Synthesis and luminescent properties of novel organic luminescent materials based on carbazole derivatives

Article abstract of DOI:10.1007/s11696-018-0465-z

Four novel host materials C1–C4 based on carbazole derivatives have been designed, synthesized, characterized, and applied as organic light-emitting devices. This report presents a novel approach, which combines carbazole and aromatic hydrocarbons through methylene. The formed molecules exhibited twisted structures, which resulted in high glass transition temperatures (T_g), ranged from 90.4 to 148.0?°C. They also had high optical bandgaps (E_g), their optical energy bandgaps are determined by absorption edge technique as 3.22 to 3.50?eV, evaluated photo-physical properties of these synthesized novel chromophores, the optical properties such as maximum absorption and emission wavelengths (λ, nm), molar extinction coefficients (ε, cm^{? 1} M^{? 1}), Stoke’s shifts (Δλ_ST, nm), and quantum yields (φ_F) also studied the effect of solvent polarity on absorption and emission of these carbazole derivatives, and these compounds exhibited intense absorption bonds between 250 and 325?nm, which are attributed to the π–π* transition of the carbazole-centered units. In addition, they showed blue fluorescence in different solvents. These compounds showed bathochromic shift with the increase in the solvent polarity.

Full text of DOI:10.1007/s11696-018-0465-z

Chemical Papers
https://doi.org/10.1007/s11696-018-0465-z
SHORT COMMUNICATION
Synthesis and luminescent properties of novel organic luminescent
materials based on carbazole derivatives
Yingxiang Duan1 · Yanhua Yang1 · Bo Cen2 · Kecheng Shen2 · Xian Tao2 · Yingzhong Shen1
Received: 14 December 2017 / Accepted: 28 March 2018
© Institute of Chemistry, Slovak Academy of Sciences 2018
Abstract
Four novel host materials C1–C4 based on carbazole derivatives have been designed, synthesized, characterized, and
applied as organic light-emitting devices. This report presents a novel approach, which combines carbazole and aromatic
hydrocarbons through methylene. The formed molecules exhibited twisted structures, which resulted in high glass transition
temperatures (Tg), ranged from 90.4 to 148.0 °C. They also had high optical bandgaps (Eg), their optical energy bandgaps
are determined by absorption edge technique as 3.22 to 3.50 eV, evaluated photo-physical properties of these synthesized
novel chromophores, the optical properties such as maximum absorption and emission wavelengths (λ, nm), molar extinction
coefcients (ε, cm−1 M−1), Stoke’s shifts (ΔλST, nm), and quantum yields (φF) also studied the efect of solvent polarity on
absorption and emission of these carbazole derivatives, and these compounds exhibited intense absorption bonds between
250 and 325 nm, which are attributed to the π–π* transition of the carbazole-centered units. In addition, they showed blue
fuorescence in diferent solvents. These compounds showed bathochromic shift with the increase in the solvent polarity.
Keywords Carbazole · Methylene · Tg · Eg · The optical properties · Blue fuorescence
Introduction
challenges that need to be addressed, such as development,
device design, and electronics. OLED remains one of the
hottest areas of organic electronics.
Organic light-emitting diodes (OLEDs) have attracted exten-
sive research attention in the few years due to its appealing
commercial interest in the feld of new-generation fat-panel
displays, energy-saving lighting sources, and virtual real-
ity. Numerous works have been done to push the devel-
opment of OLEDs, since the frst device was designed by
Tang (Uoyama et al. 2012; Tang and VanSlyke 1987; Baldo
et al. 1998; Yang et al. 2015). Although OLEDs technology
has been successfully commercialized, there are still many
Carbazole and its derivatives have been extensively stud-
ied over the past two decades for their particular electrical,
electrochemical, and photo-physical properties and thermal
stability (Zhuang et al. 2012; Tsai et al. 2009). Due to their
electron-donating properties, these materials are typical
hole-transporting units that can be easily modifed with
electron-transporting units in diferent positions. There-
fore, it is more efcient and easier to inject both electrons
and holes into the deep blue emitters. Meanwhile, there is
a good balance between electrons and holes. On the other
hand, the electron-rich electro-activity of carbazole-based
molecular materials, which overlap in the small transitions
of n–π, resulted in smaller energy exchange (Justin Thomas
et al. 2001). It is reported that carbazole and its derivatives
have great promise as photoelectric functional materials for
their photo-physical and fuorescent properties in the blue
light region (Li et al. 2012; Liang et al. 2011; Kim et al.
