ꢝ
3.5 Operational stability of oxygen sensing films
The operational stability tests of the oxygen-sensing films were conducted by
15

monitoring their emission intensity change with repeated switching of the atmosphere
from O2 to N2. The oxygen sensing films demonstrate stable optical signals during the
quenching and recovering cycles in 4000 s (Fig. 7 and Fig. S6), exhibiting excellent
operational stability. In addition, fast response and recovery times were observed. The
sensing films exhibit quick response times (< 6 s when changing from 0 to 100 vol%
O2) and recovery times (< 15 s when changing from 100 to 0 vol% O2), respectively.
Fig. 7 Reversibility and emission intensity response of sensing film of Ir3 immobilized in EC
when cycling from 100% N2 to 100% O2.
4. Conclusions
In conclusion, two new fluorinated cationic cyclometalated Ir(III) complexes
have been synthesized. Their photophysical and electrochemical properties were
studied in detail. The results demonstrate that the position of fluorine at the
cyclometalating ligands strongly affects the properties of the corresponding
complexes. In comparison with the non-fluorinated Ir1, introducing a fluorine at the
5-position of the pyridyl ring results in a red-shift (21 nm) of the maxima emission
16

wavelength for Ir2, while Ir3 with a fluorine at the 6-position of the pyridyl ring
shows a dramatic blue-shift of 91 nm. Moreover, the ΦPL and the phosphorescence
lifetime of Ir3 increase effectively than those of Ir1 and Ir2. Likewise, the position of
the fluorine affects remarkably the oxygen sensitivity of the Ir(III) complexes. Among
them, Ir3 demonstrates the highest oxygen sensitivity both in CH2Cl2 and in an EC
film, respectively. These results provide a useful perspective on the development of
high-performance Ir(III) complexes.
Conflicts of interest
The authors declare no conflict of interest.
Acknowledgements
The authors thank the financial support from the National Natural Science Foundation
of China (21978042, 21421005, 21276043) and Talent Fund of Shandong
Collaborative Innovation Center of Eco-Chemical Engineering (XTCXYX02). We
thank Prof. Jianzhang Zhao at DUT for his assistance with the theoretical calculations.
17

References
[1] Decker H, Holde KEV. Oxygen and the Evolution of Life. Springer, Berlin: Heidelberg;
2011.
[2] Semenza G L. Life with oxygen. Science 2007;318:62-64.
[3] Yaseen MA, Srinivasan VJ, Sakadžić S, Wu W, Ruvinskaya S, Vinogradov SA, Boas
DA. Optical monitoring of oxygen tension in cortical microvessels with confocal
microscopy. Opt Express 2009;17:22341-22350.
[4] Sakadzić S, Roussakis E, Yaseen MA, Mandeville ET, Srinivasan VJ, Arai K,
Ruvinskaya S, Devor A, Lo EH, Vinogradov SA. Two-photon high-resolution
measurement of partial pressure of oxygen in cerebral vasculature and tissue. Nat Meth
2010;7:755-759.
[5] Fang HM, Huang SJ, Yang CL, Young KC, Yu CK. Miniaturized real-time oxygen
detection systems integrated with optical fiber by doping Ru-based fluorescence sensors.
J Med Biol Eng 2013;33:117-123.
[6] Formenti F, Chen R, Mcpeak H, Matejovic M, Farmery AD, Hahn CEW. A fibre optic
oxygen sensor that detects rapid PO2 changes under simulated conditions of cyclical
atelectasis in vitro. Respir Physio Neurobiol 2014;191:1-8.
[7] O'Riordan TC, Zhdanov AV, Ponomarev GV, Papkovsky D B. Analysis of intracellular
oxygen and metabolic responses of mammalian cells by time-resolved fluorometry. Anal
Chem 2007;79:9414-9419.
18

