Cyclometalated Platinum Complexes with Aggregation-Induced Phosphorescence Emission Behavior and Highly Efficient Electroluminescent Ability

Article abstract of DOI:10.1021/acs.chemmater.7b04708

Aggregation-induced emission (AIE) materials can exhibit intense luminescence in the aggregated or solid state, which are highly desirable for OLED application. However, only limited research results on developing AIE-active Pt^II complexes in e

Full text of DOI:10.1021/acs.chemmater.7b04708

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Article
Cyclometalated Platinum Complexes with Aggregation-
Induced Phosphorescence Emission Behavior
and Highly Efficient Electroluminescent Ability
Jiang Zhao, Zhao Feng, Daokun Zhong, Xiaolong Yang, Yong Wu, Guijiang Zhou, and Zhaoxin Wu
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b04708 • Publication Date (Web): 09 Jan 2018
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Cyclometalated Platinum Complexes with AggregationꢀInduced
Phosphorescence Emission Behavior and Highly Efficient
Electroluminescent Ability
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Jiang Zhao,† Zhao Feng,† Daokun Zhong,† Xiaolong Yang,† Yong Wu,† Guijiang Zhou,*,†, ¶ Zhaoxin
Wu,*, ‡
†
MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter,
Department of Chemistry, School of Science, Xi’an Jiaotong University, Xi’an 710049, P. R. China.
¶
State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an
710049, P. R. China.
‡ Key Laboratory of Photonics Technology for Information, School of Electronic and Information
Engineering, Xi’an Jiaotong University, Xi’an, 710049, P. R. China
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ABSTRACT
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Aggregationꢀinduced emission (AIE) materials can exhibit intense luminescence in aggregated or
solid state, which are highly desirable for OLED application. However, only limited research results
on developing AIEꢀactive PtII complexes in electroluminescence (EL) application have been reported
so far. Herein, a series of AIEꢀactive PtII(C^N)(Nꢀdonor ligand)Cl complexes have been developed.
Their chemical structures have been determined by NMR, MS and Xꢀray crystallography
characterization. Theoretical results including the frontier molecular orbitals, the simulated UVꢀvis
spectra and natural transition orbitals (NTO) have been employed to insightfully interpret their
photophysical properties. It has also been found that much higher degree of molecular aggregation in
these PtII complexes should be required to induce phosphorescent AIE than that for the fluorescent
AIE behavior. These AIEꢀactive PtII complexes can exhibit very strong emission in poly(methyl
methacrylate) (PMMA) films with the phosphorescence quantum yield (Фp) of ca. 72.0%, while that
in dilute solution is just about 4.7%. Accordingly, based on these AIEꢀactive PtII complexes, the
optimized OLEDs fabricated by the simple solutionꢀprocessed strategy can achieve high EL
efficiencies with a maximal external efficiency (ηext) of 28.4%, a maximal current efficiency (ηL) of
75.9 cd Aꢀ1 and a maximal power efficiency (ηP) of 62.7 lm Wꢀ1. Considering the rarity for EL
investigations of the AIEꢀactive PtII complexes, the concerned results realized by these
PtII(C^N)(Nꢀdonor ligand)Cl complexes should provide valuable clues for exploring AIEꢀactive PtII
phosphorescent complexes with high EL performance.
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INTRODUCTION
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Organic lightꢀemitting diodes (OLEDs) have obtained sustainable research interest both from
academic and industrial researchers considering their promising application in flat panel displays and
solidꢀstate lighting sources.1ꢀ4 Organometallic platinum complexes are one of the most promising
candidates as phosphor materials within the OLEDs’ emission layer (EML) for their attractive
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features such as high phosphorescent quantum yield (Фp), relatively short triplet state lifetime τp)
(
and tunable emission color. 5ꢀ9 Platinum complexes can harvest both the singlet and triplet excitons
for light emission since the efficient intersystem crossing process induced by the strong spinꢀorbit
coupling effect of the platinum atom. Hence, the maximal internal quantum efficiency (IQE) of
phosphorescent emitters can reach 100%, leading to substantial improvement of their
electroluminescent (EL) efficiency.10, 11 According to the d8 system of the PtII atom, it can be
coordinated with various organic ligands to achieve the preferred coordination number 4 with a
spuare planar geometry. Recently, different organic ligands including 1,3ꢀbis(2ꢀpyridyl)benzene
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(N^C^N),12,
6ꢀphenylꢀ2,2'ꢀbipyridine (N^N^C),14,
2,6ꢀdiphenylpridine (C^N^C)16and
2ꢀphenylpyridine(C^N, ppy)7, 17, 18 as well as their derivatives have been adopted to synthesize
platinum complexes. Among these platinum complexes, ppyꢀtype platinum complexes in presence of
one βꢀdiketonato ancillary ligand (PtII(C^N)acac) have been extensively developed for OLED
applications due to their tunable photoelectric properties by using different peripheral substituent
groups.7, 19 Similar to the fluorescent materials, the emission of some ppyꢀtype cyclometalated PtII
complexes can be quenched in high concentration solution or aggregation state, known as
aggregationꢀcaused quenching effect (ACQ), which immensely hinders their application in OLED
field.20ꢀ22 Hence, efforts have to be devoted to bypass this obstacle for improving EL performance of
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PtII complexes.
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In 2001, Tang et al discovered the very different lightꢀemitting behaviors of some silole
derivatives in solution and solid states. Weak or nonꢀemissive in dilute solution but highly
luminescent when their molecules aggregate in concentrated solutions or solid state, which has been
defined as aggregationꢀinduced emission (AIE).23ꢀ25 This discovery offers exciting inspirations to
design and synthesize PtII complexes with high EL performance, changing the emission behavior
from ACQ to AIE.26ꢀ28 To date, some PtII complexes with AIE character have been reported.21, 29ꢀ32
Huang’s group has established a series of ppyꢀtype AIEꢀactive PtII complexes for bioimaging
applications.33 Liu et al reported a series of PtII diimine acetylide complexes with good AIE nature
and optical power limiting property.34 Yam’s group developed two unsymmetric
bipyridine–PtII–alkynyl complexes with AIE nature, which have been employed in OLED application
showing maximal ηL of 18.4 cd Aꢀ1 and ηext of 5.8%.35 After replacing the βꢀdiketonato ancillary
ligand of PtII(C^N)acac with one chloride and Nꢀdonor ligand, AIEꢀactive heteroleptic
PtII(C^N)(Nꢀdonor ligand)Cl complexes can be obtained26, 27, which have been employed to
investigate their structural features,36 biological activity26, 37, 38 and catalytic performance,39 etc. To
the best of our knowledge, the EL studies of the PtII complexes with AIE character are still of
shortage. Therefore, the exploring of AIEꢀactive PtII complexes for EL application can be
significantly meaningful. Similar to the PtII(C^N)acac complexes, the photoelectric properties of the
PtII(C^N)(Nꢀdonor ligand)Cl complexes can be readily tunable by manipulating the C^N and
Nꢀdonor ligands, showing a good potential in EL field. However, apart from two complexes with
good EL performance developed by our group40 and two complexes with a maximal ηL of 0.5 cd Aꢀ1
41, studies on the EL properties of the PtII(C^N)(Nꢀdonor ligand)Cl complexes are very sparse. Hence,
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it is very worthwhile to study the EL properties of this class of AIEꢀactive PtII(C^N)(Nꢀdonor
ligand)Cl complexes. Given the significant influence of charge balance between electrons and holes
from opposite electrodes on OLEDs’ performance, it is highly desirable that PtII complexes can
exhibit good charge carrier injection/transporting features.42ꢀ44 Considerable research efforts have
been devoted by Zhou et al to describe that IrIII and PtII complexes can achieve efficient EL
performance by chelating with functionalized ppyꢀtype ligands bearing –NPh2, –POPh2 and –SO2Ph
moieties showing more balanced charge carrier injection/transporting abilities.7, 42, 45ꢀ47 Hence, the
introduction of these functional moieties into the AIEꢀactive PtII complexes should be advisable for
their EL improvement.
