Novel 1,8-naphthalimide derivatives for standard-red organic light-emitting device applications

Article abstract of DOI:10.1039/c5tc00409h

Three red-emissive D-π-A-structured fluorophores with an aromatic amine as the donor, ethene-1,2-diyl as the π-bridge, and 1,8-naphthalimide as the acceptor subunit, namely, (E)-6-(4-(dimethylamino)styryl)-2-hexyl-1H-benzo[de]isoquinoline-1,3(2H)-dione (Nap1), (E)-2-(2,6-di(isopropyl)phenyl)-6-(4-(dimethylamino)styryl)-1H-benzo[de]isoquinoline-1,3(2H)-dione (Nap2) and (E)-2-(2,6-di(isopropyl)phenyl)-6-(2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]quinolin-9-yl)vinyl)-1H-benzo[de]isoquinoline-1,3(2H)-dione (Nap3), were designed and synthesized. In-depth investigations on the correlations between their molecular structures and photophysical characteristics revealed that the presence of an electron-rich 4-dimethylaminophenyl donor moiety in compound Nap1 could endow it with a red emission (e.g., λ_PLmax = 641 nm in the host-guest blend film with a 14 wt% guest composition); moreover, the replacement of the n-hexyl group of Nap1 bonding to the imide nitrogen atom for a more bulky 2,6-di(isopropyl)phenyl one would result in compound Nap2 with more alleviated concentration quenching. Alteration of the 4-(dimethylamino)phenyl donor subunit of Nap2 into a more electron-donating 1,1,7,7-tetramethyljulolidin-9-yl substituent would render compound Nap3 with more improved chromaticity (e.g., λ_PLmax = 663 nm in a 14 wt% guest-doped film). Consequently, Nap3 could not only emit standard-red fluorescence with satisfactory chromaticity, but it also showed suppressed intermolecular interactions. Using Nap3 as the dopant, a heavily doped standard-red organic light-emitting diode (OLED) with the device configuration of ITO/MoO₃ (1 nm)/TcTa (40 nm)/CzPhONI:Nap3 (14 wt%) (20 nm)/TPBI (45 nm)/LiF (1 nm)/Al (80 nm) was fabricated, and the Commission Internationale de L'Eclairage coordinates, maximum external quantum efficiency and maximum current efficiency of this OLED were (0.67,0.32), 1.8% and 0.7 cd A^-1, respectively. All these preliminary results indicated that 1,8-naphthalimide derivatives could act as quite promising standard-red light-emitting materials for OLED applications. This journal is

Full text of DOI:10.1039/c5tc00409h

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Materials Chemistry C
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Novel 1,8-naphthalimide derivatives for
standard-red organic light-emitting device
Cite this:DOI: 10.1039/c5tc00409h
applications†
a
b
a
a
b
a
Shuai Luo,‡ Jie Lin,‡ Jie Zhou, Yi Wang, Xingyuan Liu,* Yan Huang,
a
a
Zhiyun Lu* and Changwei Hu*
Three red-emissive D–p–A-structured fluorophores with an aromatic amine as the donor, ethene-1,2-diyl
as the p-bridge, and 1,8-naphthalimide as the acceptor subunit, namely, (E)-6-(4-(dimethylamino)styryl)-2-
hexyl-1H-benzo[de]isoquinoline-1,3(2H)-dione (Nap1), (E)-2-(2,6-di(isopropyl)phenyl)-6-(4-(dimethylamino)-
styryl)-1H-benzo[de]isoquinoline-1,3(2H)-dione (Nap2) and (E)-2-(2,6-di(isopropyl)phenyl)-6-(2-(1,1,7,7-tetra-
methyl-2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]quinolin-9-yl)vinyl)-1H-benzo[de]isoquinoline-1,3(2H)-dione
(
Nap3), were designed and synthesized. In-depth investigations on the correlations between their
molecular structures and photophysical characteristics revealed that the presence of an electron-rich
-dimethylaminophenyl donor moiety in compound Nap1 could endow it with a red emission (e.g.,
PL max = 641 nm in the host–guest blend film with a 14 wt% guest composition); moreover, the replacement
4
l
of the n-hexyl group of Nap1 bonding to the imide nitrogen atom for a more bulky 2,6-di(isopropyl)phenyl
one would result in compound Nap2 with more alleviated concentration quenching. Alteration of the
4
-(dimethylamino)phenyl donor subunit of Nap2 into a more electron-donating 1,1,7,7-tetramethyljulolidin-
-yl substituent would render compound Nap3 with more improved chromaticity (e.g., lPL max = 663 nm in
9
a 14 wt% guest-doped film). Consequently, Nap3 could not only emit standard-red fluorescence with
satisfactory chromaticity, but it also showed suppressed intermolecular interactions. Using Nap3 as the
dopant, a heavily doped standard-red organic light-emitting diode (OLED) with the device configuration of
Received 11th February 2015,
Accepted 11th April 2015
ITO/MoO
3
(1 nm)/TcTa (40 nm)/CzPhONI:Nap3 (14 wt%) (20 nm)/TPBI (45 nm)/LiF (1 nm)/Al (80 nm) was
fabricated, and the Commission Internationale de L’Eclairage coordinates, maximum external quantum
ꢀ1
efficiency and maximum current efficiency of this OLED were (0.67,0.32), 1.8% and 0.7 cd A , respectively.
All these preliminary results indicated that 1,8-naphthalimide derivatives could act as quite promising
standard-red light-emitting materials for OLED applications.
