A series of short axially symmetrically 1,3,6,8-tetrasubstituted pyrene-based green and blue emitters with 4-tert-butylphenyl and arylamine attachments

Article abstract of DOI:10.1016/j.dyepig.2016.03.020

A series of short axially symmetrically 1,3,6,8-tetrasubstituted pyrene-based derivatives (4a-4c) with two 4-tert-butylphenyl segments at 1,8-positions and two hole-transporting arylamine attachments at 3,6-sites of pyrene core were designed and synthesized based on stepwise synthetic strategy. These compounds were structurally characterized and their photoelectric properties were investigated by spectroscopy, electrochemical and theoretical studies. The results show that the incorporation of different arylamine units into pyrene core can tune the electronic structures of the compounds, and polysubstituted configuration of pyrene can effectively suppress the intermolecular π-π interactions and consequently leads to relatively high absolute fluorescence quantum efficiencies. These compounds exhibit high thermal stability and can form morphologically stable amorphous thin films with glass transition temperature in the range of 156-189 °C. Solution-processed non-doped green and blue organic light-emitting diodes with maximum luminance efficiencies 4.14 cd A^-1 and 2.41 cd A^-1 were achieved for 4b and 4c, respectively.

Full text of DOI:10.1016/j.dyepig.2016.03.020

Dyes and Pigments 130 (2016) 106e115
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
A series of short axially symmetrically 1,3,6,8-tetrasubstituted pyrene-
based green and blue emitters with 4-tert-butylphenyl and arylamine
attachments
*
Ran Zhang, Yun Zhao, Tengfei Zhang, Lu Xu, Zhonghai Ni
School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou 221116, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 January 2016
Received in revised form
A series of short axially symmetrically 1,3,6,8-tetrasubstituted pyrene-based derivatives (4ae4c) with
two 4-tert-butylphenyl segments at 1,8-positions and two hole-transporting arylamine attachments at
3,6-sites of pyrene core were designed and synthesized based on stepwise synthetic strategy. These
25 February 2016
compounds were structurally characterized and their photoelectric properties were investigated by
spectroscopy, electrochemical and theoretical studies. The results show that the incorporation of
different arylamine units into pyrene core can tune the electronic structures of the compounds, and
Accepted 10 March 2016
Available online 14 March 2016
polysubstituted configuration of pyrene can effectively suppress the intermolecular pep interactions and
Keywords:
Pyrene-based
Symmetrical
Tetrasubstituted
Synthesis
consequently leads to relatively high absolute fluorescence quantum efficiencies. These compounds
exhibit high thermal stability and can form morphologically stable amorphous thin films with glass
ꢀ
transition temperature in the range of 156e189 C. Solution-processed non-doped green and blue
ꢁ
1
ꢁ1
organic light-emitting diodes with maximum luminance efficiencies 4.14 cd A and 2.41 cd A were
achieved for 4b and 4c, respectively.
Property
OLED
©
2016 Elsevier Ltd. All rights reserved.
1
. Introduction
supramolecular structures and the optimization of functional units
through the introduction of bulky peripheral attachments into the
suitable positions of pyrene core [11e14]. Indeed, a larger number
of pyrene-based compounds with interesting photoelectric prop-
erties and various molecular architectures and supramolecular
structures have been designed and synthesized up to date [15e17].
Some of them have been exploited as efficient hole-transporting or
host emitting materials for high performance OLEDs [18e22].
However, the reported pyrene derivatives are almost 1-substituted
pyrenes, 1,6-disubstituted and 1,3,6,8-tetrasubstituted pyrenes
with the same group, and some non-symmetrically 1,3,6,8-
tetrasubstituted pyrenes with different peripheral segments at
1,6- and 3,8-postions of pyrene ring [23,24]. The purely 1,8-
disubstituted pyrenes are very limited due to the synthetic chal-
lenge [8]. Naturally, to the best of our knowledge, there is still no
report on short axially symmetrically 1,3,6,8-tetrasubstituted pyr-
enes with different peripheral attachments at 1,8- and 3,6-
positions of pyrene core, which can not only offer more novel
compounds but also provide one effective strategy for molecular
electronic tuning, interface control and optimization of film mor-
phologies of pyrene-based photoelectric materials.
Organic light-emitting diodes (OLEDs) have attracted extensive
attention over the last two decades because of their potential ap-
plications in flat panel displays and general lightings [1e3]. The
main research focuses in this field include the design and synthesis
of new materials with excellent photoelectric properties and the
efficient device fabrication techniques [4e7]. Among the many
famous candidate materials for OLEDs, polyaromatic pyrene has
been paid everlasting attention due to its pure blue fluorescence
with high quantum yield, long fluorescence lifetime, excellent
thermal stability, and high charge carrier mobility [8e10]. How-
ever, pyrene and its some derivatives tend to form excimers in solid
state through pep stacking due to the flat structure of pyrene core,
giving rise to the red-shifted emission, the decrease of fluorescence
quantum yield and the degrade of colour purity, which brings a big
obstacle for the solid state applications of such pyrene-based
compounds. To obtain excellent pyrene-based OLED materials,
the most effective strategies involve the control of the
*
Corresponding author.
E-mail address: nizhonghai@cumt.edu.cn (Z. Ni).
Recently, we designed and synthesized three novel short axially
http://dx.doi.org/10.1016/j.dyepig.2016.03.020
143-7208/© 2016 Elsevier Ltd. All rights reserved.
0

R. Zhang et al. / Dyes and Pigments 130 (2016) 106e115
107
symmetrically 1,3,6,8-tetrasubstituted pyrenes with two different
functional attachments at 1,8- and 3,6-positions, respectively,
through the stepwise derivatization and functional transformation
strategy using pyrene as the precursor (Scheme 1). The introduc-
tion of 4-tert-butylphenyl groups into the 1,8-positions of pyrene
ring can be beneficial to the separation of pure 1,8-disubstitutted
pyrenes and their solution-processed property. The incorporation
of different arylamines is to improve the hole-injection and trans-
porting properties of the emitting materials. Herein, we report the
design, synthesis and photoelectrical properties of the new series of
short axially symmetrically 1,3,6,8-tetrasubstituted pyrenes with
two different functional attachments (4ae4c). In addition,
solution-processed OLEDs with two representative compounds as
non-doped emitting layer were investigated.