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Herein, four AIEꢀactive heteroleptic PtII(C^N)(Nꢀdonor ligand)Cl type complexes are presented,
their preparation, thermal stability, electrochemistry and photophysical properties have been
investigated in detail and were corroborated by theoretical calculations. The OLEDs as well as single
carrier devices based on the concerned PtII complexes are fabricated by solution process and their EL
properties are studied. The AIE property and balanced charge carrier injection/transporting ability of
the concerned PtII complexes should be significantly beneficial for the EL enhancement. This
research should offer critical clues for developing novel phosphorescent PtII complexes with AIE
nature and promoting their application in EL field.
EXPERIMENTAL SECTION
General Information. Under nitrogen atmosphere, all reactions were carried out with the use of
standard Schlenk techniques. The solvents were purified by standard methods under dry nitrogen
before use. All commercially available reagents were used as received unless otherwise stated. The
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reactions were monitored by thinꢀlayer chromatography (TLC) with Merck preꢀcoated aluminum
plates. Flash column chromatography and preparative TLC were carried out using silica gel. The
detailed information for the physical characterization, Xꢀray crystallography, theoretical calculation
and OLED fabrication and measurements are provided in the Supplementary Information (SI).
Synthesis. Preparation details of the cyclometalating ppyꢀtype ligands LꢀN, LꢀP and LꢀS were
according to the reported literature.7 The synthesis details for some related compounds are presented
in the Supplementary Information (SI).
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General Procedures for the Synthesis of Nꢀdonor Ligands: Under a N2 atmosphere,
4ꢀbromopyridine hydrochloride (1.0 equiv), the corresponding borate compound or boronic acid (1.2
o
equiv, Scheme S1), and Pd(PPh3)4 (5 molꢀ%) were heated to 110 C in a mixture of 10 mL 2M
Na2CO3 and 25 mL degassed THF for 24 h. After cooling to room temperature, the mixture was
extracted with dichloromethane, the organic phase was dried over Na2SO4 and the solvent was
removed under reduced pressure. The residue was obtained as a crude product, which was
chromatographed on a silica column to produce a pure product.
Nꢀ4ꢀN: (Yield: 62%); 1H NMR (400 MHz, CDCl3, δ): 8.62 (d, J = 6.40 Hz, 2 H), 7.52 (d, J = 8.80
Hz, 4 H), 7.30 (t, J = 8.40 Hz, 4 H), 7.15–7.13 (m, 6 H), 7.8 (t, J = 7.20 Hz, 2 H); 13C NMR (100
MHz, CDCl3, δ): 150.16, 148.92, 147.63, 147.22, 130.91, 129.41, 127.61, 124.96, 123.58, 122.89,
120.86; FABꢀMS (m/z): 322 [M]+; Elemental analysis calcd (%) for C23H18N2: C 85.68, H 5.63, N
8.69; found: C 85.57, H 5.55, N 8.58.
1
Nꢀ4ꢀS: (Yield: 68%); H NMR (400 MHz, CDCl3, δ): 8.71 (d, J = 1.60 Hz, 2 H), 8.08ꢀ8.05 (m, 2
H), 8.01ꢀ7.98 (m, 2 H), 7.76–7.74 (m, 2 H), 7.62ꢀ7.58 (m, 1 H), 7.57ꢀ7.52 (m, 2 H), 7.48ꢀ7.46 (m, 2
H); 13C NMR (100 MHz, CDCl3, δ): 150.55, 146,37, 143.06, 142.00, 141.29, 133.41, 129.40, 128.46,
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127.97, 127.72, 121.72; FABꢀMS (m/z): 295 [M]+; Elemental analysis calcd (%) for C17H13NO2S: C
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Nꢀ4ꢀP: (Yield: 52%); H NMR (400 MHz, CDCl3, δ): 8.70 (d, J = 5.60 Hz, 2 H), 7.82ꢀ7.77 (m, 2
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H), 7.73ꢀ7.68(m, 6 H), 7.57 (t, J = 7.60 Hz, 2 H), 7.51ꢀ7.47 (m, 6 H); 31P NMR (162 MHz, CDCl3, δ)
28.65; 13C NMR (100 MHz, CDCl3, δ): 150.42, 147,06, 141.58, 133.87, 132.89, 132.65, 132.12,
131.96, 128.64, 127.13, 121.65; FABꢀMS (m/z): 355 [M]+; Elemental analysis calcd (%) for
C23H18NOP: C 77.74, H 5.11, N 3.94; found: C 77.62, H 4.95, N 3.86.
General Procedures for the Synthesis of PtII Complexes: Under a N2 atmosphere, the organic
o
cyclometalating ligands (LꢀN, LꢀP and LꢀS, 1.1 equiv) and K2PtCl4 (1 equiv) were heated to 90 C
and stirred for 18 h in 15 mL mixture of H2O and 2ꢀethoxyethanol (v/v = 1 : 3). Then the reaction
mixture was cooled to room temperature, and water was added. The precipitate of ꢀꢀchloroꢀbridged
dimer was collected and dried under vacuum. Without further purification, the dimer and Nꢀdonor
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ligands (Nꢀ4ꢀN, Nꢀ4ꢀP and Nꢀ4ꢀS) were added in 10 ml chloroform and heated to 50 C for 12 h.
After cooling to room temperature, the mixture was extracted with dichloromethane. The organic
phase was dried over anhydrous Na2SO4 and the solvent was removed under reduced pressure. The
residual was purified by flash column chromatography using appropriate eluent.
NꢀPtꢀP: yellow solid (Yield: 79%). 1H NMR (400 MHz, CDCl3, δ): 9.59 (d, J = 5.60 Hz, 1H), 8.83
(d, J = 6.40 Hz, 2H), 7.86 (d, J = 8.40 Hz, 1H), 7.84 (d, J = 8.00 Hz, 1H), 7.77–7.70 (m, 5H),
7.64–7.59 (m, 4H), 7.54–7.50 (m, 5H), 7.33–7.27 (m, 3H), 7.17 (t, J = 7.60 Hz, 4H), 7.04 (d, J =
7.60 Hz, 5H), 6.92 (t, J = 7.20 Hz, 2H), 6.80 (dd, J = 2.40, 8.4 Hz, 1H), 5.93 (d, J = 2.00 Hz, 1H);
13C NMR (100 MHz, CDCl3, δ): 166.81, 153.91, 151.11, 149.38, 148.37, 147.17, 142.26, 139.63,
138.61, 137.76, 135.33, 134.31, 133.06, 132.96, 132.09, 131.99, 131.40, 129.11, 128.77, 128.65,
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127.12, 127.00, 125.44, 124.55, 123.85, 123.51, 120.58, 117.56, 116.80; 31P NMR (162 MHz, CDCl3,
δ): 28.35; FABꢀMS (m/z): 930.1572 [M+Na]+, 871.2030 [M–Cl]+; Elemental analysis calcd (%) for
C46H35ClN3OPPt: C 60.89, H 3.89, N 4.63; found: C 60.81, H 3.79, N 4.56.