2006; Boudreault et al. 2009). Carbazole and its derivatives
not only can be used as small molecules in fuorescent mate-
rials, but also can be designated as phosphorescent materi-
als and thermally activated delayed fuorescence materials
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s11696-018-0465-z) contains
supplementary material, which is available to authorized users.
* Yingzhong Shen
yz_shen@nuaa.edu.cn
1
Applied Chemistry Department, School of Material
Science and Engineering, Nanjing University
of Aeronautics and Astronautics, Nanjing 210016,
People’s Republic of China
2
Jiangsu MO Opto-Electronic Material Co. Ltd.,
Qinglongshan Branch Road 1, Zhenjiang New District,
Zhenjiang 212132, People’s Republic of China
Vol.:(0123456789)
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Chemical Papers
(Wang et al. 2011; Leyla et al. 2017; Gupta et al. 2010; Chen
et al. 2006; Diaz et al. 2001). In recent years, the synthesis
of carbazole derivatives and their applications become the
highlight in blue light-emitting organic light-emitting diodes
(Treutler and Ahlrichs 1995; Becke. 1993; Kim et al. 2010).
In this report, we have designed a series of small carbazole-
related materials as organic luminescent materials and stud-
ied the concept of molecular design, optical physical prop-
erties, and thermal stability. Electrochemical properties are
also discussed. Finally, the trends and development trends of
carbazole organic light-emitting devices in the foreseeable
future are also introduced (Li et al. 2011; Tao et al. 2008;
Sasabe et al. 2009).
Synthesis of 3,6‑dibromocarbazole (2)
A mixture of carbazole (1.00 g, 6.00 mmol) and silica gel
(8.00 g, 133.33 mmol) was added to CH2Cl2 (60 mL) at room
temperature, N-bromosuccinimide (1.07 g, 12.00 mmol),
added to the above system, the mixture was reacted for 24 h
at room temperature, then fltering and evaporating of the
solvent, and the residue was extracted with n-hexane/ethyl
acetate (10:1, v/v) to generate a white solid. 1.71 g, yield:
88%.
1
Mp: 204–206 °C. H NMR (400 MHz, DMSO-d6, δ,
ppm): 11.60 (s, 1H, –NH), 8.44 (d, J = 8.8 Hz, 2H), 7.54
(d, J = 8.9 Hz, 2H), 7.48 (d, J = 8.7 Hz, 2H). 13C NMR
(101 MHz, DMSO-d6, δ, ppm): 139.3, 129.2, 123.9, 123.8,
113.8, 111.5. Anal. calcd for C12H7Br2N: C, 44.35; H, 2.17;
Br, 49.17; N, 4.31. Found: C, 44.28; H, 2.15; Br, 49.14; N,
4.29.
Experimental
General information
Synthesis of 3,6‑dibromo‑9‑(2‑bromobenzyl)‑9H‑carbazole
(3)
The NMR spectra were recorded on a Bruker Advance 400
1
spectrometer at resonant frequencies of 400 MHz for H
and 101 MHz for 13C nuclei using DMSO-d6 or CDCl3 as
the solvent. Uncorrected melting points were observed in
sealed capillaries. UV–visible absorption spectroscopy
was obtained on a UV-2550 UV–Vis spectrophotometer.
