The dependence of oxygen sensitivity on the molecular structures of Ir(iii) complexes and their application for photostable and reversible luminescent oxygen sensing

Article abstract of DOI:10.1039/c9ra02277e

Three Ir(iii) complexes IrC1, IrC2, and IrC3 substituted with 4-(diphenylamino)phenyl (TPA), 4-(9H-carbazol-9-yl)phenyl (Cz1), and 9-phenyl-9H-carbazol-3-yl (Cz2) moieties were prepared and fully characterized as phosphorescent emitters. In comparison with Ir(ppy)₃, introduction of TPA, Cz1, and Cz2 moieties strongly improved the oxygen sensitivities of IrC1-IrC3. Short-decayed IrC1 with I₀/I₁₀₀ of 168.6 and KappSV of 202.2 bar^-1 in THF exhibited the highest sensitivity for oxygen. TPA and Cz moieties caused remarkable collision radius variations of the Ir(iii) complexes with 2.13 ± 0.08 for σ_IrC1/σ_Ir(ppy)3, 1.24 ± 0.06 for σ_IrC2σ_Ir(ppy)3, and 1.54 ± 0.08 for σ_IrC3σ_Ir(ppy)3. For demonstrating the dependence of oxygen sensitivity on the molecular structure of the oxygen-sensitive probes (OSPs), the delocalization of spin populations (DSPs) has been applied for the first time to confirm the collision radius variations of Ir(iii) complexes. Remarkable DSPs were found on the TPA, Cz1, and Cz2 moieties with the spin population (percentage of the spin population) of 0.23210 (11.61%), 0.08862 (4.43%), and 0.13201 (6.60%), respectively. And strong linear correlations (R² = 0.997) between the collision radius variations and spin population on TPA and Cz moieties were apparent. The DSPs could be used to describe the dependence of oxygen sensitivity on the molecular structure of the OSPs. For achieving real-time oxygen sensing, the photostability, oxygen sensing performance, and operational stability of IrC1-IrC3 and Ir(ppy)₃ immobilized in ethyl cellulose (EC) were investigated. The IrC1-EC film demonstrated outstanding photostability after 60 min of irradiation and excellent operational stability for continuous oxygen monitoring with no attenuation of the original emission intensity in 4000 s. This study quantified and analyzed the dependence of oxygen sensitivity on the molecular structure of Ir(iii) complexes for the first time and illustrated a feasible approach to achieve high-efficiency sensors for real-time monitoring of oxygen.

Full text of DOI:10.1039/c9ra02277e

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PAPER
The dependence of oxygen sensitivity on the
molecular structures of Ir(III) complexes and their
application for photostable and reversible
luminescent oxygen sensing†
a
Cite this: RSC Adv., 2019, 9, 15370
Yang Xing,
Chengfang Qiao,b Xinmin Li,c Chun Li,a Honghao Wang,a Fayun Li,a
ad
a
*
*
Ling Di
and Zhanxu Yang
Three Ir(III) complexes IrC1, IrC2, and IrC3 substituted with 4-(diphenylamino)phenyl (TPA), 4-(9H-
carbazol-9-yl)phenyl (Cz1), and 9-phenyl-9H-carbazol-3-yl (Cz2) moieties were prepared and fully
characterized as phosphorescent emitters. In comparison with Ir(ppy)3, introduction of TPA, Cz1, and
Cz2 moieties strongly improved the oxygen sensitivities of IrC1–IrC3. Short-decayed IrC1 with I0/I100 of
app
SV
168.6 and K
of 202.2 barꢀ1 in THF exhibited the highest sensitivity for oxygen. TPA and Cz moieties
caused remarkable collision radius variations of the Ir(III) complexes with 2.13 ꢁ 0.08 for sIrC1/sIr(ppy)
,
3
1.24 ꢁ 0.06 for sIrC2/sIr(ppy) , and 1.54 ꢁ 0.08 for sIrC3/sIr(ppy) . For demonstrating the dependence of
3
3
oxygen sensitivity on the molecular structure of the oxygen-sensitive probes (OSPs), the delocalization
of spin populations (DSPs) has been applied for the first time to confirm the collision radius variations of
Ir(III) complexes. Remarkable DSPs were found on the TPA, Cz1, and Cz2 moieties with the spin
population (percentage of the spin population) of 0.23210 (11.61%), 0.08862 (4.43%), and 0.13201
(6.60%), respectively. And strong linear correlations (R2 ¼ 0.997) between the collision radius variations
and spin population on TPA and Cz moieties were apparent. The DSPs could be used to describe the
dependence of oxygen sensitivity on the molecular structure of the OSPs. For achieving real-time
oxygen sensing, the photostability, oxygen sensing performance, and operational stability of IrC1–IrC3
and Ir(ppy)3 immobilized in ethyl cellulose (EC) were investigated. The IrC1-EC film demonstrated
outstanding photostability after 60 min of irradiation and excellent operational stability for continuous
oxygen monitoring with no attenuation of the original emission intensity in 4000 s. This study quantified
and analyzed the dependence of oxygen sensitivity on the molecular structure of Ir(III) complexes for the
first time and illustrated a feasible approach to achieve high-efficiency sensors for real-time monitoring
of oxygen.
