Synthesis and structure-property correlation of blue fluorescence isomer emitters based on rigid pyrazine-bridged carbazole frameworks

Article abstract of DOI:10.1039/c9nj04043a

Three novel blue fluorescence isomer emitters, namely 2,5-bis(6,9-bis(4-(tert-butyl)phenyl)-9H-carbazol-3-yl)pyrazine (TCz-3PA-TCz), 9-(5-(6,9-bis(4-(tert-butyl)phenyl)-9H-carbazol-3-yl)pyrazin-2-yl)-3,6-bis(4-(tert-butyl)phenyl)-9H-carbazole (TCz-3,9PA-T

Full text of DOI:10.1039/c9nj04043a

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Synthesis and structure–property correlation of
blue fluorescence isomer emitters based on rigid
pyrazine-bridged carbazole frameworks†
Cite this: New J. Chem., 2019,
43, 16629
Lingjuan Wei, Jie Li, Kai Xue, Shanghui Ye
and Hongji Jiang
*
Three novel blue fluorescence isomer emitters, namely 2,5-bis(6,9-bis(4-(tert-butyl)phenyl)-9H-carbazol-
3-yl)pyrazine (TCz-3PA-TCz), 9-(5-(6,9-bis(4-(tert-butyl)phenyl)-9H-carbazol-3-yl)pyrazin-2-yl)-3,6-
bis(4-(tert-butyl)phenyl)-9H-carbazole (TCz-3,9PA-TCz) and 2,5-bis(3,6-bis(4-(tert-butyl)phenyl)-9H-
carbazol-9-yl)pyrazine (TCz-9PA-TCz), with a donor–acceptor–donor structure were synthesized and
characterized. TCz-3,9PA-TCz in the thin solid powder state showed much higher thermal stability than
TCz-3PA-TCz and TCz-9PA-TCz. The maximum photoluminescent emission wavelengths of TCz-3PA-TCz,
TCz-3,9PA-TCz and TCz-9PA-TCz in toluene solution were 423, 435 and 454 nm, which were aggregation-
induced quenching active. The absolute photoluminescent quantum yields of doped films for TCz-3PA-TCz,
TCz-3,9PA-TCz and TCz-9PA-TCz were 10.75%, 4.46% and 16.70%, which were considerably improved
relative to those of the pure film. The driving voltage, maximum external quantum efficiency, current
efficiency, power efficiency and luminance of doped OLEDs based on TCz-9PA-TCz fabricated by spin
coating were 4.0 V, 2.67%, 4.12 cd Aꢀ1, 2.46 lm wꢀ1 and 8052 cd mꢀ2. Furthermore, the doped OLEDs
with TCz-3,9PA-TCz as a dopant fabricated by thermal deposition in vacuum showed a driving voltage
of 3.5 V, maximum luminance of 6369.67 cd m2 and maximum external quantum efficiency of 3.85%
at a brightness of 100 cd mꢀ2. The linking model analogous to TCz-3,9PA-TCz was validated to be a
more feasible way to combine pyrazine with carbazole moieties to produce highly efficient and stable
electroluminescent blue light materials.
Received 4th August 2019,
Accepted 2nd October 2019
DOI: 10.1039/c9nj04043a
rsc.li/njc
high-efficiency organic blue materials is greatly limited by their
inherent wide band gap, carrier charge transporting imbalance
Introduction
Due to their advantages of low driving voltage, high luminous and low efficiency in the solid state.11,12 Therefore, the synthesis
efficiency, fast response, flexibility, being ultra-thin and so on, of highly efficient and stable organic blue light materials is still
organic light-emitting diodes (OLEDs) are thought to be the next- one of the focuses of OLEDs.
generation technology for flat panel displays and solid-state light-
To obtain efficient and stable organic blue light materials
ing sources.1–3 Depending on the light-emitting mechanism, for high performance OLEDs, researchers have presented a
organic light-emitting materials can be classified into fluorescent synthesis methodology of organic donor–acceptor (D–A) con-
and phosphorescent materials.4–8 Pure organic molecules with jugated molecules to finely adjust the emission wavelength,
thermally activated delayed fluorescence (TADF) properties are efficiency and lifetime of the obtained emitters.13 Through the
considered the third-generation materials for OLEDs, because of asymmetric D–A molecular design, it can make the electron
their ability to effectively harvest both singlet and triplet excitons injection and transporting capability of the emitting layer fully
for radiative transitions.9,10 With the vigorous developments of match with the energy levels of electrodes and interfacial layers.
all kinds of ways of light emission, efficient red and green The D–A molecular framework can form a carrier transporting
materials have been quite mature, but the molecular design of channel inside a single molecule, which can promote intra-
molecular charge transfer (ICT) to some degree, thereby improving
Nanjing University of Posts and Telecommunications, Institute of Advanced
Materials, Key Laboratory for Organic Electronics and Information Displays and
Jiangsu Key Laboratory for Biosensors, Jiangsu National Synergetic Innovation
Center for Advanced Materials, Nanjing 210023, China.