Fourier transform infrared (FT-IR) spectra of intermedi-
ates and monomer were obtained from KBr pellets with
a Nicolet NEXUS-470 FT-IR spectrometer. Diferential
scanning calorimetry (DSC) studies were performed using
a Netzsch DSC 214, at a heating rate of 10 °C min−1 from 40
to 300 °C under a nitrogen atmosphere, respectively. Tg was
determined from the second heating scan. The fuorescence
and UV–Vis absorption spectra of solutions were measured
at room temperature. The cyclic voltammetry properties
were studied on a CHI 660b electrochemical workstation.
Cyclic voltammetry (CV) was carried out using nitrogen-
purged N,N-dimethylformamide (DMF) for the reduction
and dichloromethane for the oxidation scan at room tem-
perature with a CHI voltammetric analyzer. Tetrabutylam-
monium perchlorate (n-Bu4NClO4) (0.1 M) was used as the
supporting electrolyte. The conventional three-electrode
confguration consists of a platinum working electrode, a
platinum wire auxiliary electrode, and a saturated calomel
electrode (SCE). Cyclic voltammograms were obtained at a
scan rate of 100 mV s−1.
3,6-dibromocarbazole (3.25 g, 10.00 mmol) was added to
DMF (30 mL) at room temperature. It was stirred for 30 min
and to this solution was then added 2-bromobenzyl bromide
(4.30 g, 70.00 mmol) and K2CO3 (8.00 g, 70.00 mmol).
The reaction temperature was controlled at about 60 °C.
The mixture was stirred at this temperature for 5 h and then
poured into distilled water (200 mL). The precipitate was
collected by fltration, washed with water (3×100 mL), and
air-dried. The product was recrystallized from ethanol to
give yellow solid 3.30 g. Yield: 67%.
Mp: 225–228 °C. 1H NMR (400 MHz, CDCl3, δ, ppm):
8.51 (s, 2H), 7.74 (d, J=8.8 Hz, 2H), 7.60 (d, J=8.9 Hz,
2H), 7.48 (d, J = 8.7 Hz, 2H), 7.21 (s, 1H), 7.12 (d,
J=8.8 Hz, 2H), 5.70 (s, 2H, –CH2). 13C NMR (101 MHz,
DMSO-d6, δ, ppm): 139.7, 139.6, 135.9, 133.3, 129.9, 129.6,
128.6, 127.5, 124.1, 123.7, 122.2, 112.4, 112.2, 46.8. Anal.
calcd for C19H12Br3N: C, 46.19; H, 2.45; Br, 48.52; N, 2.84.
Found: C, 46.16; H, 2.43; Br, 48.50; N, 2.83.
General procedure for synthesis
of 9‑([1,1′‑biphenyl]‑2‑ylmethyl)‑3,6‑diphenyl‑9H‑carba‑
zole (C1) and other fnal products 9‑((4′‑(naphthalen‑1‑yl)‑
[1,1′‑biphenyl]‑ 2‑yl)methyl)‑3,6‑bis(4‑(naphthalen‑1‑yl)‑
phenyl)‑9H‑carbazole (C2),4,4′‑(9‑((4′‑ (diphenylamin
o)‑[1,1′‑biphenyl]‑2‑yl)methyl)‑9H‑carbazole‑3,6‑ diyl)
bis(N,N‑diphenylaniline) (C3), 9,9′‑((9‑((4′‑(8aH‑9λ4‑carbaz
ol‑9‑yl)‑ [1,1′‑biphenyl]‑2‑yl)methyl)‑9H‑carbazole‑3,6‑diyl)
bis(4,1‑phenylene))bis(8aH‑9λ4‑carbazole) (C4)
Synthesis
The starting material, arylboronic acids including phenylb-
oronic acid (A1), [4-(diphenylamino)phenyl]boronic acid
(A2), and 4-(1-naphthyl)phenylboronic acid (A3) were pur-
chased and [4-(9H-carbazol-9-yl)phenyl]boronic acid (A4),
was used without further purifcation.