performed with a CHI660A electrochemical work station. All
measurements were carried out at room temperature with a con-
ventional three-electrode configuration consisting of a glassy car-
bon working electrode, a platinum auxiliary electrode and a
calomel reference electrode. The solvent in all experiments was dry
dichloromethane and the supporting electrolyte was 0.1 M tetra-
butylammonium hexafluorophpsphate. The glass-transition tem-
g
peratures (T ) of the compounds were determined with differential
scanning calorimetry (DSC) under a nitrogen atmosphere by using a
ꢀ
DSC6000 (PerkinElmer). Samples were heated to 400 C at a rate of
ꢀ
ꢁ1
ꢀ
ꢁ1
10 C min and cooled at 10 C min then heated again under the
same heating conditions as used in the initial heating process.
d
Decomposition temperature (T ) were determined with thermog-
ravimetric analysis (TGA) under a nitrogen atmosphere by using a
ꢀ
DTG-60AH (Shimadzu). Samples were heated to 800 C at a rate of
10
ꢀ
ꢁ1
C min . The ground state geometries of all molecules were
fully optimized using density functional theory (DFT) at the B3LYP/
-31G (d, p) level, as implemented in Gaussian 09W software
2
. Experimental section
6
2.1. General
package [25]. The electronic transitions were calculated using the
time-dependent DFT (B3LYP) theory and the 6-31G (d, p) basis set.
Dichloromethane was distilled from calcium hydride. All
chemicals used were purchased from commercial sources and used
1
13
2.2. OLED fabrication and performance evaluation
without further purification. H and C NMR spectra were collected
on a Bruker-400 MHz or Bruker-600 MHz spectrometer in CDCl
3
Poly(ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:
PSS) (Baytron PAI 4083) and 1,3,5-tri[(3-pyridyl)-phen-3-yl]ben-
zene (TmPyPB) were purchased from Heraeus Precious Metals
GmbH Co. KG and Luminescence Technology Corp., respectively.
The OLEDs have a structure of indium tin oxide (ITO)/PEDOT:PSS
solution with TMS as an internal standard. Mass spectra were ob-
tained on a Bruker Ultraflextreme MALDI TOF/TOF mass spec-
trometer. Elemental analyses (C, H, N) of the dried solid samples
were carried out using an Elementary Vario El analyser. UVeVis
spectra were recorded on Shimadzu UV-3600 with a UVeVISeNIR
spectrophotometer. Emission spectra were performed by a HITACHI
fluorescence spectrometer (F-4600). The absolute fluorescence
(
(
40 nm)/4b or 4c (40 nm and 50 nm)/TmPyPB (50 nm)/LiF (1 nm)/Al
150 nm) [26]. For the device fabrication, a 40 nm-thick PEDOT:PSS
layer was spin-coated from an aqueous dispersion of PEDOT:PSS
onto the pre-cleaned ITO substrate and then annealed at 120 C for
F
quantum yields (F ) were determined by FM-4P-TCSPC Transient
ꢀ
State Fluorescence Spectrometer using an integrating sphere for
dilute dichloromethane solution and thin films which obtained by
drop-casting on quartz plate. Cyclic voltammetry experiments were
30 min in air condition. Subsequently, the EML was spin-coated
with two different thickness from its fresh tetrahydrofuran solu-
tion at a spin speed of 1200 rpm or 1500 rpm and then annealed at
ꢀ
1
00 C for 30 min to remove the residual solvent at nitrogen at-
mosphere in a glove box. Finally, the structure of TmPyPB(50 nm)/
LiF(1 nm)/Al(150 nm) was thermally deposited in sequence in a
ꢁ
4
vacuum chamber at a pressure less than 4 ꢂ 10 Pa through a
2
shadow mask with an array of 14 mm openings. The current
density-voltage-luminance (IeVeL) characteristics were measured
using a Keithley source measurement unit (Keithley 2400) with a
calibrated silicon photodiode. The EL spectra of the devices were
measured using a SpectraScan PR650 spectrophotometer. All
measurements were carried out at room temperature under
ambient conditions.
2
2
.3. Synthesis
.3.1. Synthesis of 1,8-bis(4-tert-butylphenyl)pyrene (2a)
Under the atmosphere of nitrogen, the mixture of 1,8-
dibromopyrene (85%) and 1,6-dibromopyrene (15%) (1.80 g,
mmol) [27], 4-tert-butylphenylboronic acid (2.14 g, 12 mmol),
Pd(PPh (0.12 g, 0.1 mmol) and 2 M potassium carbonate solution
12 mL) were dissolved in toluene (80 mL) and refluxed for 24 h.
5
3 4
)
(
After the reaction finished, the mixture was poured into water and
extracted with dichloromethane. The organic layer was dried over
MgSO and the solvent was removed under vacuum. Then, the
4
crude products were purified by silica gel column chromatography
using hexane/dichloromethane mixture as eluent afforded a
mixture of 2a and 2b. Finally, the mixture was recrystallized from
dichloromethane/ethanol (1:2) solution and the pure compound 2a
1
was obtained as white needle-like crystals. Yield: 1.7 g, 73%.
NMR (400 MHz, CDCl
2H), 8.03 (d, J ¼ 7.6 Hz, 2H), 7.59 (d, J ¼ 16 Hz, 8H), 1.48 (s, 18H).
H
Scheme 1. The strategy for new symmetrically pyrene-based photoelectric materials,
where arylamine attachment is diphenylamino (4a), 4-(diphenylamino)phenyl (4b)
and 4-(carbazol-9-yl)phenyl (4c).
3
)
d
8.25 (d, J ¼ 8.0 Hz, 2H), 8.21 (s, 2H), 8.13 (s,
13
C

108
R. Zhang et al. / Dyes and Pigments 130 (2016) 106e115
Scheme 2. Procedures for the syntheses of compounds 4ae4c.
1
.0
4a
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
4a
4b
4
4
b
c
4c
0.8
0.6
0.4
0.2
0.0
4
a
4b 4c
2
50
300
350
400
450
500
550
400
450
500
550
600
650
Wavelength (nm)
Wavelength (nm)
Fig. 1. Absorption spectra (left) and normalized emission spectra (right) of the compounds 4ae4c recorded in dichloromethane at 1 ꢂ 10ꢁ5 M concentration.
þ
NMR (101 MHz, CDCl
3
)
d
138.22 (s), 137.54 (s), 136.93 (s), 130.80 (s),
124.42 (s), 34.70 (s), 31.51 (s). MALDI TOF-MS: m/z 466.270 [M] .