DOI: 10.1039/c5tc00409h
www.rsc.org/MaterialsC
with phosphorescent OLEDs, fluorescent ones are more suitable
1
. Introduction
with respect to display applications due to their faster response as
2
Owing to their merits like solid-state self-emission, wide viewing well as lower efficiency roll-off. For full-color display applications,
angle, facile color tunability and processability, organic light- it is necessary to develop red, green, and blue fluorescent OLEDs
emitting devices (OLEDs) have been considered as quite competi- with high efficiency and appropriate chromaticity, yet compared
1
3a
3b
tive candidates for flat-panel display applications. In comparison with that of blue and green devices, the performance of red
fluorescent OLEDs with good chromaticity is unsatisfactory. For
example, although the current efficiency (CE) of red OLEDs with
a
Key Laboratory of Green Chemistry and Technology of Ministry of Education,
Commission Internationale de L’Eclairage (CIE) coordinates of
College of Chemistry, Sichuan University, Chengdu, 610064, P. R. China.
ꢀ1 4
(
0.63,0.37) could reach 11.5 cd A , their chromaticity is far from
E-mail: luzhiyun@scu.edu.cn, changweihu@scu.edu.cn
b
standard-red whose CIE coordinates should be close to (0.67,0.33)
according to the stipulation from the National Television Stan-
dards Committee. As far as standard-red OLEDs are concerned,
although according to a technical report from Idemitsu Kosan Co.,
State Key Laboratory of Luminescence and Applications, Changchun Institute of
Optics, Fine Mechanics and Physics, Chinese Academy of Sciences,
Changchun 130033, P. R. China. E-mail: liuxy@ciomp.ac.cn
†
Electronic supplementary information (ESI) available: Lippert–Mataga plots of
Nap1–3, cyclic voltammograms of Nap1–3, EL spectra of devices I–III, current
ꢀ
1
1
13
high performance devices with CE of 11.0 cd A
could be
efficiency–current density characteristics of devices I–III, H NMR, C NMR,
FT-IR, and HRMS spectra of Nap1–3. See DOI: 10.1039/c5tc00409h
5
achieved, no detailed information about the molecular structures
of the emitting materials and the device structures could be found.
‡
These authors contributed equally.
This journal is ©The Royal Society of Chemistry 2015
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In scientific literature reports, the performance of standard-red
OLEDs is rather poor with the maximum CE values lower than
ꢀ
1
3
4
.0 cd A and external quantum efficiencies (EQEs) less than
6
.0%. In addition, due to the notorious concentration quenching
of the guest fluorophores, the best standard-red OLED bearing
ꢀ
1
guest–host doping structure (CEmax: 1.6 cd A ) has just a low
6d
doping-level of 2 wt%; hence the manufacturing process should
7
be precisely controlled to acquire good device reproducibility.
Consequently, it is highly desired to exploit novel high perfor-
mance standard-red fluorophores with suppressed concentration
6c,8
quenching, so that OLEDs with relatively high doping-levels or
even self-hosted device structure could be achieved.
Fig. 1 Molecular structures of NIM, Nap1, Nap2 and Nap3.
In general, red electrofluorescent materials could be classified
into two main categories, namely, compounds bearing polycyclic
aromatic hydrocarbon (PAHs) structures and compounds showing
intramolecular charge-transfer (ICT) characteristic with D–p–A
molecular structures (where D denotes electron-donor and A
emit intense orange fluorescence in the solid state (lPL max
=
24
597 nm). To construct fluorophores with more red-shifted
photoluminescence (PL) emission bands than that of NIM,
herein, we altered the 4-(diphenylamino)phenyl (DPAP) donor
segment of NIM to be a more electron-rich 4-(dimethylamino)-
phenyl (DMAP) or 1,1,7,7-tetramethyljulolidin-9-yl (TMJ) donor
segment, and constructed three objective compounds (Nap1–3,
structures shown in Fig. 1). Moreover, to alleviate the inter-
molecular interactions, a bulky 2,6-di(isopropyl)phenyl substi-
tuent was bonded to the imide nitrogen atom of Nap2 and
Nap3. As expected, Nap1–3 all could emit standard-red fluores-
cence in the thin solid film state, and Nap2 and Nap3 showed
more suppressed concentration quenching than Nap1. Using
Nap3 as the guest dopant, a high performance standard-red
OLED with a relatively heavy doping-level of 14 wt% was achieved.
The device had CIE coordinates of (0.67,0.32), EQEmax of 1.8%,
8
–10
denotes electron-acceptor).
To acquire standard-red fluores-
cence, PAHs compounds should possess relatively large conju-
1
1a
11b,c
gation systems, e.g., polyacenes and porphyrins.
However,
to prepare these compounds, delicate synthetic and purifying
1
2
procedures have to be involved. In addition, serious concen-
tration quenching is often observed in these PAHs due to their
1
0
intense intermolecular interactions. By contrast, ICT-featured
red fluorophores bearing D–p–A molecular skeletons have
attracted much attention due to their more facile molecular
1
3
tailoring. Currently, the reported red electrofluorescent ICT-
compounds could be assorted, according to their acceptor
1
4
constructive units, as maleimide derivatives, benzothiadiazole
1
5
6f,16
derivatives, fumaronitrile derivatives,
4
and 4-dicyanomethylene-
ꢀ1
and a CEmax of 0.7 cd A , indicating that 1,8-naphthalimide
1
7
H-pyran derivatives, etc. However, despite the fact that
derivatives should be promising candidates as high performance
standard-red EL materials.