Received 25th March 2019
Accepted 12th May 2019
DOI: 10.1039/c9ra02277e
rsc.li/rsc-advances
(Winkler titration4 and electroanalytical5), luminescent
sensing has become extremely signicant in the last two
1. Introduction
decades due to the overwhelming advantages of its nonde-
structive characteristics, sensitive luminescence response,
excellent reversibility, and versatility of formats (lm, ber,
nanoparticle, etc.).6 Such type of exible approach depends
on the dynamic (diffusion-controlled) luminescence (phos-
phorescence or uorescence) quenching of oxygen-sensitive
probes (OSPs).7,8 The transition metal complexes are OSPs
of choice, including Ir(III),9–12 Pt(II),13,14 Ru(II)15,16 complexes,
and Pd(II) porphyrins.17,18
Oxygen plays an essential role in evolution and reproduction,
and is indispensable for all forms of living organisms.1 The
determination of oxygen has attracted special attention in
biomedicine and environmental monitoring.2,3 When
compared with traditional methods of oxygen determination
aCollege of Chemistry, Chemical Engineering, and Environmental Engineering,
Liaoning Shihua University, Fushun 113001, China. E-mail: diling@lnpu.edu.cn;
zxyanglnpu@163.com
In general, the oxygen sensitivities of luminescence
sensors are roughly proportional to luminescent decay times
(s) of OSPs.19,20 And state of the art luminescent sensors
consisted of OSPs possessing luminescent decay times in
a wide range of milliseconds to microseconds.21 Efforts have
been made to design OSPs with long decay times.8,22,23
bCollege of Chemical Engineering and Modern Materials, Shangluo University,
Shangluo 726000, China
cSchool of Pharmacy, Zunyi Medical University, Zunyi, 563000, China
dState Key Laboratory of Fine Chemicals, Dalian University of Technology, Linggong
Road 2, Dalian 116024, China
† Electronic supplementary information (ESI) available: Phosphorescent emission
decay curves and cyclic voltammogram. See DOI: 10.1039/c9ra02277e
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Matrices have been widely used to provide microenvironment (diphenylamino)phenyl (TPA), 4-(9H-carbazol-9-yl)phenyl
for OSPs as encapsulation and regulate the determination (Cz1), and 9-phenyl-9H-carbazol-3-yl (Cz2) moieties. Molec-
range of oxygen.24,25 High oxygen permeability of matrices ular structures and synthetic routes of IrC1, IrC2, IrC3 and
commonly provided high sensitivity and low limit of detec- Ir(ppy)3 were illustrated in Scheme 2.
tion (LOD).21,26–28 Luminescent decay times and oxygen
sensitivities of OSPs are sometimes not in direct proportion
(short-decayed OSPs with high oxygen sensitivities or vice
versa) in the same matrix with the same approach of prepa-
2. Experimental
2.1. Materials and methods
ration. Astoundingly, the abnormal phenomena occurred
frequently in the related researches.9,10,29–31 For instance,
Ir(III) complexes Ir(III)-a and Ir(III)-b were immobilized in
a nano aluminum based matrix (Scheme 1A). In comparison
to short-decayed Ir(III)-a, lower oxygen sensitivity of Ir(III)-
b with long decay time was showed.9 A t-butyl substituted
Ir(III) complex Ir(III)-d was prepared based on Ir(III)-c (Scheme
1B). The oxygen sensitivity of short-decayed Ir(III)-c was 6-fold
higher than that of long-decayed Ir(III)-d.10 Zhao et al. con-
ducted a research on the two Pt(II) complexes Pt(II)-a and
Pt(II)-b with phenylthiazo and thiazo-coumarin ligands,
respectively (Scheme 1C). And there was a similar phenom-
enon that short-decayed Pt(II)-a showed considerably high
oxygen sensitivity than that of Pt(II)-b.29 Coincidentally,
parallel results were obtained in our previous reports.32,33
Therefore, the molecular structure of OSPs has potential
inuence which should not be ignored on the oxygen
sensitivity.
Carbazole and triphenylamine moieties have drawn a wide
range of attention in organic light-emitting diodes
(OLEDs)34–38 and dye-sensitized solar cells (DSSCs) due to
intriguing properties of high light-to-electrical energy
conversion efficiencies, strong electron-donating nature and
good hole-transporting abilities.39–41 Herein, we report
a systematic quantication and analysis of the dependence of
oxygen sensitivity on the molecular structure of OSPs by the
modication of traditional Ir(III) complex Ir(ppy)3 with 4-
The corresponding boronic acids, Pd(OAc)2, Pd(PPh3)4, and
IrCl3$3H2O were purchased from Alfa Aesar. Other puried
reagents were purchased from Greagent. 1H NMR and 13C
NMR spectra were measured on a BRUKER AVANCE III HD
spectrometer (400 MHz). High-resolution MS were measured
with a 1290/6224 HPLC/MS spectrometer.20 The UV/Vis
spectrometer (cary 5000) and uorescence spectrophotom-
eter (cary eclipse G9800A) of Agilent Technology were used to
record absorption spectra and emission spectra. The excited-
state lifetime curves were recorded on LP920 Laser Flash
photolysis apparatus of Edinburgh Instruments Ltd (excita-
tion wavelength: 355 nm, pulse width: 6.4 ns, frequency:
3 Hz, single pulse energy: 1 mJ). The 2273 electrochemical
workstation of Princeton Applied Research was used for the
cyclic voltammetry tests.