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NꢀPtꢀS: yellow solid (Yield: 76%). 1H NMR (400 MHz, CDCl3, δ): 9.58 (d, J = 6.00 Hz, 1H), 8.85
(d, J = 6.80 Hz, 2 H), 8.10 (d, J = 8.4 Hz, 2 H), 8.01 (d, J = 7.60 Hz, 2 H), 7.75 (t, J = 7.2 Hz, 1 H),
7.65–7.61 (m, 3 H), 7.56 (t, J = 7.60 Hz, 2 H), 7.50 (d, J = 8.00 Hz, 1 H), 7.32 (d, J = 2.20, 1 H),
7.24 (d, J = 6.80 Hz, 2 H), 7.17 (t, J = 4.00 Hz, 4 H), 7.05 (t, J = 6.80 Hz, 5 H), 6.91 (t, J = 7.20 Hz,
2 H), 6.80 (dd, J = 2.00, 4.20 Hz, 1 H), 5.91 (d, J = 2.00 Hz, 1 H); 13C NMR (100 MHz, CDCl3, δ):
166.75, 154.04, 151.09, 149.33, 147.57, 147.15, 143.03, 142.17, 140.99, 140.96, 138.68, 137.75,
133.64, 129.51, 129.10, 128.61, 127.96, 127.80, 125.44, 124.58, 123.75, 123.59, 123.33, 120.62,
117.59, 116.85; FABꢀMS (m/z): 873.1991 [M+Na]+, 811.1591 [M–Cl]+; Elemental analysis calcd (%)
for C40H30ClN3O2PtS: C 56.70, H 3.57, N 4.96; found: C 56.59, H 3.61, N 4.85.
PꢀPtꢀN: yellowish green solid (Yield: 72%). 1H NMR (400 MHz, CDCl3, δ): 9.75 (d, J = 5.20 Hz,
1 H), 8.65 (d, J = 6.80 Hz, 2 H), 7.88 (t, J = 4.00 Hz, 1 H), 7.73 (d, J = 8.00 Hz, 1 H), 7.58–7.53 (m,
6 H), 7.50 (d, J = 8.80 Hz, 2 H), 7.38–7.28 (m, 11 H), 7.24–7.12 (m, 10 H), 6.57 (d, J = 12.40 Hz, 1
H); 13C NMR (100 MHz, CDCl3, δ): 166.06, 153.05, 151.73, 150.20, 149.03, 148.25, 146.70, 139.05,
138.61, 134.26, 134.16, 133.15, 133.00, 132.04, 131.94, 129.62, 128.30, 127.70, 127.54, 127.46,
127.36, 125.60, 124.34, 123.28, 123.15, 123.00, 122.14, 121.67, 119.06; 31P NMR (162 MHz, CDCl3,
δ): 29.43; FABꢀMS (m/z): 930.1569 [M+Na]+, 871.2015 [M–Cl]+; Elemental analysis calcd (%) for
C46H35ClN3OPPt: C 60.89, H 3.89, N 4.63; found: C 60.78, H 3.82, N 4.59.
SꢀPtꢀN: yellowish green solid (Yield: 78%). 1H NMR (400 MHz, CDCl3, δ): 9.75 (d, J = 6.00 Hz,
1 H), 8.79 (d, J = 6.80 Hz, 2 H), 7.91–7.86 (m, 1 H), 7.77 (d, J = 8.00 Hz, 2 H), 7.75 (t, J = 7.20 Hz,
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1 H), 7.71–7.66 (m, 2 H), 7.62 (d, J = 4.80 Hz, 2 H), 7.58 (d, J = 4.80 Hz, 2 H), 7.56–7.49 (m, 2 H),
7.41 (t, J = 8.00 Hz, 2 H), 7.34 (t, J = 7.60 Hz, 4 H), 7.27–7.12 (m, 8 H), 6.97 (d, J = 2.80 Hz, 1 H);
13C NMR (100 MHz, CDCl3, δ): 166.16, 153.32, 151.83, 150.31, 149.74, 149.42, 146.73, 143.07,
141.61, 141.43, 139.16, 133.02, 129.59, 129.07, 127.87, 127.61, 127.59, 125.56, 124.28, 123.61,
123.44, 122.53, 122.43, 121.91, 119.38; FABꢀMS (m/z): 871.1681 [M+Na]+, 811.1598 [M–Cl]+;
Elemental analysis calcd (%) for C40H30ClN3O2PtS: C 56.70, H 3.57, N 4.96; found: C 56.61, H 3.50,
N 4.87.
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RESULTS AND DISCUSSION
Synthesis and Structural Characterization. The synthetic procedures for the Nꢀdonor ligands
the and heteroleptic PtII(C^N)(Nꢀdonor ligand)Cl complexes are shown in Scheme 1 and 2. The
cyclometalating ppyꢀtype ligands LꢀN, LꢀP and LꢀS are synthesized by the reported method.7 As the
key compounds, the Nꢀdonor ligands are prepared from the Suzuki crossꢀcoupling catalyzed by
Pd(PPh3)4 between 4ꢀbromopyridine hydrochloride and the corresponding borate compounds or
boronic acid (BꢀP, BꢀS and BꢀN). The PtII complexes are obtained in high yields by a twoꢀstep
protocol. The relevant cyclometalating ppyꢀtype ligands reacted with K2PtCl4 to afford the
ꢀꢀchloroꢀbridged dimer complexes, which are then converted to the final complexes by reaction with
the Nꢀdonor ligands under mild condition. These airꢀstable compounds are isolated in high purity as
solids by column chromatography on silica gel with the proper eluent.
Scheme 1. Synthesis Protocol for the Functional Nꢀdonor Ligands
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N
P
B(OH)2
N
P
N
N
N
B–N
N–4–N
N–4–P
N–4–S
P
Br
•
N
HCl
O
B
O
O
O
O
O
O
B B
O
O
Pd(PPh3)4
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B–P
B–S
Pd(dppf)Cl2
O
B
O
O
S
O
S
Br
O
S
O
O
O
Scheme 2. Synthesis Protocol for the PtII(C^N)(Nꢀdonor ligand)Cl Complexes
O
P
N
N
Pt
Pt
N–Pt–P
N
N–4–P
N
N
Cl
Cl
Cl
N
N
CH3Cl, 50 oC
Pt
Pt
Cl
N
N
N–Pt–S
N
P
N
P
L–N
N–4–S
N
S
O
P
O
O
O
O
N
K2PtCl4
N
N
2ꢀethoxyethanol : H2O = 3 :1
Cl
Cl
P
O
Pt
Pt
Pt
Pt
L–P
N
N
N
Pt
Pt
P–Pt–N
N–4–N
N
N
N
Cl
CH3Cl, 50 oC
Cl
Cl
N
Cl
N
O
S
S–Pt–N
N
O
O
O
L–S
S
S
O
S
O
O
O
The 1H, 13C and 31P NMR spectra of all compounds are thoroughly recorded. All the NMR spectra
are provided in the SI (Fig S1, S2 and S3). In the H NMR spectra for all the PtII complexes, the
1
resonance signals with δ at ca. 9.58 (for NꢀPtꢀP and NꢀPtꢀS) and 9.75 ppm (for PꢀPtꢀN and SꢀPtꢀN)
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can be assigned the protons adjacent to N atom on the pyridine ring of the cyclometalating ppyꢀtype
ligands. The resonance peaks with the chemical shift at ca. 8.65 to 8.79 ppm can be ascribed to
protons in the proximity of the N atom on the pyridine ring of the Nꢀdonor ligands. The ratio of these
two kinds of protons is 1 : 2, which is in good agreement with the structural features of these PtII
complexes. The other resonance signals in the low field region come from the aromatic groups of
cyclometalating ppyꢀtype ligands and Nꢀdonor ligands. The resonance peak with δ at ca. 28.35 and
29.43 ppm in the 31P NMR spectra of NꢀPtꢀP and PꢀPtꢀN can be assigned to the –POPh2 groups,
respectively. In addition, the MS spectra have been also collected to characterize the PtII complexes,
which gave the respective molecular ion peaks clearly in Figure S4 (SI). At the meantime, single
crystals of SꢀPtꢀN were obtained by slow diffusion of its solution in CHCl3/THF (v/v = 1 : 1) into
hexane, and single Xꢀray crystallography was employed to investigated the structure of SꢀPtꢀN. Its
ORTEP drawing is illustrated in Figure 1, and the detailed structural data are summarized in Table S1
and S2 (SI). As shown in Figure 1, the PtII atom with a square planar coordination geometry is
coordinated by a bidentate ppyꢀtype ligand (LꢀS, C^N) and two monodentate ligands (Nꢀdonor
ligand, Nꢀ4ꢀN and chloride). The coordination around the Pt center is distorted tetragonal geometry
with the transꢀN, N and transꢀC, Cl disposition. The bonds chelating around the Pt atom exhibit
typical bond lengths comparing with some similar complexes.48, 49 Generally, it can be concluded
that the heteroleptic PtII(C^N)(Nꢀdonor ligand)Cl have been successfully prepared according to all of
the spectral results and characterizations showing good consistency.