1
,8-naphthalimide is a widely used acceptor subunit for con-
1
8
19
structing high performance green and yellow ICT-featured
electroluminescent (EL) materials, quite limited success has
been achieved in terms of high performance orange and red
ones. In 2003, using 1,8-naphthalimide and quinoxaline as the
2
. Experimental section
20
2.1 General information
acceptor segments, Lee et al. reported an naphthalimide lumi-
nogen, but the OLED based on it could only emit orange-red EL All the reagents involved in the synthetic procedures were com-
21
with lEL max of 608 nm. Subsequently, Tian et al. reported that mercially available and used without purification unless other-
D–A fluorophores with 1,8-naphthalimide as the acceptor subunit wise stated. Cyclohexane (CHX), tetrachloromethane, toluene (Tol),
could act as red EL materials, yet both the chromaticity and the chloroform, tetrahydrofuran (THF), dichloromethane (DCM),
performance of the OLED were unsatisfactory (lEL max = 620 nm, acetone, N,N-dimethyl formamide (DMF) were of analytical grade
ꢀ
2
1
13
Lmax (maximum brightness) is 15 cd m ). Although in 2005, and were distilled freshly prior to use. H NMR and C NMR
22
Cheng et al. fabricated red OLEDs with CIE coordinates of spectra were obtained on a BRUKER AVANCE AV II-400 MHz
0.63,0.36) using 1,8-naphthalic anhydride derivatives as light- spectrometer in DMSO-d or CDCl using TMS as the internal
(
6
3
emitting materials, the CEmax of these devices was as low as standard. High resolution MS spectra were obtained on a
ꢀ
1
0
.6 cd A . Nevertheless, our recent study has revealed that a Shimadzu LCMS-IT-TOF spectrometer. FT-IR spectra were
,8-naphthalimide derivative could act as a high performance obtained on a Perkin-Elmer 2000 infrared spectrometer with
1
orange EL guest material, and the corresponding OLED shows a KBr pellets under an ambient atmosphere. UV-Vis absorption
ꢀ1
ꢀ2 23
CEmax of 7.2 cd A , EQEmax of 3.6%, and Lmax of 16 800 cd m
Despite the fact that the CIE coordinates of this device was just spectrophotometer. PL spectra were obtained on a Perkin-Elmer
0.56,0.44), its high performance has triggered our speculation LS55 fluorescence spectrophotometer at 298 K. The absolute PL
.
spectra were obtained on a Perkin-Elmer Lambda 950 scanning
(
that through rational molecular design, 1,8-naphthalimide deri- quantum yields (QYs) of both solution and film samples were
vatives may also act as quite promising standard-red EL materials. determined on a FluoroMax-4 spectrofluorometer (Horiba Jobin
Recently, it has been reported that the D–p–A-structured 1,8- Yvon) equipped with an integrating sphere and a digital photo-
naphthalimide derivative NIM (structure shown in Fig. 1) could meter. Both the fluorimeters have been corrected for the wavelength
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dependence of the sensitivity of the detectors and throughput of then a solution of 3 (0.4 g, 1.6 mmol) in 5.0 mL anhydrous
the monochromators. The Nap–CzPhONI composite as well as degassed THF was added. After being stirred for 20 min, the
the Nap neat thin film samples were spin-coated from their yellow suspension was poured into saturated aqueous ammonium
corresponding chloroform solutions with a concentration of chloride, and then extracted by petroleum ether (4 ꢁ 15 mL). The
ꢀ
1
1
4
0 mg mL at a speed of 1500 rpm on quartz substrates for organic layer was dried over anhydrous Na SO , and concentrated
2 4
0 s. Cyclic voltammetry measurements were performed in in vacuum. The resulting crude product was purified by silica gel
ꢀ
4
ꢀ1
anhydrous acetonitrile (5 ꢁ 10 mol L ) solutions of Nap1–3 column chromatography (eluent: petroleum ether/ethyl acetate =
ꢀ
1
using 0.10 mol L Bu
a N atmosphere on a LK 2010A electrochemical work station at CDCl
room temperature, and the three-electrode cell comprised a Pt 10.8 Hz,QCH), 5.51 (1H, dd, J
working electrode, a Pt wire counter electrode, and a Ag/AgNO3 (d, J = 10.0 Hz, J = 1.6 Hz, 1H, QCH), 3.18 (t, J = 6.0 Hz, 6H,
4
NClO
4
as the supporting electrolyte under 20/1, v/v) to give 4 as a yellowish liquid (0.3 g, 50%). d
; Me Si) 7.07 (2H, s, ArH), 6.62 (1H, dd, J = 17.2 Hz, J
= 17.6 Hz, J = 1.6 Hz,QCH), 4.99
H
(400 MHz;
2
3
4
1
2
=
1
2
1
2
(
0.1 M in acetonitrile) reference electrode. A ferrocene/ferrocenium –NCH ), 1.45 (t, J = 6.0 Hz, 1H, Ar–CH ), 1.30 (s, 12H, –CH ).
2
2
3
redox couple was employed as the external standard.