E-mail: iamhjjiang@njupt.edu.cn
the carrier transporting capacity of the molecule as a whole.14–16
On the other hand, the D–A molecular framework exhibits the
properties of electron acceptors when attached to strong electron
donors, and exhibits the properties of electron donors when
attached to strong electron acceptors. The strength of pushing–
pulling electrons can be balanced by changing electron donors
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c9nj04043a
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and acceptors, which makes the fine control of molecular energy and 2,5-dibromo-pyrazine purchased from Shanghai Energy
levels easy and straightforward. Huang et al.17–19 proposed a Chemical were used directly in the experiment without further
strategy to control the molecular energy levels of polymers purification, and the solvents used in the experiments were
through a p–n diblock structure by using an alkoxy group as purified by standard methods. Anhydrous tetrahydrofuran and
an electron-donating group and a cyano group as an electron- toluene were distilled over sodium and benzophenone under a
withdrawing group to push and pull the electron cloud in the nitrogen atmosphere.
main chain of poly(1-methyl-4-vinylbenzene), which made the
General information. 1H-NMR and 13C-NMR spectra were
lowest unoccupied molecular orbital (LUMO) energy levels ꢀ3.58 measured on a Varian Mercury Plus 400 NMR spectrometer
to ꢀ2.7 eV and the highest occupied molecular orbital (HOMO) with tetramethyl silane as a reference. In the NMR tests, CDCl3
energy levels ꢀ5.5 to ꢀ4.8 eV. Furthermore, as a typical feature (chemical shift set to 7.26 ppm) was used to dissolve the
of organic TADF materials, ICT characteristics can reduce the samples. The molecular weights of the samples were acquired
electronic exchange energy and achieve a small singlet–triplet using a Bruker Daltonics Flex matrix-assisted laser desorption
splitting energy.20,21 In 2017, Adachi et al.22 reported TADF time-of-flight mass (MALDI-TOF-MS) analysis spectrometer.
OLEDs based on triazine and carbazole units, in which the Ultraviolet (UV) visible absorption spectra were measured by
electronic interaction was very important for the TADF mechanism using a UV-3600 SHIMADZU UV-vis-NIR spectrophotometer.
between triazine and carbazole. The deep blue light device Photoluminescent (PL) spectra were measured by using a LS
achieved high external quantum efficiencies over 19.2% and 55 fluorescence luminescence spectrophotometer. The absolute
Commission International de L0Eclairage (CIE) coordinates of photoluminescent quantum yields (PLQYs) were determined by
(0.15, 0.10). However, the ICT characteristic sometimes also using a */FLS920 steady state transient fluorescence system.
causes the emitting wavelength of the materials to red shift and The concentration of the solution for the tests is 10ꢀ5 mol Lꢀ1
,
the full width at half-maximum becomes wider, resulting in and the thin solid film used was obtained by spin coating the
unexpected poor color purity. Our group has long committed to dichloromethane solution (10ꢀ3 mol Lꢀ1) of the emitters on
exploring new organic electroluminescent blue light materials quartz plates. The glass transition temperatures (Tg) of the
for high-tech applications.23–25 Carbazole and its derivatives, samples were characterized by using a PerkinElmer Diamond
owing to their aromatic biphenyl structures with a wide energy differential scanning calorimeter (DSC) at a heating rate of
gap in the backbones and highly efficient electroluminescence 10 1C minꢀ1 in a scanning range of 25–200 1C under a nitrogen
coupled with a high hole mobility and good processability, atmosphere. Thermal 5% weight loss analysis was performed
have emerged as an attractive class of conjugated materials on a SEIKO EXSTAR 6000TG/DTA 6200 at a heating rate of
for organic blue light-emitting materials, hole-transporting 10 1C minꢀ1 under a nitrogen atmosphere. Cyclic voltammetry
materials and hosts for phosphorescent and TADF OLEDs.26 (CV) experiments were performed by using a three-electrode
Similarly, diazine compounds contain a benzene ring in which structure of a glassy carbon electrode, a platinum wire electrode
two of the C–H fragments have been replaced by isolobal and a silver reference electrode. Oxidation and reduction poten-
nitrogen, producing three electron-deficient isomers of pyrida- tials were measured in dichloromethane and tetrahydrofuran
zine, pyrimidine and pyrazine. The availability of specific and with tetrabutylammonium hexafluorophosphate (0.1 mol Lꢀ1) as
highly region-selective coupling reactions of carbazole and the supporting electrolyte and ferrocene as the internal standard
diazine moieties is expected to provide a rich variety of tailored at a scan rate of 100 mV sꢀ1
materials to finely tune the general optoelectronic properties in Device fabrication and measurements. The OLED structure
.
a wide range.27 To the best of our knowledge, there are few fabricated by spin coating is ITO/PEDOT:PSS (30 nm)/CBP:TCz-
examples about the structure–property correlation of blue nPA-TCz (55 nm)/TPBi (35 nm)/Ca:Ag, where 4,40-bis(9-
fluorescence isomer emitters based on rigid pyrazine-bridged carbazolyl)-1,10-biphenyl (CBP) was used as a host with doping
carbazole frameworks with an ICT characteristic. In this report, TCz-nPA-TCz at a concentration of 5% by weight (devices A1, A2
three novel blue fluorescent isomer emitters TCz-3PA-TCz, and A3); poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
TCz-3,9PA-TCz and TCz-9PA-TCz with ICT characteristics were (PEDOT:PSS) and 2,20,200-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-
synthesized and characterized. The results illustrated that the benzimidazole) (TPBi) were used as hole transporting and
thermal stability, photophysical properties, energy levels and electron transporting layers. The OLED structure fabricated
OLED performance were strongly correlated with the chemical by thermal deposition in vacuum is ITO/MoO3 (1 nm)/TAPC
structure of the isomer emitters.
(20 nm)/mCP (10 nm)/DPEPO:TCz-3,9PA-TCz/TmPyPB (40 nm)/
LiF (0.7 nm)/Al (120 nm), where TCz-3,9PA-TCz is doped in a
high triplet energy host of bis[2-(diphenylphosphino)phenyl]-
ether oxide (DPEPO) at a concentration of 45% by weight.