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Figure 1. ORTEP drawing of SꢀPtꢀN with thermal ellipsoids drawn at the 30% probability level. H
atoms are omitted for clarity.
Thermal Properties and Photophysical Characterizations. The thermal properties of the
heteroleptic PtII(C^N)(Nꢀdonor ligand)Cl complexes are investigated by thermogravimetric analysis
(TGA) and different scanning calorimetry (DSC) under a nitrogen flow. The TGA results show their
thermal stability with the decomposition temperature (Td) in the range from ca. 303 to 326 oC (Table
1, Figure S5), which are similar to the PtII(C^N)acac type complexes. 7 The DSC traces have revealed
o
their glassꢀtransition temperature (Tg) in the range from ca. 109 to 128 C (Table 1, Figure S6). The
good thermal properties will guarantee their stability in fabrication and working process of the
concerned EL devices.
Table 1 Photophysical and Thermal Data for the Heteroleptic PtII(C^N)(Nꢀdonor ligand)Cl
Complexes.
Emission
λem (nm)
Φ
293 K
Absorption a
(293K)
λabs (nm)
τ d
Td/Tg
Compd
c
solution a
film
solution c
ΦF/ΦP
film ΦP
PMMA/neat
solution
298 K/77 K
film
(oC)
293 K/77 K
PMMA b/neat
PMMA/neat
255 (4.61), 274 (4.67),
NꢀPtꢀP 338 (4.37), 377 (4.12),
426 (4.21)
253 (4.41), 274 (4.47),
NꢀPtꢀS 341 (4.17), 380 (3.95),
423 (4.02)
4.2 ns (447 nm)
0.3 ꢁs (543 nm)
/2.2 ꢁs (537 nm)
3.8 ns (450 nm)
0.2 ꢁs (543 nm)
/2.3 ꢁs (537 nm)
4.6 ns (449 nm)
0.4 ꢁs (502 nm)
/1.9 (503 nm)
1.1 ꢁs (533 nm)
/0.9 ꢁs (550 nm)
447, 543/537
533/550
5.0%/4.7% 60.7%/45.1%
6.5%/3.2% 68.0%/36.3%
5.6%/4.1% 57.1%/37.6%
325/122
315/128
303/109
0.9 ꢁs (535nm)
/0.7 ꢁs (557 nm)
450, 543/537
535/557, 580
252 (4.54), 293 (4.42),
PꢀPtꢀN
449, 502, 536
/503, 538
505, 541
/508, 547
1.2 ꢁs (505 nm)
/0.8 ꢁs (508 nm)
328 (4.02), 390(4.53)
4.7 ns (447 nm)
0.5 ꢁs (503 nm)
/2.1ꢁs (507 nm)
254 (4.57), 293 (4.48),
SꢀPtꢀN
447, 503, 538
/507, 539
508, 544
/515, 550
1.3 ꢁs (508 nm)
/0.7 ꢁs (515 nm)
7.8%/4.2% 72.0%/34.1%
326/117
327 (4.09), 393 (4.58)
Measured in CH2Cl2 at a concentration of 10ꢀ5 M, log ε values are shown in parentheses. Measured in PMMA film with 6 wtꢀ% doping level.
a
b
c
Measured in degassed CH2Cl2 relative to facꢀ[Ir(ppy)3] (ΦP= 97%, λex=360 nm),50 the absolute ΦPs are obtained from Hamamstu Photonics K.K., E7536
model measurement system using their PMMA films with 6 wtꢀ% doping level and neat films. d Measured in degassed CH2Cl2 solutions, films with a 369
nm picosecond LED as the excitation source.
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UVꢀVis absorption spectra for these PtII complexes are investigated in CH2Cl2 solution with a
concentration of 10ꢀ5 mol Lꢀ1 at 293 K and the corresponding data are compiled in Table 1. As
depicted in Figure 2, there are two main absorption bands in their UVꢀVis spectra, showing similarity
with respect to the reported results.51 The first absorption bands can be safely assigned to the
spinꢀallowed ligandꢀcentered (LC) πꢀπ* transition due to their large extinction coefficients (log
ε >4).18, 52 These PtII complexes present similar absorption behavior in the highꢀenergy region before
ca. 350 nm. The second absorption bands in lowꢀenergy region ranging from ca. 350 to 450 nm for
these PtII complexes exhibit distinctive differences. The maximum absorption peaks for PꢀPtꢀN and
SꢀPtꢀN are located at ca. 390 nm, which are obviously more intense than that of NꢀPtꢀP and NꢀPtꢀS
centered at ca. 425 nm. To directly interpret the absorption behavior of these PtII complexes, the
simulated UVꢀVis spectra (Figure 3) with discrete vertical vibronic transitions and the
timeꢀdependent density functional theory (TDꢀDFT) calculations for all of these PtII complexes have
been carried out. Figure 4 displays the distribution patterns of the highest occupied molecular
orbitals (HOMOs), the lowest unoccupied molecular orbitals (LUMOs), HOMOꢀ1 and LUMO+1.
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Table 2 summarizes the detailed TDꢀDFT results for these PtII complexes.
NꢀPtꢀP
NꢀPtꢀS
1.0
PꢀPtꢀN
SꢀPtꢀN
0.8
0.6
0.4
0.2
0.0
250
300
350
400
450
500
550
Wavelength (nm)
Figure 2. UVꢀVis absorption spectra for these PtII complexes in CH2Cl2 at 293 K.
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As displayed in Figure 3, the simulated UVꢀVis spectra from theoretical calculations show
excellent agreement with the experimental ones for all the PtII complexes in terms of both absorption
wavelength and lineꢀshape, indicating the validity of the calculation method. For NꢀPtꢀP and NꢀPtꢀS,
the simulated UVꢀVis results have shown that the low energy absorption bands located at ca. 425 nm
with strong vertical vibronic transitions are from H→L+1 transition. As shown in Figure 4 and Table
2, the HOMOs for NꢀPtꢀP and NꢀPtꢀS are almost entirely located on the LꢀN ligand (96.77% and
97.07%, respectively, Table 2), while the LUMOs are mainly distributed on the Nꢀdonor ligands
(94.36% for Nꢀ4ꢀP and 96.32% for Nꢀ4ꢀS, Table 2). The LUMO+1s are mainly contributed from the
ppy moiety on the LꢀN ligands. Therefore, the absorption bands with low energy located at ca. 425
nm for NꢀPtꢀP and NꢀPtꢀS can be attributed to the intraligand charge transfer (ILCT) features from
–NPh2 moiety to ppy moiety within the LꢀN ligand. As for PꢀPtꢀN and SꢀPtꢀN, the obviously intense
absorption bands centered at ca. 390 nm can be assigned to H→L and H→L+1 transitions with large
percentage of vertical vibronic transitions. The Nꢀdonor ligand Nꢀ4ꢀN gives a substantial
contribution to the HOMOs for complexes PꢀPtꢀN and SꢀPtꢀN (98.39% and 98.69%, Table 2). The
ppy units on the ligand LꢀP and LꢀS hold the primary distribution of their LUMOs and the
4ꢀphenylpyridine unit on the ligand Nꢀ4ꢀN gives the main contribution to the LUMO+1s.