(E)-6-(4-(Dimethylamino)styryl)-2-hexyl-1H-benzo[de] isoquino-
line-1,3(2H)-dione (Nap1). A Schlenk flask was charged with a
2
.2 OLED fabrication and measurements
mixture of 2 (0.13 g, 0.9 mmol), 5 (0.30 g, 0.9 mmol), Pd(OAc)
3.5 mg, 0.02 mmol), P(o-tolyl) (8.3 mg, 0.03 mmol), triethyl-
2
(
3
Indium-tin oxide (ITO) coated glass substrates were cleaned by
ultrasonic mixing successively in alcohol, acetone, methanol,
and deionized water, followed by UV-ozone oxygen plasma
treatment for 2 min before use. Organic functional layers were
amine (6.0 mL) and N,N-dimethylformamide (DMF) (7.0 mL).
The reaction mixture was heated at 90 1C for 24 h under argon.
After being cooled to room temperature, the mixture was
poured into water (100 mL), extracted by dichloromethane
(4 ꢁ 20 mL) and then organic phase was combined and washed
ꢀ
4
thermo-evaporated in vacuum (3 ꢁ 10 Pa) with a deposition
ꢀ
1
rate of 0.1 nm s . After deposition of the organic layers, the
LiF–Al cathode was prepared first by thermal deposition of a
2 4
with brine and then dried over anhydrous Na SO . After the
solvent was removed under vacuum, the crude product was
purified by column chromatography over a silica gel (eluent:
petroleum ether/dichloromethane = 15/1, v/v), followed by
recrystallization from a mixture of cyclohexane and dichloro-
methane to afford pure Nap1 as a red solid (0.1 g, 30%). dH
LiF thin film (1 nm) followed by the deposition of an Al layer
2
(80 nm). The active area of the OLEDs was 1 ꢁ 1 mm . The
thicknesses of the organic layers and the cathode were controlled
using a quartz crystal thickness monitor. The luminance–voltage–
current density (L–V–J) characteristics of the OLEDs were measured
with a Keithley 2611 Source Meter and a PR705 Spectra Colori-
meter, which can also record EL spectra and CIE coordinates
accurately. All the measurements on the devices were carried out
in an ambient atmosphere without further encapsulation.
(
(
400 MHz; DMSO-d ; Me Si) 9.00 (1H, d, J = 9.2 Hz, ArH), 8.53
6 4
1H, d, J = 7.2 Hz, ArH), 8.45 (1H, d, J = 8.0 Hz, ArH), 8.21 (1H, d,
J = 8.4 Hz, ArH), 7.96 (1H, d, J = 16.0 Hz,QCH), 7.89 (1H, t, J
.6 Hz, J = 8.0 Hz, ArH), 7.71 (2H, d, J = 8.4 Hz, ArH), 7.55 (1H,
d, J = 16.0 Hz,QCH), 6.89 (2H, d, J = 8.4 Hz, ArH), 4.05 (2H, t, J =
1
=
7
2
7
.2 Hz, –NCH
2
), 3.00 (6H, s, –NMe
2
), 1.64 (m, 2H, –CH
2 3
, –CH ),
2.3 Synthesis
1.31 (m, 6H, –CH
2
), 0.87 (m, 3H, –CH
3
). d (100 MHz; CDCl ;
C
3
The synthetic routes to Nap1–3 are outlined in Scheme 1. Me Si) 164.4, 164.2, 150.9, 142.4, 135.6, 131.2, 131.0, 130.1,
4
2
5
26
27 28
Intermediates 2, 3, 5,
6
were synthesised according to 129.4, 128.9, 128.5, 126.3, 124.8, 123.1, 122.9, 118.4, 112.2, 40.5,
40.3, 31.6, 28.1, 26.8, 22.6, 14.1. FT-IR nmax/cm 1655 (CQO),
,1,7,7-Tetramethyl-9-vinyl-2,3,6,7-tetrahydro-1H,5H-pyrido- 1358 (C–N). HRMS (ESI) m/z: calcd for [M + H] : C H N O ,
ꢀ
+
1
literature reports.
+
+
1
28 31 2 2
ꢀ
1
[
3,2,1-ij]quinolone (4). n-Butyl lithium (1.0 mL of 2.5 mol L in 427.2380; found, 427.2365.
n-hexane solution) was added dropwise to a suspension of
(E)-2-(2,6-Di(isopropyl)phenyl)-6-(4-(dimethylamino)styryl)-
3 3
PPh CH Br (0.90 g, 2.3 mmol) in anhydrous degassed THF 1H-benzo[de]isoquinoline-1,3(2H)-dione (Nap2). Compound
(10.0 mL). The yellow solution was allowed to stir for 30 min, Nap2 was prepared as a red solid with a yield of 50% using
a similar procedure for the synthesis of Nap1, but with 5
rather than 6 as the reactant. dH (400 MHz; DMSO-d ; Me Si)
6
4
9
(
.11 (1H, d, J = 8.8 Hz, ArH), 8.59 (1H, d, J = 6.8 Hz, ArH), 8.50
1H, d, J = 8.0 Hz, ArH), 8.25 (1H, d, J = 8.0 Hz, ArH), 8.01 (1H,
d, J = 16.0 Hz,QCH), 7.93 (1H, t, J = 6.4 Hz, J = 8.0 Hz, ArH),
.73 (2H, d, J = 8.8 Hz, ArH), 7.60 (1H, d, J = 15.6 Hz, QCH),
.45 (1H, t, J = 7.6 Hz, ArH), 7.33 (2H, d, J = 7.6 Hz, ArH), 6.78
), 2.66 (2H, m, Ar–
CH) 1.05 (12H, d, J = 6.8 Hz, –CH ). d (100 MHz; CDCl3;
1
2
7
7
(
2H, d, J = 8.8 Hz, ArH), 3.00 (6H, s, –NMe
2
3
C
Me Si) 164.4, 164.2, 145.7, 142.8, 135.9, 131.7, 131.6, 131.1,
4
1
1
30.4, 129.6, 129.6, 129.4, 128.6, 126.4, 124.0, 123.1, 123.0,
ꢀ
1
20.4, 118.4, 112.4, 40.4, 29.1, 24.0, 8.1. FT-IR nmax/cm 1661
+
+
(
CQO), 1352 (C–N). HRMS (ESI) m/z: calcd for [M + H] :
+
Scheme 1 Synthetic routes to the objective compounds.