A hole transporting layer of 1,1-bis[(di-4-tolylamino)phenyl]cyclo-
hexane (TAPC) and an electron transporting layer of 1,3,5-tris(3-
Experimental section
Materials and measurements
Materials. 3,6-Dibromocarbazole, 1-tert-butyl-4-phenylboronic pyridyl-3-phenyl)benzene (TmPyPB) were introduced into the
acid, tetrakis(triphenylphosphine)palladium (Pd(PPh3)4), 9H-carb- device to achieve efficient exciton recombination, and N,N0-
azole, 1-tert-butyl-4-iodobenzene, N-bromosuccinimide, tetra- dicarbazolyl-3,5-benzene (mCP) layers were inserted between
butylammonium bromide (TBAB), boranoic acid pinacol ester the TAPC layer and emitting material layer to block the excitons.
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The current density–brightness–voltage of the device was 2H), 7.28–7.26 (d, J = 8.0 Hz, 2H), 1.44 (m, 9H); MALDI-TOF, m/z:
recorded on a system consisting of a Keithley source measure- theoretical value: 461.01, experimental value: 461.28.
ment unit (Keithley 2400 and Keithley 2000) and a calibrated
3-Bromo-6,9-bis(4-(tert-butyl)phenyl)-9H-carbazole30. 3-Bromo-
silicon photodiode. The electroluminescent spectra and CIE 6,9-bis(4-(tert-butyl)phenyl)-9H-carbazole was synthesized following
coordinates were measured using a PR-655 spectral scanning the same synthetic procedure for 3,6-bis(4-(tert-butyl)phenyl)-
spectrometer. All measurements were conducted at ambient 9H-carbazole. 9-(4-tert-Butyl-benzene)-3,6-dibromo-9H-carbazole
temperature.
(6.0 g, 13.14 mmol), 1-tert-butyl-4-phenylboronic acid (2.34 g,
13.14 mmol), Pd(PPh3)4 (0.9 g, 0.78 mmol), TBAB (2 mg,
Synthesis of the target compounds
0.005 mmol), potassium carbonate aqueous solution (30 mL,
9-(4-(tert-Butyl)phenyl)-9H-carbazole28,29
.
A
mixture of 2 mol Lꢀ1, 60 mmol), 80 mL deoxygenated toluene. 2.5 g
9H-carbazole (4.0 g, 23.9 mmol), 1-tert-butyl-4-iodobenzene white powder was obtained in a yield of 38%. 1H-NMR of
(7.0 g, 26.9 mmol), Cu (0.5 g, 7.5 mmol), potassium carbonate compound 3-bromo-6,9-bis(4-(tert-butyl)phenyl)-9H-carbazole
solid (10.2 g, 60 mmol) and 60 mL nitrobenzene was added to a (400 MHz, CDCl3): d = 8.29 (t, 2H), 7.67–7.60 (m, 5H), 7.53–
150 mL flask. The mixtures were refluxed at 180 1C for 24 h 7.43 (m, 6H), 7.31 (s, 1H), 1.46–1.42 (m, 9H), 1.41–1.37 (m, 9H),
under a nitrogen atmosphere. After nitrobenzene was distilled MALDI-TOF, m/z: theoretical value: 510.21, experimental
off under reduced pressure, the residue was extracted with value: 510.63.
H2O and dichloromethane three times. The solvent in the
3,9-Bis(4-(tert-butyl)phenyl)-6-(4,4,5,5-tetramethyl-1,3,2-dioxa-
organic layer was evaporated under reduced pressure. borolan-2-yl)-9H-carbazole31. A mixture of 3-bromo-6,9-bis(4-tert-
The crude product was purified by column chromatography butyl-benzene)-9H-carbazole (2.0 g, 7.33 mmol), boranoic acid
(petroleum ether/dichloromethane by volume = 20 : 1) to give pinacol ester (1.85 g, 7.33 mmol), [1,1-bis(diphenylphosphino)-
4.6 g 9-(4-(tert-butyl)phenyl)-9H-carbazole as a white solid in a ferrocene]palladium dichloride (0.1 g, 0.73 mmol), potassium
yield of 64%. 1H-NMR of compound 9-(4-(tert-butyl)phenyl)-9H- acetate solid (1.0 g, 10 mmol) and 40 mL 1,4-dioxane was added
carbazole (400 MHz, CDCl3): d = 8.16–8.14 (d, J = 8.0 Hz, 2H), to a 100 mL flask. The mixture was refluxed at 80 1C for 24 h
7.62–7.60 (d, J = 8.0 Hz, 2H), 7.50–7.48 (d, 2H), 7.44–7.40 (dd, under a nitrogen atmosphere. After completion of the reaction,
J1 = 8.0 Hz, J2 = 16.0 Hz, 4H), 7.29–7.28 (d, J = 4.0 Hz, 2H), 1.44 the residue was extracted with H2O and dichloromethane three
(m, 9H).
3,6-Bis(4-(tert-butyl)phenyl)-9H-carbazole30
times. The solvent in the organic layer was evaporated under
mixture of reduced pressure. The crude product was purified by column
.
A
3,6-dibromocarbazole (3.0 g, 9.25 mmol), 1-tert-butyl-4-phenyl- chromatography (petroleum ether : ethyl acetate by volume =
boronic acid (4.0 g, 22.4 mmol), Pd(PPh3)4 (0.9 g, 0.78 mmol), 10 : 1). A white powder of 2.4 g was obtained in a yield of 59%.