Consequently, ligand to ligand charge transfer (LLCT) transition (from Nꢀ4ꢀN ligand to LꢀP and LꢀS
ligands) and ILCT transition (from –NPh2 moiety to 4ꢀphenylpyridine moiety within the Nꢀ4ꢀN
ligand) should be responsible for the low energy absorption bands in the UVꢀVis spectra of PꢀPtꢀN
and SꢀPtꢀN. For all of these PtII complexes, the contribution from the dπ orbitals of Pt centers to their
HOMOs is negligible (Table 2), which should result from the strong electronꢀdonating property of
–NPh2 moiety, excluding the participation of the PtII center to the HOMO contribution.45 So, no
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noticeable MLCT transitions contribute to their UVꢀVis spectra.
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8
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
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H→L+1 (88.6%)
H→L+1 (87.5%)
250
300
350
400
450
500
550
250
300
350
400
450
500
550
Wavelength (nm)
Wavelength (nm)
PꢀPtꢀN
SꢀPtꢀN
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
H→L+1 (76.8%)
H→L+1 (82.2%)
H→L(86.1%)
H→L (91.2%)
250
300
350
400
450
500
550
250
300
350
400
450
500
550
Wavelength (nm)
Wavelength (nm)
Figure 3. Experimental UVꢀVis absorption spectra (red profile) and simulated spectra (black profile)
with discrete vertical vibronic transitions for these PtII complexes.
Figure 4. Molecular orbital (MO) patterns for these PtII complexes based on their optimized S0
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Table 2. TDꢀDFT Results for These PtII Complexes Based on Their Optimized S0 Geometries
Contribution percentages of
metal dπ orbitals and π orbitals of
Main configuration of S0→S1
Main configuration of S0→T1
a
Complexes
MO
excitation/Ecal/λcal/f a
excitation/Ecal/λcal
ligands to MOs (%)
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Pt
LꢀN
90.35 4.06
3.45 94.36
96.77 0.05
Nꢀ4ꢀP
Cl¯
H→L (37.1%), H→L+1
(50.7%)
L+1
5.25
0.33
H→L (98.4%)
NꢀPtꢀP
L
H
Hꢀ1
1.86
2.30
0.34
0.88
33.65
Cl¯
2.453 Ev
505 nm
0.0081
2.309 eV
537 nm
34.98 31.23 0.14
Pt
LꢀN
90.40 4.08
1.73 96.32
97.07 0.05
Nꢀ4ꢀS
H→L (74.8%), H→L+1
(20.0%)
L+1
5.17
0.34
H→L (98.8%)
NꢀPtꢀS
PꢀPtꢀN
L
H
Hꢀ1
1.65
2.11
0.30
0.77
33.40
2.287 eV
542 nm
0.0045
2.237 eV
554 nm
34.97 31.50 0.13
Pt
LꢀP
Nꢀ4ꢀN Cl¯
H→L (63.8%), H→L+1
(10.0%)
L+1
2.83
25.29 71.63
0.25
H→L (94.7%)
L
H
Hꢀ1
3.73
1.09
69.77 26.03
0.47
0.32
35.00
2.869 eV
432 nm
0.0284
2.524 eV
491 nm
0.20
98.39
36.36 28.28 0.36
Pt
LꢀS
Nꢀ4ꢀN Cl¯
H→L (59.3%), H→L+1
(12.2%)
L+1
2.34
15.79 81.63
0.24
H→L (95.4%)
SꢀPtꢀN
L
H
Hꢀ1
3.67
0.92
80.19 15.69
0.46
0.23
36.95
2.853 eV
435 nm
0.0315
2.490 eV
498 nm
0.16
98.69
35.95 26.88 0.23
a
H→L represents the HOMO to LUMO transition. Ecal, λcal and f represent calculated excitation energy, calculated
absorption wavelength and oscillator strength, respectively. The oscillator strength of S0→T1 is zero due to the
spinꢀforbidden of singletꢀtriplet transition under TDꢀDFT calculation in the Gaussian program without considering the
spinꢀorbit coupling.
The PL spectra for these PtII complexes are recorded in both CH2Cl2 and film at 293 K. As
displayed in Figure 5, all of these complexes emit much weaker luminescence in CH2Cl2 solutions
compared with in films under irradiation with 365 nm UV light. In CH2Cl2 solutions (Figure 5a and
S8), two types of emission bands can be observed. The emission bands located at ca. 450 nm for all
PtII complexes should come from the ligandꢀcentered (LC) πꢀπ* transition. Due to the nanosecond
scale lifetime (ca. 4 ns, Table 1), this highꢀenergy emission band should be fluorescence induced by
the singlet excited states (S1).36 On the contrary, the emission bands at longer wavelengths than 500
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nm for these PtII complexes may be ascribed to the triplet excited state (T1) emission for their much
longer lifetime in the order of microseconds (Table 1).51, 53 The phosphorescence intensities of these
complexes are weaker than the fluorescence intensities, indicated by the lower phosphorescence
quantum yield (Φp) than the fluorescence quantum yield (ΦF) (Table 1). At room temperature (RT),
in dilute solution, the dꢀd characters in excited states of the PtII chloride complexes are obvious due
to the thermal activation.54, 55 The lowꢀlying metalꢀcentered dꢀd excited states are rarely emissive
because they provide a nonradiative pathway via molecule distortion in excited states attributed by
the weak ligand field.56, 57 Besides, the structural distortion from D4h symmetry to D2d symmetry of
the square planar PtII complexes also results in nonradiative process, leading to low emission
efficiencies in dilute solution.5, 11, 56, 58 To support these point views, the PL properties of these
complexes in dilute solution at 77 K are investigated. As shown in Figure 5b and S9, all of these PtII
complexes exhibit highly strong phosphorescence emission, which is more than 10 times higher than
at RT under the same measurement conditions, showing much longer phosphorescence lifetime of ca.
2.0 ꢁs (Table 1). In cooperation with all the PL results in solutions, it can be drawn a conclusion that
these PtII complexes emit intense fluorescence compared with the phosphorescence in CH2Cl2
solution at RT, which is due to the faster radiative decay rates of S1 states than the T1 states, induced
by the free rotation of moieties, dꢀd metalꢀcentered states and D4h to D2d structural distortions.
However, at the low temperature of 77 K and in doped PMMA films, all these PtII complexes exhibit
intense phosphorescence emission due to the rotational moieties and the structural distortions in
excited states are efficiently restrained, promoting the radiative efficiency of the T1 states (Table 1,
Figure 5 and S9). Besides, the high efficiency of ISC process form S1 to T1 state is another reason for
the strongly intense phosphorescence at 77 K.
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(b)
(a)
NꢀPtꢀP
NꢀPtꢀS
PꢀPtꢀN
SꢀPtꢀN
500
400
300
200
100
0
4000000
7000
3500000
6000
Solution
PMMAfilm
3000000
5000
NꢀPtꢀP
NꢀPtꢀS
PꢀPtꢀN
SꢀPtꢀN
NꢀPtꢀP
NꢀPtꢀS
PꢀPtꢀN
SꢀPtꢀN
2500000
4000
2000000
3000
1500000
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1000000
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0
500000
0
400
450
500
550
600
650
350
400
450
500
550
600
650
Wavelength (nm)
Wavelength (nm)
Figure 5. (a) PL spectra for all PtII complexes under same measurement conditions in CH2Cl2
solution at 293 K with a concentration of 10ꢀ5 M and in PMMA film with 6 wtꢀ% doping level. (b)
PL spectra for all PtII complexes in CH2Cl2 solution at 77 K with a concentration of 10ꢀ5 M.
