Iridium(iii) complexes with the dithieno[3,2-: B:2′,3′- d] phosphole oxide group and their high optical power limiting performances

Article abstract of DOI:10.1039/d0dt00102c

A new 2-phenylpyridine-type (ppy-type) ligand with the dithieno[3,2-b:2′,3′-d]phosphole oxide (DTPO) group has been successfully synthesized. Based on this novel ligand, three cyclometalated iridium(iii) complexes (P-Ir-P, P-Ir-T and P-Ir-C) are synthesized with symmetrical and unsymmetrical structures. Photophysical results reveal that these cyclometalated iridium(iii) complexes can show weak near-infrared (NIR) phosphorescence emission with wavelengths of 739 nm for P-Ir-P, 750 nm for P-Ir-T and 746 nm for P-Ir-C. Importantly, transient absorption characterization shows that these cyclometalated iridium(iii) complexes can exhibit strong excited state absorption in the range of ca. 520 to 700 nm, indicating their optical power limiting (OPL) potential in this wavelength range. Open-aperture Z-scan against a 532 nm laser shows their OPL ability in the order of P-Ir-P > P-Ir-C > P-Ir-T. Complex P-Ir-P shows an even better OPL ability than the state-of-the-art OPL material C₆₀, indicating the important potential application of these cyclometalated iridium(iii) complexes as new OPL materials.

Full text of DOI:10.1039/d0dt00102c

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DOI: 10.1039/D0DT00102C
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Iridium(III) complexes with dithieno[3,2-b:2ʹ,3ʹ-d]phosphole oxide
group and their high optical power limiting performances †
Zhao Liu,a Yanmin Xu,b Ling Yue,a Ming Li,b Xiaolong Yang,a Yuanhui Sun,a Lihe Yan,*b Guijiang Zhou*a
Received 00th January 20xx,
Accepted 00th January 20xx
A new 2-phenylpyridine-type (ppy-type) ligand with dithieno[3,2-b:2ʹ,3ʹ-d]phosphole oxide (DTPO) group has been
successfully synthesized. Based on this novel ligand, three cyclometalated iridium(III) complexes (P-Ir-P, P-Ir-T and P-Ir-C)
are synthesized with symmetrical and unsymmetrical structures. Photophysical results reveal that these cyclometalated
iridium(III) complexes can show weak near-infrared (NIR) phosphorescence emission with 739 nm for P-Ir-P, 750 nm for P-
Ir-T and 746 nm for P-Ir-C, respectively. Importantly, transient absorption characterization shows that these cyclometalated
iridium(III) complexes can exhibit strong excited state absorption in the range from ca. 520 to 700 nm, indicating their optical
power limiting (OPL) potential in this wavelength range. Open-aperture Z-scan against 532 nm laser shows their OPL ability
in the order of P-Ir-P > P-Ir-C > P-Ir-T. The complex P-Ir-P even show better OPL ability than the state-of-the-art OPL material
C60, indicating important potential of these cyclometalated iridium(III) complexes as new OPL materials.
DOI: 10.1039/x0xx00000x
www.rsc.org/
sensors in the occasion of suddenly exposed to laser beams, ppy-type
cyclometalated iridium(III) complexes have been employed to
investigate OPL ability as well.31-34 Due to the strong spin-orbit
coupling (SOC) effect induced by the Ir(III) center, ppy-type
cyclometalated iridium(III) complexes can show high triplet or
Introduction
Among all the phosphorescent transition-metal complexes, 2-
phenylpyridine(ppy)-type iridium(III) complexes have attracted
numerous research efforts both in academic and industry for their
high phosphorescent quantum yield, tunable emission color and easy
purification.1-6 So, ppy-type cyclometalated iridium(III) complexes
can show great potential in organic light-emitting diodes (OLEDs),7-10
photodynamic therapy,11,12 solar cells,13,14 photovoltaics,15
bioimaging16-18 and photocatalysis19,20 etc. Attributed to the spin-
orbital coupling effect of Ir(III) ions, these complexes can show
efficient intersystem crossing from singlet states to triplet states,
endowing these novel iridium complexes excellent phosphorescent
ability even at room temperature.21-24 Importantly, the
photophysical and functional properties of these ppy-type
cyclometalated iridium(III) complexes can be effectively tuned by the
functionalization of their major and/or the auxiliary ligands around
the Ir(III) coordination spheres.23,25 Hence, ppy-type cyclometalated
iridium(III) complexes can show diverse properties to meet the
requirements in various fields aforementioned.
phosphorescent quantum yield indicated by their high
24,35
phosphorescent or triplet emission ability.
If the triplet states
possess high absorption ability, the high triplet quantum should
benefit their OPL behavior based on reverse saturable absorption
(RSA) mechanism for nano-second laser.33,36 In recent years, Sun’s
group have employed several ppy-type cyclometalated iridium(III)
complexes to explore their OPL potential.11,33,37 It has been found
that these complexes can exhibit obvious triplet state absorption and
induce obvious OPL effect,32,37 indicating their potential as new OPL
materials. To the best of our knowledge, all the reported OPL
materials based on the ppy-type cyclometalated iridium(III)
complexes adopt the symmetric structure with two identical ppy-
type major ligands.37,34 Hence, developing ppy-type cyclometalated
iridium(III) complexes with new structures should be very necessary
for obtaining novel OPL materials.
