The synthesis and photoluminescence characteristics of novel blue light-emitting naphthalimide derivatives

Article abstract of DOI:10.1016/j.dyepig.2010.01.003

Substitution at the 4-position of 1,8-naphthalimide with electron-donating phenoxy or tert-butyl modified phenoxy groups, novel naphthalimide derivatives were obtained which emitted blue fluorescence with emission peaks of 425-444?nm in chloroform solution under UV irradiation, with highest relative photoluminescence quantum efficiency of 0.82. When in solid film, only compounds that contained ortho-tert-butylphenoxy substituents displayed blue photoluminescence of 438-451?nm, with highest absolute fluorescence quantum yield of 0.29; whereas other compounds showed greenish blue fluorescence at 471-478?nm, with highest absolute fluorescence quantum yield of 0.42. Cyclic voltammetry studies revealed that the molecules have low-lying energy levels of the lowest unoccupied molecular orbital (LUMO) ranging from -3.29?eV to -3.24?eV, and energy levels of the highest occupied molecular orbital (HOMO) ranging from -6.26?eV to -6.16?eV, suggesting they may possess good electron-transporting or hole-blocking properties. The findings indicate that the molecules offer potential as dopants as well as non-doping light-emitting materials with good electron injection capabilities for fabrication of blue or greenish blue organic light-emitting diodes.

Full text of DOI:10.1016/j.dyepig.2010.01.003

Dyes and Pigments 86 (2010) 190e196
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
The synthesis and photoluminescence characteristics of novel
blue light-emitting naphthalimide derivatives
,
Yi Wang a, Xiaogen Zhang a, Bing Han b, Junbiao Peng b, Shiyou Hou a, Yan Huang a
*
,
,
Huiqin Sun c, Minggui Xie a, Zhiyun Lu d
*
a College of Chemistry, Sichuan University, Chengdu 610064, PR China
b Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Guangzhou 510640, PR China
c Analytical and Testing Center, Sichuan University, Chengdu 610064, PR China
d Key Laboratory of Green Chemistry and Technology, Minister of Education, Sichuan University, Chengdu 610064, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Substitution at the 4-position of 1,8-naphthalimide with electron-donating phenoxy or tert-butyl
modified phenoxy groups, novel naphthalimide derivatives were obtained which emitted blue
fluorescence with emission peaks of 425e444 nm in chloroform solution under UV irradiation, with
highest relative photoluminescence quantum efficiency of 0.82. When in solid film, only compounds that
contained ortho-tert-butylphenoxy substituents displayed blue photoluminescence of 438e451 nm, with
highest absolute fluorescence quantum yield of 0.29; whereas other compounds showed greenish blue
fluorescence at 471e478 nm, with highest absolute fluorescence quantum yield of 0.42. Cyclic voltam-
metry studies revealed that the molecules have low-lying energy levels of the lowest unoccupied
molecular orbital (LUMO) ranging from ꢀ3.29 eV to ꢀ3.24 eV, and energy levels of the highest occupied
molecular orbital (HOMO) ranging from ꢀ6.26 eV to ꢀ6.16 eV, suggesting they may possess good
electron-transporting or hole-blocking properties. The findings indicate that the molecules offer
potential as dopants as well as non-doping light-emitting materials with good electron injection
capabilities for fabrication of blue or greenish blue organic light-emitting diodes.
Received 10 October 2009
Received in revised form
7 January 2010
Accepted 8 January 2010
Available online 18 January 2010
Keywords:
1,8-Naphthalimide
Synthesis
Blue light-emission
Photoluminescence
Fluorescence quantum yield
Electron affinity
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
temperatures [3,9]. Therefore, intense research efforts have focused
on the development of non-doped, pure blue light-emitting
Owing to their potential application in flat panel displays and
solid-state light sources, the development of organic light-emitting
diode (OLED) materials has attracted much attention over the past
twenty or so years. In the context of full-colour OLEDs, efficient blue
emitters are of especial interest because blue OLEDs display much
poorer performance compared to red and green emitters [1e5]. The
luminous efficiency of a blue OLED with CIE x,y coordinates of 0.16,
0.12, respectively, is only 1.57 lm Wꢀ1 [6], while that of green
emitter is 26.2 lm Wꢀ1[1]. Moreover, as concentration quenching is
a common characteristic of fluorescent dyes, a doping procedure is
generally employed during the fabrication of OLEDs [1,2,7,8].