2.2. Synthesis section
Synthetic procedure of ligand L1, L2, and L3. 0.5 mmol of 2-
(3-bromophenyl)pyridine (116.5 mg), 0.75 mmol of corre-
sponding boronic acid, 2.0 equiv. of K2CO3 (138.0 mg),
Pd(PPh3)4 (5.0 mol%, 0.025 mmol, 28.9 mg), and THF/H2O
(10 mL/6 mL) were mixed and stirred at 85 ꢂC in N2 for 6
hours. Then 15 mL brine was added into the mixture. Then
the mixture was extracted with ethyl acetate (EA, 4 ꢃ 10 mL).
The organic phase was condensed and the corresponding
product was isolated by column chromatography using
petroleum ether/EA (50 : 1 for L1, 20 : 1 for L2, and L3, v : v).
Scheme 1 Chemical structures of referenced Ir(III) and Pt(II) OSPs. Structure variations is shown with red.
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Scheme 2 Molecular structures and synthetic routes of ligands and Ir(III) complexes. (A) (3-Bromophenyl)boronic acid, Pd(OAc)2, K2CO3,
C2H5OH/H2O (3 : 1, v : v), 85 ꢂC, 3 hours. (B) RB(OH)2, Pd(PPh3)4, K2CO3, THF : H2O 5 : 3 (v : v), 80 ꢂC in N2, 6 hours. (C) IrCl3$3H2O, C2H5-
OCH2CH2OH/H2O (3 : 1, v : v), 110 ꢂC in N2, 36 hours. (D) The corresponding ligands, Na2CO3, glycerol, 200 ꢂC in N2, 48 hours.
L1, N,N-Diphenyl-30-(pyridin-2-yl)-[1,10-biphenyl]-4-amine. reacted with 1.2 equiv. (0.48 mmol) corresponding ligand (L1
1
Yield: 82.2%; white solid. H NMR (400 MHz, CDCl3) d 8.72 191 mg, L2 190 mg, L3 190 mg, respectively) and 5 equiv. of
ꢂ
(d, 1H), 8.22 (d, 1H), 7.93 (s, 1H), 7.79 (d, 2H), 7.63 (s, 1H), Na2CO3 (4 mmol, 424 mg) in 25 mL glycerol at 200 C for 2
7.60–7.50 (m, 3H), 7.27 (s, 5H), 7.17 (s, 3H), 7.15 (s, 3H), 7.05 days under N2. Upon cooling to room temperature, the
(d, 2H).42
mixture was poured into water and extracted with CH2Cl2 (3
L2, 9-(30-(Pyridin-2-yl)-[1,10-biphenyl]-4-yl)-9H-carbazole. ꢃ 25 mL). The organic phase was condensed and the corre-
1
Yield: 79.1%; white solid. H NMR (400 MHz, CDCl3) d 8.78 sponding product was isolated by column chromatography
(d, 1H), 8.38 (s, 1H), 8.16 (d, 2H), 8.03 (d, 1H), 7.93 (d, 2H), using dichloromethane.33
7.88 (s, 2H), 7.78 (d, 1H), 7.70–7.60 (m, 3H), 7.52–7.40 (m,
IrC1: yield: 77.4%; yellow powder. 1H NMR (400 MHz,
4H), 7.31 (t, 3H). 13C NMR (101 MHz, CDCl3) d 157.1, 149.6, CDCl3) d 7.96 (s, 3H), 7.86 (s, 3H), 7.64–7.57 (m, 6H), 7.50 (s,
140.9, 140.8, 140.1, 139.8, 137.1, 137.0, 129.4, 128.7, 127.8, 3H), 7.48 (d, 3H), 7.23 (s, 6H), 7.20 (s, 3H), 7.12 (s, 12H), 7.10–
127.3, 126.2, 125.9, 123.4, 122.4, 120.8, 120.3, 119.9, 109.8. 7.02 (m, 12H), 7.00 (s, 3H), 6.99–6.94 (m, 6H), 6.91–6.86 (m,
HRMS (ESI): calc. for C29H21N2: 397.1705 [M + H]+. Found: 3H). 13C NMR (101 MHz, CDCl3) d 169.7, 167.6, 146.9, 143.2,
397.1702 [M + H]+.
