Influence of integrated alkyl-chain length on the mesogenic and photophysical properties of platinum-based metallomesogens and their application for polarized white OLEDs

Article abstract of DOI:10.1016/j.dyepig.2016.05.024

In this context, a series of platinum complexes containing a triphenylene unit, namely [Formula presented], was prepared and characterized. Platinum complex Pt6 shows a clearly column liquid crystalline property, confirmed by differential scanning calorimetry, polarized optical microscopy and X-ray diffraction. These platinum complexes only display monomolecular emission both in solution and in neat film, an effect attributed to the presence of carbazole as a bulky group disfavoring aggregation of the complexes. Hole mobilities in the range of 10^?5–10^?6?cm²?V^?1 s^?1 were obtained for the annealed platinum complexes films. It was found that the method to align the emissive layer has a crucial role on the performance of the devices. The first example of polarized phosphorescent white organic light-emitting diode (OLED) with polarized ratio of 1.4 was achieved in Pt6-based device. This research opens up a new aspect of phosphorescent metallomesogens application for polarized white light emission.

Full text of DOI:10.1016/j.dyepig.2016.05.024

Accepted Manuscript
Influence of integrated alkyl-chain length on the mesogenic and photophysical
properties of platinum-based metallomesogens and their application for polarized
white OLEDs
Yafei Wang, Jiang Fan, Junwei Shi, Hongrui Qi, Etienne Baranoff, Guohua Xie,
Qingguo Li, Hua Tan, Yu Liu, Weiguo Zhu
PII:
S0143-7208(16)30214-5
10.1016/j.dyepig.2016.05.024
DYPI 5256
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 2 April 2016
Revised Date: 11 May 2016
Accepted Date: 14 May 2016
Please cite this article as: Wang Y, Fan J, Shi J, Qi H, Baranoff E, Xie G, Li Q, Tan H, Liu Y, Zhu W,
Influence of integrated alkyl-chain length on the mesogenic and photophysical properties of platinum-
based metallomesogens and their application for polarized white OLEDs, Dyes and Pigments (2016),
doi: 10.1016/j.dyepig.2016.05.024.
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ACCEPTED MANUSCRIPT
Graphical Abstract
1

ACCEPTED MANUSCRIPT
Influence of Integrated Alkyl-Chain Length on the Mesogenic
and
Photophysical
Properties
of
Platinum-Based
Metallomesogens and Their Application for Polarized White
OLEDs
Yafei Wang†ǁ*, Jiang Fan†, Junwei Shi†, Hongrui Qi†, Etienne Baranoff∆, Guohua Xie‡*,
Qingguo Liꢀ, Hua Tan†, Yu Liu†, Weiguo Zhu†*
†College of Chemistry, Key Lab of Environment-Friendly Chemistry and Application of the Ministry of Education,
Xiangtan University, Xiangtan 411105, China
‡Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Hubei Key Lab on Organic
and Polymeric Optoelectronic Materials, Department of Chemistry, Wuhan University, Wuhan 430072, China.
∆ School of Chemistry, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK
ꢀcollege of Chinese medicines, Guangzhou University of Chinese medicine, Guangzhou 510640, China
ǁInstitute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and
Devices, South China University of Technology, Guangzhou 510640, China
*To whom correspondence should be addressed
(G. Xie) xgh-008@163.com
(W. Zhu) zhuwg18@126.com
(Y. Wang) qiji830404@hotmail.com
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Abstract
In this context, a series of platinum complexes containing a triphenylene unit, namely
Pt5-Pt8, was prepared and characterized. Platinum complex Pt6 shows a clearly
column liquid crystalline property, confirmed by differential scanning calorimetry,
polarized optical microscopy and X-ray diffraction. These platinum complexes only
display monomolecular emission both in solution and in neat film, an effect attributed
to the presence of carbazole as a bulky group disfavoring aggregation of the
complexes. Hole mobilities in the range of 10–5–10–6 cm2 V–1 s–1 were obtained for the
annealed platinum complexes films. It was found that the method to align the
emissive layer has a crucial role on the performance of the devices. The first example
of polarized phosphorescent white OLED with polarized ratio of 1.4 was achieved in
Pt6-based device. This research opens up a new aspect of phosphorescent
metallomesogens application for polarized white light emission.
Keywords: Platinum metallomesogens; Liquid crystalline; Luminescence; Polarized
OLEDs
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1. Introduction
Over the last two decades, square planar d8 platinum complexes have attracted
intensive attention as phosphors in organic light-emitting diodes (OLEDs) due to their
high spin-orbit coupling constants and diverse excited states, such as metal-to-ligand
charge transfer (MLCT), metal-metal-to-ligand charge transfer (MMLCT), ligand
center charge transfer (LC), interligand charge transfer (ILCT) [1-4]. To date, OLEDs
using platinum complexes with high efficiency across the whole visible and
near-infrared region have been realized [5-8]. Yet, these devices generally emit light
without polarization, which makes them unsuitable for applications such as the
backlight of liquid crystal displays (LCDs) and three-dimension (3D) imaging
systems [9-12].
Polarized emission is usually obtained with a polarizer and a non-polarized light
source. Consequently, more than 50% of the produced light is wasted due to
absorption by the polarizer. To solve this issue and improve the energy efficiency of
polarized light sources, imparting liquid crystalline properties to luminescent
materials has been intensively studied as a strategy to directly obtain polarized
emission [13]. Since the first example of linearly polarized electroluminescence (EL)
was reported by Dyreklev et al. using an aligned polymer film [14], the research
focusing on polarized emission from fluorescent liquid crystal molecules and
polymers has flourished [15-22]. However, fluorescent materials can harvest only
singlet excitons, which further limits the efficiency of electroluminescent devices.
Phosphorescent square planar platinum complexes appear very promising in view
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of achieving efficient polarized electroluminescence: by harvesting both singlet and
triplet excitons they could lead to 100% internal quantum efficiency and their planar
structure favor the formation of liquid crystalline phases and promotes charge
mobility in OLEDs. Efforts have been particularly devoted to developing arylpyridine
derivative based platinum metallomesogens [23-36]. Although these platinum
metallomesogens showed intense emission both in solution and in solid state [37-41],
the investigation of the polarized emission is still limited, with only few reported
examples. The first study of the polarized EL emission using mesogenic platinum
complex was reported by Liu et al., in which a R (polarized ratio) of 2, maximum
luminance of 2000 cd/m2 and current efficiency of 2.4 cd/A were observed [38].