With aim to clearly elucidate the PL behaviors of these PtII complexes, the natural transition
orbitals (NTO) have been obtained based on the optimized T1 geometries for their S0→T1 excitation.
Figure 6 displays the NTO pattern and Table 3 presents the concerned NTO results for all of these
PtII complexes. The NTO results for the NꢀPtꢀP have shown that both hole (H) and particle (P)
orbitals are mainly located on the ligand LꢀN (93.95% and 90.31%, respectively). Hence, the large
percentage of H→P transition (98.70%) should suggest that ILCT transition process induces the
phosphorescence emission of NꢀPtꢀP. For NꢀPtꢀS, the ligand LꢀN gives a major contribution to its
hole and particle orbitals and the ligand Nꢀ4ꢀS (17.75%) slightly contributes to its particle orbital as
well. So, the ILCTꢀdominated transition together with slight LLCT features should account for the
phosphorescent bands in the solution PL spectrum of NꢀPtꢀS, showing agreement with its structured
PL spectra (see Figure S8). From the data in Table 3, no noticeable MLCT transition process should
be responsible for the phosphorescent signals in the PL spectra of NꢀPtꢀP and NꢀPtꢀS because of the
negligible contribution of the Pt center in their NTOs (5.10% and 3.90%, respectively). Differently,
in PꢀPtꢀN and SꢀPtꢀN, the Pt centers give some contribution to their hole orbitals (22.82% and
23.19%, respectively), while the ligand LꢀP and LꢀS predominantly accounts for the distributions of
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both hole and particle orbitals (Figure 6 and Table 3). Consequently, the ILCTꢀdominated transition
with some MLCT characters should be responsible for the phosphorescent feature of PꢀPtꢀN and
SꢀPtꢀN. So, PꢀPtꢀN and SꢀPtꢀN can exhibit slightly less structured features in the lineꢀshape of their
phosphorescent spectra (Figure S8).
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Figure 6. Natural transition orbital (NTO) patterns for S0 → T1 excitation for these PtII complexes
based on their optimized T1 geometries.
Table 3. NTO Results for These PtII Complexes Based on Their Optimized T1 Geometries.
Contribution percentages of metal dπ orbitals and π orbitals of
Complexes
NTOa
ligands to NTOs (%)
Pt
5.10
3.90
Pt
LꢀN
93.95
90.31
LꢀN
Nꢀ4ꢀP
0.11
Cl
NꢀPtꢀP
H
P
0.84
0.54
Cl
5.34
Nꢀ4ꢀS
0.12
NꢀPtꢀS
PꢀPtꢀN
SꢀPtꢀN
H
P
4.92
3.50
Pt
94.05
78.30
LꢀP
0.92
0.46
Cl
17.75
Nꢀ4ꢀN
0.11
H
P
22.82
5.64
Pt
69.82
92.74
LꢀS
7.25
0.70
Cl
0.93
Nꢀ4ꢀN
0.10
H
P
23.19
68.74
7.97
0.69
5.34
93.05
0.92
a H and P represent NTO hole and particle orbital, respectively.
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The UVꢀVis results and theoretical calculations reveal the obvious charge transfer (CT) character
including the ILCT and LLCT features of the excited states in these PtII complexes, inducing
solvatochromic effect in different polarity solvents. Therefore, the PL spectra of these PtII complexes
in different solvents (toluene, CH2Cl2, THF, MeCN and DMF) have been investigated under the
same measurement conditions. An obvious solvatochromic effect can be observed by changing the
polarity of the solvents and these PtII complexes display structureless PL spectra in high polarity
solvent (Figure 7), indicating the CT character of their excited states. Besides, the fluorescence bands
are generally redꢀshifted with increasing the polarity of the solvents. In general, the phosphorescence
intensity of these complexes is higher in the solvents with low polarity (toluene, CH2Cl2 and THF)
than that in high polarity ones (MeCN and DMF) (Figure 7). Clearly, increasing the polarity of the
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solvent gives rarely positive effect on the phosphorescence enhancement of these complexes.
100000
NꢀPtꢀP
Toluene
CH2Cl2
NꢀPtꢀS
Toluene
CH2Cl2
90000
81000
72000
63000
54000
45000
36000
27000
18000
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MeCN
DMF
THF
MeCN
DMF
0
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550
600
650
450 Wa5v0e0length (nm)
Wavelength (nm)
Toluene
CH2Cl2
SꢀPtꢀN
PꢀPtꢀN
Toluene
CH2Cl2
1100000
990000
880000
770000
660000
550000
440000
330000
220000
110000
0
1100000
990000
880000
770000
660000
550000
440000
330000
220000
110000
0
THF
MeCN
DMF
THF
MeCN
DMF
400
450
500
550
600
650
400
450
500
550
600
650
Wavelength (nm)
Wavelength (nm)
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Figure 7. PL spectra for all PtII complexes under same measurement conditions in different solvents
at 293 K with a concentration of 10ꢀ5 M
6
7
8
9
From the PL spectral difference in both rigid PMMA matrix and dilute solution (Figure 5a), it may
suggest that the aggregation state makes a positive influence on enhancing the phosphorescent
emission of these PtII complexes, i.e. these PtII complexes may be AIEꢀactive. To confirm the AIE
property of these PtII complexes, the PL spectra of these PtII complexes in tetrahydrofuranꢀwater
(THFꢀH2O) mixture with different H2O volumetric fractions (fw) have been measured (Figure 8). For
NꢀPtꢀP, two main emission peaks located at ca. 470 nm and 530 nm can be observed in THF
solution. With increasing the fw, the PL intensities for the two peaks are increasing as well. When the
fw reaches 80%, the fluorescence emission signal at 470 nm starts to decrease, while the
phosphorescent emission band centered at 530 nm is greatly increased. At the fw of 90%, the
emission signal for the peak of 470 nm has nearly disappeared, only the emission at 530 nm can be
observed, showing a peak PL intensity of ca. 10 times higher than that of in THF solution. As for
NꢀPtꢀS, similar emission peaks located at ca. 470 nm and 530 nm can be detected in THF solution as
well. With increasing the fw, the PL intensities for the two peaks initially increase (fw = 10% and
20%), and then begin to decrease with the fw of 30%. When the fw increases to 90%, the fluorescent
emission signal for the peak at 470 nm is absent, only the phosphorescence emission peak at 530 nm
can be detected with much higher intensity than that in THF solution. As for PꢀPtꢀN and SꢀPtꢀN,
only one main emission peak at ca. 445 nm can be obviously observed in their THF solution. When
the fw is 10%, the PL intensities are the highest. Starting from the fw of 20%, the emission peaks at ca.
445 nm are redꢀshifted to ca. 470 nm with PL intensity decreasing. At the fw of 90%, the
phosphorescence signal can be detected for PꢀPtꢀN and SꢀPtꢀN with the similar intensity comparable
to that of NꢀPtꢀS (Figure 8).
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(b)
f (vol %)
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w
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Wavelength (nm)
Wavelength (nm)
18000
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12000
9000
6000
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25000
20000
15000
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560000
480000
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320000
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160000
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(vol %)
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(vol %)
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0%
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10%
20%
30%
50%
60%
70%
80%
90%
10%
20%
30%
50%
70%
90%
400 450 500 550 600 650 700
400 450 500 550 600 650 700
350
400
450
500
550
600
650
700
350 400 450 500
550 600 650 700
Wavelength (nm)
Wavelength (nm)
Figure 8. PL spectra in THFꢀH2O mixture with different H2O volumetric fractions under same
measurement condition ((a) for NꢀPtꢀP, (b) for NꢀPtꢀS, (c) for PꢀPtꢀN and (d) for SꢀPtꢀN, when fw =
0%, concentration is 2× 10ꢀ4 M).
