Two Symmetrically Bis-substituted Pyrene Derivatives: Synthesis, Photoluminescence, and Electroluminescence

Article abstract of DOI:10.1002/cjoc.201500385

Two kinds of cyanophenyl terminated pyrene derivatives for organic light-emitting diodes were synthesized and characterized by UV/Vis, fluorescence (FL), ¹H NMR, MALDI-TOF, CV and TGA. Both compounds exhibited blue photoluminescence and high fluorescent quantum yield of 85% and 75% in solutions. Due to the presence of acetylene spacer, the compound distinguishes itself by high coplanarity, high thermal stability, little Stokes' shift and clear excimer formation in the solid state from the acetylene-free compound. In order to suppress the molecular aggregation, the electroluminescent properties were studied by doing the materials in PVK. The result proved that energy transfer happened from the host PVK to the materials. Two kinds of cyanophenyl terminated pyrene derivatives for OLEDs were synthesized and characterized. Both compounds exhibited blue photoluminescence and high fluorescent quantum yield of 85% and 75% in solutions. The result proved that energy transfer happened from the host PVK to the materials.

Full text of DOI:10.1002/cjoc.201500385

FULL PAPER
DOI: 10.1002/cjoc.201500385
Two Symmetrically Bis-substituted Pyrene Derivatives:
Synthesis, Photoluminescence, and Electroluminescence
Xiaojie Gong,a Xiang Xie,b Naiwu Chen,a Chaoyue Zheng,a Jie Zhu,a
Runfeng Chen,*,b Wei Huang,a,b and Deqing Gao*,a
a Key Laboratory of Flexible Electronics & Institute of Advanced Materials, Jiangsu National Synergistic
Innovation Center for Advanced Materials, Nanjing Tech University, Nanjing, Jiangsu 211816, China
b Key Laboratory for Organic Electronics & Information Displays and Institute of Advanced Materials,
Nanjing University of Posts and Telecommunications, Nanjing, Jiangsu 210003, China
Two kinds of cyanophenyl terminated pyrene derivatives for organic light-emitting diodes were synthesized and
1
characterized by UV/Vis, fluorescence (FL), H NMR, MALDI-TOF, CV and TGA. Both compounds exhibited
blue photoluminescence and high fluorescent quantum yield of 85% and 75% in solutions. Due to the presence of
acetylene spacer, the compound distinguishes itself by high coplanarity, high thermal stability, little Stokes’ shift
and clear excimer formation in the solid state from the acetylene-free compound. In order to suppress the molecular
aggregation, the electroluminescent properties were studied by doing the materials in PVK. The result proved that
energy transfer happened from the host PVK to the materials.
Keywords pyrene, organic light-emitting diode, aggregation, energy level, doping
Introduction
arrange pyrene units orthogonal to each other and got
current efficiency of 10.5 cd/A and maximum external
quantum efficiency of 6.4% of a blue non-doped OLED.
A few reports are available by doping pyrene deriva-
tives in a host material to avoid the aggregation until
now.[17-19] Wee et al.[17] reported a series of 1- and
6-position bis-substituted pyrene derivatives as the
dopants; among them, the compound with strong elec-
tron-withdrawing CN group presented the best elec-
troluminescent property with 9,10-di(2-naphthyl)anth-
racene as the host. Thomas et al.[19] developed pyrene-
fluorene chromophores with an acetylene linkage and
blue or yellow electroluminescence was obtained with
4,4-bis(9H-carbazole-9-yl)biphenyl (CBP) as the host.
Introduction of acetylene unit as spacer between
polyaromatic hydrocarbons (PAH) helps to extend the
conjugation system and acquire high planarity. Both
acetylene bridged tetra-substituted pyrene-fluorene and
pyrene-pheyl compounds presented attractive optical
properties.[19,20] Feng et al. reported a serial of pyrene-
based Y-shaped blue emitters[21] and 1,3,5,9-tetracyano-
phenyl-7-tert-butylpyrene,[22] and found that both CN
substituted derivatives had high thermal stability and
excellent solid-state fluorescence quantum yield.