In our previous work, mononuclear and dinuclear platinum-based metallomesogens
bearing 2-phenylpyridine ligand showed a polarized ratio up to 10.7 in PL emission
after annealing the complexes on a pre-aligned polyimide film [40,41]. Considering
the stacking of discotic mesogens into extended one-dimensional column liquid
crystals [42], most recently, we successfully combined discotic mesogens such as
triphenylenes with platinum complex and thus obtained new metallomesogens with
columnar phases [43,44]. Encouraged by these results, a new series of platinum
bearing triphenylene and carbazole units, namely Pt5-Pt8, were designed and
prepared (Chart 1). In order to explore the structure-property relationship, the
alkyl-chains between triphenylene moiety and platinum skeleton varied from
conjugation to non-conjugation. To enhance the emission efficiency and the thermal
stability, carbazole unit was grafted onto the cyclometalated ligand. All these platinum
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1
complexes were characterized by H NMR, 13C NMR, MALDI TOF-MS and
elemental analysis. The mesomorphic behavior was studied by differential scanning
calorimetry (DSC), polarized optical microscopy (POM) and X-ray diffraction (XRD),
while the optophysical properties were investigated with UV-visible absorption, PL
emission, cyclic voltammetry (CV) and hole mobility measurements. Polarized
OLEDs were fabricated using Pt5 and Pt6 as the dopants, respectively, and the first
example of polarized white emission using platinum complex was obtained for Pt6
doped device.
2. Experimental section
2.1. Materials and measurement
All reagents were purchased from J&K Chemical and Aladdin companies. All
reactions were carried out under N2 atmosphere. Compounds
6
and
monohydroxypentaalkoxytriphenylene (10) were reported in previous literatures
[42,45]. 1H NMR and 13C NMR spectra were acquired using a Bruker Dex-400 NMR
instrument using CDCl3 as a solvent. Mass spectra (MS) were recorded on a Bruker
Autoflex MALDI-TOF instrument using dithranol as a matrix. Elemental analysis was
determined by Vario EL III. The UV-vis absorption and PL spectra were measured
with a Varian Cray 50 and Perkin-Elmer LS50B luminescence spectrometer,
respectively. Solutions of ppyPtacac (ФPL = 0.15) in 2-methyltetrahydrofuran were
used as a reference [4]. The equation Фs = Фr(ƞs2ArIs/ ƞr2AsIr) was used to calculate the
quantum yields. Thermogravimetric analysis (TGA) was carried out with a
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NETZSCH STA449 from 25 to 600 °C at a 20 °C/min heating rate under N2
atmosphere. Differential scanning calorimerty (DSC) was measured at the phase
transition temperature with a rate of 10 °C/min on the first cooling and second heating
process. Polarized optical microscopy (POM) was recorded the birefringent
o
phenomenon with a rate of 1 C/min on cooling process. X-ray diffraction was
measured by Bruker D8 Discover diffractometer with a 2D Vantec detector. The
sample was mounted in a capillary in a bespoke heating environment-a hollow
graphite furnace-with temperature control via an Eurotherm controller.
2.2. Devices fabrication and characterization
A layer of poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS)
was spin-coated on the ITO glass substrate after UV-ozone treatment. After baking of
PEDOT:PSS at 120 °C for 10 mins, another layer of poly(N-vinylcarbazole) (PVK)
was spin-coated directly and then baked at 120 °C for another 10 mins. In order to
obtain polarized emission, we managed to pattern the emitting layer via different
methods (annealing or rubbing, see below) after spin-coating of the platinum complex
doped poly(9,9-dioctylfluorene) (PFO) onto the hole transporting layer PVK. An
electron transporting layer of bis-4,6-(3,5-di-3-pyridylphenyl)-2-methylpyrimidine
(B3PYMPM) was thermally evaporated on the emitting layer after deliberate
treatments. To compare the influence of the treatment of the emitting layer on the
polarized emission, we constructed several devices with different treatments of the
emitting layer. The device configurations are listed as below:
Devices I: ITO/PEDOT:PSS (30 nm)/PVK (30 nm)/PFO:Pt6 (80:20, 70 nm, no
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baking)/B3PYMPM(50 nm)/Ca (100 nm)/Al (100 nm).
Devices II: ITO/PEDOT:PSS (30 nm)/PVK (30 nm)/PFO:Pt6 (80:20, 70 nm, baked at
120°C for 10 min)/B3PYMPM (50 nm)/Ca (100 nm)/Al (100 nm).
Devices III: ITO/PEDOT:PSS (30 nm)/PVK (30 nm)/PFO:Pt6 (80:20, 70 nm, rubbed
and then baked at 120°C for 10 min)/B3YMPM (50 nm)/Ca (100 nm)/Al (100 nm).
Devices IV: ITO/PEDOT:PSS (30 nm)/PVK (30 nm)/PFO:Pt6 (80:20, 70 nm, pressed
and then baked at 120°C for 10 min)/B3YMPM (50 nm)/Ca (100 nm)/Al (100 nm).
All the devices were encapsulated under a nitrogen atmosphere using UV curable
epoxy. The current-voltage-luminance characteristics were collected with a PR735
spectrascan spectrometer and a Keithley 2400 programmable source meter. The
(polarized) EL spectra were recorded by an Ocean Optics USB2000 spectrometer. The
EL intensities parallel and perpendicular to the rubbing direction (along the ITO stripe)
could be distinguished by aligning a linear polarizer inserted between the OLEDs and
the spectrometer at two mutually perpendicular directions which were denoted as H
(horizon) and V (vertical).
2.3. Hole mobilities
The hole mobility was measured using the space charge limited current (SCLC)
model with a devices configuration of ITO/PEDOT:PSS (40 nm)/Active Layer (100
nm)/MoO3 (10 nm)/Ag (100 nm). The active layer is annealed for 30 mins. The
PEDOT:PSS was spin-coated onto the ITO glass at 4000 rpm and then baked at 160
oC for 15 mins in air. The active layer was spin-cast from chloroform (20 mg/mL) at
1000 rpm. A MoO3 and Ag cathode was then thermally evaporated under vacuum (∼
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10–7 torr) through a shadow mask defining an active device area of ∼0.03 cm2. The
SCLC is described by: J = (9/8)ε0εrꢀ(V2/d3), where J is the current density, d is the
film thickness of the active layer, µ0 is the hole mobility, εr is the relative dielectric
constant of the transport medium, ε0 is the permittivity of free space (8.85 × 10–12
m–1), V is the internal voltage in the device.
