Substituent effect on color tuning of red light emission in photoluminescence and electroluminescence of red fluorophore

Article abstract of DOI:10.1166/jnn.2010.2962

Typical small red light-emitting molecules for organic light emitting diodes (OLEDs) were highly susceptible to fluorescence concentration quenching in solid state. Red fluorophores, (2Z, 2'Z)-3, 3'-[4,4''-bis(dimethylamino)-1, 1':4',1''-terphenyl-2',5'-diyl]bis(2-phenylacrylonitrile)(ABCV -P), (2E, 2'E)-3,3'-[4,4''-bis(dimethylamino)-1,1':4',1''-terphenyl-2',5'-diyl] bis[2-(2-thienyl)acrylon itrile] (ABCV-Th) and (2Z, 2'Z)-3,3'-[4,4''- bis(dimethylamino)-1,1':4',1''-terphenyl-2',5'-diyl]bis[2-(2-naphthyl)acrylo nitrile] (ABCV-Np), capable of preventing fluorescence concentration quenching were designed and synthesized. These compounds have intramolecular charge transfer (ICT) properties which were estimated by measurement of UV-Visible absorption and photoluminescence (PL) emission spectra with variation of solvent polarity (n-Hexane/Chloroform = 99/1, 1/1; Chloroform; Methylene chloride). The magnitude of ICT for ABCV-Th was measured to be the largest and that for ABCV-Np was slightly larger compared to that for ABCV-P The magnitude of ICT resulted in a shift of peak wavelength of PL emission. Therefore, this result well supported substituent effect on the color change of PL emission. The peak wavelengths of photoluminescence for ABCV-P, ABCV-Np and ABCV-Th were observed to be 607.5, 611.5 and 617.5 nm, respectively, and those of EL spectra were measured to be 612.5, 619.5, 621.0 nm, respectively. The emission maxima of PL and EL spectra for these red fluorescent compounds were well correlated with substituent effect on ICT for them. Copyright

Full text of DOI:10.1166/jnn.2010.2962

Copyright © 2010 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 10, 6805–6810, 2010
Substituent Effect on Color Tuning of
Red Light Emission in Photoluminescence and
Electroluminescence of Red Fluorophore
Gweon Young Ryu1, Sang Gu Lee2, Sung Eui Shin2, Jong Jin Park1,
Seungho Park1, and Dong Myung Shin2ꢀ∗
1Department of Mechanical and System Design, Hongik University, Seoul, 121-791, Korea
2Department of Chemical Engineering, Hongik University, Seoul, 121-791, Korea
Typical small red light-emitting molecules for organic light emitting diodes (OLEDs) were highly
susceptible to fluorescence concentration quenching in solid state. Red fluorophores, (2Z,
2ꢀZ)-3, 3ꢀ-[4,4ꢀꢀ-bis(dimethylamino)-1,1ꢀ:4ꢀ,1ꢀꢀ-terphenyl-2ꢀ,5ꢀ-diyl]bis(2-phenylacrylonitrile) (ABCV-P),
(2E,
2ꢀEꢀ-3,3ꢀ-[4,4ꢀꢀ-bis(dimethylamino)-1,1ꢀ:4ꢀ,1ꢀꢀ-terphenyl-2ꢀ,5ꢀ-diyl]bis[2-(2-thienyl)acrylonitrile]
(ABCV-Th) and (2Z, 2ꢀZꢀ-3,3ꢀ-[4,4ꢀꢀ-bis(dimethylamino)-1,1ꢀ:4ꢀ,1ꢀꢀ-terphenyl-2ꢀ,5ꢀ-diyl]bis[2-(2-
naphthyl)acrylonitrile] (ABCV-Np), capable of preventing fluorescence concentration quenching
were designed and synthesized. These compounds have intramolecular charge transfer (ICT)
properties which were estimated by measurement of UV-Visible absorption and photoluminescence
(PL) emission spectra with variation of solvent polarity (n-Hexane/Chloroform = 99/1, 1/1; Chloro-
form; Methylene chloride). The magnitude of ICT for ABCV-Th was measured to be the largest and
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that for ABCV-Np wIaPs:s1li1g7ht.l2y5la5r.g2e5r3c.2o3m6paOrend: Mtootnha, t1f5orFAeBbC2V0-P1.6T1h6e:1m1a:g1n0itude of ICT resulted
Copyright: American Scientific Publishers
in a shift of peak wavelength of PL emission. Therefore, this result well supported substituent effect
on the color change of PL emission. The peak wavelengths of photoluminescence for ABCV-P,
ABCV-Np and ABCV-Th were observed to be 607.5, 611.5 and 617.5 nm, respectively, and those
of EL spectra were measured to be 612.5, 619.5, 621.0 nm, respectively. The emission maxima
of PL and EL spectra for these red fluorescent compounds were well correlated with substituent
effect on ICT for them.