TBAB (2 mg, 0.005 mmol), potassium carbonate aqueous 1H-NMR of compound 3,9-bis(4-(tert-butyl)phenyl)-6-(4,4,5,5-
solution (30 mL, 2 mol Lꢀ1, 60 mmol), and 80 mL deoxygenated tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-carbazole (400 MHz, CDCl3):
toluene was added to a 200 mL flask. The mixture was heated to d = 8.68 (s, 1H), 8.40 (s, 1H), 7.84 (s, 1H), 7.69–7.57 (m, 5H), 7.53–7.33
90 1C and refluxed under a nitrogen atmosphere for 24 h. After (m, 6H), 1.41–1.37 (m, 30H), MALDI-TOF, m/z: theoretical value:
evaporation of the solvent, the residue was diluted with H2O 557.35, experimental value: 556.77.
and extracted with dichloromethane. The solvent in the organic
TCz-3PA-TCz. TCz-3PA-TCz was synthesized following the
layer was evaporated under reduced pressure. The crude product same synthetic procedure for 3,6-bis(4-(tert-butyl)phenyl)-9H-
was purified by column chromatography (petroleum ether : ethyl carbazole.30 3,9-Bis(4-(tert-butyl)phenyl)-6-(4,4,5,5-tetramethyl-
acetate by volume = 10: 1). 2.4 g pale yellow powder was obtained 1,3,2-dioxaborolan-2-yl)-9H-carbazole (1.00 g, 1.8 mmol),
in a yield of 60%. 1H-NMR of compound 3,6-bis(4-(tert-butyl)- 2,5-dibromo-pyrazine (0.214 g, 0.9 mmol), potassium carbonate
phenyl)-9H-carbazole (400 MHz, CDCl3): d = 8.32–8.31 (d, J = aqueous solution (30 mL, 2 mol Lꢀ1, 60 mmol), Pd(PPh3)4
4.0 Hz, 2H), 8.16 (s, 1H), 7.66 (m, 6H), 7.50 (m, 6H), 1.45–1.39 (0.1 g, 0.1 mmol), TBAB (2 mg, 0.005 mmol) and 10 mL
(m, 18H). MALDI-TOF, m/z: theoretical value: 431.26, experi- deoxygenated toluene. 0.3 g yellow powder was obtained in a
mental value: 431.35.
yield of 25%. 1H-NMR of compound TCz-3PA-TCz (400 MHz,
3,6-Dibromo-9-(4-(tert-butyl)phenyl)-9H-carbazole29. 9-(4-tert- CDCl3): d = 9.29–9.27 (d, J = 8.0 Hz, 2H), 8.97–8.96 (d, J = 4.0 Hz,
Butyl-benzene)-9H-carbazole (4.0 g, 13.4 mmol), N-bromosuccin- 2H), 8.54–8.52 (d, J = 8.0 Hz, 2H), 8.20–8.18 (d, J = 8.0 Hz, 2H),
imide (7.0 g, 40.0 mmol) and 45 mL N,N-dimethylformamide 7.82–7.50 (m, 22H), 1.54–1.42 (m, 36H). 13C-NMR of compound
were added to a 100 mL flask. The mixture was stirred at room TCz-3PA-TCz (100 MHz, CDCl3); d = 29.74, 31.47, 34.56, 34.88,
temperature for 12 h. The reaction solution was dissolved in 110.42, 110.59, 118.91, 123.92, 124.21, 125.82, 126.48, 126.89,
ethyl acetate and further washed with H2O several times. The 126.99, 134.60, 133.75 138.97, 140.99, 142.27, 149.64, 150.50,
solvent in the organic layer was evaporated under reduced 150.81. MALDI-TOF, m/z: theoretical value: 938.53, experimental
pressure. The crude product was purified by column chromato- value: 938.74. Elemental analysis: calcd (%) for C68H66N4: C 86.95,
graphy (petroleum ether : ethyl acetate by volume = 20: 1). A pale H 7.08, N 5.96; found: C 86.79, H 7.22, N 5.81.
1
yellow powder of 4.8 g was obtained in a yield of 80%. H-NMR
L1. L1 was synthesized following the same synthetic procedure
of compound 3,6-dibromo-9-(4-(tert-butyl)phenyl)-9H-carbazole for TCz-3PA-TCz.30 3,9-Bis(4-tetra-tert-butyl-benzene)-6-(4,4,
(400 MHz, CDCl3): d = 8.19 (d, J = 2.0 Hz, 2H), 7.62–7.60 (d, J = 5,5-tetramethyl-[1,3,2]dioxapentyl boron alkyl)-9H-carbazole
8.0 Hz, 2H), 7.50–7.48 (d, J = 8.0 Hz, 2H), 7.42–7.40 (d, J = 8.0 Hz, (0.668 g, 1.2 mmol), 2,5-dibromo-pyrazine (0.714 g, 3 mmol),
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potassium carbonate aqueous solution (30 mL, 2 mol Lꢀ1
,
139.32, 140.23, 141.04, 146.17, 149.72, 149.87. MALDI-TOF, m/z:
60 mmol), Pd(PPh3)4 (0.1 g, 0.1 mmol), TBAB (2 mg, 0.005 mmol) theoretical value: 938.53, experimental value: 938.75. Elemental
and 10 mL deoxygenated toluene. 0.42 g white powder was analysis: calcd (%) for C68H66N4: C 86.95, H 7.08, N 5.96; found:
obtained in a yield of 58%. 1H-NMR of compound L1 (400 MHz, C 86.83, H 7.25, N 5.91.