2.4. Synthesis
F
3,6-diacetyl-9H-carbazole (1)
9H-carbazole (3.0 g, 17.96 mmol) was added to the solution of aluminum chloride
o
(6.8 g, 51.1 mmol) in CH2Cl2 (100 mL) and the mixture was cooled to 0 C. Acetyl
o
chloride (3.8 g, 48.7 mmol) was then added dropwise into the mixture at 0 C. After
12 h, the solvent was removed by rotary evaporation. The residue was purified by
1
recrystallization with ethanol to obtain gray solid 1 (2.24 g, 50%). H NMR (CDCl3,
400 MHz, TMS), δ (ppm): 8.78 (s, 2H), 8.68 (s, 1H), 8.15 (d, J = 8.2 Hz, 2H), 7.50 (d,
J = 8.48 Hz, 2H), 2.7 (s, 6H).
3,6-diethyl-9H-carbazole (2)
A mixture of compound 1 (2.8 g, 11.1 mmol), aluminum chloride (5.93 g, 44.4 mmol)
o
and tetrahydrofuran (THF) (40 mL) was cooled to 0 C. LiAlH4 (1.0 g, 33.3 mmol)
was added in small batches. The reaction mixture was stirred for 4 h at room
temperature (RT). The solvent was removed by rotary evaporation, and the residue
was purified by silica gel column chromatography using petroleum ether (PE)/CH2Cl2
1
(V:V, 8/1) as eluent to give 2 as a yellow solid (1.0 g, 40%). H NMR (CDCl3, 400
MHz, TMS), δ (ppm): 7.88 (s, 2H), 7.32 (d, J = 8.21 Hz, 2H), 7.24 (d, J = 8.77 Hz,
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2H), 2.85-2.79 (t, J = 7.52 Hz, 4H), 1.36-1.11 (m, 6H).
9-(4-bromophenyl)-3,6-diethyl-9H-carbazole (3)
A mixture of compound 2 (1.4 g, 6.27 mmol), 1-bromo-4-iodobenzene (2.6 g, 9.4
mmol), potassium carbonate (2.5 g, 18.81 mmol), 1,10-phenanthroline monohydrate
(372 mg, 1.88 mmol) in o-xylene (100 mL) was refluxed for 24 h under nitrogen
atmosphere. After cooling to RT, the reaction mixture was poured into water and
extracted with CH2Cl2. The organic layer was collected and dried with MgSO4. The
solvent was removed by rotary evaporation, and the residue was purified by silica gel
column chromatography using PE as eluent to give as faint yellow solid 3 (1.68 g,
1
71%). H NMR (d-DMSO, 400 MHz, TMS), δ (ppm): 8.04 (s, 2H), 7.99 (d, J = 8.2
Hz, 2H), 7.84 (d, J = 8.4 Hz, 2H), 7.58 (d, J = 8.4 Hz, 2H), 7.43 (d, J = 8.2 Hz, 2H),
2.78 (d, J = 7.4 Hz, 4H), 1.30-1.26 (m, 6H).
3,6-diethyl-9-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan)phenyl)-9H-carbazole
(4)
n-BuLi (1.6 M in hexane, 5.4 mL, 8.64 mmol) was added dropwise into a solution of
o
3 (1.09 g, 2.88 mmol) in THF (30 mL) at –78 C under nitrogen atmosphere. The
mixture was stirred at –78 oC for 2 h, then the solution was stirred for 30 mins at RT.
2-Isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (1.07 g, 5.76 mmol) was added
o
at –78 C. The mixture was then allowed to room temperature and stirred overnight.
The reaction was quenched by water and the mixture was extracted with CH2Cl2. The
organic layer was collected and washed with water. After removing the solvent, the
residue was purified by silica gel column chromatography using PE as eluent to give
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as brown solid 4 (480 mg, 38%). 1H NMR (CDCl3, 400 MHz, TMS), δ (ppm): 8.06 (d,
J = 7.9 Hz, 2H), 7.96 (s, 2H), 7.61 (d, J = 7.9 Hz, 2H), 7.40 (d, J = 8.3 Hz, 2H), 7.26
(d, J = 8.4 Hz, 2H), 2.87 (d, J = 7.5 Hz, 4H), 1.53-1.20 (m, 18H).
9-(4-(5-bromopyridin)phenyl)-3,6-diethyl-9H-carbazole (5)
A mixture of compound 4 (480 mg, 1.1 mmol), 2,5-dibromopyridine (260 mg, 1.65
mmol), cesium carbonate (1.0 g, 3.3 mmol), tetrakis(triphenylphosphine)palladiun(0)
(38 mg, 0.032 mmol) and THF (40 mL) was refluxed for 24 h in nitrogen atmosphere.
After cooling to RT, the mixture was extracted with CH2Cl2 and washed with water.
The solvent was removed by rotary evaporation. The residue was purified by silica gel
column chromatography using CH2Cl2/PE (V:V, 8/1) as eluent to give as faint yellow
1
solid 5 (160 mg, 32%). H NMR (CDCl3, 400 MHz, TMS), δ (ppm): 8.82 (s, 1H),
8.21 (d, J = 8.68 Hz, 2H), 7.96 (d, J = 8.78 Hz, 2H), 7.76-7.70 (m, 4H), 7.42 (d, J =
8.2 Hz, 2H), 7.08 (d, J = 7.48 Hz, 1H), 6.94 (d, J = 8.4 Hz, 1H), 2.90-2.85 (m, 4H),
0.90 (m, 6H).