The reaction mixture containing intermediate 3 (289 mg,
1 mmol), the corresponding Phenylboronic Acid for
A1, [4-(diphenylamino)phenyl]-boronic acid for A2,
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Chemical Papers
4-(1-naphthyl)phenylboronic acid for A3, and [4-(9H-car-
bazol-9-yl)phenyl]boronic acid for A4 (1.1 mmol), toluene
(10 mL), H2O (5 mL), K2CO3 (2.5 mL, 5 mmol), and tetra-
kis- (triphenylphosphino)palladium(0) (28 mg, 0.02 mmol)
was refuxed under nitrogen atmosphere for 6 h. After the
reaction fnished, the reaction mixture was cooled to room
temperature and diluted by water (20 mL). The reaction
mixture was separated into organic layer and aqueous layer,
and the layer of which was extracted with dichloromethane
(3×20 mL). After the combined organic layers were washed
with brine (50 mL), dried over anhydrous magnesium sul-
fate, and fltered, the solvent was removed under reduced
pressure. Then, the residue was isolated by column chro-
matography over silica using petroleum ether/ethyl acetate
(10:1, v/v) as eluent and further purifed by recrystalliza-
tion in methanol/chloroform to give pure products as white
solids.
6.93 (d, J = 2.1 Hz, 1H), 5.89 (s, 2H, –CH2–). 13C NMR
(101 MHz, CDCl3, δ, ppm): 141.0, 140.9, 140.7, 140.4,
139.6, 139.1, 138.2, 137.9, 134.3, 133.8, 133.7, 132.7,
132.6, 130.3, 129.5, 128.6, 128.4, 128.3, 128.1, 127.7,
127.7, 127.5, 126.8, 126.4, 126.3, 126.1, 125.9, 125.5,
125.5, 132.7, 45.5. Anal. calcd for C73H54N4: C, 88.81; H,
5.51; N, 5.68. Found: C, 88.79; H, 5.50; N, 5.66.
9,9′‑((9‑((4′‑(8aH‑9λ4‑carbazol‑9‑yl)‑[1,1′‑biphenyl]‑2
‑yl)methyl)‑9H‑carbazole‑3,6‑diyl) bis(4,1‑phenylene))
bis(8aH‑9λ4‑carbazole) (C4)
Yield: 65%. Mp: 267–269 °C. 1H NMR (400 MHz, DMSO-
d6, δ, ppm): 8.42 (s, 2H), 8.08 (d, J=8.5 Hz, 6H), 7.79 (d,
J=8.2 Hz, 4H), 7.69 (d, J=7.7 Hz, 2H), 7.68 (t, J=7.7 Hz,
8H), 7.43 (t, J=7.9 Hz, 7H), 7.35 (t, J=8.0 Hz, 8H), 7.28
(t, J = 7.5 Hz, 2H), 7.21 (t, J = 7.9 Hz, 8H), 7.14 (s, 1H),
5.58 (s, 2H, –CH2–). 13C NMR (101 MHz, CDCl3, δ, ppm):
139.9, 139.9, 139.8, 139.6, 139.4, 138.5, 136.1, 135.1,
133.1, 131.0, 129.5, 129.4, 127.4, 126.7,126.3, 125.9, 125.0,
124.9, 124.5, 119.3, 119.3, 118.8, 108.8, 108.6, 44.6. Anal.
calcd for C73H48N4: C, 89.36; H, 4.93; N, 5.71. Found: C,
89.34; H, 4.90; N, 5.69.