CDCl3): d = 8.92 (s, 1H), 8.85 (s, 1H), 8.75 (s, 1H), 8.45 (s, 1H),
TCz-9PA-TCz. TCz-9PA-TCz was synthesized following the same
8.04 (s, 1H), 7.72–7.68 (m, 4H), 7.56–7.48 (m, 6H), 7.05 (s, 1H), synthetic procedure for 9-(4-(tert-butyl)phenyl)-9H-carbazole.28,29
1.48–1.44 (m, 18H). MALDI-TOF, m/z: theoretical value: 587.19, 3,6-Bis(4-tert-butyl-benzene)-9H-carbazole (0.38 g, 0.88 mmol),
experimental value: 588.14.
2,5-dibromo-pyrazine (0.11 g, 0.5 mmol), Cu (0.5 g, 7.5 mmol),
TCz-3,9PA-TCz. TCz-3,9PA-TCz was synthesized following potassium carbonate solid (10.2 g, 60 mmol) and 6 mL nitro-
the same synthetic procedure for 9-(4-(tert-butyl)phenyl)-9H- benzene. 0.31 g yellow powder was obtained in a yield of 73%.
carbazole.28,29 3,6-Bis(4-tert-butyl-benzene)-9H-carbazole (1.2 g, 1H-NMR of compound TCz-9PA-TCz (400 MHz, CDCl3): d = 1.00
2.7 mmol), L1 (0.42 g, 0.7 mmol), Cu (0.5 g, 7.5 mmol), potassium (s, 1H), 8.42 (s, 2H), 8.10 (d, J = 8.0 Hz, 2H), 7.82–7.80 (d, J =
carbonate solid (10.2 g, 60 mmol) and 15 mL nitrobenzene. 8.0 Hz, 2H), 7.75–7.73 (d, J = 8.0 Hz, 4H), 7.55 (m, 7H), 6.45
1
0.4 g yellow powder was obtained in a yield of 67%. H-NMR (m, 4H), 6.15 (m, 4H), 5.90 (m, 4H), 1.50–1.40 (m, 36H).
of compound TCz-3,9PA-TCz (400 MHz, CDCl3): d = 9.35–9.30 13C-NMR of compound TCz-9PA-TCz (100 MHz, CDCl3);
(s, 1H), 9.20 (s, 1H), 9.00 (s, 1H), 8.60 (s, 1H), 8.41–8.40 (d, J = d = 29.72, 31.27, 31.46, 31.53, 110.02, 110.37, 118.28, 118.93,
4.0 Hz, 2H), 8.06–8.04 (d, J = 8.0 Hz, 2H), 7.80–7.70 (m, 11H), 120.88, 123.86, 124.21, 125.61, 125.80, 126.42, 126.50, 126.87,
7.60–7.45 (m, 11H), 1.45–1.40 (m, 36H). 13C-NMR of compound 126.90, 133.70, 138.94, 143.22, 149.58, 150.79, 157.27. MALDI-TOF,
TCz-3,9PA-TCz (100 MHz, CDCl3); d = 31.48, 34.58, 34.90, m/z: theoretical value: 938.53, experimental value: 938.62.
110.53, 110.72, 111.60, 118.66, 125.37, 125.84, 126.03, 126.50, Elemental analysis: calcd (%) for C68H66N4: C 86.95, H 7.08,
126.94, 126.99, 133.85, 134.54, 134.95, 138.70, 138.90, 138.99, N 5.96; found: C 86.76, H 7.14, N 5.83.
Scheme 1 Synthetic route and chemical structures of the target emitters.
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Results and discussion
Synthesis and thermal properties
a fixed concentration of 10ꢀ5 mol Lꢀ1 (Fig. 2). Detailed proper-
ties are summarized in Table 1. The strong absorption peaks
appearing at 290–350 nm can be ascribed to the p–p* transition
of the carbazole framework. The weak absorption peaks at
350–450 nm can be classified as ICT transitions from carbazole
units to the central nitrogen heterocyclic pyrazine ring.32,33
Based on the UV absorption spectra of the three compounds
in toluene solution, the optical energy band gaps of compounds
TCz-3PA-TCz, TCz-3,9PA-TCz and TCz-9PA-TCz were calculated
to be 2.94, 2.90 and 2.74 eV, respectively. In the thin solid film
state, the ICT absorption of the three compounds gradually
weakens or even disappears compared to the solution state.
The PL emission spectra of the neat films are red shifted about
40–50 nm relative to those of the solution state, which may be
due to the p–p stacking of the solid film.34 After the three
emitters are doped into CBP at a weight ratio of 5%, the
full width at half-maximum of the PL spectra of TCz-3PA-TCz,
TCz-3,9PA-TCz and TCz-9PA-TCz in the film state is narrowed and
blue shifted. The absolute PLQYs of TCz-3PA-TCz, TCz-3,9PA-TCz
and TCz-9PA-TCz solid powder were 3.78%, 1.55% and 6.18%, and
the PLQYs of the three emitters doped into CBP at a weight ratio of
5% were 10.75% for TCz-3PA-TCz, 4.46% for TCz-3,9PA-TCz and
16.7% for TCz-9PA-TCz. The PLQY of TCz-9PA-TCz was the highest
among the three emitters due to the reduced intermolecular
interaction caused by stronger steric hindrance.