Organic light-emitting diodes (OLEDs) have been
widely used in display such as mobile phones and digi-
tal cameras since the first report by Tang et al.[1] They
have attracted increasing industrial and academic inter-
est owing to their high technological potential for next
generation full-color flat panel displays and solid-state
lighting.[2-4]
Among blue light-emitting materials, pyrene deriva-
tives have rich electronic and photophysical properties
and have been exploited as organic light-emitting and
semiconductor materials.[5-11] The optical feature of the
pyrene-based materials is that fluorescence quantum
efficiency is high in dilute solution but much lower in
condensed state due to heavy aggregation and excimer
formation from planer molecular configuration. For
example, 1,3,6,8-tetrakis(3,5-dimethylphenyl)pyrene has
fluorescence quantum yield of 86% in cyclohexane but
just 20% in film.[12] The well studied 1,3,6,8-tetraphe-
nylpyrene has high fluorescence quantum yield of 90%
in solution, but it showed poor OLED performance with
maximum external quantum efficiency of 0.5%.[6,13-15]
One strategy to suppress the aggregation is to design
twisted molecular conformation.[6,14,16] Moorthy et al.[6]
inhibited aggregation by introducing multiple methyl
and methoxyl groups on 1,3,6,8-tetraarylpyrenes mole-
cule. Chan et al.[16] used 1,3-alt-calix[4]arene scaffold to
On the base of above analysis of literatures, we want
to develop thermally stable and highly fluorescent effi-
cient blue-emitting materials. Here we report the syn-
*
E-mail: iamdqgao@njtech.edu.cn, iamrfchen@njupt.edu.cn; Tel.: 0086-025-83587984, Fax: 0086-025-83587984
Received May 20, 2015; accepted June 14, 2015; published online July 22, 2015.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201500385 or from the author.
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FULL PAPER
thesis of two symmetrically bis-substituted pyrene de-
rivatives 1 and 2 with CN group terminated. 1 has two
directly bound cyanophenyl on pyrene core and 2 has
acetylene unit inserted between pyrene core and
cyanophenyl. The compounds were fully characterized
and the electroluminescence property was studied by
doping the material in poly(vinyl carbazole) (PVK) as
the emitting layer in the OLEDs.
filtration. 1,6-Dibromopyrene was purified by recrystal-
lization from toluene for several times with the yield of
50%. H NMR (CDCl3, 300 MHz, 298 K) δ: 8.46 (d,
J＝9 Hz, 2H), 8.27 (d, J＝9 Hz, 2H), 8.12 (d, J＝9 Hz,
2H), 8.06 (d, J＝9 Hz, 2H).
