Synthesis, photophysical and electrochemical properties of a blue emitter with binaphthalene and carbazole units

Article abstract of DOI:10.1016/j.saa.2018.04.067

A blue emitter, 3,3′-(2,2′-dimethoxy-[1,1′-binaphthalene]-6,6′-diyl)bis(9-benzyl-9H-carbazole), was synthesized by Suzuki coupling reaction. The photophysical properties of the emitter in solution were firstly investigated by UV–Vis absorption and fluorescence emission techniques. The results indicate that the emitter has excellent optical and electron transfer properties. The maximum absorption and emission peaks of the emitter are 302 nm and 406 nm with 67.4% fluorescence quantum yield in chloroform, respectively. Thermal stability study reveals that the emitter has a good thermal stability (Td > 330 °C, Tg > 160 °C). Electrochemical Redox properties of the emitters were measured by cyclic voltammetry, and the energy gaps of highest occupied molecular orbital and the lowest unoccupied molecular orbital levels are in good agreement with the results of theoretical calculation. Furthermore, the multilayer electrochemcial device with the emitter was fabricated and its properties were explored. The wavelength of electroluminescence for the device with this emitter locates at 428 nm. These results indicate the emitter as a deep blue-emitting material has promising application in organic light-emitting diode devices.

Full text of DOI:10.1016/j.saa.2018.04.067

Accepted Manuscript
Synthesis, photophysical and electrochemical properties of a blue
emitter with binaphthalene and carbazole units
Lixia Guo, Xiaoju Wang, Liheng Feng
PII:
DOI:
Reference:
S1386-1425(18)30379-2
doi:10.1016/j.saa.2018.04.067
SAA 16052
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
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Received date: 28 July 2017
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29 April 2018
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ACCEPTED MANUSCRIPT
Synthesis, photophysical and electrochemical properties of a
blue emitter with binaphthalene and carbazole units
Lixia Guoa1, Xiaoju Wangb1, Liheng Fenga
aSchool of Chemistry and Chemical Engineering, Shanxi University, Taiyuan, 030006, P.R. China.
bInstitute of Molecular Science, Chemical Biology and Molecular Engineering, Laboratory of Education Ministry,
Shanxi University, Taiyuan 030006, PR China;
Abstract: A blue emitter, 3,3'-(2,2'-dimethoxy-[1,1'-binaphthalene]-6,6'-diyl)bis(9-
benzyl-9H-carbazole), was synthesized by Suzuki coupling reaction. The
photophysical properties of the emitter in solution were firstly investigated by UV-Vis
absorption and fluorescence emission techniques. The results indicate that the emitter
has excellent optical and electron transfer properties. The maximum absorption and
emission peaks of the emitter are 302 nm and 406 nm with 67.4 % fluorescence
quantum yield in chloroform, respectively. Thermal stability study reveals that the
emitter has a good thermal stability (Td>330 oC, Tg>160 oC). Electrochemical Redox
properties of the emitters were measured by cyclic voltammetry, and the energy gaps
of highest occupied molecular orbital and the lowest unoccupied molecular orbital
levels are in good agreement with the results of theoretical calculation. Furthermore,
the multilayer electrochemcial device with the emitter was fabricated and its
properties were explored. The wavelength of electroluminescence for the device with
this emitter locates at 428 nm. These results indicate the emitter as a deep blue-
emitting material has promising application in organic light-emitting diode devices.
Keywords: Blue emitter, Property, Suzuki reaction, Naphthalene, Carbazole

E-mail address: lhfeng@sxu.edu.cn (L. Feng); Tel.:+86 351 7010904; Fax: +86 351 7011688.
1The authors of Lixia Guo and Xiaoju Wang contributed equally to this work.
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1. Introduction
Since the discovery of organic light emitting diodes (OLEDs), it has drawn
considerable attention because of their delocalized electronic structures, large -
conjugated backbone, easy availability, high efficiency and the potential applications
in full-color flat panel displays and solid state lighting [1-5]. In the performance of the
devices, light-emitting materials play an essential role. So far, researchers have made
every effort to design and prepare a wide variety of light-emitting materials [6-21].