However, this procedure results in poor device durability, as the
dopants may aggregate, stack, and crystallize, resulting in phase
separation under prolonged electric stress and at high working
materials with high fluorescence quantum yield in solid states.
Among the various kinds of OLED emitters, 1,8-naphthalimide
derivativeshave attracted much attentionowingtotheirgood optical,
thermal and chemical stabilities, as well as high photoluminescence
(PL) quantum efficiency in solution [10e13]. By introducing different
electron-donating substituents, such as N-substituted groups [14,15],
C-substituted groups [16e18], and O-substituted groups [19], in the
4-position of 1,8-naphthalimides, their PL emission colour can be
readily tuned from yellowish green to pure blue. Furthermore, it has
been reported that naphthalimide derivatives generally have high
electron affinity due to the existence of an electron-deficient
centre [20e24] and should display good electron-transporting or
hole-blocking capabilities that are appropriate for balanced carrier
injection in OLEDs.
By employing naphthalimide derivatives as emitters, efficient
OLEDs have been obtained [13,14,25e29], such emitters having
employed C-substituted groups (eg arylalkynyl [28], arylalkenyl
[29]) or N-substituted groups (eg arylamino [14], alkylamino [26])
in the 4-postion and electroluminescence (EL) colours varying
* Corresponding authors. Tel.: þ86 28 85410059; fax: þ86 28 85413601.
E-mail addresses: huangyan@scu.edu.cn (Y. Huang), luzhiyun@scu.edu.cn
(Z. Lu).
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.01.003

Y. Wang et al. / Dyes and Pigments 86 (2010) 190e196
191
from greenish blue to orange have been secured. Although it is
reported that 4-alkyloxy or 4-aryloxy substituted naphthalimides
exhibit pure blue fluorescence in solution [19,30], there is, to our
knowledge, only one report employing such kind of naph-
thalimide, i.e., 4,5-diethoxy-1,8-naphthalimide, as EL material [31].
The dye gives pure blue PL emission at 460 nm, but shows green
EL peak at 520 nm, and the performance of the OLED is quite poor
(35 cd mꢀ2 at 100 mA cmꢀ2). The red-shifted EL emission as well
as the low device efficiency may be ascribed to the concentration
quenching of the planar dyes caused by aggregation and close
stacking.
In order to exploit blue emitters with reduced propensity of
concentration quenching, rigid phenoxy substituents were intro-
duced into the 4-position of naphthalimides [19,32], and tert-butyl
substituents were introduced at different positions of the phenoxy
groups to achieve more bulky molecules and alter the conjugation
length as well. In this work, as it has been reported that H-bonds
can be formed between hydroxy and carbonyl groups in N-(2-
hydroxyethyl)-1,8-naphthalimide [33], the influence that H-bonds
may have on photophysical characteristics has been investigated by
modifying the carboximide site with N-(2-hydroxyethyl) or N-(2-
acetoxyethyl) substituents.
a fluorescence spectrophotometer (F-7000 from Hitachi), while
absolute PL quantum yields in neat films were determined with an
integrating sphere (IS80 form Labsphere) equipped together with
a digital photometer (S370 from UDT). In the measurement of PL
spectra and PL efficiencies, different excitations were chosen under
the guidance of absorbance peaks to assure enough absorption. All
the thin films were prepared by the following procedure: the
samples were dissolved in chloroform with the concentration of
40 mg mLꢀ1, then spin-coated onto quartz substrates at a speed of
1700 rpm, the thickness of the films was about 100 nm. Melting
points were measured by a differential scanning calorimeter
(DSC200PC from NETZSCH). The cyclic voltammograms reported
here were recorded with a CHI830B electrochemical workstation at
a constant scan rate of 50 mV sꢀ1. The measurements were
performed in 1 mmol Lꢀ1 acetonitrile solutions of the samples with
tetrabutylammonium hexafluorophosphate solution (0.1 mol Lꢀ1 in
acetonitrile) as supporting electrolyte at room temperature. The
cyclic voltammetric system was constructed using a three-elec-
trode undivided electrochemical cell consisted of a Pt working
electrode, a Pt-wire counter electrode, and a Ag/AgNO3 reference
electrode (0.1 mol Lꢀ1 in CH3CN) under protection of nitrogen, and
each measurement was calibrated with an internal standard,
ferrocene/ferrocenium redox system [34].