L3, 9-Phenyl-3-(3-(pyridin-2-yl)phenyl)-9H-carbazole. 125.3, 123.9, 123.1, 121.2, 121.0, 118.5. HRMS (ESI) for
142.9, 139.3, 136.8, 136.4, 136.1, 131.2, 130.1, 129.1, 128.4,
1
Yield: 77.2%; white solid. H NMR (400 MHz, CDCl3) d 8.77
C
87H64IrN6, calc.: 1385.4822 [M + H]+. Found: 1385.4820 [M +
(d, 1H), 8.46 (s, 1H), 8.37 (s, 1H), 8.22 (d, 1H), 7.96 (d, 1H), H]+.42
7.85 (t, 2H), 7.81–7.72 (m, 2H), 7.66–7.56 (m, 5H), 7.52–7.46
IrC2: yield: 70.4%; yellow powder. 1H NMR (400 MHz,
(m, 2H), 7.43 (d, 2H), 7.31 (dt, 2H). 13C NMR (101 MHz, CDCl3) d 8.14 (t, 9H), 8.05 (t, 6H), 7.88 (d, 6H), 7.68 (d, 6H),
CDCl3) d 157.6, 149.6, 142.6, 141.3, 140.5, 139.8, 137.6, 136.9, 7.63–7.57 (m, 6H), 7.49–7.39 (t, 18H), 7.30 (d, 6H). 13C NMR
133.2, 129.9, 129.2, 128.0, 127.5, 127.1, 126.1, 126.0, 125.6, (101 MHz, CDCl3) d 166.2, 150.2, 141.3, 141.1, 138.6, 137.8,
125.2, 123.9, 123.5, 122.2, 120.9, 120.5, 120.1, 119.0, 110.0, 135.4, 135.0, 129.7, 129.2, 128.4, 127.9, 127.0, 125.3, 122.4,
109.9. HRMS (ESI): calc. for C29H21N2: 397.1705 [M + H]+. 121.3, 120.2, 119.6, 119.1, 108.4. HRMS (ESI) for C87H58IrN6,
Found: 397.1703 [M + H]+.
calc.: 1379.4352 [M + H]+. Found: 1379.4347 [M + H]+.
Synthetic procedure of Ir(III) complexes IrC1, IrC2, and IrC3.
IrC3: yield: 75.2%; yellow powder. 1H NMR (400 MHz,
The corresponding ligand 0.8 mmol (L1 318.4 mg, L2 CDCl3) d 8.39 (s, 3H), 8.17 (d, 6H), 8.10 (d, 6H), 8.05 (s, 6H),
316.8 mg, L3 316.8 mg, respectively) was reacted with 0.6 7.68 (d, 6H), 7.60 (d, 12H), 7.47–7.39 (m, 12H), 7.30 (d, 6H).
equiv. of IrCl3$3H2O (0.48 mmol, 169 mg) in C2H5OCH2- 13C NMR (101 MHz, CDCl3) d 167.5, 150.8, 143.3, 142.9, 140.1,
ꢂ
CH2OH/H2O (12 mL/4 mL) at 110 C for 36 h under N2. The 138.8, 136.7, 135.4, 134.4, 133.3, 130.0, 128.8, 127.7, 126.3,
solution was condensed to afford a powder of iridium 125.9, 124.8, 124.2, 122.7, 122.5, 121.8, 121.3, 119.3, 118.8,
chloride-bridged dimer. Without purication, the dimer was
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Fig. 1 (A) UV-vis spectra and (B) luminescent spectra of IrC1–IrC3 and Ir(ppy)3.
Table 1 Photophysical parameter of IrC1–IrC3 and Ir(ppy)3
a
Emission b
Absorption
d
e
f
Complexes
Ir(ppy)3
labs (nm) (3, 104 mol Lꢀ1 cmꢀ1
)
lem (nm)
s (ms) c
Fp
kr (105 sꢀ1
)
knr (105 sꢀ1
)
284 (3.75), 379 (1.12), 411 (0.71), 456
(0.27), 490 (0.12)
511
2.38
0.40
1.68
2.52
IrC1
IrC2
332 (9.48), 404 (1.32), 472 (0.27)
290 (9.31), 327 (7.15), 340 (6.34), 389
(0.95), 447 (0.25)
522
516
2.42
2.76
0.53
0.50
2.19
1.81
1.94
1.81
IrC3
299 (11.69), 349 (2.62), 397 (0.82), 421
(0.51)
525
3.19
0.50
1.57
1.56
a
b
In THF at a concentration of 10ꢀ5 mol Lꢀ1
.
In degassed THF. c lex ¼ 355 nm. d Relative to Ir(ppy)2(acac) (Fp ¼ 0.34). e kr ¼ Fp/s. f knr ¼ (1/s) ꢀ kr.
117.7, 117.1, 108.8, 108.7. HRMS (ESI) for C87H58IrN6, calc.: emission maxima of IrC1 was shied to a longer wavelength
1379.4352 [M + H]+. Found: 1379.4342 [M + H]+.
by TPA moieties. The phosphorescence quantum yields (Fp)
of IrC1 (0.53), IrC2 (0.50), IrC3 (0.50) were enhanced
compared to Ir(ppy)3 (0.40) (Table 1). The luminescent decay
times (s) were in the sequence of IrC3 (3.19 ꢁ 0.02 ms) > IrC2
(2.76 ꢁ 0.01 ms) > IrC1 (2.42 ꢁ 0.03 ms) > Ir(ppy)3 (2.38 ꢁ 0.02
ms). The s in microseconds inferred that the emissions were
phosphorescent (Table 1 and Fig. S1, see ESI†).44 Radiative
(kr) and nonradiative (knr) decay rates were calculated by Fp
and s. And the radiative lifetimes srad(1/kr) for IrC1–IrC3 and
Ir(ppy)3 were 4.57 ms, 5.52 ms, 6.40 ms, and 5.95 ms, respec-
tively (Table 1). Compared to IrC1, the extended srad of IrC2
and IrC3 was possibly attributed to enhanced conjugation
effects which was caused by Cz moieties.45 The changes in
emission color were quantied with CIE coordinates of
(0.2750, 0.6268) for Ir(ppy)3, (0.3164, 0.6341) for IrC1,
(0.3286, 0.6090) for IrC2, and (0.3242, 0.6332) for IrC3
(Fig. S2, see ESI†).