6-(4-(3,6-diethyl-9H-carbazol-9-yl)phenyl)nicotinaldehyde (7)
A mixture of compound 4 (500 mg, 1.2 mmol), 2-bromine-5-pyridinecarboxaldehyde
(271 mg, 1.46 mmol), tetrakis(triphenylphosphine)palladium(0) (42 mg, 0.036 mmol),
cesium carbonate (4.14 g, 3.63 mmol) and toluene/ethanol (15 mL/15 mL) was heated
o
to 80 C for 24 h in nitrogen atmosphere. After cooling to RT, the mixture was
extracted with CH2Cl2 (3 × 20 mL). The organic layer was collected and washed with
water. The solvent was removed by rotary evaporator, and the residue was purified by
silica gel column chromatography using PE/ethyl acetate (EA) (V:V, 10/1) as eluent to
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1
give as a faint yellow solid 7 (270 mg, 55%). H NMR (CDCl3, 400 MHz, TMS), δ
(ppm): 8.72 (s, 1H), 8.20 (d, J = 8.63 Hz, 2H), 7.92 (d, J = 8.76 Hz, 2H), 7.72-7.69 (m,
4H), 7.40 (d, J = 8.21 Hz, 2H), 7.06 (d, J = 7.42 Hz, 1H), 6.92 (d, J = 8.30 Hz, 1H),
2.88-2.83 (m, 4H), 1.30 (m, 6H).
(6-(4-(3,6-diethyl-9H-carbazol-9-yl)phenyl)pyridin-3-yl)methanol (8)
Compound 7 (270 mg, 0.66 mmol) and NaBH4 (112 mg, 1.66 mmol) in
toluene/ethanol (15 mL/15 mL) was stirred for 6 h at RT. Water was added to quench
the reaction, and the mixture was extracted with ethyl acetate (3 × 20 mL). The
organic layer was collected, washed with water and then dried with anhydrous MgSO4.
After removed the solvent, the residue was purified by silica gel column
1
chromatography using EA as eluent to give as yellow solid 8 (210 mg, 78%). H
NMR (CDCl3, 400 MHz, TMS), δ (ppm): 8.72 (s, 1H), 8.20 (d, J = 8.28 Hz, 2H), 7.94
(s, 2H), 7.85 (d, J = 8.04 Hz, 1H), 7.82 (d, J = 8.08 Hz, 1H), 7.67 (d, J = 8.28 Hz, 2H),
7.40 (d, J = 8.32 Hz, 2H), 7.24 (d, J = 8.04 Hz, 2H), 4.84 (s, 2H), 2.87-2.81 (m, 4H),
1.35 (t, J = 7.52 Hz, 6H).
9-(4-(5-((6-bromohexyloxy)methyl)pyridin-2-yl)phenyl)-3,6-diethyl-9H-carbazole
(9)
A mixture of compound 8 (250 mg, 0.61 mmol ), 1,6-dibromohexane (179 mg, 0.74
mmol), potassium iodide (3.05 mg, 0.018 mmol), saturated sodium hydroxide (73 mg)
and actone (20 mL) was refluxed 24 h in nitrogen atmosphere. After cooling to RT,
the mixture was extracted with CH2Cl2 (3 × 20 mL). The organic layer was collected,
washed with water and dried with anhydrous MgSO4. The solvent was removed by
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rotary evaporation, and then the residue was purified by silica gel column
chromatography using PE/EA (V:V, 10/1) as eluent to give as yellow solid 9 (70 mg,
20%). 1H NMR (CDCl3, 400 MHz, TMS), δ (ppm): 8.68 (s, 1H), 8.19 (d, J = 7.16 Hz,
2H), 7.94 (s, 2H), 7.81 (s, 2H), 7.67 (d, J = 7.08 Hz, 2H), 7.40 (d, J = 7.72 Hz, 2H),
7.25 (s, 2H), 4.58 (s, 2H), 3.54 (s, 2H), 3.42 (s, 2H), 3.19 (s, 2H), 2.84 (t, J = 6.88 Hz,
4H ), 1.89-1.43 (m, 6H), 1.34 (t, J = 6.4 Hz, 6H).
Compound LG-1
A mixture of compound 5 (220 mg, 0.48 mmol), compound 6 (220 mg, 0.48 mmol),
tetrakis (triphenylphosphine)palladiun(0) (17 mg, 0.014 mmol), cesium carbonate (2
mol/L, 6 mL) and toluene 20 mL was heated to 80 oC for 24 h in nitrogen atmosphere.
After cooling to RT, the reaction was quenched by water and the mixture was
extracted with CH2Cl2. The organic layer was collected, washed with water and dried
with anhydrous MgSO4. The solvent was removed by rotary evaporation and the
residue was purified by silica gel column chromatography using PE/CH2Cl2 (V:V,
1
10/8) as the eluent to give as yellow solid LG-1 (290 mg, yield 54%). H NMR
(CDCl3, 400 MHz, TMS), δ (ppm): 9.20 (s, 1H), 8.72 (s, 1H), 8.61 (d, J = 8.6 Hz, 1H),
8.31 (d, J = 8.34 Hz, 2H), 8.21 (d, J = 6.18 Hz, 3H), 7.98 (t, J = 7.9 Hz, 3H), 7.87 (d,
J = 5.44 Hz, 3H), 7.72 (d, J = 8.34 Hz, 2H), 7.44 (d, J = 8.32 Hz, 2H), 7.28 (m, 2H),
4.28 (t, J = 7.48 Hz, 8H), 2.88-2.83 (m, 4H), 1.96 (t, J = 7.16 Hz, 8H), 1.61-1.25 (m,
40H), 0.92 (m, 18H). 13C NMR (100 MHz, CDCl3), δ (ppm): 14.05, 15.51, 16.41,
22.67, 25.98, 26.22, 28.95, 29.13, 29.33, 31.90, 63.92, 66.64, 68.26, 68.55, 69.58,
69.70, 69.84, 107.41, 109.65, 114.09, 118.98, 120.26, 121.28, 123.40, 123.91, 124.72,
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126,95, 128.19, 128.80, 129.47, 131.69, 134.85, 135.32, 136.07, 137.56, 139.06,
139.47, 148.47, 149.39, 150.03, 155.25. MALDI-MS (m/z): calcd. for C77H98N2O4
[M+] 1114.75, found 1114.87.