2. Experimental
2.2. Syntheses
2.1. Materials and instrumentation
The synthetic route to the target molecules is shown in Fig. 1.
Intermediate 2 was prepared according to the reported procedure
[35]. Compounds 3ae3d were prepared via nucleophilic substitu-
tion reactions of phenol and substituted phenols with 2;
compounds 4ae4d were obtained by further acetylation of 3ae3d.
All chemicals were purchased from Aldrich and Acros Chemical
Co., and were used without further purification. The solvents were
of analytical grade and freshly distilled before use. N, N-dime-
thylformamide (DMF) and dimethylsulfoxide (DMSO) were dried
with CaH2 and freshly distilled before use.
1H NMR and 13C NMR spectra were recorded on a BRUKER (AVII-
400) spectrometer in CDCl3 using TMS as internal standard. High
resolution TOF-MS spectra were recorded on a Waters Q-TOF-Pre-
miter instrument. UVevis absorption spectra of the 10ꢀ5 mol Lꢀ1
solutions as well as spin-coated films on quartz plates were
collected with a UVevisible spectroscopy (HP8453A from Agilent
Co.). Photoluminescence spectra were recorded with a CCD spec-
trography (INTRASPEC IV from Oriel Co.). Relative PL efficiencies in
chloroform solution were determined by using 10ꢀ5 mol Lꢀ1
2.2.1. 4-Phenoxy-N-(2-hydroxyethyl)-1,8-naphthalimide (3a)
To a solution of 0.32 g intermediate 2 (1.0 mmol) dissolved in
40 mL of anhydrous DMF were added phenol 0.20 g (2 mmol) and
anhydrous potassium carbonate 0.54 g (4 mmol). The mixture was
stirred at 100 ꢁC for 90 min under protection of nitrogen, after the
reactant was cooled down, 40 mL of water was added, the yellow
precipitate was collected and purified by column chromatograph
(eluent: petroleum ether/ethyl acetate ¼ 2/1) to afford pale yellow
solid. Yield: 75.7%. Mp: 157e158 ꢁC; 1H NMR (400 MHz, CDCl3)
d
quinine sulfate in 0.1 mol Lꢀ1 H2SO4 (
4
¼ 0.55) as the standard with
(ppm): 8.74(d, J ¼ 8.0 Hz, 1H), 8.67(d, J ¼ 7.2 Hz, 1H), 8.46(d,
O
N
O
O
N
O
O
O
Ar
Br
HOCH2CH2NH2
H2O
Br
OH
Method1
OH
O
O
or Method2
2
3
1
O
N
O
O
(CH3CO)2O
refulx
Ar
O
O
4
t-Bu
t-Bu
O
O
O
O
ArO =
t-Bu
t-Bu
a(3~4)
b(3~4)
c(3~4)
d(3~4)
Fig. 1. The synthetic route to naphthalimide derivatives (Method1: DMF/K2CO3; Method2: DMSO/KOH).

192
Y. Wang et al. / Dyes and Pigments 86 (2010) 190e196
J ¼ 8.4 Hz, 1H), 7.80(t, J ¼ 8.0 Hz, 1H), 7.50(t, J ¼ 8.0 Hz, 2H), 7.32
(t, J ¼ 7.2 Hz, 1H), 7.20(d, J ¼ 8.0 Hz, 2H), 6.91(d, J ¼ 8.0 Hz, 1H), 4.46
(t, J ¼ 5.2 Hz, 2H), 3.99(t, J ¼ 5.2 Hz, 2H); 13C NMR (100 MHz, CDCl3)
4.47 (t, J ¼ 5.2 Hz, 2H), 3.98 (t, J ¼ 5.2 Hz, 2H), 1.41 (s, 9H); 13C NMR
(100 MHz, CDCl3)dd(ppm): 165.25, 164.66, 160.31, 153.38, 142.11,
132.21, 131.16,129.87, 128.91, 126.64, 125.60, 124.15, 122.44, 122.02,
115.94, 110.80, 61.97, 42.86, 34.83, 30.42, 29.68；TOF-MS: m/z
390.1693 (Mþ1), Calc. for MW: 390.1705.
d(ppm): 165.25, 164.65, 160.21, 154.67, 133.22, 132.20, 130.45,
129.73, 128.93, 126.52, 125.72, 123.87, 122.27, 120.84, 116.13, 110.51,
61.95, 42.77; TOF-MS: m/z 334.1062 (Mþ1), Calc. for MW: 334.1079.