The vast majority of luminescent materials used in OLEDs are organic small
molecules, oligomers, metal complexes and polymers containing a variety of aromatic
and π-conjugated units [22-25]. Among various OLEDs, the blue-emitting material is
desired, especially the deep blue emitters. Therefore, there is an increasing need to
develop stable, high-intensity, efficient deep blue emitters for OLEDs application.
Carbazole is a typical hole-transporting group. The materials including carbazole
units have high temperature resistance and strongly intense luminescence. Based on
these properties, when introducing carbazole group to the core structure of OLED
emitter, the morphological and thermal stabilities of this material can be vastly
improved [26-28]. Additionally, the materials with carbazole display stronger
brightness and higher stability [29]. Carbazole and its derivations have been
frequently used to design novel host materials in OLED owing to their high triplet
energy level and low oxidation potential [30-34]. Due to its ten carbon atoms ring and
delocalization conjugated bond, naphthalene is quite stable and usually uses as a blue
emitter. The wavelength of naphthalene always takes place red-shift by conjugating
with other organic molecules. Furthermore, polycyclic aromatic hydrocarbons
exhibited intense fluorescence in many situations because of their rigid planar [35]. It
was also used in manufacturing dyes, water purification, fluorescent probe and other
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organic compounds [36-40]. As we all know that the balance of the transport of hole
and injection of electron is imperative for obtaining an excellent OLEDs device. So, it
is necessary to obtain an excellent OLED device by combining the electron-transfer
group and hole-transfer group in the same compound [41-44].
In view of the above idea, in here, we reported a proficient conjugated material
containing carbazole and naphthalene units (MNBC) as an emitter in the OLED
device. In this emitter, carbazole unit is a hole-transfer group. Naphthalene group not
only acts as electron-transfer role but also is a supplier of blue light emission.
Through UV-Vis absorption and fluorescence emission techniques, the photophysical
properties of the emitter were investigated. The thermostability and electrochemical
property of MNBC also were studied. In order to develop the application, the OLED
device with the material as an emitter was fabricated. These results indicate that the
emitter is a promising and desired deep blue-emitting material in organic light-
emitting diodes (OLEDs).
2. Experimental
2.1 Materials and instruments
Unless otherwise stated, all chemical reagents were obtained from commercial
supplier and were used without further purification. Benzyl bromide, 9H-carbazole,
N-Bromosuccinimide (NBS), toluene and dibromine were obtained by Aladdin
(Tianjin, China). Bis(pinacolato)diboron, (R)-2,2'-dimethoxy-1,1'-binaphthalene,
Pd(PPh3)4 and Pd(dppf)Cl2 were all purchased from Aldrich (Steinheim, Germany).
1
Bruker ARX400 spectrometer was used for measuring H NMR and 13C NMR of
products with chemical shifts reported as ppm (TMS as an internal standard). Mass
spectra (MS) were acquired on an Agilent 6510 Q-TOF LC/MS instrument (Agilent
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Technologies, Palo Alto, CA) with an electrospray ionization (ESI) source. Elemental
analyses was measured on a EuroVector EA3000 elemental analyzer. The UV-Vis
absorption and fluorescence emission spectra were reported on a HITACHI UH5300
and F-4600 spectrophotometers, respectively. The excitation and emission slit widths
were both 5.0 nm. The glass transition temperature of the emitter was determined by
DSC using a DSC-Q10 instrument under nitrogen atmosphere. The decomposition
temperature corresponding to 5 % weight loss was detected by Perkine Elmer Pyris 1
TGA thermal analyzer. Cyclic voltammetry (CV) measurement was determined on
three-electrode AUTOLAB (model PGSTAT30) workstation in a solution of
Bu4NClO4 (0.1 M) in acetonitrile at a scan rate of 50 mV/s at room temperature.