1
4,4,5,5-Tetramethyl-2-(4-cyanophenyl)-1,3,2-di-
oxaborolane Following the literature,[23] the mixture
of 4-bromobenzenecarbonitrile (1.82 g), potassium ace-
tate (2.34 g, 30 mmol), bis(pinacontato) diboron (2.80 g,
11 mmol), Pd(dppf)Cl2 (0.036 g, 0.030 mmol) in DMSO
(25 ml) was heated under N2 at 80 ℃ overnight. The
reaction mixture was cooled to room temperature, then
cool water was added. The solution was extracted by
ethyl acetate and the combined organic phase was dried
over anhydrous MgSO4. After the solvent was evapo-
rated and with column chromatography (silica gel, ethyl
acetate/petroleum ether＝1/8, V/V) the product was col-
Experimental
General
Pyrene, 4-bromobenzenecarbonitrile, methyltrioc-
tylammonium chloride, tetrakis(triphenylphosphine)-
palladium(0) (Pd(PPh3)4), cuprous iodide (CuI), [1,1'-
bis(diphenylphosphino)ferrocene]palladium(II) chloride
(Pd(dppf)Cl2), triphenylphosphine (PPh3), trans-di-
chlorobis(triphenylphosphine)palladium(II) (Pd(PPh3)2-
Cl2) and trimethylsilyl acetylene (TMSA) were pur-
chased from Energy Chemical. Bis(pinacontato) diboron
(B2Pin2) was purchased from Sukailu Chemical. Bro-
mine, potassium acetate, DMSO, anhydrous MgSO4,
diisopropylamine, sodium carbonate, and potassium
carbonate were purchased from Sinopharm Chemical
Reagent. They were used without any further purifica-
tion. Poly(3,4-ethylnenedioxythiophene):poly(styrene-
sulfonate) (PEDOT:PSS) was obtained from Lumi-
nescnece Technology Corp., PVK was obtained from
Sigma-Aldrich, 1,3,5-tri(1-phenyl-1H-benzo[d]imida-
zol-2-yl)phenyl (TPBI) was obtained from Nichem Fine
Technology Co. Ltd. Anhydrous toluene and THF were
obtained by distilling the corresponding solvent previ-
ously treated with sodium and diphenylmethanone.
1
lected as white solid with the yield of 75%. H NMR
(CDCl3, 300 MHz, 298 K): 7.88 (d, J＝6 Hz, 2H), 7.64
(d, J＝9 Hz, 2H), 1.35 (s, 12H).
Compound 1 The mixture of 1,6-dibromopyrene
(0.36 g, 1 mmol), 4,4,5,5-tetramethyl-2-(4-cyanophe-
nyl)-1,3,2-dioxaborolane (0.6 g, 3 mmol), one drop of
methyl trioctyl ammonium chloride, dry toluene (25 mL)
and 2 mol/L Na2CO3 aqueous solution (25 mL) was
bubbled with N2 for 10 min. Then Pd(PPh3)4 (46 mg,
0.04 mmol) was added. The mixture was heated at 100
℃ for 24 h. The mixture was cooled to room tempera-
ture and the solution was extracted by dichloromethane.
The combined organic phase was dried over anhydrous
MgSO4 and evaporated. The crude product was purified
by column chromatography (silica gel, dichlorometh-
ane/petroleum ether＝1/1, V/V) to get compound 1 as
1
1
greenish yellow solid with the yield of 33%. H NMR
The H NMR (300 MHz) spectra were recorded on
(CDCl3, 300 MHz, 298 K) δ: 8.26 (d, J＝9 Hz, 2H),
8.11 (s, 4H), 7.98 (d, J＝6 Hz, 2H), 7.88 (d, J＝6 Hz,
4H), 7.77 (d, J＝6 Hz, 4H). MS (MALDI-TOF) m/z:
calcd for C30H16N2 404.47, found 404.97 (M＋).
Bruker AMX400 spectrometer in CDCl3. UV/Vis spec-
tra were recorded on Shimadzu UV-1750. Fluorescence
spectra were recorded on Hitachi F-4600. Fluorescence
quantum yields (Фf) were calculated using coumarin 1
as primary standard (Фf＝0.99 in ethyl acetate). Cyclic
voltammetry (CV) experiments were performed with a
CHI-621B electrochemical analyzer. All measurements
were carried out at room temperature with a conven-
tional three electrode configuration consisting of a
platinum working electrode, an auxiliary electrode and a
non-aqueous Ag/AgNO3 reference electrode. The sol-
vent was CHCl3 in CV measurement and CH2Cl2 in
other experiments. The supporting electrolyte was 0.1
mol/L tetrabutylammonium hexafluorophosphate. TGA
measurements were performed on a Perkin Elmer TGA7
thermal analyzer at a heating rate of 10 ℃/min under a
dry nitrogen flow.