34 35 2 2
C H N O , 503.2693; found, 503.2676.
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(E)-2-(2,6-Di(isopropyl)phenyl)-6-(2-(1,1,7,7-tetramethyl-2,3,6,7-
tetrahydro-1H,5H-pyrido[3,2,1-ij]quinolin-9-yl)vinyl)-1H-benzo[de]-
isoquinoline-1,3(2H)-dione (Nap3). Compound Nap3 was prepared
as a dark red solid with a yield of 24% using a similar proce-
dure as the synthesis of Nap1, but with 4 and 6 rather than
2
and 5 as the reactants. The crude product was purified by
column chromatography over a silica gel (eluent: petroleum
ether/dichloromethane = 15/1, v/v), followed by recrystallization
from a mixture of n-hexane and dichloromethane three times.
d
H
(400 MHz; DMSO-d
6 4
; Me Si) 9.16 (1H, d, J = 8.4 Hz, ArH),
8.60 (1H, d, J = 7.2 Hz, ArH), 8.49 (1H, d, J = 8.0 Hz, ArH), 8.25
(1H, d, J = 8.0 Hz, ArH), 7.95 (2H, m, ArH,QCH), 7.57 (1H, d, J =
1
7
–
6.0 Hz,QCH), 7.52 (2H, s, ArH), 7.45 (1H, t, J = 7.6 Hz, ArH),
1
.34 (2H, d, J = 7.6 Hz, ArH), 3.23 (4H, t, J
NCH ), 2.67 (2H, m, Ar–CH), 1.73 (4H, t, J
Ar–CH ), 1.32 (12H, s, –CH
100 MHz; CDCl ; Me Si) 164.5, 164.2, 145.7, 143.2, 137.0,
1
= 6.4 Hz, J
2
= 6.0 Hz,
2
1
= 5.6 Hz, J
2
= 6.0 Hz,
2
3 3 C
), 1.06 (12H, d, J = 4.0 Hz, –CH ). d
(
3
4
1
1
31.8, 131.6, 131.1, 130.5, 129.6, 129.4, 126.3, 123.9, 123.3,
23.1, 122.8, 119.9, 117.0, 46.8, 36.4, 32.3, 30.8, 29.1, 24.0.
ꢀ
1
+
FT-IR nmax/cm 1661 (CQO), 1576 (C–N). HRMS (ESI) m/z:
calcd for [M + H] : C H N O , 611.3632; found, 611.3618.
+
+
4
2
47 2 2
3. Results and discussion
3.1 Photophysical properties in dilute solutions
The UV-Vis absorption spectra of Nap1–3 in dilute solutions of
different polarities are illustrated in Fig. 2, and the corre-
sponding data are summarized in Table 1. In a similar solvent,
the absorption maximum (labs max) of Nap2 is slightly red-
shifted (B7 nm) compared with that of Nap1, but the labs max
of Nap3 was considerably red-shifted (430 nm) compared to
those of Nap1 and Nap2. Therefore, the replacement of the n-hexyl
bonding to the imide nitrogen atom into a 2,6-di(isopropyl)
Fig. 2 Normalized absorption spectra of the three objective compounds
in solvents with different polarities. (a) Nap1; (b) Nap2; and (c) Nap3.
phenyl group would just bring a little effect on the conjugation in toluene; lPL max = 680 nm in chloroform; lPL max = 722 nm in
length of these compounds, but the presence of a TMJ rather than DCM; and lPL max = 750 nm in acetone), indicating a strong ICT
a DMAP donor subunit in Nap3 would endow it with a much character of the lowest singlet excited states of Nap1–3. This
extended p-conjugation system. As expected, in every solvent, the deduction was further confirmed by the Lippert–Mataga plots
absorption bands of all the three objective compounds were red- of Nap1–3 (see Fig. S1, ESI†), since good linear correlations
24
shifted compared to that of NIM, confirming that the bandgaps between the solvent polarity parameter and Stokes shifts could
of Nap1–3 are narrower than that of NIM, which should be chiefly be achieved in all these Nap systems. It should be pointed out
attributed to the more electron-donating capability of DMAP and that in solvents with moderate polarity like chloroform and
TMJ groups than that of DPAP. With an increase in solvent tetrahydrofuran (THF), Nap1–3 could emit red fluorescence
polarity from cyclohexane (CHX) to dichloromethane (DCM), with high PL quantum yields (PLQY, FPL). For example, in
the absorption bands of Nap1–3 showed 20–30 nm red-shifts, THF solution, both Nap1 and Nap2 showed lPL max values
indicative of the ICT character of the three compounds in their 4650 nm and FPL values 40.5, and the chloroform solution
2
0,29
ground states.