Keywords: ABCV-P, ABCV-Np, ABCV-Th, Intramolecular Charge Transfer (ICT), The Color
Change, Concentration Quenching, Stokes Shift.
1. INTRODUCTION
device performance have been reported.6–9 However, the
red fluorescent molecular materials have remained to
be the weakest part in realizing the full color display
with low efficiency and poor color purity.3ꢀ10 Concen-
tration quenching due to the interaction among the
molecules at high concentration is common and serious
problem for small molecule-based OLEDs. Most red
light-emitting materials including pyran-containing donor–
acceptor family, such as 4-(dicyanomethylene)-2-methyl-
Organic light-emitting diodes (OLEDs) have attracted
major attentions in recent years and are now being con-
sidered as the next-generation flat-panel displays due
to their advantages, such as low operating voltage, low
power consumption, self-emission, high contrast, fast
response time, wide-viewing angle, ultrathin structure
and light weight.1–3 Light-emitting materials are one of
the primary substances for OLEDs. Numerous fluores-
cent materials, including both the host-emitters and the
dopants, have been known and developed since reports
from Kodak on green OLEDs in 1987 and the green
and red doped OLEDs in 1989.4–5 A number of green
and blue emissive materials used in OLEDs with high
6-[p-(N,N-dimethylamino)-styryl]-4H-pyran
(DCM1),
4-(dicyanomethylene)-2-methyl-6- (julolidine-4-yl-vinyl)-
4H-pyran (DCM2) and 4-(dicyanomethylene)-2-tert-
butyl-6-(1, 1, 7, 7-tetramethyljulolidyl-9-enyl)-4H-pyran
(DCJTB), are highly prone to fluorescence concentration
quenching due to aggregation in solid state.3ꢀ10–12 A uni-
versal method for solving the concentration quenching
problem of red emissive materials in OLED application
∗Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2010, Vol. 10, No. 10
1533-4880/2010/10/6805/006
doi:10.1166/jnn.2010.2962
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Substituent Effect on Color Tuning of Red Light Emission in PL and EL of Red Fluorophore
Ryu et al.
is the use of them as a dopant in the host-guest doped
emitter system. In doped red OLEDs, the control of the
dopant concentration is very critical because inefficient
energy transfer from the host will result in a mixture
of two emission profiles and contribute to an unsatu-
rated red emission.12 Furthermore, the optimum dopant
concentration is usually low, commonly no greater than
2%, and the effective doping range is extremely narrow
and commonly no greater than 0.5% of the optimum
concentration.10 Therefore, preventing of concentration
quenching became important issue in development of red
fluorescent materials.
2,5-Dibromoterephthalaldehyde (I) was prepared by the
method of Evers and Moore.13
4,4ꢀꢀ-Bis(dimethylamino)- 1,1ꢀ:4ꢀ,1ꢀꢀ- terphenyl- 2ꢀ,5ꢀ-
dicarbal-dehyde (II) was obtained from the reaction of
2,5-dibromote-rephthalaldehyde (I) with 4-(dimethyl-
amino)phenylboronic acid by Suzuki coupling. The mix-
ture of 2,5-dibromoterephthalaldehyde (I) (500 mg, 1.71
mmol), 4-(dimethylamino)phenylboronic acid (622 mg,
3.77 mmol), Pd(PPh3ꢁ4 (79 mg), Na2CO3 (540 mg) and
Aliquat 336 (0.3 ml) in mixed solvent of toluene (40 ml)-
H2O (25 ml)-THF (10 ml) was mildly refluxed under N2
atmosphere with stirring for 3 hr. The reaction mixture
was cooled, filtered and washed with water and n-hexane.