2.2 Synthesis of monomers and emitter MNBC
2.2.1 Synthesis of monomer1 and monomer 2
The 9-benzyl-3-bromo-9H-carbazole (monomer 1) and 2,2'-(2,2'-dimethoxy-[1,1'-
binaphthalene]-6,6'-diyl)bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) (monomer 2)
were prepared according to our previous reported methods [45,46]. The
characterization data were listed as follows: 9-benzyl-3-bromo-9H-carbazole
(monomer 1): 1H NMR (CDCl3, 400 MHz) δ 8.302 (d, J=12.8 Hz, 1H), 8.198 (s, 1H),
7.854 (t, J=12.8, 9.6 Hz, 1H), 7.460-7.364 (m, 3H), 7.358-7.221 (m, 4H), 7.162 (t,
13
J=10.8, 9.6 Hz, 2H), 5.513 (s, 2H); C NMR (CDCl3, 100 MHz) δ 142.84, 140.66,
135.94, 132.43, 127.57, 127.38, 126.32, 125.91, 123.38, 120.63, 119.54, 118.99,
102.45, 44.48; EI-MS for C19H14BrN (m/z) 335.2 [M+], 337.1 [M++2].
2,2'-(2,2'-dimethoxy-[1,1'-binaphthalene]-6,6'-diyl)bis(4,4,5,5-tetramethyl-1,3,2-
1
dioxaborolane) (monomer 2): H NMR (d-CDCl3, 400 MHz) δ 8.421 (s, 2H), 8.053
(d, J=9.2 Hz, 2H), 7.586 (d, J=7.6 Hz, 2H), 7.468 (d, J=9.2 Hz, 2H), 7.102 (d, J=8.4
Hz, 2H), 3.759 (s, 6H), 1.382 (s, 24H); 13C NMR (d-DMSO, 100 MHz) δ 25.02,
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56.69, 83.69, 99.99, 113.93, 119.30, 124.30, 128.59, 130.33, 130.83, 135.72, 136.51,
155.92; EI-MS for C34H40B2O6 (m/z) 567.2 [M++2], 566.1 [M++1], 565.4 [M+].
2.2.2 Synthesis of 3,3'-(2,2'-dimethoxy-[1,1'-binaphthalene]-6,6'-diyl)bis(9-benzyl-
9H-carbazole)(MNBC)
A mixture of 2.83 g 2,2'-(2,2'-dimethoxy-[1,1'-binaphthalene]-6,6'-diyl)bis-
(4,4,5,5-tetramethyl-1,3,2-dioxaborolane)1,6-dibromopyrene (0.5 mol), 3.36 g 9-
benzyl-3-bromo-9H-carbazole (1.0 mol) in toluene (30.0 mL) and 2.0 M Na2CO3
(10.0 mL) was stirred for 30 min at room temperature under nitrogen atmosphere.
Then, Pd(PPh3)4 (0.1 g) catalyst was quickly added to the suspension and heated to
110 oC for 48 h under nitrogen atmosphere. After cooling it to room temperature, the
solvent was removed under reduced pressure. The crude product was obtained
through extraction with dichloromethane, drying with anhydrous sodium sulfate and
desolventizing. And then, it purified by column chromatography on silica gel with
dichloromethane/petroleum ether (1/2, v/v) as the eluant to afford MNBC as
tephrosious solids (1.58 g 38.4 %). 1H NMR (CDCl3, 400 MHz) δ 8.435 (d, J=7.6 Hz,
2H), 8.195 (t, J=4.8, 6.6 Hz, 4H), 8.114 (t, J=9.6, 7.6 Hz, 2H), 7.790 (t, J=4.8, 4.4 Hz,
2H), 7.762 (t, J=4.8, 6.4Hz, 2H), 77.554 (d, J=9.6Hz, 2H), 7.478-7.403 (m, 6H),
7.376-7.270 (m, 8H), 7.199-7.179 (d, J=8.0 Hz, 6H), 5.562 (s, 4H), 3.842 (s, 6H); 13C
NMR (CDCl3, 100 MHz) δ 154.94, 141.11, 140.07, 137.13, 132.88, 132.82, 129.69,
129.62, 128.82, 127.51, 126.66, 126.44, 125.86, 123.59, 123.19, 120.49, 119.59,
119.33, 118.99, 114.68, 109.16, 105.04, 57.03, 46.71; EI-MS for C60H44N2O2 (m/z)
824.2 [M+]. Element Analysis for C60H44N2O2 (Mol. Wt.: 825.02) calcd.: C, 87.35; H,
5.38; N, 3.40; found: C, 86.88; H, 5.43; N, 3.32.