As aforementioned, photophysical properties of the ppy-type
cyclometalated iridium(III) complexes can be tuned by the chemical
structures of their ppy-type ligands. Our recent results have
indicated that new unsymmetrical ppy-type cyclometalated
iridium(III) complexes bearing two different ppy-type major ligands
and one acetylacetone (acac) auxiliary ligand can possess excited
states with different feature from those of their symmetric
counterparts.1,23,38 According to the OPL mechanisms, OPL effect is
typically induced by the strong absorption ability of the excited
states.12,31,36 Hence, adopting unsymmetrical structure should
represent a new outlet to tune the OPL performance of the ppy-type
cyclometalated iridium(III) complexes.
Recently, inspired by the success of other type transition-metal
complexes, such as Pt(II) acetylides,26-30 as promising optical power
limiting (OPL) materials for protecting both human eyes and optical
a. MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed
Matter, State Key Laboratory for Mechanical Behavior of Materials, Department
of Chemistry, School of Science, State Key Laboratory for Mechanical Behavior of
Materials, Xi’an Jiaotong University, Xi’an 710049, P. R. China.
E-mail: zhougj@mail.xjtu.edu.cn
b. Key Laboratory for Physical Electronics and Devices of the Ministry of Education
& Shaanxi Key Lab of Information Photonic Technique, School of Electronics &
Information Engineering, Xi'an Jiaotong University, Xi'an, 710049, China.
E-mail: liheyan@mail.xjtu.edu.cn
† Electronic supplementary information (ESI) available: Synthetic of the
intermediate compounds and theoretical fitting of the Z scan results.
See DOI: 10.1039/x0xx00000x
Dithieno[3,2-b:2ʹ,3ʹ-d]phosphole oxide (DTPO) represents a kind
of conjugated organic system formed by fusing both two electron-
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rich thiophene segments and one electron-deficient phosphole oxide connected to a computer. The TA data were recorded as a function
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42
DOI: 10.1039/D0DT00102C
unit. Hence, many organic DTPO-based derivatives had been of the delay time between the pump and probe pulses.
developed to show unique photophysical properties.39-41 However,
to the best of our knowledge, DTPO unit has not been introduced
into ppy-type iridium(III) complexes. So, in this research, DTPO unit
has been employed to develop new ppy-type ligand. With the DTPO-
based ligand, new ppy-type cyclometalated iridium(III) complexes
adopting symmetric and unsymmetrical structures have been
developed. Through the diverse structures, donor (D) and acceptor
(A) have been introduced into the cyclometalated iridium(III)
complexes to investigate their effect on OPL behavior and get
structure-activity information. Critically, these novel cyclometalated
Computational details
Geometrical optimizations were conducted using the popular B3LYP
density functional theory (DFT). The effective core potentials with
the LanL2dz basis set were employed for the Ir atom,50,51 whereas
the basis set used for C, H, O, N, S and P atoms was 6-31G (d, p). The
energies of the excited states of the complexes were computed by
TD-DFT (time-dependent density functional theory) based on the
ground-state geometries. All the calculations were carried out by
using the Gaussian 09 program.43
iridium(III) complexes with DTPO group can outperform the state-of- Synthesis
the-art OPL material C60, indicating their potential in the field of OPL.
Synthetic procedures for 3,3ʹ,5,5ʹ-tetrabromo-2,2ʹ-bithiophene,44
3,3ʹ-dibromo-2,2ʹ-bithiophene,45 dithieno[3,2-b:2ʹ,3ʹ-d]phosphole
oxide41 and 2-bromodithieno[3,2-b:2ʹ,3ʹ-d]phosphole oxide46 follow
the literature methods. Hence, their synthetic details have been
provided in the ESI.†
Experimental
General information
All the reactions were conducted under a nitrogen atmosphere and
no special precautions were required during the workup. The
solvents were purified by routine procedures and distilled under dry
nitrogen before use. Commercially available reagents were used
without further purification unless otherwise stated. The reactions
were monitored by thin-layer chromatography (TLC) purchased from
Merck & Co, Inc. Flash column chromatography and preparative TLC
were carried out using silica gel from Shenghai Qingdao (200–300
mesh).
L-P
To the solution of 2-bromodithieno[3,2-b:2ʹ,3ʹ-d]phosphole oxide
(0.37 g, 1.0 mmol), 2-(tri-nbutylstannyl)pyridine (0.41 g, 1.1 mmol)
and Pd(PPh3)4 (64 mg) as catalyst in toluene (15 mL) under N2
atmosphere was heated to 110 oC overnight. Upon cooling to room
temperature, the resulting solution was removed under reduced
pressure. The crude product was purified on a silica gel column using
ethyl acetate and CH2Cl2 (1:1, v/v) as eluent to give the product as
yellow solid (0.26 g, 71%). 1H NMR (400 MHz, CDCl3):  (ppm) 8.55 (d,
J = 4.8 Hz, 1H), 7.80-7.48 (m, 2H), 7.69 (td, J = 8.0, 2.0 Hz, 1H), 7.58-
7.53 (m, 3H), 7.47-7.42 (m, 2H), 7.34 (dd, J = 3.2, 4.8 Hz, 1H), 7.19-
7.18 (m, 2H); 13C NMR (150 MHz, CDCl3):  (ppm) 151.56, 149.64,
149.09 (d, J = 13.2 Hz), 146.85 (d, J = 22.05 Hz), 146.18 (d, J = 23.85
Hz), 139.68 (d, J = 111.0 Hz, 1H), 138.85 (d, J = 111.75 Hz), 136.84,
132.55 (d, J = 30.0 Hz), 130.92 (d, J = 11.1 Hz), 129.90, 129.11(d, J =
15.06 Hz), 128.99 (d, J = 13.05 Hz), 126.12 (d, J = 15.0 Hz), 122.60,
122.44 (d, J = 15.0 Hz), 118.52; 31P NMR (162 MHz, CDCl3):  (ppm)
18.61; FAB-MS (m/z): 365 [M]+; Elemental analysis calcd (%) for
C19H12NOPS2: C 62.45, H 3.83; found: C 62.36, H 3.75.