[8] Zhang KY, Gao P, Sun G, Zhang T, Li X, Liu S, Zhao Q, Lo KK, Huang W.
Dual-phosphorescent iridium(III) complexes extending oxygen sensing from hypoxia to
hyperoxia. J Am Chem Soc 2018;140:7827-7834.
[9] Hempel AW, O'Sullivan MG, Papkovsky DB, Kerry JP. Assessment and use of optical
oxygen sensors as tools to assist in optimal product component selection for the
development of packs of ready-to-eat mixed salads and for the non-destructive
monitoring of in-pack oxygen levels using chilled storage. Foods 2013;2
[10] Vu CH, Won K. Novel water-resistant UV-activated oxygen indicator for intelligent
food packaging. Food Chem 2013;140 52-56.
[11] Arrieta MP, López J, Ferrándiz S, Peltzer MA. Novel water-resistant UV-activated
oxygen indicator for intelligent food packaging. Polym Test 2013;32 760-768.
:213-224.
:
:
[12] Nielsen AH, Hvitvedjacobsen T, Vollertsen J. Effects of pH and iron concentrations on
sulfide precipitation in waste water collection systems. Water Environ Res
2008;80:380-384.
[13] Koshiba Y, Nakamura Y, Ito D, Yokoyama T, Okazaki S, Nakagawa H, Arai T.
Development of a durable fiber-optic oxygen sensor for harsh underground
environments. Talanta 2010;82:1495-1499.
[14] Jouanneau S, Recoules L, Durand MJ, Boukabache A, Picot V, Primault Y, Lakel A,
Sengelin M, Barillon B, Thouand. Methods for assessing biochemical oxygen demand
(BOD): A review. Water Res 2014;49:62-82.
[15] Mclamore ES, Garland JL, Mackowiak C, Desaunay A, Garland N, Chaturvedi P,
Taguchi M, Dreaden K, Catechis J, Ullman JL. Development and validation of an open
19

source O2-sensitive gel for physiological profiling of soil microbial communities. J
Microbiol Methods 2014;96 62-67.
[16] Winkler LW. Die Bestimmung des im wasser gelösten sauerstoffes. Ber Der Deuts
Chem Ges 1888;21 2843-2854.
[17] Rice A, Costa ADS, Labrincha JA, Marques FMB. Electrochemical filters for oxygen
sensors with improved surface properties. Ionics 1997;3 36-43.
[18] Stetter JR, Li J. Amperometric gas sensors a review. Chem Rev, 2008,108
[19] Wang X, Wolfbeis S. Optical methods for sensing and imaging oxygen: materials,
spectroscopies and applications, Chem Soc Rev 2014;43 3666-3761.
:
:
:
:
352-366.
:
[20] Shi C, Li Q, Zou L, Lv Z. Yuan A, Zhao Q. A single anion based red emitting
cationic diiridium(III) complex bearing a pyrimidine based bridging ligand for oxygen
sensing. Eur J Inorg Chem 2018;1131-1136.
[21] Liu S, Wei L, Guo S, Jiang J, Zhang P, Han J, Ma Y, Zhao Q. A naphthalene-based
two-photon fluorescent probe for selective and sensitive detection of thiophenols. Sens
Actuators B 2018;262:436-443.
[22] Wang X, Stolwijk JA, Lang T, Sperber M, Meier RJ, Wegener J, Wolfbeis OS. Ultra-small,
highly stable, and sensitive dual nanosensors for imaging intracellular oxygen and pH in
cytosol. J Am Chem Soc 2012;134:17011-17014.
[23] Wang B, Zhang L, Li B, Li Y, Shi Y, Shi T. Synthesis, characterization, and oxygen sensing
properties of functionalized mesoporous silica SBA-15 and MCM-41 with a Pt(II)-porphyrin
complex. Sens Actuators B 2014;190:93-100.
[24] Mills A, Graham A, Rourke C. A novel, titania sol-gel derived film for luminescence-based
20