9‑([1,1′‑biphenyl]‑2‑ylmethyl)‑3,6‑diphenyl‑9H‑carbazole
(C1)
Yield: 73%. Mp: 168–170 °C. 1H NMR (400 MHz, DMSO-
d6, δ, ppm): 8.60 (s, 2H), 7.81 (d, J=7.2 Hz, 4H), 7.72 (d,
J=7.5 Hz, 2H), 7.59 (t, J=7.3 Hz, 2H), 7.49 (t, J=2.0 Hz,
5H), 7.47 (d, J=7.3 Hz, 2H), 7.42 (t, 4H), 7.34 (t, J=7.5 Hz,
1H), 6.54 (d, J=7.8 Hz, 1H), 5.64 (s, 2H, –CH2–). 13C NMR
(101 MHz, DMSO-d6, δ, ppm): 141.4, 141.1, 140.6, 140.5,
134.9, 132.1, 130.5, 129.7, 129.3, 129.1, 128.2, 128.0,
127.7, 127.1, 127.0, 126.0, 125.5, 123.5, 119.4, 110.1, 44.6.
Anal. calcd for C37H27N: C, 91.51; H, 5.60; N, 2.88. Found:
C, 91.48; H, 5.59; N, 2.85.
Results and discussion
Synthesis
The synthetic procedure of these materials is shown in
Scheme 1. All these four compounds are synthesized through
a Suzuki cross-coupling reaction between 3,6-dibromo-
9-(2-bromobenzyl)-9H-carbazole (3) and the correspond-
ing carbazole-containing boronic acid. These target com-
pounds, C1, C2, C3, and C4, were then prepared at yields
of 65–78%. All these compounds could be easily purifed
by column chromatography and recrystallization to reach a
high purity for research and applications. The structures of
the newly synthesized compounds were determined by 1H
and 13C NMR spectroscopy and elemental analysis.
9‑((4′‑(naphthalen‑1‑yl)‑[1,1′‑biphenyl]‑2‑yl)methyl)‑3,6‑bi
s(4‑(naphthalen‑1‑yl)phenyl)‑9H‑ carbazole (C2)
Yield: 77%. Mp: 264–266 °C. 1H NMR (400 MHz, DMSO-
d6, δ, ppm): 8.85 (s, 2H), 8.29 (d, J = 7.2 Hz, 5H), 8.09
(d, J = 9.3 Hz, 4H), 7.74 (dd, J = 9.3 Hz, 3H), 7.58 (d,
J = 8.2 Hz, 5H), 7.53 (m, 7H), 6.91 (d, J = 1.8 Hz, 1H),
5.89 (s, 2H, –CH2–). 13C NMR (101 MHz, CDCl3, δ, ppm):
133.8, 132.6, 129.5, 128.4, 128.3, 128.2, 127.7, 127.7,
126.3, 125.9, 125.5, 125.4, 109.5, 45.4. Anal. calcd for
C67H45N: C, 93.13; H, 5.25; N, 1.62. Found: C, 93.08; H,
5.24; N, 1.59.
NMR and IR spectra
Figure 1 shows the 1H NMR spectra of 2 and 3 before and
after the deprotonation of the N–H group. The deprotonation
of the N–H group of the compound 2 and the appearance
of the –CH2– group of the compound 3 were reacted with
2-bromobenzyl bromide. The disappearance of the signal
corresponding to N–H group from the 1H NMR spectra of
solutions of the compounds indicated that the deprotonation
proceeded quantitatively.