2.2.6. 4-(2-tert-butylphenoxy)-N-(2-acetoxyethyl)-1,8-
naphthalimide (4c)
2.2.2. 4-Phenoxy-N-(2-acetoxyethyl)-1,8-naphthalimide (4a)
0.48 g (1.3 mmol) 3a was mixed with 25 mL of acetic anhydride
and stirred at 130 ꢁC for 2 h. After the reactant was cooled down,
the pale yellow precipitate was filtered and recrystallized from
ethanol to afford pale yellow solid. Yield: 67.0%. Mp: 177e178 ꢁC;
Compound 4c was synthesized with the same procedure as 4a,
using 3c and acetic anhydride as starting materials. Yield: 72.0%,
Mp: 96e98 ꢁC. 1H NMR (400 MHz, CDCl3)
d(ppm): 8.76e8.74(m,
1H), 8.70e8.68(m, 1H), 8.48(d, J ¼ 8.4 Hz, 1H), 7.84e7.80(m, 1H),
7.55e7.53(m, 1H), 7.30e7.28(m, 1H), 7.25e7.22(m, 1H), 7.00e6.98
(m, 1H), 6.91(d, J ¼ 8.4 Hz, 1H), 4.51e4.48(m, 2H), 4.45e4.22(m,
1H NMR (400 MHz, CDCl3)
d(ppm): 8.74e8.72 (m, 1H), 8.69e8.67
(m, 1H), 8.47(d, J ¼ 8.4 Hz, 1H), 7.82e7.78 (m, 1H), 7.52e7.48 (m,
2H), 7.32 (t, J ¼ 7.6 Hz,1H), 7.20 (d, J ¼ 7.6 Hz, 2H), 6.92 (d, J ¼ 8.4 Hz,
1H), 4.49 (t, J ¼ 5.2 Hz, 2H), 4.43 (t, J ¼ 5.2 Hz, 2H), 2.02(s, 3H); 13C
2H), 2.02(s, 3H), 1.41(s 9H); 13C NMR (100 MHz, CDCl3)
d(ppm):
171.05, 164.49, 163.79, 160.17, 153.42, 142.11, 133.50, 133.12, 132.07,
131.39, 129.90, 128.79, 127.72, 126.63, 124.21, 122.51, 116.03, 110.78,
61.71, 38.98, 34.84, 30.42, 20.89; TOF-MS: m/z: 432.1826 (Mþ1),
Calc. for MW: 432.1811.
NMR (100 MHz, CDCl3)d(ppm): 170.96, 164.43, 163.76, 160.04,
154.84, 132.97, 132.03, 130.42, 129.83, 128.77, 126.51, 125.62, 124.04,
122.44, 120.76, 116.37, 110.65, 61.68, 38.98, 29.68; TOF-MS: m/z
376.1211(Mþ1), Calc. for MW: 376.1185.
2.2.7. 4-[2,4-di(tert-butyl)]phenoxy-N-(2-hydroxyethyl)-1,8-
naphthalimide (3d)
2.2.3. 4-[(4-tert-butylphenoxy)]-N-(2-hydroxyethyl)-1,8-
naphthalimide (3b)
Compound 3d was synthesized with the same procedure as 3c,
using 2,4-di(tert-butyl)phenol and 2 as starting materials. Yield:
Compound 3b was synthesized with the same procedure as 3a,
using 4-tert-butylphenol and 2 as starting materials. Yield: 54.0%.