3. Result and discussion
3.1. Photophysical properties
UV-visible absorption of IrC1–IrC3 and Ir(ppy)3 have been
measured in THF at room temperature (Fig. 1A and Table 1).
Strong bands up to 310 nm attributed to p–p* transitions of
intraligands. Bands of lower-energy absorption at 350 nm
were assigned to metal-to-ligand charge transfer transitions
(MLCT). The spin-forbidden transitions of triplet states
ascribed to weak bands at 400 nm with absorption coeffi-
cients of 2000 Mꢀ1 cmꢀ1. The similar band positions and
signicant differences of absorption coefficients of IrC1–IrC3
and Ir(ppy)3 were observed. In comparison with Ir(ppy)3,
intensity of the rst band at 280 nm and the second band at
350 nm of IrC1–IrC3 were enhanced which was attributed to
p–p* transition and MLCT of intraligands. It is probably due
to the enhanced delocalization of the excited electron on
3.2. Electrochemical properties
TPA, Cz1, and Cz2 moieties which was conrmed by theo- The electrochemical behaviors of IrC1–IrC3 and Ir(ppy)3 were
retical calculations (Fig. 2).43
studied by cyclic voltammetry (Table 2). The introduction of
The emission maxima were in region from 511 to 525 nm TPA moieties leaded to more negative oxidation potential
which was excited in the MLCT bands (Fig. 1B and Table 1). (0.78 V) and higher HOMO energy level (ꢀ5.18 eV) of IrC1.
In comparison to Ir(ppy)3 (511 nm), introduction of TPA and And the introduction of Cz moieties showed more obvious
Cz moieties showed bathochromic shi of emission maxima cathodic shis of oxidation potentials of IrC2 (0.72 V) and
of IrC1 (522 nm), IrC2 (516 nm), and IrC3 (525 nm). The IrC3 (0.67 V) in comparison with Ir(ppy)3 (0.84 V). Ir(ppy)3
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Fig. 2 Electron density isosurfaces (0.02 a.u.) of frontier molecular orbitals of Ir(III) complexes.
Table 2 Electrochemical data and frontier orbital energy levels of IrC1–IrC3 and Ir(ppy)3
Ir(III)
complexes
Eoonx seta (V)
EHOMO (eV)
ELUMO (eV)
ledge (nm)
E00e (eV)
Eo*x ðVÞ
b
c
d
f
Ir(ppy)3
IrC1
IrC2
0.84
0.78
0.72
0.67
ꢀ5.24
ꢀ5.18
ꢀ5.12
ꢀ5.07
ꢀ2.96
ꢀ3.02
ꢀ2.89
ꢀ2.96
544
574
556
587
2.28
2.16
2.23
2.11
ꢀ1.44
ꢀ1.38
ꢀ1.51
ꢀ1.44
IrC3
a
Onset oxidation potential. 0.1 mol Lꢀ1 [Bu4N]PF6 was dissolved in degassed THF. The standard electrode was SCE. b HOMO energy level, EHOMO
(eV) ¼ ꢀe(4.4 + Eoonx set). c LUMO energy level, ELUMO ¼ EHOMO + E00
.
Absorption edge data, which were measured from solid UV-vis spectra. e Zero–
d
f
on set
ox
46
zero emission energy level. E00 ¼ 1240/ledge
.
Oxidation potential of excited state: Eo*x ¼ E
ꢀ E00
:
and IrC1 demonstrated quasi-reversible reduction waves, polarizable continuum model (PCM ¼ THF) were applied to
which exhibited electrochemical stability and relative diffi- optimize the ground-state geometries of Ir(III) complexes20
culty of oxidation. While IrC2 and IrC3 showed irreversible (Fig. 2). TPA moieties of IrC1 take no-co-planar geometry.
reduction waves (Fig. S3, see ESI†). Therefore all the HOMO Contrastively, large p-conjugation were presented on Cz1 and
energy levels were calculated by the onset oxidation poten- Cz2 moieties of IrC2 and IrC3 just as expected. It was clear
tials (Eooxn set) (Table 2). The data of zero–zero emission energy that 2-phenylpyridine moieties signicantly contributed the
levels (E00) were calculated by the absorption edge data (ledge
)
LUMOs of IrC1–IrC3 and Ir(ppy)3. And none of electron pop-
of solid UV-vis spectra (Fig. S4, see ESI†). The data of E00 with ulation on Ir(III) atom, TPA, and Cz moieties of LUMOs. Ir(III)
order of Ir(ppy)3 (2.28 eV) > IrC2 (2.23 eV) > IrC1 (2.16 eV) > atoms and benzene rings of 2-phenylpyridines mainly
IrC3 (2.11 eV) were in accordance with the order of emission contributed the HOMOs, while pyridine rings slightly
maxima in Fig. 1B.