Compound LG-2
A mixture of compound 10 (822 mg, 0.93 mmol), compound 9 (353 mg, 0.62 mmol),
potassium carbonate (855 mg, 6.2 mmol) and potassium iodide (51 mg, 0.31 mmol) in
acetone (20 mL) was refluxed for 24 h in nitrogen atmosphere. After cooled to RT, the
mixture was extracted with CH2Cl2 (3 × 30 mL). The organic layer was collected,
washed with water and dried with anhydrous MgSO4. The solvent was removed by
rotary evaporation and was purified by silica gel column chromatography using
1
PE/EA (V:V, 10/1) as eluent to give as yellow solid LG-2 (574 mg, 54%). H NMR
(CDCl3, 400 MHz, TMS), δ (ppm): 8.67 (s, 1H), 8.14 (d, J = 7.8 Hz, 2H), 7.94 (s, 2H),
7.83-7.77 (m, 6H), 7.71 (d, J = 7.76 Hz, 2H), 7.62 (d, J = 7.76 Hz, 2H), 7.38 (d, J =
8.16 Hz, 2H), 7.25 (s, 2H), 4.58 (s, 2H), 4.22-4.21 (m, 12H), 3.5 (s, 2H), 2.85 (s, 4H),
1.92 (m, 12H), 1.36-1.15 (m, 60H), 0.80( m, 15H). 13C NMR (100 MHz, CDCl3), δ
(ppm): 14.15, 14.60, 16.42, 22.81, 26.0, 26.31, 29.41, 29.73, 29.82, 31.90, 32.71,
60.25, 69.38, 69.56, 72.18, 76.20, 106.23, 107.38, 109.52, 111.12, 117.23, 118.90,
121.24, 122.11, 123.70, 126.21, 133.12, 136.47, 138.36, 139.70, 146.52, 147.87,
148.26, 149.90. MALDI-MS (m/z): calcd. for C92H128N2O7 [M+] 1374.01, found
1374.34.
Generation procedures for synthesis platinum complexes
Cyclometalated ligand (LG-1/LG-2) (2.2 eq) and K2PtCl4 (1.0 eq) in a mixture of
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chloroform (4 mL), distilled water (2 mL) and 2-ethoxyethanol (6 mL) was heated to
o
80 C for 24 h in nitrogen atmosphere. After cooling to RT, the reaction mixture was
extracted with CH2Cl2, and the organic layer was collected and dried with Na2SO4.
After removed the solvent, the dimer was obtained and used to the next step without
any further purification.
A mixture of dimer (1.0 eq), ancillary ligand (3.0 eq), sodium carbonate (10.0 eq) and
o
2-ethoxyethanol was heated to 100 C for 24 h in nitrogen atmosphere. After cooling
to RT, the reaction mixture was poured into water and extracted with CH2Cl2. The
organic layer was collected and dried with anhydrous Na2SO4. The organic solvent
was removed by rotary evaporation and the residue was passed through a flash silica
gel column to give the platinum complexes.
Pt5: PE/CH2Cl2 (V:V, 2/1), yellow solid, (40 mg, yield 12.5%). 1H NMR (CDCl3, 400
MHz, TMS), δ (ppm): 9.49 (s, 1H), 8.69 (s, 1H), 8.65 (d, J = 8.4 Hz, 1H), 8.26 (d, J =
8.1 Hz, 1H), 8.25 (s, 1H), 8.11 (s, 1H), 7.97 (s, 1H), 7.87 (t, J = 8.0 Hz, 4H), 7.80 (d,
J = 8.2 Hz, 3H), 7.70 (d, J = 8.0 Hz, 1H), 7.54 (d, J = 8.2 Hz, 2H), 7.33 (d, J = 7.8 Hz,
2H), 5.5 (s, 1H). 4.28 (t, J = 7.48 Hz, 8H), 2.88-2.84 (m, 4H), 1.96 (t, J = 7.16 Hz,
8H), 1.61-1.12 (m, 44H), 0.92 (m, 18H). 13C NMR (100 MHz, CDCl3), δ (ppm):
14.14, 16.50, 22.73, 26.25, 26.28, 26.29, 27.13, 28.46, 28.99, 29.38, 29.40, 29.49,
29.54, 29.62, 31.90, 69.40, 69.53, 69.65, 70.00, 102.53, 107.00, 110.31, 118.74,
120.81, 121.68, 123.15, 123.29, 123.58, 123.96, 124.06, 124.50, 124.87, 125.96,
128.88, 129.35, 133.17, 135.59, 136.27, 138.16, 139.56, 143.12, 145.46, 149.19,
149.29, 149.97, 150.15, 165.16, 184.41, 185.56. MALDI-MS (m/z): calcd. for
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C82H104N2O6Pt [M+] 1407.75, found 1407.84. Anal. Calcd for C82H104N2O6Pt: C,
69.91; H, 7.44; N, 1.99. Found: C, 70.07; H, 7.41; N, 1.90.
1
Pt6: PE/CH2Cl2 (V:V, 10/7), yellow solid (90 mg, yield 37.6%). H NMR (CDCl3,
400 MHz, TMS), δ (ppm): 9.72 (s, 1H), 8.74 (s, 1H), 8.57 (d, J = 8.3 Hz, 1H), 8.06 (d,
J = 8.3 Hz, 1H), 8.05 (s, 1H), 7.88 (s, 1H), 7.78-7.76 (m, J = 8.3 Hz, 8H), 7.73-7.72
(d, J = 6.4 Hz, 6H), 7.45 (d, J = 7.52 Hz, 1H), 7.31 (m, 2H), 6.67 (d, J = 8.4 Hz, 2H),
6.2 (d, J = 8.2 Hz, 2H), 6.5 (s, 1H), 4.30-4.24 (t, J = 6.3 Hz, 8H), 4.30-4.24 (t, J = 6.3
Hz, 8H), 3.79 (t, J = 6.3 Hz, 4H) 2.91-2.90 (m, 4H), 1.98 (t, J = 6.6 Hz, 8H),
1.68-1.27 (m, 78H), 0.93-0.85 (m, 18H). 13C NMR(100 MHz, CDCl3), δ (ppm): 14.05,
14.06, 16.52, 22.66, 22.69, 26.28, 29.29, 29.36, 29.38, 29.43, 29.52, 29.56, 29.60,
29.63, 29.70, 29.75, 31.90, 31.94, 67.84, 69.04, 69.63, 69.80, 69.90, 107.49, 110.80,
113.92, 114.31, 118.71, 123.32, 123.84, 124.01, 124.34, 124.39, 124.66, 126.02,
128.54, 128.66, 128.82, 129.51, 131.81, 133.89, 134.46, 135.75, 139.06, 145.54,
149.37, 149.49, 149.86, 150.12, 161.10, 161.41, 177.11, 177.60. MALDI-MS (m/z):
calcd. for C116H156N2O8Pt [M+] 1900.15, found 1900.20. Anal. Calcd for
C116H156N2O8Pt: C, 73.27; H, 8.27; N, 1.47. Found: C, 72.23; H, 8.09; N, 1.48.