Physical characterization
UV/vis spectra were measured on a Perkin-Elmer UV-vis NIR
Spectrometer Lambda 950. The photoluminescent (PL) properties
and the phosphorescent lifetimes of the iridium(III) complexes were
obtained using a spectrophotometer from Edinburgh Instruments
(FLS920). Cyclic voltammetry was performed using a Princeton
Applied Research model 2273A potentiostat with a glassy carbon
working electrode, a platinum sheet counter electrode, and a
platinum-wire reference electrode with the scan rate of 50 mV s-1.
The supporting electrolyte used was 0.1 M [nBu4N]BF4 solution in
General procedure for the synthesis of iridium(III) complexes
degassed CH2Cl2 with ferrocene (Fc) as the calibrant. 1H, 13C and 31
P
Under N2 atmosphere, L-P (1.1 equiv), L-P/L-T/L-C (1.1 equiv) and
IrCl3· nH2O (60 wt%, 1.0 equiv) were heated to 110 oC in the mixture
of THF and H2O (3:1, v/v) for 16 h. The reaction mixture was then
cooled to room temperature and water was added. After extracted
with CH2Cl2, the organic phase was dried over anhydrous Na2SO4 and
solvent was removed under reduce pressure. The μ-chloro-bridged
dimer was obtained as colored residue. After stirring the reaction
mixture of acetylacetone and tBuOk in CH2Cl2 for half an hour under
N2 atmosphere. The μ-chloro-bridged dimer was added, then the
reaction mixture was stirred for 2 h. The resulting solution was
removed under vacuum to obtain the crude solid production which
was purified with thin layer chromatography on silica gel with
appropriate eluents to obtain the complex.
NMR spectra were recorded in CDCl3 on a Bruker Avance 400 or 600
MHz spectrometer and chemical shifts were referenced to the
1
solvent residual peak at d 7.26 ppm for H and 77.0 ppm for 13C,
respectively. Elemental analyses were performed on a Flash EA 1112
elemental analyzer. Mass spectra (MS) were recorded on a Finnigan
MAT SSQ710 spectrometer. The thermal gravimetric analysis (TGA)
was conducted on a METTLER TOLEDO TGA2 star system under
nitrogen with a heating rate of 20 K min-1. Transient absorption (TA)
behaviors were characterized by femtosecond (fs) time-resolved
pump-probe technique with pump pulses centered at 400 nm with
energies of 3.2 mJ cm-2. In the TA measurements, the output of the
laser was split into two beams: the stronger beam was used as the
excitation light and the other beam was focused into a sapphire plate
with 1 mm thickness to generate the broad band white-light probe
pulses of 1k Hz from 450 to 750 nm. The transmitted probe pulses
from the sample were collected by a fibre-coupled spectrometer
1
P-Ir-P. Yield: 50%. H NMR (600 MHz, CDCl3): δ ( ppm) 8.58 (d, J
=3.6 Hz, 1H), 8.24 (d, J = 3.7 Hz, 1H), 8.06 (d, J = 5.16 Hz, 2H), 7.58 (td,
J = 7.8, 1.2 Hz, 1H), 7.45 (m, 2H), 7.35-7.32 (m, 2H), 7.23-7.21 (m, 2H)
, 7.17-7.14 (m, 2H), 7.10-7.08 (m, 1H), 6.97-6.87 (m, 5H), 6.82-6.85
2 | J. Name., 2012, 00, 1-3
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(m, 2H), 6.32 (d, J = 1.8 Hz, 1H), 5.22 (s, 1H), 1.87 (s, 3H), 1.73 (s, 3H); 125.35, 124.76, 124.16, 122.56, 119.52, 118.99, 118.29, 117.33,
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31
13C NMR (150 MHz, CDCl3) δ (ppm) 185.57, 184.09, 166.42, 163.26, 115.05, 100.29, 28.76, 28.55; P NMR (16D2OMI: H10z.,10C3D9C/Dl30):DT0(0p1p0m2C)
150.84, 148.16, 145.18, 144.08, 140.27, 137.69, 136.94, 136.43, 17.78; FAB-MS (m/z): 977 [M]+; Elemental analysis calcd (%) for
131.12, 130.14, 129.92 (d, J = 11.1 Hz), 128.85, 128.16 (d, J = 13.5 Hz), C47H35IrN3O3PS2: C 57.77, H 3.61, N 4.30; found: C 57.66, H 3.56, N
125.62, 125.35 (d, J = 13.5 Hz), 125.12, 125.44, 124.53, 123.27, 4.19.