contributed the HOMOs. The electron delocalization was
increased on the propeller structures of IrC1. While the p-
conjugation reduced the electron delocalization on Cz moie-
ties of HOMOs of IrC2 and IrC3. These frontier molecular
orbital energies in Fig. 2 and Table 2 were not over-interpreted
3.3. Theoretical calculations
Density functional theory (DFT) calculations (B3LYP level,
mixed basis sets of 6-31G* and LanL2DZ) based on the
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because of acceptable differences between experimental and
calculated data.47
I
s
f1
f2
¼
¼
þ
(2)
I0
s0
1 þ KSV1
p
O2
1 þ KSV2
p
O2
3.4. Luminescence sensing of molecular oxygen
Decline ratios of emission intensity of IrC1–IrC3 and
Ir(ppy)3 were tted by the two-site model in Fig. 4 and Table
The Stern–Volmer relationship (eqn (1)) described the
S1 (see ESI†). The I0/I100 of IrC1–IrC3 and Ir(ppy)3 were 168.6,
diffusion-controlled
luminescent
quenching
process
app
115.1, 158.1, and 79.4, respectively. And K
of IrC1–IrC3
SV
between OSPs and oxygen. I and s are emission intensities
and luminescent decay times of OSPs, respectively. I0 and s0
are corresponding values in the absence of oxygen. kq means
the bimolecular quenching constant and [O2] expresses the
molar concentration of molecular oxygen. The Stern–Volmer
plots (SVPs) are gotten with slopes of kqs0 by linear tting.
and Ir(ppy)3 were 202.2 barꢀ1, 144.1 barꢀ1, 181.1 barꢀ1, and
119.6 barꢀ1. IrC1 demonstrated the highest oxygen sensitivity
than others. The oxygen sensitivities were in the order of IrC1
> IrC3 > IrC2 > Ir(ppy)3. Hence, introducing TPA and Cz
moieties distinctly made Ir(III) complexes more sensitive to
oxygen. Noticeably, large quenchable portions (f1) of IrC1
(0.998), IrC2 (0.999), and IrC3 (0.999) were improved
compared with that of Ir(ppy)3 (0.985), which demonstrated
considerable linearity of SVPs and more homogeneous
I0
I
s0
s
¼
¼ 1 þ kqs0½O2ꢄ
(1)
In order to drastically eliminate the inuence of hetero-
geneity and accurately characterize original oxygen sensitiv-
ities, puried THF was used to disperse Ir(III) complexes. The
emission intensities of IrC1–IrC3 and Ir(ppy)3 were obviously
weaken with incremental concentration of O2 in the air
volume of 0–100%. 6.8% of oxygen concentration could
quench over 81% of original emission intensities. Notewor-
thily, above 90% of original emission intensity of IrC1 was
quenched with 6.8% of oxygen which demonstrated high
oxygen sensitivity (Fig. 3).
The two-site model48 was applied to t the SVPs (eqn (2)).
Where f1 and f2 are the quenchable and unquenchable frac-
tions, respectively. And KSV1 and KSV2 are quenching rate
constants for corresponding fractions. f1 + f2 ¼ 1. pO means
2
oxygen partial pressure. Oxygen sensitivities of OSPs are illus-
Fig. 4 SVPs for IrC1–IrC3 and Ir(ppy)3 in THF (fitted by two-site
model).
app
app
SV
trated by the weighted constant K
(K
¼ f1KSV1 + f2KSV2).
SV
Fig. 3 Dependence of luminescent spectra of Ir(III) complexes in THF (10ꢀ5 mol Lꢀ1) of 0–100% O2.
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ÂÀ Á
Ã
I0
microenvironment. The limit of detections (LODs) of IrC1–
IrC3 and Ir(ppy)3 in THF were 0.27 mbar, 0.40 mbar, 0.28
mbar, and 0.43 mbar, respectively (see ESI†).
ꢀ 1 s2
s1
s2
I
ÂÀ Á
I0
1 ꢀ 1 s1
Ã
¼
(4)
I
2
The ratios of collision radii between IrC1–IrC3 and
3.5. Variations of the collision radius
Ir(ppy)3 (sIr(III)/sIr(ppy) ) represented the collision radius vari-
3
Winnik et al. reported the fundamental expression of lumi-
ations of OSPs. With luminescent decay times, the sIr(
/
)
III
nescent oxygen sensing (eqn (3)).7 In eqn (3), PO means
2
s
have been calculated by measuring data of I0 and I100
Ir(ppy)3
oxygen permeability of solvents or matrices. NA means the
constant of Avogadro. a expresses the constant of lumines-
cent quenching probability. s represents the collision radii of
ve times in parallel (Table S2, see ESI†). The ratios of
collision radii were 2.13 ꢁ 0.08 for sIrC1/sIr(ppy) , 1.24 ꢁ 0.06
3
for sIrC2/sIr(ppy) , and 1.54 ꢁ 0.08 for sIrC3/sIr(ppy) , respec-
3
3
OSPs to molecular oxygen. In eqn (3), PO describes the
2
tively, with the sequence of IrC1 > IrC3 > IrC2 > Ir(ppy)3
(Fig. 5). In contrast to Ir(ppy)3, s of IrC1 was obviously
increased by TPA moieties. And in comparison with IrC1, the
introduction of Cz1 and Cz2 moieties markedly decreased the
collision radii of IrC2 and IrC3, respectively.
