Dimesitylboryl-functionalized fluorene derivatives: Promising luminophors with good electron-transporting ability for deep blue organic light-emitting diodes

Article abstract of DOI:10.1016/j.dyepig.2013.09.042

Two dimesitylboryl-functionalized fluorene derivatives are synthesized and characterized. They show deep blue fluorescence in both solution and film states, with narrow spectral profiles and excellent solid-state emission efficiencies up to 94%. Due to th

Full text of DOI:10.1016/j.dyepig.2013.09.042

Dyes and Pigments 101 (2014) 136e141
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Dimesitylboryl-functionalized fluorene derivatives: Promising
luminophors with good electron-transporting ability for deep blue
organic light-emitting diodes
a
b
,
a
a
a
a
c
*
*
Xiaofei Xu , Shanghui Ye
, Bairong He , Bin Chen , Jiayun Xiang , Jian Zhou , Ping Lu ,
a
,
a
,
*
**
Zujin Zhao , Huayu Qiu
a
College of Material, Chemistry and Chemical Engineering, Hangzhou Normal University, Hangzhou 310036, China
Key Laboratory for Organic Electronics & Information Displays (KLOEID) and Institute of Advanced Materials (IAM), Nanjing University of Posts &
b
Telecommunications, Nanjing 210046, China
c
State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 16 August 2013
Received in revised form
Two dimesitylboryl-functionalized fluorene derivatives are synthesized and characterized. They show
deep blue fluorescence in both solution and film states, with narrow spectral profiles and excellent solid-
state emission efficiencies up to 94%. Due to the presence of the boron atoms, both luminophors possess
low-lying LUMO energy levels as revealed by theoretical calculation. They can function as both light
emitter and electron transporter in organic light-emitting diodes. A device fabricated from one of the
new luminophors radiates deep blue light with a maximum emission at 432 nm (CIE ¼ 0.179, 0.128), and
20 September 2013
Accepted 28 September 2013
Available online 10 October 2013
ꢀ
2
ꢀ1
affords a maximum luminance of 10,320 cd m , and a peak efficiency of 3.4 cd A .
Keywords:
Organoboron
Ó 2013 Elsevier Ltd. All rights reserved.
Fluorene derivatives
Blue fluorescence
Electronic structure
Electroluminescence
Organic light-emitting diodes
1
. Introduction
luminophors often lead to low electron affinities. Therefore, the
charge injection, particularly electron injection, into blue emit-
ters becomes inefficient, which undermines the efficiency of EL
devices.
The search for blue luminescent materials with high emission
efficiency is a subject of current interest, for they are essential in
high quality full color displays and white lighting [1]. To respond
to the demand, various blue luminophors derived from dis-
tyrylarylene, fluorene, anthracene, pyrene, oligoquinoline, phe-
nanthro[9,10-d]imidazole and tetraphenylethene have been
designed and studied, some of which have promising prospect in
efficient blue organic light-emitting diodes (OLEDs) [2]. How-
ever, compared with green and red luminescent materials that
have been well-developed, most blue emitters are still not
satisfactory. The performance of blue electroluminescence (EL)
devices needs to be further improved in terms of efficiency and
color purity. The intrinsic wide energy gaps (ꢁ3 eV) of most blue
Recently, boryl-functionalized
p-conjugated luminophors are
attracting considerable attention because of their great potential
in nonlinear optics [3], organic electronics [4] and anion sensors
[5]. The presence of boryl groups has been demonstrated to be
beneficial to the increase of electron affinity of materials due to
the vacant low-lying p orbital on the boron center, which al-
p
lows them to function as excellent electron acceptors [6,7].