Removal of the solvents and drying under high vacuum
afforded 600 mg (1.61 mmole) of the product (II) as a
light yellow to orange solid and further purification was
not required. Yield: 94%.
In this work, a new red fluorophore, (2Z, 2ꢀZꢁ-3,3ꢀ-[4,
4ꢀꢀ-bis(dimethylamino)-1, 1ꢀ:4ꢀ, 1ꢀꢀ-terphenyl-2ꢀ, 5ꢀ-diyl]bis
[2-(2-naph-thyl)acrylonitrile] (ABCV-Np), was synthe-
sized in order to study on color tuning of emission from
that molecular system which was controlled by varia-
tion of substituent in the molecule. For the purpose of
quantitative analysis of degrees of color change in light
emission by substituent effect, UV-visible absorption and
photoluminescence (PL) spectra of fluorophores having
various substituents with variation of solvent polarity were
measured.
1H-NMR (500 MHz, CDCl3) ꢂ (ppm) 10.105 (s, 2H),
8.060 (s, 2H), 7.315 (d, 4H), 6.830 (d, 4H), 3.048 (s, 12H);
13C-NMR (125 MHz, CDCl3) ꢂ (ppm) 192.85, 192.80,
150.45, 143.64, 136.36, 131.07, 129.88, 123.00, 112.15,
40.33.
(2Z, 2ꢀZ)-3, 3ꢀ-[4, 4ꢀꢀ-bis(dimethylamino)-1, 1ꢀ:4ꢀ, 1ꢀꢀ-
terphenyl-2ꢀ, 5ꢀ-diyl] bis [2-(2-naphthyl) acrylonitrile]
(ABCV-Np) was prepared from the Knoevenagel
reaction of 4,4ꢀꢀ-bis(dimethylamino)-1,1ꢀ:4ꢀ,1ꢀꢀ-terphenyl-
2ꢀ,5ꢀ-dicarbaldehyde (II) with 2-naphthylacetonitrile.
2. EXPERIMENTAL DETAILS
2.1. Synthesis
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The mixture of 4,4ꢀꢀ-bis(dimethylamino)-1,1ꢀ:4ꢀ,1ꢀꢀ-
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Scheme 1 showed the synthetic procedure of red-emissive
material, ABCV-Np.
ꢀ
ꢀ
Copyright: American Scientific Publishers
terphenyl-2 ,5 -dicarbaldehyde (II) (1.40 g, 3.76 mmol),
2-naphthylacetonitrile (1.51 g, 9.03 mmol) and sodium
ethoxide (prepared by reaction of 207 mg of Na with
20 ml of absolute EtOH) in 340 ml of EtOH was stirred
at room temperature for 4 days. The red solid formed in
the reaction mixture was filtered and washed with water,
EtOH and n-Hexane. The filtrate was further purified
by recrystallization in CHCl3/Acetone. Removal of the
solvents and drying under high vacuum afforded 1.98 g
(2.95 mmole) of ꢁthe product (III) as a red solid. Melt-
All solvents involved in the experiments were reagent
grade and were purified by the usual methods before use.
The molecular structure of ABCV-Np was confirmed by
1
the H and 13C-NMR spectra of its precursors and by H1
and MS (FAB) spectra of ABCV-Np.
1
ing point: 305.5 C Yield: 78.5%; H-NMR (500 MHz,
CDCl3ꢁ ꢂ (ppm) 8.281 (s, 2H), 8.188 (s, 2H), 7.913
(m, 2H), 7.853 (m, 4H), 7.807 (s, 2H), 7.683 (dd, 2H),
7.533 (m, 4H), 7.462 (d, 4H), 6.840 (d, 4H), 3.029
(s, 12H); MS (FAB) calcd. for C48H38N4 (M+ꢁ m/z 670,
found 670.