dependence of oxygen sensitivity on matrices. And high
values of PO caused high oxygen sensitivities of OSPs.26 s0
2
illustrates the dependence of oxygen sensitivity on the pho-
tophysical properties of OSPs. Long-lived s0 leaded to high
oxygen sensitivities of OSPs.19,21 The variations of collision
radii (s) describe the dependence of oxygen sensitivity on
molecular structures of OSPs. OSPs with high values of s
should be more sensitive to oxygen.
In this research, the delocalization of spin populations
(DSPs) was for the rst time applied to conrm the collision
radius variations of Ir(III) complexes. The spin densities of T1-
state IrC1–IrC3 and Ir(ppy)3 were calculated using unre-
stricted DFT calculations (B3LYP level, mixed basis sets of 6-
31G* and LanL2DZ) (Fig. 6 and Table S3, see ESI†).49,50 The
spin-density contours of IrC1–IrC3 and Ir(ppy)3 were domi-
nated by p orbitals with spin population Pspin (percentages of
I0
I
s0
s
4paNA
¼
¼ 1 þ
ðPO Þðss0ÞpO
(3)
2
2
1000
According to eqn (3), the ratio of collision radii (s1/s2)
between two OSPs was obtained by the expression (eqn (4)),
reads as
Fig. 7 Spin-density integral curves of the optimized T1-state geometry
of IrC1–IrC3 and Ir(ppy)3 along the Z axis.
Fig. 5 The ratios of collision radii for IrC1–IrC3 versus Ir(ppy)3.
Fig. 6 Spin-density contours (0.003 a.u.) of IrC1–IrC3 and Ir(ppy)3. Magenta and green contours mean positive and negative phase. Arrows
exhibited the spin population variations of on N atoms.
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spin population, Pspin%
)
of 1.66310 (83.15%), 1.59881 Notably, considerable DSPs were demonstrated on TPA
(79.94%), 1.62016 (81.00%) and 1.55760 (77.88%), respec- moieties than Cz1 and Cz2 moieties in the range of 17.5 to
tively. One of intraligands of Ir(III) complexes signicantly 27.5 bohr (Fig. 7). The integral curves conrmed the collision
contributed the spin-orbital contours. Remarkable DSPs were radius variations of Ir(III) complexes in Fig. 5.
found on the TPA, Cz1, and Cz2 moieties with Pspin (Pspin%) of
To demonstrate the relationship between the collision
0.23210 (11.61%), 0.08862 (4.43%), and 0.13201 (6.60%), radius variations and the DSPs of Ir(III) complexes. The ratios
respectively (Table S3, see ESI†). TPA moieties caused more of collision radii (sIr(III)/sIr(ppy) ) and the spin population
3
intense DSPs than Cz moieties. The DSPs could be visually (Pspin) on TPA, Cz1, and Cz2 moieties of IrC1–IrC3 were tted
observed on the spin-density contours of N atoms on TPA, with linearity (Fig. 8). Strong linear correlations between the
Cz1, and Cz2 moieties with Pspin of 0.06098, 0.00542, and ratios of collision radii and Pspin were apparent with R2 of
0.02332, respectively (Fig. 6). The spin-density integral curves 0.997. Hence, the DSPs could be the descriptor for the
of IrC1–IrC3 and Ir(ppy)3 were plotted along Z axis (Fig. 7).49 dependence of oxygen sensitivity on the molecular structure
Compared with Ir(ppy)3, spin population on Ir(III) atoms and of Ir(III) complexes. In comparison with Ir(ppy)3, introduction
2-phenylpyridine moieties of IrC1–IrC3 declined in the range of TPA and Cz moieties effectively improved the DSPs on
of 0 to 13 bohr, and then increased from 13 bohr to 27.5 bohr intraligands resulting in increase of collision radii of Ir(III)
which exhibited intense DSPs on TPA, Cz1, and Cz2 moieties. complexes. Despite the upward tendency of luminescent
Fig. 8 The linear fitting curve between the ratios of collision radii Fig. 10 Luminescent attenuation of Ir(III) films under continuous
(sIr(III)/sIr(ppy) ) and the spin population (Pspin) of IrC1–IrC3.
irradiation.
3
Fig. 9 Schematic illustration for the fabrication of IrC1-EC film (0.5 wt%) as sample and oxygen sensing measurements. (a) EC (9.95 mg) and THF
(0.90 mL) were mixing with 0.10 mL IrC1-THF solution (0.5 mg mLꢀ1). (b) The mixture (0.10 mL) was painted on a vitreous quartz sheet (S ¼ 1.54
mm2). (c) For the formation of the thin film, the solution was volatilized for 1 day at room temperature. (d) The SEM cross profile of IrC1-EC film.