1
30.47 (s), 129.07 (s), 128.38 (s), 127.72 (s), 127.35 (s), 125.36 (s),
Elemental analysis: anal. calcd for C36
C, 92.59; H, 7.37.
H34: C, 92.66; H, 7.34; Found:
þ
1
25.29 (s), 124.74 (s), 21.29 (s). MALDI TOF-MS: m/z 466.271 [M] .
Elemental analysis: anal. calcd for C36 34: C, 92.66; H, 7.34; Found:
C, 92.62; H, 7.38. The by-product 2b was obtained as white crystals.
H
1
2.3.2. Synthesis of 3,6-dibromo-1,8-bis(4-tert-butylphenyl)pyrene
3)
H NMR (400 MHz, CDCl
3
)
d
8.27 (d, J ¼ 9.2 Hz, 2H), 8.22 (d,
(
J ¼ 7.6 Hz, 2H), 8.04 (dd, J ¼ 15.7, 8.6 Hz, 4H), 7.62 (s, 8H), 1.48 (s,
13
2
Br (6.3 mmol, 1.13 g) in 15 mL DMF was added dropwise into a
solution of compound 2a (3 mmol, 1.40 g) in 15 mL DMF. After
stirring for 15 h at room temperature, water was added. The
18H). C NMR (101 MHz, CDCl
3
)
d
150.13 (s), 138.31 (s), 137.75 (s),
130.30 (s), 128.90 (s), 127.83 (s), 127.33 (s), 125.33 (t, J ¼ 6.3 Hz),

R. Zhang et al. / Dyes and Pigments 130 (2016) 106e115
109
Table 1
Physical parameters of the compounds 4ae4c.
a
a
c
b
c
ꢁ1
d/E
e
HOMO/LUMO(eV)f
g
ꢀ
Com.
l
abs (nm)
l
em (nm)/(
F
F
(%))
l
em (nm)/(
F
F
(%))
Stokes shift (cm
)
E
g
g
cal. (eV)
g d
T /T ( C)
4a
4b
4c
258, 300, 437
251, 301, 405
241, 294, 391
488 (67.43)
483 (87.45)
437 (95.43)
491 (29.31)
485 (56.47)
464 (73.87)
2349
3944
2692
2.50/2.99
2.78/3.10
2.96/3.23
ꢁ5.16/e2.66
ꢁ5.65/e2.87
ꢁ5.74/e2.81
183/409
189/500
156/514
a
b
c
Measured in dichloromethane.
Measured as drop-coated film.
Absolute quantum yield measured using integrating sphere.
d
e
f
Calculated from the absorption edge, E
Obtained from the quantum chemical calculation using TDDFT/B3LYP/6-31G (d,p).
HOMO values were deduced from the relation: HOMO ¼ Eox þ 4.8. LUMO values were calculated from the relation band LUMO ¼ HOMO e E
g
¼ 1240/
onset
l .
g
.
g
ꢀ
ꢁ1
2
Obtained from DSC/TGA measured at 10 C min under N .
precipitated solid was collected and used without further purifi-
2.3.4. Synthesis of 3,6-bis[4-(diphenylamino)phenyl]-1,8-bis(4-
tert-butylphenyl)pyrene (4b)
A mixture of the compound 3 (1.24 g, 2 mmol), 4-(diphenyla-
3 4
mino)phenylboronicacid (1.21 g, 4.2 mmol), Pd(PPh ) (46 mg,
0.04 mmol) and 2 M potassium carbonate (4.2 mL) in toluene
(40 mL) was heated to reflux for 24 h under nitrogen. After the
reaction finished, the mixture was poured into water and extracted
with dichloromethane. Then, the organic layer was dried over
1
3
cation. H NMR (600 MHz, CDCl ) d 8.59 (s, 2H), 8.32 (s, 2H), 8.13 (s,
1
3
2
d
1
3
H), 7.58e7.53 (m, 8H), 1.44 (s, 18H). C NMR (151 MHz, CDCl
150.81 (s), 138.95 (s), 136.61 (s), 132.24 (s), 130.19 (s), 129.01 (s),
28.26 (s), 127.39 (s), 126.04 (s), 125.54 (s), 125.43 (s), 34.72 (s),
3
)
þ
1.44 (s). MALDI TOF-MS: m/z 624.339 [M] . Elemental analysis:
anal. calcd for C36 : C, 69.24; H, 5.17; Found: C, 69.19; H, 5.19.
H32Br
2
4
MgSO and the solvent was removed under vacuum. Finally, the
crude products were purified by silica gel column chromatography
using hexane/dichloromethane mixture as eluent afforded 4b as a
green powder solid. And then the compounds was recrystallized
2
.3.3. Synthesis of 3,6-bis(N,N-diphenylamino)-1,8-bis(4-tert-
butylphenyl)pyrene (4a)
Under the atmosphere of nitrogen, a mixture of compound 3
1
from dichloromethane/ethanol (1:1) mixture. Yield: 1.69 g, 89%. H
(
1.24 g, 2 mmol), diphenylamine (0.64 g, 3.8 mmol), bis(dibenzy-
3
NMR (600 MHz, CDCl ) d 8.33 (s, 2H), 8.23 (s, 2H), 8.06 (s, 2H), 7.65
lideneacetone)palladium (Pd(dba)
2
) (50 mg, 0.08 mmol), P(t-Bu)
3
(
d, J ¼ 8.4 Hz, 4H), 7.58 (dd, J ¼ 8.4, 2.4 Hz, 8H), 7.34 (t, J ¼ 7.9 Hz,
(
(
0.1 g/mL in toluene, 0.24 mL, 0.12 mmol), sodium tert-butoxide
0.38 g, 4 mmol) and toluene (50 mL) were heated at 80 C for 8 h.
13
ꢀ
8H), 7.27e7.24 (m, 12H), 7.09 (t, J ¼ 7.3 Hz, 4H). 1.45 (s, 18H).
NMR (151 MHz, CDCl 150.17 (s), 147.78 (s), 147.06 (s), 138.17 (s),
37.13 (s), 136.74 (s), 135.18 (s), 131.48 (s), 130.34 (s), 129.54 (s),
29.35 (s), 129.06 (s), 128.25 (s), 128.03 (d, J ¼ 8.3 Hz), 126.16 (s),
25.32e125.23 (m), 124.50 (s), 123.49 (s), 122.98 (s), 34.68 (s), 31.49
C
3
) d
After the reaction finished, the mixture was poured into water and
extracted with dichloromethane. Then, the organic layer was dried
over MgSO and the solvent was removed under vacuum. The crude
4
products were purified by silica gel column chromatography using
hexane/dichloromethane mixture as eluent. The product 4a was
1
1
1
þ
(s). MALDI TOF-MS: m/z 952.474 [M] . Elemental analysis: anal.