2.2. Measurement
1H-NMR and 13C-NMR spectra were recorded on a Var-
ian Unity INOVA 500 spectrometer operating at 499.761
MHz and 125.701 MHz, respectively. A mass spectrum
(FAB-MS) was measured on JEOL, JMS-AX505WA, HP
5890 Series II Hewlett-Packard 5890A (capillary column)
using standard conꢁditions. The melting point was mea-
sured to be 305.5 C by differential scanning calorime-
try (DSC) using a TA Instruments, USA (Q-1000) with
a scan rate of 5ꢁC/min over the temperature range of
Scheme 1. The synthesis of ABCV-Np
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Ryu et al.
Substituent Effect on Color Tuning of Red Light Emission in PL and EL of Red Fluorophore
0
ꢁC∼350 ꢁC. UV-visible absorption and PL emission
spectra were measured by HP model 8453 and Perkin
Elmer LS-55, respectively.
2.3. Fabrication of OLED and EL Measurement
The devices with the structure of ITO (100 nm)/NPB
(40 nm)/red emitter (30 nm)/BCP (10 nm)/Alq3
(20 nm)/Liq (2 nm)/Al (100 nm) was fabricated by the
high-vacuum thermal deposition (8×10−7 Torr) of organic
materials onto the surface of indium tin oxide (ITO)-coated
glass substrate. The deposition rates were 1.0∼1.1 Å/sec
for organic materials, 0.1 Å/sec for lithium quinolate (Liq)
and 10 Å/sec for aluminum (Al) cathode. The EL emis-
sion spectra were recorded on Perkin Elmer LS-55 and
device performance was measured using Keithley 236 and
CHROMA METER CS-100A instruments.
Fig. 2. The Variation of substituent at the carbon atom attached
electron-withdrawing cyano group.
Therefore, preventing the fluorescence quenching is cru-
cial for enhancing the dopant concentration and doping
concentration range. ABCV-Np was designed to prevent
this concentration quenching. As shown in Figure 1, The
ABCV-Np molecule is almost nonpolar due to the cross
arrangements of antiparallel, electron donor–donor and
acceptor-acceptor pairs in the molecule. Furthermore, the
molecular structure of ABCV-Np is highly noncoplanar
due to its distorted molecular geometry caused by its
twisted terphenyl backbone and the steric crowding on its
central benzene ring being a part of that terphenyl.
Figure 2 showed the concept of color tuning by the
control of the electron-accepting ability depending on sub-
stituent at carbon atom linked to the strong electron-
withdrawing cyano group.
In our previous works which were studies on red
OLEDs and dependence of fluorescence quenching upon
molecular structure, some optoelectronic properties of
ABCV-Th and ABCV-P were reported.20–22 As discussed
previously, ABCV-P, ABCV-Np and ABCV-Th were
designed to prevent concentration quenching and Figure 3
showed the PL emission spectra of them in powder. Unlike
3. RESULTS AND DISCUSSION
The conjugation system, the strength of electron donor
and electron acceptor, the rigidity of the molecule and the
steric hindrance has to be considered in the design of an
intramolecular charge-transfer (ICT) fluorescent molecule.
The molecular structures of the newly synthesized ABCV-
Np and the widely reported red dopants, DCM2 and
DCJTB,14–18 were shown in Figure 1. The typical red
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dopants, DCM2 and DCJTB, with both strong electron
donor and electron acceptor groups posses a high fluo-
rescence quantum yield10 and have ICT character. DCM-
type red light-emitters, such as DCM2 and DCJTB, suffer
from severe fluorescence concentration quenching in solid
state due to both large dipole moments caused by pyran-
containing donor–acceptor structures and their molecular
planarities. These quenching phenomena limited the opti-
mum dopant concentration and the effective doping range
to a low doping concentration and narrow doping concen-
tration range.10ꢀ19
Copyright: American Scientific Publishers
Fig. 3. PL spectra of ABCV-P, ABCV-Np and ABCV-Th excited at
Fig. 1. The molecular structures of DCM2, DCJTB and ABCV-Np.
J. Nanosci. Nanotechnol. 10, 6805–6810, 2010
340 nm in powder.