Introduction of boryl groups, typically dimesitylboryl ones, into
a
p-conjugated framework is an effective approach to create
luminescent materials with improved electron transporting
ability. The generated materials can serve as bifunctional ma-
terials as both light emitters and electron transporters for OLEDs
[
4a,4b,8]. On the other hand, fluorene is a popular building block
*
Corresponding author. Tel.: þ86 571 28867898; fax: þ86 571 28867899.
to create luminescent materials, for its excellent photo-
luminescence (PL) property, fast charge mobility and high
thermal stability. In addition, chemical modifications can be
** Corresponding authors.
E-mail addresses: yeshh@iccas.ac.cn (S. Ye), zujinzhao@gmail.com (Z. Zhao),
huayuqiu@gmail.com (H. Qiu).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.09.042

X. Xu et al. / Dyes and Pigments 101 (2014) 136e141
137
easily made at the 2,7, and 9-positions of the fluorene ring,
which allows incorporation of diverse functional groups. Herein,
2.2. Device fabrication and measurements
In general procedure, indium-tin oxide (ITO)-coated
glass substrates were etched, patterned, and washed with deter-
gent, deionized water, acetone, and ethanol in turn. Poly(3,4-
two tailored fluorene derivatives bearing dimesitylboryl func-
a
0
tional end-caps are designed. 9,9-Diphenylfluorene and 9,9 -
spirobifluorene are adopted to build the main
backbones because of their rigid and non-planar conformations,
which are conducive to prevention of intermolecular in-
p-conjugated
ethylenedioxythiophene):poly(4-styrenesulfonate)
(PEDOT:PSS,
Ò
p
e
p
Baytron P Al 4083) was spin coated onto the ITO substrates under
ambient atmosphere yielding a layer of 10 nm, and then it was
teractions [2d,9]. Their synthesis and characterization are
described, their thermal stability, energy levels and optical
properties are depicted, and the applications in non-doped blue
OLEDs are demonstrated.
ꢂ
baked at 120 C for 30 min to remove residual water. All the organic
layers were subsequently thermally deposited in a vacuum cham-
ꢀ
5
ber at a base pressure of 8 ꢃ 10 Pa. After that a 100-nm-thick Ag
capping layer was deposited through a 2-mm-wide opening array
to fulfill the device fabrication. In all devices, a thin layer of Ca about
2
. Experimental
8
nm was deposited to enhance the electron injection ability on the
Ag cathode. A quartz crystal oscillator placed near the substrate was
used to monitor the thickness of each layer, which was calibrated ex
situ using Ambios Technology XP-2 surface profilometer. The active
2
.1. General
THF was distilled from sodium benzophenone ketyl under dry
2
area of device was 6 mm . EL spectra and chromaticity coordinates
nitrogen immediately prior to use. Compounds 3e5 were prepared
by the literature methods [2d,10]. All other chemicals and reagents
were purchased from Aldrich and used as received without further
purification. H and C NMR spectra were measured on a Bruker
AV 400 spectrometer in deuterated chloroform using tetrame-
were measured with a SpectraScan PR650 photometer. Current
densityꢀvoltageꢀluminance measurements were recorded simul-
taneously using a Keithley 4200 semiconductor parameter analyzer
combined with a Newport multifunction 2835-C optical meter,
with luminance was measured in the forward direction. All device
characterizations were carried out under ambient laboratory air at
room temperature.
1
13
thylsilane (TMS;
surements were made with an RF-5301pc spectrofluorometer
Shimadzu, Kyoto, Japan) equipped with a xenon lamp. UVevis
d
¼ 0) as internal reference. Fluorescence mea-
(
absorption spectra were recorded on a Shimadzu UV-2450 spec-
trophotometer. The high resolution mass spectra (HRMS) were
recorded on a GCT premier CAB048 mass spectrometer operated
in MALDI-TOF mode. Thermogravimetric analysis (TGA) was car-
2.3. Synthesis
2.3.1. 2,7-Bis(4-(dimesitylboryl)phenyl)-9,9-diphenylfluorene
ried out on a TA TGA Q5000 under dry nitrogen at a heating rate of
((DMesB)
A mixture of 3 (0.81 g, 2.2 mmol), 4 (0.48 g, 1 mmol), Pd(PPh
(0.11 g, 0.1 mmol), and K CO (1.1 g, 8.0 mmol) in toluene/ethanol/
water (100 mL, 8/1/1 v/v/v) was heated to reflux for 12 h under
nitrogen. Then the reaction mixture was cooled to room
2
DPF)
ꢂ
10 C/min. The ground-state geometries were optimized using the
3 4
)
density functional theory with B3LYP hybrid functional at the basis
set level of 6-31G(d). All the calculations were performed using
Gaussian 09 package.
2
3
OH
OH
1
2
) n-BuLi
1) n-BuLi
B
F
B
Br
B
B
2
3
) B(OMe)3
) HCl
)
Br
Br
1
2
3
3
Br
Br
Pd(PPh3)
4
B
B
K CO
2
3
4
(
DMesB) DPF
2
3
Br
Br
Pd(PPh3)4
K CO
B
B
2
3
5
(
DMesB) SF
2
2 2
Scheme 1. Synthetic routes to (DMesB) DPF and (DMesB) SF.

138
X. Xu et al. / Dyes and Pigments 101 (2014) 136e141
Table 1
100
Optical properties, thermal stabilities and energy levels of (DMesB)
DMesB) DPF.
2
SF and
(
2
e
f
l
abs (nm)
Solna
SF 367
l
em (nm)
F
F
(%)
T
d
E
g,opt
80
60
40
20
0
Soln (fwhm)a Film (fwhm)b Solnc Filmd
ꢂ
C
eV
(
DMesB)
2
412,432 (56) 420,428 (53) 98
414,430 (62) 419,427 (54) 96
94
93
379 3.06
377 3.05
(DMesB)
2
DPF 369
a
In THF solution (10
m
M).
Film drop-casted on quartz plate.
Fluorescence quantum yield determined in THF solutions using 9,10-
¼ 90% in cyclohexane) as standard.
Fluorescence quantum yield of the amorphous film measured using an inte-
b
c
(
DMesB) SF
2
(DMesB) DPF
2
diphenylanthracene (F
F
d
grating sphere.
e
f
Decomposition temperature corresponding to 5% weight loss.
Optical energy bandgap obtained from onset of absorption spectrum.
2
6
H, J ¼ 8.0 Hz), 7.46e7.35 (m, 10H), 7.13e7.09 (m, 2H), 6.99 (s, 2H),
100
200
300
400
500
13
.81e6.78 (m, 10H), 2.28 (s, 12H), 1.97 (s, 24H). C NMR (100 MHz,
o
Temperature ( C)
3
CDCl ), d (TMS, ppm): 150.0, 148.5, 144.0, 141.8, 141.1, 140.8, 140.5,
1
38.6, 136.9, 128.1, 127.9, 127.2, 126.4, 124.2, 122.7, 120.5, 120.1, 66.1,
Fig. 1. TGA thermograms of (DMesB)
a heating rate of 10 C/min.
2
SF and (DMesB)
2
DPF recorded under nitrogen at
þ
23.5, 21.2. HRMS (MALDI-TOF): m/z 964.5362 (M , calcd 964.5351).
ꢂ
Anal. Calcd for C73
H
66
B
2
: C, 90.87; H, 6.89. Found: C, 90.72; H, 6.78%.
temperature and poured into water. The organic layer was extrac-
ted with dichloromethane, and the combined organic layers were
washed with a saturated brine solution and water, and dried over
anhydrous magnesium sulfate. After filtration and solvent evapo-
ration, the residue was purified by silica-gel column chromatog-
(DMesB) SF
A
2
(
DMesB) DPF
2
raphy using hexane/dichloromethane as eluent. White solid of
1
(
(
(
2
1
DMesB)
400 MHz, CDCl
m, 4H), 7.66 (br, 8H), 7.30e7.22 (m, 10H), 6.82 (s, 8H), 2.31 (s, 12H),
.02 (s, 24H). 13C NMR (100 MHz, CDCl
), (TMS, ppm): 152.3,145.7,
44.3, 141.8, 140.8, 140.3, 139.5, 138.6, 137.1, 128.4, 128.2, 127.0,
26.8, 126.5, 124.9, 120.7, 65.7, 23.5, 21.3. HRMS (MALDI-TOF): m/z
2
DPF was obtained in 83% yield (0.8 g, 0.83 mol). H NMR
3
),
d
(TMS, ppm): 7.84 (d, 2H, J ¼ 7.6 Hz), 7.69e7.67
3
d
1
9
þ
66.5515 (M , calcd 966.5507). Anal. Calcd for C73
68 2
H B : C, 90.68;
H, 7.09. Found: C, 90.60; H, 7.01%.
0
2
(
.3.2. 2,7-Bis(4-(dimesitylboryl)phenyl)-9,9 -spirobifluorene
(DMesB) SF)
The procedure was analogous to that described for
2
325
375
425
475
525
575
625
675
1
(
DMesB)
2 3
DPF. White solid, yield 80%. H NMR (400 MHz, CDCl ),
Wavelength (nm)
d
(TMS, ppm): 7.92 (d, 2H, J ¼ 7.6 Hz), 7.85 (d, 2H, J ¼ 7.6 Hz), 7.68 (d,
(
DMesB) SF
B
2
(
DMesB) SF
2
(DMesB) DPF
2
(DMesB) DPF
2
325
375
425
475
525
575
625
675
275
325
375
425
475
Wavelength (nm)
Wavelength (nm)
Fig. 3. PL spectra of (DMesB)
(B) films, excited at 350 nm.
2 2
SF and (DMesB) DPF in (A) THF solutions (10 mM) and
2 2
Fig. 2. Absorption spectra of (DMesB) SF and (DMesB) DPF in THF solutions (10 mM).

X. Xu et al. / Dyes and Pigments 101 (2014) 136e141
139
Fig. 4. Optimized molecular structures and molecular orbital amplitude plots of HOMO and LUMO energy levels of (DMesB)
2
SF and (DMesB)
2
DPF calculated by B3LYP/6-31G(d).
3
. Results and discussion
(DMesB)
2
DPF, respectively. The fluorescence quantum yields (
F
F )
of (DMesB)
2
SF and (DMesB) DPF are as high as 98 and 96%,
2
3.1. Synthesis
measured using 9,10-diphenylanthracence as standard [11]. Both
luminophors are also highly emissive in the solid state. The films of
The chemical structures and synthesis of dimesitylboryl-
2 2
(DMesB) SF and (DMesB) DPF show deep blue emission peaked at
functionalized fluorene derivatives are shown in Scheme 1. Inter-
mediate 3 was prepared by the literature methods [10]. The treat-
ment of 3 with 4 [2d], and 5 [2d] in the presence of a palladium
around 419e428 nm, with narrow fwhm of 53 and 54 nm,
respectively, which are nearly the same as the PL in solutions
(Fig. 3B). Remarkably high
an integrating sphere [12] from the films of (DMesB)
(DMesB) DPF. Such efficient solid-state deep blue emission should
be partially attributed to the non-planar conformation that pre-
vents close -stacking and suppresses the formation of detrimental
species, such as excimers.
F
F
values of 94% and 93% are attained by
catalyst in
a
basic medium generated the target products,
SF, respectively, in good yields. The
2
SF and
(
DMesB) DPF, and (DMesB)
2
2
2
chemical structures were confirmed by NMR, high resolution mass
spectrometry and elemental analysis. The thermal stability of both
final products was examined by thermogravimetric analysis (TGA).
p
ꢂ
High decomposition temperatures of 379 and 377 C were recorded
2 2
for (DMesB) SF and (DMesB) DPF, corresponding to 5% weight loss
3.3. Electronic structure
of initial weight (Fig. 1). The results demonstrate that both lumi-
nophors are thermally stable for film preparation by vacuum
deposition.
The theoretical calculations by B3LYP/6-31G(d) reveal that
both luminophors adopt a highly twisted conformation (Fig. 4).
The presence of mesityl substituents in addition to bulky groups
at the 9,9-posisitons of the fluorene unit can hinder close
packing and suppress intermolecular interactions effectively,
rendering the luminophors stable and intense deep blue emis-
sion in both dispersive and congregate states. The HOMOs of
3.2. Optical properties
(
DMesB)
and 369 nm (Fig. 2), respectively, in dilute THF solutions, associated
to the * transition of the molecules. The optical energy
2 2
SF and (DMesB) DPF show absorption maxima at 367
pe
p
2 2
(DMesB) SF and (DMesB) DPF are mainly concentrated at the p-
bandgaps of both luminophors are w3.0 eV (Table 1), calculated
from the onsets of their absorption spectra. They luminesce intense
deep blue light, with emission peaks ranged from 412 to 432 nm, in
dilute THF solutions (Fig. 3A and Table 1). The emission peaks of
both luminophors are very narrow. The full widths at half
conjugated backbone comprised of the central fluorene unit and
two phenyl rings at the 2,7-posisitons. The LUMOs, however, are
delocalized to the four mesityl substituents, due to the p ep*
p
conjugation between empty pp orbital of boron and
p* orbitals
of the aromatic rings. Such distinctive electronic structures give
maximum (fwhm) are 56, and 62 nm for (DMesB)
2
SF, and
rise to low LUMO energy levels of ꢀ2.16 and ꢀ2.17 eV for
(
2 2
DMesB) SF and (DMesB) DPF, respectively, indicating that
electron-injection and transport are favored in the present
luminophors.
Table 2
2 2
EL performances of OLEDs based on (DMesB) SF and (DMesB)
DPF.a
3.4. Electroluminescence
EL (fwhm)
nm)
CIE
V
on
L
max
h
C,max
h
C@1000
2
(
(V) (cd/m ) (cd/A) (cd/A)
The excellent solid-state emission efficiencies and low-lying
energy levels of (DMesB) SF and (DMesB) DPF encourage us to
2 2
investigate their applications in non-doped OLEDs. Multilayer
OLEDs with a configuration of ITO/PEDOT:PSS (10 nm)/NPB
Device I
(
(
DMesB)
DMesB)
2
SF
414, 432 (62) 0.173, 0.105 6.7
0.177, 0.133 8.3
7910
8880
2.1
2.7
1.6
2.2
2
DPF 434 (69)
Device II
(
(
DMesB)
DMesB)
2
SF
412, 432 (65) 0.179, 0.128 5.2 10,320
0.183, 0.143 4.6 9700
3.4
2.3
1.9
1.6
(
(
30 nm)/CBP (10 nm)/EML (50 nm)/Ca:Ag (Device I) were fabricated
2
DPF 434 (71)
0
NPB ¼ N,N-bis(1-naphthyl)-N,N-diphenylbenzidine; CBP ¼ 4,4 -
a
Device configuration: ITO/PEDOT:PSS (10 nm)/NPB (30 nm)/CBP (10 nm)/EML
bis(9H-carbazol-9-yl)biphenyl;
EML
¼
(DMesB)
SF and (DMesB)
2
SF
or
DPF
(
50 nm)/Ca:Ag (Device I) and ITO/PEDOT:PSS (10 nm)/NPB (30 nm)/CBP (10 nm)/
EML (30 nm)/TPBI (20 nm)/Ca:Ag (Device II). Abbreviation:
EL ¼ EL maximum,
C,max ¼ maximum
(
DMesB)
2
DPF). In the devices I, (DMesB)
2
2
l
ꢀ2
function as bifunctional layer of both light emitter and electron
transporter. PEDOT:PSS is used as hole-injection layer, NPB is
V
on ¼ turn-on voltage at 1 cd m , Lmax ¼ maximum luminance,
h
current efficiency, and
h
C@1000 ¼ current efficiency at 1000 cd mꢀ2.

140
X. Xu et al. / Dyes and Pigments 101 (2014) 136e141
adopted as hole-transporting layer, and CBP is introduced as
interlayer between hole-transporting and light-emitting layers to
promote hole-injection into and confine excitons within the light-
emitting layer. Table 2 lists the performances of the devices. Figs. 5
and 6 illustrate the EL spectra and the characteristic curves of the
To further improve the device performances, devices II with a
configuration of ITO/PEDOT:PSS (10 nm)/NPB (30 nm)/CBP
(10 nm)/EML (30 nm)/TPBI (20 nm)/Ca:Ag were fabricated
(TPBI ¼ 2,2 ,2 -(1,3,5-benzinetriyl)tris(1-phenyl-1-H-benzimid-
azole)). The TPBI layer was added in order to improve the elec-
tron injection and transport. The devices II show similar EL
spectra to devices I but lower turn-on voltages and higher cur-
rent densities (Fig. 6C) than devices I. The device II of
0
00
2 2
devices. (DMesB) SF and (DMesB) DPF exhibit stable deep blue
light in EL devices. Their EL maxima are located at 432 and 436 nm
at 11 V, corresponding to the Commission International de
l’Eclairage (CIE) chromaticity coordinates of (0.173, 0.105) and
(DMesB)
2
SF is turned on at 5.2 V, and radiates deep blue light
(
(
0.177, 0.133), respectively. The fwhm of the EL spectra of
DMesB) SF and (DMesB) DPF are 62 and 69 nm, which are as
with CIE coordinates of (0.179, 0.128) at 11 V. The values of Lmax
ꢀ2
ꢀ1
2
2
and
are enhanced in comparison with those of device I. The devices II
based on (DMesB) DPF also show good performances, with a
hC,max are 10,320 cd m , and 3.4 cd A , respectively, which
narrow as those of PL spectra in solid films. The almost identical
spectra reveal that the EL and PL emissions are originated from the
same decay of singlet excitons. The EL emissions of both lumino-
phors are much bluer than those of the organoboron derivatives in
the literature [4a,4b,8]. The maximum luminance (Lmax) and cur-
2
ꢀ ꢀ1
2
L
max of 9700 cd m , and a hC,max of 2.3 cd A (Table 2). These
results indicate that both luminophors are excellent emitters for
blue OLEDs.
rent efficiency (
h
C,max) attained by the devices I are 7910 and
ꢀ2
ꢀ1
8
880 cd m , and 2.1 and 2.7 cd A , demonstrating that
(
DMesB)
2
SF and (DMesB)
2
DPF perform outstandingly as bifunc-
tional materials.
A
device I
device II
325
375
425
475
525
575
625
675
Wavelength (nm)
B
device I
device II
3
25
375
425
475
525
575
625
675
Wavelength (nm)
Fig. 5. EL spectra of (A) (DMesB)
1 V (device I: ITO/PEDOT:PSS (10 nm)/NPB (30 nm)/CBP (10 nm)/EML (50 nm)/Ca:Ag;
device II: ITO/PEDOT:PSS (10 nm)/NPB (30 nm)/CBP (10 nm)/EML (30 nm)/TPBI
20 nm)/Ca:Ag).
2
SF and (B) (DMesB)
2
DPF in devices I and II at a bias of
Fig. 6. Plots of (A) luminance versus voltage, (B) current efficiency versus luminance,
and (C) current density versus applied voltage in multilayer EL devices (device I: ITO/
PEDOT:PSS (10 nm)/NPB (30 nm)/CBP (10 nm)/EML (50 nm)/Ca:Ag; device II: ITO/
PEDOT:PSS (10 nm)/NPB (30 nm)/CBP (10 nm)/EML (30 nm)/TPBI (20 nm)/Ca:Ag).
1
(

X. Xu et al. / Dyes and Pigments 101 (2014) 136e141
141
4
. Conclusions
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006;12(10):2758e71;
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2
(
In summary, two thermally stable luminophors from
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solid films, with deep blue emissions peaked at w430 nm and
4574e85;
(
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F
F
values up to 94% in films. Due to the presence of
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(
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p
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Acknowledgments
biphenyl-NPh(1-naphthyl)): a multifunctional molecule for electrolumines-
cent devices. Chem Mater 2005;17(1):164e70;
We acknowledge the financial support from the National Nat-
ural Science Foundation of China (51273053, 21104012, 21284034
and 61106017), the Natural Science Foundation of Zhejiang Prov-
ince (Y4110331), the Program for Changjiang Scholars and Inno-
vative Research Teams in Chinese Universities (IRT 1231) and the
Project of Zhejiang Key Scientific and Technological Innovation
Team (2010R50017).
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