Pt7: PE/CH2Cl2 (V:V, 1/1), yellow solid (300 mg, 57%). 1H NMR (CDCl3, 400 MHz,
TMS), δ (ppm):8.67 (s, 1H), 8.14 (d, J = 7.8 Hz, 2H), 7.94 (s, 2H), 7.83-7.77 (m, 6H),
7.71 (d, J = 7.76 Hz, 2H), 7.62 (d, J = 7.76 Hz, 2H), 7.38 (d, J = 8.16 Hz, 2H), 7.25 (s,
1H), 5.42 (s, 1H), 4.58 (s, 2H), 4.22-4.21 (m, 12H), 3.5 (s, 2H), 2.85 (s, 4H), 1.92 (m,
12H), 1.36-1.15 (m, 66H), 0.80 (m, 18H). 13C NMR (100 MHz, CDCl3), δ (ppm):
14.15, 16.65, 22.73, 25.89, 25.99, 26.26, 26.39, 29.43, 29.56, 29.62, 29.68, 29.73,
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31.90, 31.97, 67.79, 69.25, 69.85, 69.91, 70.67, 107.47, 110.51, 114.14, 118.77,
123.38, 123.84, 124.19, 125.99, 128.76, 128.85, 135.71, 137.42, 139.57, 148.92,
149.03, 149.11, 184.39, 185.74. MALDI-MS (m/z): calcd. for C97H134N2O9Pt [M+]
1665.97, found 1666.3, 1567.22. Anal. Calcd for C97H134N2O9Pt: C, 69.88; H, 8.10; N,
1.68 Found: C, 67.94; H, 7.94; N, 1.72.
Pt8: PE/CH2Cl2 (V:V, 1/1), yellow solid, (70 mg, 30.7%). 1H NMR (CDCl3, 400 MHz,
TMS), δ (ppm): 9.13 (s, 1H), 8.06 (d, J = 7.9 Hz, 2H), 7.94 (s, 2H), 7.83-7.77 (m, 8H),
7.71 (d, J = 7.76 Hz, 2H), 7.62 (d, J = 7.76 Hz, 2H), 7.38 (d, J = 8.16 Hz, 2H), 7.25 (s,
2H), 6.95 (d, J = 8.15 Hz, 2H), 6.68-6.66 (m, 3H), 4.6 (s, 2H), 4.20 (m, 14H), 3.99 (t,
J = 5.8 Hz, 2H), 3.89 (t, J = 7.75 Hz, 2H), 3.63 (s, 2H), 2.90-2.85 (m, 4H), 1.92 (m,
12H), 1.40-1.24 (m, 99H), 0.88 (m, 27H). 13C NMR (100 MHz, CDCl3), δ (ppm):
14.12, 16.50, 22.71, 25.97, 26.33, 27.07, 28.24, 28.98, 29.36, 29.42, 29.71,31.88,
53.45, 69.59, 69.73, 69.86, 70.92, 106.74, 107.44, 110.24, 118.74, 121.78, 123.23,
123.43, 123.47, 123.51, 124.05, 124.98, 125.95, 128.29, 131.34, 135.58, 138.01,
138.39, 139.53, 140.95, 142.98, 147.60, 148.95, 149.04, 149.24, 149.30, 149.36,
167.15. MALDI-MS (m/z): calcd. for C131H186N2O11Pt [M+] 2159.96, found 2159.65,
1568.14. Anal. Calcd for C131H186N2O11Pt: C, 72.84; H, 8.68; N, 1.30. Found: C,
72.20; H, 8.69; N, 1.36.
3. Results and discussion
3.1. Synthesis and characterized
The synthetic route of platinum complexes is depicted in Scheme 1. Starting from
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the commercially available carbazole, intermediate 3 was obtained by Friedel-Crafts
acylation and subsequent reduction reaction in 28% yield in two steps [46]. The
following borylation of compound 3 with 2-isopropoxy-4,4,5,5-tetramethyl-
1,3,2-dioxaborolane in the presence of n-BuLi gave intermediate 4 in 38% yield. Then,
Suzuki coupling reaction between compound 4 and 2,5-dibromopyridine or
6-bromonicotinaldehyde yielded precursors 5 and 7, respectively. Intermediate 5
reacted with compound 6 to afford cyclometalating ligand LG-1 by Suzuki coupling
reaction in the presence of Pd(PPh3)4. Compound 7 was reduced with NaBH4 in
toluene/ethanol solution at room temperature and then converted to intermediates 9
using 1,6-dibromohexane as the reactant. Cyclometalating ligand LG-2 was obtained
by etherification reaction between monohydroxypentaalkoxytriphenylene (10) and
intermediate 9 using potassium carbonate in acetone solution. Target platinum
complexes were finally synthesized according to the conditions reported by our group
[40,41]. All the platinum complexes were confirmed by 1H NMR, 13C NMR, MALDI
TOF-MS and elemental analysis (ESI†).
3.2. Thermal properties
The thermal stability of all platinum complexes was evaluated by TGA under a
nitrogen atmosphere. All platinum complexes possess good thermal stability, with the
o
decomposition temperature (Td, 5% weight loss) above 270 C, which is much better
o
than the analogous platinum complexes TppyPtacac (162 C) and TppyPtPhacac (191
oC) [43]. As seen from Table 1, the platinum complexes with dibenzoylmethane
(DBM) moiety (Pt6 and Pt8) are much more stable than the platinum complexes with
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acetylacetone (acac) ligand (Pt5 and Pt7). Additionally, the trend in thermal stability
(Pt5<Pt7) clearly illustrates that the alkyl-chains length between triphenylene and
platinum core has a positive impact on the thermal stability.
The mesomorphic behavior of these platinum complexes was investigated by DSC,
POM and XRD. DSC scan was initially carried out to study the thermotropic property
under nitrogen atmosphere. The DSC curves exhibit distinct patterns as a function of
the molecular structure (ESI†). As for the platinum complexes with acac ligand (Pt5
and Pt7), no clear endothermic peak and exothermic peak are found upon heating and
cooling curves (Table 1 and ESI†), which is similar to the previously reported
triphenylene-based compounds [47-50]. In contrast, the platinum complexes with
DBM ligand possess obviously phase transition peak owing to the more periphery
alkyl chains. Pt6 shows two endothermic peaks indicating multiple phase transitions
upon heating process. However, the phase transition is not observed on cooling from
DSC [45,49,51]. Pt8 shows two endothermic peaks and one exothermic peak on
cooling, respectively. Notably, the clearing point decreases with the increased length
of the alkyl-chain between the triphenylene and the platinum core complex.