2.3 Fluorescence and UV-Vis measurements
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The stock solution of compound MNBC was 1.010-2 mol/L in CHCl3.
Corresponding,
the
stock
solutions
of
N,N-dimethylaniline
(DMA),
dimethylterephthalate(DMTP) and C60 were all 1.010-2 mol/L in organic solvents,
respectively. Through the dilution method, various working solutions were prepared.
The working solution (1.010-5 mol/L) was placed in a quartz cuvette with 1 cm path,
and the total volume of it was 2.0 mL. The stock solutions of DMA, DMTP and C60
were added to the MNBC working solution, respectively. After shaking for 30 s, the
new spectra were measured. The volume added of DMA, DMTP and C60 did not
exceed 3% of the total. All of the experiments were performed at barometric pressure
and room temperature.
2.3 Device fabrication
The vacuum-deposition technique was used to fabricate the multilayer OLEDs. In
the device, ITO layer acted as anode and organic layers was fabricated by high-
vacuum (5×104 Pa) thermal evaporation onto a glass (3 cm×3 cm) substrate
percolated. Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT: PSS)
served as the hole-transfer layer (HTL). The electron-blocking layer was 4,4-bis(N-
carbazolyl)-1,10-biphenyl (CBP). The emitter MNBC acted as an emitting layer. The
electron-transport layer (ETL) was 1,3,5-tris(N-phbenylbenzimidazol-2-yl) benzene
(TBPI). The cathode was the LiF/Al. All organic layers were placed according the
prescribed order. Thermal deposition rates for organic materials, LiF and Al were 0.5
Å·s-1, 0.5·Å s-1and 1.0 Å·s-1, respectively. The active area of the device was 9 mm2.
The electroluminescent spectra were reported on a Hitachi MPF-4 fluorescence
spectrometer. The voltage-current density characteristic of OLEDs was obtained on
Keithley 2400 Source Meter. The characterization of luminance-voltage was
measured with a 3645 DC power supply combined with a 1980 A spot photometer
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and the data was recorded simultaneously. All measurements were conducted at room
temperature.
3. Results and discussion
3.1 Synthesis of MNBC
The emitter MNBC was prepared by Suzuki coupling reaction of monomer 1 and
monomer 2 in the presence of palladium catalyst with 38.4 % yield. Carbazole units in
monomer 1 acts as a hole transport group, and naphthalene unit in monomer 2 is an
electron transport group. The detail synthetic routes for monomers and MNBC were
described in Scheme 1. The two monomers were synthesized by our previously
reported methods. The structures of monomers and MNBC were characterized and
verified by 1H NMR, 13C NMR, Mass spectra (MS) and elemental analysis (EA). The
MNBC has a poor solubility in routine solvents (such as ethanol, acetone, ethyl
acetate, etc.) because of its larger rigid conjugated structure.
3.2 Photophysical properties of MNBC
The photophysical properties of MNBC in CHCl3 were studied by UV-Vis
absorption and fluorescence emission spectra (Fig.1). As can be seen from Fig. 1, the
two major absorption peaks of compound MNBC locate at 249 nm and 302 nm, which
mainly comes from the characteristic of vibronic pattern for the carbazole groups,
respectively. Based on the maximum absorption wavelength, the optical energy band
gap (4.10 eV) of MNBC was obtained by the empirical equation [Eg=1240/λabs].