of Nap3 displayed a lPL max of 680 nm and a FPL of 0.45. With
Consistent with the absorption characteristics of NIM and further increased solvent polarity to acetone, the PL emission
Nap1–3, the PL emission maximum (lPL max) of Nap3 was more spectra of Nap1–3 were all observed to shift towards the near
bathochromically-shifted (B30 nm) than that of Nap2 or Nap1 infrared region approaching B750 nm, but their FPLs were
in every solvent, the lPL max of Nap2 was just slightly (3–5 nm) found to decrease drastically, which might arise from the much
red-shifted compared to Nap1, and the lPL maxs of Nap1–3 are intensified non-radiative internal conversion processes in these
9
significantly red-shifted compared to NIM (Fig. 3 and Table 1). compounds due to their much lowered energy gaps. Never-
In comparison with their absorption spectra, the fluorescent theless, taking into consideration that Nap1–3 could emit
spectra of Nap1–3 show more significant positive solvatochro- highly efficient red fluorescence in solvents with medium
mism (e.g., for Nap3: lPL max = 553 nm in CHX; lPL max = 622 nm polarity, they might act as promising red EL materials.
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Table 1 Photophysical data of the three objective molecules in solvents with different polarities (1.0 ꢁ 10ꢀ5 mol Lꢀ1
)
a
NIM
Nap1
Nap2
Nap3
Solvent
CHX
l
abs max (nm)
l
PL max (nm)
F
PL
l
abs max (nm)
l
PL max (nm)
F
PL
l
abs max (nm)
l
PL max (nm)
F
PL
labs max (nm) lPL max (nm) F
PL
—
—
448
461
450
—
—
—
552
624
607
—
—
—
440
450
514
543
580
634
655
669
698
0.22 447
0.29 457
0.42 468
0.62 479
0.59 473
0.51 477
0.08 471
517
546
585
637
658
672
702
0.16 479
0.28 488
0.42 505
0.64 511
0.54 510
0.50 519
0.08 510
553
567
622
680
696
722
750
0.24
0.42
0.24
0.45
0.29
0.09
0.00
CCl
Tol
4
0.81 461
0.36 471
0.54 466
CHCl
THF
3
CH Cl
2 2
—
470
Acetone 448
658
0.06 464
a
Photophysical data derived from ref. 24; labs max: absorption maximum; lPL max: PL emission maximum; FPL: absolute PL quantum yield.
Fig. 3 Normalized PL emission spectra of the three objective compounds
in solvents with different polarities. (a) Nap1; (b) Nap2; and (c) Nap3.
Fig. 4 Normalized PL emission spectra of the Nap–CzPhONI blend films
(lex = 390 nm) and the crystalline powder samples as well as the neat film
samples of Nap1–3 (lex = 490 nm). (a) Nap1; (b) Nap2; and (c) Nap3.
3
.2 PL emission properties in crystalline powder and thin film
was slightly broader and red-shifted than that of the crystalline
powder sample, which should be attributed to the different
states
To investigate their potential as red light-emitting materials in molecular conformations in the crystalline and amorphous
3
0
OLEDs, PL emission spectra of Nap1–3 in crystalline powder states.
and thin film states were obtained. As shown in Fig. 4 and
Despite the fact that in dilute solutions, the fluorescence
Table 2, in both powder and film states, all the three com- band of Nap2 was slightly red-shifted compared to Nap1, its
pounds could emit standard-red fluorescence with lPL max of lPL max was observed to be 6 nm blue-shifted compared to Nap1
660–700 nm, and Nap3 shows the most red-shifted emission in the neat film state (669 nm vs. 675 nm), and its FPL was also
band. In each case, the PL spectrum of the film sample of Nap higher than that of Nap1 (0.02 vs. 0.01). Hence, we could infer
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Table 2 Fluorescence data and PLQYs of Nap–CzPhONI blend films, efficiency between CzPhONI and Nap3 should not have been as
neat films and crystalline powders of the three objective molecules
high as that between CzPhONI and Nap1/Nap2.
In comparison with those of the neat films, the FPLs of the
Nap1
Nap2
Nap3
guest–host blend film samples were much improved (Table 2).
For all the three compounds, the highest PLQYs of the blend
films were obtained at the lowest doping-level of 2 wt%. With
increasing guest doping ratios from 2 wt% to 14 wt%, the FPL of
a
a
a
Doping ratio
wt%)
l
PL max
l
PL max
l
PL max
(nm)
a
a
a
(
(nm)
F
PL
(nm)
F
PL
F
PL
2
4
1
613
622
641
678
659
0.37
0.29
0.16
0.01
0.01
608
612
632
673
659
0.46
0.38
0.27
0.02
0.04
646
655
663
703
680
0.23
0.21
4
0.15 Nap1-based film dropped considerably from 0.37 to 0.16, while
Neat film
Powder
0.01
0.02
the FPLs of Nap2- and Nap3-based samples were lowered from
0.46 to 0.27 and from 0.23 to 0.15, respectively, both were less
a
l
ex = 390 nm for the blend films; lex = 490 nm for the neat films and
significant than that of the Nap1-based one, indicating that
Nap2 and Nap3 showed more alleviated self-quenching relative
to Nap1. Consequently, owing to the concurrent presence of a
strong electron-donating TMJ group and a bulky 2,6-di(isopropyl)-
phenyl substituent, Nap3 possesses not only an extended
p-conjugation system, but also alleviated concentration quenching;
hence, it is a perspective candidate as a standard-red EL material.
crystalline powders.