51.0%, Mp: 160e162 ꢁC. 1H NMR (400 MHz, CDCl3)
d(ppm):
Mp: 174e175 ꢁC. 1H NMR (400 MHz, CDCl3)
d
(ppm): 8.76e8.73(m,
8.76e8.74 (m, 1H), 8.70e8.68 (m, 1H), 8.48 (d, J ¼ 8.4 Hz, 1H), 7.81
(t, J ¼ 8.4 Hz, 1H), 7.54 (s, 1H), 7.30e7.27 (m, 1H), 6.93 (t, J ¼ 8.4 Hz,
2H), 4.47 (t, J ¼ 5.2 Hz, 2H), 3.98 (t, J ¼ 5.2 Hz, 2H),1.38 (d, J ¼ 9.2 Hz,
1H), 8.69e8.67(m, 1H), 8.47(d, J ¼ 8.4 Hz, 1H), 7.80(t, J ¼ 8.0 Hz, 1H),
7.51e7.48(m, 2H), 7.14e7.11(m, 2H), 6.93(d, J ¼ 8.0 Hz, 1H), 4.47(t,
J ¼ 5.2 Hz, 2H), 3.99(t, J ¼ 5.2 Hz, 2H), 1.38(s, 9H); 13C NMR
18H); 13C NMR (100 MHz, CDCl3)
d(ppm): 165.35, 164.77, 160.58,
(100 MHz, CDCl3)
d
(ppm): 164.30, 163.71, 159.52, 151.10, 147.79,
150.77, 148.32, 141.09, 133.44, 132.20, 129.87, 129.01, 126.57, 124.92,
124.56, 124.10, 122.38, 121.45, 115.64, 110.62, 62.06, 42.77, 34.99,
132.29, 131.16, 128.71, 128.00, 126.25, 125.41, 122.81, 121.21, 119.28,
114.83, 109.22, 61.02, 41.76, 33.58, 30.44, 28.67; TOF-MS: m/z
390.1693 (Mþ1), Calc. for MW: 390.1705.
34.80, 31.55, 30.50; TOF-MS: m/z 446.2299 (Mþ1), Calc. for MW
:
446.2331.
2.2.4. 4-[(4-tert-butylphenoxy)]-N-(2-acetoxyethyl)-1,8-
naphthalimide (4b)
2.2.8. 4-[2,4-di(tert-butyl)]phenoxy-N-(2-acetoxyethyl)-1,8-
naphthalimide (4d)
Compound 4b was synthesized with the same procedure as 4a,
using 3b and acetic anhydride as reactants. Yield: 76.0%. Mp:
Compound 4d was synthesized with the same procedure as 4a,
using 3d and acetic anhydride as starting materials. Yield: 53.0%,
154e155 ꢁC. 1H NMR (400 MHz, CDCl3)
d(ppm): 8.74 (d, J ¼ 8.0 Hz,
Mp: 108e109 ꢁC. 1H NMR(400 MHz, CDCl3)
d(ppm): 8.67(d,
1H), 8.68 (d, J ¼ 7.2 Hz, 1H), 8.47 (d, J ¼ 8.4 Hz, 1H), 7.79 (t,
J ¼ 8.0 Hz, 1H), 7.51e7.48 (m, 2H), 7.13e7.10 (m, 2H), 6.93 (d,
J ¼ 8.4 Hz, 1H), 4.49 (t, J ¼ 5.2 Hz, 2H), 4.43 (t, J ¼ 5.2 Hz, 2H), 2.01
J ¼ 8.4 Hz, 1H), 8.61(d, J ¼ 7.6 Hz, 1H), 8.40(d, J ¼ 8.4 Hz, 1H), 7.73(t,
J ¼ 8.4 Hz, 1H), 7.46(s, 1H), 7.22e7.20(m, 1H), 6.88e6.83(m, 2H),
4.42(t, J ¼ 5.2 Hz, 2H), 4.36(t, J ¼ 5.2 Hz, 2H), 1.95(s, 3H), 1.32(d,
(s, 3H), 1.38 (s, 9H); 13C NMR (100 MHz, CDCl3)
d(ppm): 171.02,
J ¼ 12.4 Hz, 18H); 13C NMR (100 MHz, CDCl3)
d(ppm): 169.98, 163.47,
164.50, 163.83, 160.40, 152.22, 148.76, 133.07, 132.02, 129.80,
128.87, 127.26, 126.42, 123.94, 122.36, 120.27, 116.03, 110.30, 61.69,
38.95, 34.60, 31.47, 20.88; TOF-MS: m/z: 432.1823 (Mþ1), Calc. for
MW: 432.1811.
162.78, 159.35, 149.82, 147.21, 140.04, 132.13, 130.97, 128.86, 127.79,
125.49, 123.85, 123.50, 123.14, 121.43, 120.36, 114.74, 109.60, 60.68,
37.92, 33.96, 33.75, 30.51, 29.46, 19.85; TOF-MS: m/z: 488.2374
(Mþ1), Calc. for MW: 488.2437.