4,4′‑(9‑((4′‑(diphenylamino)‑[1,1′‑biphenyl]‑2‑yl)
methyl)‑9H‑carbazole‑3,6‑diyl)bis(N,N‑diphenylaniline)
(C3)
Yield: 78%. Mp: 258–260 °C. 1H NMR (400 MHz, DMSO-
d6, δ, ppm): 8.85 (s, 2H), 8.29 (d, J = 8.4 Hz, 6H), 8.09
(d, J = 8.2 Hz, 4H), 7.92 (dd, J = 7.7 Hz, 5H), 7.74 (d,
J = 7.7 Hz, 6H), 7.55 (m, 19H), 7.31 (d, J = 2.0 Hz, 9H),
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Chemical Papers
Scheme 1 Synthesis of compounds C1–C4
Fig. 1 H NMR spectra of the DMSO-d6 solutions of before and after the deprotonation of the N–H group
Figure 2 shows the IR spectra of C1–C4 and the FT-IR
spectra of carbazole derivatives C1–C4 presented the
characteristics absorption of C–H stretching located at
3068–3028 cm−1, 2961–2905 cm−1. The C=C stretch-
ing located at 1603–1586 cm−1, the bending vibration of
–CH2– bond located at 1486–1469 cm−1, and C-H out-
plane bending vibration located at 900–650 cm−1, respec-
tively. All spectral data are consistent with the expected
structure.
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Chemical Papers
These morphologies and thermal stability are desirable
for host materials, because they inhibit aggregation and
phase separation and ensure the amorphous nature of the
solid flm state.
Photo‑physical properties
The electronic absorption and photoluminescence (PL) spec-
tra of these compounds in CH2Cl2 solution at room tempera-
ture are presented in Fig. 4. The wavelength of maximum
absorption of C1, C2, C3, and C4 in dilute CH2Cl2 solu-
tions (1×10−6 mol·L−1) was observed at 247, 318, 293, and
264 nm, respectively, due to the carbazole-centered π–π*
transition of the unit. The weak absorption in the range of
326–348 nm for C1, which is typically observed in carba-
zole-containing molecules, can be attributed to n–π* transi-
tions of the lone-pair electrons to the entire conjugated back-
bone around the carbazole unit. In contrast to this molecule,
the absorption of C2, C3, and C4 covering the range of
280–370 nm is much stronger. Because of the π-conjugation
enhancement of the phenyl ring in this molecule, the π–π*
transition energy is reduced to lower than that of n–π* transi-
tion energy and dominates the absorption spectrum. Their
optical energy bandgaps were determined by absorption
edge technique as 3.33, 3.22, 3.32, and 3.50 eV, respectively.
Upon optical emission at the absorption maxima, C1, C2,
C3, and C4 emit purple–blue fuorescence with emission
peaks at 383, 413, 388, and 408 nm, respectively. The large
red shift of from C1 to C2, C4 is understandable, because
the π-conjugation expansion and asymmetry of the molecule
must result in a lower energy S1-excited state.
Fig. 2 IR spectra of C1–C4
It should be noted that the emission maximum of C2
shows a relatively large blue shift of about 20 nm as com-
pared to the other two molecules C1, C3. (Table 2) Then,
as shown in Table 3, the larger Stokes shift value of C2
observed in dichloromethane is 69 nm. The quantum yield
of the carbazole derivative was calculated and its fuores-
cence quantum yield was measured by quinine sulfate with
a quantum yield at λex = 285 nm. The quantum yield (ΦF)
was determined by the following formula:
Fig. 3 DSC curves of C1–C4. (DSC traces at the second heating
cycle heating rate 10 °C min−1, nitrogen atmosphere
Thermal properties
(
)/(
)
.
ꢀF = ꢀstd × FAstdn2
FstdAn2std
The thermal and morphological stabilities of these
compounds were investigated by diferential scanning
calorimetry (DSC), as shown in Fig. 3. When the super-
cooled isotropic sample of each compound was reheated
for the second time, a glass transition was detected with
glass transition temperatures (Tg) of 90.4, 129.1, and
109.6–148.0 °C for C1, C2, C3, and C4 (Table 1), in order
of decreasing twisting resistance and increasing molecular
weight. All of these host materials exhibited a Tg higher
than the widely used main material, mCP (Tg = 60 °C),
indicating improved amorphous stability. Its morphologi-
cal stability is the result of the introduction of methylene.
F and Fstd refer to the area under the fuorescence emis-
sion curve for samples and standards; A and Astd refer to the
absorbance of each sample and standard at the excitation
wavelength; and n and nstd refer to the refractive index of the
solvent used for the sample and standard.