3
.3 Electrochemical properties
To estimate the energy levels of the frontier orbitals of Nap1–3,
Fig. 5 Device configuration and energy band diagram of devices I, II, and their electrochemical properties were investigated by cyclic
ꢀ
+
4
ꢀ1
III (left); and molecular structure of the host compound CzPhONI (right).
voltammetry (CV) in degassed 5 ꢁ 10 mol L anhydrous
acetonitrile solutions with the Fc/Fc redox couple as the
external standard, and the cyclic voltammograms are shown
that the presence of the more bulky 2,6-di(isopropyl)phenyl in Fig. S2 (in ESI†). During the anodic scan from 0 V to 0.50 V,
rather than a n-hexyl group in Nap2 should be propitious to the both Nap1 and Nap2 showed reversible oxidation waves with
+
suppression of intermolecular interactions of these fluoro- half wave potentials (E1/2) of 0.24 V vs. Fc/Fc , but Nap3 showed
+
phores, and hence could promote less concentration quenching. a reversible oxidation wave with E1/2 of 0.05 V vs. Fc/Fc . Upon
Although the FPLs of these neat film samples were rather low cathodic scanning from 0 V to ꢀ1.95 V, Nap1 showed a
+
(
r0.02), which would limit their potential as high-performance reversible reduction wave with E1/2 of ꢀ1.70 V vs. Fc/Fc , while
non-doped red EL luminogens, the relatively high FPLs of Nap1–3 both Nap2 and Nap3 showed reversible reduction waves with
+
in medium-polarity solvents spurred us to prepare guest–host
E
1/2 of ꢀ1.66 V vs. Fc/Fc . Hence, the calculated electrochemical
composite films using Nap1–3 as dopants. As the compound band-gaps of Nap1–3 were 1.94 eV, 1.90 eV and 1.71 eV in
CzPhONI (structure shown in Fig. 5) we have developed recently sequence, which were consistent with their optical bandgaps
is a high performance host material for an orange naphthalimide (2.19 eV, 2.17 eV and 1.85 eV for Nap1–3 in sequence) deduced
23
guest compound; herein, we chose it as the host material to from the onset of their UV-Vis absorption spectra in dilute
fabricate Nap–CzPhONI guest–host composite films with different acetonitrile solutions (Fig. S3 in ESI†).
+
doping-levels.
In comparison with the Fc/Fc redox couple whose energy
31
As shown in Fig. 4 and Table 2, for all the three compounds, level is ꢀ4.88 eV in vacuum, the HOMO energy levels of Nap1–3
with increasing concentration of Nap from 2 wt% to 14 wt%, were calculated to be ꢀ5.12 eV, ꢀ5.12 eV and ꢀ4.93 eV, and their
the lPL max of the corresponding Nap–CzPhONI composite film LUMO energy levels are calculated to be ꢀ3.18 eV, ꢀ3.22 eV and
red-shifted gradually. Excitingly, at a relatively high doping- ꢀ3.22 eV, respectively. As the HOMO and LUMO energy levels of
23
level of 14 wt%, the film samples with Nap1 and Nap2 as guests CzPhONI are ꢀ5.60 eV and ꢀ3.10 eV, respectively, the HOMO
could emit red fluorescence with lPL max of 630–640 nm and the energy difference between CzPhONI and Nap1–3 was 0.48 eV, but
14 wt% Nap3 composite film could emit standard-red fluores- that between the HOMOs of CzPhONI and Nap3 was as large as
cence with satisfactory chromaticity (lPLmax = 663 nm). Analo- 0.67 eV. Hence, CzPhONI should be a more ideal host material for
gous to the PL emission property of their neat films, in every Nap1–2 than Nap3, which may account for the more efficient
doping-level, the lPLmax of Nap2 was observed to be blue-shifted energy transfer process between CzPhONI and Nap1–2 than Nap3.
for 5–10 nm than that of Nap1, validating the more suppressed
3.4 Electroluminescent properties
intermolecular interactions in Nap2 than Nap1. In composite
films with Nap1 and Nap2 as guests, no emission from the Based on these photophysical and electrochemical experimental
23
CzPhONI host (lPLmax B500 nm) could be discerned at each results, thermo-evaporated OLEDs with a Nap3–CzPhONI com-
doping-level, indicative of the efficient energy transfer between posite film as light-emitting layer (EML) were fabricated. The
3
the host and guest compounds. While for CzPhONI/Nap3 com- device structure was ITO/MoO (1 nm)/TcTa (40 nm)/CzPhONI:
posite films, the emission band from CzPhONI was discernible at Nap3 (x wt%) (20 nm)/TPBI (45 nm)/LiF (1 nm)/Al (80 nm),
0
00
a relatively low doping-level of 2 wt%; hence, the energy transfer wherein TcTa (4,4 ,4 -tri(N-carbazolyl)triphenylamine) served as
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the hole-transporting material (HTM) and TPBI (1,3,5-tris(1- broadened and blue-shifted EL spectra of these devices. In
phenyl-1H-benzo[d]imidazol-2-yl)benzene) served as the electron- addition, although the emission from CzPhONI is discernible
transporting material (ETM). The energy level diagram of the in the PL spectrum of the 2 wt% Nap3–CzPhONI composite
devices is shown in Fig. 5. According to the PL characterization film, it could not be observed in the EL spectrum of device I,
results, device I, II and III with Nap3 concentrations of 2 wt%, indicative of the more efficient energy transfer and/or charge
4
wt% and 14 wt%, respectively, were prepared. The EL spectra carrier trapping on Nap3 in the EL process. As shown in Fig. 6b,
and the luminance–voltage–current density (L–V–J) characteristics under similar driving voltages, the current density of device II
of devices I–III are shown in Fig. 6, and some representative EL was lower than that of device I, but that of the more heavily
performance data are summarized in Table 3.