oxygen sensing. Sens Actuators B 2014;190:907-912.
[25] Xue R, Behera P, Xu J, Ciapiano MS, Lannutti. Polydimethylsiloxane core–polycaprolactone
shell nanofibers as biocompatible, real-time oxygen sensors. Sens Actuators
2014;192:697-707.
B
[26] Xu X, Xiao H, Deng A. A series of phosphorescent Cu(I) complexes and their oxygen sensing
performance in SBA-15 silica matrix. Opt Mater 2014;36:1542-1554.
[27] Gao Y, Chem T, Yamamoto S, Miyashita T, Mitsuishi M. Superhydrophobic Porous surfaces:
dissolved oxygen sensing. ACS Appl Mater Interfaces 2015;7:3468-3472.
[28] Borisov SM, Pommer R, Svec J, Peters S, Novakova V, Klimant I. New red-emitting Schiff
base chelates: promising dyes for sensing and imaging of temperature and oxygen via
phosphorescence decay time. J Mater Chem C 2018;6:8999-9009.
[29] Sánchez JFF, Cannas R, Spichiger S, Steiger R, Keller UES. Novel nanostructured materials
to develop oxygen-sensitive films for optical sensors. Anal Chim Acta. 2006;566:271-282.
[30] Tian N, Lenkeit D, Pelz S, Fischer LH, Escudero D, Schiewek R, Klink D, Schmitz OJ,
González L, Schäferling M. Structure-property relationship of red and green-emitting
iridium(III) complexes with respect to their temperature and oxygen sensitivity. Eur J Inorg
Chem 2010;4875-4885.
[31] Choung K, Marroquin K, Teets T. Cyclometalated iridium-BODIPY ratiometric O2 sensors.
Chem Sci 2019;10:5124-5132.
[32] Laskar I, Chen TM. Tuning of Wavelengths:ꢀ Synthesis and photophysical studies of iridium
complexes and their applications in organic light emitting devices. Chem Mater
2004;16:111-117.
21

[33] Kwon T, Cho H, Kim M, Kim J, Kim J, Lee K, Park S, Shin I, Kim H, Shin D. Color tuning
of cyclometalated iridium complexes through modification of phenylpyrazole derivatives and
ancillary ligand based on ab Initio calculations. Organometallics 2005;24:1578-1585.
[34] Fujita T, Fuchibe K, Ichikawa J. Transition metal mediated and catalyzed C-F bond
activation by fluorine elimination. Angew Chem Int Ed 2019;58:390-402.
[35] Wang J, Rosello M, Aceña J, Pozo C, Sorochinsky A, Fustero S, Soloshonok V, Liu H,
Fluorine in pharmaceutical industry: fluorine-containing drugs introduced to the market in
the last decade (2001-2011). Chem Rev 2014;114:2432-2506.
[36] Moon HC, Lodge TP, Frisbie CD. Solution-processable electrochemiluminescent ion gels for
flexible, low-voltage, emissive displays on plastic. J Am Chem Soc 2014;136:3705-3712.
[37] Yang Y, Guo L, Ge X, Zhu T, Chen W, Zhou H, Zhao L, Liu Z. The Fluorine Effect in
Zwitterionic Half-Sandwich Iridium(III) Anticancer Complexes. Inorg Chem
2020;59:748-758.
[38] Chi Y, Chou P. Transition-metal phosphors with cyclometalating ligands: fundamentals and
applications. Chem Soc Rev 2010;39:638-655.
[39] Boyaala R, Touzani R, Roisnel T, Dorcet V, Caytan E, Jacquemin D, Boixel J, Guerchais V,
Doucet H, Soule J. Catalyst-controlled regiodivergent C-H arylation site of fluorinated
2-arylpyridine derivatives: application to luminescent iridium(III) complexes. ACS Catal
2019;9:1320-1328.
[40] Zhao Q, Zhou X, Cao T, Zhang K, Yang L, Liu S, Liang H, Yang H, Li F, Huang W.
Fluorescent/phosphorescent dual-emissive conjugated polymer dots for hypoxia bioimaging.
Chem Sci 2015;6:1825-1831.
22