1
121.45, 120.04, 119.59, 119.10, 118.20 (d, J = 12.0 Hz), 118.16,
P-Ir-C. Yield: 25%. H NMR (400 MHz, CDCl3):  (ppm) 8.30 (d, J =
117.22, 110.35, 100.62, 28.56, 28.42; 31P NMR (162 MHz, CDCl3): δ ( 5.6 Hz, 1H), 8.04 (d, J = 7.6 Hz, 2H), 7.88 (d, J = 8.4 Hz, 1H), 7.79-7.72
ppm) 18.33; FAB-MS (m/z): 1020 [M]+; Elemental analysis calcd (%) (m, 3H), 7.65 (td, J = 7.4, 1.2 Hz, 1H), 7.54 (t, J = 7.2 Hz, 1H), 7.48-7.31
for C43H29IrN2O4P2S4: C 50.63, H 2.87, N 2.75; found: C 50.53, H 2.79, (m, 9H), 7.20 (t, J = 3.8 Hz, 2H), 7.09-7.06 (m, 2H), 6.93 (dd, J =2.4, 4.8
N, 2.66.
Hz, 1H), 6.84 (td, J = 4.8, 1.3 Hz, 1H), 6.47 (t, J = 6.6 Hz, 1H), 6.29 (d, J
P-Ir-T. Yield: 22%. 1H NMR (400 MHz, CDCl3):  (ppm) 8.18 (d, J =5.6 =1.6 Hz, 1H), 5.23 (s, 1H), 1.84 (s, 3H), 1.76 (s, 3H); 13C NMR (100 MHz,
Hz, 1H), 7.74 (dd, J= 9.2, 7.2 Hz, 2H), 7.65 (d, J= 8.0 Hz, 1H), 7.54-7.48 CDCl3):  (ppm) 184.44, 184.27, 168.52, 163.46, 150.34 (d, J = 4.5 Hz),
(m, 2H), 7.40-7.26 (m, 5H), 7.21-7.14 (m, 5H), 7.08 (dd, J = 4.8, 3.2 Hz 149.24, 148.25 (d, J = 10.5 Hz), 144.57, 144.37, 144.34, 143.87 (d, J =
1H), 6.97-6.93 (m, 7H), 6.74 (td, J = 6.4, 1.0 Hz, 1H), 6.48 (dd, J = 8.4, 23.50 Hz), 141.85, 141.13, 140.43 (d, J = 19.5 Hz), 140.25, 137.66,
2.4 Hz 1H), 6.34 (t, J = 6.4 Hz, 1H), 5.57 (d, J =2.4 Hz, 1H), 5.17 (s,1H), 137.20, 131.99, 131.90, 131.79 (d, J = 19.5 Hz), 131.28, 129.59,
1.78 (s, 3H), 1.73 (s, 3H); 13C NMR (100MHz, CDCl3): δ (ppm) 184.23, 128.27, 128.18, 128.08, 125.46, 125.38, 124.63, 123.23, 120.86,
184.02, 168.76, 163.21, 150.32 (d, J =19.5 Hz), 148.71 (d, J = 28.85 119.84, 119.44, 119.26, 119.16, 117.30, 110.73, 100.46, 29.34, 28.6;
Hz), 148.01 (d, J = 29.85 Hz), 147.49, 147.36, 144.54, 144.00, 143.84, 31P NMR (162 MHz, CDCl3):  (ppm) 17.96; FAB-MS (m/z): 975 [M]+;
143.77, 143.28, 141.53 (d, J = 107.7 Hz), 140.43 (d, J = 20.25 Hz), Elemental analysis calcd (%) for C47H35IrN3O3PS2: C 57.89, H 3.41, N
139.13, 136.74, 132.00, 131.81 (d, J = 11.10 Hz), 131.73, 131.29, 4.31; found: C 57.81, H 3.46, N 4.22.
128.77, 128.34, 128.12 (d, J = 27.3 Hz), 127.8 (d, J = 14.1 Hz), 125.44,
Br
Br
S
S
S
S
S
S
Zn
Br2
CH3COOH/HCl
CHCl3/CH3COOH
Br Br
i) nBuLi, -78 o
Br Br
C
ii) PhPCl2, -78 o
C
iii) H2O2, r.t.
N
Br
S
S
SnBu3
N
Pd(PPh3)4
S
S
NBS
DMF
P
P
P
O
O
O
S
S
L-P
Scheme 1. Synthetic pathway of ligand L-P.
adjacent to the nitrogen atom in the pyridine ring. The td peak at ca.
7.69 ppm is induced by the proton on the carbon atom at the para-
position phosphine atom in the pendent phenyl ring of the
phosphole oxide unit. The peak with  at 18.61 ppm is induced by
the phosphole oxide unit of DTPO group in the 31P NMR spectrum of
L-P.
After obtaining L-P, the designed symmetric complex P-Ir-P had
been prepared according to the reported two-step pathway (Scheme
2). The unsymmetrical counterparts P-Ir-T and P-Ir-C were prepared
by similar synthetic strategy by adding two kinds of ppy-type ligand
L-P and L-T/L-C in the first step for synthesis of Ir(III) μ-chloro-bridged
dimers. Then, the dimers were treated with the mixture of
acetylacetone and tBuOK to obtain the unsymmetrical complexes P-
Ir-T and P-Ir-C. Clearly, in the synthesis of unsymmetrical complexes,
the symmetrical complexes can be formed as well. Fortunately, they
can be separated easily through either column or TLC
chromatography due to the difference in molecular polarity. In their
Results and discussion
Synthetic strategies and structural characterization
Chemical structures and detailed synthetic method of the ppy-type
ligand L-P with DTPO group are shown in Scheme 1. In order to obtain
the designed complexes, the synthesis of L-P is crucial. The synthesis
of L-P was started with bromination of 2,2’-bithiophene to obtain
3,3’,5,5’-tetrabromo-2,2’-bithiophene, which was converted into
3,3’-dibromo-2,2’-bithiophene through selective debromination
with Zn/CH3COOH-HCl in a yield of ca. 80%. Through lithiation in
anhydrous THF at -78 °C, subsequent treatment with
dichlorophenylphosphine and finally adding H2O2, 3,3’-dibromo-2,2’-
bithiophene had been cyclized into DTPO, which was employed to
synthesize 2-bromodithieno[3,2-b:2ʹ,3ʹ-d]phosphole oxide. Finally,
through Stille coupling reaction, L-P could be obtained in a yield of
1
71%. In the H NMR spectrum of L-P, the doublet peak at ca. 8.55
ppm can be assigned to the proton attaced to the carbon atom
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1H NMR spectra (Figure S1 in Electronic supplementary information, unit of DTPO group. As shown in Figure 1, the single-crystal X-ray
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ESI), the signals at 8.58 and 8.24 ppm for P-Ir-P, 8.18 and 7.74 ppm crystallography further confirms the absoluDteOcI:h1e0m.10ic3a9l/Dst0rDuTc0tu0r1e02oCf
for P-Ir-T and 8.30 and 8.04 ppm for P-Ir-C can be assigned to the the complex P-Ir-C. Clearly, two different ppy-type ligands and one
protons attached to the carbon atoms adjacent to the nitrogen atom acetylacetone anion centering on Ir(III) atom adopt a distorted
in the pyridyl ring of the two ppy-type ligands. The acetylacetone octahedral chelating skeleton with cis-C,C, cis-O,O and trans-N,N
auxiliary ligand (acac) has been indicated by the three singlet peaks disposition. The DTPO group adopts rigid planar geometry with
at ca. 5.2, 1.8 and 1.7 ppm, respectively. In their 31P NMR spectra phenyl ring and P=O bond sticking out of its molecular planar. The
(Figure S1), all the complexes can exhibit resonance peak with  ca. detail of related crystal data and selected bond angles and lengths of
18 ppm which can be attributed to the P atom in phosphole oxide P-Ir-C are summarized in Table S1 and S2.
O
O
N
N
Ir
N
P
S
S
O
S
O
P
P
S
S
S
O
P-Ir-P
L-P
O
O
N
N
P
N
N
Ir
S
i) IrCl3.nH2O, THF/H2O2
+
ii) Acetylacetone, tBuOK
S
O
N
S
P
O
N
S
L-P
L-T
P-Ir-T
N
O
O
N
N
Ir
N
S
O
P
L-C
N
S
P-Ir-C
Scheme 2. Synthetic pathway of the cyclometalated iridium(III) complexes with DTPO group.
450 nm extended to the visible region should be induced by the
charge-transfer (CT) transition process.
Thermal and photophysical properties
Both thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) have been employed to characterize thermal
properties of these cyclometalated iridium(III) complexes under a
nitrogen follow. The TGA results show that these complexes possess
high thermal stability with decomposition temperature > 300 oC,
indicating their good thermal stability.
UV-vis absorption spectra of these cyclometalated iridium(III)
complexes can be seen in Figure 2. There are two kinds of obvious
absorption bands in their absorption spectra. According to the
analogues in literature references,47,48 the intense absorption bands
before 450 nm can be assigned reasonably to the spin-allowed –*
transitions of the ppy-type organic ligands, which can also be
ascertained by their high extinction coefficients (logɛ ca. 4.0) (Table
1). On the contrary, the weaker low-energy absorption bands after
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Figure 1. ORTEP drawing of P-Ir-C with thermal ellipsoids drawn at
structure-less spectral line-shape has indicated charge transfer (CT)
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DOI: 10.1039/D0DT00102C
feature of the NIR phosphorescent band (Figure 3).
the 25% probability level (H atoms are omitted for clarity).
With regard to feature of the NIR phosphorescent emission,
natural transition orbital (NTO) analysis was carried out for the S0 →
T1 excitation based on the optimized T1 geometries of these
iridium(III) complexes (Figure 5 and Table 3). We will focus on the
NTO results to elucidate the phosphorescent behaviors of these
iridium(III) complexes. From the NTO pattern in Figure 5, it is obvious
that S0 → T1 excitation of these iridium(III) complexes can have
rendered the broad structure-less spectral line-shape for the NIR
phosphorescence (Figure 3). Reasonably, electron-donating ability of
carbazole and diphenylamine units the will promote the Hole →
Particle transition (i.e. S0 → T1 transition) to lead lower T1 energy-
level of P-Ir-T and P-Ir-C than P-Ir-P. Hence, P-Ir-T (ca. 750 nm) and
P-Ir-C (ca. 746 nm) should exhibit longer phosphorescent wavelength
that P-Ir-P (ca. 739 nm), which has been indicated by the
experimental result (Table 1).
The UV-vis absorption and photoluminescence (PL) spectra of
these complexes are shown in Figure 2 and 3, respectively. In order
to illustrate the absorption behavior of these cyclometalated
iridium(III) complexes, TD-DFT calculations have been carried out.
The pattern of key molecular orbitals (MO) and detailed calculation
results for these iridium(III) complexes are shown in Figure 4 and
Table 2, respectively. Due to their large coefficient in CI expansion (>
0.9) with the corresponding lowest excited states, the
HOMO→LUMO (H→L) transition can show the feature of the low-
energy absorption bands of these iridium(III) complexes. From the
MO pattern shown in Figure 4, the H→L transition in P-Ir-P mainly
show intraligand charge transfer (ILCT) feature (Figure 4). In addition,
due to the different contributions to HOMO (ca. 15.07%) and LUMO
(ca. 1.39%) from the dπ orbital of the Ir(III) center (Table 2), some
metal-to-ligand charge transfer (MLCT) feature can be associated
with the H→L transition in P-Ir-P as well. So, the low-energy
absorption bands of P-Ir-P can exhibit ILCT feature with some MLCT
character. Differently, for both P-Ir-T and P-Ir-C, their HOMOs mainly
locate on the π orbitals of L-T for P-Ir-T and L-C for P-Ir-C, while their
LUMOs are contributed by the π* orbitals of L-P, showing obvious
ligand-to-ligand charge transfer (LLCT) features. Accordingly, the
low-energy absorption bands in the UV-vis spectra of P-Ir-T and P-Ir-
C should mainly show LLCT character. Due to the strong electron-
donating ability associated with carbazole and diphenylamine units,
the lowest energy transition in P-Ir-T and P-Ir-C is mainly from π
orbitals of carbazole and diphenylamine units to the π* orbitals of
L-P containing electron-accepting moiety, restraining the
involvement of dπ orbital from Ir(III) center. As a result, LLCT
character can associate with the low-energy absorption bands in the
UV-vis spectra of P-Ir-T and P-Ir-C.
(a)
P-Ir-P
P-Ir-T
P-Ir-C
1.8x104
1.5x104
1.2x104
9.0x103
6.0x103

3.0x103
0.0
300
400
500
600
700
Wavelength (nm)
Figure 2. UV-vis spectra for cyclometalated iridium(III) complexes in
CH2Cl2 solution at 298 K.
(b)
1.0
0.8
0.6
0.4
0.2
0.0
P-Ir-P
P-Ir-T
P-Ir-C
Under UV light irradiation at 450 nm, two emission bands can be
observed in the PL spectra of these complexes (Figure 3). Their high-
energy emission bands at ca. 530 nm should be the metal-disturbed
singlet emission of ligand L-P due to the nanosecond lifetime and
their great resemblance. Clearly, the low-energy emission band
should be phosphorescent emission indicated by their remarkably
large Stokes shifts (Table 1). The phosphorescent wavelengths for
these complexes are 739 nm for P-Ir-P, 750 nm for P-Ir-T and 746 nm
for P-Ir-C, respectively (Table 1). Hence, these complexes can show
near-infrared (NIR) phosphorescence emission. Obviously, the broad
500 550 600 650 700 750 800 850
Wavelength (nm)
Figure 3. PL spectra for cyclometalated iridium(III) complexes in
CH2Cl2 solution at 298 K.
Table 1 Photophysical and thermal data of the cyclometalated iridium(III) complexes.
Absorption λabs (nm) a
Emission λem (nm) a
τ(ns)b
298 K
PLQY
298 K
Td
Compound
298 K
298 K
(oC)
252 (4.11), 288 (4.02), 315 (3.94), 384 (4.01)
429 (3.80), 490 (3.34), 502 (3.34)
254 (4.31), 288 (4.23),383 (4.23), 427 (4.02),
492 (3.54), 506 (3.55)
258(4.33), 287 (4.11), 331 (3.99), 346 (3.99)
400 (3.80), 426 (3.75), 484 (3.46), 500(3.47)
P-Ir-P
P-Ir-T
P-Ir-C
519/739
517/750
523/746
5.15/N.A.
7.02/N.A.
5.16/N.A.
0.09
0.06
0.09
345
306
331
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aMeasured in CH2Cl2 at a concentration of 10−5 M, logε values are shown in parentheses. bMeasured in CH2Cl2 with the excitation source was a 405
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DOI: 10.1039/D0DT00102C
nm picosecond LED (The τ for the NIR phosphorescence could not be determined due to their very weak intensities).
Figure 4. Molecular orbital (MO) patterns for these cyclometalated iridium(III) complexes based on their optimized S0 geometries.
Table 2 TD-DFT results for these cyclometalated iridium(III) complexes based on their optimized S0 geometries.
Main configuration of
Main configuration of S0→S1
excitation/cal (nm) /Ecal/f b
Contribution percentages of metal d orbitals and 
Complex
MO
S0→T1 excitation/Ecal/cal
orbitals of ligand to MOs (%)a
(nm)b
Ir
L-P
1.33
97.36
9.99
74.21
L-P
0.63
98.37
0.89
57.09
L-P
L-P
95.89
1.07
72.74
12.74
L-T
94.36
0.17
97.15
13.74
L-C
94.30
0.20
acac
0.49
0.18
2.19
1.70
acac
0.69
0.16
0.17
14.23
acac
0.72
1.75
0.12
1.74
L+1
L
H
2.26
1.39
15.07
11.31
Ir
4.31
1.29
1.77
26.23
Ir
H→L (95.88%)
2.5054 eV
494.87
H→L +1 (81.07%)
P-Ir-P
1.7326 eV
715.6
0
H-1
0.0191
L+1
L
H
H→L (94.83%)
2.187 eV
566.90
H→L+1 (64.07%)
1.7388 eV
713.04
P-Ir-T
P-Ir-C
H-1
0.0034
0
L+1
L
H
4.50
1.37
1.40
21.18
0.46
98.25
0.84
67.60
H→L (97.61%)
2.4662eV
502.73
H→L+1 (76.11%)
1.7494 eV
708.7
0
97.63
9.39
H-1
0.0063
aH→L denotes transition from HOMO to LUMO, while H→L+1 represents transition from HOMO to LUMO+1. bEcal, cal and f denote calculated excitation
energy, calculated emission wavelength and oscillator strength, respectively. The oscillator strength of S0→T1 is zero owing to the spin-forbidden
character of singlet-triplet transition under TD-DFT calculation in the Gaussian program with no consideration of spin-orbital coupling.
Electrochemical properties
that for P-Ir-C is ca. 0.54 V (Table 4). Differently, besides Eox at ca.
0.52 V, P-Ir-T can possess another oxidation possess with lower Eox
at ca. 0.43 V, which can be assigned to the oxidation of electron-rich
diphenylamine unit. The oxidation possess with Eox at ca. 0.5 V for
the three complexes should be induced by the oxidation process of
their Ir(III) center. Owing to two electron-withdrawing DTPO
moieties coordinated with Ir(III) center, P-Ir-P can exhibit higher Eox
In order to characterize the electrochemical properties of these
cyclometalated iridium(III) complexes with DTPO unit, cyclic
voltammetry (CV) measurements have been carried out under a
nitrogen atmosphere, calibrated with a ferrocene/ferrocenium
(Fc/Fc+) redox couple as an internal reference. From the date in Table
3, P-Ir-P shows the first oxidation potential Eox at ca. 0.56 V, while
6 | J. Name., 2012, 00, 1-3
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of Ir(III) center than P-Ir-T and P-Ir-C (Table 4). In addition, these second quasi-reversible reduction wave should be induced by the
View Article Online
DOI: 10.1039/D0DT00102C
cyclometalated iridium(III) complexes can show two reduction reduction of pyridyl group in the ppy-type ligands of these
processes. The first reversible reduction should be assigned to the cyclometalated iridium(III) complexes.
reduction process of DTPO unit, locating at ca. -2.0 V (Table 4). The
Table 3 NTO results for these cyclometalated iridium(III) complexes based on optimized T1 geometries.
Contribution percentages of metal d orbitals and  orbitals of ligand to NTOs (%)
Complexes
NTO a
Ir
L-P
95.02
98.55
L-P
L-P
0.84
0.27
L-T
acac
0.20
0.20
acac
P-Ir-P
H
P
3.92
0.96
Ir
P-Ir-T
H
P
4.97
0.73
Ir
4.32
2.09
93.61
97.07
L-P
94.95
97.53
0.11
0.27
L-C
0.61
0.23
0.26
0.09
acac
0.20
0.15
P-Ir-C
H
P
a H and P represent NTO hole and particle orbital, respectively
Table 4 Redox properties of these cyclometalated iridium(III) complexes.
Compound
Epa (V)
Epc (V)
EHOMO c (eV)
ELUMO c (eV)
EgCV (eV)
P-Ir-P
P-Ir-T
P-Ir-C
0.56a
0.43a, 0.52a
0.54a
-1.92b, -2.29a
-2.05b, -2.40a
-1.91b, -2.41a
-5.22
-5.11
-5.16
-2.88
-2.75
-2.89
2.34
2.36
2.27
a Irreversible or quasi-reversible. The value was derived from the anodic or cathodic peak potential. b Reversible. The value was
set as E1/2
LUMO levels are derived from the E1/2 potential of the first reversible reduction wave.
.
c HOMO levels are calculated according to the onset potential of the first irreversible oxidation wave, respectively. d
Figure 5. Natural transition orbital (NTO) patterns for S0 → T1 excitation for these cyclometalated iridium(III) complexes based on their
optimized T1 geometries.
Optical power limiting behaviors
iridium(III) complexes. Therefore, it should be very necessary to
investigate their OPL performances in view of both developing new
OPL materials and providing valuable structure–activity information
for guiding the design of highly efficient OPL materials based on
cyclometalated iridium(III) complexes. It is well-known that forming
Donor(D)-Acceptor(A) structure is very critical for optimizing NLO
properties. DTPO can act as electron acceptor. Obviously,
introducing DTPO unit to the iridium(III) complexes should form new
D-A structures to furnish improved OPL behaviours. Hence, the OPL
Attributing the strong SOC effect to heavy metal centers, these
cyclometalated iridium(III) complexes have shown NIR
phosphorescent ability, indicating the formation of triplet excited
states (T1). Based on reverse saturable absorption (RSA) mechanism
of T1 states for a nano-second laser, the long-live T1 states can show
the potential of furnishing good OPL ability to these cyclometalated
iridium(III) complexes. To the best of our knowledge, OPL properties
have not been reported for unsymmetrical cyclometalated
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properties of these cyclometalated iridium(III) complexes have been (T1) via intersystem crossing (ISC) process. Undoubtedly, the T1 states
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investigated.