doped device III is comparable with that of device I. Conse-
Devices I–III all displayed bias-independent EL spectra with quently, it should be the energy transfer mechanism that
lEL max of 636 nm, 644 nm and 657 nm (vide Fig. 6a and Fig. S4 dominates the EL emission process in device I and II whose
in ESI†), and CIE coordinates of (0.62,0.37), (0.65,0.34) and guest doping-levels are relatively low, but it should be the direct
(
0.67,0.32), respectively. Hence, devices I and II can emit red charge carrier trapping mechanism that governs the EL process
3
2
EL, but device III shows standard-red EL with satisfactory in device III. In fact, according to the energy level diagram of
chromaticity. Compared with the corresponding PL spectra of the devices (Fig. 5), Nap3 should act as efficient hole-carrier
their EML, the EL spectra of all the devices are much broadened traps due to its much higher HOMO energy level than that of
and slightly blue-shifted. Because the emission band of Nap3 the host and HTM.
correlated highly with the environmental polarity and the EML
All the three devices display comparable turn-on voltages of
of the OLEDs was as thin as 20 nm, carrier recombination 3.1 V. Device I exhibits relatively high EL performance with
ꢀ2
ꢀ1
may have occurred not only within the EML, but also at the Lmax, CEmax and EQEmax of 10 900 cd m , 1.9 cd A and 2.1%,
TcTa/EML or EML/TPBI interfaces; this may account for the respectively, indicating that Nap3 is a quite promising guest
compound for OLED applications. However, the chromaticity
of this device was still unsatisfactory. With increasing doping-
level to 4 wt%, the resulting device II showed EL that approaches a
standard-red emission with Lmax, CEmax and EQEmax values of
ꢀ
2
ꢀ1
6
600 cd m , 1.1 cd A and 1.8%, respectively. For device III with
a further increased doping-level, it could emit standard-red EL
ꢀ
2
ꢀ1
with a Lmax of 2660 cd m , CEmax of 0.7 cd A , and EQEmax of
.8%. It should be pointed out that compared with the state-of-art
1
standard-red electrofluorescent OLEDs, the PLQYs of the active
layers using Nap3 as a guest dopant were unsatisfactory, which
may eventually limit the efficiency of the devices. Nevertheless, as
the dilute chloroform solution of Nap3 could emit standard-red
fluorescence with a FPL value of B0.45, the PLQY of the EML with
Nap3 as the guest compound might be enhanced drastically if
more appreciate host material were used. Moreover, the device
structure, doping level and layer thickness used here have not
been optimized for either low driving voltage or high efficiency,
thus much improved EL performance should be expected after
further optimization has been carried out to resolve these issues.
4. Conclusions
Using electron-rich arylamino groups as the donor constructive
units, three red-emissive naphthalimide derivatives were designed
and synthesized, and correlations between the molecular struc-
tures and photophysical properties of these compounds have
been investigated. The results indicated that the presence of a
more electron-donating 1,1,7,7-tetramethyljulolidin-9-yl in these
Fig. 6 (a) EL spectra of the three devices (under driving current density of
ꢀ1
0
.5 mA cm ); (b) luminance–voltage–current density characteristics of
the devices.
Table 3 EL characteristics of the devices using Nap3 as the guest dopant fluorophores would endow them with more red-shifted emission
bands, while the presence of a bulky 2,6-di(isopropyl)phenyl
L
max
EQEmax CEmax
l
EL max
(nm)
ꢀ2
ꢀ1
group on the imide nitrogen atom of these luminogens would
reduce their intermolecular interactions. Consequently, the objec-
tive compound Nap3 bearing both 1,1,7,7-tetramethyljulolidin-9-yl
Device
V
on (V) (cd m
)
(%)
(cd A
)
CIE (x,y)
I
II
III
3.1
3.1
3.1
10 900
2.1
1.8
1.8
1.9
1.1
0.7
636
644
657
(0.62,0.37)
(0.65,0.34)
6600
2660
(0.67,0.32) and 2,6-di(isopropyl)phenyl groups not only showed standard-red
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fluorescence with satisfactory chromaticity, but it also showed
suppressed concentration quenching behaviour. Using Nap3
as the guest dopant, a heavily doped standard-red OLED was
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achieved with CIE coordinates, EQE
and CEmax values of
max
ꢀ1
(
0.67,0.32), 1.8% and 0.7 cd A , respectively. To the best of our
knowledge, this is the first example of a standard-red-emissive
,8-naphthalimide derivative for OLED applications. All these
1
preliminary results indicated that through rational molecular
design, 1,8-naphthalimide derivatives could act as quite promising
standard-red light-emitting materials for OLED applications.
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We acknowledge the financial support for this study by the
National Natural Science Foundation of China (Project No.
21372168, 21190031 and 21321061) and the CAS Innovation
Program. We are also grateful to the Comprehensive Training
Platform of Specialized Laboratory, College of Chemistry, SCU
and the Analytical and Testing Center, SCU for providing NMR,
FT-IR and HRMS data of the intermediates and objective
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