In addition, C2 in dichloromethane exhibited the highest
quantum yield value of 0.63 (Table 3), and other compounds
also showed larger values. Because they have strong fuo-
rescence, electron-donating groups can increase the fuo-
rescence intensity.
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Chemical Papers
Table 1 Physical data of C1,
C2, C3, and C4
Compounds
λabs (nm)a
ꢀem (nm)a
Eg (eV)b
HOMO/LUMO (eV)c
Tg °C
max
C1
233
262
301
233
318
232
293
313
233
264
301
383
416
3.33
−5.58/−2.22
90.4
C2
C3
C4
3.22
3.32
3.51
−5.38/−2.16
129.1
109.6
148
388
408
−5.37/−2.05
−5.51/−2.01
aAbsorption and fuorescence wavelengths in dilute dichloromethane solutions
bThe optical bandgap, calculated by the absorption edge technique
cDetermined using electrochemical potentials and optical bandgaps
at 415 nm in ethyl acetate and at 420 nm in DMF, and C3
emits at 391 nm in ethyl acetate and at 398 nm in DMF.
These compounds show a red shift as the solvent polar-
ity increases, indicating that the maximum absorption is
strongly infuenced by the solvent polarity of all compounds
(Table 2). The compound C4 emits at around 415 nm in
most of the listed solvents, but it has red shifted emission
in DMF (427 nm), due to the π-conjugation of the molecule
increases.
Emission spectra were recorded by exciting the 1×10−6
M solutions at their maximum absorption wavelengths, and
for excitation spectra, their maximum emission wavelengths
were used. The emission characteristics of these carbazole
derivatives were also studied and the results obtained are
summarized in Table 2. Figures 5, 6, 7, and 8 represent the
emission overlay spectra of C1–C4. The emission of C1 and
C3 in majority of the solvents lies in the range 390–400 nm,
the emission of C2 in majority of the solvents lies in the
range 415–420 nm, and the emission of C4 in majority of
ethyl acetate, THF, and dichloromethane lies in the range
415–420 nm, but in DMF, a large red shift is observed at
427 nm.
Fig. 4 UV–Vis absorption and PL spectra of C1–C4 in dilute
CH2Cl2 solutions at 293 K
Table 2 Photo-physical properties of the C1–C4 in diferent solvents
em
em
em
em
∕
nm
∕
∕
∕
nm
nm
nm
Solvents
C1 ꢀ
C2 ꢀ
C3 ꢀ
C4 ꢀ
max
max
max
max
Ethyl Acetate
THF
Dichloromethane 383
Acetone
Ethanol
DMF
390
392
415
416
416
415
421
420
391
398
388
393
386
398
401
409
408
416
417
427
Electrochemical properties
386
413
396
The electrochemical properties of these compounds were
measured by cyclic voltammetry (CV) measurements in
deoxygenated (DCM) and dimethylformamide (DMF)
solutions containing 0.1 M tetrabutylammonium per-
chlorate (n-Bu4NClO4) as the supporting electrolyte. The
cyclic voltammograms is shown in Fig. 9. During the
anodic scanning in DCM solution, each compound under-
went a quasi-reversible oxidation process that should be
assigned to the oxidation of the electron-donating carba-
zole ring in each molecule. The additional reduction wave
Efect of solvent polarity on fuorescence emission
The efect of solvent polarity on the fuorescence properties
of the carbazole derivatives was also studied. They show the
longest wavelength absorption maxima in DMF. C1 emits
at 390 nm in ethyl acetate and at 396 nm in DMF, C2 emits
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Chemical Papers
Table 3 Absorption and fuorescence emission data for compounds
molar extinction coefcients (ε, l mol−1 cm−1), Stokes’ shifts, Δλ
(nm), in dichloromethane for 1×10−6
abs
∕
nm
C1–C4 maximum absorption wavelengths (ꢀ
max
), maximum emis-
M
max
), maximum excitation wavelengths (ꢀ
em
em
∕
nm
∕
nm
sion wavelengths (ꢀ
),