As depicted in Scheme 1, Suzuki and Ullmann reactions
were used to synthesize blue fluorescence isomer emitters
TCz-3PA-TCz, TCz-3,9PA-TCz and TCz-9PA-TCz, and their
1
chemical structures were fully characterized by using H-NMR,
13C-NMR and MALDI-TOF-MS spectra (Fig. S1–S18, ESI†). The
thermal stability of TCz-3PA-TCz, TCz-3,9PA-TCz and TCz-9PA-
TCz in the thin solid powder states was measured by DSC and
thermogravimetric analysis (TGA) measurements (Fig. 1). The
Tg values of TCz-3PA-TCz, TCz-3,9PA-TCz and TCz-9PA-TCz thin
solid powders were 99, 101 and 100 1C, and their 5% weight
loss temperatures were 200, 393 and 171 1C for TCz-3PA-TCz,
TCz-3,9PA-TCz and TCz-9PA-TCz. These results indicate that the
TCz-3,9PA-TCz thin solid powder shows much higher thermal
stability than those of TCz-3PA-TCz and TCz-9PA-TCz, and OLEDs
based on TCz-3,9PA-TCz can be prepared both by spin-coating
and thermal deposition in vacuum.
Photophysical properties
To gain insight into the photophysical properties of emitters
TCz-3PA-TCz, TCz-3,9PA-TCz and TCz-9PA-TCz, we tested their
UV absorption and PL emission spectra in different solvents at
We studied the PL emission spectra of the three compounds
in a mixture of tetrahydrofuran and water with different water
volume fractions at a fixed concentration of 10ꢀ5 mol Lꢀ1 and the
PL emission spectra are shown in Fig. S19 (ESI†). It illustrated that
their PL emission intensities in the mixture showed a decreasing
trend with the increase of the water volume fraction. Their blue
light emission was aggregation-induced quenching active, and
no expected aggregation-induced emission was observed among
three emitters.35 As for the emission behavior, we also obtained
the PL emission spectra of the three compounds in solvents with
different polarities. The emission spectra of the three compounds
showed red shifts to some degree with the increase of the solvent
polarity, and we constructed the Lippert–Magada curve to
further characterize this phenomenon36–38 (Fig. 3):
2
2Dmge
1
hcðnabs ꢀ nfluoÞ ¼
Df þ C
(1)
(2)
(3)
4pe0 a3
e ꢀ 1
n2 ꢀ 1
Df ¼
ꢀ
2e þ 1 2n2 þ 1
ꢀ
ꢁ
1=3
3M
a ¼
4Npd
where h is the Planck constant, nabs and nfluo are the absorption
and photoluminescence of the three compounds in wave-
numbers, e0 is the vacuum permittivity, and a is the Onsagar
cavity radius (7.19 Å) calculated from the Avogadro constant N,
the speed of light (c), a constant (C) and the density of the
compound (d = 1.0 g cmꢀ3). The detailed data are listed in
Table S1 (ESI†). The dipole moment mge between S0 and S1 can be
calculated to be 1.02 D for TCz-3PA-TCz, 0.8 D for TCz-3,9PA-TCz
Fig. 1 TGA (a) and DSC (b) curves for compounds TCz-3PA-TCz, TCz-
3,9PA-TCz and TCz-9PA-TCz in the thin solid powder state.
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Fig. 2 UV-vis absorption and PL emission spectra of compounds TCz-3PA-TCz (a), TCz-3,9PA-TCz (b) and TCz-9PA-TCz (c) in different solvents and PL
emission spectra in film of TCz-nPA-TCz doped into BCP on quartz at a weight ratio of 5% (d).
Table 1 The key thermal, optical and electrochemical properties of TCz-3PA-TCz, TCz-3,9PA-TCz and TCz-9PA-TCz
Absm (nm) Absm (nm) PLm (nm) PLm (nm) Eg (eV) Td/Tg (1C) LUMO/HOMOd (eV) Eg (eV) S1 (eV) PLQYg,h (%)
a
b
a
b
c
e
f
Emitters
TCz-3PA-TCz
TCz-3,9PA-TCz 290/391
TCz-9PA-TCz
292/370/390 291/408
423
435
454
474/545
475/558
472/552
2.94
2.90
2.74
200/99
393/101
171/100
ꢀ3.19/ꢀ6.11
ꢀ3.36/ꢀ6.10
ꢀ3.21/ꢀ5.81
2.92
2.74
2.60
3.13
3.10
3.02
3.78/10.75
1.55/4.46
6.18/16.7
288/404
289/404
298/394
a
b
c
Measured in toluene solution (10ꢀ5 mol Lꢀ1). Measured in a spin-coated film state. The optical energy band gaps calculated from the UV-vis
d
e
f
onset absorption in toluene solvent. According to the CV curve. Calculated according to Eg = EHOMO ꢀ ELUMO in d. Single energy levels calculated
g
from the PL emission in toluene solvent. PLQYs of solid powder measured using an integrating sphere with an excitation wavelength of 400 nm.
h
PLQYs of films (TCz-nPA-TCz doped in CBP at a weight ratio of 5%) measured using an integrating sphere with an excitation wavelength of 400 nm.