2.2.5. 4-(2-tert-butylphenoxy)-N-(2-hydroxyethyl)-1,8-
3. Results and discussion
naphthalimide (3c)
To a solution of 2-tert-butylphenol 0.75 g (5 mmol) in 20 mL of
anhydrous DMSO was added 0.35 g (6.25 mmol) potassium
hydroxide. The resulting mixture was stirred at room temperature
for 30 min under protection of nitrogen, then a solution of 0.80 g
(2.5 mmol) 2 dissolved in 20 mL DMSO was added dropwise over
30 min. The mixture was stirred at room temperature for 3 h,
neutralized to pH ¼ 7 by 1 mol Lꢀ1 hydrochloric acid, and then
extracted with 20 mL ꢂ 3 ethyl acetate. The organic layer was
washed with water, dried under anhydrous magnesium sulfate,
then vacuum-vaporized to remove solvent. The residue was puri-
fied by column chromatograph (petroleum ether/ethyl acetate ¼ 8/
1) to obtain pale yellow solid. Yield: 41.2%, Mp: 179e180 ꢁC. 1H NMR
3.1. Synthesis and characterization
All the target dyes are synthesized via nucleophilic substitutions
of phenol derivatives with 4-bromo-1,8-naphthalimides, and the
nucleophilicity of the phenols would have strong effect on the
reaction conditions. For example, 3a and 3b can be prepared using
K2CO3 as base with good yields, while 3c and 3d could not be
obtained with satisfied yields until more alkaline KOH is used,
because the nucleophilic reagents of 2-tert-butylphenol and 2,4-di
(tert-butyl)phenol have more steric hindrance compared to phenol
and 4-tert-butylphenol.
The chemical structures of the resulting naphthalimides are
confirmed by 1H NMR, 13C NMR and high resolution TOF-MS
spectra. All the molecules have good solubility in common organic
solvents such as dichloromethane, chloroform, tetrahydrofuran,
(400 MHz, CDCl3)
1H), 8.48 (d, J ¼ 8.0 Hz, 1H), 7.83 (t, J ¼ 8.0 Hz, 1H), 7.54e7.48 (m,
d
(ppm): 8.76 (d, J ¼ 8.4 Hz, 1H), 8.70 (d, J ¼ 7.2 Hz,
1H), 7.29e7.25 (m, 2H), 7.01e6.98 (m, 1H), 6.91(d, J ¼ 8.4 Hz, 1H),

Y. Wang et al. / Dyes and Pigments 86 (2010) 190e196
193
Table 1
UVevis absorption, photoluminescence, and melting point data of the target naphthalimides.
Compound
Melting point (ꢁC)
In solution
In film
labs (nm)a
lPL (nm)a
4PL
FWHM (nm)
labs (nm)
lPL (nm)
4PL
FWHM (nm)
b
c
3a
4a
3b
4b
3c
4c
3d
4d
157e158
177e178
174e175
154e155
208e210
96e98
366
365
369
367
369
364
374
372
429
425
435
429
432
427
444
442
0.55
0.82
0.27
0.46
0.66
0.46
0.02
0.03
71
67
67
73
69
68
65
66
369
367
369
368
369
366
369
370
478
473
471
475
448
438
451
441
0.14
0.15
0.42
0.11
0.04
0.04
0.29
0.24
95
92
99
91
76
61
72
74
167e168
128e129
a
Solution in 10ꢀ5 mol Lꢀ1 CHCl3.
b
4PL is relative PL quantum efficiency of naphthalimide solution in 10ꢀ5 mol Lꢀ1 CHCl3 which is determined by using 10ꢀ5 mol Lꢀ1 quinine sulfate in 0.1 mol Lꢀ1 H2SO4
¼ 0.55) as the standard reference.
(
4
c
4PL is absolute PL quantum efficiency of naphthalimide solid film, which is determined with an integrating sphere together with a digital photometer.
chlorobenzene, dioxane and toluene. The melting points data of
can be observed that the eight compounds have quite similar
the eight molecules determined by DSC are summarized in Table 1,
the results indicate that most of them have high melting points of
>150 ꢁC.
UVevis absorption characteristics. The maximum absorption peaks
(labs), which can be assigned to the p-p* electronic transitions,
situate in 364e374 nm both in solution and solid films. Compared
with the objective molecules, intermediate 2 has its labs located in
340 nm with a shoulder peak of 354 nm, implying the replacement
of electron-withdrawing bromo-substituent with electron-
donating phenoxy groups would result in prolonged conjugation
length. The resembled absorption spectra of the dyes indicate that
the incorporation of tert-butyl with phenoxy groups would has
limited effect on their electronic energy levels. But one point should
be noted is that compounds 3d and 4d, which both have two tert-
butyl modified phenoxy groups, show more red-shifted labs of
374 nm and 372 nm in solution, giving evidence that tert-butyl acts
as weak electron-donating substituent here.