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Substituent Effect on Color Tuning of Red Light Emission in PL and EL of Red Fluorophore
Ryu et al.
Fig. 4. UV-Visible absorption and PL spectra of ABCV-P, ABCV-Np
and ABCV-Th in chloroform (1×10−5 M).
typical red dopants, such as DCM2 and DCJTB, their
bright PL emissions were observed. The PL emission
spectra of them well elucidated that the serious fluores-
cent concentration quenching due to molecular polarity
and planarity doesn’t exist any more in these fluorescent
molecules.
Fig. 5. Normalized UV-Visible absorption spectra of ABCV-P,
ABCV-Np and ABCV-Th in solution (1 × 10−5 M) with variation of
solvent polarity.
Figure 4 showed the UV-Visible absorption and PL
emission spectra of ABCV-P, ABCV-Np and ABCV-Th
in chloroform (1 × 10−5 M). The PL emission spectra of
ABCV-P, ABCV-Np and ABCV-Th exhibited large Stokes
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As shown in Figure 6, fluorescence emission spectra
of ABCV-P, ABCV-Np and ABCV-Th were dramatically
shifts due to considerable bathochromic shifts in emis-
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sion wavelengths, which were attributed to the presence
of photo-induced ICT excited states23–25 in the molecules.
Owing to the large Stokes shifts, there is hardly an overlap
between absorption and emission of light by the ABCV-P,
ABCV-Np and ABCV-Th molecules. This leads to little
self-absorption of the emitted light so that the fluorophores
are advantageous for OLED application.
Copyright: American Scientific Publishers
shifted to red as increasing the solvent polarity. As pre-
viously discussed, UV-Visible absorption spectra of these
fluorophores are not almost affected by variation of sol-
vent polarity and, therefore, this suggests an ICT character
is associated with excited states of these fluorescent com-
pounds. The fluorescence emission maxima of ABCV-P,
ABCV-Np and ABCV-Th in approximately nonpolar sol-
vent (n-Hexane/Chloroform = 99/1) were measured to be
Figure
5 showed UV-Visible absorption spectra
of ABCV-P, ABCV-Np and ABCV-Th in solution
(1 × 10−5 M) with variation of solvent polarity.
n-Hexane/CHCl3 (99/1) was regarded as “an approxi-
mately nonpolar solvent system” in that measurement
because these fluorescent compounds insoluble in absolute
n-Hexane.
As shown in Figure 5, UV-Visible absorption band of
ABCV-Th was shifted to red with increase of solvent
polarity. Those shifts of ABCV-P and ABCV-Np were also
observed but the magnitude of shifts was smaller com-
pared to that of ABCV-Th. Furthermore, the degree of
shifts of absorption bands was very small for all these fluo-
rescent compounds. This low influence of solvent polarity
on UV-Visible absorption spectra was interpreted as the
ground states of these compounds were not almost affected
by the solvent polarity.
Figure 6 showed the effect of solvent polarity on flu-
orescence emission spectra for ABCV-P, ABCV-Np and
ABCV-Th and their peak wavelengths of emission with
variation of solvent polarity were summarized in Table I.
Fig. 6. Normalized PL spectra of ABCV-P, ABCV-Np and ABCV-Th
(1×10−5 M) with variation of solvent polarity.
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Ryu et al.
Substituent Effect on Color Tuning of Red Light Emission in PL and EL of Red Fluorophore
Table I. Fluorescence emission maxima of ABCV-P, ABCV-Np and
ABCV-Th in solution (1×10−5 M) with various solvent polarity.
those in chloroform (1.1 D), the less polar solvent. This
indicates that the decreasing order of the magnitude of
ICT is ABCV-Th (2-thienyl), ABCV-Np (2-naphthyl) and
ABCV-P (phenyl). Therefore, this supported that fluores-
cence emission wavelengths of these compounds could
be tunable to a various range of red-light emission by
variation of substituent at the carbon atom bonded to
electron-withdrawing cyano group.