(e) Two tube flowmeters for the mixture of oxygen and nitrogen. (f) The IrC1-EC film on the vitreous quartz sheet was put in a cuvette for oxygen
sensing measurements.
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decay times from IrC1 to IrC3, the increase of collision radius
of IrC1 resulted in outstanding oxygen sensitivity.
3.6. Photostability measurements
The photostability of OSPs is always of great concern for
practical applications such as high-light densities determi-
nation or long-time continuous monitoring of oxygen. In this
work, easily available supporting matrix ethyl cellulose (EC)
was applied to immobilize Ir(III) complexes, respectively
(Fig. 9a–c). The thickness of EC lms is 5.62 mm (Fig. 9d).
Ir(III) lms were irradiated with Xe lamp under high power
density of 27.3 W mꢀ2 in air. And photostabilities of Ir(III)
lms were measured in N2 every 10 minutes.51 The data in
Fig. 10 indicated that emission intensities of Ir(III) lms were
attenuated through continuous irradiation. IrC1 demon-
strated higher photostability than that of Ir(ppy)3, IrC2 and
IrC3. About 27.8% of IrC1, 52.8% of IrC2, 45.3% of IrC3, and
44.9% of Ir(ppy)3 were destroyed aer 60 min of irradiation.
The photostabilities were in the sequence of IrC1 > Ir(ppy)3 z
IrC3 > IrC2. And the sequence was correlated well with the
order of excited-state oxidation potentials ðEo*xÞ in Table 2,
which determined the photo-oxidation reactivity of OSPs.46
Thus, the introduction of TPA moieties raises Eo*x which
makes IrC1 more stable to continuous irradiation.
Fig. 13 Reversibility trace curve of IrC1-EC film to 0% O2/100% O2
saturation cycles in 4000 s.
3.7. Real-time monitoring of oxygen
The luminescent responses of Ir(III) lms were studied by
gradually increasing concentration of O2. The Ir(III) lms in
0% O2 (100% N2) exhibited intense room-temperature phos-
phorescence. And emission intensities of Ir(III) lms were
attenuated stepwise with incremental concentration of O2
app
SV
(Fig. 11). K
and f1 values were summarized in Fig. 12 and
Table S4 (see ESI†) which were tted by eqn (2). The I0/I100 for
Ir(III) lms of IrC1, IrC2, IrC3, and Ir(ppy)3 were 33.0, 8.2,
app
SV
16.5, and 9.7 with K
of 32.9 barꢀ1, 18.5 barꢀ1, 23.6 barꢀ1
and 8.8 barꢀ1. The IrC1-EC lm exhibited outstanding oxygen
sensitivity than that of other lms. And the sequence of
oxygen sensitivities was IrC1 > IrC3 > IrC2 > Ir(ppy)3, which
was consistent with the sequence of oxygen sensitivities in
THF (Fig. 4). The f1 values of IrC1 (0.999), IrC3 (0.978), and
Ir(ppy)3 (0.997) lms were higher than that of IrC2 (0.911)
which showed considerable linearity and perfect micro-
homogeneity and resulted in increased oxygen sensitivities.
3.8. Operational stability of oxygen sensing lm
Because of the excellent oxygen sensing performance and
high photostability, the operational stability of IrC1-EC lm
was measured by switching O2 and N2 atmospheres in 4000 s
(Fig. 13). The values of I0 and I100 was constant and the cycles
of quenching and recovery were perfectly reversible with no
attenuation of original emission intensity which exhibited
excellent operational stability. The response time of 3.7 s and
recovery time of 6.1 s (96.6% recovery of original intensity)
indicated quick responses of IrC1-EC lm to molecular
oxygen. Therefore, IrC1-EC lm demonstrated outstanding
photostability and operational stability which satised on-
line continuous monitoring of molecular oxygen.
Fig. 11 Time-dependent luminescent responses of Ir(III) films with
incremental concentration of oxygen.
4. Conclusions
In summary, new TPA and Cz moieties contained Ir(III)
complexes IrC1, IrC2, and IrC3 were prepared and fully char-
acterized as OSPs for luminescent sensing of oxygen. The ratios
of collision radii (sIr(III)/sIr(ppy) ) of IrC1–IrC3 to Ir(ppy)3 were 2.13
3
Fig. 12 SVPs for Ir(III) oxygen sensing films (fitted by two-site model). ꢁ 0.08, 1.24 ꢁ 0.06, and 1.54 ꢁ 0.08, respectively. Remarkable
15378 | RSC Adv., 2019, 9, 15370–15380
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DSPs were found on the TPA, Cz1, and Cz2 moieties with the 13 L. Liu, D. Huang, S. M. Draper, X. Yi, W. Wu and J. Zhao,
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3
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Conflicts of interest
There are no conicts to declare.
Acknowledgements
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The author thank the nancial supports of Liaoning Shihua
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Natural Science Foundation of China (21703135), and the
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