Synthesis of compound 2
4-Ethynylbenzonitrile According to the litera-
ture,[24] 4-bromobenzenecarbonitrile (1.82 g, 10 mmol)
was dissolved in the mixture of toluene (20 mL) and
diisopropylamine (10 mL). The system was bubbled
with N2 for 10 min and then Pd(PPh3)4 (577.8 mg, 0.50
mmol) and CuI (95.2 mg, 0.50 mmol) were added.
TMSA (1.2 g, 12 mmol) was added by syringe. The
mixture was heated under N2 at 80 ℃ overnight. The
reaction was cooled to room temperature, and then
NH4Cl aqueous solution was added. The solution was
extracted by dichloromethane. The combined organic
phase was dried over anhydrous MgSO4 and evaporated.
The crude product was purified by column chromatog-
raphy (silica gel, dichloromethane/petroleum ether＝1/3,
V/V) to get yellow solid (0.92 g, 93%). The yellow solid
(0.598 g, 3.0 mmol) was added to a mixture of THF (7
mL) and MeOH (7 mL), then K2CO3 (4.2 g, 30 mmol)
was added. The mixture was stirred at room temperature
Synthesis of compound 1
1,6-Dibromopyrene A solution of bromine (2.0 ml)
in dichloromethane (50 mL) was dropwise added to a
solution of pyrene (4.0 g) in dichloromethane (70 mL) at
room temperature over 2 h. The solution was then
stirred overnight. The white precipitate was collected by
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Two Pyrene Derivatives: Synthesis, Photoluminescence, and Electroluminescence
overnight. NH4Cl aqueous solution was added and the
product was extracted with dichloromethane. The com-
bined organic phase was dried over anhydrous MgSO4
and then evaporated. The crude product was purified by
column chromatography (silica gel, dichloromethane/
petroleum ether＝1/2, V/V) to get pure 4-ethynylben-
zonitrile as white solid with the yield of 85%.1H NMR
(CDCl3, 300 MHz, 298 K) δ: 7.56－7.64 (m, 4H), 3.30
(s, 1H).
devices was 0.09 cm2. The EL spectra and bright-
ness–current density–voltage characteristics of devices
were measured by using a SpectraScan PR 650 spectro-
photometer and a computer-controlled voltage-current
source, Keithley 2400, under ambient condition.
Results and Discussion
The synthesis of two pyrene derivatives 1 and 2
started with bromination of pyrene (as shown in Figure
1). The product, 1,6-dibromopyrene, was obtained by
recrystallization in toluene with several times. The bo-
ron reagent, 4,4,5,5-tetramethyl-2-(4-cyanophenyl)-
1,3,2-dioxaborolane, was synthesized with 4-bromo-
benzenecarbonitrile as the starting material. 1,6-Dibro-
mopyrene and the boron reagent reacted by Suzuki cou-
pling mechanism to produce compound 1 with the yield
of 33%. The intermediate for the synthesis of compound
2, 4-ethynylbenzonitrile, was prepared by subsequent
Sonogashira coupling reaction and the cleavage of
acetylene protecting group trimethylsilyl with the yield
of 79%. 1,6-Dibromopyrene and 4-ethynylbenzonitrile
reacted by Sonogashira coupling mechanism to produce
compound 2 with the yield of 25%. Due to large planer
structure, 1 and 2 are only slightly soluble in toluene
and CH2Cl2.
Compound 2 The mixture of 1, 6-dibromopyrene
(0.36 g, 1 mmol) and 4-ethynylbenzonitrile (0.40 g, 2.5
mmol) in dry THF (25 ml) and diisopropylamine (10
mL) was bubbled with N2 for 10 min. Then PPh3 (50 mg,
0.2 mmol), CuI (25 mg, 0.2 mmol) and Pd(PPh3)2Cl2
(50 mg, 0.05 mmol) were added. The mixture was
heated at 80 ℃ for 24 h. The mixture was cooled to
room temperature and the solution was extracted by di-
chloromethane. The combined organic phase was dried
over anhydrous MgSO4 and evaporated. The crude
product was purified by column chromatography (silica
gel, dichloromethane/petroleum ether＝2/1, V/V) to get
compound 2 as yellow solid (100 mg) with the yield of
1
25%. H NMR (CDCl3, 300 MHz, 298 K) δ: 8.67 (d,
J＝9 Hz, 2H), 8.18－8.31 (m, 6H), 7.78 (d, J＝9 Hz,
4H), 7.72 (d, J＝9 Hz, 4H). MS (MALDI-TOF) m/z:
calcd for C36H16N2 452.52, found 453.20 (M＋).
To gain a deep insight into the structure-property re-
lationship at the molecular level, density functional the-
ory (DFT) calculations were performed to evaluate the
positions and energies of frontier orbits for 1 and 2, us-
ing the Gaussian 03 program at the B3LYP/6-31G(d)
level. For 1 and 2, the largest coefficients of HOMO
orbitals are mainly positioned on the pyrene core, and
the coefficients of LUMO orbitals are spread across the
entire π-conjugated skeleton, as the case of 1,3,5,9-tetra-
cyanophenyl-7-tert-butylpyrene.[22] As observed from
the geometries, phenyl twists in relative to pyrene in 1
with the dihedral angle of 52.3°, while phenyl and
pyrene in 2 are coplanar. The extended conjugation sys-
tem by inserting acetylene may explain why 2 has a
smaller energy gap than 1.
Thermogravimetric analysis (TGA) was carried out
under nitrogen atmosphere to evaluate the thermal sta-
bility of 1 and 2. The onset decomposition temperatures
(Td) of 1 and 2 were estimated to be 191.8 ℃ for 1 and
256.2 ℃ for 2. It indicates that the acetylene linkage in
2 serves as a spacer, relieves the steric strain between
the pyrene and phenyl units and enhances the stability,
in agreement with acetylene linked pyrene-fluorene
compound.[19]
OLED device fabrication and measurement
Patterned indium tin oxide (ITO) substrates (10 Ω/ ,
Thin Film Devices, Inc.) were cleaned sequentially in an
ultrasonic bath of the detergent, deionized water, ace-
tone and ethanol. After dried in an oven at 120 ℃ for 20
min, the substrates were treated with UV-Ozone for
about 5 min. A certain volume of the stock solution of 1
(or 2) with concentration of 1 mg/mL in chlorobenzene
was added into the stock solution of PVK with concen-
tration of 12 mg/mL in chlorobenzene and a mixture
solutions with concentration of 8% (w/w) of 1 (or 2)
were ready for spin coating. A thin layer of PEDOT:PSS
of 30 nm thick was prepared by spin coating onto the
ITO at 600 r/min for 6 s and 3000 r/min for 60 s and
baked at 130 ℃ for 30 min. A layer of the emitting ma-
terial (PVK, 1 doped PVK, or 2 doped PVK) was
spin-coated on the top of PEDOT:PSS layer. The sam-
ples were then loaded into a deposition chamber. A se-
quence of TPBI (30 nm), LiF (1 nm) and Al (100 nm)
were thermally deposited at a vacuum of 3×105 mbar.
Electroluminescent (EL) devices fabricated in this work
had a configuration of device A: ITO/PEDOT:PSS
(30 nm)/1:PVK (8%, w/w) (50 nm)/TPBI (30 nm)/Al
(100 nm), device B: ITO/PEDOT:PSS (30 nm)/2:PVK
(8%, w/w) (50 nm)/TPBI (30 nm)/Al (100 nm), and de-
vice C: ITO/PEDOT:PSS (30 nm)/PVK (50 nm)/TPBI
(30 nm)/Al (100 nm); where PEDOT:PSS was the hole
injection material, TPBI was the electron transporting
material and PVK was the hole transporting and host
material. The film thickness was controlled by a quartz
oscillating thickness monitor. The emission area of the
UV-vis absorption spectra and PL spectra in dilute
CH2Cl2 solution and in powder were carried out at room
temperature. In CH2Cl2 solution (Figure 3), 1 has the
absorption spectrum at 242, 289 and 366 nm which is
associated with the π-π* transition of the pyrene back-
bone, and emission spectrum at 427 nm. With extended
conjugation structure, 2 has fine-structured absorption
spectrum with maximum at 432 nm which is associated
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with the π-π* transition of the conjugated backbone, and
emission spectrum at 448 nm and a shoulder at 475 nm,
which resembles to those of 1,3,6,8-tetrakis[(triiso-
propylsilyl)ethynyl]pyrene[25] and 1,6-bis(phenylethynyl)-
pyrene-phenothiazine dyad.[26] Compared with 1, 2 has
both bathochromicly shifted absorption and emission
peaks because acetylene extends the conjugation system,
as reported pyrene-acetylene compounds.[27] It is ob-
servable that 2 has the Stokes’ shift of 826.7 cm−1 and 1
has the Stokes’ shift of 3903.2 cm−1, suggesting the re-
maining planar configuration for the singlet ground state
(S0) and the first excited singlet ground state (S1) states
of 2.[13] In powder, 1 has an emission peak at 478 nm
while 2 has a broad emission peak at 537 nm with a
shoulder at 561 nm, indicating that 2 has strong
J-aggregation through intermolecular π-π stacking in the
solid state.[25] 2 shows a strong bathochromic shift of 89
nm of the emission in powder from that in solution, be-
ing much larger than 51 nm in the situation of 1. This
suggests that the excimers between pyrene moieties of 2
are formed in the solid state. It could be explained that 2
has higher coplanarity and has more tendency to have
strong π-π staking in the solid, in agreement with the
literature.[12] This experimental result gets supported by
the DFT theoretical calculations (Figure 2). 1 has a large
dihedral angle of 52.3° between phenyl and pyrene ring,
thus resulting in severe steric congestion. The phenyl
and pyrene ring in 2 are coplanar and the planar mo-
lecular structure is prone to close packing and excimer
formation in the solid state, thereby resulting in the
drastically bathochromic shift in the PL spectrum of its
film in comparison with its emission in solution. The
fluorescence quantum yields of both compounds in
CH2Cl2 solution are high, 85% from 1 and 75% from 2,
respectively (see Table 1). Relatively lower value from
2 than 1 can be explained by the observation that 2 has
larger overlap of absorption and emission spectra and
therefore stronger self absorption (Figure 3). The emis-
sion of 1 and 2 in a serial of from-nonpolar-to-polar
solvents was tested (Figure S3) and only minimal sol-
vatochromism was found in contrast to the reported re-
sult,[28] suggesting small change of polarity from the
ground state to the excited state of 1 and 2 due to sym-
metrical substitution.[29]
The electrochemical property of 1 and 2 was exam-
ined by cyclic voltammetry (CV). The measurements of
the compounds displayed one reversible oxidation proc-
ess in CHCl3 solution, but no obvious reduction peaks
were detected, as the reported observations.[12] So, we
only adopted anodic scan from 0 to 2.0 V to detect the
(a)
O
B
NC
Br
NC
O
(b)
(c)
NC
Br
NC
TMS
NC
NC
O
B
NC
O
(e)
Br
(d)
1
CN
NC
Br
(f)
NC
2
CN
Figure 1 Synthesis of compound 1 and 2: (a) Pd(dppf)Cl2, B2Pin2, KOAc, DMSO, 80 ℃, 75%; (b) Pd(PPh3)4, TMSA, CuI, i-Pr2NH,
toluene, 93%; (c) K2CO3, MeOH/THF, 85%; (d) Br2, 50%; (e) Pd(PPh3)4, K2CO3, H2O/toluene, 33%; (f) Pd(PPh3)2Cl2, i-Pr2NH, CuI,
PPh3, toluene, 25%.
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Two Pyrene Derivatives: Synthesis, Photoluminescence, and Electroluminescence
Table 1 Photophysical properties, frontier molecular orbital energy levels and thermal stability
e
λabs Solna
λem Solna
Solidb
ФF Solnc
HOMOd
LUMOd
Td
1
2
366, 289, 242
432, 413, 281
427
478
85%
−5.70
−2.62
191.8
256.2
448, 475
538, 561
75%
−5.71
−2.94
a In dichloromethane solution (ca. 10 µmol/L). b In powder at a quartz substrate. The fluorescent quantum yield (ФF) was determined in
dichloromethane using Coumarin 1 as primary standard (Фf＝0.99 in dichloromethane). d Determined by cyclic voltammetry and optical
c
e
energy band gap obtained from the onset of absorption spectra. Decomposition temperature corresponding to 5℅ weight loss measured
by TGA.
meeting the requirement of Förster energy transfer.[31]
The film of 1 or 2 doped PVK was prepared by
spin-coating the blend of 1 or 2 and PVK. 1 doped PVK
has PL emission at 439 nm and no emission of PVK,
meaning the complete energy transfer from PVK to 1. 2
doped PVK has PL emission at 455 nm and 478 nm
which are attributed to 2, and a shoulder at 375 nm
which is the contribution of PVK, indicating that the
Figure 2 Spatial distributions of the compounds 1 and 2 frontier
orbits.
energy transfer from PVK to 2 is not complete at the
doping concentration of 8%.
Figure 3 UV/Vis absorption spectra (left) and PL spectra (right)
in dilute CH2Cl2 solution, and PL spectra (right) in powder on
quartz substrates of 1 and 2.
HOMO.[22] 1 and 2 have nearly identical onset oxidation
potential (Eonset), meaning that they have identical
HOMO level. From Eonset the HOMO energy levels
[HOMO＝–(4.4＋Eonset)] are calculated as −5.70 and
−5.71 eV for 1 and 2, respectively. The LUMO energies
were evaluated using standard approximations by the
formula: HOMO＝LUMO−Egopt, as –2.62 and –2.94 eV
for 1 and 2, respectively. The optical energy band gap
(Egopt) was obtained from the onset of the absorption
Figure 4 (a) PL spectra of PVK, 1:PVK and 2:PVK in films
prepared by spin-coating the corresponding solution at the con-
centration of 8% on quartz substrates; (b) EL spectra of device A
and B measured at 22 V, compared with EL spectrum of device C
at 13 V.
spectra.
Considering a strong π-π staking of both two com-
pounds which results in intermolecular aggregation and
deterioration of the electroluminescence property, a
doping method with PVK as the host in the emissive
layer was adopted. PVK is an excellent hole transport-
ing material. In agreement with the report,[30] the PL
emission of PVK film extends from 350 to 600 nm with
a maximum at 404 nm (As shown in Figure 4 (a)) and
well overlaps with the absorption spectra of 1 or 2,
As shown in Figure 4 (b), device A has a strong
emission at 460 nm and device B has a strong peak at
468 nm. It is obvious that the emission of the device A
is attributed to that of 1 and the emission of device B is
attributed to that of 2 compared with device C which
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emits electroluminescence emission at 420 nm from
PVK. Three devices have a common shoulder emission
at 610 nm which is ascribed to the excimer emission
from PVK. Compared with PL emission of 2 doped
PVK, it is interesting that PL emission at 375 nm which
is attributed to emission of PVK does not appear in EL
spectrum, meaning the energy transfer from PVK to 2 is
complete upon an applied bias. Either A or B has a sta-
ble emission spectrum at increasing applied voltage. It is
noted that the emission from PVK did not appear,
meaning that energy transfer from PVK to the com-
pounds was efficient at a low concentration of 8%, in
contrast to the literature reporting complete energy
transfer at high concentration of 32% due to small over-
lap between PVK and the dopant.[32]
trap the excitons generated from a host in a dopant.[19]
We consider to test other hosts to boost the performance
of these two blue-light emitting.
Chang et al.[12] reported that the 1,3,6,8-tetrakis(3,5-
dimethylphenyl)pyrene had a strong aggregation and the
excimer which was formed upon applied voltage exhib-
ited efficient electroluminescence properties. It is worth
trying the undoped 1 or 2 as the emitting material and
studying the electroluminescence from excimer.
Scheme 1 Schematic drawing of device A (a) and B (b)
(a)
-2.00
-2.20
-2.62
-2.80
PEDOT
:PSS
1: PVK
LiF/Al
-4.20
ITO
TPBI
-4.70
-5.50
-5.70
-5.10
-6.30
-2.80
Figure 5 Current density-voltage (J-V) (left) and lumines-
cence-voltage (L-V) (right) characteristics of device A and B.
(b)
-2.00
-2.94
-2.20
J-V and L-V of devices are shown in Figure 5. De-
vice A has a turn-on voltage of 9 V and the highest
brightness of 553 cd/cm2 at 21 V. Device B has a
turn-on voltage of 6 V and the highest brightness of 227
cd/cm2 at 20 V. In comparison, device B has higher
current density than device A till the applied bias of 18
V. This is attributed to two factors: easy electron injec-
tion from TPBI to 2 and high carrier transporting ability
from 2 compared with 1. As shown in Scheme 1, LU-
MO of 2 is −2.94 eV and stays lower than that of
LUMO at −2.80 eV of TPBI, while the LUMO of 1 is
−2.62 eV and stays higher than LUMO at −2.80 eV of
TPBI. This means that electrons can be injected more
easily into the 2:PVK layer than into the 1:PVK layer.
In addition, 2 should have higher carrier transporting
ability than 1 due to high coplanarity. Therefore, device
B exhibited higher current density and lower turn-on
voltage than device A. However, Device B is not good
as device A in terms of the luminance and efficiency.
The highest current efficiency of 0.22 cd•A−1 from de-
vice B is lower than that of 0.36 cd•A−1 from device A.
This could be explained by the relatively lower fluores-
cence efficiency of 75% of 2 than that of 85% of 1. It is
noted that HOMO levels of 1 and 2 are lower than −5.50
eV of PVK in Scheme 1, indicating mismatch of energy
level which can not meet the requirement to efficiently
PEDOT
:PSS
LiF/Al
2:PVK
-4.20
ITO
TPBI
-4.70
-5.50
-5.71
-5.10
-6.30
Device A: ITO/PEDOT:PSS (30 nm)/1:PVK (8%, w/w) (50
nm)/TPBI (30 nm)/Al (100 nm), device B: ITO/PEDOT:PSS (30
nm)/2:PVK (8%, w/w) (50 nm)/TPBI (30 nm)/Al (100 nm). En-
ergy levels of the functional materials are cited from the litera-
ture.[33]
Conclusions
Two pyrene-based materials were designed and syn-
thesized. Compound 1 with directly linked phenyl and
pyrene has large dihedral angle, and compound 2 with
972
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© 2015 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2015, 33, 967—973

Two Pyrene Derivatives: Synthesis, Photoluminescence, and Electroluminescence
[13] Berlman, I. B. J. Phys. Chem. 1970, 74, 3085.
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is coplanar. Optical analysis and DFT calculation con-
firmed that compound 2 is prone to excimer formation
in the solid state due to high coplanarity. The materials
exhibited blue photoluminescence and high fluorescent
quantum yield of 85% and 75% in solution. The elec-
troluminescence properties were studied with 1 or 2
doped PVK as the emitting layer. The electrolumines-
cent emission was attributed to the material itself and it
was deduced that the energy transfer from PVK to the
pyrene derivatives happened.
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Acknowledgement
This work was supported by the National Natural
Science Foundation of China (No. 21371096).
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www.cjc.wiley-vch.de
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