Suggested by the data in Figure 7, increasing the polarity of solvent cannot show positive effect on
enhancing the phosphorescence intensities of these complexes. As the fw increasing, the polarity of
the mixed solvent (THFꢀH2O) rises up as well. However, in Figure 8, the emission intensities are
obviously enhanced. Hence, the solvatochromic effect on the emission enhancements of these
complexes in different THFꢀH2O mixture should be excluded. The emission intensity enhancements
in the THFꢀH2O mixture of these complexes should be attributed to the aggregates generated by
adding water. To verify this perspective, the absorption behavior of these complexes in the mixture of
THF and H2O has been characterized. As shown in Figure 9, from 10% to 70% of fw, no obvious
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changes in absorption wavelength are observed. As the fw increasing to 90%, the absorption bands
are redꢀshifted to longer wavelength region. Besides, the lowꢀenergy absorption bands show a
levelꢀoff tail feature due to the lightꢀscattering effects of aggregates formed in the mixed solvent.
Clearly, after these PtII complexes aggregated in the mixture with increasing H2O contents, the
concentration of the dissolved PtII complex is greatly decreased, resulting in the inefficiency of light
absorption.59 As a result, these complexes show a decreasing absorption intensities when the fw
increases. Many research results based on both AIEꢀactive organic compounds and PtII complexes
have proved that the nanoꢀaggregate suspensions of these compounds generated in high fw conditions
should induce both the redꢀshift effect and levelꢀoff tail in their UVꢀVis spectra due to the
lightꢀscattering effect of the formed aggregates.23, 31, 33, 35, 60, 61 To confirm the existence of
nanoꢀaggregates with the fw increasing, the dynamic light scattering (DLS) experiments for these
complexes have been studied using the mixture at the fw of 90%. DLS results reveal that the
aggregates with average diameter of less than 200 nm are formed in the medium of 90% fw (Figure
S11). Based on both the PL and UVꢀVis results in same THF : H2O mixture, together with the DLS
results at the fw of 90% mixture, it can be concluded that the nanoꢀaggregates of these PtII complexes
have been generated in THF : H2O system with the fw increasing, they are the main reason behind the
PL enhancement in Figure 8.
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5.0
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
f
f
w
0.6
0.4
0.2
0.0
w
(
Vol%
0%
)
(Vol%)
(a)
0.6
0.3
0.0
(b)
0%
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
10%
30%
50%
70%
90%
10%
30%
50%
70%
90%
300
350
400
A
450
350
400
A
450
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250
(d)
300
400
450
Wave3le50ngth (nm)
250
300
400
450
Wav3e5l0ength (nm)
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
f
f
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
(c)
w
(
Vol%
0%
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0%
10%
30%
50%
70%
90%
10%
30%
50%
70%
90%
250
300
400
450
300Wavelength (4n0m0 )
Wave3le50ngth (nm)
250
350
450
Figure 9. UVꢀVis spectra in THFꢀH2O mixture with different H2O volumetric fractions under same
measurement conditions ((a) for NꢀPtꢀP, (b) for NꢀPtꢀS, (c) for PꢀPtꢀN and (d) for SꢀPtꢀN, when fw
= 0%, concentration is 7.9× 10ꢀ5 M).
By restraining the rotational Nꢀdonor ligands Nꢀ4ꢀP and Nꢀ4ꢀS as well as the –NPh2 moiety on
ligand LꢀN in the aggregated state, the enhanced radiative decay of the T1 states can account for the
obvious phosphorescence enhancement with the increasing of fw in the solutions of NꢀPtꢀP and
NꢀPtꢀS (Figure 8). On the contrary, the PꢀPtꢀN and SꢀPtꢀN exhibit lower phosphorescence
enhancement, the lower molecular aggregation degree in the THFꢀH2O system should be responsible
for this result. The intramolecular rotation and D4h to D2d distortion become much easier due to the
lower molecular aggregation degree of PꢀPtꢀN and SꢀPtꢀN in THFꢀH2O system, resulting in lower
phosphorescence emission induced by T1 states. Even though, the phosphorescence intensity of
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PꢀPtꢀN and SꢀPtꢀN is still in the similar order to that of NꢀPtꢀS (Figure 8). In short, the PL results in
different THFꢀH2O mixture should prove that these PtII complexes are AIEꢀactive.
7
8
9
Clearly indicated by Figure 8, all complexes initially exhibit a fluorescent intensity enhancement
in the THFꢀH2O system with lower fw and then the phosphorescence emission increases with the fw
increasing. It indicates that the phosphorescent emission enhancement should need much higher
aggregation degree among the molecules of these PtII complexes to more effectively restrain the
rotation of both Nꢀdonor ligand and functional groups, guaranteeing much higher radiative efficiency
of the T1 states.
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As is well known, the phosphorescent materials employed in OLEDs are typically doped into a
solid matrix, i.e. host material. Hence, it is more desirable that the AIE behaviors of these PtII
complexes should be investigated in doped film to evaluate its potential in EL field. On this basis,
these PtII complexes have been doped with PMMA with doping level of 6 wtꢀ%, 10 wtꢀ%, 20 wtꢀ%,
40 wtꢀ% and 60 wtꢀ%, these doped PMMA films and their neat films are employed to characterize
their AIE properties. The nonꢀemissive PMMA has been selected as rigid matrix to avoid the effect
of hostꢀtoꢀguest energyꢀtransfer on the phosphorescent intensity which is also generated by the AIE
process as aforementioned. As depicted in Figure 5a, 10 and S10, all of these PtII complexes emit
intense phosphorescence emission in films. From the doping level of 6% to 10% or/and 20%, these
PtII complexes show obvious enhancement in the phosphorescence intensity. Continuously increasing
the doping level to 40%, the phosphorescence intensities of these complexes begin to decrease and
reach the lowest in the neat films. In addition, the phosphorescence lifetimes also low down with
increasing the contents of the PtII complexes. These PL results should indicate the structural
distortion (D4h to D2d) in excited state of these PtII complexes.
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Cleary indicated by the PL results in films, the restrictions of the rotational moieties and the D4h to
D2d distortions can be relayed on the PMMA content. In the doped films with high PMMA contents
i.e. low doping level of the PtII complex, the structural distortion in excited states of these complexes
can be greatly restrained by the PMMA matrix. So, the radiative decay of the T1 states is more
efficient, achieving obviously stronger phosphorescence. Besides, the restraint of the rotational
moieties in these complexes also promotes the radiative decay of the T1 states. However, with low
PMMA contents i.e. high doping level of the PtII complex, the rotations of the moieties in these
complexes and the structural distortions become much easier, the T1 states prefer to the nonradiative
decay process, leading to the lower phosphorescence intensity. Of course, the tripletꢀtriplet
quenching might also be one of the possible reasons to induce lower phosphorescence intensity of
the films with high dopant contents.5, 11 The much shorter phosphorescence lifetimes of these
complexes in films with the doping level of 40%, 60% and neat films than that of in 6%, 10 % and
20 % films should represent a good support. The neat films can still receive the highest Φp of ca.
45.0%, still showing nearly 10 times higher than these in dilute solutions. In the doped films at
doping level of 6 wt.%, the phosphorescence intensity is increased by ca. 45ꢀ to 50ꢀfold compared
with that of in CH2Cl2 solutions (Figure 5a). The SꢀPtꢀN exhibits the highest Φp of ca. 72.0% among
all these PtII complexes. The Φp for NꢀPtꢀS is 68.0%, which is 21.5 times higher than that of its dilute
solution. All these inspiring PL results suggest that these PtII complexes are AIEꢀactive. This AIE
nature also can be confirmed by the vivid image comparison of the luminescence between in solution
and PMMA film (Figure 5a). The phosphorescence of these complexes is obviously enhanced in the
solid state. Furthermore, they possess much higher AIE ability in the doped film, which should be
favorable for OLED application.