max
em
em
∕
nm
abs
∕
nm
∕
Compounds
Δλ/nm
εmax
φ
nm
ꢀ
ꢀ
ꢀ
max
max
max
C1
C2
C3
C4
262
318
313
301
383
416
388
408
332
347
329
350
51
69
59
58
20439
35600
109600
32740
0.3651
0.6342
0.5148
0.6203
Fig. 5 Emission spectra of C1 in diferent solvents
Fig. 7 Emission spectra of C2 in diferent solvents
Fig. 8 Emission spectra of C2 in diferent solvents
Fig. 6 Emission spectra of C2 in diferent solvents
energies were determined from the onset potential of the
frst oxidation wave (Eooxnset) according to the formula of
EHOMO = − (Eooxnset + 4.4) as ca. −5.58, −5.38, −5.37, and
−5.51 eV, respectively. The almost identical HOMO levels
of these molecules are reasonable, since the carbazole ring
stands for the exclusive and the common hole-transporting
of 0.80–1.00 eV was detected for each compound in the
reduction process. For nonprotected carbazole derivatives
at 3,6 sites, this additional reduction peak was frequently
observed in the previous reports (Gong et al. 2012; Deng
et al. 2012). No reduction wave was detected for all these
compounds during the cathodic scan to −2.0 V. The HOMO
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compounds have the longest emission maxima in DMF. All
derivatives displayed large Stokes’ shift values and excellent
photostabilities. Thus, the maximum Stokes shift value of
C2 observed in dichloromethane is 69 nm. In addition, C2
also exhibited the highest quantum yield value of 0.69 in
dichloromethane. These materials can be applied as organic
light-emitting devices due to their excellent thermal and
morphological stability, suitable energy levels, and hole-
transporting capabilities The formed molecules exhibited
twisted structure, which resulted in high glass transition
temperatures (Tg), ranged from 90.4 to 148.0 °C. They also
have high optical bandgaps (Eg), whose optical energy band-
gaps are determined by the absorption edge technique to be
3.33, 3.22, 3.32, and 3.50 eV, respectively. Therefore, our
research may promote the development of general-purpose
hole-transporting materials for OLEDs. They also had high
optical bandgaps (Eg), whose optical energy bandgaps are
determined by the absorption edge technique as 3.33, 3.22,
3.32, and 3.50 eV, respectively. Therefore, our research may
promote the development of the universal hole-transporting
materials for the development of OLEDs.
Fig. 9 Cyclic voltammograms of C1–C4 measured in DCM (anodic)
and DMF (cathodic) at a scan rate of 100 mV s−1
group in all these molecules. The HOMO energy levels
of these compounds are close to the HOMO levels of the
widely used hole transport material 1,1-bis[(di-4-tolylamino)
phenyl]cyclohexane (TAPC, 5.50 eV), indicating small hole
injection barrier from TAPC to the emitting layer when
these compounds are used as the hosts in emitting layer.
During the cathodic scan in DMF solution, each compound
detected an irreversible reduction signal with a very similar
onset potential. The LUMO energies were calculated from
the HOMO energies and the optical bandgaps (Eg), accord-
ing to the formula of ELUMO =(EHOMO +Eg) as ca. − 2.22,
−2.16, −2.05, and −2.01 eV, respectively. In addition, Eg
was determined as the onset potential diference between
the frst oxidation and reduction waves multiplied by the
electron charge (e) for each compound as 3.20, 3.09, 3.13,
and 3.30 eV. All of these electrochemical and electronic data
are listed in Table 1.
Acknowledgements A project funded by the Priority Academic Pro-
gram Development of Jiangsu Higher Education Institutions.
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