3.2. Photophysical properties
The photophysical characteristics of the objective molecules
are investigated in dilute CHCl3 solutions (10ꢀ5 mol Lꢀ1) and
thin solid films as well. The data of UVevis absorption and PL
emission peaks are summarized in Table 1.
3.2.1. UVevis absorption properties
The UVevis absorption spectra of the objective dyes are shown
in Fig. 2 (in 10ꢀ5 mol Lꢀ1 CHCl3 solution) and Fig. 3 (in solid films). It
Fig. 2. UVevis absorption spectra of 2, 3ae3d and 4ae4d in 10ꢀ5 mol Lꢀ1 CHCl3
solution.
Fig. 3. UVevis absorption spectra of 3ae3d and 4ae4d in thin solid film state.

194
Y. Wang et al. / Dyes and Pigments 86 (2010) 190e196
3.2.2. Photoluminescence properties
The PL spectra of the target molecules in 10ꢀ5 mol Lꢀ1 CHCl3
solution and neat films are shown in Fig. 4 and Fig. 5, respectively,
PL quantum yield (4PL) data of solution and films are tabulated in
Table 1. Upon UV photoexcitation, intermediate 2 exhibits quite
weak PL in solution owing to the existence of the electron-with-
drawing bromo-substituent, and the maximum emission peak
(
lPLmax) locates at 406 nm. While the replacement of bromo- with
electron-donating phenoxy-substituents results in dramatically
improved PL intensity due to the formation of typical D- -A
p
molecular structures, the emission colour is red-shifted to pure
blue with lPLmax of 425e444 nm and narrow full width at half
maximum (FWHM) of 65e73 nm. The fluorescence properties in
solid films, however, differ from those in solution due to the
condensing aggregation. Intrinsic relationships between molecular
structure and PL properties have been investigated in detail.
Compounds 3a and 4a with planar phenoxy at 4-position show
pure blue PL in solution (lPLmax < 430 nm), with high PL efficiencies
of 0.55 and 0.82. While in solid films, the two compounds show
dramatically bathochromic shifted (w50 nm) lPLmax of 478 nm and
473 nm, with much broadened FWHM of >90 nm, and the emission
colour is greenish blue. Meanwhile, 3a and 4a have much lower 4PL
(<0.15) in solid films. These results confirm that there exists intense
intermolecular aggregation and stacking in condensed states of 3a
and 4a, due to their relatively planar structures.
Compounds 3b and 4b, which have para-tert-butylphenoxy
groups, also give pure blue PL in solution, but their 4PL drop greatly
to 0.27 and 0.46, this can be ascribed to the free rotation of tert-
butyl groups that quenches the singlet excited states of the fluo-
rescent molecules [36]. Their PL emission peaks and colours in solid
films are similar with those of 3a and 4a, yet the 4PL of 3b is as high
as 0.42, suggesting that the introduction of 4-(para-tert-butylphe-
noxy) to naphthalimide skeleton is an effective way for diminishing
concentration quenching, but has limited effect on the tuning of
emission colour.
Fig. 4. PL spectra of 2, 3ae3d and 4ae4d in 10ꢀ5 mol Lꢀ1 CHCl3 solution (lexc ¼ 340 nm
for 2, and lexc ¼ 376 nm for 3 and 4).
By altering the para-tert-butyl substituents to ortho-ones, the
resulting compounds, 3c and 4c, show pure blue PL both in
solution and solid films with lPLmax < 450 nm and relative narrow
FWHM of 61e76 nm. This can be attributed to the twist of phenyl-
O-naphthalimide conjugation backbone caused by the bulky ortho-
tert-butyl, which would result in more rigid non-planar molecules
with less conjugation lengths. The rigid structures would bring
about good 4PL in solution, but poor 4PL in condensed state due to
the partly interrupted conjugation system. The experiment results
are consistent with our hypothesis: the 4PL of 3c and 4c are good
(0.46e0.66) in solution, while as low as 0.04 in films.
The introduction of another para-tert-butyl into 3c and 4c gives
compounds 3d and 4d. They have longer lPLmax of 444 nm and
442 nm in CHCl3 solution compared to that of the other six
compounds, confirming that they have longer conjugation systems,
which is in accordance with the UVevis absorption characteristics.