[41] Chen Y, Liu C, Wang L. Effects of fluorine substituent on properties of cyclometalated
iridium(III) complexes with a 2,2'-bipyridine ancillary ligand. Tetrahedron 2019;75:130686.
[42] Xing Y, Wang L, Liu C, Jin X. Effects of fluorine and phenyl substituents on oxygen
sensitivity and photostability of cyclometalated platinum(II) complexes. Sens Actuators B
2020;304:127378.
[43] Lai SW, Hou YJ, Che CM, Pang HL, Wong KY, Chang CK, Zhu N. Electronic spectroscopy,
photophysical properties, and emission quenching studies of an oxidatively robust
perfluorinated platinum porphyrin. Inorg Chem 2004;43:3724-3732.
[44] Yu M, Zhao Q, Shi L, Li F, Zhou Z, Yang H, Yi T, Huang C. Cationic iridium(iii) complexes
for phosphorescence staining in the cytoplasm of living cells. Chem Commun
2008;2115-2117.
[45] Suárez M, Curchod BF, Tavernelli I, Rothlisberger U, Scopelliti R, Jung I, Censo DD,
Grätzel M, Sánchez JF, Gutiérrez AF, Nazeeruddin MK, Baranoff E. Nanocomposites
containing neutral blue emitting cyclometalated iridium(III) emitters for oxygen sensing.
Chem Mater 2012;24:2330-2338.
[46] Liu C, Song X, Rao X, Xing Y, Wang Z, Zhao J, Qiu J. Novel triphenylamine-based
cyclometalated platinum(II) complexes for efficient luminescent oxygen sensing. Dyes Pigm
2014;101:85-92.
[47] Liu C, Yu H, Rao X, Lv X, Jin Z, Qiu J. Bis-cyclometalated Ir(III) complexes with a
diphenylamino group: design, synthesis, and application in oxygen sensing. Dyes Pigm
2017;136:641-647.
[48] Wang L, Gao Z, Liu C, Jin X. A diphenylamino-substituted cationic cyclometalated Ir(III)
23

complex: its aggregation-induced phosphorescent emission and oxygen sensing properties.
Mater Chem Front 2019;3:1593-1600.
[49] Liu C, Rao X, Song X, Qiu J, Jin Z. Palladium-catalyzed ligand-free and aqueous Suzuki
reaction for the construction of (hetero)aryl-substituted triphenylamine derivatives. RSC Adv
2013;3:526-531.
[50] Yu H, Liu C, Yu Z, Zhang L, Xiu J. Effect of ancillary ligands on the properties of
diphenylphosphoryl-substituted cationic Ir(III) complexes.
2017;5:3519-3527.
J
Mater Chem
C
[51] Zhou Y, Gao H, Wang X, Qi H. Electrogenerated chemiluminescence from heteroleptic
iridium(III) complexes with multicolor emission. Inorg Chem 2015;54:1446-1453.
[52] Lee W, Kwon T, Kwon J, Kim J, Lee C, Hong J. Effect of main ligands on organic
photovoltaic performance of Ir(III) complexes. New J Chem 2011;35:2557-2563.
[53] Ho CL, Wong WY, Zhou GH, Yao B, Xie ZY, Wang LX. Solution-processible
multi-component cyclometalated iridium phosphors for high-Efficiency orange-emitting
oleds and their potential use as white light sources. Adv Funct Mater 2007;54:2925-2936.
[54 Liu L, Wang X, Hussain F, Zeng C, Wang B, Li Z, Kozin I, Wang S. Multiresponsive
tetradentate phosphorescent metal complexes as highly sensitive and robust luminescent
oxygen sensors: Pd(II) versus Pt(II) and 1,2,3-triazolyl versus 1,2,4-triazolyl. ACS Appl
Mater Interfaces 2019;11:12666-12674.
[55] Liu C, Yu H, Xing Y, Gao Z, Jin Z. Photostable ester-substituted bis-cyclometalated cationic
iridium(III) complexes for continuous monitoring oxygen. Dalton Trans 2016;45:734-741.
[56] Carraway ER, Demas JN, Degraff BA, Bacon JR. Photophysics and photochemistry of
24

oxygen sensors based on luminescent transition-metal complexes. Anal Chem
1991;63:337-342.
25

Research Highlights
1. Two novel fluorinated cationic cyclometalated Ir(III) complexes were synthesized.
2. The position of fluorine affects the properties of Ir(III) complexes strongly.
3. The 6-fluorinated Ir(III) complex exhibits the highest oxygen sensitivity.

Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests: