Aggregation-induced phosphorescence emission (AIPE) behaviors in PtII(C^N)(N-donor ligand)Cl-type complexes through restrainedD2ddeformation of the coordinating skeleton and their optoelectronic properties

Article abstract of DOI:10.1039/d0tc05527a

A series of four-coordinated Pt^II(C^N)(N-donor ligand)Cl-type complexes have been synthesized through combination between C^N-type and N-donor ligands with different sizes. Photophysical features, electrochemical behaviors and electroluminescent (EL) performances have been investigated in detail. Critically, the relationship between the size of organic ligands and aggregation-induced phosphorescence emission (AIPE) behaviors for these Pt^II(C^N)(N-donor ligand)Cl-type complexes has been characterized. With extending the dimensions of the C^N-type and/or N-donor ligands, the AIPE of these Pt^II(C^N)(N-donor ligand)Cl-type complexes is more likely to show up with lower H₂O volumetric fractions (f_w) in the THF solution of these complexes. These unique AIPE experimental results have clearly revealed a new AIE mechanism called restrainedD_2ddeformation of the coordinating skeleton of these Pt^II(C^N)(N-donor ligand)Cl-type complexes from square-planar (D_4h) in the ground states to the tetrahedron (T_d) skeleton in the excited states. In addition, solution-processed organic light-emitting diodes (OLEDs) based on these AIPE emitters have been fabricated to characterize their EL potential. Impressive EL efficiencies with the maximum external quantum efficiency (η_ext) of 25.2%, current efficiency (η_L) of 53.9 cd A^?1and power efficiency (η_P) of 43.5 lm W^?1can be achieved, indicating great potential of these Pt^II(C^N)(N-donor ligand)Cl-type AIPE emitters in the field of OLEDs. Importantly, this research has proposed a new AIE mechanism to promote the development of new phosphorescent AIPE complexes with great potential in the field of OLEDs.

Full text of DOI:10.1039/d0tc05527a

Journal of
Materials Chemistry C
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PAPER
Aggregation-induced phosphorescence emission
(AIPE) behaviors in PtII(C^N)(N-donor ligand)Cl-
type complexes through restrained D2d
deformation of the coordinating skeleton
and their optoelectronic properties†
a
Cite this: J. Mater. Chem. C, 2021,
9, 2334
Hua Yang,a Huiying Li,a Ling Yue,
Xi Chen,a Dongdong Song,a Xiaolong Yang,a
Yuanhui Sun,a Guijiang Zhou *a and Zhaoxin Wu
b
A
series of four-coordinated PtII(C^N)(N-donor ligand)Cl-type complexes have been synthesized
through combination between C^N-type and N-donor ligands with different sizes. Photophysical
features, electrochemical behaviors and electroluminescent (EL) performances have been investigated in
detail. Critically, the relationship between the size of organic ligands and aggregation-induced
phosphorescence emission (AIPE) behaviors for these PtII(C^N)(N-donor ligand)Cl-type complexes has
been characterized. With extending the dimensions of the C^N-type and/or N-donor ligands, the AIPE
of these PtII(C^N)(N-donor ligand)Cl-type complexes is more likely to show up with lower H2O
volumetric fractions (fw) in the THF solution of these complexes. These unique AIPE experimental results
have clearly revealed a new AIE mechanism called restrained D2d deformation of the coordinating
skeleton of these PtII(C^N)(N-donor ligand)Cl-type complexes from square-planar (D4h) in the ground
states to the tetrahedron (Td) skeleton in the excited states. In addition, solution-processed organic
light-emitting diodes (OLEDs) based on these AIPE emitters have been fabricated to characterize their
EL potential. Impressive EL efficiencies with the maximum external quantum efficiency (Zext) of 25.2%,
current efficiency (ZL) of 53.9 cd Aꢀ1 and power efficiency (ZP) of 43.5 lm Wꢀ1 can be achieved,
indicating great potential of these PtII(C^N)(N-donor ligand)Cl-type AIPE emitters in the field of OLEDs.
Importantly, this research has proposed a new AIE mechanism to promote the development of new
phosphorescent AIPE complexes with great potential in the field of OLEDs.
Received 25th November 2020,
Accepted 4th January 2021
DOI: 10.1039/d0tc05527a
rsc.li/materials-c
nonradiative decay and hence bring strong emission. In most
cases, emissive materials are applied in the solid state. Hence, AIE
Introduction
Since the first discovery by Tang in silole-type molecules,1 emitters can show inherent advantages in various fields, such as
aggregation-induced emission (AIE) has been shown as a new bio-imaging,10–14 sensing,15–17 tracing,18,19 electroluminescence20–22
strategy to develop emissive materials.2–9 For AIE emitters, in etc. Importantly, AIE should open a new outlet for developing
dilute solution, the motion of the groups in these molecules emissive materials.
will quench the emission signal through nonradiative decay
Besides the organic analogs developed at early stage, phos-
pathways to induce very weak emission. In contrast, in a solid phorescent organometallic complexes have also been found to
or viscous matrix, their group motion can be restrained to avoid show AIE behavior.23–26 Owing to their enhanced phosphore-
scence emission in the aggregation state, the AIE behavior
associated with phosphorescent organometallic complexes is
called aggregation-induced phosphorescence emission (AIPE).
Definitely, this discovery offers exciting inspiration to develop
new emissive materials with AIE properties. Zhao et al. had
synthesized a series of 2-phenylpyridine type (C^N-type) AIPE-
active Pt(II) complexes for bioimaging applications.27 Yam’s
group had developed two bipyridine-Pt(II)-alkynyl complexes
with AIPE behavior.28 Critically, these Pt(II) complexes can show
a School of Chemistry, MOE Key Laboratory for Nonequilibrium Synthesis and
Modulation of Condensed Matter, State Key Laboratory for Mechanical Behavior
of Materials, Xi’an Jiaotong University, Xi’an 710049, P. R. China.
E-mail: zhougj@xjtu.edu.cn
b Key Laboratory for Physical Electronics and Devices of the Ministry of Education,
Faculty of Electronic and Information Engineering, Xi’an Jiaotong University,
Xi’an 710049, P. R. China
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0tc05527a
2334 | J. Mater. Chem. C, 2021, 9, 2334ꢀ2349
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decent electroluminescent (EL) performances with a maximal CDCl3 on a Bruker Avance 400 MHz spectrometer. Chemical
external quantum efficiency (Zext) of 5.8% and current efficiency shifts were referenced to the solvent residual peak at d 7.26 ppm
1
(ZL) of 18.4 cd A
.
ꢀ1 28 A series of Pt(II) diimine acetylide complexes for H and 77.0 ppm for 13C NMR spectra, respectively. UV-vis
with good AIPE nature had been synthesized by Liu et al. to show absorption spectra were measured on a PerkinElmer Lambda
optical power limiting properties.29–31 Recently, a new category of 950 spectrophotometer. Emission spectra and lifetimes for the
PtII(C^N)(N-donor ligand)Cl-type complex has been found to obtained complexes were recorded on an Edinburgh Instruments,
readily show AIPE feature.32 Generally, these PtII(C^N)(N-donor Ltd (FLSP 920), fluorescence spectrophotometer. Phosphorescence
ligand)Cl-type complexes with AIPE ability had been employed to quantum yields (Fp) were measured in CH2Cl2 solution against
obtain mesoporous emissive materials,33 label cells,34 fabricate fac-[Ir(ppy)3] (Fp = 0.97) standard at 293 K.36 The thermal gravi-
organic light-emitting diodes (OLEDs)35 etc. For all these Pt(II) metric analysis data were acquired on a NETZSCH STA 409C
complexes with AIPE behavior, they generally share a very similar instrument. Cyclic voltammetry (CV) was carried out with a
structural feature of bearing rotatable or flexible moieties which Princeton Applied Research model 273A potentiostat with
can quench phosphorescence emission of these complexes in 100 mV sꢀ1 scan rate. All the CV measurements were conducted
dilute solution by their rotation or vibration to promote the in a three-electrode compartment cell with a Pt-sheet counter
nonradiative decay of the excited states of these complexes.32–35 electrode, a glassy-carbon working electrode, and an Ag/AgCl
In contrast, rotation or vibration of these groups can be effec- reference electrode. The supporting electrolyte was a 0.1 M acetoni-
tively blocked in the aggregate state to promote the radiative trile solution of [nBu4N]BF4, containing ferrocene as the internal
decay of the excited states of these complexes and hence induce reference. The data of elemental analyses were obtained on a Flash
strong phosphorescence emission. So, the AIPE phenomenon EA 1112 elemental analyzer. Fast atom bombardment (FAB) mass
can be observed in these Pt(II) complexes. Clearly, the design spectra were recorded on a Finnigan MAT SSQ710 system.
conception of AIPE molecules follows that of their fluorescent
Computational details
organic AIE counterparts. For sure, this situation will set an
obstacle for the design and synthesis of novel AIPE molecules. DFT calculations using the B3LYP method were conducted for
Hence, developing a new molecular design strategy should be of all of the obtained Pt(II) complexes. The 6-31G* basis set was
great importance for obtaining new AIPE molecules.
employed for C, H, N, and Cl atoms, while effective core
Herein, a series of AIPE-active heteroleptic PtII(C^N)(N-donor potentials applied for Pt atoms were a LanL2DZ basis set.37,38
ligand)Cl-type complexes have been presented with different Excitation behaviors were obtained through the TD-DFT calcula-
C^N and N-donor ligands. It has been found that the square tions on the basis of the optimized ground state (S0) geometries.
planar coordinating skeleton with D4h symmetry character of Additionally, the first triplet state (T1) geometries were optimized
these Pt(II) complexes in the ground state can undergo D2d using the UB3LYP method and analysis of the natural transition
deformation to form a tetrahedron coordinating skeleton with orbital (NTO) was conducted for S0 - T1 excitation. All of the
Td symmetry character in the excited state. The D2d deformation calculations were carried out using the Gaussian 09 program.39
associated with the coordinating skeleton of these Pt(II) complexes
OLED fabrication and measurements
can effectively promote the nonradiative decay of their excited
states to furnish low phosphorescence quantum yield (FP) in dilute The pre-cleaned ITO glass substrates were treated with ozone
solution. However, the D2d deformation process can be restrained for 20 min. Then, a 55 nm-thick hole injection layer of PED-
effectively in the aggregate state of these Pt(II) complexes to bring OT:PSS was spin-coated onto the surface of ITO glass. After the
high FP. Hence, these PtII(C^N)(N-donor ligand)Cl-type complexes ITO glass substrates with a PEDOT:PSS layer were cured at
should follow a new AIE mechanism of restraining the D2d 120 1C for 30 min in air, the emitting layer (30 nm) was
deformation in their excited states to show AIPE behavior. In obtained by spin-coating a chloroform solution of each of the
addition, these AIPE Pt(II) complexes can also show great potential Pt(II) complexes and 4,40-N,N0-dicarbazole-biphenyl (CBP) at
in OLEDs based on their impressive EL performances. Obviously, various concentrations. The obtained ITO chip was dried in a
this research should offer critical information for developing novel vacuum oven at 50 1C for 20 min and transferred to the
AIPE molecules and promoting their application in the EL field.
deposition system for organic and metal deposition. TPBi
(1,3,5-tris-(N-(phenyl)-benzimidazole)-benzene) (55 nm), LiF
(0.7 nm) and Al cathode (150 nm) were successively evaporated
at a base pressure less than 10ꢀ6 Torr. The EL spectra and CIE
coordinates were recorded using a PR650 spectra colorimeter.
The J–V–L curves of the devices were recorded using a Keithley
Experimental
General information
All commercially available chemicals used were not subjected 2400/2000 source meter and the luminance was measured
to further purification. All solvents for the reactions were dried using a PR650 SpectraScan spectrometer. All of the experiments
and distilled in a proper way. The reactions were checked with and measurements were conducted under ambient conditions.
thin-layer chromatography (TLC) materials purchased from
Procedures for the synthesis of Pt(II) complexes
Merck & Co., Inc. Flash column chromatography and preparative
TLC plates were made from silica gel from Shenghai Qingdao Under the N2 atmosphere, organic cyclometalating ligands (L-1,
(300–400 mesh). 1H and 13C NMR spectra were recorded in L-2, and L-3, 1.1 equiv., Scheme S1, ESI†) and K2PtCl4 (1.0 equiv.)
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were heated to 90 1C and stirred for 24 h in 16 mL of mixture of 7.92–7.68 (m, 11H), 7.50 (d, J = 4.40 Hz, 3H), 7.43 (t, J = 7.20 Hz,
H2O and 2-ethoxyethanol (v/v = 1 : 3). Then the reaction mixture 1H), 7.35 (t, J = 3.60 Hz, 2H), 7.22 (t, J = 3.20 Hz, 1H), 6.84
was cooled to room temperature, and water was added. The (s, 1H). 13C NMR (100 MHz, CDCl3, d): 166.37, 154.02, 151.45,
precipitate of the m-chloro-bridged dimer was collected and dried 149.62, 143.26, 139.84, 138.52, 136.40, 135.07, 134.84, 130.78,
under vacuum. Without further purification, the dimer and 130.33, 129.54, 129.01, 128.70, 128.19, 128.09, 127.52, 127.15,
N-donor ligands (N1, N2, and N3, Scheme S2, ESI†) were added 127.10, 126.66, 124.56, 123.58, 123.34, 123.20, 122.58, 118.90.
in 15 mL of CHCl3 and heated to 45 1C for 12 h. After cooling to FAB-MS (m/z): 688 [M + Na]+. Elemental analysis calcd (%) for
room temperature, the mixture was extracted with CH2Cl2. The
C32H23ClN2Pt: C 57.70, H 3.48, N 4.21; found, C 57.60, H 3.53,
organic phase was dried over anhydrous Na2SO4 and the solvent N 4.12.
1
was removed under reduced pressure. The residual was purified
C2. Yellow solid; yield: 65%. H NMR (400 MHz, CDCl3, d):
9.83 (d, J = 5.70 Hz, 1H), 9.16 (d, J = 6.80 Hz, 2H), 8.18 (d, J = 7.70 Hz,
by flash column chromatography using an appropriate eluent.
1
A1. Orange solid; yield: 44%. H NMR (400 MHz, CDCl3, d): 2H), 8.01 (d, J = 8.40 Hz, 3H), 7.91 (s, 2H), 7.81 (d, J = 8.40 Hz, 2H),
9.83 (d, J = 4.70 Hz, 1H), 9.00 (d, J = 5.00 Hz, 2H), 8.43 (d, J = 8.70 7.78 (t, J = 7.70 Hz, 3H), 7.54–7.32 (m, 7H), 7.34 (d, J = 6.90 Hz, 2H),
Hz, 1H), 7.93 (t, J = 7.30 Hz, 1H), 7.87 (t, J = 7.10 Hz, 1H), 8.32 (d, 7.23 (s, 1H), 6.89 (s, 1H). 13C NMR (100 MHz, CDCl3, d): 166.41,
J = 8.20 Hz, 1H), 7.76 (d, J = 3.50 Hz, 1H), 7.54–7.38 (m, 5H), 7.16 154.80, 151.51, 149.14, 144.27, 140.45, 139.93, 138.71, 136.45, 135.12,
(t, J = 6.0 Hz, 1H), 6.50 (d, J = 8.4 Hz, 1H). 13C NMR (100 MHz, 134.83, 130.86, 128.79, 128.73, 128.69, 127.81, 127.19, 126.70, 126.26,
CDCl3, d): 168.20, 154.10, 152.40, 145.47, 138.85, 138.54, 124.66, 123.80, 123.53, 123.31, 122.67, 120.56, 118.99, 109.77. FAB-MS
138.07, 132.09, 130.63, 130.22, 129.75, 129.23, 127.48, 126.30, (m/z): 777 [M + Na]+. Elemental analysis calcd (%) for C38H26ClN3Pt: C
123.88, 122.35, 121.80, 121.06. FAB-MS (m/z): 536 [M + Na]+. 60.44, H 3.47, N 5.56; found, C 60.35, H 3.52, N 5.49.
1
Elemental analysis calcd (%) for C20H15ClN2Pt: C 46.75, H 2.94,
A3. Yellow solid; yield: 39%. H NMR (400 MHz, CDCl3, d):
9.71 (d, J = 5.80 Hz, 1H), 9.11 (d, J = 5.00 Hz, 2H), 7.98 (t, J = 7.70 Hz,
N 5.45; found, C 46.66, H 2.86, N 5.39.
1
B1. Orange solid; yield: 37%. H NMR (400 MHz, CDCl3, d): 1H), 7.82 (t, J = 6.70 Hz, 1H), 7.73 (d, J = 8.90 Hz, 1H), 7.50 (t, J =
9.85 (d, J = 4.80 Hz, 1H), 9.02 (d, J = 6.40 Hz, 2H), 8.45 (d, J = 8.40 6.60 Hz, 2H), 7.43 (s, 1H), 7.31–7.19 (m, 4H), 7.13 (t, J = 5.9 Hz, 1H),
Hz, 1H), 8.25 (d, J = 8.40 Hz, 1H), 7.89 (t, J = 6.40 Hz, 1H), 7.84– 6.67 (s, 1H), 2.02 (q, J = 7.40 Hz, 4H), 0.33 (t, J = 7.30 Hz, 6H). 13
C
7.67 (m, 9H), 7.55–7.39 (m, 6H), 7.17 (t, J = 6.80 Hz, 1H), 6.65 NMR (100 MHz, CDCl3, d): 167.44, 154.27, 151.47, 150.73, 145.29,
(d, J = 8.40 Hz, 1H). 13C NMR (100 MHz, CDCl3, d): 168.07, 143.58, 143.40, 140.00, 140.55, 138.67, 137.83, 127.45, 126.60,
153.81, 152.27, 149.78, 145.39, 143.29, 139.83, 138.70, 138.43, 126.00, 122.88, 121.59, 121.42, 119.62, 118.19, 55.84, 32.74, 8.58.
134.81, 131.99, 130.52, 130.12, 129.65, 129.27, 129.03, 128.20, FAB-MS (m/z): 630 [M + Na]+. Elemental analysis calcd (%) for
128.12, 127.53, 127.34, 127.16, 123.74, 123.51, 122.23, 121.68, C27H25ClN2Pt: C 53.33, H 4.14, N 4.61; found, C 53.26, H 4.22,
120.95. FAB-MS (m/z): 688 [M + Na]+. Elemental analysis calcd N 4.53.
1
(%) for C32H23ClN2Pt: C 57.70, H 3.48, N 4.21; found, C 57.63,
H 3.51, N 4.11.
B3. Yellow solid; yield: 54%. H NMR (400 MHz, CDCl3, d):
9.73 (d, J = 5.20 Hz, 1H), 9.12 (d, J = 6.80 Hz, 2H), 7.87 (d, J = 8.40 Hz,
C1. Orange solid; yield: 56%. 1H NMR (400 MHz, CDCl3, d): 9.86 2H), 7.81 (d, J = 8.00 Hz, 3H), 7.75 (d, J = 6.80 Hz, 3H), 7.69 (d, J =
(d, J = 4.70 Hz, 1H), 9.08 (d, J = 6.70 Hz, 2H), 8.46 (d, J = 8.70 Hz, 7.20 Hz, 2H), 7.51 (t, J = 7.20 Hz, 2H), 7.44 (d, J = 4.00 Hz, 2H), 7.34
1H), 8.26 (d, J = 8.30 Hz, 1H), 8.18 (d, J = 7.70 Hz, 2H), 7.98 (d, J = (d, J = 7.20 Hz, 1H), 7.30–7.18 (m, 3H), 7.14 (t, J = 6.10 Hz, 1H), 6.99
8.40 Hz, 2H), 7.89 (t, J = 8.30 Hz, 1H), 7.80 (d, J = 8.40 Hz, 3H), 7.75 (s, 1H), 2.03 (q, J = 7.20 Hz, 4H), 0.34 (t, J = 7.40 Hz, 6H). 13C NMR
(d, J = 6.70 Hz, 2H), 7.53 (t, J = 9.40 Hz, 3H), 7.48–7.40 (m, 4H), 7.34 (100 MHz, CDCl3, d): 167.42, 154.10, 151.46, 150.70, 149.60,
(t, J = 7.0 Hz, 2H), 7.18 (t, J = 7.20 Hz, 1H), 6.66 (d, J = 8.40 Hz, 1H). 145.30, 143.60, 143.41, 143.31, 141.02, 140.51, 139.85, 138.64,
13C NMR (100 MHz, CDCl3, d): 168.08, 154.01, 152.27, 149.22, 134.84, 129.02, 128.23, 128.09, 127.52, 127.42, 127.16, 126.60,
145.37, 140.40, 139.92, 138.71, 138.45, 134.74, 131.99, 130.53, 123.27, 122.81, 121.72, 121.42, 119.70, 118.18, 55.82, 32.73, 8.57.
130.11, 129.63, 129.19, 128.66, 127.75, 127.36, 126.20, 123.76, FAB-MS (m/z): 782 [M + Na]+. Elemental analysis calcd (%) for
123.61, 122.23, 121.68, 120.95, 120.52, 120.49, 109.70. FAB-MS C39H33ClN2Pt: C 61.62, H 4.38, N 3.68; found, C 61.52, H 4.43,
(m/z): 777 [M + Na]+. Elemental analysis calcd (%) for C38H26ClN3Pt: N 3.57.
1
C 60.44, H 3.47, N 5.56; found, C 60.34, H 3.53, N 5.47.
C3. Yellow solid; yield: 61%. H NMR (400 MHz, CDCl3, d):
1
A2. Yellow solid; yield: 51%. H NMR (400 MHz, CDCl3, d): 9.77 (d, J = 4.90 Hz, 1H), 9.21 (d, J = 6.80 Hz, 2H), 8.21 (d, J = 7.70
9.83 (d, J = 5.80 Hz, 1H), 9.11 (d, J = 5.00 Hz, 2H), 7.98 (t, J = Hz, 2H), 8.05 (d, J = 8.50 Hz, 2H), 7.89–7.77 (m, 6H), 7.57 (d, J =
7.70 Hz, 2H), 7.91 (dd, J = 3.80 Hz, 2H), 7.79 (dd, J = 3.50 Hz, 8.20 Hz, 2H), 7.50 (t, J = 6.40 Hz, 3H), 7.41–7.30 (m, 5H), 7.24 (t,
1H), 7.51 (t, J = 6.60 Hz, 2H), 7.46 (t, J = 3.80 Hz, 1H), 7.41– J = 5.60 Hz, 1H), 7.17 (t, J = 5.90 Hz, 1H), 6.89 (s, 1H), 2.07 (q, J =
7.31 (m, 2H), 7.23 (dd, J = 5.6 Hz, 1H), 6.70 (s, 1H). 13C NMR 7.30 Hz, 4H), 0.38 (t, J = 7.40 Hz, 6H). 13C NMR (100 MHz,
(100 MHz, CDCl3, d): 166.36, 154.19, 151.44, 144.19, 138.63, CDCl3, d): 167.44, 154.32, 151.50, 150.77, 149.11, 145.39,
137.84, 136.38, 135.03, 130.77, 128.72, 128.56, 127.11, 126.65, 143.64, 143.47, 141.04, 140.52, 140.42, 139.93, 138.72, 134.81,
126.04, 124.58, 123.20, 122.58, 118.91. FAB-MS (m/z): 536 [M + 128.70, 127.80, 127.49, 126.62, 126.24, 123.78, 123.42, 122.92,
Na]+. Elemental analysis calcd (%) for C20H15ClN2Pt: C 46.75, H 121.71, 121.48, 120.54, 120.52, 119.71, 118.24, 109.75, 55.86,
2.94, N 5.45; found, C 46.68, H 3.05, N 5.37.
B2. Yellow solid; yield: 46%. H NMR (400 MHz, CDCl3, d): calcd (%) for C45H36ClN3Pt: C 63.64, H 4.27, N 4.95; found, C
9.83 (d, J = 5.60 Hz, 1H), 9.10 (d, J = 6.80 Hz, 2H), 8.01 (s, 1H), 63.56, H 4.35, N 4.88.
32.76, 8.61. FAB-MS (m/z): 907 [M + Na]+. Elemental analysis
1
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Scheme 1 Synthetic scheme for the PtII(C^N)(N-donor ligand)Cl-type complexes.
Results and discussion
Synthesis and characterization
shown in Scheme 1. Firstly, the C^N-type ligands were cyclome-
talated with K2PtCl4 in the mixture of 2-ethoxyethanol and water
with a volume ratio of 3 : 1 at 90 1C. By this way, the corres-
ponding Pt(II) m-chloro-bridged dimers with different C^N-type
ligands were obtained to prepare the target PtII(C^N)(N-donor
ligand)Cl-type cyclometalated complexes by refluxing the mixture
of the Pt(II) m-chloro-bridged dimer and the corresponding
N-donor ligand in CHCl3.
Synthesis schemes for C^N-type ligands and N-donor ligands
are shown in Schemes S1 and S2 (ESI†). Synthetic procedures
for C^N-type ligands and N-donor ligands are provided in the
ESI† according to the reported method.40–43 The synthetic pro-
cedures for these PtII(C^N)(N-donor ligand)Cl-type complexes are
Table 1 Photophysical and thermal data of the PtII(C^N)(N-donor ligand)Cl-type complexes
a
Emission lem [nm] solution
293 K/film PMMA
FP solutionb/filmc 293 K tP solutiond/filme [ms] 293 K Td [1C]
a
Complexes Absorption labs [nm] 293 K
A1
B1
C1
A2
B2
C2
A3
B3
C3
a
258 (4.92), 279 (4.73), 329 (4.43), 612, 643/600, 639
355 (4.33), 421 (3.97), 438
285 (4.79), 307 (4.86), 350 (4.46), 611, 649/600, 639
411 (3.99), 421 (4.02), 438
279 (5.01), 343 (4.84), 358 (4.81), 611, 647/600, 639
419 (4.20), 440 (4.21)
275 (4.78), 311 (4.69), 339 (4.26), 521, 564, 605/524, 564, 605
387 (3.68), 402 (3.67)
275 (5.05), 309 (5.06), 345 (4.53), 521, 565, 615/524, 564, 605
388 (3.95), 406 (3.94)
276 (4.92), 316 (4.76), 345 (4.65), 520, 564, 610/524, 564, 605
405 (3.80), 413 (3.72)
276 (4.54), 305 (4.49), 326 (4.71), 540, 584, 622/542, 584, 625
396 (4.02), 412 (4.06)
0.5%/19.6%
1.5%/17.7%
3.0%/19.7%
0.1%/19.0%
0.2%/19.8%
0.6%/21.5%
7.2%/67.7%
7.7%/70.0%
11.4%/70.2%
0.6/7.6
205
279
245
215
265
311
220
297
318
0.56/6.2
0.99/5.9
0.68/14.9
0.66/6.3
0.73/2.5
0.78/12.7
0.78/6.8
0.81/7.5
278(4.83), 299 (5.02), 319 (5.08),
349 (4.70), 414 (4.27)
540, 582, 620/542, 584, 625
279 (4.93), 331 (4.99), 349 (4.85), 541, 583, 620/542, 584, 625
390 (4.27), 417 (4.22)
Measured in CH2Cl2 at a concentration of 10ꢀ5 M, log e values are shown in parentheses, and measured in the PMMA film with a 6 wt% doping
b
c
level. Relative to fac-[Ir(ppy)3] (Fp = 97%), lex = 360 nm. Doped PMMA film of the 6 wt% doping-level with a thickness of ca. 200 nm on quartz.
Measured in an integrating sphere, lex = 430 nm. Measured in degassed CH2Cl2 at a concentration of 10ꢀ5 M. Measured in the doped PMMA
d
e
film of the 6 wt% doping-level with a thickness of ca. 200 nm on quartz, lex = 400 nm.
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All the obtained PtII(C^N)(N-donor ligand)Cl-type complexes
have been characterized by 1H and 13C NMR spectra. All the
NMR spectra are provided in the ESI† (Fig. S1 and S2). For all
the complexes, in their 1H NMR spectra, the resonance peak
located at around 9.8 ppm can be assigned to the proton on the
carbon atom adjacent to the chelated nitrogen atom in the
pyridyl ring of the C^N-type ligands. Another characteristic set
signal at ca. 9.1 ppm should be induced by the two protons
attached to the two carbon atoms connected with the coordi-
nated nitrogen atom in the N-donor ligand. In addition, for
complexes A3, B3 and C3, the two sets of signals at ca. 2.1 and
0.3 ppm can be ascribed to the protons of the two ethyl groups
on the 9-position of the fluorene unit. All these spectral data have
indicated the successful synthesis of these PtII(C^N)(N-donor
ligand)Cl-type complexes.
Thermal and photophysical properties
The thermal properties of these PtII(C^N)(N-donor ligand)Cl-
type complexes have been characterized by thermogravimetric
analysis (TGA) under a nitrogen flow. The TGA results revealed
their good thermal stability with decomposition temperature
(Td) in the range from 205 to 318 1C (Table 1 and Fig. S3, ESI†).
Td above 200 1C should make them suitable for application in
various fields, such as AIE investigation and phosphorescent
emitters in OLEDs in terms of stability.
In CH2Cl2 solution with a concentration of ca. 10ꢀ5 mol Lꢀ1 at
293 K, UV-vis absorption behaviors of these PtII(C^N)(N-donor
ligand)Cl-type complexes are depicted in Fig. 1 and the corres-
ponding data are shown in Table 1. Obviously, there are two
kinds of absorption bands in their UV-vis absorption spectra
(Fig. 1). The high-energy absorption bands before ca. 360 nm can
be safely assigned to the spin-allowed p–p* transition of both
C^N-type and N-donor ligands.44 With increasing conjugation of
the N-donor ligands, the bathochromic effect can be observed in
the high-energy absorption bands (Fig. 1). Besides the strong
absorption bands, much weaker ones in the long wavelength
region can be observed as well (Fig. 1 and Table 1). The low-
energy weak absorption bands should be induced by the charge-
transfer (CT) excitation processes of these PtII(C^N)(N-donor
ligand)Cl-type complexes.45 In addition for each set of complexes
with the same C^N-type ligand, they typically show a very similar
spectral line-shape for the weak absorption bands.
Fig. 1 UV-vis absorption spectra of these PtII(C^N)(N-donor ligand)Cl-
type complexes in CH2Cl2 solution recorded at 293 K. (a) A1, B1 and C1; (b)
A2, B2 and C2; and (c) A3, B3 and C3.
Based on the theoretical results obtained by time-dependent
density functional theory (TD-DFT) calculations, due to the
overwhelming contribution of over 94% (Table 2), the HOMO -
LUMO (H - L) transitions of these PtII(C^N)(N-donor ligand)Cl-
type complexes can be employed to interpret the feature of their absorption bands should possess primarily both ligand-to-ligand
S0 - S1 transitions corresponding to their major absorption charge-transfer (LLCT) feature together with some metal-to-ligand
bands. From their molecular orbital (MO) patterns (Fig. 2), their charge-transfer (MLCT) features. Apart from LLCT and MLCT
HOMOs are mainly contributed by the p orbitals of the coordi- features, for complexes A1, A2 and A3, intraligand charge-
nated N-free aromatic unit in the C^N-type ligands as well as the transfer (ILCT) in the C^N-type ligand can contribute the character
dp orbitals of the Pt(II) centers (Fig. 2 and Table 2). However, their of their low-energy CT absorption bands as well. For each set of
LUMOs are located on the p* orbitals of the pyridyl units in the three complexes possessing the same C^N-type complexes, their
N-donor ligands. In addition, for complexes A1, A2 and A3, the p* H - L transitions similarly involve a Pt(II) center, a C^N-type ligand
orbitals of the pyridyl rings in the C^N-type ligands can also give and the pyridyl unit in the N-donor ligand (Fig. 2). As a result, they
some contribution to their LUMOs. Hence, their low-energy CT can exhibit similar low-energy CT absorption bands (Fig. 1).
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Table 2 Theoretical calculation results for these PtII(C^N)(N-donor ligand)Cl-type complexes based on their optimized S0 geometriesa
Contribution percentages of metal dp orbitals and
p orbitals of ligands to MOs [%]
Main configuration of S0 - S1 Main configuration of S0 - T1
excitation/Ecal/lcal/f a
excitation/Ecal/lcal
a
Complexes MO
A1
L+1
L
Pt
Cl
N1
L-1
H - L (95.08%)
2.776 eV
446.6 nm
H - L (72.91%)
H - L+1 (10.70%)
Hꢀ1 - L (6.93%)
2.118 eV
3.94
5.05
23.63
0.27
0.81
14.05
26.78
73.90
22.25
0.10
0.27
21.89
71.89
62.22
59.89
H
f = 0.0330
Hꢀ1 13.06
585.3 nm
B1
C1
A2
B2
C2
A3
B3
Pt
Cl
N2
L-1
H - L (94.87%)
2.652 eV
467.5 nm
H - L (72.53%)
H - L+1 (12.73%)
Hꢀ1 - L (6.17%)
2.694 eV
L+1
L
H
5.87
2.19
24.17
0.44
0.50
14.49
26.83
15.18
83.67
0.10
0.28
78.51
13.64
61.24
59.95
f = 0.0101
Hꢀ1 12.94
460.2 nm
Pt
Cl
N3
L-1
H - L (96.33%)
2.615 eV
474.2 nm
H - L (71.32%)
H - L+1 (15.05%)
2.654 eV
L+1
L
H
5.94
2.35
23.82
0.42
0.55
14.22
0.19
14.09
84.12
0.11
79.55
12.97
61.85
0.16
f = 0.0067
467.2 nm
Hꢀ1 0.64
99.01
Pt
Cl
N1
L-2
H - L (96.35%)
2.809 eV
441.4 nm
H - L (62.48%)
Hꢀ1 - L (22.95%)
Hꢀ1 - L+1 (3.04%)
2.684 eV
L+1
L
H
2.90
2.20
29.59
0.37
0.27
20.43
8.25
75.70
17.87
0.18
0.33
21.03
79.66
49.80
84.64
f = 0.0176
Hꢀ1 6.79
461.9 nm
Pt
Cl
N2
L-2
H - L (95.15%)
2.658 eV
466.5 nm
H - L (73.39%)
H - L+1 (4.68%)
Hꢀ1 - L (3.46%)
2.614 eV
L+1
L
H
2.46
1.79
29.97
0.11
0.35
20.78
8.23
9.77
87.79
0.19
0.84
87.66
10.07
49.06
83.77
f = 0.0092
Hꢀ1 7.16
476.3 nm
Pt
Cl
N3
L-2
H - L (97.32%)
2.640 eV
469.6 nm
H - L (79.82%)
H - L+1 (2.30%)
2.604 eV
L+1
L
H
2.53
1.89
29.78
0.12
0.37
20.44
0.18
8.73
88.61
0.19
99.01
88.62
9.13
49.59
0.17
f = 0.0088
476.2 nm
Hꢀ1 0.64
Pt
Cl
N1
L-3
H - L (94.13%)
2.845 eV
435.7 nm
H - L (68.65%)
H - L+1 (10.05%)
Hꢀ1 - L (9.94%)
2.366 eV
L+1
L
H
3.51
3.45
27.88
0.36
0.47
18.44
17.79
70.97
24.69
0.09
0.17
25.16
71.39
53.59
72.19
f = 0.0352
Hꢀ1 9.85
524.1 nm
Pt
Cl
N2
L-3
85.98
9.04
52.25
72.89
H - L (95.18%)
2.685 eV
461.8 nm
H - L (73.85%)
H - L+1 (11.75%)
Hꢀ1 - L (6.12%)
2.710 eV
L+1
L
H
4.09
1.90
28.60
0.29
0.36
19.06
17.44
9.64
88.70
0.09
0.21
f = 0.0062
Hꢀ1 9.46
457.6 nm
C3
Pt
Cl
N3
L-3
H - L (96.09%)
2.645 eV
468.8 nm
H - L (73.39%)
H - L+1 (13.70%)
2.667 eV
L+1
L
H
4.17
2.03
28.22
0.30
0.39
18.76
0.20
8.16
89.76
0.09
98.95
87.37
7.82
52.93
0.18
f = 0.0045
464.9 nm
Hꢀ1 0.67
a
H - L represents the HOMO to LUMO transition. Ecal, lcal and f represent the calculated excitation energy, calculated emission wavelength and
oscillator strength, respectively. The oscillator strength of S0 - T1 is zero due to the spin-forbidden singlet–triplet transition under TD-DFT
calculation in the Gaussian program without the consideration of spin-orbital coupling.
Photoluminescence spectra of all the PtII(C^N)(N-donor PMMA matrix. Hence, phosphorescence wavelengths are
ligand)Cl-type complexes are measured in both CH2Cl2 solution mainly up to the C^N-type ligands (Fig. 3). Complexes A1, B1
and PMMA film at 293 K (Fig. 3). Under irradiation by 360 nm and C1 with C^N-type ligand L-1 show nearly identical phos-
UV light, phosphorescence emission can be observed (Fig. 3). phorescence maxima of ca. 610 nm, while 520 nm for A2, B2
Through comparison of the difference in phosphorescence and C2 with L-2 has been observed together with 540 nm for A3,
quantum yields (FP) for solution and film samples (Table 1), B3 and C3 (Fig. 3 and Table 2). It has been shown that the
it has been found that the phosphorescence signal can be electronic structure of the C^N-type ligands of this type
greatly enhanced when these complexes are doped into a rigid complex has a dominant impact on their photophysical
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Fig. 2 Molecular orbital (MO) patterns (isocontour value = 0.030) of these PtII(C^N)(N-donor ligand)Cl-type complexes on the basis of their optimized
S0 geometries.
properties.46–48 Although the HOMO and LUMO patterns of L-1 C2, a significant redshift is detected in the emission band of
and L-2 are similar, the electron density on their coordinating complexes A1, B1 and C1. For complexes A3, B3, and C3, their
carbon atoms to the Pt(II) center is quite different.48 The yellow phosphorescence should be induced by the fluorenyl in
electron density on the coordinating carbon atom in L-1 is ligand L-3 with large conjugation. In order to figure out the
much higher than that in L-2. This means that the chelated phosphorescence emission behavior of these PtII(C^N)(N-donor
carbon atom in L-1 has a stronger electron-donating ability to ligand)Cl-type complexes, natural transition orbital (NTO) analysis
promote the CT process and lower the energy-level of the has been carried out to interpret their S0 - T1 excitation on the
emissive CT states. So, compared with complexes A2, B2, and basis of their optimized T1 geometries.
Fig. 3 Photoluminescence spectra of these PtII(C^N)(N-donor ligand)Cl-type complexes at 293 K. (a) Solution and (b) doped PMMA film.
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Fig. 4 Natural transition orbital patterns (isocontour value = 0.030) for S0 - T1 excitation for these PtII(C^N)(N-donor ligand)Cl-type complexes on the
basis of their optimized T1 geometries.
Based on the distribution patterns of NTO hole and particle should not be induced by the water added, but incurred by the
orbitals for these complexes, both hole and particle orbitals are aggregates generated by adding water. So, these PtII(C^N)(N-donor
mainly resident on the C^N-type ligands with nearly no contribu- ligand)Cl-type complexes can exhibit obvious AIPE behavior.
tion from the N-donor ligands (Fig. 4 and Table 3). Hence, the
Through adding water stepwise to the THF solution of these
phosphorescence of these complexes should shows ligand Pt(II) complexes, the phosphorescence signal increases slowly
centered 3p–p* character involving ILCT feature from the when the value of fw is low. However, upon increasing fw
coordinated hydrocarbon aromatic ring to the pyridyl unit further, the intensity of the phosphorescence signal can be
(Fig. 4). Clearly, this theoretical result can provide a perfect enhanced rapidly. We define a parameter of f cw, at which the
explanation to the great similarity among the spectral line- phosphorescence intensity begins to be enhanced notably.
shape for the phosphorescence emission of the complexes Based on the results of Fig. 5, it can be clearly seen that f wc
bearing the same C^N-type ligand. In addition, the structured values have a close relationship with the chemical structures of
spectral line-shape for their phosphorescence emission at both both N-donor ligands and C^N-type ligands (Fig. 6). With the
293 K (Fig. 3) and 77 K (Fig. S4, ESI†) also indicated the ligand- elongation of the N-donor ligands from both longitudinal and
centered character for the phosphorescence signal from these transverse dimensions, the f cw values decrease (Fig. 6), indicating
PtII(C^N)(N-donor ligand)Cl-type complexes. Clearly, there is a that the AIPE is much more likely to occur. For example, the f cw of
good consistency between experimental and theoretical results, A1 with pyridine (N1) as the N-donor ligand is about 70%, while
indicating the validity of the theoretical calculations.
the f cw of B1 with 4-([1,10-biphenyl]-4-yl)pyridine (N2) as the
N-donor ligand is about 50%. With even larger 9-(4-(pyridin-4-
yl)phenyl)-9H-carbazole (N3) as the N-donor ligand, the f cw of C1
can be as low as 40% (Fig. 6). Also, the same trend has been
found in complexes A2, B2 and C2 with the f cw values falling in
the similar order A2 ( f wc ca. 60%) 4 B2 (f wc ca. 50%) 4 C2 ( f cw ca.
30%) upon increasing the size of the N-donor ligand. With a
much larger fluorene-based C^N-type ligand, very low f cw of 10%
can induce AIPE behavior in A3, B3 and C3. So, it can be clearly
seen that increasing the size of both C^N-type and N-donor
ligands, the AIPE behavior of these can been incurred easily in
these PtII(C^N)(N-donor ligand)Cl-type complexes.
AIPE behaviors and mechanism
From the FP difference in both dilute solution and rigid PMMA
film (Table 1), the AIPE properties of these PtII(C^N)(N-donor
ligand)Cl-type complexes should be indicated. In order to
confirm the AIPE properties of these Pt(II) complexes, their PL
spectra in the tetrahydrofuran–water (THF–H2O) mixture with
different H2O volumetric fractions (fw) have been measured
(Fig. 5). Upon increasing the fw, the phosphorescence intensity
of these PtII complexes is enhanced as well (Fig. 5). It has been
found that solvent with high polarity typically disfavors the
phosphorescence emission of this kind of complex.27,32 Hence,
the phosphorescence enhancement of these Pt(II) complexes
Based on the structural characteristics of these PtII(C^N)(N-donor
ligand)Cl-type complexes, it might be only right and proper that
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Table 3 NTO results for these PtII(C^N)(N-donor ligand)Cl-type com-
plexes based on their optimized t1 geometries
processes and quench the emission signal in dilute solution.49–51
This coordinating skeleton deformation from the ground state to
the excited state is called the D2d deformation. Reasonably, there
should be a great chance for the D2d deformation of the square-
planar Pt(II) complexes to be effectively restrained in either the
rigid matrix or aggregate state to block the nonradiative decay
processes and recover their emission. By this way, the square-
planar Pt(II) complexes can furnish a new AIPE mechanism
called restrained D2d deformation of the coordinating skeleton
of the excited state in aggregated state. In order to the evaluate
the role played by the D2d deformation in the AIPE behavior of
these PtII(C^N)(N-donor ligand)Cl-type complexes, both AIPE
results and photophysical data have been analyzed.
In order to show the mechanism of the restrained D2d
deformation in the excited state, theoretical calculations have
been carried out for a model Pt(II) complex with 2-phenyl-
pyridine and pyridine as the C^N ligand and the N-donor
ligand, respectively. A local minimum (Min) on the ground
state (S0) surface and a local Min on the excited state (S1) surface
were optimized by CAM-B3LYP and TDA-CAM-B3LYP for the
model complex, respectively (Fig. 7). The former (S0 Min) and
latter (S1 Min) of the model complex can adopt planar and
tetrahedron coordinating skeletons, respectively. The dihedral
angle between the Pt1–C6–N19 plane (P1) and the Pt1–N20–Cl31
plane (P2) is very close to zero (4.71), while P1 is almost
perpendicular to P2 at the S1 Min (P1/P2 dihedral angle is
93.11) (Fig. 7). Moreover, according to the natural orbital analysis,
the located S1 Min has a d - p* character which is also found in
the excited S3 state of the S0 Min. This means that the energy levels
of the excited states may be exchanged during the D2d deformation
process.
Therefore, an approximate path between the located S0 Min
and S1 Min was computed by the linear interpolation to
investigate the D2d deformation process of the coordinating
skeleton (Fig. 8). Then, the approximate potential energy curves
(PECs) of the ground (S0) and excited states (S1–S5) were
computed by CAM-B3LYP and TDA-CAM-B3LYP, respectively.
As shown in Fig. 8, the S1–S5 states are close to each other
around the S0 Min, which means it is easy for the internal
conversion to occur at the S0-Min in the photo-excited model
complex. The shape and the transition properties of the PECs
for S0, S1 and S2 imply a S1/S2 conical intersection (CI) and a
S0/S1 CI may exist around +P1/P2 = 251 and S1 Min (+P1/P2 =
93.11), respectively. Through the S1/S2 CIs, the S2 and the higher
excited species of the model complex with a planar coordinat-
ing configuration can decay to the S1 states and form a tetra-
hedral coordination on the S1 surface and finally decay to the S0
state by the S0/S1 CI without radiation (Fig. 8).
Based on the theoretical results for the model complex, it
can be expected that the D2d deformation of the coordinating
skeleton for the excited states of these PtII(C^N)(N-donor
ligand)Cl-type complexes in dilute solution can quench their
phosphorescence signals, which can be recovered in the aggregate
state by restraining the D2d deformation of their coordinating
skeleton. Critically, the AIPE results in Fig. 5 and 6 should provide
a solid basis for this expected new AIPE mechanism. Clearly,
Contribution percentages of metal dp orbitals and
p orbitals of ligand to NTOs [%]
Complexes NTOa
A1
B1
C1
A2
B2
C2
A3
B3
Pt
12.12
5.49
Cl
2.95
0.69
N1
0.07
2.09
L-1
84.86
91.73
H
P
Pt
12.59
5.49
Cl
3.13
0.70
N2
0.07
2.77
L-1
84.21
91.04
H
P
Pt
12.49
5.45
Cl
3.13
0.71
N3
0.07
3.41
L-1
84.31
90.43
H
P
Pt
5.91
0.37
Cl
1.88
0.01
N1
0.26
0.63
L-2
91.95
98.99
H
P
Pt
6.46
0.37
Cl
2.13
0.01
N2
0.29
1.15
L-2
91.12
98.47
H
P
Pt
6.26
0.36
Cl
2.04
0.01
N3
0.27
1.21
L-2
91.43
98.42
H
P
Pt
14.22
3.75
Cl
4.12
0.39
N1
0.06
70.97
L-3
81.60
25.16
H
P
Pt
15.34
3.72
Cl
4.67
0.41
N2
0.07
4.61
L-3
79.92
91.26
H
P
C3
Pt
15.37
3.70
Cl
4.76
0.41
N3
0.07
5.80
L-3
79.80
90.09
H
P
a
H and P represent NTO hole and particle orbitals, respectively.
their AIPE behavior should be induced by the rotation of the
N-donor ligands in the dilute solution, which can be effectively
restrained in the rigid matrix or aggregated state. However, if the
AIPE behavior of these Pt(II) complexes has been induced by the
restrained intramolecular rotation (RIR) of the N-donor ligands,
the complexes with the same C^N-type ligand should exhibit a
similar phosphorescence quantum yield (FP) in dilute solution,
since the N-donor ligands have not involved in the radiative
excited states (Fig. 4) and the pyridyl units in the N-donor ligands
of N1, N2 and N3 should rotate in a similar degree. In contrast,
upon increasing the size of the N-donor ligands, the FP can be
effectively enhanced, especially for the complexes with
2-naphthylpyridine-type ligands (Table 1). In addition, aggre-
gates of these PtII(C^N)(N-donor ligand)Cl-type complexes
formed in the THF–H2O mixture can exhibit a similar size (Fig.
S5 in the ESI†). Hence, based on these AIPE behaviors and
photophysical results, different AIPE mechanisms might dominate
the AIPE behavior of these PtII(C^N)(N-donor ligand)Cl-type
complexes. It had been expected that the square-planar (D4h
)
coordinating skeleton of Pt(II) complexes with mono-dental
ligands can undergo deformation to form a tetrahedron (Td)
skeleton in their excited states to induce nonradiative decay
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Fig. 5 PL spectra of these PtII(C^N)(N-donor ligand)Cl-type complexes in the THF–H2O mixture with different fw under the same measurement
conditions. (a) A1, (b) B1, (c) C1, (d) A2, (e) B2, (f) C2, (g) A3, (h) for B3 and (i) C3.
increasing the size of both C^N and N-donor ligands, the D2d deformation of their coordinating skeleton. As a result, low
deformation of the coordinating skeleton process is definitely fraction of water ( fw ca. 10%) has enhanced their phosphores-
subject to greater resistance. Accordingly, the AIPE phenomenon cence emission in dilute THF solution (Fig. 6).
should occur much easier. Encouragingly, the AIPE results in
Based on both the experimental and theoretical results
Fig. 5 and 6 have indicated the expectation. Upon increasing the obtained, the AIPE behavior of these PtII(C^N)(N-donor
size of the N-donor ligands, the value of f cw exhibits a decreasing ligand)Cl-type complexes should be dominated by the new
trend from A1 ( f wc ca. 70%) to B1 ( f cw ca. 50%) and then to C1 ( f wc mechanism of the restrained D2d deformation of their coordi-
ca. 40%) (Fig. 6), indicating the enhanced easiness of AIPE nating skeleton in the aggregated state, in which the deforma-
occurrence. In addition, very similar results have been obtained tion of the square-planar (D4h) coordinating skeleton of the
in A2 ( f cw ca. 60%), B2 ( f wc ca. 50%) and C2 ( f wc ca. 30%) (Fig. 6). Pt(II) center to tetrahedron (Td) can be effectively restrained to
As the terminal group of the N3 ligand, the carbazole unit facilitate radiative decay of the excited states (Fig. 9). In our
can expand the transverse size of the N3 ligand. It will disfavor previous investigation of the AIPE properties of PtII(C^N)(N-donor
the D2d deformation of the coordinating skeleton of these ligand)CRCAr-type complexes, the new restrained D2d deforma-
PtII(C^N)(N-donor ligand)Cl-type complexes. Hence, both C1 and tion AIE mechanism had been proposed based on theoretical
C2 can exhibit an even lower f cw value. For complexes A3, B3 and results.26 Obviously, these results should provide the first experi-
C3, they possess the C^N ligand with a large-size fluorene unit, mental evidence to support the new AIE mechanism for these
which should definitely exert much greater resistance to the D2d phosphorescent complexes.
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Fig. 8 Reaction path from the Franck–Condon points of S0 Min to the
tetrahedral S1 Min by the linear interpolation for the model complex. The
energies of the ground and singlet excited states were computed by CAM-
B3LYP and TDA-CAM-B3LYP, respectively.
Fig. 6 H2O volumetric fractions at which phosphorescence intensity
begins to be enhanced notably (fcw) for these PtII(C^N)(N-donor
ligand)Cl-type complexes in the THF–H2O mixture.
Fig. 9 Proposed AIPE mechanism of the restrained D2d deformation in
these PtII(C^N)(N-donor ligand)Cl-type complexes.
Fig. 7 CAM-B3LYP located S0 Min and TDA-CAM-B3LYP located S1 Min
for the model complex. The red triangle represents the Pt1–C6–N19 plane
(P1) while the blue triangle represents the Pt1–N20–Cl31 plane (P2).
atmosphere. Table 4 shows the outcomes of the CV measure-
ments. In the anodic scan, all of the Pt(II) complexes can give an
irreversible or quasi-reversible oxidation wave, ranging from ca.
0.55 to 0.76 V which can be ascribed to the oxidation of the Pt
Based on the AIPE mechanism of the restrained D2d defor-
mation in these PtII(C^N)(N-donor ligand)Cl-type complexes
aforementioned, the D2d deformation of these square-planar
Pt(II) complexes can be effectively inhibited in the PMMA film,
thereby preventing the non-radiative decay process and increas-
Table
4
Electrochemical properties of these PtII(C^N)(N-donor
ligand)Cl-type complexes
c
c
Compound
Epa [V]
Epc [V]
EHOMO [eV]
ELUMO [eV]
ing their rigidity and hence leading to
a lengthened
A1
B1
C1
A2
B2
C2
A3
B3
C3
0.61
0.55
0.72
0.63
0.74
0.76
0.58
0.62
0.58
ꢀ2.23a
ꢀ5.41
ꢀ5.35
ꢀ5.52
ꢀ5.43
ꢀ5.54
ꢀ5.56
ꢀ5.38
ꢀ5.42
ꢀ5.38
ꢀ2.57
ꢀ2.68
ꢀ2.72
ꢀ2.56
ꢀ2.64
ꢀ2.58
ꢀ2.53
ꢀ2.66
ꢀ2.68
lifetime.32,52 Based on their lifetimes in Table 1, all these
Pt(II) complexes can exhibit a much longer lifetime of Z2.5 ms
in doped PMMA film with a low doping level of 6 wt% compared
with the data in the range from ca. 560 to 990 ns obtained in
dilute deoxygenated CH2Cl2 solution.
ꢀ2.12a, ꢀ2.38b
ꢀ2.08b, ꢀ2.34b
ꢀ2.24a, ꢀ2.40a
ꢀ2.16a, ꢀ2.39a
ꢀ2.22a, ꢀ2.39b
ꢀ2.27b, ꢀ2.41b
ꢀ2.14a, ꢀ2.44a
ꢀ2.12b, ꢀ2.35b
Electrochemical properties
a
b
The electrochemical properties of these PtII(C^N)(N-donor
ligand)Cl-type complexes have been investigated in acetonitrile
by the cyclic voltammetry (CV) strategy calibrated against the
ferrocene/ferrocenium (Fc/Fc+) redox couple under a nitrogen
Reversible. The value was set as E1/2
.
Irreversible or quasi-reversible.
The value was derived from the anodic or cathodic peak potential.
c
HOMO levels are calculated according to first oxidation potential from
CV data. LUMO levels are derived from the onset potential of the first
reduction wave from the CV data.
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Fig. 10 Device structure of the solution-processed OLEDs with these PtII(C^N)(N-donor ligand)Cl-type AIPE emitters.
center in these complexes (Table 4 and Fig. S6 in the ESI†). ability of these new PtII(C^N)(N-donor ligand)Cl-type complexes
Obviously, these oxidation potentials (Epas) are shown in higher to benefit their EL performances.
position region compared with those of the PtII(C^N)(acac)-type
complexes.47,48 This result can be assigned to the lower electron
Electroluminescence properties
To evaluate the electroluminescence ability of these PtII(C^N)(N-
density on the Pt centers in these PtII(C^N)(N-donor ligand)Cl-
type complexes due to the coordinatoin with electron-
withdrawing Clꢀ and N-donor ligands. In the cathodic scan,
donor ligand)Cl-type complexes, solution-processed OLEDs have
reduction processes have been observed for these PtII(C^N)(N-
been constructed with the configuration of ITO/PEDOT:PSS
(55 nm)/Pt x wt%:CBP (30 nm)/TPBi (55 nm)/LiF (0.7 nm)/
donor ligand)Cl-type complexes.47.48 The potentials for these
Al (150 nm) (Fig. 10). In such a device configuration, the well-
known CBP has been selected as the host material for the AIPE
emitters and the hole injection layer (HIL) is fabricated by spin-
reduction processes are located in the range from ca. ꢀ2.1 to
ꢀ2.4 V, lower than those of the PtII(C^N)(acac)-type complexes.
This result should indicate the enhanced electron-injection
Fig. 12 Current density–voltage–luminance (J–V–L) characteristics for
Fig. 11 EL spectra for the optimized OLEDs at ca. 10 V.
the optimized OLEDs.
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coating the solution of PEDOT:PSS. The TPBi layer is employed to data of these solution-processed OLEDs are provided in Table 5.
fulfill both hole-blocking and electron-transporting functions of For the AIPE emitters of A3, B3 and C3 with the same C^N-type
the OLEDs, while LiF serves as an electron-injection layer.
ligand, the optimal EL performances have been achieved at their
With much higher FP of ca. 70%, A3, B3 and C3 have been doping level of 8 wt% in the emission layer of the OLEDs. The
selected as AIPE emitters for the OLEDs. In addition, with devices with A3 as the emitter can achieve the optimal EL capacity
functional N-donor ligand N3, C1 and C2 have been employed in device A3-2 with a maximum external quantum efficiency (Zext
)
to fabricate OLEDs as well. With a suitable voltage applied, of 23.3%, a maximum current efficiency (ZL) of 49.4 cd Aꢀ1 and a
OLEDs with these AIPE emitters can emit strong electropho- maximum power efficiency (ZP) of 36.2 lm Wꢀ1, while the highest
sphorescence from green to yellow color (Fig. 11).
EL data for device B3-2 with B3 as the emitter are 25.2% (Zext),
Current density–voltage–luminance (J–V–L) and EL efficiency– 51.2 cd Aꢀ1 (ZL) and 39.2 lm Wꢀ1 (ZP), respectively (Table 5,
luminance curves of these solution-processed OLEDs are depicted Fig. 13a and b). The optimized device C3-2 with C3 as the emitter
in Fig. 12 and 13 as well as in the ESI† (Fig. S7 and S8). The EL can achieve EL efficiencies of 23.9% (Zext), 53.9 cd Aꢀ1 (ZL) and
43.5 lm Wꢀ1 (ZP) (Table 5 and Fig. 13c), which might be
attributed to the N-donor ligand N3 with a functional carbazole
group showing hole injection/transporting ability. For compar-
ison, EL abilities of C1 and C2 with N-donor ligand N3 have been
characterized as well. Clearly, the optimized devices C1-2 (4.8%,
10.8 cd Aꢀ1 and 7.5 lm Wꢀ1) and C2-2 (18.7%, 46.7 cd Aꢀ1 and
35.1 lm Wꢀ1) (Fig. S8, ESI†) exhibit inferior EL efficiencies with
respect to device C3-2. It seems that the C^N-type ligands can
play a crucial role in determining the EL performances of these
PtII(C^N)(N-donor ligand)Cl-type AIPE emitters. However, it
should be noted that the low EL performances achieved by the
AIPE emitter C1 can also be ascribed to its red-emission feature.
Recently, Maji and co-workers reported AIE molecules of a
mono-cyclometalated Ir(III) complex for EL research. The high-
est EL efficiency of 0.35% (Zext) can be achieved.53 Cibian and
co-workers successively developed complex Zn2(4-bromo-N,N0-
diphenylbenzamidinate N-oxide)4 and the concerned OLEDs can
show EL performance with a maximum efficiency of 1.12 cd Aꢀ1
(ZL) and 0.30 lm Wꢀ1 (ZP).54 Li and co-workers designed and
synthesized two AIE-active complexes based on unsymmetrical
bipyridine–platinum(II)–alkynyl. The final OLEDs can give a max-
imum efficiency of 5.8% (Zext), 18.4 cd Aꢀ1 (ZL), and 8.0 lm Wꢀ1
(ZP).55 In comparison with the reported EL results, these
PtII(C^N)(N-donor ligand)Cl-type complexes can show great
potential in the EL field.
Conclusions
New PtII(C^N)(N-donor ligand)Cl-type complexes have been
designed and synthesized successfully. Bearing C^N-type and
N-donor ligands of different sizes, the new AIE mechanism
of the restrained D2d deformation of their square-planar coor-
dinating skeleton in the aggregated state has been shown
experimentally in these PtII(C^N)(N-donor ligand)Cl-type com-
plexes. In addition, these AIPE complex emitters can exhibit
impressive EL performances in solution-processed OLEDs with
Z
ext of 25.2%, ZL of 53.9 cd Aꢀ1 and ZP of 43.5 lm Wꢀ1, showing
great potential as new EL materials. Clearly, this research
not only represents a valuable attempt at developing AIPE
materials based on the inherent characteristics of the coordi-
nating skeleton of four-coordinated Pt(II) complexes, but also
provides a new outlet to obtained new AIPE emitters with high
performances.
Fig. 13 Relationship between EL efficiencies and luminance for the
optimized devices. (a) Device A3-2, (b) device B3-2 and (c) device C3-2.
2346 | J. Mater. Chem. C, 2021, 9, 2334ꢀ2349
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Table 5 EL performance of the electrophosphorescent OLEDs
Luminance Lmax [cd mꢀ2
]
Zext [%]
ZL [cd Aꢀ1
]
ZP [lm Wꢀ1
]
lmax [nm]
a
d
Device
Dopant
Vturn-on [V]
A3-1
A3 (6.0 wt%)
4.5
6112 (16.5)
13.4 (4.5)
8.8b
28.5 (4.5)
18.6
12.2
19.9 (4.5)
6.7
3.5
544 (0.49, 0.47)
5.7c
A3-2
A3-3
B3-1
B3-2
B3-3
C1-1
C1-2
C1-3
C2-1
C2-2
C2-3
C3-1
C3-2
A3 (8.0 wt%)
A3 (10.0 wt%)
B3 (6.0 wt%)
B3 (8.0 wt%)
B3 (10.0 wt%)
C1 (6.0 wt%)
C1 (8.0 wt%)
C1 (10.0 wt%)
C2 (6.0 wt%)
C2 (8.0 wt%)
C2 (10.0 wt%)
C3 (6.0 wt%)
C3 (8.0 wt%)
C3 (10.0 wt%)
4.0
3.6
4.0
3.6
3.7
4.1
4.5
4.2
3.7
3.8
3.6
3.0
3.3
3.0
7971 (16.0)
8943 (16.5)
7552 (19.0)
8561 (20.0)
8798 (19.5)
1832 (19.0)
2078 (19.5)
1768 (19.5)
10066 (20.5)
10184 (20.0)
12929 (20.0)
9345 (17.5)
10905 (18.0)
10056 (18.5)
23.3 (5.0)
16.6
9.9
49.4 (5.0)
34.6
21.1
36.2 (4.5)
13.3
6.7
544 (0.49, 0.47)
544 (0.49, 0.48)
544 (0.47, 0.51)
544 (0.47, 0.50)
544 (0.47, 0.51)
608 (0.61, 0.35)
608 (0.61, 0.36)
608 (0.60, 0.36)
520 (0.45, 0.47)
520 (0.45, 0.46)
520 (0.46, 0.47)
544 (0.48, 0.46)
544 (0.48, 0.46)
544 (0.49, 0.47)
22.2 (4.5)
15.4
10.1
47.3 (5.0)
32.5
33.4
36.7 (3.6)
13.2
14.0
12.8 (5.5)
12.4
10.6
26.0 (4.1)
24.6
21.4
19.7 (4.0)
10.5
6.9
25.2 (5.0)
22.9
18.0
51.2 (5.0)
46.4
36.5
39.2 (3.6)
21.1
11.6
22.7 (5.5)
22.0
17.6
46.1 (5.5)
44.7
35.8
34.1 (3.7)
21.9
11.6
3.9 (4.5)
1.2
0.5
8.7 (4.5)
2.7
1.2
6.5 (4.4)
0.7
0.2
4.8 (4.4)
1.7
0.8
10.8 (4.3)
3.9
1.9
7.5 (4.3)
1.1
0.4
3.0 (4.4)
1.4
0.6
6.8 (4.5)
3.1
1.3
4.7 (4.5)
0.8
0.3
13.1 (5.0)
9.0
6.5
32.6 (5.0)
22.3
16.2
24.7 (4.0)
7.5
4.3
18.7 (5.0)
11.4
7.8
46.7 (5.0)
29.6
19.4
35.1 (4.0)
10.3
5.6
16.9 (4.5)
8.0
5.1
42.2 (4.5)
19.4
12.8
32.6 (4.0)
6.1
3.5
22.8 (5.0)
20.6
14.0
46.8 (5.0)
46.5
31.3
33.4 (4.0)
22.6
12.7
23.9 (5.0)
22.6
15.9
53.9 (5.0)
51.0
35.8
43.5 (3.3)
28.5
14.2
C3-3
19.7 (4.0)
19.1
44.5 (4.0)
43.2
42.7 (3.0)
26.1
14.1
31.3
12.6
a
b
Maximum values of the devices. Values in the parentheses are the voltages at which they were obtained. Values were collected at ca. 100 cd mꢀ2
.
c
d
Values collected at ca. 1000 cd mꢀ2
.
Values were collected at 10 V and CIE coordinates (x, y) are shown in parentheses.
Conflicts of interest
Acknowledgements
The authors declare that they have no known competing This work was supported by the National Natural Science
financial interests or personal relationships that could have Foundation of China (21875179, 51803163, 21602170, and
appeared to influence the work reported in this paper.
21572176), the Natural Science Foundation of Shaanxi Province
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19 K. Li, W. Qin, D. Ding, N. Tomczak, J. L. Geng, R. R. Liu,
J. Z. Liu, X. H. Zhang, H. W. Liu, B. Liu and B. Z. Tang, Sci.
Rep., 2013, 3, 1150.
20 X. Y. Shen, Y. J. Wang, E. G. Zhao, W. Z. Yuan, Y. Liu, P. Lu,
A. J. Qin, Y. G. Ma, J. Z. Sun and B. Z. Tang, J. Phys. Chem. C,
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21 L. Yang, Y. Zhang, X. Y. Zhang, N. Q. Li, Y. W. Quan and
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22 Y. Y. Zhan, Z. W. Yang, J. H. Tan, Z. P. Qiu, Y. Y. Mao, J. He,
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24 W. Z. Duan, Q. S. Liu, Y. M. Huo, J. C. Cui, S. W. Gong and
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(2019JZ-29 and 2019JQ-188), the China Postdoctoral Science
Foundation (2020M673369 and 2016M600778), and the Key
Laboratory Construction Program of Xi’an Municipal Bureau
of Science and Technology (201805056ZD7CG40). The character-
ization assistance from the Instrument Analysis Center of Xi’an
Jiaotong University and the financial supporting from State Key
Laboratory for Mechanical Behavior of Materials are also
acknowledged.
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