Like 3c and 4c, 3d and 4d also give pure blue PL both in solution and
solid films, but have much dropped 4PL (<0.03) in solution which
may be ascribed to the free rotation of tert-butyl groups, and much
improved 4PL of >0.24 in films which can be assigned to the
effective inhibition of concentration quenching.
Additionally, 4a, 4b and 4d have better 4PL compared to 3a, 3b
and 3d, respectively, implying that the end capping of hydroxy
group by acetylation may be beneficial to the enhancement of 4PL in
CHCl3 solution.
3.2.3. Electrochemical properties
To gain information on the charge injection capabilities, the
electrochemical behavior of the target dyes was investigated by
cyclic voltammetry in 1 mmol Lꢀ1 acetonitrile solutions. As
Fig. 5. PL spectra of 3ae3d and 4ae4d in thin solid film state (lexc ¼ 370 nm).

Y. Wang et al. / Dyes and Pigments 86 (2010) 190e196
195
levels ranging from ꢀ6.26 to ꢀ6.16 eV, suggesting they have high
electron affinities, and are potentially electron-transporting or
hole-blocking blue electroluminescent materials. Their electro-
chemical band gaps (Eegc), calculated from cyclic voltammetry data
ox
red
(E
ꢀ E
), are 2.95e3.01 eV, somewhat in good accordance
onset
onset
with the optical band gaps (Eogpt) estimated from the onset
absorption wavelength (lonset) of the solution (3.03e3.08 eV).
4. Conclusion
By introducing phenoxy or tert-butyl modified phenoxy into
their 4-positions, and incorporation of N-(2-hydroxyethyl) or N-(2-
acetoxyethyl) substituents with their carboximide sites, eight blue
light-emitting 1,8-naphthalimides are prepared, and the relation-
ships between molecular structures and photophysical properties
are investigated. The results indicate that the compounds
comprising 2-(tert-butyl)phenoxy substituent would give pure blue
PL emission in solid films, and the addition of an extra 4-(tert-butyl)
would lead to enhanced PL quantum yield in film. The resulting
compound, 4-[2,4-di(tert-butyl)]phenoxy-N-(2-hydroxyethyl)-1,8-
naphthalimide, has lPLmax of w450 nm and high 4PL of 0.29, which
is suitable as emitting material for fabrication of non-doping
blue OLEDs. Moreover, all these compounds exhibit pure blue PL
emission in chloroform solution with the highest 4PL of 0.82, yet the
introduction of 4-[2,4-di(tert-butyl)phenoxy] substituents would
lower the 4PL in solution. Electrochemical measurements indicate
that the objective compounds have high electron affinities of
3.29e3.24 eV, implying they have good electron injection capabil-
ities. All these results suggest that these novel naphthalimides are
perspective blue dopants or non-doping blue light emitters with
good electron-transporting properties for fabrication of blue
OLEDs.
Fig. 6. Cyclic voltammograms of 4ae4d in 1 mmol Lꢀ1 CH3CN solution on Pt con-
taining 0.1 mol Lꢀ1 of Bu4NPF6 as supporting electrolyte and ferrocene/ferrocenium as
calibrant, referenced with Ag/AgNO3 electrode.
compounds 3ae3d all have free hydroxy groups which may form
hydrogen bonds with the solvent, herewith, only the cyclic vol-
tammograms (CV) of the four acetyl-end-capped dyes, i.e., 4ae4d,
are given (shown in Fig. 6). All the four compounds exhibit
quasireversible reduction waves upon the cathodic sweeps, with
onset reduction potentials of ꢀ1.51wꢀ1.56 V vs Ag/AgNO3, while
the oxidation processes exhibit irreversible waves when sweep
anodically, the onset potentials of oxidation for the dyes are
located at 1.33e1.43 V vs Ag/AgNO3. The HOMO and LUMO energy
levels of the target molecules can be estimated from the following
equations [37]:
Acknowledgements
ox
This work was financially supported by the National Natural
Science Foundation of China under project No. 20672076.
HOMO [ ðL4:8 L E
ðvs Fc=FcDÞÞeV
ðvs Fc=FcDÞÞeV
onset
red
LUMO [ ðL4:8 L E
onset
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ox
red
the vacuum level, and E
(vs Fc/Fcþ)/E
(vs Fc/Fcþ) are
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