and 0.85 D for TCz-9PA-TCz. The relatively slight change in the of TCz-3PA-TCz, TCz-3,9PA-TCz and TCz-9PA-TCz were calcu-
dipole moment mge of TCz-3,9PA-TCz and TCz-9PA-TCz suggested lated to be ꢀ6.10, ꢀ6.09 and ꢀ5.81 eV.42–47 Similarly, according
a weak ICT characteristic existing between the carbazole and to the reduction potentials and the equation: ELUMO = ꢀ4.75 ꢀ
pyrazine units in the two emitters,39,40 which was beneficial Ered (eV), the reduction potentials were calculated to be
to suppress dipole–dipole interaction induced quenching in ꢀ1.33, ꢀ1.52 and ꢀ1.30 V for TCz-3PA-TCz, TCz-3,9PA-TCz
OLED applications.41
and TCz-9PA-TCz. Subsequently, the LUMO energy levels
of TCz-3PA-TCz, TCz-3,9PA-TCz and TCz-9PA-TCz were calcu-
lated to be ꢀ3.19, ꢀ3.36 and ꢀ3.21 eV. The N-linked model
Electrochemical properties
The electrochemical properties of the three emitters were in TCz-9PA-TCz can elevate the HOMO energy level to some
examined by cyclic voltammetry, and the results are shown degree, while the 3,9-linked model in TCz-3,9PA-TCz can
in Fig. 4. The oxidation onsets of compounds TCz-3PA-TCz, make the LUMO energy level deeper when compared with that
TCz-3,9PA-TCz and TCz-9PA-TCz were observed at 1.36, 1.35 of TCz-3PA-TCz. The energy band gaps can be calculated to
and 1.06 V. According to the oxidation potentials and the be 2.92, 2.74 and 2.60 eV for TCz-3PA-TCz, TCz-3,9PA-TCz and
equation: EHOMO = ꢀ4.75 ꢀ Eox (eV), the HOMO energy levels TCz-9PA-TCz, which is consistent with the optical band gaps
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efficiency of devices A1, A2 and A3 is 1.86 cd Aꢀ1, 2.39 cd Aꢀ1
and 4.12 cd Aꢀ1. The external quantum efficiency of devices A1,
A2 and A3 is 2.19%, 2.30% and 2.64% at a luminance of
100 cd mꢀ2. The general performance of the TCz-9PA-TCz
doped device is much better than that of TCz-3PA-TCz
and TCz-3,9PA-TCz, which illustrates that the low mge does
not play a unique role in suppressing the dipole–dipole
interaction induced quenching in the doped emitting layer,
and the external quantum efficiency of the devices is roughly
consistent with the absolute PLQYs of emitters TCz-3PA-TCz,
TCz-3,9PA-TCz and TCz-9PA-TCz in the undoped and doped
solid powder states. The PLQY of TCz-3PA-TCz is higher than
that of TCz-3,9PA-TCz in both the solid state and the doped film,
respectively. The EQE of device A2 is higher than that of device
A3, which may be caused by the reduced quenching of the
emission layers.
Fig. 3 Lippert–Magada plots for compounds TCz-3PA-TCz, TCz-3,9PA-
TCz and TCz-9PA-TCz. The y and x values in the fitted equation represent
the Stokes shift and solvent orientation.
OLED devices by thermal deposition in vacuum
The thermal stability results indicated that TCz-3,9PA-TCz had
obviously higher thermal stability. We then prepared four sets of
OLED devices with optimized structures consisting of ITO/MoO3
(1 nm)/TAPC (20 nm)/mCP (10 nm)/DPEPO:TCz-3,9PA-TCz
(15 nm for B1, 20 nm for B2, 25 nm for B3 and 30 nm for B4)/
TmPyPB (40 nm)/LiF (0.7 nm)/Al (120 nm) by thermal deposition
in vacuum, in which we doped TCz-3,9PA-TCz in a high triplet
energy host of DPEPO with a concentration of 45% by weight. All
devices B1–B4 show blue emission with electroluminescent
peaks at around 452 nm, whose CIE values are (0.161, 0.152)
for B1, (0.164, 0.160) for B2, (0.168, 0.168) for B3 and (0.169,
0.169) for B4. As shown in Fig. 6(b), the current efficiency
of devices B1, B2, B3 and B4 at a luminance of 100 cd mꢀ2 is
4.16 cd Aꢀ1, 4.72 cd Aꢀ1, 5.06 cd Aꢀ1 and 5.13 cd Aꢀ1. Due to the
blue emission, the external quantum efficiency of devices B1, B2,
B3 and B4 at a luminance of 100 cd mꢀ2 is 3.49%, 3.74%, 3.79%
and 3.85%. Device B4 with a film thickness of 30 nm shows a
driving voltage of 3.5 V, maximum luminance of 6369.67 cd mꢀ2
and external quantum efficiency of 3.85% at a luminance of
100 cd mꢀ2. As shown in Fig. S24 and S25 of the ESI,† a
constituting fragment of isomer emitters, namely 3,6-bis-(4-tert-
butyl-phenyl)-9-phenyl-9H-carbazole, has been synthesized, whose
melting point and 5% weight loss temperature are 252 and 355 1C.
The device performance illustrates that the maximum electro-
luminescence peak of the device based on 3,6-bis-(4-tert-butyl-
phenyl)-9-phenyl-9H-carbazole is obviously blue shifted and the
general efficiency is inferior to the device based on the isomer
emitters synthesized in this research.
Fig. 4 The oxidation curves in dry dichloromethane (right) and reduction
curves in dry tetrahydrofuran (left) of compounds TCz-3PA-TCz, TCz-
3,9PA-TCz and TCz-9PA-TCz.
obtained from the UV absorption spectra of the three com-
pounds in toluene solution.