Accordingly, the fluorescence emission peak of MNBC locates at 388 nm with a
shoulder peak in 406 nm. The fluorescent quantum yield (Φ) of MNBC is 67.4 % in
chloroform with quinine sulfate in 0.10 M sulfuric acid as the reference. In order to
investigate the solvent effect of MNBC, the fluorescence spectra of MNBC in
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different organic solvents were investigated and listed in Fig.S1. From Fig.S1, the
maximum emission wavelength (λem) of MNBC occurred gradually red-shifted (30-50
nm) with increase of solvent polarity. Similar, the half bandwidth (λem½) also had a
slight increase with the increase of the solvent polarity. Additionally, the electron
transfer property of MNBC in solution also was studied. In this aspect, the
interactions of MNBC with electron donor (DMA), electron acceptor (DMTP) and C60
were obtained by the titration method, respectively. These results are summarized in
Fig.S2a-d. These data indicate that the MNBC has a good electronic transmission
capacity, which is beneficial for OLED fabrication.
3.3 Thermal properties of MNBC
Thermal property is a powerful parameter for testing the OLED materials. The
thermal properties of MNBC were tested by using differential scanning calorimeter
(DSC) with a heating rate of 5 °C/min under nitrogen atmosphere. The test exhibits
that MNBC shows good thermostability because of its large molecular masses and
rigid structure. The decomposition temperature (Td, 5 % weight loss) of MNBC is
181ºC (Fig.2). Accordingly, the glass transition temperatures (Tg) of MNBC is 152
ºC. The high thermal stability of MNBC will contribute to the device performance
(longevity).
3.4 Electrochemical property and theoretical calculation
The electrochemical property is also a way to evaluate the performance of the
material. Cyclic voltammograms (CV) were performed with a voltammetric analyzer
(CHI660E) with saturated calomel electrode as the reference electrode. The
electrochemical experiment was carried out in the supporting electrolyte under
nitrogen atmosphere at room temperature. The supporting electrolyte was a solution
of acetonitrile containing 0.1 M tetrabutylammonium perchlorate (n-Bu4NClO4). The
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concentration of MNBC was 1.0 mM. As shown in Fig.3, the electrochemical window
possesses a reversible reduction peak. The reversible oxidation peak is about +0.38
eV and the reversible reduction peak is about -1.25eV, which mainly attributes to the
oxidation of the carbazole groups and electron injection into the vacant naphthalene.
From the empirical equation [HOMO= -(Eox+4.5+0.24)eV], we can get the value of
the HOMO energy level is -5.12 eV, which indicates the electron distribution mainly
comes from the carbazole moiety. These results further demonstrate that this MNBC
can be used as an efficient hole-injection materials. From the maximum absorption
peak and empirical equation [Eg=1240/λabs], we can obtain the energy gap (Eg) of the
MNBC is 4.10 eV. Accordingly, the LUMO energy level of MNBC is -0.90 eV,
which is obtained by the difference between EHOMO and Eg.
In order to verify the experiment results, the energy levels of MNBC in the
acetonitrile solvent were calculated by using density functional theory (DFT) method
with the basis sets 6-31G(d) on Gaussian 09 software [47-48]. The geometries and
electron density distributions of the HOMO (highest occupied molecular orbital) and
LUMO (lowest unoccupied molecular orbital) energy levels of MNBC were described
in Fig.3. Based on the conversion relation (1 a.u.= 27.2114 eV)[49], the value of
EHOMO (-5.20 eV) was obtained by the calculated result of EHOMO (-0.19117 a.u.). And,
electron density in HOMO state locates on the whole conjugated skeleton.
Accordingly, ELUMO is calculate as -1.19 eV. The electron density in LUMO state
locates mainly on the naphthalene moiety. The results are similar to those in
electrochemical. Besides, the Mulliken charges and their distribution of MNBC were
also studied in Fig.S3. As shown in Fig.S3, the large negative values in the BMNC
molecules are N86, N58, O25, and O26, respectively. This is due to the electron-
withdrawing ability of these atoms. Correspondingly, the C atoms attached to these
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atoms are large positive values. Regardless of C4 and C10, C82 and C48 or C85 and
C51, the positions of these atoms are corresponding and their values are similar,
which is attributed to the higher symmetry of the structure of BMNC. The dipole
moment of MNBC is 1.5738 debye. Furthermore, the excited-state absorption
spectrum of MNBC in chloroform was calculated by TDDFT, with the hybrid B3LYP
functional and the 6-31G(d) basis set. The absorption spectrum calculated by
theoretical was shown in Fig.S4. The absorption wavelength of the compound in
chloroform is located at 310 nm, which matches with the result by UV-Vis
spectrophotometer.