1
calcd for C72H60N : C, 90.72; H, 6.34; N, 2.94. Found: C, 90.68; H,
obtained as a yellow powder solid. Yield: 1.29 g, 81%. H NMR
400 MHz, CDCl 8.18 (s, 2H), 8.10 (s, 2H), 7.85 (s, 2H), 7.54 (s, 8H),
.22e7.20 (m, 8H), 7.09 (d, J ¼ 8.0 Hz, 8H), 6.94 (t, J ¼ 7.2 Hz, 4H),
.42 (s, 18H). 13C NMR (101 MHz, CDCl
148.50 (s), 140.50 (d,
J ¼ 10.9 Hz), 138.99 (s), 130.53 (s), 129.70 (s), 129.17 (s), 128.36 (s),
28.08 (s), 127.43e127.11 (m), 124.95 (s), 123.76 (s), 122.07 (s),
2
6.39, N, 2.92.
(
7
1
3
) d
3
)
d
2.3.5. Synthesis of 3,6-bis[4-(carbazol-9-yl)phenyl]-1,8-bis(4-tert-
butylphenyl)pyrene (4c)
The compound was synthesized with the same method as for
compound 4b by using (4-(9H-carbazol-9-yl)phenyl)boronic acid
(1.21 g, 4.2 mmol). Accordingly, a yellow solid was obtained and
1
þ
121.79 (s). MALDI TOF-MS: m/z 800.574 [M] . Elemental analysis:
anal. calcd for C60
H, 6.59; N, 3.43.
52 2
H N : C, 89.96; H, 6.54; N, 3.50; Found: C, 89.89;
then recrystallized from dichloromethane/ethanol (1:1) mixture.
1
Yield: 1.61 g, 85%. H NMR (600 MHz, CDCl
3
)
d
8.44 (s, 2H), 8.34 (s,
2
4
4
d
1
1
H), 8.23e8.20 (m, 6H), 7.97 (d, J ¼ 8.4 Hz, 4H), 7.80 (d, J ¼ 8.3 Hz,
H), 7.71 (d, J ¼ 8.2 Hz, 4H), 7.64e7.61 (m, 8H), 7.50 (t, J ¼ 7.6 Hz,
1.0
0.8
0.6
0.4
0.2
0.0
4a
4b
4c
13
H), 7.36 (t, J ¼ 7.4 Hz, 4H), 1.48 (s, 18H). C NMR (151 MHz, CDCl
150.44 (s), 140.91 (s), 140.29 (s), 137.95 (s), 137.57 (s), 136.92 (s),
36.29 (s), 132.14 (s), 130.39 (s), 129.70 (s), 129.07 (s), 128.54 (s),
28.26 (s), 128.06 (s), 126.95 (s), 126.14 (s), 126.05 (s), 125.66 (s),
25.39 (s), 125.34 (d, J ¼ 2.8 Hz), 123.51 (s), 120.41 (s), 120.07 (s),
3
)
1
þ
109.91 (s), 31.50 (s). MALDI TOF-MS: m/z 948.444 [M] . Elemental
analysis: anal. calcd for C72
C, 91.05; H, 5.89, N, 3.03.
56 2
H N : C, 91.10; H, 5.95; N, 2.95. Found:
3
. Results and discussion
3.1. Synthesis
The synthetic routes of the intermediates and target compounds
are shown in Scheme 2 using pyrene as the starting material. Firstly,
a mixture of 1,8-dibromopyrene (1a, 85%) and 1,6-dibromopyrene
4
00
450
500
550
600
650
Wavelength (nm)
(1b, 15%) was obtained from pyrene after six times recrystalliza-
Fig. 2. Normalized emission spectra of the compounds 4ae4c recorded as thin films.
tion. Even though, we still start the experiments using the mixture

110
R. Zhang et al. / Dyes and Pigments 130 (2016) 106e115
Table 2
Absorption and emission properties of compounds 4ae4c in different solvents.
l
max (nm)/
em (nm)/Stokes shift (cmꢁ1)
Com.
l
TOL
DCM
THF
CHL
MeCN
DMF
DMSO
4
4
4
a
b
c
441/480/1842
405/460/2952
391/437/2692
437/488/2391
405/483/3987
391/437/2692
438/484/2170
401/473/3796
387/438/3009
437/483/2179
402/470/3590
386/437/3023
433/494/2852
398/506/5362
389/435/2718
437/497/2763
405/508/5006
394/440/2653
438/500/2831
409/511/4880
395/443/3742
1
1
1
0
0
0
0
0
.4
1
.0
.8
TOL
DCM
THF
TOL
DCM
THF
.2
.0
.8
.6
.4
.2
.0
0
CHL
CHL
MeCN
DMF
DMSO
MeCN
DMF
DMSO
0.6
0
.4
.2
0
0.0
300
350
400
450
500
550
450
500
550
600
Wavelength (nm)
Wavelength (nm)
Fig. 3. UVevis absorption (left) and Emission spectra (right) of the compound 4a recorded in different solvents at 1 ꢂ 10ꢁ5 concentration.
0
0
0
0
0
.4
.3
.2
.1
.0
1
.0
TOL
DCM
THF
TOL
DCM
THF
0.8
CHL
CHL
MeCN
DMF
DMSO
MeCN
DMF
DMSO
0.6
0.4
0.2
0.0
300
350
400
450
500
550
450
500
550
600
650
700
Wavelength (nm)
Wavelength (nm)
Fig. 4. UVevis absorption (left) and Emission spectra (right) of the compound 4b recorded in different solvents at 1 ꢂ 10ꢁ concentration.
5
1
.0
TOL
DCM
THF
TOL
DCM
THF
0.6
0.4
0.2
0.0
CHL
0.8
0.6
CHL
MeCN
DMF
DMSO
MeCN
DMF
DMSO
0
0
.4
.2
0.0
300
350
400
450
500
400
450
500
550
Wavelength (nm)
Wavelength (nm)
Fig. 5. UVevis absorption (left) and Emission spectra (right) of the compound 4c recorded in different solvents at 1 ꢂ 10ꢁ5 concentration.
because it is possible to obtain the final pure 1,8-disubstituted
pyrene-based compounds in the flowing steps based on their
obvious difference in solubility. Secondly, the mixture of dibro-
mopyrenes were transformed to
a mixture of 1,8-(4-tert-

R. Zhang et al. / Dyes and Pigments 130 (2016) 106e115
111
more effectively reduces the
p
-conjugation of the molecule. In
MeCN
DMSO
5
5
4
4
3
3
2
2
500
000
500
000
500
000
500
000
4
4
4
4
4
a
b
c
a
b
DMF
addition, compounds 4ae4c show small Stokes shifts
ꢁ
1
(2349e3944 cm in dichloromethane), indicating less energy loss
during the relaxation process and thereby ensuring efficient fluo-
rescence. As shown in Fig. 2, the emission spectra of 4a and 4b in
thin films exhibit a small bathochromic shift (3 nm and 4 nm)
relative to those in dichloromethane solution, which indicates that
the two compounds exhibit a very similar conformation in both
states [11]. While compound 4c shows an obvious red-shift
DCM
THF
4c
CHL
TOL
(
27 nm), suggesting that there exists slightly strong intermolec-
ular interactions in film state. The absence of characteristic
pyrene excimer peak in these PL spectra evidences that the co-
planarity and -conjugation of the compounds have been sup-
pep
p
pressed to some extent by the twist between the functionalized
moieties and pyrene core.
The absolute fluorescence quantum yields
ae4c in dichloromethane and in film state were determined using
an integrating sphere. These compounds show high s in
dichloromethane with the range from 67.43 to 95.43%. The s of
F
F s of compounds
32
34
36
38
40
E (30)
42
44
46
4
F
F
T
F
F
compounds 4ae4c in film states (29.31e73.87%) show a decreasing
Fig. 6. Correlation between the Stokes shift and solvent parameter E
T
(30) for com-
pounds 4ae4c.
tendency compared to those in dichloromethane due to the inter-
molecular
an obviously lower
p
e
p
interactions in film states. Compound 4a exhibits
in film state than those of 4b and 4c, which
F
F
butylphenyl)pyrene (2a) and 1,6-(4-tert-butylphenyl)pyrene (2b)
through Suzuki coupling reaction. There is a significant difference
of solubility in dichloromethane and ethanol (1:2) solution for 2a
and 2b, and therefore the pure key intermediate 2a was relatively
readily obtained. Thirdly, compound 2a was brominated with liquid
bromine, yielding the another key intermediate 3,6-dibromo-1,8-
bis(4-tert-butylphenyl)pyrene (3) in a very high yield of 95%,
which was used directly without further purification. Finally, the
target products 4ae4c were prepared by BuchwaldeHartwig and
Suzuki coupling reaction between the compound 3 and the corre-
sponding arylamine or aryl boronic acid with yields of 80e90%.
These compounds show excellent solubility in organic solvents,
which makes them promising candidates for solution-processed
OLEDs.
indicates that the structure of diphenylamine directly attached on
the pyrene backbone has a relatively weak effect on interrupt the
pep stacking of the molecule. The F s of compounds 4b (56.47%)
F
and 4c (73.87%) in film states are very high, which makes them
having great potential application in solid state luminescence.
To better understand the effect of environments on the elec-
tronic spectroscopic of the compounds 4ae4c, we studied the ab-
sorption and emission spectra of the three compounds in different
polar solvents such as toluene (TOL), dichloromethane (DCM),
tetrahydrofuran (THF), chloroform (CHL), acetonitrile (MeCN), N,N-
dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). The
pertinent data are summarized in Table 2. The changes in absorp-
tion and emission profiles for compounds 4ae4c with different
solvent polarities are illustrated in Figs. 3e5. In the absorption
spectra, compounds 4ae4c are insensitive to the solvent polarity
with the shift below 11 nm. In the emission spectra, the emission
maximum of compound 4c has no significant change with the
different solvent polarities, suggesting a small transition dipole
moment [21,28]. However, for compounds 4a and 4b, the solvent
dependence in the emission spectra is remarkable, with large
bathochromic shifts of 20 nm for 4a, 51 nm for 4b, which indicates
that their excited state are more stable in polar solvent probably
due to the separation of charges in the higher energy state [29]. The
Stokes shifts of compounds 4ae4c in different solvents were
calculated to know the structural reorganization occurring during
electronic excitation. Stokes shift values in different solvents for the
compounds follow the order 4c < 4a < 4b. Larger Stokes shift
observed for 4b indicates appreciable structural reorganization due
to photo-excitation from ground state to excited state [30].
Furthermore, the relationships of Stokes shift against the solvent
3.2. Photophysical properties
The UVevis absorption and photoluminescence (PL) spectra of
compounds 4ae4c in dichloromethane are shown in Fig. 1. The
detailed photophysical data are presented in Table 1. The three
compounds feature the longest wavelength absorptions at ca.
3
91e437 nm attributed to the charge transfer from amine to pyrene
and the absorptions at ca. 294e301 nm ascribed to pyrene localized
* transitions. The absorption spectrum of compound 4a ex-
pep
hibits a relative large red-shift compared with those of 4b and 4c,
suggesting that the pyrene core directly attached by diphenylamine
unit can increase the
p-electron conjugation of the molecule. The
energy gaps (E ) of compounds 4a, 4b and 4c estimated from the
g
onset of the absorption spectra are 2.50, 2.78 and 2.96 eV, respec-
tively, indicating that the different arylamines attached to the
pyrene core can effectively tune the
p
-conjugation of the com-
T
parameter E (30) of the compounds 4ae4c were investigated as
pounds. The PL spectra of compounds 4a, 4b and 4c in dichloro-
methane display emission peak in green, bluish-green and deep
blue region, respectively. It indicates that the introduction of
different arylamines into the pyrene core has obvious influence on
the fluorescence properties of the compounds. Compound 4b ex-
hibits a slightly blue-shift compared with that of 4a, which dem-
onstrates that the incorporation of phenyl ring between the
diphenylamine and the pyrene core increases the molecule twist
shown in Fig. 6. The slopes of the correlation plots are in the order
of 4c (27.90) < 4a (90.77) < 4b (196.64), which indicates that more
pronounced intramolecular charge transfer is occurred in com-
pound 4b.
3.3. Theoretical calculation
To deeply investigate the electronic structures, orbital energies,
electron densities of the highest occupied molecular orbital
(HOMO) and lowest occupied molecular orbital (LUMO) states of
the compounds 4ae4c, as well as to understand the absorption
and reduces the
p-conjugation. The PL spectrum of compound 4c
shows obvious blue-shift compared with those of 4a and 4b, which
suggests that the more rigid and planar conformation of carbazole

112
R. Zhang et al. / Dyes and Pigments 130 (2016) 106e115
Fig. 7. Electronic distributions observed for the frontier orbitals for the compounds 4ae4c.
characteristics, density functional theory (DFT) calculations (B3LYP/
-31G (d, p) basis set) were performed on the three compounds
with the Gaussian 09W software package [25]. Three-dimensional
optimized geometries and electronic distributions in the frontier
molecular orbitals of compounds 4ae4c are shown in Fig. 7 and the
electron-rich pyrene core. It suggests that the substituent of 3,6-bis
[4-(carbazol-9-yl)phenyl] on pyrene ring has a little effect on the
HOMO of the molecule. Moreover, the TDDFT calculations of com-
pounds 4ae4c have been carried out and the predicted vertical
transitions and their assignments are summarized in Table S1. The
6
HOMOeLUMO energy gaps (E
g
cal.) were calculated and are pre-
results reveal that the S
1 0 1
states (S /S transition) of compounds
sented in Table 1. The calculated structures of compounds 4ae4c
reveal that all molecules adopt non-coplanar conformations. The
planar pyrene twists by the substituents on the 1-, 3-, 6-, 8- posi-
tion of the pyrene ring. Such structural characteristic is beneficial to
OLED application by suppressing the undesirable intermolecular
4ae4c are dominated by the HOMO/LUMO transition.
3.4. Electrochemical and thermal studies
The electrochemical properties of compounds 4ae4c were
pe
p
interactions and the crystallization hindrance to build a uni-
form amorphous morphology in the film state. The -electrons in
the HOMOs of compounds 4a and 4b are mainly distributed on the
,6-bis(N,N-diphenylamino)pyrene and 3,6-bis[4-di(phenylamino)
studied by cyclic voltammetry (CV) measurements and the onset
oxidation potentials of the compounds were calculated using
ferrocene as standard. The data of HOMO, LUMO energy levels and
p
3
g
band gap energy (E ) are summarized in Table 1. The HOMO values
phenyl]pyrene moieties, respectively. For compound 4c, the HOMO
orbital is mainly contributed by the pyrene ring and a little by the
of compounds 4a, 4b and 4c estimated from the onset potentials of
oxidation peak are ꢁ5.16 eV, e5.65 eV and ꢁ5.74 eV, respectively.
3
,6-bis[4-(carbazol-9-yl)phenyl] unit. In the LUMOs of the com-
The LUMO values of these compounds calculated by HOMO and E
are in the range from ꢁ2.66 to ꢁ2.87 eV.
g
pounds 4ae4c, the excited electrons are localized primarily on the

R. Zhang et al. / Dyes and Pigments 130 (2016) 106e115
113
the materials used in these devices are shown in Fig. 8 and the
essential performance electric parameters are demonstrated in
Table 3. These data indicate that the hole-injection into the emit-
ting layer of 4c-based devices (devices I and II) were less effective
than 4b-based devices (devices III and IV) because of the larger
barrier between the PEDOT:PSS (ꢁ5.2 eV) and 4c (ꢁ5.74 eV) than
Emitters
Tickness
nm)
4
0
50
4
0 or 50
(
-2
-3
-4
-5
-6
-7
-2.2
-
2.7
-
2.81
-
2.87
4
b (ꢁ5.65 eV). The effective of electron-injection into the emitting
layer of 4c- and 4b-based devices are almost equal due to the
similar electron-injection barrier between TmPyPB (ꢁ2.7 eV) and
4
c (ꢁ2.81 eV), 4b (ꢁ2.87 eV), respectively.
-
4.1
The current densityevoltageeluminance (IeVeL) characteris-
tics and the current densityeefficiency curves of the devices IeIV
are shown in Fig. 9. The turn-on voltages of the devices I, II, III and
IV are very low with the values of 2.9, 2.9, 4.5 and 4.1 eV, respec-
tively, which indicates that the performance of the devices is
decent. Devices I and II show relatively lower turn-on voltages than
those of devices III and IV, which may be ascribed to the better
charge carrier transporting ability of compound 4b. The device II
using 4b as emitting material shows the best device performance
LiF/Al
-
4.8
ITO
-
5.2
-
5.65
-5.74
2
with a maximum brightness (Lmax) of 8741 cd/m , a maximum
-6.7
luminance efficiency (hmax) of 4.03 cd/A, a maximum power effi-
ciency (PE) of 2.89 lm/W. The 4c-based device with the emitting
Fig. 8. Structure of the non-doped devices and energy levels of materials 4b and 4c.
Table 3
EL performance: ITO/PEDOT: PSS (40 nm)/EML (40 nm and 50 nm)/TmPyPB (50 nm)/LiF (1 nm)/Al (150 nm).
a
b
FWHMc
d
ꢁ2
e
f
Devices
Emitters
V
on (V)
ELmax (nm)
L
max at voltage (cd m /V)
h
max (cd/A)
PE (lm/W)
CIE (x,y)
I
II
III
IV
4b (50 nm)
4b (40 nm)
4c (50 nm)
4c (40 nm)
2.9
2.9
4.5
4.1
480
480
465
464
63
54
90
88
7878/8.6
8741/8.0
2438/11.0
3550/9.6
4.14
4.03
1.89
2.41
2.86
2.89
0.95
1.34
(0.153, 0.328)
(0.147, 0.294)
(0.174, 0.258)
(0.163, 0.246)
a
ꢁ2
.
Turn-on voltage (V) at a luminance of 1 cd m
Emission maximum.
b
c
d
e
f
Full-width at half maximum.
ꢁ2
Maximum luminance (cd m ) at the applied voltage (V).
ꢁ1
Luminance efficiency (cd A ).
Maximum power efficiency.
The thermal properties of the compounds 4ae4c were investi-
layer thickness of 40 nm (device IV) exhibits a relatively lower
device performance with a Lmax of 3550 cd/m , a hmax of 2.41 cd/A, a
2
gated by thermal gravimetric analysis (TGA) and differential scan-
ning calorimetry (DSC) under a nitrogen atmosphere, the relevant
data are shown in Table 1 and Fig. S1. These compounds show high
PE of 1.34 lm/W. Generally, the EL efficiencies of 4c-based devices
are lower than those of 4b-based devices, which may derive from
the higher hole-injection barrier of 4c (ꢁ0.54 eV) than that of 4b
(ꢁ0.45 eV). From the EL spectra shown in Fig. 9(b), the devices I and
thermal stability and the decomposition temperatures (T
sponding to 5% weight loss are 409, 500 and 515 C for compounds
d
) corre-
ꢀ
4
a, 4b and 4c, respectively. DSC thermograms of the three com-
II emit a strong bluish-green emission with lEL all peaked at 480 nm
pounds display remarkably high glass transition temperature (T
g
)
and the corresponding CIE coordinates are (0.153, 0.328) and (0.147,
0.294), respectively. While the devices III and IV exhibit a sky-blue
emission with maximum emission peaks at 465 nm, 464 nm and
the CIE coordinates of the devices are (0.174, 0.258) and (0.163,
0.246), respectively. The electroluminescence (EL) spectra of all
devices match with their corresponding PL spectra, indicating that
the EL purely originate from the emitting layers. Furthermore, it is
worth to note that there is almost no emission shoulder at the
longer wavelength and no significant change in the EL spectra upon
varying driving voltages for all devices, which indicates that the
holeeelectron pairs for recombination are well confined in the
emitter layer.
ꢀ
values at 183, 189 and 156 C, respectively, and no crystallization
and melting peaks being observed at higher temperatures, indi-
cating an excellent amorphous glass state stability [31]. The high
thermal and morphological stabilities of these compounds are ad-
vantageous for the construction of stable and long-life OLED
devices.
3.5. Electroluminescence
Two representative compounds 4b and 4c were employed as the
non-doped emitting layers (EMLs) in OLEDs with the following
configuration: ITO/PEDOT: PSS (40 nm)/EML (40 nm and 50 nm)/
TmPyPB (50 nm)/LiF (1 nm)/Al (150 nm). Poly(3,4-
ethylenedioxythiophene):ploy(4-styrenesulfonate) (PEDOT: PSS)
act as the hole injection layer, 1,3,5-tri[(3-pyridyl)-phen-3-yl]ben-
zene (TmPyPB) is used as an electron-transporting layer, the ma-
terials 4b and 4c are conducted as emitters, respectively. The
emitting materials were spin-coated with two different thickness
from its fresh THF solution. The structures and the energy levels of
4. Conclusions
In conclusion, a series of short axially symmetrically 1,3,6,8-
tetrasubstituted pyrene-based compounds (4ae4c) with 4-tert-
butylphenyl and hole-transporting arylamine attachments have
been elaborately designed and successfully synthesized. The planar
pyrene twists by the substituents on the pyrene ring, which can

114
R. Zhang et al. / Dyes and Pigments 130 (2016) 106e115
1
0
0
0
0
.0
.8
.6
.4
.2
4
b (50 nm)
10
(
a)
8
6
4
2
0
00
00
00
00
I 4b (50 nm)
4b (40 nm)
4c (50 nm)
4c (40 nm)
(
b)
II 4b (40 nm)
III 4c (50 nm)
IV 4c (40 nm)
1
0
0
1
10
10
0.0
2
3
4
5
6
7
8
9
10
11
450
500
550
600
650
700
Voltage (V)
Wavelength (nm)
5
3
2
1
0
I
4b (50 nm)
I 4b (50 nm)
II 4b (40 nm) (c)
(d)
II 4b (40 nm)
III 4c (50 nm)
IV 4c (40 nm)
4
3
2
1
0
III 4c (50 nm)
IV 4c (40 nm)
0
50
100
150
200
250
300
350
0
50
100
150
200
250
Current Density (mA/cm )
Current Density (mA/cm )
Fig. 9. The current densityevoltageeluminance curves with varying thickness of the compounds 4b and 4c (a); The electroluminescent spectrum of the compounds 4b and 4c (b);
Luminance efficiency of the compounds 4b and 4c (c); Power efficiency of the compounds 4b and 4c (d).
effectively suppress the intermolecular
p
e
p
interactions of the
[3] Tao YT, Yang CL, Qin JG. Organic host materials for phosphorescent organic
light-emitting diodes. Chem Soc Rev 2011;40:2943e70.
molecules. Incorporation of different arylamine units into the pyr-
ene core can tune the electronic structures of these compounds and
then influences their photoelectric properties. All the three com-
pounds exhibit high PL absolute quantum efficiencies both in
dichloromethane (67.43e95.43%) and film states (29.31e73.87%).
The three compounds are also thermally and morphologically sta-
[4] Sasabe H, Kido JJ. Development of high performance OLEDs for general
lighting. J Mater Chem C 2013;1:1699e707.
[5] Yang XL, Xu XB, Zhou GJ. Recent advances of the emitters for high perfor-
mance deep-blue organic light-emitting diodes. J Mater Chem C 2015;3:
913e44.
[
6] Huang J, Tang RL, Zhang T, Li QQ, Yu G, Xie SY, et al. A new approach to
prepare efficient blue AIE emitters for undoped OLEDs. Chem Eur J 2014;20:
5317e26.
ꢀ
ꢀ
ble, showing high T (409e514 C) and T values (156e189 C).
d
g
[7] Hong YN, Lam JWY, Tang BZ. Aggregation-induced emission. Chem Soc Rev
These compounds with a symmetrical structure show good solu-
bility and film-forming abilities, which is beneficial to form ho-
mogeneous and smooth films for OLED device by conventional
solution processing techniques. Solution processed double-layer
OLEDs using two representative compounds 4b and 4c as non-
doped emitting layer were fabricated and they exhibit good de-
vice performance. More short axially symmetrically as well as the
long axially symmetrically pyrene-based compounds with the
excellent properties are carrying out in our lab.
2
011;40:5361e88.
[8] Figueira-Duarte TM, Müllen K. Pyrene-based materials for organic electronics.
Chem Rev 2011;111:7260e314.
[
9] Wang ZQ, Xu C, Wang WZ, Dong XM, Zhao BT, Ji BM. Novel pyrene derivatives:
synthesis, properties and highly efficient non-doped deep-blue electrolumi-
nescent device. Dyes Pigments 2012;92:732e6.
[
10] Zhang ZM, Zhao Y, Zhang R, Zhang LF, Cheng WQ, Ni ZH. Design and synthesis
of new series of tetra(polycyclic aryl)ethenes: achieving aggregation-
induced emission and efficient solid-state photoluminescence. Dyes Pig-
ments 2015;118:95e101.
a
[11] Hu JY, Ni XL, Feng X, Era M, Elsegood MRJ, Teat SJ, et al. Highly emissive hand-
shaped -conjugated alkynylpyrenes: synthesis, structures, and photo-
p
physical properties. Org Biomol Chem 2012;10:2255e62.
[12] Feng X, Hu JY, Tomiyasu H, Seto N, Redshaw C, Elsegood MRJ, et al. Synthesis
and photophysical properties of novel butterfly-shaped blue emitters based
on pyrene. Org Biomol Chem 2013;11:8366e74.
Acknowledgements
This work was supported by the Fundamental Research Funds
for the Central Universities (2015XKZD08) and the Priority Aca-
demic Program Development of Jiangsu Higher Education
Institutions.
[
[
[
13] Feng X, Hu JY, Yi L, Seto N, Tao Z, Redshaw C, et al. Pyrene-based Y-shaped
solid-state blue emitters: synthesis, characterization, and photoluminescence.
Chem Asian J 2012;7:2854e63.
14] Feng X, Hu JY, Iwanaga F, Seto N, Redshaw C, Elsegood MRJ, et al. Blue-
emitting butterfly-shaped 1,3,5,9-tetraarylpyrenes: synthesis, crystal struc-
tures, and photophysical properties. Org Lett 2013;15:1318e21.
15] Figueira-Duarte TM, Simon SC, Wagner M, Druzhinin SI, Zachariasse KA,
Mullen K. Polypyrene dendrimers. Angew Chem Int Ed Engl 2008;47:
Appendix A. Supplementary data
1
0175e8.
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2016.03.020.
[16] Liang ZQ, Li YX, Yang JX, Ren Y, Tao XT. Suppression of aggregation-induced
fluorescence quenching in pyrene derivatives: photophysical properties and
crystal structures. Tetrahedron Lett 2011;52:1329e33.
[
17] Zhao ZJ, Chen SM, Lam JWY, Lu P, Zhong YC, Wong KS, et al. Creation of highly
efficient solid emitter by decorating pyrene core with AIE-active tetraphe-
nylethene peripheries. Chem Commun 2010;46:2221e3.
References
[
18] Keawin T, Prachumrak N, Namuangruk S, Pansay S, Kungwan N, Maensiri S,
et al. Efficient bifunctional materials based on pyrene- and triphenylamine-
functionalized dendrimers for electroluminescent devices. RSC Adv 2015;5:
[
[
1] Tang CW, VanSlyke SA. Organic electroluminescent diodes. Appl Phys Lett
987;51:913e5.
2] Farinola GM, Ragni R. Electroluminescent materials for white organic light
1
73481e9.
emitting diodes. Chem Soc Rev 2011;40:3467e34682.

R. Zhang et al. / Dyes and Pigments 130 (2016) 106e115
115
[
[
19] Lee H, Kim B, Kim S, Kim J, Lee J, Shin H, et al. Synthesis and electrolumi-
nescence properties of highly efficient dual core chromophores with side
groups for blue emission. J Mater Chem C 2014;2:4737e47.
20] Kotchapradist P, Prachumrak N, Tarsang R, Jungsuttiwong S, Keawin T,
Sudyoadsuk T, et al. Pyrene-functionalized carbazole derivatives as non-
doped blue emitters for highly efficient blue organic light-emitting diodes.
J Mater Chem C 2013;1:4916e24.
[25] Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA, Cheeseman JR.
Gaussian 09, revision A.02. Wallingford CT: Gaussian, Inc.; 2009.
[26] Wang SM, Wang XD, Yao B, Zhang BZ, Ding JQ, Xie ZY, et al. Solution-pro-
cessed phosphorescent organic light-emitting diodes with ultralow driving
voltage and very high power efficiency. Sci Rep 2015. http://dx.doi.org/
10.1038/srep12487.
[27] Grimshaw J, Trocha-Grimshaw J. Characterisation of 1,6- and 1,8-
dibromopyrenes. J Chem Soc Perkin Trans 1972;1:1622e3.
[28] Reichardt C, Welton T. Solvent effects in organic chemistry. Wiley-VCH Verlag
GmbH & Co KGaA; 1988. p. 359e424.
[29] Kapoor N, Thomas KRJ. Fluoranthene-based triarylamines as hole-
transporting and emitting materials for efficient electroluminescent devices.
New J Chem 2010;34:2739e48.
[30] Karthik D, Thomas KRJ, Jou JH, Kumar S, Chen YL, Jou YC. Deep-blue emitting
pyrene-benzimidazole conjugates for solution processed organic light emit-
ting diodes. RSC Adv 2015;5:8727e38.
[
21] Chercka D, Yoo SJ, Baumgarten M, Kim JJ, Müllen K. Pyrene based materials for
exceptionally deep blue OLEDs. J Mater Chem C 2014;2:9083e6.
22] Zhang Y, Ng TW, Lu F, Tong QX, Lai SL, Chan MY, et al. A pyrene-phenan-
throimidazole derivative for non-doped blue organic light-emitting devices.
Dyes Pigments 2013;198:190e4.
[
[
[
23] Wee KR, Ahn HC, Son HJ, Han WS, Kim JE, Cho DW, et al. Emission color tuning
and deep blue dopant materials based on 1,6-bis(N-phenyl-p-(R)-phenyl-
amino)pyrene. J Org Chem 2009;74:8472e5.
24] Niko Y, Kawauchi S, Otsu S, Tokumaru K, Konishi GI. Fluorescence enhance-
ment of pyrene chromophores induced by alkyl groups through
s
-
p
conju-
[31] Thangthong A, Prachumrak N, Tarsang R, Keawin T, Jungsuttiwong S,
Sudyoadsuk T, et al. Blue light-emitting and hole-transporting materials based
on 9,9-bis(4-diphenylaminophenyl)fluorenes for efficient electroluminescent
devices. J Mater Chem 2012;22:6869e77.
gation: systematic synthesis of primary, secondary, and tertiary alkylated
pyrenes at the 1, 3, 6, and 8 positions and their photophysical properties. J Org
Chem 2013;78:3196e207.