OLED devices fabricated by the solution spin coating method
In order to explore the electroluminescence performance of
compounds TCz-3PA-TCz, TCz-3,9PA-TCz and TCz-9PA-TCz,
we have fabricated blue OLED devices consisting of ITO/PED-
OT:PSS (30 nm)/CBP:TCz-nPA-TCz (55 nm)/TPBi (35 nm)/Ca:Ag
(TCz-3PA-TCz for A1, TCz-3,9PA-TCz for A2 and TCz-9PA-TCz
for A3) by solution spin coating. The electroluminescence
spectra, current density–voltage–luminance characteristics, current
efficiency–current density characteristics and external quantum
efficiency–current density characteristics of the blue devices are
Conclusions
shown in Fig. 5. The device performance is summarized in Table 2. In summary, three novel blue fluorescence isomer emitters
All devices show blue emission with peaks at 448, 444 and TCz-3PA-TCz, TCz-3,9PA-TCz and TCz-9PA-TCz based on rigid
464 nm, whose CIE values are (0.163, 0.121) for A1, (0.157, pyrazine-bridged carbazole frameworks were synthesized and
0.103) for A2 and (0.163, 0.205) for A3. As shown in Fig. 5(b), the characterized. TCz-3,9PA-TCz in the thin solid powder state
maximum luminance of devices A1, A2 and A3 is 3252 cd mꢀ2
,
showed much higher thermal stability than that of TCz-3PA-
4404 cd mꢀ2 and 8052 cd mꢀ2, and the maximum current TCz and TCz-9PA-TCz. All three isomer emitters display weak
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Fig. 5 Current density–voltage–luminance characteristics (a), current efficiency–luminance characteristics (b), electroluminescent spectra (c) and
external quantum efficiency–luminance (d) characteristics of the devices A1, A2 and A3.
Table 2 Summary of the blue OLED performance based on TCz-3PA-TCz, TCz-3,9PA-TCz and TCz-9PA-TCz by solution spin coating and thermal
deposition in vacuum
100 cd mꢀ2
1000 cd mꢀ2
EQEe
OLEDs Vona/V lems/nm PEmaxb/lm wꢀ1 CEmaxc/cd Aꢀ1 PEb/lm wꢀ1 CEc/cd Aꢀ1 PEb/lm wꢀ1 CEc/cd Aꢀ1 Lmaxd/cd mꢀ2 (100 cd mꢀ2) CIE(x, y)
A1
A2
A3
B1
B2
B3
B4
4.3
3.9
4.0
3.3
3.5
3.5
3.5
448
444
464
452
452
452
452
0.84
1.41
2.46
2.33
2.47
2.64
2.59
1.86
2.39
4.12
4.16
4.72
5.06
5.13
0.83
1.29
2.41
2.33
2.47
2.64
2.59
1.61
2.04
3.89
4.16
4.72
5.06
5.13
0.68
1.11
1.79
1.13
1.16
1.24
1.17
1.65
2.19
3.48
2.96
3.26
3.56
3.51
3252.32
4404.31
8052.0
5608.23
4283.74
4823.04
6369.67
2.19
2.30
2.64
3.49
3.74
3.79
3.85
(0.163, 0.121)
(0.157, 0.103)
(0.163, 0.206)
(0.161, 0.152)
(0.164, 0.160)
(0.168, 0.168)
(0.169, 0.169)
a
b
c
d
e
Driving voltage. PE. CE. Lmax
. EQE are the maximum power efficiency, current efficiency, brightness and external quantum efficiency.
solvatochromism and aggregation-induced emission quenching interaction induced quenching in the doped emitting layer, and
characteristics. The N-linked model in TCz-9PA-TCz can elevate the external quantum efficiency of the devices is roughly con-
the HOMO energy level to some degree, while the 3,9-linked sistent with the absolute PLQYs of TCz-3PA-TCz, TCz-3,9PA-TCz
model can make the LUMO energy level deeper when compared and TCz-9PA-TCz in the undoped and doped solid film states.
with that of TCz-3PA-TCz. The driving voltage, maximum exter- Furthermore, the optimized OLED devices fabricated by thermal
nal quantum efficiency, maximum current efficiency, maximum deposition in vacuum with TCz-3,9PA-TCz as a dopant emitter
power efficiency and maximum luminance of doped OLEDs showed the highest external quantum efficiency of 3.85% at
based on TCz-9PA-TCz fabricated by spin coating were 4.0 V, a luminance of 100 cd mꢀ2, and the linking model analogous
2.67%, 4.12 cd Aꢀ1 and 8052 cd mꢀ2, which were much better to TCz-3,9PA-TCz was validated to be a more feasible way to
than those based on TCz-3PA-TCz and TCz-3,9PA-TCz. The low combine pyrazine with carbazole moieties to produce highly
mge does not play a unique role in suppressing dipole–dipole efficient and stable electroluminescent blue light materials.
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Fig. 6 Current density–voltage–luminance characteristics (a), current efficiency–luminance characteristics (b), power efficiency–luminance charac-
teristics (c) and external quantum efficiency–luminance characteristics (d) of devices B1–B4. Inset: The electroluminescent spectra of devices B1–B4.
7 S. O. Jeon and J. Y. Lee, J. Mater. Chem., 2012, 22(21),
10537–10541.
8 Q. Zhang, D. Fang, H. Jiang, X. Zhang and H. Zhang, Org.
Conflicts of interest
The authors declare no competing financial interest.
Electron., 2015, 27, 173–182.
9 Q. Zhang, J. Li, K. Shizu, S. Huang, S. Hirata, H. Miyazaki and
C. Adachi, J. Am. Chem. Soc., 2012, 134(36), 14706–14709.
Acknowledgements
Project supported by the Major Research Program from the
State Ministry of Science and Technology (No. 2012CB933301),
the National Natural Science Foundation of China (No.
21574068) and the Priority Academic Program Development
of Jiangsu Higher Education Institutions (No. YX03001).
10 Q. Q. Liang, C. M. Han, C. B. Duan and H. Xu, Adv. Opt.
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