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NꢀPtꢀS
NꢀPtꢀP
4550000
3640000
2730000
1820000
910000
0
τP
6 wt% 0.9 ꢀ
τP
1.1 ꢀ
2.2 ꢀ
1.8 ꢀ
4000000
3000000
2000000
1000000
s
s
s
s
s
s
s
s
s
s
s
s
6 wt%
10 wt%
20 wt%
40 wt% 1.4 ꢀ
60 wt%
1.0 ꢀ
neat film 0.9 ꢀ
10 wt%
20 wt%
40 wt%
60 wt%
neat film
1.5 ꢀ
1.1 ꢀ
0.8ꢀ
0.7 ꢀ
0.7 ꢀ
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0
450
500
550
600
650
450
600
650
50W0 avel5e5n0gth (nm)
Wavelength (nm)
τP
τP
1.3 ꢀ
1.4 ꢀ
1.1 ꢀ
0.9 ꢀ
0.8 ꢀ
0.7 ꢀ
7600000 PꢀPtꢀN
6650000
SꢀPtꢀN
5000000
6 wt% 1.2 ꢀ
10 wt%
20 wt%
40 wt%
60 wt%
neat film
s
s
s
s
s
s
s
6 wt%
1.8ꢀ
s
10 wt%
20 wt%
40 wt%
60 wt%
neat film
4000000
3000000
2000000
1000000
0
1.4 ꢀ
s
5700000
1.1ꢀ
0.9 ꢀ
0.8 ꢀ
s
s
s
4750000
3800000
2850000
1900000
950000
0
450
500
550
600
650
450
500
550
600
650
Wavelength (nm)
Wavelength (nm)
Figure 10. PL spectra for these PtII complexes in different doping rate PMMA films and neat films.
As is wellꢀestablished, the restrained intramolecular rotation (RIR) can be a reasonable mechanism
for the AIE nature (Figure 11).23 In aggregated or solid state, the rotation of the Nꢀdonor ligands
along with the –NPh2, –POPh2 and –SO2Ph moieties in the cyclometalating ligand can be effectively
restrained by the rigid PMMA matrix or the PtII complexes themselves. Hence, these complexes in
aggregated or solid state, the RIR effect inhibits the nonradiative decay process, and enhances the
radiative decay of the T1 states, achieving strong phosphorescence emission. As is well known, such
as increasing solvent viscosity and decreasing solution temperature, these external controls exhibit
positive effect on the restraint of the intramolecular rotation.62 For these PtII complexes, the obvious
phosphorescence enhancements at 77 K should provide a good support for this RIR effect due to the
rotating units are restrained at low temperature (Figure 5b and S9). Besides, the rotating units in
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these complexes also can be efficiently restrained by increasing of solvent viscosity according to the
obvious phosphorescence enhancements obtained from the PL investigation of the dilute solutions
(10ꢀ5 M) under different viscosity conditions (Figure S12). Taking all the PL results into
consideration, the RIR effect together with restrained structural distortion should be the main reason
for these PtII complexes showing very strong phosphorescent emission in the aggregated or solid
state.
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RIR
(restrained intramolecular rotation)
a
c
c
b
d
a
d
b
Figure 11. Mechanism for the concerned AIEꢀactive PtII complexes and the PL luminescence images
under 365 nm light illumination in a concentration of 10ꢀ5 M together with PMMA film with a doping
of 6 wtꢀ%: (a) for NꢀPtꢀP, (b) for NꢀPtꢀS, (c) for PꢀPtꢀN and (d) for SꢀPtꢀN
.
Electrochemical Properties
Under nitrogen atmosphere in an acetonitrile solution, the cyclic voltammetry (CV) calibrated
against the ferrocene/ferrocenium (Fc/Fc+) redox couple as an internal reference has been applied to
characterize the electrochemical properties of these PtII complexes. The results are presented in Table
4 and Figure S7. In the anodic scan, the first oxidation potential (Epa) with reversible feature ranging
from ca. 0.36 to 0.48 V can be assigned to the oxidation of the electronꢀrich –NPh2 moiety,7, 63, 64
which is absent in their parent complex ppyꢀPtꢀpy. The NꢀPtꢀP shows the lowest Epa at ca. 0.36 V
due to the dπꢀπ conjugation effect with the Pt center (Table 2). Besides, the second irreversible
oxidation potential ranging from ca. 0.57 to 0.67 V can be ascribed to the oxidation of the Pt center
in these complexes, since it is similar to the reported results for other PtII complexes.7, 18, 65, 66 In
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Page 29 of 45
Chemistry of Materials
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associating with the Epa results, the good hole injection (HI) ability should be conferred to these PtII
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complexes owing to their much lower Epa than that of the parent complex ppyꢀPtꢀpy
.
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In the cathodic scan, all of these PtII complexes exhibit two reduction waves. The first cathodic
peak potential (Epc) at ca. ꢀ2.03 V should come from the reduction of the –POPh2 or –SO2Ph
moieties in these complexes (ꢀ2.03 V and ꢀ2.24 V for –POPh2 group in NꢀPtꢀP and PꢀPtꢀN, ꢀ2.21 V
and ꢀ2.37 V for –SO2Ph moiety in NꢀPtꢀS and SꢀPtꢀN, respectively).47, 67 In addition, the reduction
peaks in the more negative region at ca. ꢀ2.50 V can be assigned to the reduction of pyridyl group in
the complexes. These data are similar to that of ppyꢀPtꢀpy and the reported results.47 As a result, the
LUMO levels of these complexes range from ca. ꢀ2.43 to ꢀ2.77 eV and are much lower than those of
their parent complex ppyꢀPtꢀpy and the reported Pt(ppy)acac type complexes,7 The concerned
results indicate that the –POPh2 or –SO2Ph functional moieties should offer good electronꢀinjection
(EI) abilities to these PtII complexes. Taking both of the oxidation and reduction results into
consideration, the functional groups –NPh2, –POPh2 and –SO2Ph should endow these PtII complexes
with good charge carrier injection ability, which can be very desirable for improving the EL
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performance of the concerned PtII complexes.
Table 4. Electrochemical Properties for the PtII Complexes.
Epa
Epc
(V) b
HOMO
(eV) c
LUMO
(eV) d
Eg
(eV) e
Compounds
(V)
NꢀPtꢀP
NꢀPtꢀS
PꢀPtꢀN
SꢀPtꢀN
0.36 a, 0.61 b
0.44 a, 0.57 b
0.40 a, 0.62 b
0.48 a, 0.67 b
ꢀ2.03, ꢀ2.44
ꢀ2.21, ꢀ2.49
ꢀ2.24, ꢀ2.52
ꢀ2.37, ꢀ2.64
ꢀ2.40
ꢀ5.16
ꢀ5.24
ꢀ5.20
ꢀ5.28
ꢀ5.63
ꢀ2.77
ꢀ2.59
ꢀ2.56
ꢀ2.43
ꢀ2.40
2.39
2.65
2.64
2.85
3.23
ppyꢀPtꢀpy
0.83 b
a
b
c
Reversible. The value was set as E1/2
.
Irreversible or quasiꢀreversible. The value was derived from the cathodic peak potential.
d
HOMO levels are calculated according to the first oxidation potential from CV data, HOMO = ꢀ (Epa + 4.8) eV. LUMO levels are
obtained from the onset potential of the first reduction wave from the CV data, LUMO = ꢀ (Epc + 4.8) eV. CV energy gap Eg =
e
LUMO – HOMO.
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