possess much longer lifetime than duratiDonOI:o1f0.t1h0e39/laDs0eDrT0p0u1l0se2sC,
furnishing enough time to the T1 states for extensive absorbing
energy of laser irradiance to be excited to a higher triplet states (Tn)
and induce OPL effect (Figure S4). Clearly, P-Ir-P can outperform the
unsymmetrical complexes P-Ir-T and P-Ir-C as OPL materials for 532
nm laser and even show better OPL ability than the state-of-the-art
OPL material C60 (Figure 7). Compared with P-Ir-T, the unsymmetrical
analog P-Ir-C can possess enhanced OPL performance which is
comparable to that of C60. Hence, the OPL ability of these
cyclometalated iridium(III) complexes fells in the order of P-Ir-P > P-
Ir-C > P-Ir-T, showing excellent consistency with their transient
absorption ability at 532 nm (Figure 6). The OPL ability of these
cyclometalated iridium(III) complexes based on the RSA mechanism
can be characterized by using the figure of merit factor σex/σo=
lnTsat/lnTo, where σo and σex represent the ground-state absorption
cross-section and the effective excited-state absorption cross-
section, respectively. Accordingly, the σex/σo values of P-Ir-P, P-Ir-T
and P-Ir-C are 7.34, 4.02 and 6.17, respectively. Based on their
absorption behavior (Figure 2), P-Ir-T shows stronger ground-state
absorption ability to furnish higher σo than that of P-Ir-P and P-Ir-C.
Accordingly, P-Ir-T possesses the lowest σex/σo among the three
complexes. So, it seems that introducing electron-donating group to
the ppy-type iridium(III) complexes might not represent a feasible
way to optimizing their OPL ability.
To investigate the potential of these cyclometalated iridium(III)
complexes as OPL materials, their transient absorption behavior was
studied. The transient absorption difference spectra for these
cyclometalated iridium(III) complexes are presented in Figure 6. As
shown in Figure 6, these complexes exhibit broad and strong broad
positive absorption bands from 520 to 700 nm and obvious bleaching
bands from 480 to 520 nm. The bleaching bands are also in
agreement with the ground-state absorption spectra, which can be
attributed to the depletion of the ground CT states. These results
indicate that these cyclometalated iridium(III) complexes can exhibit
strong excited state absorption in the range from 520 to 700 nm,
since their excited-state absorption (ESA) is stronger than the
ground-state absorption. Hence, significant RSA can occur in this
spectral region.
Based on the UV-vis spectra of these cyclometalated iridium(III)
complexes (Figure 2), it can be clearly seen that they display very low
ground-state absorption at 532 nm. Hence, their absorption
behaviors are adaptable to characterize their OPL properties against
532 nm laser beam. Together with their transient absorption
behavior, these cyclometalated iridium(III) complexes should
possess strong OPL effect for 532 nm laser beam. On this basis, The
OPL performances of these cyclometalated iridium(III) complexes
have been carried out in CH2Cl2 solution with a high linear
transmittance (T0) of ca. 85% through open-aperture Z-scan method.
0.010
P-Ir-P
P-Ir-T
532 nm
P-Ir-C
0.005
0.000
-0.005
In the Z-scan measurement (Figure S2), the incident irradiance
upon the solution samples was changed according to their Z-position
(against focal point Z0). When the sample stays far from the focal
point with the weak incident laser irradiance, the transmittance (T)
of the iridium(III) complexes solution can remains constant,
exhibiting linear optical property (i.e. obeying Beer's law) (Figure 7).
However, when the sample is gradually moved towards the focus
with the incident laser irradiance increase (Figure S1), the T of the
sample decreases to show OPL effect (Figure 7).
500
550
600
650
700
Wavelength (nm)
According to the good fitness between the Z-scan data and RSA
mechanism (Figure S3), the OPL behavior of these cyclometalated
iridium(III) complexes can be attributed to the reverse RSA of their T1
states (Figure S3). The molecules in the ground state (S0) can be
excited to the first singlet states (S1) by weak absorbing energy of
laser irradiance. Meanwhile, as a result of strong SOC effect induced
by the Ir(III) center. The S1 states can convert to the first triplet states
Figure 6. Time-resolved nanosecond transient difference absorption
spectra of these cyclometalated iridium(III) complexes in CH2Cl2
solution. λex = 400 nm. (c = 1 × 10 −5 M).
8 | J. Name., 2012, 00, 1-3
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1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
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Acknowledgements
This work was supported by the National Natural Science Foundation
of China (Nos. 21572176, 21875179, 21602170 and 51803163), the
Natural Science Foundation of Shaanxi Province (2019JZ-29), the
China Postdoctoral Science Foundation (2016M600778), the Shaanxi
Province Postdoctoral Science Foundation (2017BSHEDZZ03), the
Fundamental Research Funds for the Central Universities (Nos.
xjj2017099, xjj2016061 and cxtd2015003), and the Key Laboratory
Construction Program of Xi’an Municipal Bureau of Science and
Technology (201805056ZD7CG40). The characterization assistance
from Axin Lu and Gang Chang in the Instrument Analysis Center of
Xi’an Jiaotong University was also acknowledged.
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Journal Name
ARTICLE
TOC
View Article Online
DOI: 10.1039/D0DT00102C
Iridium(III) complexes with dithieno[3,2-
b:2ʹ,3ʹ-d]phosphole oxide group and their
high optical power limiting performances
Zhao Liu, Yanmin Xu, Ling Yue, Ming Li,Xiaolong Yang, Yuanhui
Sun, Lihe Yan, Guijiang Zhou
New cyclometalated iridium(III) complexes with dithieno[3,2-b:2ʹ,3ʹ-
d]phosphole oxide group can show higher optical power limiting
ability than C60 for 532 nm laser.
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