3.5 Electroluminescence Devices
In order to evaluate MNBC as an emitter in OLEDs, a multilayers non-doped
device: ITO/PEDOT:PSS(40nm)/MNBC(21nm)/TPBI(40nm)/LiF(1nm)/Al (200nm)
was fabricated (Fig.4a). The electroluminescence (EL) spectra of the device were
determined under different voltages and listed in Fig.4b. From Fig.4b, we can find
that the emission peak of the device locates at 428 nm and the wavelength does not
change with the increase of voltages, which indicates the MNBC can emit a deep blue
light and has a good stability in OLED device. The voltage-luminance and voltage-
current density curves of the device were listed in the Fig. 4c and 4d, respectively.
From these data, we can find the turn-on voltage, maximum luminance, maximum
current efficiency and maximum power efficiency of the device are 5.5 V, 535 cd m-2
(at 15 V), 0.659 cd A-1 (at 6 V) and 0.345 l m·W-1(at 6 V), respectively. Owing to the
experiments carry out under ordinary laboratory conditions and the experimental dates
were not reflected the optimized devices, by changing the layer thickness, optimizing
the process conditions, we trust the performance of the device will be improved.
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4. Conclusion
In conclusion, we have successfully designed and synthesized a novel deep blue-
emitting OLED material. The studies of photophysical properties show that the
MNBC can emit a deep blue light with a higher quantum yield in solution. The
electrochemical and thermal properties of MNBC also were investigated carefully.
The results indicates that this compound MNBC processes good -electron
delocalization and conjugation features, high thermal stability and glass transition
temperature, excellent optical and electrical characteristics. The device with the
MNBC emitter was fabricated and emitted deep blue light. Further photoelectric
device tests of the emitter are in progress and reported subsequently.
Acknowledgment
The work described in this paper was supported by the National Nature Science
Foundation (No.21601111, 21571116 and 21371110), Youth Science Foundation of
Shanxi Province (No.201601D021054), Scientific and Technologial Innovation
Programs of Higher Education Institutions in Shanxi (No. 2017108), Program for the
Outstanding Innovative Teams of Higher Learning Institutions of Shanxi (No. 2017-
07), and SanJin Scholars Support Plan under Special Funding (No. 2017-06). We also
thank the Scientific Instrument Cente of Shanxi University for assistances during the
characterizations of the compounds.
Appendix. Supplementary data
Other fluorescence spectroscopy (PDF) associated with this article can be found in the
online version, at doi:
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The figures and scheme captions
Scheme 1. The synthesis routes of monomer 1, 2 and compound MNBC.
Fig. 1 The UV-Vis absorption and fluorescence emission spectra of MNBC in CHCl3.
Fig. 2 TGA thermogram of MNBC with a heating rate of 5 oC/min under nitrogen
atmosphere.
Fig. 3 Cyclic voltammogram of MNBC and molecular orbital surfaces of the HOMO
and LUMO for MNBC. The supporting electrolyte is 0.10 M Bu4NClO4 in
acetonitrile and the scan rate is 0.10 V/s.
Fig. 4 Properties of the device with the MNBC emitter. a) the schematic diagram of
the device (ITO/PEDOT:PSS/MNBC/CBP/TPB/LiF/Al); b) The electroluminescence
spectra of MNBC in the device under different voltages; c) Plot of voltage versus
luminance for blue-light emitting device; d) Plot of voltage versus current density for
blue-light emitting device.
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Scheme 1
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Fig. 1
Abs
Em
1.0
0.8
0.6
0.4
0.2
0.0
240 280 320 360 400 440 480 520
Wavelength (nm)
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Fig. 2
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Fig. 3
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Fig. 4
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Graphical Abstract
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Highlights
A deep blue emitting OLED material was designed and prepared.
The emitter possesses good thermal stability and photophysical properties.
The device with the emitter exhibits good optical and electrical characteristics.
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