Based on the DSC results, POM results demonstrated that only Pt6 showed fluidity
and liquid crystal characters, with birefringence upon cooling. As shown in Figure 1,
complex Pt6 exhibits a branched leaf-like texture when cooling from the isotropic
liquid, indicating a typical columnar phase [45,52]. Although Pt5 presents an
obviously birefringence phenomenon, it also seems to be some small crystals (Figure
1). In contrast, for complexes Pt7 and Pt8, we did not detect any fluidity and
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birefringence during heating and cooling (ESI†), in which both complexes possess
crystalline state. Clearly, the length of alkyl chain between platinum complex skeleton
and triphenylene unit has a crucial role on the mesomorphic behavior.
In order to further confirm the columnar phase of the platinum complexes, Pt5 and
Pt6 were selected as representative examples to measure temperature-dependent
X-ray diffraction (Figure 2). At 54 oC, as shown in Figure 2, the XRD pattern of Pt6
shows one strong reflection at small angle range together with several small scatter
peaks at low angle and high angle ranges, a feature of glassy (or partially crystalline)
o
structure. When Pt6 was heated to 185 C, all the peaks disappear because of the
o
formation of the isotropic phase. When the temperature is decreased to 116 C from
isotropic liquid, only a sharp reflection at 2θ of 3.2o (27.6 Å) appears with a diffuse
reflection at around 2θ of 18.4o (4.8 Å), typical of the liquid crystalline state and
assigned to the liquid-like order of the peripheral alkoxy chains [53,54]. Combined
o
with DSC and POM observations, and compared to the XRD pattern at 54 C, the
disappearing of the diffraction peaks in the range of 5-15o (2θ) and clearly changed
XRD pattern at 116 oC indicates the formation of a mesophase. Due to the absence of
more diffraction peaks in the high angle range, the exact mesophase structure could
not be calculated. Nevertheless, according to literature reports about
triphenylene-based compounds we assign the structure to a columnar phase [55-57].
However, for complex Pt5, the temperature-dependent XRD could not offer more
evidences for the liquid crystalline state. Therefore, Pt5 was mainly tentative assigned
to crystal structure.
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3.3. Optical Properties
The absorption spectra of the complexes were measured in dichloromethane
(CH2Cl2, 10–5 M) at room temperature (Figure 3), and the data are listed in Table 2.
Each platinum complex shows three well-resolved absorption bands in the range of
250-500 nm. The intense absorption band from 250 to 310 nm with high molar
absorptivities (ε) on the order of 105 M−1 cm−1 is assigned to the spin-allowed π-π*
electron transitions of the cyclometalating ligand. Compared to Pt5 and Pt6, the other
platinum complexes show additional triphenylene absorption bands at about 280 nm,
demonstrating the weak ground-state coupling between the platinum core and
triphenylene unit due to the aliphatic spacer unit [58]. The moderately intense
absorption band ranging from 310 to 370 nm (ε = 104 M−1 cm−1) is attributed to
intramolecular charge-transfer (ICT) and metal-to-ligand charge-transfer transitions
(1MLCT). The low-energy absorption band (ꢁ370 nm, 103-104 M−1 cm−1) is ascribed
3
to MLCT transition according to previous report [58]. It is noted that the platinum
complexes with DBM ligand (Pt6 and Pt8) exhibit a stronger absorption in the range
of 300-370 nm than the platinum with acac moiety (Pt5 and Pt7). This result could be
referred to electronic transitions involving the DBM moiety. Due to the expanded
π-conjugation, Pt5 and Pt6 display a red-shifted absorption in low-energy region
compared to other platinum complexes. Therefore the grafted aliphatic chain in
cyclometalating ligand has a significant influence on their absorption property.
As shown in Figure 4, all platinum complexes display resolved vibrational
emission spectra, generated from π-π* electron transitions. Pt7 and Pt8 show similar
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emission spectra centered at 512 nm with a shoulder at about 547 nm. On the other
hand, the PL spectra of Pt5 and Pt6 are red-shifted to 558 nm with respect to that of
Pt7 and Pt8 caused by the expended π-conjugation. Compared to the analogous
platinum complex TppyPtPhacac and TppyPtacac [43], Pt5 and Pt6 exhibit 8 nm red
shift spectra due to the additional carbazole unit results in more effective
intramolecular charge transfer transition, whereas Pt7 and Pt8 show hypochromatic
shift spectra owing to the low conjugation caused by alkyl chains. The
photoluminescence quantum yields (Φ) of these platinum complexes are in the range
of 0.2-0.4 in degassed CH2Cl2. Interestingly, the emission peaks show little red shift
in neat films relative to those in solution (ESI†). It is quite different from generally
square planar platinum complex that Pt5-Pt8 did not show distinct excimer emission
in neat film, indicating the periphery alkyl and additional carbazole unit have an
impact on suppressing the intermolecular aggregation.
3.4. Electrochemical Properties
The electrochemical properties of platinum complexes were evaluated by cyclic
voltammetry in CH2Cl2 solution using ferrocene/ferrocenium (Fc/Fc+) as an internal
standard. All platinum complexes show the irreversible oxidation potentials between
0.39 V and 0.6 V (vs Fc/Fc+, ESI†). Based on the oxidation potentials and optical
band gap (optEg), correspondingly, their highest occupied molecular orbital (HOMO)
and lowest unoccupied molecular orbital (LUMO) levels are estimated via the
formula of EHOMO = –(4.8 + Eox) eV and ELUMO = –(EHOMO
–
optEg) eV [59],
respectively. As seen from Table S1, the ancillary ligand with DBM unit has a
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stabilizing effect on HOMO and LUMO of the platinum complexes. All platinum
complexes possess the energy band gaps from 2.63 to 2.82 eV (Table S1). Pt5 and
Pt6 display relatively narrow energy band gaps compared to other platinum
complexes, owing to the extended π-conjugation, which is also supported by
absorption and emission properties.
3.5. Hole Mobilities
The hole mobilities of platinum complexes were also measured by the space charge
limited current (SCLC) method with a device configuration of ITO/PEDOT:PSS (40
nm)/platinum complex (100 nm)/MoO3 (10 nm)/Ag (100 nm). For the devices treated
with thermal annealing (120oC for 30 mins), these platinum complexes exhibit
moderate hole mobilities in the range of 10–5–10–6 cm2 V–1 s–1 (Table S2, ESI†).