flu
ꢃ
(nm)
max
Solvent
ABCV-P
ABCV-Np
ABCV-Th
n-Hexane/chloroform (99:1)
n-Hexane/chloroform (1:1)
Chloroform
539.0
585.5
605.0
628.0
548.5
592.5
608.0
634.0
555.5
601.0
615.0
648.5
Methylene chloride
Figure 7 showed electroluminescence (EL) emis-
sion spectra of OLEDs with the device structures of
ITO/NPB (40 nm)/red emitters (30 nm)/BCP (10 nm)/Alq3
(20 nm)/Liq (2 nm)/Al, in which red emitters were
ABCV-P, ABCV-Np and ABCV-Th, respectively. EL
emission spectra of ABCV-P (ꢃmax = 612ꢅ5 nm), ABCV-
Np (ꢃmax = 619ꢅ5 nm) and ABCV-Th (ꢃmax = 621ꢅ0 nm)
also showed the similar tendency to PL emission spectra
of them.
539.0, 548.5 and 555.5 nm, respectively. In this nonpolar
solvent, locally excited (LE) states of these compounds
would be the lowest excited energy states and, thus, those
emission maxima were approximately originated from LE
emissions for them, respectively.
As shown in Figure 2, these fluorescent com-
pounds all have the same electron-donating group (N,N-
dimethylamino group) and, thus, the controlling the degree
of ICT would be carried out by variation of substituents,
such as phenyl, 2-naphthyl and 2-thenyl, at the car-
bon atom linked to electron-withdrawing cyano group.
In Figure 6, fluorescence emission maxima of ABCV-P,
ABCV-Np and ABCV-Th in chloroform (ꢄm = 1.1 D)26
were measured to be 605.0, 608.0 and 615.0 nm, respec-
tively. In methylene chloride (ꢄm = 1.8 D),26 the more
polar solvent, those emission maxima were strongly red-
shifted to 628 (ABCV-P), 634 (ABCV-Np) and 648.5 nm
4. CONCLUSION
In conclusion, we synthesized red fluorescent compounds,
ABCV-P, ABCV-Np and ABCV-Th, which were designed
to prevent fluorescence concentration quenching and to
have ICT character, and measured their optoelectronic
properties to investigate substituent effect on tuning of
photoluminescence and electroluminescence emission col-
ors in these compounds. Measurement of UV-Visible
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(ABCV-Th), respectively. This result indicates that ICT
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absorption and PL emission spectra of these fluorophores
states of ABCV-P, ABCV-Np and ABCV-Th are more sta-
bilized by polar solvent than nonpolar solvent and become
the lowest excited energy states and, thus, the larger
bathochromic shifts of emission spectra are observed.
In methylene chloride (1.8 D), the more polar solvent,
fluorescence emission peak wavelengths were shifted to
longer wavelengths by 23.0 (ABCV-P), 26.0 (ABCV-Np)
and 33.5 nm (ABCV-Th), respectively, with respect to
Copyright: American Scientific Publishers
with variation of solvent polarity supported that these
fluorescent compounds possessed ICT characters. The
degree of ICT with variation of substituent at the carbon
atom bonded to electron-withdrawing cyano group for this
molecular system could be also evaluated by measurement
of PL emission spectra in various solvent. As the mag-
nitude of ICT was increased, the larger Stokes shift was
generated and, thus, PL emission was shifted to a longer
wavelength. The result from measurement of EL emis-
sion spectra was well correlated with that of PL emission
spectra. Therefore, it is concluded that, in this molecu-
lar system, adjustment of electron-withdrawing ability by
variation of substituent would become a good tool for
tuning of PL and EL emission color of these fluorescent
compounds.
Acknowledgment: This work was supported by the
Brain Korea 21 Project and Seoul Research and Business
Development Program (10555).
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Fig. 7. Normalized EL spectra of ABCV-P, ABCV-Np and ABCV-Th
with the device structures of ITO (100 nm)/NPB (40 nm)/red emitters
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Received: 20 September 2008. Accepted: 3 July 2009.
Delivered by Publishing Technology to: McMaster University
IP: 117.255.253.236 On: Mon, 15 Feb 2016 16:11:10
Copyright: American Scientific Publishers
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J. Nanosci. Nanotechnol. 10, 6805–6810, 2010