Complex Pt5 shows the best hole mobility up to 2.1 × 10–5 cm2 V–1 s–1. The results
imply that the hole mobilities decreases with the increased aliphatic chains between
the platinum core and triphenylene.
3.6. Polarized Devices
In order to systematically investigate the polarized EL of the platinum complexes,
Pt5 and Pt6 were selected as the emitter. Poly(9,9'-dioctylfluorene) (PFO) was chosen
as the host matrix because of its outstanding liquid crystalline and polarized emission
[20,21]. Initially, the polarized OLEDs were fabricated with the configuration of
ITO/PEDOT:PSS (30 nm)/PVK (30 nm)/PFO:platinum complex (94:6, 70 nm,
rubbed)/B3YMPM (50 nm)/Ca (100 nm)/Al (100 nm). The polarized EL spectra
recorded at 10 mA/cm2 are depicted in Figure 5 in terms of the emission intensity
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parallel (H) and perpendicular to (V) the rubbing direction. In the case of Pt5-doped
device, the polarized emission, with a dichroic ratio (R = ELH/ELV) of about 5, is only
observed from blue emission of PFO (Figure 5a). For the Pt6-doped device, both
blue emission from PFO and orange emission from platinum complex show polarized
emission (Figure 5b). The R was evaluated to be 17.5 at 430 nm and 1.4 at 569 nm,
respectively. This result indicates that complex Pt5 has an inferior self-assemble
property than Pt6 in PFO film, which is also confirmed that Pt5 could be crystal.
Notably, the emission parallel to the rubbing direction is stronger than that
perpendicular to the rubbing direction, implying the PFO and platinum complexes
could be aligned with the rubbing [38].
To elucidate the effect of molecular order on polarized emission, a series of devices
based on Pt6 were fabricated with different procedures to prepare the aligned
emissive layer (Experimental Section). The EL data of these devices are listed in
Table S3. The aligned emissive layer in device I was processed without any special
o
procedures. The emissive layer was annealed at 120 C in device II. The emissive
layer of device III was uniaxial rubbed for surface alignment, and then annealed at
o
120 C, while the emissive layer in device IV was prepared via pressing on the
o
surface, followed by annealing at 120 C. Furthermore, to improve the quality of
white light compared to the previous devices having a large blue component from
PFO, the amount of Pt complex was increased to 20% in these devices in order to
increase the red component.
Apparently, the EL emission varied with aligned emissive layer in different
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polarized devices (Figure 6 and ESI†). As depicted in Figure 6, Device I shows a
broad EL emission from 400 nm to 750 nm. The blue emission in the range of
400-480 nm is attributed to host matrix PFO, while the low-energy emission (635 nm)
are assigned to emission from electrically excited bimolecular species (electromers
and electroplexes) as they are not observed in the PL emission in neat film [60]. The
whole EL spectrum exhibits polarization dependence with R of 1.75 at 439 nm and
1.34 at 635 nm, respectively. This broad emission from device I has Commission
Internationale de L’Eclairage (CIE) coordinates (0.43, 0.35), implying a nearly white
emission. To the best of our knowledge, this is the first example of polarized white
emission based on platinum complex.
When the emissive layer in devices II-IV was processed with thermal annealing,
the deep red component of the emission spectra attributed to electromers/electroplexes
significantly increases in intensity compared to device I (ESI†). Obviously, the
annealed emitting layer has a significant effect on the EL emission, attributed to more
ordered molecules and effective energy transfer between host matrix and guest.
Device II shows the polarized EL with R of about 1 only at 670 nm, while device III
displays the polarized emission at both blue emission (R = 5.5, 432 nm) and red
emission (R = 1.1, 655 nm). However, the polarization dependence was not detected
in device IV, which infers that it is difficult to align the emission film by pressing
method. Compared to device I, devices II-IV show inferior polarized emissions,
likely due to the poor alignment of the emissive layer. The CIE coordinates of devices
II-IV have minor change from (0.51, 0.37) to (0.59, 0.35) in the reddish orange range.
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Although these platinum metallomesogens show the charming polarized emission,
the devices performances are still very low. Device I exhibits the best performance
with a turn-on voltage of 6 V, a maximum luminance of 100 cd/m2 and a current
efficiency of 0.46 cd/A. Even though the efficiency achieved in these polarized
devices is not fascinating compared to that of state-of-the-art phosphorescent OLEDs,
it provides the strategy to achieve polarized white emission, which is necessary for the
application of backlight in liquid crystal displays.
4. Conclusions
In summary, a series of novel platinum complexes were synthesized and
characterized. Complex Pt6 rather than Pt5, Pt7 and Pt8 can effectively form liquid
crystalline, confirmed by DSC, POM and XRD. Compared to the reported
TppyPtacac and TppyPtPhacac, this novel platinum complex showed better thermal
stability and liquid crystal property. Additionally, the mesophase disappeared as the
length of the alkyl-chain between platinum skeleton and triphenylene unit increased.
The excimer emission can be effectively suppressed by periphery chains and
additional carbazole units under opto-excitation. However, emission from electrically
excited bimolecular species is observed in devices as a deep red, broad band. The
exact nature of these species is not clear at this stage. As a result, the first example of
polarized white electroluminescence rom a phosphorescent dopant was achieved with
R of 1.4 in Pt6-based OLEDs. Importantly, the polarized EL was sharply dependent
on the aligned emissive layer. Even though the performance of the polarized OLEDs
is still frustrated, it is significant that current studies open up a new aspect of
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phosphorescent metallomesogens application for the backlight of liquid crystal
displays.
Acknowledgements
The authors sincerely thank Christopher Cabry and Prof. Duncan W. Bruce for XRD
measurements. We are grateful for the financial support from the National Natural
Science Foundation of China (21202139, 51473140 and 51273168), the Open Fund of
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the State Key Laboratory of Luminescent Materials and Devices (South China
University of Technology) (2015-skllmd-08), the European Union (MC-IIF-329199),
and the Key Fund Project of Hunan Provincial Education Department (13A133).
.
Supporting Information
The Supporting Information is available free of charge on the Publications website.
1
Detailed information on H NMR, TOF-MS, TGA, DSC, POM, XRD, CV curves,
hole mobility and polarized EL spectra.
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